# Would a Alcubierre drive produce a gravity wave wake?

1. Feb 17, 2016

### newjerseyrunner

Discussion of gravity waves got me thinking about objects that seriously distort spacetime and I started wondering about the Alcubierre drive idea. Let's make the assumption that negative energy density is in fact possible and another species has figured out how to do it. What outside-observable effects would it produce as it moved?

2. Feb 17, 2016

### CalcNerd

None. It is a Sci-fy FTL travel device that functions perfectly in Star Trek.

3. Feb 17, 2016

### newjerseyrunner

How is that possible? Could you explain where all of that energy goes, or does the expansion and contraction on either side completely cancel out?

Also, please leave Star Trek out of this, I don't want talk about fictional drives, I'm not even really interested in the drive itself, I'm asking about Alcubierre's solutions to Einstein's equations, which is not sci-fi, but valid physics. Please keep this purely mathematical.

4. Feb 18, 2016

### CalcNerd

The Wiki page on the Alcubierre drive delves into the physics and mathematics far better than I can. Someone took the time to create a Wiki page, have you looked that over. Not that Wiki is an authoritive source, but it is a start.

5. Feb 18, 2016

### PAllen

Alclubierre 'warp drive' is one of a whole class of similar solutions in GR where entering and leaving a 'warp bubble' you can travel FTL relative to paths through nearby approximately flat spacetime not going through the bubble (or tube, or presumably other topologies). As is the case in SR/GR, if FTL is allowed, time travel is allowed - by combining two warp drives you can created a closed timelike curve and travel to your own past. There is nothing controversial about any of this in GR except whether it has any relation to our universe. These solutions entail large amounts of negative energy, manipulated in specific ways; there is no evidence this possibility is possible in principle in our universe.

Thus, the OP is a completely legitimate GR physics (or mathematical physics if you believe the negative energy requirements rule it out of being physics) question. Unfortunately, I have no idea of the answer or else I would have responded earlier. I made a couple of brief searches to see if this question had been investigated, but found nothing. A heuristic guess is that it would not produce GW in isolation, but as it is used by some rocket to travel from one star to another, there would be a small amount of GW emitted by the departure process and the arrival process. Do not attach much weight to this guess.

6. Feb 18, 2016

### Ontophobe

I don't know, but consider this reasoning: Michelson and Morley failed to detect any "ether drag." This didn't make sense to them until Einstein proposed that the speed of light is constant for all inertial reference frames. We believe that g-waves travel at c also. Given that the Earth doesn't experience a drag, I assume the ship wouldn't create a wake behind it either. But the Earth isn't traveling FTL inside a space-time bubble, so maybe that changes things

7. Feb 18, 2016

### newjerseyrunner

The earth does produce a wake, all objects with mass do, that's why orbiting objects create waves. The earth is simply tiny and slow moving, so it's gravity wake is unmeasurable. The amplitude of the gravity wave of the Earth-Sun orbit would be 1 part in 10^25.

Of course the ship itself would produce a gravity wake (assuming it's got mass) but I'm more interesting in side effects from the extreme warping of space in front and behind the bubble.

8. Feb 18, 2016

### PAllen

No, the earth does NOT produce a wake. The co-orbiting system produces GW that do not originate from either the earth or the sun, and the distantly observed point of origination would not be to exactly the earth or the sun. If the earth were isolated in the universe, it would produce no GW. You can even have an accelerating rocket and produce exactly no GW if the thrust is such as not to produce change in quadropole moment of the system. The Kinnersley photon rocket is an exact solution of an accelerating rocket that produces no GW because the anisotropy is exclusively dipolar.

See: http://arxiv.org/abs/gr-qc/9412063

So my guess, without finding any references on this, or doing any computations, remains that a warp buble with contained rocket operating in an empty universe (or far enough away from everything else) produces no GW.

9. Feb 18, 2016

### PAllen

The earth does feel a slight drag from the emission of GW by the earth/sun system. All orbits of massive bodies in GR are, in principle, unstable due to GW.

10. Feb 18, 2016

### Staff: Mentor

At least for the start and destination, I would be very surprised if you could generate volumes with the necessary spacetime warping without inducing any side-effects.
No idea about the theoretical journey in between. An experimental realization (if possible at all) won't be perfect, so I would expect gravitational waves there as well.

11. Feb 18, 2016

### Staff: Mentor

And what happens to all the stuff during the journey? There is no real vacuum so all the protons and what else is there might cause major trouble.

12. Feb 18, 2016

### Staff: Mentor

Show me an Alcubierre drive with all the details worked out so it is possible to build one and I'll tell you.

13. Feb 19, 2016

### Ontophobe

Interesting. Thanks! :)