alcubierre drive causality

The math explanation and associated formulas I wrote below are correct. Obviously they can’t observe it at Alpha Centauri five minutes later since they are looking at it 4.3 years ago. Can one use a reversible hash algorithm as a compression function. General relativity states that no two objects in the same reference frame can go faster that the speed of light relative to each other - which would otherwise cause violations of causality and time travel. The time that will have elapsed here on Earth will be much, much greater than the 14 days that elapsed on the ship. What exactly is the constraint on spacetime curvature from this argument? The Alcubierre drive, Alcubierre warp drive, or Alcubierre metric (referring to metric tensor) is a speculative idea based on a solution of Einstein's field equations in general relativity as proposed by theoretical physicist Miguel Alcubierre, by which a spacecraft could achieve apparent faster-than-light travel if a configurable energy-density field lower than that of vacuum … site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. 16. As I understand SR, observers in different inertial frames may disagree on the time difference between those two signals, but that the departure will always precede the arrival for all observers. Rev. The Alcubierre drive makes use of a supply of negative energy whose gravitation creates a carefully shaped distortion in the space-time of General Relativity, nicknamed a "warp bubble" because Alcubierre was a Star Trek fan. His idea is now called the Alcubierre Drive. While the distortion was present one could then make the journey between those stars quickly. Author has 23.2K answers and 39M answer views. Close. You can then dress up the calculation with language about "metric engineering" and you'll have a paper as publishable as Alcubierre's. What aspect of GR explicitly allows violations of causality? All you need is to have two of these warp drives going in opposite directions relative to a static observer with intersecting paths. What does cause problems is causal loops, that is, timelike or lightlike worldlines that eventually return to their starting points. http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Everett%20-%20Warp%20drive%20and%20causality%20-%20prd950914.pdf. For instance, travelling faster than light from a single point of perception this is absolutely possible and for those nay-sayers consider this: Person A heads in one direction at 0.75*c and person B travels in the opposite directs at the same speed, neither is breaking the speed of light but taken from a relative perception of person A, person B has to be travelling at a resultant speed of 1.5*c. That much everyone should agree on. In the warp drive case we are not dealing with flat space anymore but curved space hence losing the benefit of inertial frames. Then, the warp ship is just a point moving superluminally against a SR background, and all of the causlity problems derived from there arise again. Why is light different than sound in terms of the assumptions we make regarding causality? The site may not work properly if you don't, If you do not update your browser, we suggest you visit, Press J to jump to the feed. a relativistic rocket rather than an Alcubierre drive (since the Alcubierre drive does require exotic matter, and from what I understand it isn't actually accelerating relative to quasi-inertial observers who are far away in the asymptotically flat spacetime of Alcubierre's solution). General relativity states that no two objects in the same reference frame can go faster that the speed of light relative to each other - which would otherwise cause violations of causality and time travel. Are airfoil profiles patented? Although it requires a little bit of reading, the paper explains how some formulations of the metric do not necessarily result in CTC, but imply that one could do so. where $a$ is the acceleration of the ship and $c$ is the speed of light. I suspect that when dolphus333 said in his "end edit" that a ship would take 817 years to reach Alpha Centauri for observers on Earth, this comment was falsely assuming that the relationships between time and distance for the Alcubierre drive would be identical to those of a relativistic rocket undergoing uniform proper acceleration in SR. A couple paragraphs before the "end edit" dolphus333 had brought up the case of 'uniform acceleration and no exotic matter whatsoever', i.e. No? So, the line element equation above describes a globally hyperbolic system in space time. Its lifespan can be tuned to make it predictably desintegrate at the intended destination. In other words $\alpha$ and $\beta$ describe how your ship moves through space and time per incremental step. causality might kick in in chronology-protection a la Hawking to destabilise the vacuum fields before the CTC is formed. Warp drives were a fairly vague idea until in 1994, Miguel Alcubierre found a way to make them work in General Relativity. which can be used to show that when the ship reaches Alpha Centauri, 817 years will have passed here on Earth. @innisfree: Spacetimes with closed causal curves are