Why does FTL violate causality?

[keybounce]

In the science fiction vs fantasy thread, on the subject of hard sci-fi vs soft sci-fi, I constantly see people complain that FTL violates causality and is equal to time travel.

And: Why?

Lets say I go from Sol to Alpha Centauri in less than 5 minutes. I gather information about the sun, and return back home. I then say "this is what will happen 4 years from now".

But, I still returned to Sol about 10 minutes after I left.

I've seen people point out that if your FTL travel uses your frame of reference -- specifically, the frame of reference you have before you go FTL -- then two round trips can, by changing reference frame at the stop-over -- generate a time loop.

But why does FTL have to use the originator's frame of reference? Why wouldn't it use it's own FoR that is based on the FTL medium / whatever you travel through to make the trip?

And once you have a single constant FoR for all trips, doesn't that make time travel by FTL travel impossible?

[Yora]

As science goes, anything going faster than light is magic.

That being said, hypothetical faster than light movement only "violates causality" in the sense that light travel at a speed equal to the speed of causality. If you move faster than c, you violate c.
But even then, it does not cause an effect to happen before its own cause.

[Anonymouswizard]

The short answer is 'there is no authoritive frame of reference to gauge FTL travel from'.

The slightly longer and maybe somewhat wrong (I have never actually worked with physics at this level) is that we have the origin's frame, the traveller's frame, and the destination frame, as well as a bunch of other frames because other things in the universe exist. But essentially for causality to work you need events to play out (or be detected) in the same order in each frame of reference. FTL and some fancy movement can allow you to have events in one frame play out in a different order in another frame.

A friend once tried to explain to me why this happens using the fact that once you hit relativistic speeds sizes appear to change, and you can use theoretically use this (this might have required actual FTL travel, it's been years) to get a plank in a shed half it's size, close both doors, than open the exit door and let it out because of how even here we have to disagree on the order of events, from our frame of reference we had both doors closed, while from the plank's we had to open the exit door before we could close the entrance because otherwise the plank would never fit in.

Also this might be better for the 'Mad Science and Grumpy Technology' section. This thread from there might be good reading material. In essence, you can construct an FTL system that avoids time travel, but you need to destroy Relativity to pull it off.

[Ibrinar]

There is to the best of our knowledge no preferred reference frame, but physics is the same in all reference frame. You could of course introduce one in scific but methods to explain why it doesn't work in your setting is another topic. If you can travel ftl in all frames, why it breaks causality is probably easiest to understand with instant transmission (it works with just ftl too but you actually have to do the math for the timings then.)

One important thing about relativity is that events simultaneous in one reference frame can happen in a different order in a different reference frame. (https://en.wikipedia.org/wiki/Relati...f_simultaneity) If there is an event T and event M all frames will only agree T happened after M if light of T could have reached M, otherwise some will think M happened first. (https://en.wikipedia.org/wiki/Light_cone) (if I didn't get that wrong.)

Take two planets A and B who are far away from each other but move in the same way so no speed relative to each other. Event X happens on planet A, they sent an instant message to planet B, planet B reacts by immediately informing Observer C with instant transmission. Now the observer C moves with a high fraction of C in relation to the two and in such a way that B receiving the message happens from their perspective before A sends the message (as mentioned simultaneous event aren't necessarily simultaneous from another frame) so from their perspective they received the message before Event X happened. If they now use instant transmission to alert A they can alert them of X before X happened.

[Willie the Duck]

Lets say I go from Sol to Alpha Centauri in less than 5 minutes. I gather information about the sun, and return back home. I then say "this is what will happen 4 years from now".

But, I still returned to Sol about 10 minutes after I left.

This, by itself, is why it violated causality. If you can go find out what happens 4 years from now, and go back and change what will happen, you have violated causality. That's the primary issue. While we don't know that FTL is straight up impossible (so much as we haven't found a way to do it), we really don't know that time travel violates the laws of physics, only that if it were possible, one could violate the laws of logic.

[Reddish Mage]

Shouldn't this be in the "Mad Science" forum?

