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You can't have faster than light travel without violations of causality. And while the equations of General Relatively allow for universes where causality is ro

You can't have faster than light travel without violations of causality. And while the equations of General Relatively allow for universes where causality is routinely violated, the probability that we are in such a universe seems vanishingly small. E.g., just because you can violate causality does not mean that you can go back in time and kill your grandfather. If you were to try, something would stop you, as a universe in which you kill your own grandfather is not consistent with General Relativity. A universe that gives you the means to go back in time and try to kill your grandfather, but then somehow always thwarts you, would be a very strange universe indeed. Edit: To whoever down-voted me, have you taken any classes on General Relativity? I have. In any case this is from Wikipedia: Causality is not required by special or general relativity[citation needed], but is nonetheless generally considered a basic property of the universe that cannot be sensibly dispensed with. Because of this, most physicists[who?] expect that quantum gravity effects will preclude this option.[citation needed] An alternative is to conjecture that, while time travel is possible, it never leads to paradoxes; this is the Novikov self-consistency principle. The "Novikov self-consistency principle" is the the one that states that you could try to create a time paradox, but that you would never be able to succeed. I.e., the universe would be a very strange place!

stryker 14y ago
I feel like a lot of people here misunderstand the speed of light as a cosmic speed limit. The speed of light as a speed limit is a local constraint. On a larger scale there is no such speed limit. Just because you travel faster than light on a non-local scale does not mean you break causality. For example, light "travels" more slowly through water than vacuum. Well, actually, photons travel at c always -- it's just that light gets bounced around by water molecules, getting absorbed and reabsorbed many many times before arriving at the destination. It's like driving to a destination as opposed to running to it. If the roads are curvy enough, the runner will win because he is not constrained by roads. Could it be that the Alcubierre drive gets past the roads of empty space in a way light cannot? For a better example, consider the expansion of the universe. Link: http://curious.astro.cornell.edu/question.php?number=575 http://curious.astro.cornell.edu/question.php?number=575. I quote: The fact that galaxies we see now are moving away from us faster than the speed of light has some bleak consequences, however. Astronomers now have strong evidence that we live in an "accelerating universe," which means that the speed of each individual galaxy with respect to us will increase as time goes on. If we assume that this acceleration continues indefinitely, then galaxies which are currently moving away from us faster than the speed of light will always be moving away from us faster than the speed of light and will eventually reach a point where the space between us and them is stretching so rapidly that any light they emit after that point will never be able to reach us. In other words, vacuum fills in between us and far away galaxies faster than light can traverse it. Again, locally, the derivative of position with respect to time is c, but the universe is able to "cheat" by messing around with the definition of position. If the Alcubierre drive messes with the fabric of the universe in a similar way, I don't see any reason why it cannot succeed. Here's a related link: http://curious.astro.cornell.edu/question.php?number=56 http://curious.astro.cornell.edu/question.php?number=56. I quote: Technically speaking, the speed of light limit only applies when you are in an "inertial frame" -- that is, sitting where you are, without any forces acting on you, and measuring the speed of an object that moves past a ruler and clock that you are holding in your hand. Across the large distances in the universe, however, we have a very different set of circumstances. No one is in an inertial frame, because everyone is being accelerated with respect to everyone else, due to the universe's gravitational field and the fact that the universe is expanding. In effect, the universe's expansion isn't really due to galaxies moving "through space" away from each other, but rather due to the stretching of space itself, which isn't governed by the same limits that we are. I'm not sure whether the Alcubierre drive contradicts causality, but if it does, I highly doubt that it is because of the arguments presented on this forum.
- nessus42 14y ago
  I'm not sure whether the Alcubierre drive contradicts causality, but if it does, I highly doubt that it is because of the arguments presented on this forum. The Alcubierre drive most certainly does violate causality, and this follows from just the most basic amount of Special Relativity that you might learn if only you'd spend your time learning real things, rather than spreading disinformation.
  - stryker 14y ago
