4 ms·

> If we ever invented FTL travel, even for just probes, this could be done too. If we had FTL travel, we could outrace light waves, look back, and see events i

by lutusp 14y ago

> If we ever invented FTL travel, even for just probes, this could be done too. 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. Remember that the speed of light isn't just a speed limit for light waves -- it's a barrier that sorts out temporal causes and effects. Imagine a flat plane in the time dimension called the present -- on each side of the plane is a cone. The cone in the past encloses possible causes for present events. The cone in the future encloses possible effects from events in the present. Outside the past cone are events that may not produce results in the present. Outside the future cone are effects that cannot have been caused by events in the present. The slope of the cone surfaces is the speed of light. Light cone picture: http://4.bp.blogspot.com/--VBwEngltGc/Tzniy1imbII/AAAAAAAABLw/SRpE1WvNqC4/s1600/500px-World_line.svg.png http://4.bp.blogspot.com/--VBwEngltGc/Tzniy1imbII/AAAAAAAABL...

98709870987 14y ago
>>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."
ck2 14y ago
So in normal space-time FTL is not possible. It might be possible to transmit data through shortcuts, ala wormholes even if physical FTL is not possible.
- Jach 14y ago
  The light speed limit comes up elsewhere. As best we know, you cannot transmit information faster than c. Not even with "spooky quantum stuff". There are no shortcuts. Information is just as real as matter or energy--if not more so, see John Wheeler's "it from bit"--and isn't tied to meaning, see Shannon.

by lutusp 14y ago

98709870987 14y ago

>>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."

ck2 14y ago

So in normal space-time FTL is not possible. It might be possible to transmit data through shortcuts, ala wormholes even if physical FTL is not possible.

Jach 14y ago
The light speed limit comes up elsewhere. As best we know, you cannot transmit information faster than c. Not even with "spooky quantum stuff". There are no shortcuts. Information is just as real as matter or energy--if not more so, see John Wheeler's "it from bit"--and isn't tied to meaning, see Shannon.