[Narkis]

There is to the best of our knowledge no preferred reference frame, but physics is the same in all reference frame. You could of course introduce one in scific but methods to explain why it doesn't work in your setting is another topic. If you can travel ftl in all frames, why it breaks causality is probably easiest to understand with instant transmission (it works with just ftl too but you actually have to do the math for the timings then.)

One important thing about relativity is that events simultaneous in one reference frame can happen in a different order in a different reference frame. (https://en.wikipedia.org/wiki/Relati...f_simultaneity) If there is an event T and event M all frames will only agree T happened after M if light of T could have reached M, otherwise some will think M happened first. (https://en.wikipedia.org/wiki/Light_cone) (if I didn't get that wrong.)

Take two planets A and B who are far away from each other but move in the same way so no speed relative to each other. Event X happens on planet A, they sent an instant message to planet B, planet B reacts by immediately informing Observer C with instant transmission. Now the observer C moves with a high fraction of C in relation to the two and in such a way that B receiving the message happens from their perspective before A sends the message (as mentioned simultaneous event aren't necessarily simultaneous from another frame) so from their perspective they received the message before Event X happened. If they now use instant transmission to alert A they can alert them of X before X happened.

I don't understand how this can happen as stated. Yes, C will get the message from B before they see it being sent from A, but that should have no relevance on A. From A's point of view, something happened, THEN they immediately notified B, THEN they got immediately notified by C. No causality violations whatsoever. C might be a bit confused about the order of events, but it should not be too hard to clear things out.

This, by itself, is why it violated causality. If you can go find out what happens 4 years from now, and go back and change what will happen, you have violated causality. That's the primary issue. While we don't know that FTL is straight up impossible (so much as we haven't found a way to do it), we really don't know that time travel violates the laws of physics, only that if it were possible, one could violate the laws of logic.

But you don't go back in time. You notify Earth about what Earth will SEE happening in 4 years. The event has still already happened. Even if you immediately head back to Alpha Centauri, you will get there 10 minutes after the event, unable to change a thing.

[DavidSh]

Shouldn't this be in the "Mad Science" forum?

It was, at one point, with the usual suspects complaining that a stronger version of the claim (that nobody ever made) was obvious nonsense.

The short answer is that, if Special Relativity is true, and there are no preferred inertial reference frames, and you can make an FTL move with distance X and time T, (so X/T > speed of light) with respect to one inertial frame, then you can make an FTL move with distance X and time T with respect to any inertial frame. Otherwise there would be a subset of preferred inertial reference frames. If you can string together such FTL moves with respect to multiple arbitrary inertial frames, you can travel from a point (x,t) in spacetime to a point (x,t') which is in the past with respect to (x,t) (but at the same location with respect to some inertial reference frame), allowing for a loop in time.


Furthermore, any FTL move is travel into the past with respect to some set of inertial frames, but not every sequence of FTL moves necessarily creates loops.

A more exact analysis gets into light cones, and time-like and space-like intervals.

[Z3ro]

You're making a simple mistake; as you go faster, time slows down. So as you approach then break the speed of light, weird things happen to the time axis with regards to your movement. The truth is we don't know what would happen, as we've never been able to observe anything going FTL.

In your example of going to Alpha Centauri, if you were able to do so in a way that didn't involve FTL travel (say, a wormhole or an Alcubierre warp drive, you would in fact be able to do the thing you describe, and it wouldn't be time travel. It's the FTL that's the problem.

[LibraryOgre]

This, by itself, is why it violated causality. If you can go find out what happens 4 years from now, and go back and change what will happen, you have violated causality. That's the primary issue. While we don't know that FTL is straight up impossible (so much as we haven't found a way to do it), we really don't know that time travel violates the laws of physics, only that if it were possible, one could violate the laws of logic.

But I wouldn't say you can find out what will happen in 4 years... you can find out what will be visible in 4 years, from a given point.

Let's say I go FTL, blow up Alpha Centauri, and return. In 4 years, my blowing up Alpha Centauri will be visible from Earth, because light from the Alpha Centauri Nova will take that long to reach Earth. But that doesn't mean you can go back in time to stop me, unless you can actually go back in time. The INFORMATION about the event is taking time to travel... the event itself has already happened.