    I'm going to ignore your condescension. I'm also going to assume you didn't read the rest of my post, because you presented no arguments to counter my points. I'd like to point out that the wikipedia article on the Alcubierre drive writes at length to show how this warping process does not violate causality: http://en.wikipedia.org/wiki/Alcubierre_drive http://en.wikipedia.org/wiki/Alcubierre_drive. I know you feel strongly about people not understanding special relativity and I actually agree with you on that point, but you're not addressing any of the valid counterpoints that people are making. You're simply closing your ears and exclaiming "special relativity" which only applies locally (if you want an appeal to authority, I talked about this with a former physicist who worked with Stephen Hawking). 1) The universe is expanding faster than the speed of light. In other words, galaxies are moving away from us faster than c (evidenced by the growing redshift of light emitted by galaxies when they were NOT traveling away from us faster than c). That does not violate causality (as I mentioned in my post) because the topology of space itself is changing. Locally, everything still obeys special relativity. 2) Virtual particles, EPR Paradox (I know it's not actually a paradox). (http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html http://math.ucr.edu/home/baez/physics/Quantum/virtual_partic...) My point with these phenomena is that locality and the speed of light are not very intuitive concepts. You can still have "spooky action at a distance" without violating causality.
    - nessus42 14y ago
      You're simply closing your ears and exclaiming "special relativity" which only applies locally (if you want an appeal to authority, I talked about this with a former physicist who worked with Stephen Hawking). If you come up with a mathematical "proof" and I can use its conclusion to prove that 1 + 1 = 3, then I have shown your proof to be incorrect via contradiction, no matter how hard you continue to argue for it. Special Relativity does not only apply locally. It applies to any system where information can be transmitted between two points that are in inertial frames. The universe expanding at faster than the speed of light is neither here nor there, since you cannot use the expanding universe to transmit information between two points in at FTL speeds. On the other hand, you CAN use an FTL drive to transmit information between two points in inertial frames at FTL speeds. QM spooky action at a distance does not let you transmit information between two points at FTL speeds. This is all you need to do to show that any FTL scheme violates Special Relativity: Information transmitted between two points in inertial frames at FTL speeds. If you can do that, you've either disproven causality, or you've disproven Special Relativity.
geuis 14y ago
We aren't talking about time travel. So many people confuse the idea of FTL with time travel. General Relativity talks about frames of reference. Suppose you had a base on Mars and sent a radio signal to it from Earth. It would take between 4.3 to 21 minutes to reach Mars, depending on its distance from Earth at the time. If you simultaneously jumped in your 10x warp drive and set out, you would arrive at Mars in 25.8 seconds to 1min 26 seconds, again depending on distance. In this scenario, there has been no violation of causality. You aren't arriving before you sent your radio signal. All that is happening is that you're arriving before your radio signal does. From the perspective of a telescope or radio antenna at the Mars base, you would view Earth in its state 4.3 to 21 minutes before, because it simply takes that long for the electromagnetic waves to get to you. So yes, you can travel faster than the speed of light without violating causality. If you travel in a relativistic way, i.e. speed all the way up to 99.99999% of light speed, you will most certainly run into time slowing down in your local frame, while it continues at a normal frame back at your departure point. But with a warp drive that works as described in the article, there is no changing of reference frames. Since you're warping space itself, the spacecraft isn't actually undergoing acceleration in the relativistic way of a rocket. For passengers on board, the clocks are going to run at the same rates as back home.
- politician 14y ago
  A pair of FTL communication devices (jargon: ansible) can be arranged in a configuration which would allow the future to speak to the past, violating causality.
- nessus42 14y ago
  You are not correct. If anything with information travels faster than the speed of light, then there are frames of reference in which that information arrived at the destination before it left the origin. Furthermore, unless your FTL drive is limited in ways that would make it utterly useless, any FTL drive can be used to make "closed time-like loops", which would allow travelers to meet up with their own past selves.
  - enraged_camel 14y ago
stan_rogers 14y ago
Causality, though, is a postulate in the system. If you start with the assumption that faster-than-light is impossible and build a system around that, the system you build around that assumption will always prove that it's impossible -- until something else proves otherwise.
- nessus42 14y ago
  Causality it NOT a postulate of the system. General Relativity fully allows for universes in which causality does not hold. Such universes would either turn out to be either very strange, or actual use of FTL drives would have to be extremely rare.
  - Steko 14y ago
    Pick two: relativity, causality, FTL travel.
nlh 14y ago
I'm not the down-voter, but I think you were likely down-voted because this article/discussion is not about time travel. And your first statement is that "you can't have faster-than-light travel..." is precisely what this paper discusses -- that with certain loopholes in physics (and consistent with General Relativity), faster-than-light travel is at least theoretically possible.
- nessus42 14y ago
  Faster-than-light travel allows you to travel backwards in time. This is an essential part of Special and General Relativity. Those who don't understand this and who are not ready to address its consequences, should refrain from speculating on what is and isn't possible.
  - ryandvm 14y ago
    > [Those] who are not ready to address its consequences, should refrain from speculating on what is and isn't possible Really? Do you hear yourself? I hope you're taking down badge numbers because I'm not sure that all these guys are qualified to address the consequences of faster-than-light travel.
    - nessus42 14y ago
