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rankrath
2007-11-11, 08:53 PM
Iím writing a sci-fi story for my creative writing class, and seeing as I have no real knowledge in physics to speak of, have come here for help designing ships/weapons.

As of now, the ships are mile long cigars, using rail-guns or something to propel a small chunk of highly dense matter at each other at near light speeds. These guns span the entire length of the ships, are basically magnets curled into a tube, and can be used to assist in interplanetary travel (using a planets magnetic field somehow)
Now, is this a good idea? How much energy would it require? How much damage would it do? Would magnets that large be possible to make?
Thanks in advance.

warty goblin
2007-11-11, 09:19 PM
Railguns are quite possible as weapons go. There's some things I would think about in the setting:

1) Do you want orbital bombardment? If not, I'd shy away from railguns, particularly those that fire projectiles close to light speed, since reletivistic bombardment is basically unstopable and is capable of destroying planets (or at least pretty much everything that lives on them). Another way to solve this problem is to use railguns, but make them smaller, so that even though they can target things on the surface of the planet, the individual rounds won't do such extreme damage.

2) How long do you want battles to last? If you don't want a "first to fire is the winner" type senario, I'd again shy away from this size of railgun, since a hit will most likely destroy pretty much anything- particularly if you don't have shields. Again, this problem can be solved by the use of smaller turreted railguns if you don't want energy weapons, since they would still be effective, but not one hit kill weapons.

3) Do you want some form of energy shields? Shields are a very convenient plot device, but also are scientifically somewhat weak, so you have to decide between plausibility and convenience. If you do use shields, I would plan out how much energy they can absorb and if there's any weapons that can penetrate them easily.

4)Do you use actual faster than light/hyperspace/subspace/space warping, or just very good conventional engines? If you use ftl of some sort what are the limits on it (like can it be used close to a planet or in an atmosphere? Does there exist a technology to interfere with it and force ships back into realspace?). How long do you want travel to take, between say Star Wars where ships cross the galaxy in a week or two, to Star Trek where it takes years, to a non-ftl system where it takes lifetimes?

5) Here's a website that I found quite useful when thinking about my own Sci-Fi setting and all things warfare related:
http://www.projectrho.com/rocket/rocket3x.html#kinetic

I realize that this was higher on questions that answers, but I hope the questions were useful.

Seraph
2007-11-11, 09:21 PM
forget railguns. use lasers, just dont forget to make sure they actually work the way theyre supposed to, IE travelling at the speed of light and being invisible.

Turcano
2007-11-11, 10:00 PM
Electromagnetic pulse weapons are another class of weapons you could consider, and it could even be a modification to the original weapon you described.

VeisuItaTyhjyys
2007-11-11, 10:09 PM
3) Do you want some form of energy shields? Shields are a very convenient plot device, but also are scientifically somewhat weak, so you have to decide between plausibility and convenience. If you do use shields, I would plan out how much energy they can absorb and if there's any weapons that can penetrate them easily.
A very strong electromagnetic field would work if railguns were the primary weapon used, since magnetism can really mess with a weapon wholly based in magnetism.

Talkkno
2007-11-11, 10:09 PM
If you decide to add sheilds, make them some sort exotic matter cotained by a electromatic field, so that turning them off would just be realsing the field, and it being deplated could it be litterly blown away faster then it can be replaced. See current reasearch in "Plasma sheath" for a example.

averagejoe
2007-11-11, 10:35 PM
A very strong electromagnetic field would work if railguns were the primary weapon used, since magnetism can really mess with a weapon wholly based in magnetism.

I'm pretty sure the projectile itself doesn't need to be charged, so I'm not sure what you mean when you say an electromagnetic field would help. Do you mean to throw off the firing mechanism? Because, they'd have to get pretty unrealistically close for that to work.

Also, I'd imagine that such a "shield" would end up accelerating a bunch of space debris toward the ship, which would be inconvenient to say the least.

Edit: To the OP, keep in mind that with such weapons, momentum has to be conserved, so the ship would accelerate in the opposite direction. With a cannon as big as you're proposing, the effect would be fairly profound, because momentum depends both on the size of the projectile and how fast it goes. This isn't insurmountable, just something to keep in mind. It does make a large cannon seem extravagent, and energy inefficient, though, since it would take up so much more of the ship's fuel just to accelerate.

Kojiro Kakita
2007-11-11, 10:45 PM
How about using a new Non-Newtonian solid/liquid for the shields?

BRC
2007-11-11, 10:48 PM
Maybe the ships function by firing 2 shots in opposite directions simultaniously, so the ship itself dosn't move.

VeisuItaTyhjyys
2007-11-11, 10:53 PM
Hm, I'd assumed the projectile itself would have to be magnetized, or at least magnetic, but I guess not. I stand corrected.

Nerd-o-rama
2007-11-11, 11:33 PM
A Gauss Gun/Coil Gun (http://en.wikipedia.org/wiki/Gauss_gun) would require a magnetic projectile. A Railgun (http://en.wikipedia.org/wiki/Railgun) does not.

However, a magnetic field of sufficient strength could deflect most matter, ferrous or not, and would also serve to protect ships from micrometeorites, radiation, and other hazards.

Things that are impractical to do with real physics:

Faster than Light travel: Don't even bother with this one if you want to be "different" in your adherence to science. You're going to have to make something up out of whole cloth.

Artificial gravity: Except on large ships that maintain "gravity" toward their outer hulls by rotating constantly along an axis (see 2001: A Space Odyssey). Doing it any other way is impractical unless you have an arbitrarily large power supply. Which, if you're relativistically accelerating your cannonfire...

Anything bigger than a modern space shuttle working as well in-atmosphere as out of it. By the same token, don't bother making your starships aerodynamic.

Gungnir
2007-11-11, 11:51 PM
They work well in the Halo universe (http://halo.wikia.com/wiki/Magnetic_Accelerator_Cannon).

averagejoe
2007-11-11, 11:55 PM
They work well in the Halo universe (http://halo.wikia.com/wiki/Magnetic_Accelerator_Cannon).

Yeah, but so does artificial gravity, impossible FTL, and a guy who can flip tanks one-handed. And, for some reason, cloaking while holding a big glowy sword. Seriously, what's with that?

Gungnir
2007-11-12, 12:09 AM
Well, artificial gravity is only in there because it would have been a lot harder to make the game without it, they only travel faster than light while in a dimension where normal laws of physics don't apply, we're pretty sure the tank-flipping isn't canon, and, well, I don't have an explanation for the sword. I'm honestly very confused how the sword made of white plasma is cloakable, but the little lights on the gravity hammer aren't.

Also, while we're talking about FTL travel, the system they decided on for the Halo universe is really the only thing I can think of that makes any sort of sense, at least when you don't want to waste plot time traveling through the galaxy at sub-light speeds.

averagejoe
2007-11-12, 12:11 AM
I'm honestly very confused how the sword made of white plasma is cloakable

No, that's my point, the sword isn't cloakable, but the NPC's never seem to put two and two together when a floating sword comes running at them.

Gungnir
2007-11-12, 12:19 AM
No, that's my point, the sword isn't cloakable, but the NPC's never seem to put two and two together when a floating sword comes running at them.

Oh, that's Halo 2, nowadays it turns invisible with you. Maybe it's assumed that you turn it off while not using it, kinda like how you can carry a ladder around with you in DnD without ever having to maneuver it through doors. As for the Elites, they're so pissy, and the sword is so religiously important, that they might stop caring about stealth when they see the "Demon" and zerg rush you.

Edit: Or, Covenant soldiers might assume that the invisible sword belongs to an allied Elite, because it would be ridiculous to think that a heretic would come into possession of such a Holy Weapon. That's probably more likely.

averagejoe
2007-11-12, 12:28 AM
Oh, that's Halo 2, nowadays it turns invisible with you. Maybe it's assumed that you turn it off while not using it, kinda like how you can carry a ladder around with you in DnD without ever having to maneuver it through doors. As for the Elites, they're so pissy, and the sword is so religiously important, that they might stop caring about stealth when they see the "Demon" and zerg rush you.

Edit: Or, Covenant soldiers might assume that the invisible sword belongs to an allied Elite, because it would be ridiculous to think that a heretic would come into possession of such a Holy Weapon. That's probably more likely.

Halo 1 as well. And marines are just as stupid about it. Sword-elietes wouldn't be half so lethal if marines would just shoot them. :smallsigh:

Point is, just 'cause it's in halo, doesn't prove it's a feasable weapon.

Dervag
2007-11-12, 12:28 AM
As of now, the ships are mile long cigars, using rail-guns or something to propel a small chunk of highly dense matter at each other at near light speeds. These guns span the entire length of the ships, are basically magnets curled into a tube, and can be used to assist in interplanetary travel (using a planets magnetic field somehow)
Now, is this a good idea? How much energy would it require? How much damage would it do? Would magnets that large be possible to make?
Thanks in advance.Using the planet's magnetic field is out; those fields are much too weak to accelerate any spacecraft to interplanetary speeds. Using the coilgun as a reaction drive would be more effective (firing high-speed cannonballs so get a kick from the recoil), although still not as efficient as a good conventional engine.

The weapon is viable, although I doubt you could get relativistic speeds on the projectiles that way unless you have some kind of hellaciously powerful 'sufficiently advanced' energy source in your setting.


If you decide to add sheilds, make them some sort exotic matter cotained by a electromatic field, so that turning them off would just be realsing the fieldHuh?

I'm not sure where you're getting this from; it doesn't sound like anything physically viable I've ever heard of. I can see through the spelling, but it sounds like technobabble to me. Which doesn't mean it isn't valid, just that I don't get it.


How about using a new Non-Newtonian solid/liquid for the shields?There are truly stunningly compelling reasons of physics to believe there ain't no such animal.

Gungnir
2007-11-12, 12:41 AM
Halo 1 as well. And marines are just as stupid about it. Sword-elietes wouldn't be half so lethal if marines would just shoot them. :smallsigh:

Point is, just 'cause it's in halo, doesn't prove it's a feasable weapon.

I didn't say it was completely feasible, but Bungie has a way of making things seem realistic enough that it seems like it could be done, given 500 years of technological development. The only things that don't have a good "scientific" (meaning said science doesn't exist yet) explanation are energy shields and artificial gravity.

WalkingTarget
2007-11-12, 12:00 PM
As of now, the ships are mile long cigars, using rail-guns or something to propel a small chunk of highly dense matter at each other at near light speeds...
Now, is this a good idea? How much energy would it require? How much damage would it do? Would magnets that large be possible to make?
Thanks in advance.

Well, accelerating a 2kg mass to 90% of light speed from rest takes something like 230 peta-Joules of energy (if my math is right at least, and "peta" is a million billions). This is about 30% more energy than reaches the Earth from the Sun every second. This is also more energy than was generated by the largest thermonuclear device ever detonated (the Tsar Bomba (http://en.wikipedia.org/wiki/Tsar_Bomba) test) which was about 50 megatons. So even very small projectiles take a massive amount of energy to fire at "near light speeds" but you get quite a bang for your buck as well.

If you can cover an earth-sized planet with 100% efficiency solar panels and weaponry, you could fire a five-pound projectile every few seconds which will impact for the equivalent of 50 megatons of TNT (think about 3000 times the bomb dropped on Hiroshima). Hope that helps on understanding power levels involved.

VeisuItaTyhjyys
2007-11-12, 12:25 PM
There are truly stunningly compelling reasons of physics to believe there ain't no such animal.

Yeah, but Aristotle's mathematical proof that the earth was at the centre of the universe was also stunningly compelling. It was also very, very wrong in its first step.

Icewalker
2007-11-12, 01:59 PM
Well, most awesome sci-fi ideas require ungodly amounts of energy. You just need some ridiculous energy source, like harnessing antimatter energy releases :smallbiggrin:

Of course, you could just use the antimatter as a weapon instead.

Ashtar
2007-11-12, 02:58 PM
1) Railguns firing at relativistic speeds use up a enormous amount of energy. Which points to very efficient power sources: Highly efficient fusion coupled with huge power banks, Antimatter, Total conversion (E=mc^2).

2) Unless ships have inertia-less drives, ultra high speeds or very efficient point defence systems, it is a game of who shoots first. Human intervention into such systems will be exceedingly limited, probably choosing a strategy which will then be executed by the computer (reaction speed for humans is far too slow).

3) The detection range of enemy ships is also a very important question. If you can only detect an enemy when you receive his "radiation signature" (which only travels at the speed of light), then there is close to no defence possible against shots fired by the enemy.

4) The damage of even a 200g mass at relativistic speed is such that unless you invent an impossibly hard material (such as one needed on the outside of a dyson sphere), an impact causes catastrophic damage.

Interception of such projectiles could only happen at short range (since you can't detect them before their signature arrives at the speed of light), so to protect yourself you have very little time. Autonomous ultra-high powered lasers might be able to vaporize the projectile, but then you would still get hit by a relativistic gas cloud, which might still cause immense damage. Or, if you have enough time, it might be a matter of firing an identical projectile on an opposing course, which would be very challenging.

This is all if you take the view that nothing goes FTL.

Now if you get FTL detection, that changes slightly the game. Space fights might then be two ships firing relativistic buckshots hoping to overwhelm the other's point defence systems.

WalkingTarget
2007-11-12, 02:58 PM
Well, most awesome sci-fi ideas require ungodly amounts of energy. You just need some ridiculous energy source, like harnessing antimatter energy releases :smallbiggrin:

Of course, you could just use the antimatter as a weapon instead.

