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Hypothetical effects of the hypothetical Alcubierre drive


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  gpisic said:
Another question: Is it possible to leave the galaxy with an AD ? SciFi literature mentions something like a galactic barrier, how much fiction is in this statement?

As already said. In Galactic terms, light is incredibly slow. You'll need to go much FTL to get anywhere.

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Another question:

With a sufficiently high FTL factor, could a backward time travel be possible into a time that is soon after the big bang 13 billion years ago?

The farest human looked into the sky is the "Hubble Ultra-Deep Field" image. If we had a powerfull enough AD drive to get us at this farest galaxies observed which we believe the light travelled 13.2 billion years to us in an relatively short time,

what would the AD ship observe there? An patch of galaxies with the same young age we are observing them or galaxies that are older?

Edit: Maybe we could observe better from there what happened immediately after the big bang (some 450 million years after)?

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A single AD ship in the mostly flat space-time does not make a time machine. In fact, AD by itself doesn't allow for time travel without two bubbles passing through each other, which is all sorts of trouble. If you travel at FTL speeds, the earliest you can get somewhere is right now.

  Quote
which we believe the light travelled 13.2 billion years to us in an relatively short time

It did not. It's actually 13-something billion years old. The galaxies we see are now 90-something billion light years away, because universe expands faster than light. But light still travels at light speed.

On the other hand, AD does allow to traverse otherwise untraversable wormholes and pass bellow event horizon of black holes. That opens up a whole list of time travel opportunities. The most obvious is in the Kerr metric, corresponding to a rotating black hole. It contains closed curves which allow for time travel. Unfortunately, Kerr solutions are suspect bellow event horizon. The exact solution for the interior is not known. So this isn't a guarantee. But arrangements that allow for time travel using an Alcubierre Drive do exist. So it's a theoretical possibility. If we build FTL Warp Drive, time travel would be something we'd have to eventually accept.

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As far as I've seen in "theoretically this works!" related publishing, every theoretically possible form of (backward) time travel is limited to the times after the invention of the time machine- in layman's terms, you need a "recever" to catch the time traveler and return it to foreward time- without a bucket to catch you in, you'll keep going back in time forever, unable to turn around and return to "Normal" time.

Astronomical phenomimna is the best bet in that regard, because the universe wou;d have "invented" them eons ago.

Edited by Rakaydos
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Stephen Hawking theorizes that it may very well be possible to build a time machine but to preserve causality, every time it is operated, 'something' will happen to keep it from working- it will explode, you will have a heart attack as you reach for the switch- it will always malfunction.

On a side note I have built several Time Machines... and I'll sell them to you cheap!

Edit-Not to open a whole new can of worms or anything but..

http://www.universetoday.com/112822/has-the-cosmology-standard-model-become-a-rube-goldberg-device/

rube-goldberg_and_StandardModel.jpg

Good god. I'll ask in another place how to size that down.

Edited by Aethon
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Modern field theory has no problems with time travel. Only local causality is critical. Global causality can be violated left and right.

And standard model is symmetry based. There are only a handful of "real" fields, corresponding to the fermions. All of the boson fields are gauge fields, and they are consequences of fundamental symmetries. Nothing we can do about it. These symmetries are really there.

And yes, it's getting cluttered. The full symmetry group we are dealing with is: R1,3 ⋊ SO+(1,3) × U(1) × SUL(2) × SU(3) × SU(2). The R1,3 ⋊ SO+(1,3) part of it (Poincare group) corresponds to symmetries under translation, rotation, and boosts. In other words, movement in our 3+1 space. It's also the source of gravitational force. The rest correspond to internal degrees of freedom of particle fields, and are responsible for remainder of the forces.

It's messy, but nowhere near as bad as some people seem to imply. And while there may be some structure to these symmetries, for the time being, that's all we've found.

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  K^2 said:
Modern field theory has no problems with time travel. Only local causality is critical. Global causality can be violated left and right.

Interesting, can you point me to some material? All the prescriptions I am aware of require a time-ordered slicing into spacelike hypersurfaces before you can apply QFT. And if you ignore that and apply path integral formalism naively to a spacetime with closed timelike loops, you get nasty infinities from all the particles that can inhabit those loops. They are somewhat similar to the infinities from virtual particle pairs, and in some special geometries, you can get rid of them in similar ways (first google result, not read yet), but AFAIK not in the most general case.

Edit: Single particles without interaction would be fine, that much I know. As long as local time development is unitary, global time development can be unitary too (I'm talking about the case where the closed timelike loop only exists temporarily and there are spacelike hypersurfaces in its past and future you can use to even define global time development on); simply put, your particle can traverse the loop an arbitrary number of times and you sum up the probability amplitudes. But that no longer works once interactions are allowed.

Edited by Z-Man
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  K^2 said:
Only local causality is critical. Global causality can be violated left and right.

I have no problem with the term "causality" however i do not understand what is the difference between local and global causality.

Maybe someone could enlighten me?