forbidden. "A good many physicists believe that this paradox can only be resolved by the general theory of relativity. Thats why I said locally the drive is travelling with $v < c$ which implies that something like causality is preserved locally as well. EDIT: I'm not saying I expect negative mass to be a real thing. The argument against the Alcubierre drive cannot stem from special relativity as it would only apply if there were a violation of local causality. There are all sorts of observers, not on the spacecraft or anywhere near it, who would see the events as violating causality if the craft were FTL. We verify this conjecture by … While warp drives can be used to violate causality they do not do so as a matter of course. It is not exactly certain if this means they must be in principle invalid. Should I process the data or add a new constraint to achieve the target? Local causality though is preserved. It. An interesting musing is the possible side effect as experienced in sound where a plane travels at Mach1 we get sonic booms cuased by compression of sound waves at the same speed of thier creation, would a photonic boom also happen in light? We verify this conjecture by … I'm talking about this nonsense: http://www.washingtonpost.com/news/post-nation/wp/2014/06/11/this-is-the-amazing-design-for-nasas-star-trek-style-space-ship-the-ixs-enterprise/. But in this case my question is different and more specific. An actual Alcubierre drive wouldn’t either, I suspect, since its hypothetical possibility is derived within General Relativity, which does not “break causality”. SR says that the simultaneity and relative ordering of events is dependent on the relative motion of the observer, but that no matter how it works out, if event A causes event B, no observer will see B first. Alcubierre sets out to find his warp drive metric using a 3+1 formulation of spacetime. Looks like you're using new Reddit on an old browser. How Alcubierre drives[1] move is different: they move in a seperate reference frame of space, while the space is contracting and expanding around it - the drive is actually stationary relative to that occupied space. curves using a similar idea to the one presented here.". Although it is worth noting that historically, nonsense which is mathematically consistent has, on occasion, not been nonsense. Alcubierre recently exhibited a spacetime which, within the framework of general relativity, allows travel at superluminal speeds if matter with a negative energy density can exist, and conjectured that it should be possible to use similar techniques to construct a theory containing closed causal loops and, thus, travel backwards in time. There is a later episode (or maybe a couple of them) that go into how interactions with the Higgs field make it so things with mass travel slowed than the speed of causality. Although time travel would be exciting, there is just never going to be anything that violates special relativity. It will violate causality globally. Well, the Alcubierre drive does not include closed causal curves. u/Uo42w34qY14. I'm saying we should not so eagerly dismiss it. Can you solve this creative chess problem? That's really it. However, in the previous slide he brings up the possibility that Hawking's chronology protection conjecture might be true in quantum gravity, so that solutions in general relativity involving closed timelike curves could be destroyed by quantum effects, without ruling out the possibility of constructing FTL solutions that don't involve CTCs. Granted they’re extremely theoretical, but if we could build one and travel to Alpha Centauri in five minutes by the ship’s clock, what would an observer on earth see. observers on Alpha Centauri wouldn't see the bubble reach them before a light ray sent from Earth at the same time, and which traveled outside the bubble), and thus that causality issues associated with FTL can be avoided, but in fact the Alcubierre bubble definitely does travel faster than light as seen by external observers. Were SVMs developed as a method of efficiently training neural networks? 119 2082-2089 (1960). Alcubierre recently exhibited a spacetime which, within the framework of general relativity, allows travel at superluminal speeds if matter with a negative energy density can exist, and conjectured that it should be possible to use similar techniques to construct a theory containing closed causal loops and, thus, travel backwards in time. For example, we could appeal to the chronology protection conjecture, which, though very speculative, is better motivated and more likely to be correct than anything in Alcubierre's paper. Every cent of NASA money given to his research has been nothing but a way of defrauding US taxpayers. As PhotonicBoom says, local causality is preserved, fwiw. Can anyone give me an instance of 3SAT with exactly one solution? "Person A heads in one direction at 0.75*c and person B travels in the opposite directs at the same speed, neither is breaking the speed of light but taken from a relative perception of person A, person B has to be travelling at a resultant speed of 1.5*c." - No, velocities don't add the same way in