I’ve heard physicists say it all the time. Certain laws in physics seem to allow for time travel. But it just seems so unlikely.
One of the reasons why it seems unlikely, is that it would likely lead to temporal paradoxes.
The Grandfather Paradox is an extreme example of a temporal paradox. But you realize it works on a much smaller level. Just going back in time, and saying ‘hi’ to yourself before you enter the time machine would likely lead to a temporal paradox. In fact, going back a billionth of a second could do it.
My question then, is do temporal paradoxes themselves prove time travel is impossible? And if so, how?
And on a related topic, what if a temporal paradox occured? What would happen then? Science fiction authors claim it would create a rip in the fabric of space and time. Is this true? Star Trek for one claims it would lead to an unending loop. Is that true?
If the universe is deterministic, and free will is an illusion, then everything a time-traveler does is predestined. Paradoxes won’t happen, and there is no impediment to time travel.
Personally, I find determinism to be repugnant. But that is an aesthetic judgement, not a scientific one.
I remember some documentary show in which a scientist proposed a wormhole. a billiard ball entered one end of the wormhole, and exited in its own past, knocking its past self out of the wormhole entrance. According to the scientist, this would create a feedback loop that would destroy the wormhole. Apparently, it takes an arbitrarily large (mathematicians don’t like to say “infinite”) amount of energy to create a paradox.
A lot of fiction where time travel is a feature postulate that history is pretty resilient and minor infringements (squashing the wasp that stung the president?) will not have any major effect.
What does seem fairly universal is that you have to avoid meeting yourself and that bringing artefacts from the past to the present is a no-no.
SF authors just invent rules that suit, and ignore any that don’t. The most common example is the way the space travellers usually ignore relativity
That’s always been my view too. Not for fiction, of course. Go wild if it tells a good story. But in the real universe, there is only one timeline, and what happened is what happened.
I had an idea for a sort-of time travel story once, where the actual only way to visit the past or future is through a visual technology. You can see and hear it all through VR, but not actually interact with it. The problem I had with that idea was it would render all crime impossible to carry out, because you could visit the events and see the truth at any time, and that didn’t sound all that interesting to me. I might still find a way to use it in a short story though.
(No, I don’t take that seriously. There’s something to it. There’s something to just about any classification scheme. But taking these schemes too seriously makes you incapable of seeing variations and hybrids, which is a pretty serious blind spot.)
One thing that surprises me about stories of time travel is that they never seem to take into account that the Earth moves pretty fast through space and that there is no absolute reference point: we move at a different speed relative to different reference points. So if we travel a year into the past we might find ourselves in the middle of empty space, the Earth being as far from us as Pluto at that moment, but to move to Pluto would take us several years, so we cannot reach it.
Or if we travel just five minutes into the past we might find ourselves buried under 10 miles of solid rock or 10 miles above ground with no parachute.
My favourite just now is a series by Jodi Taylor. The protagonist is an historian who travels back in time to witness important events in history. Of course, it’s a literary device to put forward her own ideas about what it might have been like in places like Troy (and a plausible theory about the ‘wooden’ horse) for example.
I’ve seen stories that do take that into account, but it’s a flawed notion anyway; by virtue of the fact that velocity is relative, there is (as you say) no such thing as absolute location, so travel through time would have to include travel through space, relative to something
Earth is moving relative to the sun; the sun is moving relative to the galactic centre; the galaxy is moving relative to other galaxies; even if there were such a thing as absolute location, a spatially stationary time jump of just a few minutes would never land you back on Earth
That is what I don’t understand. Didn’t Einstein prove everything is relative? There is no absolute speed, time, location? So there’s no fixed point in space.
But if that’s true, then where do you end up?
Me I like the Sci fi model, you just stay firmly on earth. But as someone else all ready pointed out, that’s probably just a plot ploy.
I think the simplest answer to that is that time is a dimension - just because you find a way to move differently within that dimension doesn’t mean you stop interacting with the three spatial dimensions; moving through time doesn’t excuse you from space.
(That is, without invoking wormholes and the like)
But that in itself does cause some interesting problems; if you find a way, for example, to travel into the future at a rate of 2 subjective seconds per Earth-second, then the Earth is spinning twice as fast for you now.
If the Many Worlds interpretation of quantum mechanics is true then wouldn’t time travel just shift you to a universe where whatever you did happened that way?
If you were to go back in time 200 years and be in the same place, you would need some sort of Time And Relative Dimension In Space device that could somehow identify the same location. A point in spacetime is an abstraction that is not entirely meaningful in real-world terms.
And using some sort of reference object to track is fraught because of constant change occurring in stuff. Try starting in your living room: 200 years ago the floor you are standing on almost certainly did not exist (it may have not been built, or if it was, has probably been replaced since then) so finding a durable reference object is a huge challenge.
You could try to approximate, but then when you arrive in the past, you might not be able to identify where you actually are because everything is so drastically different.
And it may be that this would confound the grandfather paradox, because once you make it into the past, you would not be able to get your bearings in a way that would allow you to bollix things up. That may be how regressive time travel would protect the timeline: by making it impossible for you to cause problems because you simply cannot effectively navigate to where you can.