      Badge numbers? Are you talking about the badge numbers of the authors of the original article? If so, I'm sure they are quite aware that their proposed drive violates causality. Some scientists are willing to bite that bullet. It's some people on HN who don't seem to understand the chose-two-out-of-three principle when it comes to Relativity, FTL, and causality, and want to willfully ignore it.
  - Evbn 14y ago
    Is FTL the same as warping space? what happens in a (reduced dimensions for intuitiveness) scenario where a universe is like a line segment, and gets bent into crossing itself, allowing jumps from one point to another? Then there is a long way and a short way to send a message, kind of like travelling through vacuum is faster than traveling through water.
rootbear 14y ago
I'm glad you brought this up. I got into this same argument on a Star Trek fan site and my opinions on the feasibility of FTL travel were not popular with that crowd. I would like to know what Alcubierre himself thinks about the causality issue. You simply can't wish it away.
deleted 14y ago
[deleted]
jonmrodriguez 14y ago
Since QED and relativity aren't consistent, and QED says that can photons travel at speeds less than and greater than c, isn't it possible that relativity is just probabilistically true but not "enforced" to be true?
- nessus42 14y ago
  Since I don't know anything about QED, I'm not equipped to answer this question. What I can say, however, is that much of what we consider to be "causality" is just a matter of probability. There's nothing in the fundamental laws of physics that says that a flower pot that fell off the window sill and broke into a million pieces an hour ago, can't now suddenly put itself back together and then fly back onto the sill, just as if nothing had ever gone awry. The fact that flower pots don't typically tend to do so, is that this turn of events is extremely unlikely. In fact, as far as fundamental physics goes, the forward arrow of time and the backward arrow of time are indistinguishable. I.e., if you were to look at a movie of subatomic particles interacting, you wouldn't be able to tell, if you didn't already know, if the film were being played forwards or being played backwards. (As a slight caveat to this--a caveat that does not detract from what I am saying, but should be mentioned for the sake of completeness--there is one tiny aspect of fundamental particle physics that breaks this time symmetry, but it is a rather subtle one. The physicists who discovered this won a Nobel prize, and as I understand it, why this symmetry should be broken--no matter how subtle the effect--is still a deep mystery.) What we consider to be the forward arrow of time comes completely from thermodynamics, not from fundamental law. E.g., if the Big Bang occurred at the end of time, rather than the beginning of time, we wouldn't be able to tell the difference, because from our point of view, that would end up being the beginning of time anyway.
  - jonmrodriguez 14y ago
    Do you think the big bang has universes on both sides of it?
- gus_massa 14y ago
  QED is consistent with Special Relativity. The standard introduction to QED starts with the successful attempt of Dirac to create a "covariant" version of the Quantum Mechanics that is consistent with Special Relativity (in that process he explained the particle spin and predicted the positrons). Then it's necessary to make the theory more complicated to explain some experiments, but the final QED theory is consistent with Special Relativity (or "covariant" in the jargon). The extensions that involve the weak and strong forces are also covariant and they form the Standard Model (with the slightly hypothetical Higgs boson). Any reasonable extension of the Standard Model is "covariant". And it would be very difficult to make the physic community consider seriously a no "covariant" theory (nothing is impossible). Nobody knows the quantum version of General Relativity, so perhaps there is a very tiny loophole, but I would not be very hopeful about it.
jxcole 14y ago
A lot of people responding to this thread do not understand relativity. The OP is correct, the fundamental problem is causality, relativity and FTL do not mix. The common way of saying it is that you can have two of the three: FTL, causality, or relativity. You cannot have all three. Here is a very nice explanation of why: http://thebestforumever.com/community/threads/tachyon-pistol-duel-thought-experiment.47326/ http://thebestforumever.com/community/threads/tachyon-pistol... Also, I've never understood how the Novikov self-consistency principle would work in reality. Causality violations can be a lot simpler than killing your grandfather. Say, for example, you set up a pool table with a pool ball and hit it through a time machine such that it went back in time and hit itself, making it miss the time machine? Would the pool ball be suddenly acted on by some invisible force? Would it explode? Would the universe just end? The whole scenario seems so implausible that addressing this theoretical concern is a lot more relevant than worrying about the energy levels required to manipulate space time.
- nessus42 14y ago