True, if anti-matter is around, then you could get the same effect as my example with one kg of each per shot (or he could set off large thermonuclear explosives and use the energy generated to power the projectile). I guess a good question for the OP is what sort of energy production is available.

I mean, the power is the same, it's a matter of the speed of the attack. Firing a nuke or anti-matter bomb would have the same final payload, but unless he's firing them at relativistic speeds they take much longer to get there (actually, that's a neat idea - shoot a wad of anti-matter at high speed so you get the kinetic damage from the impact, then the damage from the matter/anti-matter annihilation).

Of course, the small relativistic projectile only has that large of an effect if most/all of the momentum is absorbed by the target. Is the purpose to deal massive impact-event damage to, say, planets (think craters), or maybe just punch a hole straight through another ship (or two, or ten if they're all lined up)?

Ashtar
2007-11-12, 03:05 PM
Even a small mass (200g) impacting a pressurised section at close to light speed will cause hypersonic shock waves and superheating inside a ship. The mass itself will convert to plasma on impact, burning through nearly anything. It will be nasty.

Another possibility, is instead of magnetism, allow the science to advance to the level of using gravitic technology. Then you can use the ship's drive both as a reactionless thruster (for travelling) and as a reaction thruster (for combat).

If you use magnetism, you'll probably need supercooled supra conducting materials for your magnets.

Gungnir
2007-11-12, 03:13 PM
For future reference, how do you find things like how much energy it takes to propel an object of x mass at y speed? I'm sure I knew this at one point but it has completely escaped me.

averagejoe
2007-11-12, 03:19 PM
For future reference, how do you find things like how much energy it takes to propel an object of x mass at y speed? I'm sure I knew this at one point but it has completely escaped me.

Energy conversion, I would suppose. A moving object has kinetic energy (1/2)mv^2, and that's how much energy you need to put into it to accelerate it to velocity v from rest.

WalkingTarget
2007-11-12, 03:26 PM
For future reference, how do you find things like how much energy it takes to propel an object of x mass at y speed? I'm sure I knew this at one point but it has completely escaped me.

Energykinetic = ((m*c^2) / sqrt(1 - (v/c)^2)) - (m*c^2)

Where m is the object's rest mass and v is the end velocity relative to the observer.

Edit - averagejoe's equation is the classical mechanics definition of kinetic energy, this one incorporates special relativity.

Another edit - the below post has the formula in a much more readable format.

Emperor Tippy
2007-11-12, 03:27 PM
Energy conversion, I would suppose. A moving object has kinetic energy (1/2)mv^2, and that's how much energy you need to put into it to accelerate it to velocity v from rest.

That doesn't hold true with relativistic objects.

http://upload.wikimedia.org/math/c/0/9/c0968bfdb72bbbd68aa09fd36851344b.png

Thats the correct formula.

rankrath
2007-11-12, 03:59 PM
Ok, so after a bit of feedback, I've come up with an expanded design of warships/weapons;

Sigre battleships "discs of death"
Design: a larger disc, about 1K in diameter connected to a disc half that size by a shaft 100m thick and 250m long. The central core holds the ships power supply, a nuclear fusion reactor, and the bottom disc is the engine.

Weapons: railgun turrets along large disc, numerous point defense missiles.
Shields: none relies on slender profile to avoid being hit.

Carries no fighters

Calmar ships:
Design: as first post, only with a shortened rail barrel to make room for other stuff.
engines at back, power supplied by anti matter.

Weapons: aside from the main rail cannon, coil gun point defense guns.
Shields: nano particles held in a protective shield by electromagnetic field.
carries twenty-four fighters

Chimera defense stations:
Design: pinwheel, each arm is a high powered laser capable of delivering a beam powerful enough to vaporize a lake, they rely on solar power for energy, the lasers and crew quarters taking up most of the station.
Carries no fighters.

Solo
2007-11-12, 04:05 PM
Not strictly on topic, but you may find this interesting:

http://qntm.org/destroy#Methods%20for%20destroying%20the%20Earth

http://qntm.org/fictional

averagejoe
2007-11-12, 04:09 PM
That doesn't hold true with relativistic objects.

http://upload.wikimedia.org/math/c/0/9/c0968bfdb72bbbd68aa09fd36851344b.png

Thats the correct formula.

Yeah, yeah, but that one's harder to remember, and more of a pain to work with. :smalltongue: (Okay, so I completely and inexcusably forgot. So sue me. I've been doing a buncha quantum mechanics lately, and they don't take kindly to relativity around there.)

(No excuses. I'm shamed forever. The only solution is honorable suicide.)

Seraph
2007-11-12, 05:25 PM
I hate to repeat myself, but I cant ever understand this obsession with railguns and coilguns and jacketed plasma bursts and all this silliness. a decent laser defense grid would just atomize any solid-matter ammunition you fire long before it hits me.

SurlySeraph
2007-11-12, 06:00 PM
I hate to repeat myself, but I cant ever understand this obsession with railguns and coilguns and jacketed plasma bursts and all this silliness. a decent laser defense grid would just atomize any solid-matter ammunition you fire long before it hits me.

True. But a clump of gas moving at near lightspeed is still a bunch of atoms moving at near light speed, and will still do a lot of damage. Probably not as much damage as a solid projectile, but still a lot of damage. In fact, since the lasers would of course heat up the projectile to vaporize it, the atoms would hit with slightly more energy distributed over a wider area.

Emperor Tippy
2007-11-12, 06:06 PM
I hate to repeat myself, but I cant ever understand this obsession with railguns and coilguns and jacketed plasma bursts and all this silliness. a decent laser defense grid would just atomize any solid-matter ammunition you fire long before it hits me.

Yes and no.

The following assumes no FTL sensors exist (a reasonable assumption).

If a projectile is launched at .8c at a target 1 light minute away then the time from detection to impact is a bare 6 seconds. And your trying to hit a target potentially the size of a fist with a laser at most a centimeter across. If the laser turret is misaligned by even a millimeter then it will miss the missile by several kilometers.

And even if you hit the missile, you still have the remains of the missile traveling at .8c. They aren't likely to hit but they can be a pain.

Seraph
2007-11-12, 06:10 PM
True. But a clump of gas moving at near lightspeed is still a bunch of atoms moving at near light speed, and will still do a lot of damage. Probably not as much damage as a solid projectile, but still a lot of damage. In fact, since the lasers would of course heat up the projectile to vaporize it, the atoms would hit with slightly more energy distributed over a wider area.


projectile weapons are useful because all that energy is getting concentrated into one small point. a cloud probably has a good bit more mass than a magazine full of bullets, but a plane flying through one at the same speed as a bullet moves doesn't get damaged nearly as much as if someone had unloaded a gun into it while parked. when the atomized cloud hits the ship, its going to be taking up a greater amount of space (as the dispersing metal would give outward velocity to the atoms) and thus do significantly less damage over a wide area, which isnt really useful at all - otherwise, anti-carrier jets would be loaded out with napalm.

EDIT:


Yes and no.

The following assumes no FTL sensors exist (a reasonable assumption).

If a projectile is launched at .8c at a target 1 light minute away then the time from detection to impact is a bare 6 seconds. And your trying to hit a target potentially the size of a fist with a laser at most a centimeter across. If the laser turret is misaligned by even a millimeter then it will miss the missile by several kilometers.

And even if you hit the missile, you still have the remains of the missile traveling at .8c. They aren't likely to hit but they can be a pain.


computer targeting systems in a time of space combat almost certainly would be able to react within 6 seconds. and even if you can't directly hit it, you can fire multiple lasers at an intersecting point along its flightpath and let it fly into them. if it has the ability to avoid this type of configuration, that means more time to snipe it. lastly, the thing with missiles is that they usually don't do damage purely by mass, they have some kind of explosive that would likely be triggered if the missile itself it hit by a laser, meaning that the rapidly approaching cloud of molecules wont have NEARLY the same kind of destructive energy.

warty goblin
2007-11-12, 06:20 PM
The energy of the cloud of particles is, as pointed out above, the same as that of the projectile. Thus if the original had 2 kilotons of energy, the cloud will too. Sure, its more spread out, but I guanentee that a ship will notice that kind of impact when its armor melts off. For one thing, its pretty likely to serously damage any systems on the outside of the ship- like say that PD grid.

Another reason that railgun type weapons are popular is that they scale very nicely and simply. Lasers are a good bit more complex, and the really nasty kinds (gamma lasers for example) are really hard to get.

Emperor Tippy
2007-11-12, 06:21 PM
React? Yes. Whether or not they can effectively engage the target depends on far to many variables.

As for the warhead, your best option is prolly multiple bomb pumped X-Ray lasers. See Project Excalibur for more details.

Seraph
2007-11-12, 06:41 PM
Another reason that railgun type weapons are popular is that they scale very nicely and simply. Lasers are a good bit more complex, and the really nasty kinds (gamma lasers for example) are really hard to get.

of course, the benefit of a laser outside of speed is the fact that it does not work upon a principle that leads to the weapon itself being ripped to shreds by the very process of being fired, a point people like to leave out when they discuss railguns.

warty goblin
2007-11-12, 07:32 PM
of course, the benefit of a laser outside of speed is the fact that it does not work upon a principle that leads to the weapon itself being ripped to shreds by the very process of being fired, a point people like to leave out when they discuss railguns.

Current Railguns tear themselves apart true, but its not an insurmontable obstacle. Current lasers also are about 30% energy efficient, dissappating the other 70% of their energy as heat, meaning that if your laser equiped ship engaged in combat for more than a few minutes it would probably roast its own crew alive. Again, a most likely surmountable obstacle.

Gungnir
2007-11-13, 01:13 AM
By the way, you guys might want to cut back a little, HAZMAT is having trouble keeping up with the dead catgirl removal. You're gonna get a ticket.

Also, I deeply regret that my high school didn't have a physics class.

Dervag
2007-11-13, 01:25 AM
This isn't an argument about a roleplaying game, so the Law of Catgirls doesn't apply here.

Skjaldbakka
2007-11-13, 01:34 AM
Shields: nano particles held in a protective shield by electromagnetic field.

I like this concept.

Nerd-o-rama
2007-11-13, 02:25 AM
...You can't just stick "nano" in front of a random word and have it mean small. I assume you meant nanometer-scale particles? Why that particular size? Heck, the larger the particles of a barrier, the more likely they are to stop incoming projectiles, just as a factor of momentum change. That's why big ceramic plates are better at stopping bullets than a fine coating of dust.

EDIT: That was from the first post? Sorry I missed it.

TheRiov
2007-11-13, 09:29 AM
a 'cloud' of matter --plasma or otherwise is not an effective projectile weapon, unless it is contained by a magnetic bottle.

In order for something to to be in gaseous form (vaporized by laser etc) the individual molecules have to have a significant Kinetic energy. Unfortunatly, that KE is random. In fact, air molecules at room temperature have an average speed of ~500 m/s. In one second time, in a vacuum, it would expand to approximatly 1 kilometer in size.

Energy of a gaseous system:
1/2*mv^2=E

Temperature of a gas:
3/2*kT=T

solving for the velocity of an average particle:

sqrt(3*T*k/m) = v


IF you can vaporize an incoming projectile it rapidly becomes a non-threat.

The idea then is to make your projectiles large enough (or make them not absorb energy) so they cannot be vaporized.

For that you need a larger weapon.

WalkingTarget
2007-11-13, 09:41 AM
a 'cloud' of matter --plasma or otherwise is not an effective projectile weapon, unless it is contained by a magnetic bottle.

In order for something to to be in gaseous form (vaporized by laser etc) the individual molecules have to have a significant Kinetic energy. Unfortunatly, that KE is random. In fact, air molecules at room temperature have an average speed of ~500 m/s. In one second time, in a vacuum, it would expand to approximatly 1 kilometer in size.

Energy of a gaseous system:
1/2*mv^2=E

Temperature of a gas:
3/2*kT=T

solving for the velocity of an average particle:

sqrt(3*T*k/m) = v


IF you can vaporize an incoming projectile it rapidly becomes a non-threat.

The idea then is to make your projectiles large enough (or make them not absorb energy) so they cannot be vaporized.

For that you need a larger weapon.

Are those equations for a gaseous system in a stationary frame of reference? If the entire system is heading for you at .9c the velocity is considerably more than sqrt(3*T*k/m) (i.e. it's .9c + or - some fraction of sqrt(3*T*k/m) relative to the target).

Think of it like a shotgun slug vs. buckshot. Fired from the same weapon, one stays in one piece, the other has smaller particles spreading out relative to each other, but they'll both mess you up if you get in the way.

TheRiov
2007-11-13, 10:34 AM
doesn't matter. Relativity states that in a non-accellerating frame of reference its velocity in relationship to its target isnt' relevent. it will still expand.


We're not just breaking the projectile into small chunks, we're turning it into a gas.


It is all going to depend on your engagment distance. If you're firing a .9 c projectile from 30 kilometers away, the flight time of your projectile is a little more than 1 ten-thousanth of a second. Thats not even enough time to bring a laser to bear and fire let alone vaporize a target. Your mass is still lethal at that range and speed. On the other hand, if your engagement distance is on the scale of millions of kilometers, then this whole thing changes.

warty goblin
2007-11-13, 10:53 AM
doesn't matter. Relativity states that in a non-accellerating frame of reference its velocity in relationship to its target isnt' relevent. it will still expand.


We're not just breaking the projectile into small chunks, we're turning it into a gas.


