According to Wikipedia, “time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light.”
Fine. So based on the current known laws of physics or quantum mechanics, there are some mathematical formulas that solve to “go back and stop Hitler”.
And there has been endless debates about the philosophical implications of pop culture science fiction from the likes of H.G. Wells, Kurt Vonnegut, James Cameron and Robert Zemeckis. Warnings about paradoxes and sleeping with your grandmother and whatnot.
But how would time travel actually WORK in what we know of how the universe actually works?
What I mean to say is this. Let’s say I’m sitting here in the year 2029 with the rest of you. That is happening right now from my perspective. I go back in time to 1979. Does that mean that 1979 “exists” somewhere in the universe (along with every other moment in history, perhaps as some offshoot of the “Many Worlds” theory)? And if so, why is my “now” in 2029 any more “now” than that of seven year old me in 1979?
Or do I effectively have to “rewind” the entire universe (or at least from my perspective) so that basically nothing after 1970 exists yet?
Theoretically, IF one could create a wormhole and IF one could accelerate one entrance to some relativistic speed for a hundred years, one could travel backwards in time to the “younger” end. Still seems weird to me in that it still seems to require the simultaneous existence of two “universes”. One where I go into some Stargate that’s been orbiting the sun at some fraction of the speed of light while the other has been sitting there 100 years, and another universe where the second Stargate is not that old.
Basically, I’m having trouble wrapping my brain how one could go through a door and 1979 exists on the other side, even if some weird math says it might be possible.
Swap space and time in all of your questions. If I’m sitting here, but I can go through a door and be someplace else instead, does “here” really exist?
Sure, but there’s also pop-culture science fiction in which time travel means you can’t “go back and stop Hitler”: in which it’s as if a slightly-older you is already there and then, failing to stop Hitler; and in which you can, here and now, fire up a time machine and go back to take your shot, and fail, and paradox lost.
Well the answer of course is we don’t know, and can only speculate, including speculating that there is no way for macroscopic objects to do anything like travel backwards in time.
From what we can gather so far though, it looks like the universe is fine with going backwards as long as causality is preserved – you can ask kinds of weird shit, like the quantum eraser experiment, but you can’t use any of these phenomena to send the winning lottery numbers to yourself. Heck, you can send the winning numbers back as long as you’re also sending noise such that it’s impossible for your past self to read the message.
So it appears a human going back would likely be of the kind where you’re powerless to change anything, and whatever you do is what already happened.
I would suspect that the extent to which time travel exists as reality, the physical laws governing it would be such that it would be effectively useless. I could imagine that individual quantum mechanical particles could go back in time but that doing so would be uncontrollable, such that no information could be passed backwards in time, similar to the way that quantum entanglement allows fuzzy action at a distance but can’t be used to send messages faster than light. At best you could detect a closed loop in time after the fact, so no going back in time to kill Hitler.
As far as I know, faster-than-light travel with respect to the local spacetime region is itself a violation of the known laws of physics. You could have apparent time travel by which you move from point A to point B so fast that a neutral observer sees you exit from B before you walk into A. But that is just an illusion due to the relative distortion of spacetime between the observer and points A & B. It would be like the observer is watching two cameras fixed at the ends of a tunnel, where one camera has a ten second delay.
I could, of course, be very wrong about all of this.
My theory is, just as passing from point A to point B in space requires going through all points in a path between A and B, moving through time from time A to time B requires going through all points in time between A and B - and if you try to go backwards in time, you will run into something - namely, yourself - that blocks you just as a wall between space points A and B would.
My theory is, just as you can’t travel in space from point A to point B without passing through all points in a path between A and B, you can’t travel in time from time X to time Y without passing through all points in time between X and Y - and if you try to go backward, you will be stopped by something that was already there - namely, yourself - just as you would be stopped from going from A to B by, say, a wall somewhere on the path.
Whereas my theory is that in order to move from point A to point B in space, you must also move from A to B in time, and vice versa. I think it is consistent with the laws of physics to go from B to A (backwards) in time, but that would also entail everything - even your mental state - going exactly from B to A in space. Coupled with hard determinism, backwards time travel would be useless. Just like rewinding a movie - the characters won’t remember the “future” and when it is played back, nothing changes.
