Warp Drive Speed

ANarwhal

I was reading this article today: http://news.yahoo.com/warp-drive-may...161301109.html

and in it it states this:

How did they come to the figure of 10*C? Is it a matter of energy input to speed output?

Zmunkz

I'm guessing they had some ideas about the material properties in the ring and the amount of curvature they might expect to get out of it. I wonder if we can find the publication on which this story is based... Might have more info.

Glad you posted this question -- that was a cool link to read!

Zmunkz

http://en.wikipedia.org/wiki/Alcubierre_drive

More information on the math and theory is there

vega12

The author of that study has actually posted in the comments here: http://www.icarusinterstellar.org/da...eld-mechanics/. Looks like it is not published at the moment, so we can't see the details of the analysis. This also says how he came up with a reduced energy requirement: "the energy requirements can be greatly reduced by first optimizing the warp bubble thickness, and further by oscillating the bubble intensity to reduce the stiffness of space time". Interesting to see that something we have always thought of as a absolute physical restriction (the speed of light) can be overcome in some clever way without even appealing to a new theory (not counting the wormhole solution of course).

dcooper

Would warp drive be a external force or internal forces. Any experts please Help. -dcooper

K^2

Neither. The ship does not actually undergo acceleration. It's the make the mountain come to you type of deal.

HomogenousCow

how do we define the speed anyways??

Zmunkz

I never quite understood that. Where is the equal-and-opposite-reaction felt? Forces must be involved, at least around the bubble where spacetime is forced to curve. Or is it more like a wave in water and the forces are between the parts of spacetime itself?

But even in that case, wouldn't you at least experience a strong tidal and gravitational pull as the spacetime beneath you became arched between the front and back of the bubble?

HomogenousCow

Are we really taking this seriously...

K^2

Local or global? There is a huge difference, which is why this thing works at all. Local speed is limited to c. Global is not.
Absolutely. I mean, it only works on paper so far, but it's based on General Relativity, and that stuff has been verified to 11 orders of magnitude. So it's probably right. Or at least, precise enough for this to work.

Note, this is still note technologically feasible, and whether it ever will be is an open question. The new computation just bring us one step closer.

Again, the craft does not accelerate. There are no forces. You might need to have some velocity relative to destination point, which would be achieved with conventional rockets. But the warp drive itself does not provide a force of any kind.

The space-time inside the bubble is flat. No tidal forces either.

Edit: Though, I have no idea what happens to interstellar dust in all of this. I don't know if anybody tried making computations that account for it.

Nugatory

Here's a question:
Suppose, for the sake of argument that this line of thinking eventually enables us to transport a massive object, say a message written on a piece of paper, or a bomb, or a person, or whatever between two points in spacetime that are spacelike separated. (If we can't do this, it's not an especially cool and interesting "warp drive").

What good paradoxes can we construct under this assumption?

My personal definition of a good paradox: A good paradox will be short and easily described (think bug-rivet or pole-barn), will conflict strongly with our intuition of what is physically reasonable ("Hey - I just killed my grandfather!"), and will tell us something interesting when we've resolved it (think Bell's spaceship).

sophiecentaur

I think they may well be.
I think an eye of newt and toe of frog may be called for soon.

K^2

No true paradoxes, but there might be some interesting things that seem like paradoxes. Though, pole-barn paradox is in the same category, so that's probably what you are thinking of.

The biggest conceptual "problem" with FTL travel is that it violates causality. Causality is a local concept in GR, which is why Warp Drive can exist, but it's a bit hard for people to wrap their mind around global causality violations.

Look at the same pole-barn paradox. It seems like a paradox because order of events appears to switch. But in order for this to happen, the two events must be space-like separated. In other words, closing of the first barn door cannot be the cause of second door opening. If you rig the second door to open only after the first one closed, the pole will ram into the second door in every frame, because the signal required to open the door has not had time to propagate. This is effectively how pole-barn paradox is resolved. Yes, order of events is frame-dependent, but effect always follows cause in every frame.

But if instead of the light beam signal from first door to second I send information with a tiny warp drive ship, the information can actually arrive in time. I can have the second bar door open as response to first door closing, and do so in time for the pole to go through. Now the situation becomes truly bizarre from pole-bearer's perspective. He knows that the second door will only open after the first one closes, but he observes the second door opening before the first door closed. The effect precedes the cause.

That's the sort of "paradoxes" that we can expect with a warp drive.

Notice that these still don't cause any real contradictions. Yes, causality is a frame-dependent concept now. But even though there is a frame of reference in which you can have results of the lottery drawing before the drawing took place, there is no way to make use of that information.

Of course, GR also allows time travel, so apparently, knowing results in advance and being able to act on it would not be a problem either. But that's a separate discussion all together.

Nugatory

Well, I was kinda hoping for a true paradox, a genuine logical inconsistency that would spare us the search for Sophiecentaur's "eye of newt and toe of frog".

K^2

There aren't any. GR allows for space-time configurations that allow exceeding speed of light. GR is a self-consistent theory, so there cannot be any true paradoxes from that alone.

It's a different question of whether any of these things are practically achievable. Original Alcubierre Drive requires negative energy densities. I have no idea if this issue was ever resolved. But if not, that's the biggest hurdle. It might make Warp Drive an impossibility despite being entirely consistent with all other physics.

Edit: Though, GR solutions for closed world lines do exist with positive energies only. And if you can time-travel, you can FTL travel.

mrspeedybob

Does it require absolute negative energy density or can the energy density be negative relative to an arbitrary zero? For example, if I fill a chamber with ambient radiation, then somehow exclude that radiation from certain regions, will the excluded regions function as negative energy regions for the purposes of building an Alcubierre drive that carries somethi across the chamber?

K^2

I believe it would suffice. In fact, it seems that hopes of achieving negative energies in vacuum lie in finding ways of lowering vacuum energy locally. Casimir Effect is a favorite for that. This note in Wikipedia article on the effect comes with some references.

The solutions I've seen for traversible wormholes do, in fact, require negative energy densities same as Alcubierre Drive. So it would seem that the energy just has to be negative relative to the energy of the surrounding medium. Keep in mind that I'm not an expert on GR, however. I can do the math on the level required to show that Alcubierre Drive works in principle, but that's about it.

It's also worth nothing that predictions of vacuum energy based on RQFT and on measurements of cosmological constant disagree by something like 100 orders of magnitude. There is some good recent evidence that [itex]<q\bar{q}>[/itex] condensate should be excluded, but that still only brings down the error to something like 20 orders of magnitude. That suggests that there might be something important we are missing in underlying physics that leads up to Casimir Effect.