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Remember that any such device allows you to travel backwards in time. Does it still seem so plausible to you when you realize that 'c' isn't some kind of cosmic

by M_Grey 10y ago

Remember that any such device allows you to travel backwards in time. Does it still seem so plausible to you when you realize that 'c' isn't some kind of cosmic speed limit, but the speed of causality in the universe?

pavel_lishin 10y ago
Other than the fact that it seems mind-boggling, is there any reason to believe that time travel is forbidden by the laws of physics, or that causality is an absolute?
- M_Grey 10y ago
  You've heard of the progress of entropy and the "thermodynamic arrow of time" right? Time travel into the past, violating causality... it breaks the math used to formulate the most basic physics at a very fundamental level.
  - pavel_lishin 10y ago
    Does that mean that it's impossible, or that our current model of physics simply doesn't account for causality violations? We got along okay with Newton until Einstein came along and upgraded the model.
    - M_Grey 10y ago
      Definitely the latter, there's no way to say it's impossible, just incredibly unlikely. It's easy to imagine a universe in which it is possible, and that universe could be consistent with the math. The big "but" however is that our universe has none of the features we'd expect from one with the necessary curvature or exotic matter/energy. Does that make sense? I mean, if we discover that there are multiple universes, or that Block Universe is true.... who knows? Even then however, I suspect we'd run into something like Novikov Self-Consistency Principle, or we'd have noticed time travel by now.
- openasocket 10y ago
  Time travel isn't forbidden. At least, there are solutions to the Einstein field equations that allow for the existence of wormholes between two points in space-time. But that doesn't mean there are wormholes in our universe, or that wormholes could be created (that last part is a little tricky, it's not easy to talk about causality when dealing with time travel). There could well be other laws of physics that prevent time travel, but general relativity seems to allow it.
  - M_Grey 10y ago
    It's a solution that assumes things like a perfectly uniform sphere of gas collapsing from infinity (Schwarzschild geometry), without any angular momentum. Even then, the ERB isn't traversable. A lot of the talk around wormholes and time travel suffers from map-territory issues. For example, if you look at the maximally extended Schwarzschild geometry (the Kruskal-Szekeres extension) you see two asymptotically flat regions of spacetime connected by a "wormhole". It's probably just an artifact of the model though, and not anything physical. A lot of the, "Math allows" stuff probably falls into that category, which is why when it comes to relativistic masses and velocities, you can't ignore quantum effects either.
- adrianN 10y ago
  Personally, I like to believe that NP hard problems can be solved efficiently in our universe. If time travel is possible, PSPACE can be solved efficiently. That is a pretty good argument for me that time travel is not possible. http://www.scottaaronson.com/papers/ctc.pdf http://www.scottaaronson.com/papers/ctc.pdf
  - M_Grey 10y ago
    That's an interesting perspective I've never considered, coming at it from the physics end of things. I appreciate what amounts to a much more grounded and logical take on why time travel makes so little sense, and is so hard to consider seriously.
felipemnoa 10y ago
>>the speed of causality in the universe? I'm not sure it matters as long as space-time in one section is independent from another section. Maybe? Of course, I don't really know what I'm talking about since I'm not a Physicist. On a related note: As a kid, when I was still an innocent soul, I tried to build a perpetual motion machine and transmit high rate video-without first encoding it- through a phone line. It wasn't until I learned about conservation of energy and bandwidth that I understood the error of my ways. Point is, my ignorance is probably taking me astray.
- M_Grey 10y ago
  I don't think you're that far astray, but you might be missing one key factor. There is a concept in physics called the spacetime interval, which can imply one of three kinds of separation: Timelike (events which can be causally connected, i.e. events which are within each other's light-cones), Null (the worldlines followed by photons), and Spacelike (events which are causally disconnected). For an easy example of "Spacelike", anything within an event horizon has a spacelike separation from anything beyond that horizon (an extreme example). However What if you could connect those regions with this "warp drive"? You could create a causal link between regions that would otherwise be causally disconnected. You could carry information from the past to the future, or visa versa. You could essentially ignore the very fabric of spacetime. Now... maybe that's possible, but I doubt that it's possible in the context of warping gravity. If M-theory is correct in some form, or MWI is correct (basically any multiverse theory) then maybe you could do it. After all, if the universe is a lower dimensional structure embedded in a higher one, you could imagine traversing those spacelike separations in the same way that you could do things in 3D that someone bound to 2D could not. That however, depends on so many "if's" that it's basically magic.

by M_Grey 10y ago

pavel_lishin 10y ago

Other than the fact that it seems mind-boggling, is there any reason to believe that time travel is forbidden by the laws of physics, or that causality is an absolute?