  > Say, for example, you set up a pool table with a pool ball and hit it through a time machine such that it went back in time and hit itself, making it miss the time machine? Would the pool ball be suddenly acted on by some invisible force? Would it explode? Would the universe just end? No, none of these. Well, something would prevent this from happening, but whatever happened would have a perfectly reasonable explanation at the time. You have to understand, firstly, that in GR, the universe doesn't start at the beginning of time and slowly evolve until it reaches the end of time. Instead, all of space and time that ever was or ever will be, exist now and forever. And this one big constant eternal space-time is one of many possible solutions to the equations that constrain the universe. If a potential universe does not satisfy those equations, then it can't exist and therefore it won't exist. A universe in which your pool ball goes back in time and deflects itself, is not a solution to those equations, and therefore, that will never happen in any GR compliant universe. So, what might happen instead? Well, you might get bored with the experiment and give up before you get it working. Your girlfriend might come along and tell you that you are neglecting her. Burglars might steal your time machine. Or most likely, you might never invent the time machine to begin with. It's quite conceivable that in some sense a time machine is theoretically buildable, but the reason our universe is one of the solutions to GR is that no one ever actually builds one in the history of the universe. Why not, you ask? Because if they did, that universe is much less likely to be a solution to the equations, so we just so happen to find ourselves in one of the more likely universes. Another way to think about this is that perhaps all possible universes actually exist. In that case, we have to find ourselves in one of the existing universes. We can't find ourselves in a non-existing universe, since non-existing universes don't exist.
pessimist 14y ago
In General Relativity, Faster-Than-Light travel is only prohibited locally. Its possible for space-time itself to expand and shrink faster than light. This is why distant-enough galaxies are receding from us at faster than the speed of light. Mathematically, the structure of the proposed warp-drive is such that the spaceship is moving at << c locally, so no causality is violated. See the author's answer here: http://www.icarusinterstellar.org/daydreaming-beyond-the-solar-system-with-warp-field-mechanics/ http://www.icarusinterstellar.org/daydreaming-beyond-the-sol...
- nessus42 14y ago
  Nowhere does he state that causality is never violated.
Steuard 14y ago
[Background note: I'm a physics professor with a background in string theory and relativity.] You're raising a sensible point, and most of the objections raised here are invalid. But it nevertheless turns out (surprisingly, to me!) that your argument doesn't work. This topic came up a lot during the short-lived superluminal neutrino excitement last year, and one good discussion of it by Sean Carroll can be found at: http://blogs.discovermagazine.com/cosmicvariance/2011/09/24/can-neutrinos-kill-their-own-grandfathers/ http://blogs.discovermagazine.com/cosmicvariance/2011/09/24/... That post includes a link to a paper by Bob Geroch that goes into the topic in detail. The very short summary is that there is no mathematical necessity for "causality" to be defined just by relativity: the union of the relativistic definition and some other well-defined causal theory can work just as well, even if that theory involved faster-than-light travel. So thank you for raising a very good point about this topic, even if it wasn't quite accurate. And to those who've objected here: unless your objection explicitly quoted Geroch, odds are very good that they're wrong.
- Steuard 14y ago
  That being said, I'm not certain whether there's any way to avoid causality violation if the hypothetical FTL travel results entirely from within relativity itself. (Geroch's argument, as I understand it, is based entirely on combining two different theoretical structures.)
- nessus42 14y ago
  I would certainly defer to your much superior knowledge of physics. Mine is only that of an MIT student who was thoroughly indoctrinated with Special Relativity as part of MIT's general science education, and who sat in on an undergrad course on GR taught by cosmologist Max Tegmark. If I understand Bob Geroch properly (much of his paper is Sanskrit to me, but I think I understand the parts that are in English), he acknowledges that FTL particles would seem to allow you to set up "closed causal curves" (i.e., violate causality in a way that would allow you to try to generate a time paradox), but he asserts that you would never actually be able to achieve this goal due to something like the "Novikov self-consistency principle", although he doesn't mention this principle by name. This is similar to the position that many physicist think about the existence of closed time-like loops in GR. Sure, in some sense they are possible, since GR shows you how to set them up, but something like the Novikov principle would either prevent you from setting one up or from doing anything nefarious with one. So, if I understand things correctly, Geroch's claim doesn't really different too much from assertions that I made elsewhere in this thread, that sure you might be able to build an FTL drive, but if the universe lets you do this, it's not going to let you make good use of one, like, for instance, creating an FTL space-liner fleet, because if it did, you'd generate a time paradox soon enough. Or if the universe were so kind as to let us do that, we'd end up finding ourselves in a very strange world where Novikov's principle was mucking with our daily affairs in a very creepy manner.
dclowd9901 14y ago
It's not that the universe might "literally" prevent you from performing an action. It's just that our universe doesn't allow it in the first place. The coherence of the universe is a biproduct of its inherent order. I guess what I mean, without sounding too pseudo-scientific, is I believe the way it works is that, if, say, you go back in time and kill your grandfather, then what is actually happening is you are killing the person you thought was your grandfather, but they were not, in fact, your grandfather to begin with. In fact, all you would know going back is that your grandfather was murdered by someone. Let's say you believe it was you, and you have a change of heart. Decide it's not in you to kill someone, even to prove a scientific point. That doesn't mean he doesn't get killed. Nothing's changed. It's simply that someone else killed him. The answer to the question of Schroedinger's cat isn't that the cat is dead or alive, it's that it is.