I find the other way of looking at it like saying you can stop someone from hitting a wood block after you've seen it, but before the sound reaches you. You can stop a bullet from being fired if you see the muzzle flash then travel faster than sound to the sniper. It seems ridiculous to say, because we accept that the bullet has already left when we see the muzzle flash (which traveled faster than sound).

Shouldn't this be in the "Mad Science" forum?

I was thinking the same thing, but figure it's barely acceptable because of the media impact of it. But I'm flexible on it.

[Saph]

I'm not a good enough physics teacher to explain it very well, but the simple version that my friend (who is a physics teacher) has told me is that you get to pick two out of three:

1) Relativity theory
2) Faster-than-light travel
3) Causality

Dropping causality is generally considered the worst option for obvious reasons. Dropping relativity might seem appealing at first glance, but the problem is, it's been confirmed again and again over decades of work via every test that scientists have been able to devise (in fact, it's a necessary component of some of the technology you use).

So if one of the three is going to get the axe, there's a real obvious candidate.

[Ibrinar]

I don't understand how this can happen as stated. Yes, C will get the message from B before they see it being sent from A, but that should have no relevance on A. From A's point of view, something happened, THEN they immediately notified B, THEN they got immediately notified by C. No causality violations whatsoever. C might be a bit confused about the order of events, but it should not be too hard to clear things out.

If in Cs frame of reference instant transmission is possible that means that their message arrives at A in what they consider to be A s present which is before event X. Since the message arriving and event X happen in about the same location, event X is in the light cone of the message arriving and thus everyone agrees that event X happened after the message about X arriving at A.

Edit: C is not confused, order of events is frame dependent unless event A is in the light cone of event B. With any non ftl speed you can't influence anything not in your light cone, with ftl you can. The message could arrive minutes/hours prior to X. If that isn't so it in't instant in one of the frames. If A only gets the message when they send theirs then in Cs frame their reaction to Bs message is not instant.

Edit 2: The explanation Grey_Wolf_c links below is more understandable than mine I think.

[Aotrs Commander]

But I wouldn't say you can find out what will happen in 4 years... you can find out what will be visible in 4 years, from a given point.

Let's say I go FTL, blow up Alpha Centauri, and return. In 4 years, my blowing up Alpha Centauri will be visible from Earth, because light from the Alpha Centauri Nova will take that long to reach Earth. But that doesn't mean you can go back in time to stop me, unless you can actually go back in time. The INFORMATION about the event is taking time to travel... the event itself has already happened.

I find the other way of looking at it like saying you can stop someone from hitting a wood block after you've seen it, but before the sound reaches you. You can stop a bullet from being fired if you see the muzzle flash then travel faster than sound to the sniper. It seems ridiculous to say, because we accept that the bullet has already left when we see the muzzle flash (which traveled faster than sound).

Or, as an example which is basically the same thing on a smaller scale, if I just cruise out at, let's say, cruisng speed of the Aotrs starfleet on sublight engines (a touch over 10% lightspeed) which will take me about 80 minutes) and blow up the sun, it will take eight minutes for anyone to realise it on Earth - and about 80 minutes (based on the speed of a supernova, which is, as it happens about 10% c as well¹) until the shockwave hit.

No time travel is involved.

It's the whole "requires infinite energy to reach the speed of light" which is part of the problem and that the speed of light is the maximum speed of transfer of information.

FTL doesn't really have anything to do with time-travel per se, but as I understand it, (theorhetical) uncompenstated c-transit messes with the local time of the traveller (in very broad, non-technical terms).



(Compenstated c-transit is part of how Aotrs railguns can fire at 2-3 c, using the same temporal compensation as the engines. It's way out of my league, (in fact I got it so badly wrong once, I had to get of my weapons tech tom explain and even then I was mostly lost), but the lies-to-children part of it is that increasing the effective velocity beyond c doesn't actually increase the damage (because of course, if you can accelerate to c without compensation you have infinite power or whatever), but essentially mostly just drops the transit speed - which when you are firing at ranges of around a third of a lightsecond at a target moving at something between 10-60% c makes a huge difference. But that's a whole other complicated issue that, as I say, I very poorly understand, since I'm a necromancer, not a weapons tech...)





¹So, disregarding Aotrs Gate drive (which is somewhere between a wormhole and teleportation, but let's not get into that), we'd have to book it back, but the distance is enough we could get up to max safe sublight speed (roughly 60% c) to have time to gloat...