relativity that they do in classical physics--see. The cosmic expansion is one; gravitational waves are another. Dolphus333 mentioned in his answer that Alcubierre demonstrated in his paper that there is no violation of causality in the spacetime he constructed, but I believe this is just for the case of a spacetime containing a single warp bubble--if you have multiple warp bubbles moving in different directions you can indeed violate causality in a manner similar to the tachyonic antitelephone. How do you add velocities in special relativity? Apart from this error, dolphus333 seems to just assume without argument that these equations can be generalized to the case of an Alcubierre bubble, since the "end edit" first calculates the time for a relativistic rocket with an acceleration of 188G, and then goes on to say "You might point out that 188 g's would surely smush everyone against the back wall of the ship, but the beauty of the theoretical drive described is that you carry your own gravity well along with you and therefore, you're always in freefall and don't feel the acceleration." How could we travel to the nearest supermassive Black hole? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Now if person A and B look at their watches it would show the same time and date as they have both experienced the relative distortion through acceleration from the central point, hopefully, you're still with me. In physics, it is what is known as the Alcubierre Warp Drive. (I'm not a GR expert.). The expression for the time elapsed on Earth is, $$\mathrm{Earth\ time\ elapsed}= \dfrac{c}{a}\cosh\left(\dfrac{at}{c}\right),$$. Nevertheless, this does open up some interesting possibilities for futuristic space travel. Hence we need to use GR instead. The “Alcubierre Drive” (by which a spacecraft would, in fact, “surf” a locally warped segment of space-time), Robert L. Schroeder’s engineerable micro black holes hypothesis in combination with T.Towsend Brown’s “electrogravitics” (which in some cases may function in a hard vacuum), Konstantin Meyl’s superluminal scalar-longitudinal waves and Roger Shawyer’s “EM Drive … It utilizes the fact that space-time can be stretched to bypass the speed limit of the universe. [3]. ", What the article failed to mention is that the 14 days quoted is in the reference frame of the ship. So is the conclusion of all this that "exotic matter with negative mass* cannot exist, because if it did, we could use it to build an Alcubierre Drive and violate causality" or "we don't know if causality is an inviolable law, it depends on whether we find any exotic matter"? However, this does not Remember, hyperbolas look like $\dfrac{x^2}{a^2} - \dfrac{t^2}{b^2} = 1$. Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. I haven't had time to digest the material enough to say why causality isn't violated except with the very unsatisfying statement, "Well, the math works out." Rindler, W., "Hyperbolic Motion in Curved Space Time", Phys. Just my 2 cents here. This is stated specifically in the book Time Travel and Warp Drives by Allen Everett (the same physicist I mentioned above) and another physicist, Thomas Roman. to 1 light-minute, and this need not take very long. The Alcubierre drive or Alcubierre warp drive (or Alcubierre metric, referring to metric tensor) is a speculative idea based on a solution of Einstein's field equations in general relativity as proposed by Mexican theoretical physicist Miguel Alcubierre, by which a spacecraft could achieve apparent faster-than-light travel if a configurable energy-density field lower than that of … ), Edit regarding 3+1 spacetimes and causality. However, I was under the impression that any faster than light communication could result in a causality violation. if any information is sent faster than light, there will exist an inertial frame in which the signal arrives before it is sent, a blatant violation of causality. If the ship is FTL in proper time, are there not inertial frames (far from the ship) that would see the arrival as occurring before the ship's departure, clearly violating global causality? example, could change the proper separation between two stars from 1 light-year My argument still stands. Alcubierre drive: | | ||| | Two-dimensional visualization of the Alcubierre drive,... World Heritage Encyclopedia, the aggregation of the largest online encyclopedias available, and the most definitive collection ever assembled. It does. Re: Alcubierre drive « Reply #116 on: 10/15/2019 03:21 pm » In a universe without exotic matter, theoretical work shows that even traversable wormholes are probably restricted to speeds below the speed of light. Short story about survivors on Earth after the atmosphere has frozen, Recursion: Salamin and Brent equation for finding pi. In any discussion of Alcubierre's warp drive, it's important to understand that there are no rules. So strictly speaking, causality can still hold over a specific foliation. Why do I get a 'food burn' alert every time I use my pressure cooker? Now consider this in terms of causality because again this is all about perception. It only takes a minute to sign up. Outside of the warp drive region, the