Prospects for time travel
May. 30th, 2020 07:12 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
mtbcMy lay understanding of the prospects for time travel is basic and out of date. By summarizing here, perhaps I can discover how wrong I am and what interesting aspects I am missing.
There's no concept of universal time. Spatially separated events that seemed to happen at the same time from one perspective may have happened before, or after, each other from others' perspective. Starting off together with synchronized clocks does not suffice because time can pass differently for you after you part ways. If I do something like a high-acceleration journey slingshotting around a nearby star then back, time will have passed rather slower for me than for those waiting back here.
In trying to guess how time will seem to pass, with special relativity I've found it works well to imagine time as just another dimension and we always travel at lightspeed, usually largely forward in time. Time seemed to pass slower for me on the above round trip around a star because my higher relative spatial velocity meant less of that speed remained for the forward-in-time velocity component. However, achieving a right-angle turn from that time direction requires infinite energy so one can't start heading backward.
With both quantum mechanics and general relativity we have math that constrains and predicts. Especially with the former, we might not know why the math works. Still, we know what it disallows and, where we can test the math then, at least within its bailiwick, it holds. The math doesn't prohibit time travel but the difficulty of reaching a situation that would permit it may be more a physical impossibility than it is simply an engineering problem.
Black holes probably aren't useful means of time travel, whether they be charged or rotating or whatever else. Even with exotic matter in hand, the singularity so warps spacetime that usefully deploying the matter and traversing the singularity is also rather more than an engineering problem.
A relatively good prospect for time travel might be via a wormhole. These change the contiguous topology of simple spacetime to build in a shortcut between points. Given a local wormhole, one could use that relativistic trick above to accelerate one end of it around so that time has passed differently for the two ends, before bringing them back to being near each other. Perhaps one could then traverse that passage to pass forward or backward in time.
If one could do that, could one pass back in time and cause any of the classic paradoxes? My reading on this part is among the most out-of-date and hazy aspects of my understanding here but my impression is that it may be that when one tries to use the math to model such a violation, it turns out not to be possible, that consistency is preserved.
I don't know of any proposed means of creating a wormhole. It is possible that the uncertainty principle hands us extremely tiny ones that could somehow be coaxed into larger life but this probably requires manipulation of equally speculative exotic matter. Given impediments like null energy conditions, we have no strong reason to think that suitable exotic matter can be produced such as to usefully stabilize the wormholes of which we have no obvious source anyway.
In short, the math may allow time travel, it may even prevent paradoxes, but actually making it happen, in any conventional sense, is multiple dimensions away from being possible by any practical approach. For decades we have spent much money on attempting to construct a stable fusion reactor and that is categorically far easier than anything I mention above.
There's no concept of universal time. Spatially separated events that seemed to happen at the same time from one perspective may have happened before, or after, each other from others' perspective. Starting off together with synchronized clocks does not suffice because time can pass differently for you after you part ways. If I do something like a high-acceleration journey slingshotting around a nearby star then back, time will have passed rather slower for me than for those waiting back here.
In trying to guess how time will seem to pass, with special relativity I've found it works well to imagine time as just another dimension and we always travel at lightspeed, usually largely forward in time. Time seemed to pass slower for me on the above round trip around a star because my higher relative spatial velocity meant less of that speed remained for the forward-in-time velocity component. However, achieving a right-angle turn from that time direction requires infinite energy so one can't start heading backward.
With both quantum mechanics and general relativity we have math that constrains and predicts. Especially with the former, we might not know why the math works. Still, we know what it disallows and, where we can test the math then, at least within its bailiwick, it holds. The math doesn't prohibit time travel but the difficulty of reaching a situation that would permit it may be more a physical impossibility than it is simply an engineering problem.
Black holes probably aren't useful means of time travel, whether they be charged or rotating or whatever else. Even with exotic matter in hand, the singularity so warps spacetime that usefully deploying the matter and traversing the singularity is also rather more than an engineering problem.
A relatively good prospect for time travel might be via a wormhole. These change the contiguous topology of simple spacetime to build in a shortcut between points. Given a local wormhole, one could use that relativistic trick above to accelerate one end of it around so that time has passed differently for the two ends, before bringing them back to being near each other. Perhaps one could then traverse that passage to pass forward or backward in time.
If one could do that, could one pass back in time and cause any of the classic paradoxes? My reading on this part is among the most out-of-date and hazy aspects of my understanding here but my impression is that it may be that when one tries to use the math to model such a violation, it turns out not to be possible, that consistency is preserved.
I don't know of any proposed means of creating a wormhole. It is possible that the uncertainty principle hands us extremely tiny ones that could somehow be coaxed into larger life but this probably requires manipulation of equally speculative exotic matter. Given impediments like null energy conditions, we have no strong reason to think that suitable exotic matter can be produced such as to usefully stabilize the wormholes of which we have no obvious source anyway.
In short, the math may allow time travel, it may even prevent paradoxes, but actually making it happen, in any conventional sense, is multiple dimensions away from being possible by any practical approach. For decades we have spent much money on attempting to construct a stable fusion reactor and that is categorically far easier than anything I mention above.