[Gallowglass]

This, by itself, is why it violated causality. If you can go find out what happens 4 years from now, and go back and change what will happen, you have violated causality. That's the primary issue. While we don't know that FTL is straight up impossible (so much as we haven't found a way to do it), we really don't know that time travel violates the laws of physics, only that if it were possible, one could violate the laws of logic.

Really, to me, this seems like a semantics issue.

Assuming you had the ability to move from Sol to Alpha Centauri in (the frame of reference you call five minutes) then back in five more minutes, arriving 10 minutes later locale frame-of-reference after you left, you wouldn't be saying "this is what will happen four years from now" you would be saying "this is what is happening at alpha centauri concurrent with the last 10 minutes that we will observe four years from now."

[Grey_Wolf_c]

But why does FTL have to use the originator's frame of reference? Why wouldn't it use it's own FoR that is based on the FTL medium / whatever you travel through to make the trip?

Because no matter what, every Frame of Reference can tell their version of what just happened, and if FTL is involved, some FoR will tell you that things happened in a different order from what the other FoR saw.

In the thread link earlier, Douglas has a simple example of how FTL messaging will break causality. He does a much better job of explaining it that I ever can, so I thoroughly encourage you to read it.

ETA: What the keck, I'll give it a try.

There is a fueling station set 1 light Year away from Earth (the fueling station constantly matches speeds with Earth, somehow). A spaceship travelling at 50% the speed of light passes past it without stopping. But a guy at the station manages to take a picture of the space ship as it passes. The picture shows the spaceship rushing past, all its external lights blazing - which is bad, because that's a waste of energy.

That guy then sends the picture to Earth by means of his trusty FTL Messenger System. It arrives instantly, and someone on Earth then sends it to the spaceship with instructions to turn off the lights. When the captain sees the picture, he smacks his forehead and orders the lights turned off.

Now, let's examine when this happens.

From the FoR of the station, Earth is 1 LY away. So, when the message is sent, it arrives 1 year before the light from the the station arrives to Earth. So it gets sent back to the spaceship, who receives it 1 year before its light is due to arrive to Earth.

But the ship is travelling at 50% the speed of light. By the time it passes the station, it is less that a light year away from Earth, because from it's FoR, Earth is hurtling towards it at 50% the speed of light, getting closer and closer every second (so it's, say, about 6 light months away, which it'll take about 9 months to cover*). So where was the ship one light year before it arrived? Why, it hadn't reached the fueling station yet, nor would it for, say, about 4 more months. So, they turn off the lights, and four months later they pass the fueling station, where the photo is taken... except the photo doesn't show the lights on, so it doesn't get sent to Earth, and no-one tells the captain, so the lights stay on... and there goes causality.

Grey Wolf

*This is called length contraction, and it is, as you can now see, a hell of a drug.

[Frozen_Feet]

The entirety of the problem is precisely that there is no constant, universal frame of reference. If writing fiction, sure, you can arbitrarily state one exists, but as far as reality and known physics go... there isn't any, and there is no plausible form of FTL that would come with such a thing.

Because of this lack of universal frame, FTL creates events where information travels backwards between two (or more) frames, that is, causes precede effects.

Now, there are interpretations of relativity, string theory etc. where these acausal interactions aren't a big deal. But those interpretations also imply some seriously weird stuff which remains unfalsifiable at the moment.

What kind of weird stuff? Oh, just eternalism and time being an illusion, near-infinite branching universes which don't meaningfully interact, non-existence of free will, etc.

[Kato]

I don't think I can give a more concis answer than has already been given but I'll try for a very short and simpler (???) one. (It's really quite hard to give a simple, complete answer on this I think)

The problem is relativity and its lack of a universal time frame. Teleportation is cheating and doesn't matter here, it's moving while obeying the laws of relativity. The problem is, stuff is always moving so "just ignore moving objects" is not a helpful answer. If you move or just send a message FTL, a moving observer due to their different perception of time might see you or your message arrive before you send it. Which instantly leads to the possibility of them telling you, you need to get going / send the message (or not). Dang, causality broken.


