4 ms·

>>If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not

>>If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality. You are confused. You are not actually traveling to the past therefore causality is not a problem here.

lutusp 14y ago
> You are confused. I'm confused? Which of us is discussing FTL without understanding its physical meaning? > You are not actually traveling to the past ... Locate where I made this claim. > ... therefore causality is not a problem here. Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. Here's the simplest form of the relativistic equation that tells us the amount of time dilation we can expect for a given velocity v: t' = t * sqrt(1-v^2/c^2) t = time rate at v = 0 t' = time rate at v c = speed of light v = frame velocity Compute the time dilation for v > c, then get back to me. FTL means leaving the causality light cone, conventional notions of causality, and therefore all of physics: http://en.wikipedia.org/wiki/Light_cone http://en.wikipedia.org/wiki/Light_cone Quote: "Because signals and other causal influences cannot travel faster than light (see special relativity and spooky action at a distance), the light cone plays an essential role in defining the concept of causality - for a given event E, the set of events that lie on or inside the past light cone of E would also be the set of all events that could send a signal that would have time to reach E and influence it in some way." [emphasis added]
- EastLondonCoder 14y ago
  "FTL means leaving the causality light cone, conventional notions of causality, and therefore all of physics:" While I agree with your conclusion that ftl is equivalent of time travel and breaks causality I think its incorrect to say it breaks all of physics. There are several solutions to general relativity that are valid. http://en.wikipedia.org/wiki/Chronology_protection_conjecture http://en.wikipedia.org/wiki/Chronology_protection_conjectur...
  - lutusp 14y ago
    > There are several solutions to general relativity that are valid. They haven't been shown to exist in reality. Remember that science is -- must be -- empirical.
- nessus42 14y ago
  > Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. This assertion is false. GR, even without FTL, allows for closed time-like loops, which violate causality. Violations of causality are not logical contradictions like dividing by zero. (Though, of course, there are alternative mathematical systems that allow for dividing by zero. And taking the square root of a negative number is surely not impossible. You just get an Imaginary result.) It's just that a universe in which causality is violated all the time would be a very strange one, and consequently, it is reasonable to assume that if there are, for instance, closed time-like loops in the universe, they are not traversed very often.
  - lutusp 14y ago
    > GR, even without FTL, allows for closed time-like loops, which violate causality. The theory may "allow" them, but this is not to say they actually exist in reality. http://en.wikipedia.org/wiki/Causality_(physics) http://en.wikipedia.org/wiki/Causality_(physics) Quote: "In the theory of general relativity, the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause, even if the spacetime is curved." http://hipercom.inria.fr/~jacquet/retro1/crazy-science-corner/causality.html http://hipercom.inria.fr/~jacquet/retro1/crazy-science-corne... Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible."
    - nessus42 14y ago
      > Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible." And there's no evidence that they are not possible. With no evidence either way, one must remain agnostic. On the other hand, we do have evidence that closed time-like loops are possible: GR has never been observed to be wrong, and GR predicts their existence under the right circumstances. It may certainly be the case, that even though closed time-like loops are possible, they do not actually exist in our universe and never will. Paradoxical universes are ruled out by logic alone, and so the space of possible universes without closed time-like loops (or least without humans traversing them) may outnumber the space of possible universes with time-like loops in them, allowing us to make a probabilistic argument against their occurrence in our universe. My transfinite number theory is rusty, however, so YMMV.
- 98709870987 14y ago
  You are forgetting that we could "travel faster than light" using an Alcubierre Drive [1][2], or something similar. i.e. bending space so that our path is now shorter even though in our local space we are not traveling faster than light. [1] http://en.wikipedia.org/wiki/Alcubierre_drive http://en.wikipedia.org/wiki/Alcubierre_drive [2] http://techland.time.com/2012/09/19/nasa-actually-working-on-faster-than-light-warp-drive/ http://techland.time.com/2012/09/19/nasa-actually-working-on... Granted, it is all theory right now but that is how relativity started. Who knows, maybe in 100 years we'll be traveling to the stars. Nowhere does causality violation come into the picture. There is more than one way to skin a cat.

