The space elevator

[Grey_Wolf_c]

the tip of the section just below the break would likely become supersonic. Falling paper typically isn't held in 7 GPa tension before it's released.

Falling paper attempting to travel supersonically through the air would not last long. Neither would the ribbon. Heck, as someone pointed out in another thread, it doesn't take much more than a forgotten duck tape to disintegrate a supersonic jet, and those things are at least nominally designed to survive the process of going supersonic.

Grey Wolf

[Mando Knight]

Falling paper attempting to travel supersonically through the air would not last long. Neither would the ribbon. Heck, as someone pointed out in another thread, it doesn't take much more than a forgotten duck tape to disintegrate a supersonic jet, and those things are at least nominally designed to survive the process of going supersonic.

Grey Wolf

The ribbon would have very different properties from the paper--structurally, it would be closer to a multi-kilometer-long bullwhip, made of a very strong material that doesn't burn easily. Both the scale of the structure and the lack of extensive materials data on the proposed nanotube cable complicate matters--calculating the impact velocity of a space elevator's carbon nanotube cable (with, say, a 0.2cm x 5cm rectangular cross-section) that was broken at 30km altitude while under 7 GPa of tensile stress is something that's sufficiently far beyond typical aerodynamics problems that I wouldn't necessarily be confident in a single computer model's answer, but on a scale of "5 tons of falling paper" to "God's Own Bullwhip" I would definitely expect the correct answer to fall significantly closer to the latter.

[Lvl 2 Expert]

Specific gravity is used in plenty of places where the primary interest is not just buoyancy in water. The important feature of specific gravity is that it is a dimensionless measure of density. This means there is no possibility of confusion when communicating with people who use different unit systems. As long as they're used to using the term specific gravity the way it is normally used in technical communication, that is.

So the problem is people getting confused which units are being used because people don't properly name which one they're using, and the solution was introducing yet another standard specifically designed to be dimensionless so nobody will put a unit next to it? Brilliant.

[Grey_Wolf_c]

The ribbon would have very different properties from the paper--structurally, it would be closer to a multi-kilometer-long bullwhip, made of a very strong material that doesn't burn easily.

I'm sorry, what? It's made of carbon. It will burn if it tries to enter air at supersonic speeds. And the sections already in the atmosphere won't be going fast enough in the dense soup that is the atmosphere. Your claims make no sense. You need to present some kind of evidence that what you are saying is not fear mongering.

on a scale of "5 tons of falling paper" to "God's Own Bullwhip" I would definitely expect the correct answer to fall significantly closer to the latter.

OK, and I definitely expect it to fall significantly closer to the former. And I'm backed by everything I have read on the topic - literally the only time I have found it to be otherwise are simulations on an airless planet. What backs up your assertion?

Grey Wolf

[Lord Torath]

If the break is within earth's atmosphere, I'm going with "god's bullwhip" for approximately 1 second, at which point it will be "4 tons of smouldering, unconstrained feathers". The other 3996 tons are probably in space
Tug of War has a surprisingly bloody history¹, and with the space elevator, we're essentially playing Tug of War with the GSO Space Station. So yes, there will be a LOT of potential energy stored in the tension of the ribbon. But the atmosphere is big compared to that energy, and should be able to absorb that energy before it causes significant damage to the surface of the planet. I'm not exactly certain what will happen to the "half" of the ribbon in space, though. With no atmosphere to slow it down, it could be pretty destructive to anything it comes into contact with.

[georgie_leech]

If the break is within earth's atmosphere, I'm going with "god's bullwhip" for approximately 1 second, at which point it will be "4 tons of smouldering, unconstrained feathers". The other 3996 tons are probably in space
Tug of War has a surprisingly bloody history¹, and with the space elevator, we're essentially playing Tug of War with the GSO Space Station. So yes, there will be a LOT of potential energy stored in the tension of the ribbon. But the atmosphere is big compared to that energy, and should be able to absorb that energy before it causes significant damage to the surface of the planet. I'm not exactly certain what will happen to the "half" of the ribbon in space, though. With no atmosphere to slow it down, it could be pretty destructive to anything it comes into contact with.

Issac Newton is the deadliest son of a @#$% in space, after all. I think the point, as you say, is that with the energies involved with a space elevator get loose, you're in "needs specifically designed heat shielding to last any length of time." "God's own bullwhip" is a lot less scary when it destroys itself mid swing.

[halfeye]

Diamonds are also carbon, they probably burn a bit, but not that much I think.

[factotum]

Diamonds are also carbon, they probably burn a bit, but not that much I think.

It's entirely possible to burn a diamond if you get it hot enough--it's just people aren't usually daft enough to do that, since a diamond is worth considerably more as a diamond than it is as carbon dioxide.

[Rogar Demonblud]

Depends on temperature and the presence of oxygen. There are (or I guess, were, since I haven't watched in a long time) videos on YouTube of people torching gems in a chem lab to demonstrate different properties (spectrography is the one I remember).