Because it really is something that has nothing to do with what we see in our daily lives I can absolutely understand this is hard to grasp. Honestly, without decent, convincing diagrams (or formulas if you're that kind of person) I would not expect most people, me included, to entirely understand it. Here's an easy-ish explanation... Thanks, google.


(edit: and before someone tries to argue with me, I debated against this in the other thread, I was always trying to make it clear I know it requires ditching relativity or cheating in some other way and I never claimed FTL was realistic, just not that hard to imagine )

[Gallowglass]

Because no matter what, every Frame of Reference can tell their version of what just happened, and if FTL is involved, some FoR will tell you that things happened in a different order from what the other FoR saw...

*This is called length contraction, and it is, as you can now see, a hell of a drug.

I mean, you are probably right and I am probably wrong because I have certainly heard this argument before. But it never made sense to me.

So you have a station stationary at 1 LY from earth. Somehow they use some magic to keep it exactly and forever the space distance needed for light to get from the station to earth in 1 year at 186,282 miles per second.

A vessel passes the station moving at .5 C (186,282/2 miles per second) meaning at the moment it passes the station it will take 6 months for it to get to earth.

Some guy happens to take a picture at the EXACT moment both the station and the ship are at the 1 LY to earth mark. We are assuming a magic camera that takes a picture without light so he gets the picture you are positing.

Seconds to minutes are passing for both the station, the ship and the earth. The person takes the picture, sees the problem, walks over to the FTL message box and sends the message. For all three PoR we'll say a minute passes.

The station is still 1 LY from earth. Earth is still 1 LY from the station. The ship is now 1LY-1Lmi from Earth. Earth is now 1LY-1Lmi from the ship. So the SHIP is now 186,282*60 miles closer to earth than the station is.

The guy on earth gets the message, sees it, decides what to do and punches in the ship code and forwards the message. That takes another minute. At the moment he sends the message the station is still 1 LY from earth, Earth is still 1 LY from the station, but the ship is now 186,282*120 miles closer to Earth than the station and is 1LY-2Lmi from Earth and Earth is 1LY-2Lmi from the Ship.

When the ship gets the message, its moved 186,282*120 miles from the point its picture was taken. Its now that much closer to earth than when the picture was taken. Its 1LY-2LMi from earth.

I don't get why you think that the message would get to the ship WHILE it was still 1 LY from Earth just because that was when the picture was taken. Time has progressed for all three FoR.

This is assuming the instantaneous transport of the message. IF the message moved at some multiple of C then it gets messier but it still just adding time to each frame of reference based on how long each discreet activity takes.

Of course, as I said, this is most likely a failure of my imagination.

[Grey_Wolf_c]

The station is still 1 LY from earth. Earth is still 1 LY from the station. The ship is now 1LY-1Lmi from Earth. Earth is now 1LY-1Lmi from the ship. So the SHIP is now 186,282*60 miles closer to earth than the station is.

This is only true from the PoV of the station. From the PoV of the ship, Earth is significantly closer. I don't know the exact math, but it's something like 8-9 light months. What you are suggesting - that distance is equal for all frames of reference - violates General Relativity, and has been proven to not be the case. The only thing that is identical for all frames of reference is the speed of light. Not time, and not distance.

ETA: by the way, the example could work for a station that was moving with respect to Earth. I had it relatively immobile just to simplify the situation.

Grey Wolf

[keybounce]

The guy on earth gets the message, sees it, decides what to do and punches in the ship code and forwards the message. That takes another minute. At the moment he sends the message the station is still 1 LY from earth, Earth is still 1 LY from the station, but the ship is now 186,282*120 miles closer to Earth than the station and is 1LY-2Lmi from Earth and Earth is 1LY-2Lmi from the Ship.

When the ship gets the message, its moved 186,282*120 miles from the point its picture was taken. Its now that much closer to earth than when the picture was taken. Its 1LY-2LMi from earth.

I don't get why you think that the message would get to the ship WHILE it was still 1 LY from Earth just because that was when the picture was taken. Time has progressed for all three FoR.

This is my thinking as well.

I thought I mention in my first post that if you can make multiple jumps in different frames, you will get time travel/causality violation because the different light cones are at different angles (or something like that).

But if there is a special reference frame?