stryker 14y ago

I feel like a lot of people here misunderstand the speed of light as a cosmic speed limit. The speed of light as a speed limit is a local constraint. On a larger scale there is no such speed limit. Just because you travel faster than light on a non-local scale does not mean you break causality. For example, light "travels" more slowly through water than vacuum. Well, actually, photons travel at c always -- it's just that light gets bounced around by water molecules, getting absorbed and reabsorbed many many times before arriving at the destination. It's like driving to a destination as opposed to running to it. If the roads are curvy enough, the runner will win because he is not constrained by roads. Could it be that the Alcubierre drive gets past the roads of empty space in a way light cannot? For a better example, consider the expansion of the universe. Link: http://curious.astro.cornell.edu/question.php?number=575 http://curious.astro.cornell.edu/question.php?number=575. I quote: The fact that galaxies we see now are moving away from us faster than the speed of light has some bleak consequences, however. Astronomers now have strong evidence that we live in an "accelerating universe," which means that the speed of each individual galaxy with respect to us will increase as time goes on. If we assume that this acceleration continues indefinitely, then galaxies which are currently moving away from us faster than the speed of light will always be moving away from us faster than the speed of light and will eventually reach a point where the space between us and them is stretching so rapidly that any light they emit after that point will never be able to reach us. In other words, vacuum fills in between us and far away galaxies faster than light can traverse it. Again, locally, the derivative of position with respect to time is c, but the universe is able to "cheat" by messing around with the definition of position. If the Alcubierre drive messes with the fabric of the universe in a similar way, I don't see any reason why it cannot succeed. Here's a related link: http://curious.astro.cornell.edu/question.php?number=56 http://curious.astro.cornell.edu/question.php?number=56. I quote: Technically speaking, the speed of light limit only applies when you are in an "inertial frame" -- that is, sitting where you are, without any forces acting on you, and measuring the speed of an object that moves past a ruler and clock that you are holding in your hand. Across the large distances in the universe, however, we have a very different set of circumstances. No one is in an inertial frame, because everyone is being accelerated with respect to everyone else, due to the universe's gravitational field and the fact that the universe is expanding. In effect, the universe's expansion isn't really due to galaxies moving "through space" away from each other, but rather due to the stretching of space itself, which isn't governed by the same limits that we are. I'm not sure whether the Alcubierre drive contradicts causality, but if it does, I highly doubt that it is because of the arguments presented on this forum.

nessus42 14y ago
I'm not sure whether the Alcubierre drive contradicts causality, but if it does, I highly doubt that it is because of the arguments presented on this forum. The Alcubierre drive most certainly does violate causality, and this follows from just the most basic amount of Special Relativity that you might learn if only you'd spend your time learning real things, rather than spreading disinformation.
- stryker 14y ago
  I'm going to ignore your condescension. I'm also going to assume you didn't read the rest of my post, because you presented no arguments to counter my points. I'd like to point out that the wikipedia article on the Alcubierre drive writes at length to show how this warping process does not violate causality: http://en.wikipedia.org/wiki/Alcubierre_drive http://en.wikipedia.org/wiki/Alcubierre_drive. I know you feel strongly about people not understanding special relativity and I actually agree with you on that point, but you're not addressing any of the valid counterpoints that people are making. You're simply closing your ears and exclaiming "special relativity" which only applies locally (if you want an appeal to authority, I talked about this with a former physicist who worked with Stephen Hawking). 1) The universe is expanding faster than the speed of light. In other words, galaxies are moving away from us faster than c (evidenced by the growing redshift of light emitted by galaxies when they were NOT traveling away from us faster than c). That does not violate causality (as I mentioned in my post) because the topology of space itself is changing. Locally, everything still obeys special relativity. 2) Virtual particles, EPR Paradox (I know it's not actually a paradox). (http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html http://math.ucr.edu/home/baez/physics/Quantum/virtual_partic...) My point with these phenomena is that locality and the speed of light are not very intuitive concepts. You can still have "spooky action at a distance" without violating causality.
  - nessus42 14y ago
    You're simply closing your ears and exclaiming "special relativity" which only applies locally (if you want an appeal to authority, I talked about this with a former physicist who worked with Stephen Hawking). If you come up with a mathematical "proof" and I can use its conclusion to prove that 1 + 1 = 3, then I have shown your proof to be incorrect via contradiction, no matter how hard you continue to argue for it. Special Relativity does not only apply locally. It applies to any system where information can be transmitted between two points that are in inertial frames. The universe expanding at faster than the speed of light is neither here nor there, since you cannot use the expanding universe to transmit information between two points in at FTL speeds. On the other hand, you CAN use an FTL drive to transmit information between two points in inertial frames at FTL speeds. QM spooky action at a distance does not let you transmit information between two points at FTL speeds. This is all you need to do to show that any FTL scheme violates Special Relativity: Information transmitted between two points in inertial frames at FTL speeds. If you can do that, you've either disproven causality, or you've disproven Special Relativity.