lutusp 14y ago

> You are confused. I'm confused? Which of us is discussing FTL without understanding its physical meaning? > You are not actually traveling to the past ... Locate where I made this claim. > ... therefore causality is not a problem here. Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. Here's the simplest form of the relativistic equation that tells us the amount of time dilation we can expect for a given velocity v: t' = t * sqrt(1-v^2/c^2) t = time rate at v = 0 t' = time rate at v c = speed of light v = frame velocity Compute the time dilation for v > c, then get back to me. FTL means leaving the causality light cone, conventional notions of causality, and therefore all of physics: http://en.wikipedia.org/wiki/Light_cone http://en.wikipedia.org/wiki/Light_cone Quote: "Because signals and other causal influences cannot travel faster than light (see special relativity and spooky action at a distance), the light cone plays an essential role in defining the concept of causality - for a given event E, the set of events that lie on or inside the past light cone of E would also be the set of all events that could send a signal that would have time to reach E and influence it in some way." [emphasis added]

EastLondonCoder 14y ago
"FTL means leaving the causality light cone, conventional notions of causality, and therefore all of physics:" While I agree with your conclusion that ftl is equivalent of time travel and breaks causality I think its incorrect to say it breaks all of physics. There are several solutions to general relativity that are valid. http://en.wikipedia.org/wiki/Chronology_protection_conjecture http://en.wikipedia.org/wiki/Chronology_protection_conjectur...
- lutusp 14y ago
  > There are several solutions to general relativity that are valid. They haven't been shown to exist in reality. Remember that science is -- must be -- empirical.
nessus42 14y ago
> Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. This assertion is false. GR, even without FTL, allows for closed time-like loops, which violate causality. Violations of causality are not logical contradictions like dividing by zero. (Though, of course, there are alternative mathematical systems that allow for dividing by zero. And taking the square root of a negative number is surely not impossible. You just get an Imaginary result.) It's just that a universe in which causality is violated all the time would be a very strange one, and consequently, it is reasonable to assume that if there are, for instance, closed time-like loops in the universe, they are not traversed very often.
- lutusp 14y ago
  > GR, even without FTL, allows for closed time-like loops, which violate causality. The theory may "allow" them, but this is not to say they actually exist in reality. http://en.wikipedia.org/wiki/Causality_(physics) http://en.wikipedia.org/wiki/Causality_(physics) Quote: "In the theory of general relativity, the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause, even if the spacetime is curved." http://hipercom.inria.fr/~jacquet/retro1/crazy-science-corner/causality.html http://hipercom.inria.fr/~jacquet/retro1/crazy-science-corne... Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible."
  - nessus42 14y ago
    > Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible." And there's no evidence that they are not possible. With no evidence either way, one must remain agnostic. On the other hand, we do have evidence that closed time-like loops are possible: GR has never been observed to be wrong, and GR predicts their existence under the right circumstances. It may certainly be the case, that even though closed time-like loops are possible, they do not actually exist in our universe and never will. Paradoxical universes are ruled out by logic alone, and so the space of possible universes without closed time-like loops (or least without humans traversing them) may outnumber the space of possible universes with time-like loops in them, allowing us to make a probabilistic argument against their occurrence in our universe. My transfinite number theory is rusty, however, so YMMV.
98709870987 14y ago
You are forgetting that we could "travel faster than light" using an Alcubierre Drive [1][2], or something similar. i.e. bending space so that our path is now shorter even though in our local space we are not traveling faster than light. [1] http://en.wikipedia.org/wiki/Alcubierre_drive http://en.wikipedia.org/wiki/Alcubierre_drive [2] http://techland.time.com/2012/09/19/nasa-actually-working-on-faster-than-light-warp-drive/ http://techland.time.com/2012/09/19/nasa-actually-working-on... Granted, it is all theory right now but that is how relativity started. Who knows, maybe in 100 years we'll be traveling to the stars. Nowhere does causality violation come into the picture. There is more than one way to skin a cat.