M_Grey 10y ago
You've heard of the progress of entropy and the "thermodynamic arrow of time" right? Time travel into the past, violating causality... it breaks the math used to formulate the most basic physics at a very fundamental level.
- pavel_lishin 10y ago
  Does that mean that it's impossible, or that our current model of physics simply doesn't account for causality violations? We got along okay with Newton until Einstein came along and upgraded the model.
  - M_Grey 10y ago
    Definitely the latter, there's no way to say it's impossible, just incredibly unlikely. It's easy to imagine a universe in which it is possible, and that universe could be consistent with the math. The big "but" however is that our universe has none of the features we'd expect from one with the necessary curvature or exotic matter/energy. Does that make sense? I mean, if we discover that there are multiple universes, or that Block Universe is true.... who knows? Even then however, I suspect we'd run into something like Novikov Self-Consistency Principle, or we'd have noticed time travel by now.
openasocket 10y ago
Time travel isn't forbidden. At least, there are solutions to the Einstein field equations that allow for the existence of wormholes between two points in space-time. But that doesn't mean there are wormholes in our universe, or that wormholes could be created (that last part is a little tricky, it's not easy to talk about causality when dealing with time travel). There could well be other laws of physics that prevent time travel, but general relativity seems to allow it.
- M_Grey 10y ago
  It's a solution that assumes things like a perfectly uniform sphere of gas collapsing from infinity (Schwarzschild geometry), without any angular momentum. Even then, the ERB isn't traversable. A lot of the talk around wormholes and time travel suffers from map-territory issues. For example, if you look at the maximally extended Schwarzschild geometry (the Kruskal-Szekeres extension) you see two asymptotically flat regions of spacetime connected by a "wormhole". It's probably just an artifact of the model though, and not anything physical. A lot of the, "Math allows" stuff probably falls into that category, which is why when it comes to relativistic masses and velocities, you can't ignore quantum effects either.
adrianN 10y ago
Personally, I like to believe that NP hard problems can be solved efficiently in our universe. If time travel is possible, PSPACE can be solved efficiently. That is a pretty good argument for me that time travel is not possible. http://www.scottaaronson.com/papers/ctc.pdf http://www.scottaaronson.com/papers/ctc.pdf
- M_Grey 10y ago
  That's an interesting perspective I've never considered, coming at it from the physics end of things. I appreciate what amounts to a much more grounded and logical take on why time travel makes so little sense, and is so hard to consider seriously.

felipemnoa 10y ago

>>the speed of causality in the universe? I'm not sure it matters as long as space-time in one section is independent from another section. Maybe? Of course, I don't really know what I'm talking about since I'm not a Physicist. On a related note: As a kid, when I was still an innocent soul, I tried to build a perpetual motion machine and transmit high rate video-without first encoding it- through a phone line. It wasn't until I learned about conservation of energy and bandwidth that I understood the error of my ways. Point is, my ignorance is probably taking me astray.

M_Grey 10y ago
I don't think you're that far astray, but you might be missing one key factor. There is a concept in physics called the spacetime interval, which can imply one of three kinds of separation: Timelike (events which can be causally connected, i.e. events which are within each other's light-cones), Null (the worldlines followed by photons), and Spacelike (events which are causally disconnected). For an easy example of "Spacelike", anything within an event horizon has a spacelike separation from anything beyond that horizon (an extreme example). However What if you could connect those regions with this "warp drive"? You could create a causal link between regions that would otherwise be causally disconnected. You could carry information from the past to the future, or visa versa. You could essentially ignore the very fabric of spacetime. Now... maybe that's possible, but I doubt that it's possible in the context of warping gravity. If M-theory is correct in some form, or MWI is correct (basically any multiverse theory) then maybe you could do it. After all, if the universe is a lower dimensional structure embedded in a higher one, you could imagine traversing those spacelike separations in the same way that you could do things in 3D that someone bound to 2D could not. That however, depends on so many "if's" that it's basically magic.