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  Z-Man said:
Interesting, can you point me to some material? All the prescriptions I am aware of require a time-ordered slicing into spacelike hypersurfaces before you can apply QFT. And if you ignore that and apply path integral formalism naively to a spacetime with closed timelike loops, you get nasty infinities from all the particles that can inhabit those loops. They are somewhat similar to the infinities from virtual particle pairs, and in some special geometries, you can get rid of them in similar ways (first google result, not read yet), but AFAIK not in the most general case.

Can I generalize your statements to "Quantum theory of Gravity is non-renormalizable?" And yeah, it's a problem within the theory, but only at the level of us trying to make use of it. It's not that QFT requires nicely ordered spaces. It's that we can only apply QFT as a theory to such spaces. Breakdown of theory, in this case, is not an indication of a physical problem.

Yet, there are ways to deal with it. The most direct thing is not to mix gravity and particle field theory. Solve for space-time geometry, and then try to do QFT in that curved space-time. It's fairly straight forward, and you can still end up with problems that you may or may not be able to solve in that specific geometry. That's essentially what you are talking about.

A more general approach is to treat gravity as Yang-Mills Theory on Poincare group. Then you essentially have to do Quantum Gravity. As I've said above, it's not a renormalizable theory, but you can construct an effective theory which is valid up to a certain scale. Usually, Plank scale, because bellow Plank scale you're in trouble either way. So long as your space-time is "flat" on Plank scale, your theory then holds on the larger scale. There is a good paper on Arxiv that goes into more detail.

Note that this approach might not be applicable in practice to a general problem you are trying to solve. The main point here is that physics doesn't actually break there. We just end up with insane math we can't do.

P.S. By the way, general path integral formalism doesn't require time ordering. It's one of these crutches we rely on to make math easier. There are a number of theorems and equations in QFT that are derived without considering time ordering. Dyson-Schwinger Equations are an example. (They are often derived in texts with help of time ordering, but they can be derived making no such assumptions.)

  gpisic said:
I have no problem with the term "causality" however i do not understand what is the difference between local and global causality.

Maybe someone could enlighten me?

In General Relativity, a lot of limitations applied by Special Relativity globally are actually only enforced locally. It means that around any point in space-time, you can find a region of space-time for which the constraint holds. It can be a very small region, however. For example, speed of light is a global limit in Special Relativity. Nothing can go faster than light. In General Relativity, there is no such global limit. Hence the Alcubierre Drive, and the fact that Universe expands faster than light. However, in any sufficiently close proximity, two particles will never travel faster than light relative to each other. In most of the universe, that "neighborhood" spans many galaxies, but if you either stumble on some extremely curved space, or you create your own warped space with a warp drive, you can, in principle, break the light barrier.

Causality in Special Relativity is a very strong statement. It says that if event A causes event B, then in every frame of reference B follows A. In general, sequence of events can change if you travel close to speed of light, but only casually unrelated events can actually swap places. So, for example, if I flip a switch and, just by chance, somewhere on the other side of the planet a light bulb went on at the exact same time, then depending on observer, one or the other might have appeared to happen first. But if me flipping the switch was the cause of the light coming on, then from every observer's perspective, switch was flipped first, and light went on second. That's causality.

In General Relativity, such constraint is also local. So for typical space-time geometries, on the scale of individual particles you can still do time ordering. And that time ordering is frame-independent, which is very important for the field theory. But then you get to something like event horizon of a black hole, and our theory breaks down all together. On the other hand, large scale events no longer have to follow in sequence. You can go back into the past and kill your grandfather. That doesn't cause any grand cosmic paradoxes or problems with "continuity". The paradox is resolved on the quantum level, and while there are different ways to describe resolution, it's most obvious in Many World Interpretation, where you essentially end up with alternate timelines.

Both of these are relevant to Alcubierre Drive, because, in principle, you can use AD as a time machine if you have suitably curved space-time. The second part is that space-time at the bubble boundary is extremely curved. And it wasn't clear for the long time what would happen at said boundary, and if it would prevent AD from being even a theoretical possibility. Now we know that so long as bubble thickness is greater than plank scale, there aren't any fundamental problems with it. For thinner bubbles, the theory isn't clear, but odds are, we'll run into problems.

Edited by K^2
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  K^2 said:
Can I generalize your statements to "Quantum theory of Gravity is non-renormalizable?"
  K^2 said:
P.S. By the way, general path integral formalism doesn't require time ordering. It's one of these crutches we rely on to make math easier. There are a number of theorems and equations in QFT that are derived without considering time ordering. Dyson-Schwinger Equations are an example. (They are often derived in texts with help of time ordering, but they can be derived making no such assumptions.)
Yep. And if you apply path integral formalism to calculate the S-matrix from one spacelike slice to another with a noncausal region with closed timelike loops in between, your result is not unitary (unless your fields are free and non-interacting). And not just in the black hole paradox kind of way where quantum information is lost, but in the sense that probability is not conserved. That is a bit of a problem and none of the suggested resolutions I am aware of are particularly satisfying. Specifics would be for another thread, I sidetracked this one enough.