It is all going to depend on your engagment distance. If you're firing a .9 c projectile from 30 kilometers away, the flight time of your projectile is a little more than 1 ten-thousanth of a second. Thats not even enough time to bring a laser to bear and fire let alone vaporize a target. Your mass is still lethal at that range and speed. On the other hand, if your engagement distance is on the scale of millions of kilometers, then this whole thing changes.

The really interesting question is whether the cloud expands according to its perception of time, or the observer's? It would actually make a difference for something traveling that fast too.

Indon
2007-11-13, 01:27 PM
I'd like to chime in by pointing out the nature and importance of evasion in sub-light projectile space combat.

Say you're in a spaceship that's capable of 0.8c projectile weaponry, and 0.2c speed. Unless you have energy shielding, you don't want to get close to your opponent, because the first impact may well destroy your ship.

Instead, you want to stay far enough back (at least a few light minutes) so that relativity means that your opponents can not see where, exactly, you are. You can change your movement before your opponent will see it. Of course, this applies both ways.

So how do you overcome not being able to see where exactly your opponent is to hit him? There are a number of ways this problem can be approached. You could just fire immense amounts of ammunition in the direction of your target, since one hit's all you need, after all. This is the kind of strategy you'd see employed by larger, more powerful ships against weaker ships (when neither has shielding).

You could also try to predict your opponents' movement, trying to guide their direction by firing a kind of 'guiding shot' that pressures your opponent to move in the direction you want so that you can then fire your 'killing shot' at them. This is something that, likely, smaller ships, or groups of ships, would use against larger ships.

As for superenergizing the projectile as a defense, it's unlikely to work. The act of firing something out of a railgun, especially at significant sub-light speeds, is already going to provide the object with enough heat energy to render it into thinly-dispersed plasma; thus the technology to counter this must already exist in order to enable the weapon to fire in the first place.

And regarding the ship that works as a disc to lower its' profile; clever, but it has a weakness. What if it's fighting two enemies at once? The two ships can maneuver so that one gets to fire at its' big, juicy fat target area.

WalkingTarget
2007-11-13, 01:37 PM
The vapor cloud would expand at the rate given by those equations in the projectile's frame of reference. Of course, less time passes for the projectile than for the ships involved. Hmm, how much time is perceived by something traveling at .9c over a distance of one light-minute? (Edit: one light-minute as measured from a "stationary" frame of reference, two ships are 1 light minute apart, one fires a .9c projectile at the other, how much time passes for the projectile?) I don't think I have a formula for that handy.

By the way, if any of you has a computer that can track and hit a fist-sized projectile with a laser at a distance of millions of kilometers, let me know where I can get one. :smallamused:

turkishproverb
2007-11-13, 01:51 PM
Faster than Light travel: Don't even bother with this one if you want to be "different" in your adherence to science. You're going to have to make something up out of whole cloth.


Actually a recent seminar pointed out that Faster than light travel may be possible under the right conditions, partially becuase light has been found to have a variable speed in certain cercomstances, and could theoretically allow for other objects to move closer to lightspeed. Furthermore, the varous theories about how the universe is constructed allow for a number of..cutting through the cloth methods, the equivilant of tunnelign through the universe.


In addition to this, it must be remembered, that for a long time scientists claimed that SOUND had the maximum speed in the universe.

Ossian
2007-11-13, 01:56 PM
Any creative use of "gravitons". They do appear in some high imagination settings, but perhaps they have a solid background that you can exploit. A bit like shooting small gravity wells that suck the targeted ships out of its route and other nasty things (and explosions? why not, explosions are always advised)

O,

Gygaxphobia
2007-11-13, 02:00 PM
A note about rail guns:
When you're talking about vast distances, even at high speeds a projectile is very likely to miss.
You'd have to rely on a large spread of objects or highly sophisticated computers planning the shot even then a slight movement (e.g. the target dodging) can throw off the shot easily.
Guided weapons might be something to consider.

TheRiov
2007-11-13, 02:00 PM
meh. I lost my taste for gravity based weapons & shields with NJO series.

I still like the use of an artificial singularity to power an engine though.

TheRiov
2007-11-13, 02:02 PM
The vapor cloud would expand at the rate given by those equations in the projectile's frame of reference. Of course, less time passes for the projectile than for the ships involved. Hmm, how much time is perceived by something traveling at .9c over a distance of one light-minute? (Edit: one light-minute as measured from a "stationary" frame of reference, two ships are 1 light minute apart, one fires a .9c projectile at the other, how much time passes for the projectile?) I don't think I have a formula for that handy.

By the way, if any of you has a computer that can track and hit a fist-sized projectile with a laser at a distance of millions of kilometers, let me know where I can get one. :smallamused:

Its not that big a difference. You dont start seeing much in the way of relativistic effects until ~.95c (in terms of orders of magnitude change)

Emperor Tippy
2007-11-13, 02:09 PM
A note about rail guns:
When you're talking about vast distances, even at high speeds a projectile is very likely to miss.
You'd have to rely on a large spread of objects or highly sophisticated computers planning the shot even then a slight movement (e.g. the target dodging) can throw off the shot easily.
Guided weapons might be something to consider.

Why do you think I have been saying that missiles would be the primary weapon used?


If your civilization has the ability to manipulate gravity though you can do some really wicked things, depending on how well you can manipulate it.

WalkingTarget
2007-11-13, 02:31 PM
Its not that big a difference. You dont start seeing much in the way of relativistic effects until ~.95c (in terms of orders of magnitude change)

If you say so. I agree that if it spreads out enough it will cease to be a major threat. Assuming the target can spot and hit the projectile while it's still far enough away. A 2kg block of Uranium is only about 100cc. Easier to do if we're allowing FTL sensors, but under normal circumstances they'd only get 1/10th of the total travel time to react at a maximum (that is, the most time they can get is reacting as soon as they see the shot occur, by which point the shot has traveled 90% of the distance anyway).


Why do you think I have been saying that missiles would be the primary weapon used?


If your civilization has the ability to manipulate gravity though you can do some really wicked things, depending on how well you can manipulate it.

Has anyone else read Earth by David Brin? Interesting use of gravity-based technology.

Nerd-o-rama
2007-11-13, 02:43 PM
Actually a recent seminar pointed out that Faster than light travel may be possible under the right conditions, partially becuase light has been found to have a variable speed in certain cercomstances, and could theoretically allow for other objects to move closer to lightspeed. Furthermore, the varous theories about how the universe is constructed allow for a number of..cutting through the cloth methods, the equivilant of tunnelign through the universe.


In addition to this, it must be remembered, that for a long time scientists claimed that SOUND had the maximum speed in the universe.
Yes. However, I'm saying that while we can posit lasers, rail/coilguns, high-intensity magnetic fields, and even antimatter realistically, modern science really has no idea besides guesswork of how to travel faster than light. All those things can be manufactured now, it's just a question of making them efficient and weaponized. Faster than light travel is still strictly in a theoretical stage (personally, I'm a fan of artifically generated Einstein-Rosen bridges, known better as wormholes.)

Emperor Tippy
2007-11-13, 02:49 PM
There's some interesting work going on involving hyperspace. I saw an article about it in 1 of the science magazines I get about 6 months back. The people running it are apparently ready for a test but the need a couple hundred million in funding.

Gungnir
2007-11-13, 03:04 PM
You know, considering the amount of energy we (well, you guys) are dealing with, and the size of the weapons, I'd think it would be pretty easy for a computer to sense when said weapons are charging, judge the vector of the "bullet" that is about to be fired, and apply its countermeasures accordingly. You wouldn't really need FTL sensors.

warty goblin
2007-11-13, 03:15 PM
I'd like to chime in by pointing out the nature and importance of evasion in sub-light projectile space combat.

Say you're in a spaceship that's capable of 0.8c projectile weaponry, and 0.2c speed. Unless you have energy shielding, you don't want to get close to your opponent, because the first impact may well destroy your ship.



Speed is relative to an observer, and, more to the point, any sub-light speed can be achieved through applying a miniscule force for long enough. The only thing that matters is acceleration, and the current speed of the target. If you know their vector and their max acceleration you can bracket their possible locations with fire.

TheRiov
2007-11-13, 03:33 PM
If you say so. I agree that if it spreads out enough it will cease to be a major threat. Assuming the target can spot and hit the projectile while it's still far enough away. A 2kg block of Uranium is only about 100cc. Easier to do if we're allowing FTL sensors, but under normal circumstances they'd only get 1/10th of the total travel time to react at a maximum (that is, the most time they can get is reacting as soon as they see the shot occur, by which point the shot has traveled 90% of the distance anyway).



Has anyone else read Earth by David Brin? Interesting use of gravity-based technology.


Assuming my math skills havent completely deteriorated in the 10 years since I took a multivariable calculus, the energy required to fire a 2kg slug at .9c is roughly the total energy output of a good sized nuclear weapon. (4.2 x 10^16th--the Tsara Nuclear bomb (the largest ever deonated) was 2.1 x 10^17)


Any vessel chucking that much energy into a single shot is going to have some issues.

Indon
2007-11-13, 03:35 PM
Speed is relative to an observer, and, more to the point, any sub-light speed can be achieved through applying a miniscule force for long enough. The only thing that matters is acceleration, and the current speed of the target. If you know their vector and their max acceleration you can bracket their possible locations with fire.

I noted this strategy in the post you responded to; it's a good strategy for applying overwhelming force, in terms of the amount of ammunition you can put out. But covering all possible vectors at any distance you would want to engage at (so that they can't cover all your possible vectors at the same time and simultaneously kill you) would require having way more firepower than your target.

This establishes a kind of 'killing zone' around high-firepower ships, within which they are guaranteed a victory. Every ship would be capable of such a zone, but the more guns you have, the bigger it becomes. The zone is also bigger in regards to larger ships; bigger targets require less ammunition to cover all possible vectors.

It is thus in the interest of a high-firepower ship to get other ships within its' killing zone, and in the interest of low-firepower ships to pursue different firing tactics.

Though, one thing I said was wrong; a disc-shaped ship can always orient itself so that two opponents are facing its' edge. It's only three or more opponents that this would be a problem against.

WalkingTarget
2007-11-13, 03:37 PM
You know, considering the amount of energy we (well, you guys) are dealing with, and the size of the weapons, I'd think it would be pretty easy for a computer to sense when said weapons are charging, judge the vector of the "bullet" that is about to be fired, and apply its countermeasures accordingly. You wouldn't really need FTL sensors.

Well, let's say the weapon is powered by electricity (granted a lot of it).
1. get a big capacitor (and if we're already building large spaceships, why not)
2. charge said capacitor with whatever mechanism you wish
3. wait however long you feel like taking
4. release the charge to fire the weapon

Sure, if step 3 is minimal and/or always a fixed interval and always fires projectiles at the same velocity, it'd be easier to counter. That also makes the assumption that the charging of a weapon is observable (which is true in most sci-fi settings, but not necessarily always the case). If the ship's engine is always generating power anyway, why would routing some of it to a capacitor (not necessarily near the surface of the ship) be something an observer distance x away will be able to see?

WalkingTarget
2007-11-13, 03:39 PM
Assuming my math skills havent completely deteriorated in the 10 years since I took a multivariable calculus, the energy required to fire a 2kg slug at .9c is roughly the total energy output of a good sized nuclear weapon. (4.2 x 10^16th--the Tsara Nuclear bomb (the largest ever deonated) was 2.1 x 10^17)


Any vessel chucking that much energy into a single shot is going to have some issues.

Yup, see my post on page 1 in response to the OP asking what kind of energy levels this sort of thing deals with. Discussions after that have taken the power generating abilities of the ships as given (in the OP's proposed scenario).

Dervag
2007-11-13, 10:50 PM
In addition to this, it must be remembered, that for a long time scientists claimed that SOUND had the maximum speed in the universe.Yes, but they couldn't prove it.

Physicists today really can prove that the speed of light in a vacuum is a universal upper limit, with both excellent experimental results and excellent theoretical work. Any theory that provides viable FTL travel must address the issues created by chucking parts of relativity.

Among other things, it destroys our concept of causality. It will no longer be possible to say that event A definitely precedes event B or vice versa, and there will be people who (correctly) see FTL travel causing an effect before the cause of that action.


There's some interesting work going on involving hyperspace. I saw an article about it in 1 of the science magazines I get about 6 months back. The people running it are apparently ready for a test but the need a couple hundred million in funding.I'm skeptical, but it's possible. The problem is that popular science magazines often publish articles by quacks with cool-sounding results.

turkishproverb
2007-11-13, 11:25 PM
Yes, but they couldn't prove it.

Physicists today really can prove that the speed of light in a vacuum is a universal upper limit, with both excellent experimental results and excellent theoretical work. Any theory that provides viable FTL travel must address the issues created by chucking parts of relativity.

Among other things, it destroys our concept of causality. It will no longer be possible to say that event A definitely precedes event B or vice versa, and there will be people who (correctly) see FTL travel causing an effect before the cause of that action.

They thought they'd proved it before, and used all sorts've numbers to back it up. All i"m saying it, its not like its uprecedentable. And relativity really doesn't deal with the kind've causality your talking about.

Dervag
2007-11-13, 11:48 PM
They thought they'd proved it before, and used all sorts've numbers to back it up.I'm not sure you fully appreciate what it means for scientists to consider a theory 'proven' in the modern era. A convincing argument does not qualify as 'proof'. 'Proof' is what you get when the nonintuitive predictions of a theory, the ones that no sane person would expect to be true unless the theory were true, have been thoroughly checked and found good.