geuis 14y ago

We aren't talking about time travel. So many people confuse the idea of FTL with time travel. General Relativity talks about frames of reference. Suppose you had a base on Mars and sent a radio signal to it from Earth. It would take between 4.3 to 21 minutes to reach Mars, depending on its distance from Earth at the time. If you simultaneously jumped in your 10x warp drive and set out, you would arrive at Mars in 25.8 seconds to 1min 26 seconds, again depending on distance. In this scenario, there has been no violation of causality. You aren't arriving before you sent your radio signal. All that is happening is that you're arriving before your radio signal does. From the perspective of a telescope or radio antenna at the Mars base, you would view Earth in its state 4.3 to 21 minutes before, because it simply takes that long for the electromagnetic waves to get to you. So yes, you can travel faster than the speed of light without violating causality. If you travel in a relativistic way, i.e. speed all the way up to 99.99999% of light speed, you will most certainly run into time slowing down in your local frame, while it continues at a normal frame back at your departure point. But with a warp drive that works as described in the article, there is no changing of reference frames. Since you're warping space itself, the spacecraft isn't actually undergoing acceleration in the relativistic way of a rocket. For passengers on board, the clocks are going to run at the same rates as back home.

politician 14y ago
A pair of FTL communication devices (jargon: ansible) can be arranged in a configuration which would allow the future to speak to the past, violating causality.
nessus42 14y ago
You are not correct. If anything with information travels faster than the speed of light, then there are frames of reference in which that information arrived at the destination before it left the origin. Furthermore, unless your FTL drive is limited in ways that would make it utterly useless, any FTL drive can be used to make "closed time-like loops", which would allow travelers to meet up with their own past selves.
- enraged_camel 14y ago

stan_rogers 14y ago

Causality, though, is a postulate in the system. If you start with the assumption that faster-than-light is impossible and build a system around that, the system you build around that assumption will always prove that it's impossible -- until something else proves otherwise.

nessus42 14y ago
Causality it NOT a postulate of the system. General Relativity fully allows for universes in which causality does not hold. Such universes would either turn out to be either very strange, or actual use of FTL drives would have to be extremely rare.
- Steko 14y ago
  Pick two: relativity, causality, FTL travel.

nlh 14y ago

I'm not the down-voter, but I think you were likely down-voted because this article/discussion is not about time travel. And your first statement is that "you can't have faster-than-light travel..." is precisely what this paper discusses -- that with certain loopholes in physics (and consistent with General Relativity), faster-than-light travel is at least theoretically possible.

nessus42 14y ago
Faster-than-light travel allows you to travel backwards in time. This is an essential part of Special and General Relativity. Those who don't understand this and who are not ready to address its consequences, should refrain from speculating on what is and isn't possible.
- ryandvm 14y ago
  > [Those] who are not ready to address its consequences, should refrain from speculating on what is and isn't possible Really? Do you hear yourself? I hope you're taking down badge numbers because I'm not sure that all these guys are qualified to address the consequences of faster-than-light travel.
  - nessus42 14y ago
    Badge numbers? Are you talking about the badge numbers of the authors of the original article? If so, I'm sure they are quite aware that their proposed drive violates causality. Some scientists are willing to bite that bullet. It's some people on HN who don't seem to understand the chose-two-out-of-three principle when it comes to Relativity, FTL, and causality, and want to willfully ignore it.
- Evbn 14y ago
  Is FTL the same as warping space? what happens in a (reduced dimensions for intuitiveness) scenario where a universe is like a line segment, and gets bent into crossing itself, allowing jumps from one point to another? Then there is a long way and a short way to send a message, kind of like travelling through vacuum is faster than traveling through water.