I found some good places to start in the meantime, for example this old thing by Kip Thorne. Walking the quotation graph should get me up to speed. The last four references are about the loss of unitarity.

No, not really. Renormalization deals with local problems on small timescales and lengthscales, and as you correctly say, non-renormalizability is not a problem for an effective field theory. So what if you need infinite parameters to reconcile theory and experiment as long as finite parameters suffice to reconcile theory and all experiments you are interested in to good enough accuracy? The problems with closed timelike loops, on the other hand, are nonloclal problems on the lenght and timescales of the loops in your chosen metric. And yeah, I'm thinking about QFT on a fixed background metric, possibly including gravitation on the perturbative level, but that is not a requirement.
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Long range problems seem veery counter-intuitive. You can end up with CTCs once you take Poincare to be a local symmetry. And that's the only change we make going from QFT in Minkowski to Quantum Gravity. So it has to root itself in non-renormalizability somehow. And as you say, that shouldn't have any but the short-range effects. I'll read up on it, but it seems very strange. Thanks for bringing it to my attention, though.

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Sorry if this has been addressed already (I haven't had time to read the whole thread) but does anyone know how the energy requirements for the warp bubble are calculated exactly?

Like... originally Alcubierre said he would need a Jupiter-mass equivelance in energy. Then Doctor White said he could do it using 800 kg or so in mass-energy. But they never give more details on what kind of "warp trip" these energy levels would enable. Are we talking about a short hop to Pluto at 105% speed of light, or a trip to Alpha Centauri at 10x the speed of light?

Also how big a warp bubble are we talking? Would it be able to hold a ship 100 meters long? What if we want to scale it up, how much more energy do we need if we say... double the volume of the bubble?

Edited by PTNLemay
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  PTNLemay said:
Sorry if this has been addressed already (I haven't had time to read the whole thread) but does anyone know how the energy requirements for the warp bubble are calculated exactly?

Like... originally Alcubierre said he would need a Jupiter-mass equivelance in energy. Then Doctor White said he could do it using 800 kg or so in mass-energy. But they never give more details on what kind of "warp trip" these energy levels would enable. Are we talking about a short hop to Pluto at 105% speed of light, or a trip to Alpha Centauri at 10x the speed of light?

Also how big a warp bubble are we talking? Would it be able to hold a ship 100 meters long? What if we want to scale it up, how much more energy do we need if we say... double the volume of the bubble?

The duration of the trip isn't a factor. Only the bubble geometry. In practice, you'll probably end up with losses you'll have to replenish, thereby creating an energy drain that's proportional to the time you need the bubble up, but these computations don't take it into consideration. It's just the amount of energy you need up front.

I believe, Dr. White's computations were for 10c and a fairly small probe. Maybe a few meters in diameter. I don't know the exact geometry used for initial Alcubierre Drive estimate, but it doesn't matter all that much, given the scale of the result. In Alcubierre Drive, energy requirement scales as square of bubble's velocity. With other geometries, it could be a little different. On the other hand, Alcubierre's Drive is pretty insensitive to the bubble size. In other words, you don't need that much more total energy to make the bubble larger. I don't know how true it is for the Dr. White's drive.

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If an alcubierre drive bends space that means it won't affect the velocity of your ship only it's place... How you showed it in your "diagram" the driver would be screwed because he would be transported really far away from the sun. He would have his orbital path changed because it had gotten higher that means his horizontal speed would be too low for his hieght and here would fall but he would fall like a plane and come down slowly and strafe down.

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  Everten P. said:
If an alcubierre drive bends space that means it won't affect the velocity of your ship only it's place...

The exterior curvature is going to affect the ship's velocity, still. In a nutshell, it's because there is no such thing as "same velocity" in two different places on curved manifold. It's a meaningless statement. There is, however, parallel transport, and that's what will define ship's velocity after exiting warp.

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  • 2 months later...

I've been trying to research and wrap my mind around the matter of how things would be for a manned spacecraft employing an Albubierre drive. From my understanding, massless particles like photons would not appear any stranger than usual to the crew. They would not be overly redshifted, as is the case when we observe distant galaxies moving away at high relative speeds. That is because the spacecraft is in flat spacetime, and whatever path light managed to pass through the AD wavefront, it appears as it came in. Matter, however, will not. The "tidal" forces of the ship's FTL forward edge will rip matter to doll rags, as it were, just as if those atoms and molecules reached the event horizon of a black hole, or hypothetically were at the the end of the Big Rip in cosmology. Thus, the ship would receive the scattering of radiation from matter's dismemberment. If the AB drive is producing, say, 10c of spacetime warp, I'm guessing that all matter in front is converted to energy. Which is why one should avoid driving through anything more substantial than diffuse space.

I wonder what happens at the Planck Scale of vacuum energy when your AD slams it?

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