My personal theory, which preserves “free will”, is that any time travel immediately changes “the past”, which is now the new present. Nothing that happened between the 1979 of your arrival and now exists any longer. Lost, forevah lost! All those events, all those things, not only gone but never was.
And it can never be remade.
Even if your changes to the past were so small that on a macroscopic level no one can tell, you’re still not going back to the world you cane from.
Not only *can *you change the past in time travel, you MUST change the past. Your mere presence has changed things. For one, since the atoms in your body are already there in 1979, the total mass of the universe has increased. Congratulations, you’ve violated thermodynamics. But the universe doesn’t care. If it did, you couldn’t time travel. ipso facto.
Go ahead, kill Hitler. Kill your grandfather. Won’t change you - you’re still there. But now you have no connection to the current reality.
Almost all the laws of physics are symmetric in time, but some things (memory, volition) depend on time’s direction. Tommy Gold, Ph.D., F.R.S. proposed a model in which creatures from the very distant future would be traversing time in the opposite direction from us! (Agents from the far distant universe might have no way to arrive in ours — they’d first need to pass through an entropy maximum — but let’s ignore that detail.) This might provide a scientific basis for creatures like White’s Merlyn! (Physicists including Stephen Hawking once accepted the ‘Gold Universe’ as a possibility, but it is now believed to be contradicted by cosmological measurements.)
It is difficult to image how time-reversed agents could interact with the normal world. (Where would they even get food? The wheat in our fields would have the wrong causality direction to be used by Merlyn!) But if they could survive at all, they could communicate with us, though with difficulty. So we could send Merlyn a message: “When you find Hitler, kill him!” (Or “!mih llik ,reltiH dnif uoy nehW” if you will.) But how would Merlyn act on the request?
I fully expect to be ridiculed by the Board’s professional physicists. But I still have my asbestos suit on from last time.
(Please note IANA physicist, so a real physicist would probably say that I’m wrong, but I’d be interested to see exactly what mistakes I’ve made in the following essay).
Lets’s imagine some bizarre kind of reflecting device that could convert an entity like us from something going forward in time into something going backwards in time. Maybe it looks like a twist in space-time, possibly resembling a four-dimensional Moebius strip.
Under the rules of Charge, Parity, Time symmetry, an entity reflected backwards in time would be made from antimatter. So it would explode as soon as it touched anything made of matter. Ouch.
After passing through such a reflecting device, the entity concerned would be converted into antimatter, and would explode as soon as it met something travelling in the normal direction. Probably the first thing it would meet would be itself, before the conversion, travelling in the opposite direction and attempting to occupy the same space - and the result would be gamma rays.
Alternately, imagine that we get one of Tommy Gold’s backwards-travelling entities, and reflect it so that it was travelling through time in the same direction as us (so that we can communicate with it more conveniently); the first thing it would meet would be itself, going the other way, and it would annihilate. Bang!
Unfortunately, to my untrained eye, this method of time travel seems to result in the efficient creation of an antimatter bomb. Good for ordnance, not so good for temporal tourism.
Does it violate thermodynamics? If time (t) is treated as just another dimension (x,y,z), then would moving a mass from 2029 to 1979 be any different from moving the same mass across the room? You haven’t added or subtracted any matter or energy from the entire “universe”
No, but imagine a fleet of Tardises bringing refugees back from the Heat Death of the Universe to our time. They will increase the mass of our universe significantly.
Imagine that these refugees manage to live long enough to reach the Heat Death again - back they come in their time machines and repeat the process. Eventually the Current Era consists of 90% by mass of temporal refugees; this causes the mass of the universe in the Current Era to increase until it collapses in a Big Crunch.
Why stop at refugees? What if you brought the entire mass of the history of the universe to 2019? Although I suppose it would all collapse into a giant black hole where time and space are meaningless anyway.
There was no enormous crowd on the grassy knoll. Armies of faithful did not interrupt the crucifixion. There weren’t six million visitors to Max Yasgur’s farm. No welcoming committee came to Serenity Base.
Generalizing from mathematically describable quantum event states to multi-trillion molecule four dimensional transport events in two different temporal/spatial sets of loci are unrelated and entirely dissimilar levels of mathematical abstractions. Much of what can be mathematically described cannot exist.
If there could ever be time travelers, where the heck are they now?