In the case of relativity, physicists have been doing this for most of the twentieth century, both in direct tests of the theory and in experiments that would never produce the results they do without relativity. So either relativity is right, or whatever theory will replace it has to explain everything relativity explains, to the limits of our ability to measure, in addition to allowing for everything we already know.

Here's an analogy. Once upon a time, people thought the world was flat or bowl-shaped. Then they thought it was a sphere. Then, with the development of slightly more sophisticated physics, they figured out that it was actually an 'ellipsoid'- the equator bulges out a little because the Earth is spinning, sort of like the spinning dough that forms a pizza.

Finally, recently, using very precise satellite measurements, we have found that the Earth is in fact a little fatter in the southern hemisphere than the north, by a tiny tiny fraction of its overall size.

So yes, our idea of the shape of the Earth is changing, and has changed recently. But there is no chance that tomorrow scientists will discover that we were wrong all along and that the Earth is actually shaped like a taco, or a frisbee, or a cube, or that it's hollow and we live on the inside. Because for those things to be true, all of an enormous range of observations we've made would have to be wrong, dead wrong, utterly wrong. The Earth is not shaped like a taco. It may be a slightly lumpier round object than we thought, but there's no way it's anything but round.

Likewise, relativity. Any theory that replaces relativity will have to look exactly the same everywhere we've already looked to the limits of our ability to measure it. Any area in which it differs from relativity (such as in allowing FTL travel) must be a very specialized application of the theory, one that simply does not occur in the parts of nature we've observed to date.

It's possible, but don't hold your breath. Such theories are incredibly hard work to construct, and there's little or no evidence that such a theory even exists.


All i"m saying it, its not like its uprecedentable. And relativity really doesn't deal with the kind've causality your talking about.Oh yes it does. Einstein's theory of relativity is about two major things, and causality is one of them. In fact, the entire reason that Einstein's math gets weird and advanced (well, moderately advanced) is that he had to come up with a way to explain cause-and-effect while preserving the observational fact that the speed of light in a vacuum is a universal invariant (light doesn't seem to travel faster when you're charging forward into a beam of light going the other way, or slower when you're fleeing it).

warty goblin
2007-11-14, 12:01 AM
Yes, but they couldn't prove it.

Physicists today really can prove that the speed of light in a vacuum is a universal upper limit, with both excellent experimental results and excellent theoretical work. Any theory that provides viable FTL travel must address the issues created by chucking parts of relativity.

Among other things, it destroys our concept of causality. It will no longer be possible to say that event A definitely precedes event B or vice versa, and there will be people who (correctly) see FTL travel causing an effect before the cause of that action.

I'm skeptical, but it's possible. The problem is that popular science magazines often publish articles by quacks with cool-sounding results.

Order of events already is broken. Take a train moving past some stationary observer at a constant velocity. Have a person standing in a car on the train shine a light at the forward wall (in the direction that the train is moving) From the observer's point of view it will take (distance from light source to wall)/(speed of light + speed of train) for the beam of light to reach the wall of the train because for the observer the light beam is traveling at c, and the train is moving towards the light source. For the dude shining the light it will simply take (distance)/(c) for the light to reach the wall, because from his perspective the train is not moving. Thus the two people see the same event happen at different times.

Dervag
2007-11-14, 12:34 AM
First of all, you can't add anything to the speed of light that way; it flat out does not work with relativistic physics.

Second of all, that isn't a causality violation. Causality violations occur when two people cannot agree over whether an event came before or after another event that is causally linked to it. In this case, everyone will agree that the light hit the wall sometime after it left the lamp, even if they aren't confident precisely what time that is.

A causality violation would occur if, in some frame of reference, the wall got hit with light before the lamp was turned on. The reason that would present a problem is that if you can do that at all, you can do it in the wall's frame of reference, so that the wall gets hit with light before the lamp was turned on in its frame of reference.

And if you set up relays of systems capable of doing that, you can do things like send a signal that arrives at your position before you sent it.

You can't get causality violations in relativistic physics without exceeding c, while you get them effectively all the time if you do exceed c.

Skjaldbakka
2007-11-14, 12:50 AM
You can't get causality violations in relativistic physics without exceeding c, while you get them effectively all the time if you do exceed c.

I don't follow. It doesn't matter how fast a bullet is traveling, it never hits the target before the trigger is pulled. Even if it is traveling faster than light, it still can't be anywhere but in the barrel before the trigger is pulled.

averagejoe
2007-11-14, 12:54 AM
Order of events already is broken. Take a train moving past some stationary observer at a constant velocity. Have a person standing in a car on the train shine a light at the forward wall (in the direction that the train is moving) From the observer's point of view it will take (distance from light source to wall)/(speed of light + speed of train) for the beam of light to reach the wall of the train because for the observer the light beam is traveling at c, and the train is moving towards the light source. For the dude shining the light it will simply take (distance)/(c) for the light to reach the wall, because from his perspective the train is not moving. Thus the two people see the same event happen at different times.

I'd also like to add (perhaps uneccesarily) that because, in the "rest" frame, the distance from the light source to the wall and the time observed would be smaller by a factor of one over gamma. For distance d and velocity v we get:

[d/(c+v)][1-(v/c)^2]=[d(c-v)/c^2], if my algebra is correct.

Has nothing to do with causality, but there you go. I believe that's right, because you get the "moving" frame time if v goes to zero.

turkishproverb
2007-11-14, 12:59 AM
I don't follow. It doesn't matter how fast a bullet is traveling, it never hits the target before the trigger is pulled. Even if it is traveling faster than light, it still can't be anywhere but in the barrel before the trigger is pulled.

Exactly. The idea of relativistic time travel is mostly science fiction.

Dervag
2007-11-14, 02:17 AM
I don't follow. It doesn't matter how fast a bullet is traveling, it never hits the target before the trigger is pulled. Even if it is traveling faster than light, it still can't be anywhere but in the barrel before the trigger is pulled.The causality violations occur when you create a loop of faster-than-light travel.

For example, imagine two automated drone guns, each designed to fire a superluminal (FTL) bullet, presumably a very small one. Gun A is set to fire at Gun B on command; Gun B is set to fire back at Gun A when it gets hit by the bullet from Gun A.

Because of the way the math works, it is in fact straightforward to demonstrate that when Gun A and Gun B are in different frames of reference, Gun A will get hit by return fire from Gun B before Gun A itself has fired. This despite the fact that Gun B cannot fire until it is hit by Gun A.

Now, Gun A will agree that it didn't hit Gun B until after Gun A fired. And Gun B will agree that it didn't hit gun A until after Gun B fired. But Gun A will be convinced that it got hit by return fire from Gun B before it was commanded to fire the first shot of the 'battle', and that Gun B has some kind of horrible time-travelling bullets that can warp into the past. And it will be right in its own frame of reference; this is not an optical illusion.

For a further illustration of the problem, look here (http://sheol.org/throopw/tachyon-pistols.html), here (http://www.theculture.org/rich/sharpblue/archives/000089.html), and here (http://en.wikipedia.org/wiki/Closed_timelike_curve).

To summarize:
"Faster-than-light travel, relativity, and causality as human beings are wired to understand it. Pick any two."
And since relativity is amply supported by a truly gigantic range of data, the most likely thing to go in that equation if we accept FTL travel is causality as we are wired to understand it.

That doesn't mean it isn't true, or that FTL can't happen, only that if it does happen our ability to understand cause and effect is no longer reliable.


I don't follow. It doesn't matter how fast a bullet is traveling, it never hits the target before the trigger is pulled. Even if it is traveling faster than light, it still can't be anywhere but in the barrel before the trigger is pulled.Yes, I know, I'm quoting this twice; it's for a reason.

The point here is that causality violations occur only in loops with FTL travel, when an FTL signal or object travels between two different objects in different frames of reference (i.e. moving at different speeds and/or in different directions). But there are many ways to set up a loop- for instance, if an FTL ship travels from one star to another, and then comes back, there may be profound confusion over whether it arrived first or left first.


Exactly. The idea of relativistic time travel is mostly science fiction.No, it's not; it's an extremely straightforward prediction of the mathematics, and the mathematics have been demonstrated by a lot of things that just flat out would not happen if the math were genuinely wrong.

Again, this doesn't actually forbid FTL travel. What it does do is guarantee that FTL travellers will set up causality violations left and right (and yesterday and tomorrow) unless they are extremely careful. What happens when causality violations occur is unclear; obviously nobody has ever observed one. But it would be very confusing. For my own part I'm serenely confident that it won't mess up anything permanent, because I don't believe in temporal paradoxes. You may not be so confident.

averagejoe
2007-11-14, 03:06 AM
@ Dervag: Those are some extremely interesting articles. I had no idea before why FTL travel implied causality violation. It's also a bit inconvenient, as I really should be studying thermodynamics. :smallannoyed: (Seriously, no on even likes themodynamics. :smallmad: )

Skjaldbakka
2007-11-14, 03:20 AM
[QUOTE=Dervag;3516479]
math, technobabble, more math, technobabble, more math, followed by severe need for aspirin
[QUOTE]

What I can understand of that seems to take as its baseline assumption that reality is formed by perception. This is not the case. Reality is perceived by perception. This does not cause any problems with causation. Regardless of how fast you go, you still don't get back to point A before you left. What is really happening is that you are moving faster than the light reflected from you to the viewer. Which makes FTL travel effectively invisible, not time travel. Your actual, in fact location in time/space doesn't change. Just the perception of such.

EDIT- I'm not trying to be argumentative here, I am sure you physicists are probably right, but you're never going to be taken seriously if you can't explain yourselves to the laeity. EDIt- :smalltongue:

averagejoe
2007-11-14, 03:58 AM
What I can understand of that seems to take as its baseline assumption that reality is formed by perception.

No such assumption is made. When he says, "Person A observes the time of person B as X," it doesn't mean necessarily that person A is taking measurements, and because of the way light is traveling from person B he measures time as slower than it is, so that means time must be slower than it is. Person A doesn't have to make measurements at all.

Look at it like this. Say you're on a train, traveling along at some near-lightspeed constant velocity. You wouldn't notice anything different from when you were at the station, and from your point of view you would be correct. Now, say that before you left the station, you and I each took a watch, both set to exactly the same time. After you got to your destination, if you cared to call me up and compare watches, you would find my watch slightly ahead of yours. Why? Because from my point of view time is going slower for you. You don't notice anything different (and, indeed, from your point of view time is going slower for me, but you're the one switching inertial frames, while I stick to just one.) (This has been very rigorously been experimentally determined, by the way.)

What you have to get over is this notion of "absolute" time and realize that time depends on one's point of view. It's like when a person finds themselves in space, without the Earth to show them where "up" and "down" are. There isn't any "correct" orientation. It just depends on your point of veiw.

I tried to answer you as best I could. Being specific helps with answering such questions. No one's being obtuse on purpose.


EDIT- I'm not trying to be argumentative here, I am sure you physicists are probably right, but you're never going to be taken seriously if you can't explain yourselves to the laeity.

I thought we are taken seriously. I mean, we do produce tangible results, even if people don't understand why these results should occur.

Skjaldbakka
2007-11-14, 04:05 AM
Why? Because from my point of view time is going slower for you.

How is time effected by POV? Why are the watches different? I assume it has something to do with these magical inertial frames? What are they?

Dervag
2007-11-14, 04:29 AM
[QUOTE=Dervag;3516479]
math, technobabble, more math, technobabble, more math, followed by severe need for aspirin
[QUOTE]Your need for aspirin is in no way my fault. I'm sorry; this is reality. You can take it or leave it. Nobody promised you that reality would be easy to understand or intuitively obvious. I wish it were, because then I would need less aspirin.


What I can understand of that seems to take as its baseline assumption that reality is formed by perception. This is not the case. Reality is perceived by perception.You misunderstand. The problem here is that it is a manifest fact that everyone observes certain things when they look at light or objects moving around the speed of light, and that observers in different frames of reference (say, moving in opposite directions) observe different things. For example, they disagree on things like the speed of clocks, the length of objects, and the mass of objects.

Now, there are two possibilities. One is that all our observations are gibberish and reality depends on how you perceive it. This is the possibility Einstein discarded, more or less out of hand, and with reason.

The other is that the laws of nature are the same everywhere. In which case the universe isn't as simple as Galileo and Newton (and Skjaldbakka) thought it was, and that there really is something about the nature of the universe that causes observers in different frames of reference to observe different things when looking at the same object.

To figure out what this freakish 'something' is, Einstein used as a reference point the one thing that everyone in every frame of reference could agree on- the speed of light in a vacuum. No matter how wildly different your observations of the mass of objects or the speed of clocks are from mine, we will both observe the same thing if we measure the speed of a beam of light.

Einstein used that one universal invariant as an anchor to restate all the laws of physics in new terms. The new terms explain how I can disagree with you about how many seconds have passed since the last time we met (and I can, even though nothing is wrong with either your clock or my clock. They tried this).

The catch is that the new terms (well, over 100 years old now, actually) make really weird predictions about faster than light travel. Predictions that utterly violate human intuition about the universe.


This does not cause any problems with causation. Regardless of how fast you go, you still don't get back to point A before you left. What is really happening is that you are moving faster than the light reflected from you to the viewer.Here's the problem with the way you're putting it. You're assuming that everyone in the universe would agree on what happens first if we all measured 'objectively'. This assumption is implicit in your thinking; it is also implicit in the thinking of the great masters of physics during the 19th century and all earlier centuries.