[georgie_leech]

Diamonds are also carbon, they probably burn a bit, but not that much I think.

It's not even all that complicated.

[Lvl 2 Expert]

Diamonds are also carbon, they probably burn a bit, but not that much I think.

Diamonds burn at around 900 degrees C in air, and sooner in pure oxygen. (Source: a very quick google, feel free to correct me.)

Diamonds already start to convert into graphite around 700 degrees, and graphite has an ignition temperature of only 400 degrees, so whether you are actually going to see much diamond burning is a bit of a question, but it's certainly not the hardest thing ever to burn. Most metals melt don't melt until somewhere around 1500 degrees C.

Ignition temperatures for carbon nanotubes are a bit harder to come by, but this source suggests it's 610 degrees for that specific variety. So it's not going to be that much more resistant then diamond.

[DavidSh]

On the other hand, carbon-fiber reinforced carbon was used for the hottest parts on the surface of the Space Shuttle, which usually managed a successful re-entry without burning through. So it depends on details, not just on the elemental composition -- carbon, rhenium, or whatever.

[halfeye]

I wasn't saying that diamond doesn't burn, obviously everything does if it gets hot enough.

The question is, does it get through the atmosphere before it's burnt up?

For a 1000 mile wide asteroid vs a 100 mile atmosphere, the answer is obviously it's not burnt. For a mile thick cable, it's not so clear cut, for a 10 Ft thick cable it is pretty clearly the case that not much will reach the ground, but is that enough width to support a space elevator?

We can put this thing so it will fall into the Pacific, and avoid building in it's wake, the higher sections aren't a problem, we can cut them before they're a problem if it does fall.

There is a human economics problem, if the price of housing land in the "shadow" of the space elevator falls, people will build there because it's cheaper, especially because the developers won't be living in the developments they build.

[Spojaz]

If the cable breaks, you really wouldn't want to be next to it, because of the shockwave propagating downwards at around 25000 m/s, gaining energy as the tension that broke the tether (100 gigapascals!) is released the whole way. I'm not sure whether this force would shred the cable, but if not, it would absolutely destroy whatever the cable is mounted to. On both ends. Unless the station has been built to not be in the way of this happening

Once released, and the initial shockwave dispersed, the carbon nanotube cable will destroy everything unlucky enough to be in it's path simply float away. We're talking about one of the lightest solids known to man.

[gomipile]

So the problem is people getting confused which units are being used because people don't properly name which one they're using, and the solution was introducing yet another standard specifically designed to be dimensionless so nobody will put a unit next to it? Brilliant.

Well, I've been using and thinking in terms of specific gravity for most of three decades now, so, yes. I think it is a pretty brilliant and successful solution.

[Lord Torath]

So the problem is people getting confused which units are being used because people don't properly name which one they're using, and the solution was introducing yet another standard specifically designed to be dimensionless so nobody will put a unit next to it? Brilliant.

It actually is pretty useful. If a material's SG is greater than 1, it'll sink. If it's less than 1, it'll float. We humans all have a pretty decent idea of how dense water is, and so comparing the density of other materials to it gives us an instant idea of just how dense they are. 165 lb/ft³? I have no idea how dense that is. But telling me it's got a Specific Gravity of 2.65 instantly lets me know it'll sink in water, and gives me an idea of how dense it is.

Plus, if you use metric units (and really, why wouldn't you, if you're not living in one of the three backward countries that refuses to use them) SG = density in g/cm³ (through the simple magic that a gram is the mass of 1 ml of pure water at 4^oC)

[Lvl 2 Expert]

Plus, if you use metric units (and really, why wouldn't you, if you're not living in one of the three backward countries that refuses to use them) SG = density in g/cm³ (through the simple magic that a gram is the mass of 1 ml of pure water at 4^oC)

Exactly. So it doesn't change my values a lot, but it prevents me from accurately pointing out which units I'm using.

I'm sure it works fine in all those places that use it as the standard, I just thought a unitless measure was a funny solution if the problem was "people don't name their units so we never know which one they're using".

[Knaight]

It actually is pretty useful. If a material's SG is greater than 1, it'll sink. If it's less than 1, it'll float. We humans all have a pretty decent idea of how dense water is, and so comparing the density of other materials to it gives us an instant idea of just how dense they are. 165 lb/ft³? I have no idea how dense that is. But telling me it's got a Specific Gravity of 2.65 instantly lets me know it'll sink in water, and gives me an idea of how dense it is.

Plus, if you use metric units (and really, why wouldn't you, if you're not living in one of the three backward countries that refuses to use them) SG = density in g/cm³ (through the simple magic that a gram is the mass of 1 ml of pure water at 4^oC)

This seems less like an argument for specific gravity and more like an argument for just using the metric system. Which I'm entirely on board with, especially for calculations.

[georgie_leech]

This seems less like an argument for specific gravity and more like an argument for just using the metric system. Which I'm entirely on board with, especially for calculations.

SHHHHHH, if they know that, we won't be able to sneak it past the Imperialists!