rootbear 14y ago

I'm glad you brought this up. I got into this same argument on a Star Trek fan site and my opinions on the feasibility of FTL travel were not popular with that crowd. I would like to know what Alcubierre himself thinks about the causality issue. You simply can't wish it away.

deleted 14y ago

[deleted]

jonmrodriguez 14y ago

Since QED and relativity aren't consistent, and QED says that can photons travel at speeds less than and greater than c, isn't it possible that relativity is just probabilistically true but not "enforced" to be true?

nessus42 14y ago
Since I don't know anything about QED, I'm not equipped to answer this question. What I can say, however, is that much of what we consider to be "causality" is just a matter of probability. There's nothing in the fundamental laws of physics that says that a flower pot that fell off the window sill and broke into a million pieces an hour ago, can't now suddenly put itself back together and then fly back onto the sill, just as if nothing had ever gone awry. The fact that flower pots don't typically tend to do so, is that this turn of events is extremely unlikely. In fact, as far as fundamental physics goes, the forward arrow of time and the backward arrow of time are indistinguishable. I.e., if you were to look at a movie of subatomic particles interacting, you wouldn't be able to tell, if you didn't already know, if the film were being played forwards or being played backwards. (As a slight caveat to this--a caveat that does not detract from what I am saying, but should be mentioned for the sake of completeness--there is one tiny aspect of fundamental particle physics that breaks this time symmetry, but it is a rather subtle one. The physicists who discovered this won a Nobel prize, and as I understand it, why this symmetry should be broken--no matter how subtle the effect--is still a deep mystery.) What we consider to be the forward arrow of time comes completely from thermodynamics, not from fundamental law. E.g., if the Big Bang occurred at the end of time, rather than the beginning of time, we wouldn't be able to tell the difference, because from our point of view, that would end up being the beginning of time anyway.
- jonmrodriguez 14y ago
  Do you think the big bang has universes on both sides of it?
gus_massa 14y ago
QED is consistent with Special Relativity. The standard introduction to QED starts with the successful attempt of Dirac to create a "covariant" version of the Quantum Mechanics that is consistent with Special Relativity (in that process he explained the particle spin and predicted the positrons). Then it's necessary to make the theory more complicated to explain some experiments, but the final QED theory is consistent with Special Relativity (or "covariant" in the jargon). The extensions that involve the weak and strong forces are also covariant and they form the Standard Model (with the slightly hypothetical Higgs boson). Any reasonable extension of the Standard Model is "covariant". And it would be very difficult to make the physic community consider seriously a no "covariant" theory (nothing is impossible). Nobody knows the quantum version of General Relativity, so perhaps there is a very tiny loophole, but I would not be very hopeful about it.

jxcole 14y ago

A lot of people responding to this thread do not understand relativity. The OP is correct, the fundamental problem is causality, relativity and FTL do not mix. The common way of saying it is that you can have two of the three: FTL, causality, or relativity. You cannot have all three. Here is a very nice explanation of why: http://thebestforumever.com/community/threads/tachyon-pistol-duel-thought-experiment.47326/ http://thebestforumever.com/community/threads/tachyon-pistol... Also, I've never understood how the Novikov self-consistency principle would work in reality. Causality violations can be a lot simpler than killing your grandfather. Say, for example, you set up a pool table with a pool ball and hit it through a time machine such that it went back in time and hit itself, making it miss the time machine? Would the pool ball be suddenly acted on by some invisible force? Would it explode? Would the universe just end? The whole scenario seems so implausible that addressing this theoretical concern is a lot more relevant than worrying about the energy levels required to manipulate space time.

nessus42 14y ago
> Say, for example, you set up a pool table with a pool ball and hit it through a time machine such that it went back in time and hit itself, making it miss the time machine? Would the pool ball be suddenly acted on by some invisible force? Would it explode? Would the universe just end? No, none of these. Well, something would prevent this from happening, but whatever happened would have a perfectly reasonable explanation at the time. You have to understand, firstly, that in GR, the universe doesn't start at the beginning of time and slowly evolve until it reaches the end of time. Instead, all of space and time that ever was or ever will be, exist now and forever. And this one big constant eternal space-time is one of many possible solutions to the equations that constrain the universe. If a potential universe does not satisfy those equations, then it can't exist and therefore it won't exist. A universe in which your pool ball goes back in time and deflects itself, is not a solution to those equations, and therefore, that will never happen in any GR compliant universe. So, what might happen instead? Well, you might get bored with the experiment and give up before you get it working. Your girlfriend might come along and tell you that you are neglecting her. Burglars might steal your time machine. Or most likely, you might never invent the time machine to begin with. It's quite conceivable that in some sense a time machine is theoretically buildable, but the reason our universe is one of the solutions to GR is that no one ever actually builds one in the history of the universe. Why not, you ask? Because if they did, that universe is much less likely to be a solution to the equations, so we just so happen to find ourselves in one of the more likely universes. Another way to think about this is that perhaps all possible universes actually exist. In that case, we have to find ourselves in one of the existing universes. We can't find ourselves in a non-existing universe, since non-existing universes don't exist.