The universe itself is a non-moving, non-rotating reference frame -- perhaps that is special?

(It's one thing to say that all observers see the same laws of physics -- that's confirmed.
It is something else to say that you cannot detect a privileged frame -- that's an assumption, at best we can say that we don't know how to detect one.
It is something else entirely to say that a privileged frame cannot exist -- there's nothing to support that.)

[Douglas]

I mean, you are probably right and I am probably wrong because I have certainly heard this argument before. But it never made sense to me.

So you have a station stationary at 1 LY from earth. Somehow they use some magic to keep it exactly and forever the space distance needed for light to get from the station to earth in 1 year at 186,282 miles per second.

A vessel passes the station moving at .5 C (186,282/2 miles per second) meaning at the moment it passes the station it will take 6 months for it to get to earth.

Some guy happens to take a picture at the EXACT moment both the station and the ship are at the 1 LY to earth mark. We are assuming a magic camera that takes a picture without light so he gets the picture you are positing.

Seconds to minutes are passing for both the station, the ship and the earth. The person takes the picture, sees the problem, walks over to the FTL message box and sends the message. For all three PoR we'll say a minute passes.

The station is still 1 LY from earth. Earth is still 1 LY from the station. The ship is now 1LY-1Lmi from Earth. Earth is now 1LY-1Lmi from the ship. So the SHIP is now 186,282*60 miles closer to earth than the station is.

The guy on earth gets the message, sees it, decides what to do and punches in the ship code and forwards the message. That takes another minute. At the moment he sends the message the station is still 1 LY from earth, Earth is still 1 LY from the station, but the ship is now 186,282*120 miles closer to Earth than the station and is 1LY-2Lmi from Earth and Earth is 1LY-2Lmi from the Ship.

When the ship gets the message, its moved 186,282*120 miles from the point its picture was taken. Its now that much closer to earth than when the picture was taken. Its 1LY-2LMi from earth.

I don't get why you think that the message would get to the ship WHILE it was still 1 LY from Earth just because that was when the picture was taken. Time has progressed for all three FoR.

This is assuming the instantaneous transport of the message. IF the message moved at some multiple of C then it gets messier but it still just adding time to each frame of reference based on how long each discreet activity takes.

Of course, as I said, this is most likely a failure of my imagination.

There's an implicit assumption in here that there is only one correct value for "what time is it on Earth right now" at the location of the station and ship. That assumption is incorrect - the answer for "what time is it on Earth right now" depends on how quickly your frame of reference is moving towards Earth (or rather, how quickly Earth is moving towards the observer in your frame of reference). The station and the ship will give two different answers to this question - and they're both right.

That fundamental disagreement over what the current time is at a distant location is the core of the causality violation. Take that, a means of FTL communication, and a way to switch which reference frame the FTL comm is using (just accelerating the sender and/or receiver should do it), and those three ingredients add up to a way to make a message arrive back at its origin before it was sent.

[Frozen_Feet]

For all three PoR we'll say a minute passes.

This right here. This is the false assumption you're making. Precisely because there is no constant universal reference frame, a minute on the ship is not equal to minute on the station is not equal to minute on Earth. True simultaneity does not exist. Once you introduce FTL, the math actually works out to negative time passing in some frames in relation to others. To give a rough idea, your "instant messaging" device would need tachyons or similar hypothetical particles to work.

[jayem]

I mean, you are probably right and I am probably wrong because I have certainly heard this argument before. But it never made sense to me.

There is an extent where you are begging the question with the phrasing. In particular if we assume the Lorentz contractions then we need to be careful when we say distances and times as you do in the paragraph. So you keep 'causality' at the expense of '(einsteinian) relativity', and because you don't look at anything to do with c you don't see it.

For the moment lets instead assume Gallileon relativity (I.E distances/times are the same for all observers). On receipt of the signal earth instantly radio's back to the ship.

From the frame of the station, the ship has moved 1/3 of the distance. the radio beam has moved 2/3 of the distance. The radio beam has moved 2/3Light Years in 2/3 Years.

From the frame of the ship, the ship is stationary. The moving earth sends the radio wave when it is 1 LY away, it then 'chases' after the wave at 0.5C. After 2/3 Years the light wave hits the ship, having traveled 1 LY.