Unfortunately, it's wrong; there is no spoon, so to speak. There is no frame of reference that we can identify as being the right one to measure everything from. So we have to use the equations of relativity to make sense of the fact that two different people get different measurements of the same thing even when they both do everything right.

Because of that, all we can talk about meaningfully is what gets observed in a given frame of reference.

Let's say that you step into a teleporter and beam to an alien planet that is moving relative to the Earth, crossing light-years of space 'instantly'. If I could watch you arrive on the alien planet from Earth, I would see you arrive after a number of years precisely equal to the number of light-years between the planets, indicating that, in my frame of reference, you left Earth at the same time you arrived.

Great. But now you are in the alien planet's frame of reference, standing on it and not on the Earth. And let's say you immediately beam back to Earth, fulfilling the same condition of instant travel. An alien on the alien planet would think you had left Alienworld and instantly arrived on Earth.

You would be convinced that you had spent some nonzero positive amount of time traveling back and forth. And you would be right. But I, standing on Earth, would be convinced that you had arrived back home before you left in the first place, and I would be right too. If we both carried perfectly accurate clocks and logged the times of your arrival and departure, and then calculated the difference, we would confirm this. You would have recorded a positive amount of time between leaving and returning (you return after you leave, just as you and Sir Isaac Newton would expect). I would record a negative amount of time (you return before you leave).

This is very well established by the mathematics of relativity. It's not a question of one of us being fooled by you outrunning the light that tells us you're on your way. It's not a question of my clock being right and yours being broken or vice versa. We really do get perfectly accurate mutually incompatible measurements.

And to make matters worse, I can use this technique to send a message into my own past- if I give you a note, send you on a round-trip teleport to Alienworld and back, and tell you to hand me the note on your return, you will hand me the note to a younger Dervag than the Dervag who gave it to you. No kidding, you really will, based on the well-proven mathematics of relativity.


EDIT- I'm not trying to be argumentative here, I am sure you physicists are probably right, but you're never going to be taken seriously if you can't explain yourselves to the laeity.The problem here is that you've committed yourself to interpreting the findings of relativity in a way that has very little to do with what they really mean. As such, you're misunderstanding their predictions in a way that leads you to disagree with me about what those predictions are.

It's as if you were interpreting the theory of gravity as saying that "things fall" and then contradicting me when I said that it was possible for things like helicopters and balloons to fly because "things fall."


How is time effected by POV? Why are the watches different? I assume it has something to do with these magical inertial frames? What are they?Time is "effected" by point of view because, unlike the speed of light in a vacuum or the physical constants, it is not a universal invariant. Even when we measure the time between two events with identical and perfect watches, we still end up disagreeing about how long after the first event the second event happens if we're in different inertial frames.

As for inertial frames, they're about as magical as a pile of concrete. An inertial frame of reference is defined in terms of some point in space that is not accelerating. It may be moving, but it is not accelerating (bolted to a rocket engine or spinning in circles or anything like that).

In every inertial frame of reference, when the laws of physics are measured, they will be found to be the same in their relativistic form. So if I build two identical copies of a laboratory, leave one floating in interstellar space at the center of the galaxy, and send the other zooming through the universe at 99% of the speed of light at a constant speed, both labs will measure the same physical constants, the same speed of light, and the same physical laws governing the relationships between objects.

The catch is that if my frame of reference and your frame of reference are not the same, we won't always agree on measurements. Let's take a very simple example.

Imagine that you and I are in cars passing each other on a highway at constant speed of 50 miles per hour (mph). We are each in an inertial (nonaccelerating) frame of reference, anchored to our own head. You are driving from city A to city B, which is north of city A. I am driving from B to A.

In my frame of reference, I am at rest. I am just sitting in the driver's seat of my car watching the world move around me. Likewise, in your frame of reference, you are at rest. In the Earth's frame of reference, we're both moving in opposite directions and the Earth is standing still.

There is no question of any of these three frames being 'correct' or 'incorrect'. All I'm going to say here are factual observations about what you see, what I see, and what an observer standing still relative to the Earth would see.

I see you moving 100 miles an hour due north. In my frame, where I am defined to be at rest because my own head is the point where my frame of reference is anchored, you appear to be moving at a speed equal to my speed plus your speed.

Likewise, you see me moving 100 miles an hour due south. A man standing by the roadside, anchored to the Earth's frame of reference, sees us both moving in opposite directions at 50 miles an hour.

This is not an illusion. If I use a radar gun to measure your speed, the radar gun will tell me that you are going 100 mph due north. If the man by the side of the road uses an identical radar gun on your car, they will measure your speed to be 50 mph due north. If you use a radar gun on your own car while you're sitting inside it, you will measure your speed to be 0. None of us are objectively correct or mistaken in our measurements. Our radar guns are not broken. The problem is that we can only ever measure the motion of any thing relative to ourselves, making the assumption that we are standing still.

If we want to convert from one frame of reference to another, we have to use what are called 'equations of transformation' to convert positions and velocities from one frame to the next. At low speeds, these equations behave intuitively (in my frame, the Earth is moving north at 50 mph and you are moving north at 100 mph; therefore, your speed in the Earth's frame must be very very close to 100-50 = 50 mph) to a very close approximation.

At speeds near the speed of light, they stop behaving intuitively (in my frame, the Earth is going at half light speed north and you are going at 70% light speed north, and yet in the Earth's frame you are going half light speed, not 70%-50% = 20%). However, they are still predictable and easy to use.

At speeds above the speed of light, the equations of transformation not only do not behave intuitively, they break our intuition up into itty bitty little pieces and rearrange it in a sort of surrealistic collage.

turkishproverb
2007-11-14, 04:50 AM
How is time effected by POV? Why are the watches different? I assume it has something to do with these magical inertial frames? What are they?

Even assuming time was affected by point of view, it still does not explain how their claim of effect supersceding cause works.

Skjaldbakka
2007-11-14, 04:53 AM
As for inertial frames, they're about as magical as a pile of concrete.

Science is indistinguishable from magic until it is understood. That being said, I think I am beginning to understand what you are talking about:

1) Observation is an inconsitent tool to measure with, due to differences caused by POV.

2) The speed of light in a vaccum (c) is a universal constant, and is thus used to measure things with (because it is consistent).

3) Using c to measure things at speeds greater than c has non-intuitive results.

4) Because using observation would result in intuitive, but conflicting results, whereas using c results in non-intuitive, but consistent results, c is what is used, because the scientific method is all about producing consistent results.

Dervag
2007-11-14, 05:59 AM
Even assuming time was affected by point of view, it still does not explain how their claim of effect supersceding cause works.Look, as far as I'm concerned you can be as skeptical as you like as long as you're willing to check the math.

As for effect superceding cause, the problem is that relativity creates situations where we get different measurements of the same thing. When speeds greater than light get thrown into the mix, I can end up measuring a positive quantity (such as a length of time) where you measure a negative one. That really is how relativity works. The math is there if you want to check it. The experimental results are published. The derivation is as solid as a century of physicists have been able to make it. If you want to take exception to some aspect of it, fine, as long as you're willing to engage the problem on the same level they do. I am not bluffing here, nor am I trying to obscure the situation.

"Does not make sense" is not a disproof of a physical theory, because Mother Nature never promised us that she would "make sense" by the standards of a species whose physics instincts are basically limited to throwing rocks.

"Does not match experimental results" is a disproof, but it is one that relativity has never had any trouble with.

To us, it makes no sense that an effect could happen before a cause. But that's because we live in a corner of the universe where nothing forces us to confront the way relativity messes with our idea of what it means for one event to happen at the same time as another.

I would strongly suggest you review the links I provided if you want to know more about this subject and don't want to deal with the mathematics of metric tensors.


Science is indistinguishable from magic until it is understood.The problem is that this is understood, quite well, and described very thoroughly in math that has a very solid meaning and a very solid set of implications. This is not cutting-edge research anymore; the special theory of relativity has been around about as long as airplanes, and longer than wristwatches.

Unfortunately, the math required to describe it is advanced enough that not everyone has the inclination or ability to learn it, and without the math there is simply no way to explain it that doesn't boil down to "trust me" at some level. Once you get to that level of physics, math and physics can no longer be separated; math becomes part of the language in which physics is described.

Trying to explain this kind of mid-level physics without the math is like trying to get concepts across to someone who is constitutionally unable to understand, say, verbs. They're comfy with nouns. They have a big vocabulary of adjectives. And they're quite intelligent But they don't understand verbs and you can't use verbs when you talk to them.

Imagine how hard it would be to communicate with someone like that, and you see the problem a physicist faces trying to explain relativity to people who don't have three years or so of college-level math under their belts.


1) Observation is an inconsitent tool to measure with, due to differences caused by POV.

2) The speed of light in a vaccum (c) is a universal constant, and is thus used to measure things with (because it is consistent).

3) Using c to measure things at speeds greater than c has non-intuitive results.

4) Because using observation would result in intuitive, but conflicting results, whereas using c results in non-intuitive, but consistent results, c is what is used, because the scientific method is all about producing consistent results.It's not a question of using c as a yardstick to measure speeds; it's just that c is the only thing in the universe we can agree on independent of our frame of reference. To extend the car analogy, if a laser beam is zooming south along the highway, you in your car and me in my car will both measure the laser beam to be travelling at the same speed c, even though you're charging towards the beam and I'm running 'downstream' away from it.

This is itself a very nonintuitive result, but it is true (see the Michaelson-Morley experiment, 1887, for reference).

But aside from that, you've got the basics down right. For consistent laws of physics to apply everywhere in the universe, we have to take into account that c is a universal invariant. But for that to be true, everything else (mass, time, length) has to depend on the frame of reference in certain calculable ways.

And when you apply those calculations to speeds faster than light, the results totally blow away our intuition. For that matter, they act nonintuitively even at 'normal' speeds; the catch is that the part of the behavior that is nonintuitive is so small that you can approximate things closely as being intuitive.

Relativity applies even to 50 mph cars, but at those speeds the relativistic effects are something like one part in ten thousand trillion and you'll never notice them without some truly weird equipment.

Skjaldbakka
2007-11-14, 06:18 AM
I was referring to the term as magical as a tongue-in-cheek way of saying that I didn't understand it. In spite of my lack of three years of college math, I think I get the picture (now).


It's not a question of using c as a yardstick to measure speeds

I wasn't using measure in terms of yardstick, I was using measure in the more general sense. The car/laser beam analagy was very helpful though. I pretty much understand exactly what you are you talking about now. Thank You.

Dervag
2007-11-14, 06:21 AM
I was referring to the term as magical as a tongue-in-cheek way of saying that I didn't understand it. In spite of my lack of three years of college math, I think I get the picture.Sorry; I honestly couldn't tell. I get that kind of thing once in a while from people who have their tongue firmly centered in their mouths.

TheRiov
2007-11-14, 08:03 AM
From the observer's point of view it will take (distance from light source to wall)/(speed of light + speed of train) for the beam of light to reach the wall of the train because for the observer the light beam is traveling at c, and the train is moving towards the light source.

Patently false. This is one of the hardest things to understand about the speed of light, but you dont add anything to c. The beam of light will still take (distance/c) time to reach its target. (not distance/(c+v)

TheRiov
2007-11-14, 08:31 AM
found this online:

http://www.sheol.org/throopw/tachyon-pistols.html


Why FTL implies time travel (tachyon pistols)
The entire "FTL implies time travel" meme has to do with what's sometimes called "failure of simultaneity at a distance". In addition to the effects that pop treatments of relativity mention (that is, time dilation and length contraction), relativity proposes that the definition of "right now" is also different, depending on which observer's coordinate system you use. This is an effect much like the revolution of deciding that the direction "up" wasn't the same everywhere, but varied from place to place on earth. With relativity, the revolutionary notion is that the direction "futureward" (or "now-ward") isn't the same everywhere, and varies with velocity.

We can describe this effect by idealizing FTL to be "instantaneous", and describing how the more familar time dilation implies this effect. But remember, the same points apply to any FTL speed, you just have more messy arithmetic to grind through.

Consider a duel with tachyon pistols. Two duelists, A and B, are to stand back to back, then start out at 0.866 lightspeed for 8 seconds, turn, and fire. Tachyon pistol rounds move so fast, they are instantaneous for all practical purposes.

So, the duelists both set out --- at 0.866 lightspeed each relative to the other, so that the time dilation factor is 2 between them. Duelist A counts off 8 lightseconds, turns, and fires. Now, according to A (since in relativity all inertial frames are equally valid) B's the one who's moving, so B's clock is ticking at half-speed. Thus, the tachyon round hits B in the back as B's clock ticks 4 seconds.

Now B (according to relativity) has every right to consider A as moving, and thus, A is the one with the slowed clock. So, as B is hit in the back at tick 4, in outrage at A's firing before 8 seconds are up, B manages to turn and fire before being overcome by his fatal wound. And since in B's frame of reference it's A's clock that ticks slow, B's round hits A, striking A dead instantly, at A's second tick; a full six seconds before A fired the original round. A classic grandfather paradox.

Note, this is NOT a matter of when light gets to an observer, it is NOT an optical illusion. It is due to the fact that, in SR, the question of what occurs at the "same time as" something else is observer dependent.