pessimist 14y ago

In General Relativity, Faster-Than-Light travel is only prohibited locally. Its possible for space-time itself to expand and shrink faster than light. This is why distant-enough galaxies are receding from us at faster than the speed of light. Mathematically, the structure of the proposed warp-drive is such that the spaceship is moving at << c locally, so no causality is violated. See the author's answer here: http://www.icarusinterstellar.org/daydreaming-beyond-the-solar-system-with-warp-field-mechanics/ http://www.icarusinterstellar.org/daydreaming-beyond-the-sol...

nessus42 14y ago
Nowhere does he state that causality is never violated.

Steuard 14y ago

[Background note: I'm a physics professor with a background in string theory and relativity.] You're raising a sensible point, and most of the objections raised here are invalid. But it nevertheless turns out (surprisingly, to me!) that your argument doesn't work. This topic came up a lot during the short-lived superluminal neutrino excitement last year, and one good discussion of it by Sean Carroll can be found at: http://blogs.discovermagazine.com/cosmicvariance/2011/09/24/can-neutrinos-kill-their-own-grandfathers/ http://blogs.discovermagazine.com/cosmicvariance/2011/09/24/... That post includes a link to a paper by Bob Geroch that goes into the topic in detail. The very short summary is that there is no mathematical necessity for "causality" to be defined just by relativity: the union of the relativistic definition and some other well-defined causal theory can work just as well, even if that theory involved faster-than-light travel. So thank you for raising a very good point about this topic, even if it wasn't quite accurate. And to those who've objected here: unless your objection explicitly quoted Geroch, odds are very good that they're wrong.

Steuard 14y ago
That being said, I'm not certain whether there's any way to avoid causality violation if the hypothetical FTL travel results entirely from within relativity itself. (Geroch's argument, as I understand it, is based entirely on combining two different theoretical structures.)
nessus42 14y ago
I would certainly defer to your much superior knowledge of physics. Mine is only that of an MIT student who was thoroughly indoctrinated with Special Relativity as part of MIT's general science education, and who sat in on an undergrad course on GR taught by cosmologist Max Tegmark. If I understand Bob Geroch properly (much of his paper is Sanskrit to me, but I think I understand the parts that are in English), he acknowledges that FTL particles would seem to allow you to set up "closed causal curves" (i.e., violate causality in a way that would allow you to try to generate a time paradox), but he asserts that you would never actually be able to achieve this goal due to something like the "Novikov self-consistency principle", although he doesn't mention this principle by name. This is similar to the position that many physicist think about the existence of closed time-like loops in GR. Sure, in some sense they are possible, since GR shows you how to set them up, but something like the Novikov principle would either prevent you from setting one up or from doing anything nefarious with one. So, if I understand things correctly, Geroch's claim doesn't really different too much from assertions that I made elsewhere in this thread, that sure you might be able to build an FTL drive, but if the universe lets you do this, it's not going to let you make good use of one, like, for instance, creating an FTL space-liner fleet, because if it did, you'd generate a time paradox soon enough. Or if the universe were so kind as to let us do that, we'd end up finding ourselves in a very strange world where Novikov's principle was mucking with our daily affairs in a very creepy manner.

dclowd9901 14y ago

It's not that the universe might "literally" prevent you from performing an action. It's just that our universe doesn't allow it in the first place. The coherence of the universe is a biproduct of its inherent order. I guess what I mean, without sounding too pseudo-scientific, is I believe the way it works is that, if, say, you go back in time and kill your grandfather, then what is actually happening is you are killing the person you thought was your grandfather, but they were not, in fact, your grandfather to begin with. In fact, all you would know going back is that your grandfather was murdered by someone. Let's say you believe it was you, and you have a change of heart. Decide it's not in you to kill someone, even to prove a scientific point. That doesn't mean he doesn't get killed. Nothing's changed. It's simply that someone else killed him. The answer to the question of Schroedinger's cat isn't that the cat is dead or alive, it's that it is.