Now here we have two options.

One is to say of course the light moves faster, it was fired from a 'moving planet'. When we move to the ships frame we need to take that into account the light is moving at 1.5C in ship frame. And all is hunky doory as relates to the different frames (which is nice for assorted reasons), everything makes perfect sense.
However when we look at light in terms of electric and magnetic fields you end up with a hideous mess, which I can't even begin to fathom.

The other is to say that the earth-station frame is special. We expect things to break when looking at ship view. At first sight again this looks fine, things make sense in the only frame that matters.
However conservation and symmetry laws are closely related, in particular conservation of momentum is gone and Newtons 2nd law is some ghastly mess.

At this point Einstein took a third option. Basically to do what Galileo did for speed (I.E our first option) to distance and time as well, so that electo-magnetism can work. Unfortunately it's not as simple as adding it example.

The problem is we now now need to rework your example (we'll do a half assed version where we fix distances but have a constant time, this breaks when we add a third frame, einstein & lorentz did what they did for a reason)

So from the station/earth frame (radio version)
They are 1LY apart, and the spaceship meets the radio (traveling at C) at 2/3s year time (all is ok the ship Has moved 1/3 LY, the beam 2/3 LY all add up.)
In the ship frame the 'stationary' spaceship meets the radio (traveling at C) at 2/3s year time, it must have been 2/3 of a light year away.
(in proper relativity the difference is split between time and space, here we have exaggerated length contraction without time dilation).
Now switching back to your example with those distances

Spoiler: Earth-Station Frame

Show

So you have a station stationary at 1 LY from earth. Somehow they use some magic to keep it exactly and forever the space distance needed for light to get from the station to earth in 1 year at 186,282 miles per second.

A vessel passes the station moving at .5 C (186,282/2 miles per second) meaning at the moment it passes the station it will take 6 months for it to get to earth.
[as before]

Spoiler: Vessel Frame Note this isn't the right numbers

Show

So you have a station stationary at 0.666 LY from earth. Somehow they use some magic to keep it exactly and forever [this bit doesn't work, because we didn't do the job properly].

A station passes the vessel moving at .5 C (186,282/2 miles per second). At the moment it passes the station it will take 6 months for earth also to pass it.

[in this case because we've ignored the time aspect, causality is also maintained]

Spoiler: Vessel Frame, hopefully right

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So you have a station stationary at 0.866 LY from earth. Somehow they use some magic to keep it exactly and forever so light takes 0.866 Y to reach it.

A station passes the vessel moving at .5 C (186,282/2 miles per second). At the moment it passes the station it will take 6 months for earth also to pass it.

Actions that took a minute on the ship in earths frame now take 0.866 minutes.
Actions that took a minute on the earth in earths frame now take 1.15 minutes

And it is with that difference that you can play around with causality. In this example if you had two alarms that start and stop at the same time from earths frame, now the ship one now stops nearly half a minute earlier. If you tie that with further actions then things get messy

[Anymage]

This is my thinking as well.

I thought I mention in my first post that if you can make multiple jumps in different frames, you will get time travel/causality violation because the different light cones are at different angles (or something like that).

But if there is a special reference frame?

In fiction, this is a wonderful idea. Relativity is weird and unintuitive, and handwaving it for space opera simplifies a lot.

In reality, this has been experimentally disproven. Repeatedly.

The universe itself is a non-moving, non-rotating reference frame -- perhaps that is special?

(It's one thing to say that all observers see the same laws of physics -- that's confirmed.
It is something else to say that you cannot detect a privileged frame -- that's an assumption, at best we can say that we don't know how to detect one.
It is something else entirely to say that a privileged frame cannot exist -- there's nothing to support that.)

"The universe" doesn't have a clear center or preferred direction. (Everybody actually does see themselves at the exact center of the universe and thinks that everything is moving away from them at an ever increasing speed, but that's more relativistic weirdness rather than less.) Just looking at nearby galaxies, is there any reason why the Milky Way would have the right of it as opposed to the Andromeda galaxy that's coming towards us at ~68 miles per second?