As A fired that first show at tick 8, the bullet effectively teleported from A's gun to B's back instantly --- instantly according to A. But for B, who was moving at 0.866 lightspeed WRT A, B was hit in the back by the bullet 4 seconds BEFORE the bullet was fired. And again note, this is NOT due to the optical illusion of lightspeed delay in viewing A's turn-and-shoot; the light form that event wouldn't reach B until MUCH later, not tick 4.


Essentially: B kills A for doing something before A can do it.

Emperor Tippy
2007-11-14, 10:23 AM
I agree that "true" FTL travel is impossible. You can't accelerate to the speed of light or past it. However that doesn't make FTL travel impossible. Or FTL communications. Relativity allows for things to always be goign faster than light and thus don't have to accelerate (humans aren't one of those things so you can't get true FTL travel that way).

If FTL travel is possible (traveling 1 light year in 1 day for example) it will be because of weird things. Wormholes, Hyperspace, etc. Whether or not these things are possible is an unknown.

Now FTL travel or communications don't cause causality problems unless they operate by accelerating something past light speed. Going from point A to point B (100 light years away) in an instant (say if something like D&D teleportation existed) wouldn't violate causality because time would never dilate for you.

TheRiov
2007-11-14, 10:45 AM
There are still causality problems with FTL stuff. At least one thing I read suggested that even if tachyons exist (particles that travel faster than light at all times) they cannot be detected.

The problem with tachyons is that with our understanding of them, their mass, their length and all times from our point of view etc all strays into imaginary numbers. (the square-root of -1 doesnt exist, but by the math, thats what you get)

In any event:

The reason I prefer directed energy weapons--if you're firing a directed energy weapon you're not changing your vehicles momentum significantly. that sort of thing IS an issue if you're trying to fire relativistic 'slugs'

Another thing to consider--high speed weapons are NOT the most lethal. While you may punch through your target your weapon is going to KEEP going, without imparting much of its KE on the vessel as a whole. (your true goal)

(it would be akin to shooting a sniper rifle at a peice of paper. the bullet may punch through the paper, but you're going to do more damage to the paper with a much lower velocity shotgun, because more of the KE is transfered to the paper.)

warty goblin
2007-11-14, 11:32 AM
I really don't think that analogies with bullets hold with relativistic projectiles- at that speed things just don't act the same. For one thing on impact the projectile is going to convert to very high energy plasma, along with a good chunk of whatever it hits, and then splatter/burn its way through your ship. If it penetrates the hull the shockwave of its passage I'm guessing would probably kill anything inside of the ship as well- certainly everybody in the same compartment and nearby conpartments.

Fuzzy_Juan
2007-11-14, 12:09 PM
Ok, I haven't read that many of the responses cause they are very long and make my brain hurt :smalltongue:

Sci-Fi weapons and space combat...a very fun thing.

Propulsion: there are many different styles of propulsion, sublight drives and FTL drives make up the two main classes. One must take care to clairify how each works as they will dictate the technology of the species and the capabilities of the ship. sublight drives fall into 'thrusters' that operate on an action/reaction principal (eject crap out at speed and get accelerated equal and opposite relative to mass) and 'massless' propulsion (gravity manipulation, energy fields, 'solar sails', manipulation of cosmic background radiation, etc.). Thrusters are akin to rockets and will pretty much obey newtonian physics. Forms of massless propulsion can have their own laws, ignore inertia etc. These forms of propulsion are very advanced, usually requiring alot of power, energy manipulation, and more importantly don't require 'fuel' per se.

FTL drives are many and varied in theory. There is the 'warp' drive/lightspeed drive that simply manages to accelerate the vessel to lightspeed and faster in some way. Usually by warping space in some fashion. (Star trek's warp drive works by creating a moving region of space around the ship so that the ship never actually goes faster than the speed of light, thus avoiding relitavistic effects and bypassing the lightspeed barrier as well). There is the hyperdimensional approach, 'hyperspace'. In this model, the ship's FTL engines create a dimensional rift and shifts the ship into a different dimension where laws don't exactly behave the same, or in which space is different. In such dimensions, it is common that natural laws are not the same as our dimension and travel and deviation carries it's own risks. Such hyperspace travel might be 'normal and easy' enough for all ships to make their own passages, or they may beed the assistance of gates of some sort (see babylon 5 for examples). There are also the drives that warp space somehow. Either by using wormholes (artificial or existing), or by folding space in such a way that the ship never actually moves. Normally this is done with 'mysticism' or gravity manipulation that bends space on itself where the fabric of space in two distinct places touch and the ship jumps over at that time. (see Battletech universe, Dune, and Wing Commander for examples)

Do note that unless the ships ignore inertia, or have found a way to compensate, the acceleration of the ships would have to be slow, or the people inside would be killed...to speak nothing of the structure of the ship.

Weapons and shields/armor: Any ship traveling in space must have some form of shielding/armor. Micrometeorites strike spacecraft all the time and can cause serious damage. Most current spacecraft use an outter layer of shielding to break up the incomming particles and other layered thin armor and kinetic absorbent material to accept shock (wiffle shield). New technologies have proposed plasma shields or strong electromagnetic fields that would charge incomming partiicles and deflect them away from the hull. For a spacefaring race, they would have to be capable of shielding their ships from micrometeors that impact the ship at speeds in excess of what they are travelling at. Especially if they travel through 'realspace'. Craft folding space, or using hyperspace don't have to worry about particles at relativistic speeds, but anything else would have to. (just think about walking into a gust of wind...now, think if that gust of wind was moving at light speed relative to you...even if it is just gas, the result would be catastrauphic). With such ships...shields and armor would be so good, that 'normal' ballistics would be useless. Even railguns and gauss rifles would be nothing compared to what the shields must be able to withstand during normal travel. This is why in star trek that energy weapons make much more sense. In more 'lowtech' space travel where they bypass true FTL technology, bullets and small mass drivers would be effective. On such ships, it is possible that energy weapons are wasteful and not cost effective.

An example of high powered lasers, UT austin is experimenting with a chirped high frequency laser, it is capable of increasing the output of the laser many times the input power. The laser is so powersul that it does not simply burn. The energy of the laser instantly excites the electrons of any atoms in the path of the laser to such a high state of energy that the electrons fly free of the nuclei. This results in the instant transformation of any matter into the plasma state...in otherwords...it desintegrates the material on the molecular level leaving superheated plasma in it's wake that will continue to diffuse energy into surrounding material until it cools enough.

missles - for craft that only have low speed rockets, and little to no shields, missles and 'torpedos' are viable weapons. The warhead would depend on the nature of the common defense. In most cases a big enough blast would be all that is necessary. Do note that it would be best if the missle could penetrate and blow up inside the craft to maximize damage. Agaist unshielded craft, a 'frag' missle would be enough to cause damage from flying debris, but would also create a problem for all craft in the area. Against shielded craft, an energy shield or other type of charged hull would be common to missles to help penetrate the shield that would otherwise deflect the missle just like any 'normal' piece of debris. Such missles might require lock on time to adjust the 'shield' of the missle to the 'shield' of the craft to match polarity, whatever. common defense would be to rapidly shifting the nature of your shielding to confuse targeting...or just point defense weapons.

Practicality of vessels: Any vessel large enough will be fitted with bulkheads and compartmentalized as much as possible. The more sections that can operate independantly, the more damage a ship can take and still function. Redundant systems, centralization of important devices to avoid damage...internal airlocks. No 'windows' unless absolutely necessary...they are just trouble. gravity is a probem unless you are able to control gravity. 'gravity plating' is a common technique...a special material that when charged in a special way creates a unidirectional gravity field in the area above the plate (or between two plates). Or there are the centrifugal ships that have rotating sections, or ships that accelerate at 1g all the way to their destination, then roll over and accelerate at 1g to slow down.

Combat and Tactics: these mainly depend on much of the above. Small craft are useful for landing on planets and operating in gravity wells, but unless such a craft can pose a threat to a larger vessel, they are impractical for combat. Range of weapons is also very important, as is the targeting ability, weapon speed (how fast the shots travel and shoot), and defensive capability. Today, laser guided missles can hit targets from miles away and within inches...in the future, it would be silly to think that an energy weapon would miss based on a ships 'profile'. However, electronic warfare would have evolved and part of the struggle could very easily be the fooling of sensors to confuse targeting scanners. If ships can be produced 'cheaply', and shielding/armor good, combat might be much more common and brutal. Other times, a FTL ship might be a rare thing, or very costly...in 'Dune' and in the 'Battletech' universe, noone attacks a jumpship...they fight on planets, or around planets...but the jumpships are sacrosanct...without them, they are stuck.

other frills: do note that technology will do many other things than just effect war and travel. The same technology will be used by the populace and will shape their lives. Does the race know the secret of matter-energy conversion? The same technology that allows for transporter technology also allows for matter replication. nanotechnology would allow for the micronization of many things...even the manipulation of atoms and cellular structure...they would allow for crazy forms of genetic engineering and alteration, creating custom molecules, etc. Think of star trek...in a society in which food and power, shelter, water, medicine, every need is available 'for free' to all at the touch of a button...what do people do? Some maintain the 'old ways' just to do them...do nothing, devote themselves to art, science, service, teaching...

hope this helps.

WalkingTarget
2007-11-14, 02:19 PM
The reason I prefer directed energy weapons--if you're firing a directed energy weapon you're not changing your vehicles momentum significantly. that sort of thing IS an issue if you're trying to fire relativistic 'slugs'

Correct me if I'm wrong, but if you want to fire a weapon to deposit an arbitrary amount of energy at a target (be it a laser or a slug), isn't your momentum affected the same either way? This is an honest question to make sure I'm not missing something in the whole E=mc^2 thing.

Not to say that energy weapons don't have things going for them otherwise, it's just the OP specified railguns so a lot of this thread has discussed them.


Another thing to consider--high speed weapons are NOT the most lethal. While you may punch through your target your weapon is going to KEEP going, without imparting much of its KE on the vessel as a whole. (your true goal)

(it would be akin to shooting a sniper rifle at a peice of paper. the bullet may punch through the paper, but you're going to do more damage to the paper with a much lower velocity shotgun, because more of the KE is transfered to the paper.)

Yeah, I mentioned earlier that unless you're pretty sure of the impact being absorbed by the target before full penetration, a slug is less effective (better used for planet bombardment). WG's shockwave is still a phenomenon to take into consideration for shipboard lethality (might not destroy the ship, but still a threat to the relatively fragile biological entities thereon).

Dervag
2007-11-14, 02:23 PM
found this online:

http://www.sheol.org/throopw/tachyon-pistols.html

Essentially: B kills A for doing something before A can do it.I know; I already linked to it myself.


I agree that "true" FTL travel is impossible. You can't accelerate to the speed of light or past it. However that doesn't make FTL travel impossible. Or FTL communications. Relativity allows for things to always be goign faster than light and thus don't have to accelerate (humans aren't one of those things so you can't get true FTL travel that way).The problem is that if causality as we know it holds, there's no way to interact with anything moving faster than light. So you can't use pulses of it to tell someone what you had for breakfast today, nor can you use a stream of the things as a carrier wave.


Now FTL travel or communications don't cause causality problems unless they operate by accelerating something past light speed. Going from point A to point B (100 light years away) in an instant (say if something like D&D teleportation existed) wouldn't violate causality because time would never dilate for you.You're missing the problem. The problem is that if you teleport from one frame of reference to another, those two frames of reference will not agree about when you arrived regardless of whether or not you moved to get there. And if you teleport somewhere and teleport back, it is entirely possible that the teleport station's crew will disagree with you even about the question of whether you left before you arrived. Your clock will say that you did; my clock will say that you didn't, and we're both right because there is no universal frame of reference in a relativistic universe.

It's not time dilation in the sense of 'crews of starships experience less years per year of time in the outside world.' At no point does any person directly experience time travel as a result of FTL travel. However, they are quite capable of encountering past versions of themselves or other in this way... not good.

TheRiov
2007-11-14, 02:37 PM
Correct me if I'm wrong, but if you want to fire a weapon to deposit an arbitrary amount of energy at a target (be it a laser or a slug), isn't your momentum affected the same either way? This is an honest question to make sure I'm not missing something in the whole E=mc^2 thing.

E=mc^2 doesnt really apply here. That formula is if you completely annihilate mass, how much energy does it produce. (or vice versa, if you use energy to create matter, how much energy do you need to do it)

You CAN generate motion with light (light pressure) and a laser with the intesity of a nuclear weapon might produce even significant push.

But the damage a laser does doesnt come from imparted directional KE. Rather it excites the matter at the target site, causing it to heat. That heat can do things like cut. A laser as a single beam is far less destructive than a slug. The power of the laser is that you can cut with it. Punching a hole through someone's gut with a laser=not terribly likly to be fatal. Slicing them in half with one... deadly. The same applys to ships.

Its kinda like the difference between hitting someone with a baseball bat (damage is done by impacting a large mass against a target) and slicing them with a scalpel. Both do damage, both potentially fatal, but it takes far less energy to slice with a blade than to bludgeon with a baseball bat.

WalkingTarget
2007-11-14, 03:17 PM
E=mc^2 doesnt really apply here. That formula is if you completely annihilate mass, how much energy does it produce. (or vice versa, if you use energy to create matter, how much energy do you need to do it)

Uh, I thought it equated mass to energy. Anytime you have one in an equation, you can substitute in the other if you include the c^2 conversion factor. Annihilation is a nice means of explaining it to people, but is not nearly the only application.

Momentum is mass*velocity, so due to the interchangeability of mass and energy, the energy spent firing a slug at something will deflect you the same amount as firing the same amount of energy in a "pure" form. (This may be a misunderstanding in purposes here. My last question was assuming that the same amount is being used for, say, a laser and a projectile.)