background geometry still applies. Alcubierre isn't done yet, he's still got to find a metric that will fit in a 3+1 spacetime and do what he wants, (provide faster than light propulsion), but if he does, the above property of 3+1 spacetimes will guarantee causality. Two Aldubierre drives could be used to create a Closed Timelike Curve - which would violate causality. I looked into this a little bit more and this is what I have gathered. Can we prove absolutely that FTL = causality violation. The paper is amazingly well written and folks that have had a grad level general relativity class should be able to easily traipse through it. The result is that from the global perspective there is no violation of causality: it is a planned trip and the preparations take time. How does faster than light travel violate causality? A large enough gravitational wave, for Interesting fact: if I did the math correctly. So why are Alcubierre drives theoretically OK? So how is any scheme to travel faster than light, even if it may locally satisfy the equations of GR, not a blatant violation causality according to SR? Is interstellar flight possible in near future in a way that would keep our civilization alive? Alcubierre's original geometry has no causal loops. In principle, it could break down in such a way, or have coordinate singularities in places that block out "bad" regions. Ask a science question, get a science answer. What's important here is that $ds^2$ is positive and for real space, $\gamma_{ij}$ is as well. mean one can zoom around the universe at the press of a button. Opt-in alpha test for a new Stacks editor, Visual design changes to the review queues. (*: and whatever other properties are needed). where $\alpha$ is the lapse function, and is positive, and $\beta$ is the shift vector between spatial foliations. Conceptually, we demonstrate that any warp drive, including the Alcubierre drive, is a shell of regular or exotic How does “warp drive” not violate Special Relativity causality constraints? (I think we can all agree, can we not, that the threshold should be extremely high before breaking assumptions that we're in a causal universe?). In the 3+1 formulation, spacetime is described as a set of constant coordinate time spacelike hypersurfaces, (foliations, for the fancy). Together, this is enough to rule it out with close to 100% certainty as a serious possibility. The article refers to an Alcubierre drive, so the ship would move at superluminal speeds in the inertial frame of an external observer. The Alcubierre drive is one of a more general class of spacetimes (Natario 2001), and the metric describing how space curves around it is defined partly by an arbitrary choice of a function $x_s(t)$, from which we get the speed $v_s$ by differentiating: $$v_s=\frac{dx_s(t)}{dt}$$ While there are constraints on certain other parameters of the metric … http://arxiv.org/abs/gr-qc/0009013v1. Furthermore, the equation for "distance" as a function of acceleration and proper time that dolphus333 gives in the "end edit" is identical to the equation for "d" given on the relativistic rocket page here, and the equation for "Earth time elapsed" that dolphus333 gives is almost identical to the equation for t on that page, except that dolphus333 writes cosh(at/c) where the corresponding relativistic rocket equation has a factor of sinh(at/c)--probably that dolphus333 either misremembered the equation, or made a transcription error when copying it from some other source (if you use the correct equations, then if a ship travels with a uniform acceleration of 188G to Alpha Centauri 4.3 light years away in the Earth's frame, the proper time on the ship would be 14 days just as dolphus333 calculated, but the time in the Earth frame would be 4.32 years, not 817 years). @PhotonicBoom : Yes, but what of an observer in an inertial frame that is far enough away from the spacecraft that the observer is in essentially flat spacetime. Using this formula, it can be shown that at an acceleration of 188g, (188 times the acceleration due to gravity), the ship could reach Alpha Centauri in 14 days of ship time. [10] Everett, Allen E., “Warp drive and causality,” 15 Ju ne, 1996 The American Physical Society, Phys. Let's say that Earth sends a radio signal (in all directions) when the ship leaves, and Alpha Centauri sends a signal when the ship arrives there. Causality hasn't been evaded as he did not arrive earlier than he set off he travelled for a year and it would show on person B's watch, in this system. What is the time-travel implications of Alcubierre drive? Rev. This is the best argument about why building these things is impossible. It unquestionably violates causality, and that's why Harold White is a charlatan. (Pending a serious replicable experiment showing causality to be breakable. in the section on the Alcubierre solution on p. 117, the book says: The bubble and its contents could travel through spacetime at a speed faster than light, as seen by observers outside the bubble. I'll keep adding to the answer as I get more information, and hopefully everything will just evolve along.

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