And while you're right that I can't prove for certain that it isn't just mischievous pixies screwing with particles, or that you're just a brain in a jar and everything from experimental data to other people you interact with are just data fed in by your handlers, relativity makes many predictions that have been consistently verified. If you want to overturn it with "I dunno man, how can we really know for sure", create a new model that makes its own predictions and then get to work testing them.

[Lethologica]

This even suggests a way to implement that wonderful idea for fiction. Simply have FTL that is experimentally verified to only work in a particular reference frame for whatever science-fictional reason we feel like, leave everything else unchanged, and hey presto. We'll also need some way to get to relativistic speeds, so as to interact with interesting local objects as something other than a smear on the windshield, but that's no big deal.

[Darth Credence]

Lethologica - that sounds similar to the method used in Robert Kroese's Rex Nihilo books. IIRC, the idea is that there are infinite reference frames, and you create one where you are actually close to where you want to go, then reverse engineer an entire frame around that fact. Then you input that as the new reference frame for the journey, and you can travel interstellar distances quickly. It is a comedy book, so the idea isn't exactly fleshed out, and it may have been referring to changing geometry rather than dealing with relativity at all.

[Grey_Wolf_c]

The universe itself is a non-moving, non-rotating reference frame -- perhaps that is special?

Then the laws of physics as we understand them don't apply, and your original question is moot. Sure, you can imagine a universe in which Newtonian Physics apply. But in that universe, the speed of light isn't special. Going FTL just requires a sufficient amount of thrust. The speed of light would be like the speed of sound.

It'd also, I'm sure, cause all kinds of weird consequences, which is why Einstein figured out that something had to give, and it wasn't the speed of light, but you could simply hand wave that away.

(It's one thing to say that all observers see the same laws of physics -- that's confirmed.
It is something else to say that you cannot detect a privileged frame -- that's an assumption, at best we can say that we don't know how to detect one.
It is something else entirely to say that a privileged frame cannot exist -- there's nothing to support that.)

No, actually it is the other way around: physicist say there can't be a privileged frame of reference because otherwise not all observers would see the same laws of physics. For example, it could be that observers moving towards the light would measure the speed of light as being slower than observers at rest. Therefore we could find that absolute rest state by adjusting our speed until it maximized the speed of all incoming light.

Grey Wolf

[Tvtyrant]

Relativity as I understand it is based on the imbalance between the universality axiom, ie the unprovable assumption that the laws of physics remain the same and are the same everywhere.

We actually know that axiom is false, because of the Big Bang. The problem is that you can't measure changes in the laws of physics because observations become unrepeatable. Science is entirely dependent on that axiom, so if something or somewhen actually did work differently we wouldn't know.

[Grey_Wolf_c]

Relativity as I understand it is based on the imbalance between the universality axiom, ie the unprovable assumption that the laws of physics remain the same and are the same everywhere.

[citation needed]

I suppose in broad strokes all science is based on that axiom, but not relativity in particular. Relativity is based on the proven observation that the speed of light is identical for all observers regardless of relative speeds.

Grey Wolf

[Mightymosy]

I feel a lot of people miss a core principle of physics:

What IS physics?

Physics is a system of mathematical equations which try to make sense of the universe we observe around us - put it into a theoretical framework so me might try to understand it better.
Also, try to use these equations to predict how certain things in the future will turn out. And use these equations to construct machines and other devices to use the "laws" of the universe for our advantages.

Now, relativitiy theory is an awesome theory. An awesome theory that was discarded at first, until a lot of very intelligent people were ultimately proven wrong in their pre-relativistic worldview.

But kept in perspective, relativistic theory was just ONE step in the long history of physics, ultimately just one other big theory that expanded upon the ones before it.

And if you ask me: It won't be the last, either.


In other words: physics theory is best at describing things we know. Put it into equations. I would not trust physcis to predict EVERYTHING that is impossible. In fact, the historical development went in cycles: each cycle someone discovered something that went against the current equations, against the current theory. And then devised a new all-encompassing theory that explained everything better. But so far, none of them had proven to last forever. Some day, I expect someone will discover a phenomenom that breaks the concept of relativistic theory. And then people will come up with another theory that us better suited to describe reality as we - or they - know it.


The only constant in the history of humankind is that every single one who thought they knew everything has been proven wrong ultimately.