You CAN generate motion with light (light pressure) and a laser with the intesity of a nuclear weapon might produce even significant push.

Which is the point, chucking a 1kg weight at, say, 1m/s isn't going to deflect a spaceship that much, and a laser using the same amount of energy (which is very, very small) will do the same.


But the damage a laser does doesnt come from imparted directional KE. Rather it excites the matter at the target site, causing it to heat. That heat can do things like cut. A laser as a single beam is far less destructive than a slug. The power of the laser is that you can cut with it. Punching a hole through someone's gut with a laser=not terribly likly to be fatal. Slicing them in half with one... deadly. The same applys to ships.

I never said that you'd have to use a laser and a slug in the same manner.


Its kinda like the difference between hitting someone with a baseball bat (damage is done by impacting a large mass against a target) and slicing them with a scalpel. Both do damage, both potentially fatal, but it takes far less energy to slice with a blade than to bludgeon with a baseball bat.

Yes, but for a ship, how likely is a cut on the surface to be destructive to the ship (unless there are weak points like the carotid arteries that a scalpel blade can reach easily). How much work does it take to cut a person in half with a scalpel if that's the only tool you're using? The analogy doesn't entirely fit.

TheRiov
2007-11-14, 03:30 PM
Yes, but the point is you dont NEED that kind of energy with a laser.

There are several formulae for momentum of a photon.

WalkingTarget
2007-11-14, 03:39 PM
I'm aware of that. I know that lasers and projectiles can be used differently.

Momentum of a photon is it's total energy divided by c which is also the Planck constant divided by the photon's wavelength.

Do any of us know how much energy is needed for a laser to cut through a spaceship? If it's a non-trivial amount, then it will still cause a shift in the momentum of the ship firing it. That is all I am saying here.

edit - clarification - I mean, I'm not trying to debate the relative strengths and weaknesses of lasers and slug throwers.

PlasticSoldier
2007-11-14, 04:27 PM
Well it could be a trivial amount. For example you could pick a 1 atom wide cross section of the ship and turn it into plasma, then there would be nothing holding the 2 halves together.

Dervag
2007-11-14, 10:56 PM
Momentum is mass*velocity, so due to the interchangeability of mass and energy, the energy spent firing a slug at something will deflect you the same amount as firing the same amount of energy in a "pure" form. (This may be a misunderstanding in purposes here. My last question was assuming that the same amount is being used for, say, a laser and a projectile.)It is indeed a misunderstanding.

You get the most momentum transfer per unit kinetic energy for massive, slow-moving objects. You get the most kinetic energy per unit momentum for light, zippy objects.

Thus, a bullet will carry vastly more kinetic energy than a thrown baseball, even if the baseball has roughly the same amount of momentum (since it's so much heavier).

So no, it is not true that every joule of energy you throw at a target affects its momentum the same amount. You can actually prove this with real lasers and stuff, too. Energy and momentum may be, in some obscure sense, interconvertible, but they are not the same thing by any means (not least because momentum is a vector quantity with direction and energy is not).


Do any of us know how much energy is needed for a laser to cut through a spaceship? If it's a non-trivial amount, then it will still cause a shift in the momentum of the ship firing it. That is all I am saying here.The shift is trivial compared to the fact that the ship has just been bisected.

Fuzzy_Juan
2007-11-15, 01:22 AM
The energy necessary is in the terrawatt range, this can be achieved with a megawatt laser by 'chirping' the input at high amplitude which allows for the laser to avereage a much higher energy than by steady state feed. The intensisty of light in the beam will be approximately 20x the intensity of the sun I think...or it was 200, I can't remember every detail about the paper I read on the laser.

These devices generate ultrashort, ultra-high-intensity pulses with a duration of 20 to 100 femtoseconds. A typical one stage amplifier can produce pulses of up to 5 millijoules in energy at a repetition frequency of 1000 hertz, while a larger, multistage facility can produce pulses up to several joules, with a repetition rate of up to 10 Hz. Usually, amplifiers crystals are pumped with a pulsed frequency-doubled Nd:YLF laser at 527 nm and operate at 800 nm. Two different designs exist for the amplifier: regenerative amplifier and multi-pass amplifier.

Regenerative amplifiers operate by amplifying single pulses from an oscillator (see above). Instead of a normal cavity with a partially reflective mirror, they contain high-speed optical switches that insert a pulse into a cavity and take the pulse out of the cavity exactly at the right moment when it has been amplified to a high intensity. The term 'chirped-pulse' refers to a special construction that is necessary to prevent the pulse from damaging the components in the laser.


In a multi-pass amplifier, there are no optical switches. Instead, mirrors guide the beam a fixed number of times (2 or more) through the Ti-sapphire crystal with slightly different directions. A pulsed pump beam can also be multi-passed through the crystal, so that more and more passes pump the crystal. First the pump beam pumps a spot in the gain medium. Then the signal beam first passes through the center for maximal amplification, but in later passes the diameter is increased to stay below the damage-threshold, to avoid amplification the outer parts of the beam, thus increasing beam quality and cutting off some amplified spontaneous emission and to completely deplete the inversion in the gain medium.

The pulses from chirped-pulse amplifiers are most often converted to other wavelengths by means of various nonlinear optics processes.

At 5 mJ in 100 femtoseconds, the peak power of such a laser is 50 gigawatts, which is many times more than what a large electrical power plant delivers (about 1 GW). When focused by a lens, these laser pulses will destroy any material placed in the focus, including air molecules.

Such lasers in test labs require large power plants to supply megawatts of power to the test array for a very short laser burst and usually charge capacitors to store power for the burst so as not to tax the system by feeding off it directly. (read in star trek 'phasor banks'). Space craft that are capable of interstellar flight more than likely have reactors of some sort capable of terrawatt or greater power output...a warship might have a redundant reactor just for weapon systems as well as several capacitors that they can discharge through any one of the ship's hardpoints. (more likely that in simpler designs a single weapon would have multiple capacitors that could be used to fire while the others are reloading, but those capacitors are only for the one weapon. In more advanced designs, all capacitors could be shared by all hardpoints. The power generated in excess of the required energy would dictate how fast the main weapon could fire...also, the thermal tolerance of the weapon would also dictate how often it could be fired. (once the gun gets too hot, to continue to fire could cause the weapon to be damaged, or worse.)

Against shields, such a weapon would likely be absorbed by the same field used to deflect particles in space. A stong enough electromagnetic field might be enough to bend and absorb light energy. The energy spike of the weapon striking the shield may cause a feedback in the shield system that could cause harm to the shield generator in the same way a surge of electricity can fry electronics. Not all the energy may be dispursed into the shields though, some might get through (like the sun's rays breach the ozone). This energy may be strong enough to harm the hull of the vessel.

Against unguarded hull (or shields down), the laser would do damage by annihilating the matterint he vicinity of the strike. The instant vaporizing of the matter would cause a rapidly expanding region of plasma as the high energy particles from the vaporized hull followed the path of least resistance. Pockets of gas would be ignighted by the heat, and portions of nearby hull may be melted...depending on the exact nature of the material of the hull, it may be brittle enough to shatter when forced to expand that rapidly from the instant heating. Common defense against simple laser technology is a smooth mirrored hull, or a diffusing field of some sort. The counter to such techniques is to include charged particles in the stream of energy. These particles imbedded in the laser would serve as an atom smasher and literally 'ram' the hull and break up any simple 'mirror'. Most future based weapons in fantasy have a 'dual' use energy weapon like that...phasors, blasters, etc...all are not simply energy weapons, they are also particle beams as well.

The Star Trek Phasor, in the handheld variety is the perfect example of a dual use weapon that has multiple settings based on the energy output of the weapon and mix of particles. On low energy-high particles (stun) the blast works as the modern prototype PEP (pulsed energy projectile). A stream of charged particles that will cause extreme pain and knock people out. Higher energy settings can burn, blast, and at the highest settings of power evaporate.

Fuzzy_Juan
2007-11-15, 01:39 AM
oh, for firing light and how it effects a ship in space. Do note that it may be possible for a ship with a gravity drive or other advanced propulsion to have some sort of device that renders inertia moot, thus skipping the need to discuss such problems as recoil...else, the ships would not be able to accelerate very fast compared to their maximum speed (FTL travel) Even at 2 or 4 g's worth of acceleration it would take months or years to get up to speed for interstellar travel.

Now...the thing that effects one's position is momentum, not energy. momentum is P=MV where P is the momentum, M is mass of the object, and V is the speed at which it is going. The total P must be constant for newtonian physics to be obeyed. Now, we know that energy and matter are somewhat synonomus...so we can convert an amount of energy into the equivalent mass. the good ol E=Mc^2...so the mass equivalent of the energy is M=E/c^2. if you place this into the momentum equation P=M(ship)V(ship)=(E/c^2)c...

this gives you...V(ship)=E/(MC), or the change in velocity of the ship (assume initial V=0) is equal to the energy of the weapon divided by the product of the mass of the ship and the speed of light). We do a simple order of magnitude check...the enery of the laser will be in the terrawatt range 10^12. The order of magnitude of c is 10^8, the order of magnitude of a ship's mass would be 10^6 (space shuttle weighs 2,000,000+ kg)...so the result is 1/100 in order of magnitude...minimal. Even on smaller craft, a 1/1 ratio would give a few M/s change in a craft moving in Kilometers per second. On larger craft, the difference is negligible, even with more powerful lasers. A minor thruster could be used to counter any recoil if the situation demanded it.

Dervag
2007-11-15, 01:48 AM
Such lasers in test labs require large power plants to supply megawatts of power to the test array for a very short laser burst and usually charge capacitors to store power for the burst so as not to tax the system by feeding off it directly. (read in star trek 'phasor banks'). Space craft that are capable of interstellar flight more than likely have reactors of some sort capable of terrawatt or greater power output...If so, waste heat is going to be a cast-iron bitch to deal with.


These particles imbedded in the laser would serve as an atom smasher and literally 'ram' the hull and break up any simple 'mirror'. Most future based weapons in fantasy have a 'dual' use energy weapon like that...phasors, blasters, etc...all are not simply energy weapons, they are also particle beams as well.I have an extremely hard time believing that will work. Lasers and particle beams are fundamentally different, and combining them won't necessarily work well (not least because the laser will reach the target before the particles unless you have a really good rangefinder built into your synchronizer).


oh, for firing light and how it effects a ship in space. Do note that it may be possible for a ship with a gravity drive or other advanced propulsion to have some sort of device that renders inertia moot, thus skipping the need to discuss such problems as recoil...else, the ships would not be able to accelerate very fast compared to their maximum speed (FTL travel) Even at 2 or 4 g's worth of acceleration it would take months or years to get up to speed for interstellar travel.There's no way to get FTL by accelerating the ship using conventional engines; you have to cheat somehow. Therefore, the relationship between normal-space acceleration (rocket engines) and your FTL drive is more or less arbitrary.

On a side note, for a really interesting take on the inertialess drive, see Dr. Edward E. Smith's Lensman series. In that series, the inertialess "Bergenholm" drive does indeed render a ship inertialess (the term used in the series is 'free' as opposed to 'inert' for ships that do not have their Bergenholms on).

A ship's motion while 'free' is not restricted to light speed and is limited only by friction between the interstellar medium and the spacecraft's defensive screens. Collisions between 'free' and 'inert' objects lead to the free object being brought to a dead stop instantly with no effect on the inert object.

The really interesting part is that when you switch off the inertialess drive, your ship regains whatever momentum it had before you turned the drive on.

Fuzzy_Juan
2007-11-15, 02:40 AM
Yes, waste heat is a problem in all current forms of power generation, however, more efficient power conversion might be possible in the future as well as alternate forms of energy. besides, the excess heat can sometimes be channeled into heating the ship. Who knows, advanced materials in vaccum conditions might provide 99.9% heat shielding to surrounding components.

Particle beams being fired commonly use lasers to act as a targeting guide, or a tracer equivalent. When dealing with theoretical future technology, who's to say that thye don't beef up the 'targeting' laser with a full fledged beam of some sort. It might explain why the laser seems to persist for several seconds...the initial burst of energy is just light as the particles go down the stream, the energy beam punches a hole through the shields to the hull, then the particles hit scarring the surface and damaging reflectors and diffusers, then the last part of the laser pulse slams into the hull causing damage. The particles are traveling at speeds on the order of .1c, so it would not be too long in any case.

For FTL theories...I suggest AIAA2003-4990 "Future Space propulsion based on Heim's Field theory". The paper is available online, and "Guidelines for a space propulsion device based on heim's field theory (http://public.fh-wolfenbuettel.de/~haeuser/research/futureProp.html)"

While the work is being challenged by others, some of the ideas at least hold interesting prospects as to how a massless propulsion system might work. This one, proposed by the Heim's theory, creates a field that reduced the inertia of an object in space by a factor of 10^4, thus radically increasing the velocity without changing the total energy somehow...it is kinda hard to follow and I may not be quoting it right. Basically, you would get your ship up to the threshhold of relitavistic effects 0.1c, and then you kick on the FTL drive which reduces inertia and subsequently shifts your velocity into superluminal speeds. Exactly how it works...who knows...

The actual cutting edge of modern science always makes me smile with where we are in approaching the reality of Sci-fi.

WalkingTarget
2007-11-15, 10:33 AM
oh, for firing light and how it effects a ship in space. Do note that it may be possible for a ship with a gravity drive or other advanced propulsion to have some sort of device that renders inertia moot, thus skipping the need to discuss such problems as recoil...else, the ships would not be able to accelerate very fast compared to their maximum speed (FTL travel) Even at 2 or 4 g's worth of acceleration it would take months or years to get up to speed for interstellar travel.

Now...the thing that effects one's position is momentum, not energy. momentum is P=MV where P is the momentum, M is mass of the object, and V is the speed at which it is going. The total P must be constant for newtonian physics to be obeyed. Now, we know that energy and matter are somewhat synonomus...so we can convert an amount of energy into the equivalent mass. the good ol E=Mc^2...so the mass equivalent of the energy is M=E/c^2. if you place this into the momentum equation P=M(ship)V(ship)=(E/c^2)c...

this gives you...V(ship)=E/(MC), or the change in velocity of the ship (assume initial V=0) is equal to the energy of the weapon divided by the product of the mass of the ship and the speed of light). We do a simple order of magnitude check...the enery of the laser will be in the terrawatt range 10^12. The order of magnitude of c is 10^8, the order of magnitude of a ship's mass would be 10^6 (space shuttle weighs 2,000,000+ kg)...so the result is 1/100 in order of magnitude...minimal. Even on smaller craft, a 1/1 ratio would give a few M/s change in a craft moving in Kilometers per second. On larger craft, the difference is negligible, even with more powerful lasers. A minor thruster could be used to counter any recoil if the situation demanded it.

Ok, more equations now that I've tracked them all down:

The Kinetic Energy of a 2kg slug at 0.9c is 232626122052386328.97... Joules (see equation from earlier in the thread. Also, if recoil is involved, twice this is necessary due to the ship being pushed back in the other direction, right? But I'm dealing with same energy at the receiving end of the attack, so I'll use this figure for the laser)

Momentum at relativistic speeds is m*v/(sqrt(1-(v/c)^2)) which for a 2kg (rest mass) object, would be 1237987921.696... N*s.

Momentum of massless objects (like photons) is equal to E/c, so for the Energy given for firing the slug in the above example is only 775957219.21859... N*s which is over half of the momentum of the slug.

For a 20,000,000,000 kg ship (see this (http://www.tecr.com/galactica/capships/battlestar.htm) page I found on the 1.2km long Battlestar), the change in velocity from an energy blast of this size is 0.03879786... m/s (from the v=E/mc given by Fuzzy_Juan). The if the momentum of the 2kg object is imparted to an object of this mass (v=p/m) the change in velocity is 0.061899396...m/s (this is an approximation as it doesn't take the Lorentz factor into account, but it's so close to 0 that this factor is negligible)

So, yes, as has been pointed out, the momentum change is different between massed objects and massless ones, so thanks to Fuzzy_Juan, Dervag, and TheRiov for pointing it out and making me do the math. However, it's not that big of a difference. I don't care that you wouldn't fire a laser of this power (because it's unnecessarily overkill for the way in which a laser is used). My point is that there is a deflection there and it's not that big of a difference (OP stated about a 1km long ship, so I went with whatever mass I was able to find in science fiction that went along with that).

Also, if any of my math is obviously wrong, please let me know.

Fuzzy_Juan
2007-11-15, 11:07 AM
No, the math is good...the imparted momentum is not significant compared to the mass of a capitol ship...however...if you are talking about 200,000 kg. fighters, you might have to rethink some details...A small craft would be unable to handle the recoil of such a weapon, nor would it be able to remain relatively still after being struck. kinda like a 300 lbs. man shooting cans with a .22 rifle...the man doesn't move...the cans go flying. A fighter craft hit with a capitol ship weapon would be thrown for a loop at the least if not destroyed instantly.

TheRiov
2007-11-15, 11:34 AM
I think you missed the units.

c=299792458 m/s
v=c*.9
m=2
m*c^2=179751035747363528 ~ 1.8 x10^17

E(k)=(mc^2)/(sqrt(1-v/c)^2)-mc^2


I get:

E(k)=((2kg x c^2)/(sqrt(1-(.9)^2)) - (2kg x c^2)

E(K)=((2kg x c^2)/(sqrt(1-.81)) - (2kg x c^2)

E(k)=((2kg x c^2)/(.435889)- (2kg x c^2)

E(k)=412377157799749856 -

E(K)=(1.8 x 10^17)/.435889 - 1.8 x10^17

E(K)~232626968165016752 Joules

~2.3 x10^16 Joules = energy from 'rest' of your shot


your battleship-- just going newtonian here since we know our battleship isnt going to be accellerated anywhere near C by this shot:

v=sqrt(2E/m)
v=Sqrt(2*(2.3x10^16)/(2x10^10))
v=sqrt(23262696.8165016752)
v~4823 m/s

WalkingTarget
2007-11-15, 11:36 AM
No, the math is good...the imparted momentum is not significant compared to the mass of a capitol ship...however...if you are talking about 200,000 kg. fighters, you might have to rethink some details...A small craft would be unable to handle the recoil of such a weapon, nor would it be able to remain relatively still after being struck. kinda like a 300 lbs. man shooting cans with a .22 rifle...the man doesn't move...the cans go flying. A fighter craft hit with a capitol ship weapon would be thrown for a loop at the least if not destroyed instantly.

Right, but the original post of the thread specified a kilometer long ship.

Recoil of firing a 1 gram mass at .01c would throw a man-sized figure back at something like 6000 mph. I don't think anyone here said that relativistic-projectile weapons were practical on small-scale platforms (I, at least, wasn't trying to give that impression).

Edit - Sorry, read your post another time or two, yeah, if a fighter is hit by a capitol ship, it's toast. I thought we were talking about recoil of the ship firing the weapon (when the energy of the weapon that hits the target is equal).

TheRiov
2007-11-15, 11:45 AM
Now, we factor in accleration (I know its not accurate to say that accleration will be uniform down the length of a rail gun, even if it is lets figure this) Even without figuring in relativistc effects (time dilation and length contraction)
(i'm taking some approximations here cuz I want to prove my point. for this, assume c=300000000 m/s
length of battleship = 1000 m


a slug's average velocity of .45 c (half .9c) down its accelleration chamber will take 6.6666666666666666666666666666667e-6 seconds to leave the accellerator (assuming your firing chamber is the length of your battleship)

That means your accelleration faced by your ship is
723450000 m/s^2 or rougly 73972392 G's

WalkingTarget
2007-11-15, 11:54 AM
I think you missed the units.

c=299792458 m/s
v=c*.9
m=2
m*c^2=179751035747363528 ~ 1.8 x10^17


E(k)=(mc^2)/(sqrt(1-v/c)^2)-mc^2


I get:

E(k)=((2kg x c^2)/(sqrt(1-(.9)^2)) - (2kg x c^2)

E(K)=((2kg x c^2)/(sqrt(1-.81)) - (2kg x c^2)

E(k)=((2kg x c^2)/(.435889)- (2kg x c^2)

E(k)=412377157799749856 -

E(K)=(1.8 x 10^17)/.435889 - 1.8 x10^17

E(K)~232626968165016752 Joules

~2.3 x10^16 Joules = energy from 'rest' of your shot

Ok, for (1/sqrt(.19) -1)*2c^2 I get

(2.2941573389 - 1)*179751035747363528
= 232626122052386328.97
= 2.326*10^17 (check your own figure again, it's 10^17 as well, not 10^16)



your battleship-- just going newtonian here since we know our battleship isnt going to be accellerated anywhere near C by this shot:

v=sqrt(2E/m)
v=Sqrt(2*(2.3x10^16)/(2x10^10))
v=sqrt(23262696.8165016752)
v~4823 m/s

Where did you get this equation?

TheRiov
2007-11-15, 12:15 PM
Ok, for (1/sqrt(.19) -1)*2c^2 I get

(2.2941573389 - 1)*179751035747363528
= 232626122052386328.97
= 2.326*10^17 (check your own figure again, it's 10^17 as well, not 10^16)




Where did you get this equation?
formula for KE:
E=1/2 mv^2

2E= mv^2

2E/m=v^2

sqrt(2e/m)=v

WalkingTarget
2007-11-15, 12:34 PM
formula for KE:
E=1/2 mv^2

2E= mv^2

2E/m=v^2

sqrt(2e/m)=v

Ah, yes, thanks.

I thought we'd established that KE was not equal to momentum though.

For a system that starts as a 20000000002kg body with 0 velocity and you split it into a 2kg body going .9c in one direction and the 2*10^10kg body going in the opposite direction at some velocity the momentum is what is equal and opposite, not the KE.

Fuzzy_Juan
2007-11-15, 12:53 PM
correct, while energy is conserved, it can transform and do funny things...that will be conserved separately is momentum. P=MV. It is this quantity that we must deal with in spacecraft dynamics when dealing with free bodies.

In the case of the slug, the momentum of the slug is equal and opposite the momentum imparted to the spacecraft, or P = m(slug)*V(slug)= M(ship)V(ship) (P and V are actually represented by dP and dV, or delta which is the 'change in' momentum or velocity). The change in velocity of the slug is .9*c, the change in the velocity of the spacecraft is unknown and the mass of both objects is known...the change in momentum is equal.

So, you get m*0.9c=MV which boils down to V=0.9*mc/M which is around 0.027 m/s for the above case of 2kg mass and the 20,000,000,000 kg starship.

The same equations are used for collision as well as mass ejection. Conservation of momentum. So the impact case of the same projectile is not appreciably any different. A BB will never move an 18wheeler...no matter how fast it is going...there is just not enough mass compared to the larger object.

(the above didn't include complications form relitavistic effects cause I am being lazy...the order of magnitude is really what matters though)

Dervag
2007-11-16, 07:36 PM
The same equations are used for collision as well as mass ejection. Conservation of momentum. So the impact case of the same projectile is not appreciably any different. A BB will never move an 18wheeler...no matter how fast it is going...there is just not enough mass compared to the larger object.Oh yes it will.

A relativistic BB could easily carry energy on the order of that of, say, a 'bunker buster' bomb such as used by the US Air Force. Its momentum might be small, but its energy would be sufficient to volatilize the 18-wheeler, spreading its constituent atoms far and wide.

PlasticSoldier
2007-11-16, 09:00 PM
Oh yes it will.


Just to add to that I believe earlier in the thread that someone said that a 2lb. object at .8c would carry the energy of a 50 megaton nuke.

WalkingTarget
2007-11-16, 09:52 PM
Just to add to that I believe earlier in the thread that someone said that a 2lb. object at .8c would carry the energy of a 50 megaton nuke.

2kg at .9c is over 50megatons

Fuzzy_Juan
2007-11-17, 02:40 AM
well, lets do a quick crunch...a BB weighs approximately 1 gram...so 0.001 kg traveling at .9c. A famous song talks of a truck carrying 30,000 lbs. of banannas...so lets go with that...30,000 lbs is 13,608 kg...and that is just the cargo, but lets use that to be simple.

if you set up the conservation of momentum you get:

13608 * V(truck) = 0.001 * 300000000

V(truck) = 0.001 * 300,000,000 / 13,608 = 22m/s

ok...there are the numbers...I used a bad example...need a more massive object for the argument...there wasn't enough discrepancy between the mass of the BB and the mas of the truck. Now...I think it is safe to say though that lightspeed particles sufficiently low in mass will not move or harm something that is much more massive. at least not beyond the local scope.

Otherwise, space borne objects being struck by cosmic particles would be shoved out of orbit readily and battered to hell with all the cosmic particles. True that certia things do make the orbit decay over a long period of time, but that just goes to show that the imediate effect of small relitavistic particles is so smal to be negligible at any instant, but it is present, and measurable and will show up in time.

Dervag
2007-11-17, 03:00 AM
ok...there are the numbers...I used a bad example...need a more massive object for the argument...there wasn't enough discrepancy between the mass of the BB and the mas of the truck. Now...I think it is safe to say though that lightspeed particles sufficiently low in mass will not move or harm something that is much more massive. at least not beyond the local scope.Yes, but the reason doesn't have anything to do with low-mass particles being intrinsically unable to carry that much energy or momentum. Instead, the reason has to do with the physical phenomena that produce lightspeed particles. There is nothing in nature that could give an individual subatomic particle such as a proton sufficient kinetic energy and momentum to turn it into a planet-shattering juggernaut. However, there is no theoretical reason why this could not happen. Protons given sufficient velocity would carry enough momentum to knock a planet out of its orbit. Of course, to carry that much momentum they would have to carry enough energy to volatilize the planet, so talking about where the planet gets knocked after being disintegrated is kind of pointless.


Otherwise, space borne objects being struck by cosmic particles would be shoved out of orbit readily and battered to hell with all the cosmic particles. True that certia things do make the orbit decay over a long period of time, but that just goes to show that the imediate effect of small relitavistic particles is so smal to be negligible at any instant, but it is present, and measurable and will show up in time.Again, that isn't because of theoretical limits on the momentum of any one particle.

factotum
2007-11-17, 04:02 AM
The order of magnitude of c is 10^8, the order of magnitude of a ship's mass would be 10^6 (space shuttle weighs 2,000,000+ kg)

The Shuttle only weighs that much when it's sitting fully fuelled on the launch pad with the external fuel tank and SRBs fitted. The actual Orbiter (the part which goes into space) masses about 68000kg unladen, and maybe 110,000kg with full fuel and cargo aboard. One assumes a fighter craft would be considerably smaller than the Shuttle and would thus have a lower mass.