In the many-branches model, though, we’re usually talking about effectively infinite universes. Given finite time tourists, you’d expect few or none to appear in any given universe.
Obligatory SMBC references:
The least impossible method of timetravel is probably the relativistically-displaced wormhole concept. Here’s a quick explanation.
But you don’t start to get ‘time travel tourists’ until after the first time-travel wormhole is invented. In fact the universe which allows time-travel-wormholes exists in two very different states; the normal state, before time-travel-wormholes are invented, and causality is conserved; and the abnormal state, in which time-travel tourists are everywhere, and causality is an endangered concept.
One of the ways to resolve this is that if you build such a wormhole, since an infinite number of possible time travelers could come out (not travelers in the sense of full human beings in practice, just subatomic particles), you do start to get an infinite flow of travelers. The catch is, every particle the wormhole emits costs it mass-energy, so what would actually happen is that the instant you put the wormhole into a configuration that permits time travel, a flood of light comes out of it and it basically just explodes basically instantly.
I’ve often wondered that (actually it would not just be doubling, because not every choice is binary, and not every choice is independent of others - it would be increasing infinitely at every clock tick.
Or, there’s one universe that contains stuff - our perception of that stuff being arranged into time, space and the precise possibilities we have experienced, is just a viewpoint - an interpretation of the ‘stuff’. Had we made different choices, we would be experiencing the same stuff, in a different arrangement - or maybe those different arrangements of the same stuff are being experienced by different versions of us, simultaneously* (well that’s the wrong word, since the path of time is part of the stuff)
Simple really; the alternate arrangements of mass/energy manifest as what we currently call ‘Dark Matter’ and ‘Dark Energy’ - which will only be properly accessible to us when we invent time-travel.
The many-worlds interpretation of quantum mechanics only works if there’s no possibility of interaction at all, not even gravitational, between the “worlds”.
How about the practical approach: even if theoretically possible, time travel would be very hard to achieve. Hard enough to where it will never did happen. Sort of like me running a sub 2 hr marathon. Will never happen, regardless of theoretical possibility. (Frankly, neither will a 3 hr marathon)
I have a problem with the paradox being the reason for impossibility, though. The paradox resolved itself in that we wouldn’t know of the change. I invent a time machine, kill my granddad, and cease to exist. Now there’s just some people excised from reality, but the time machine makes the concept of before and after useless. At one point I existed, and then I didn’t. So what? For all we know we are missing a billion people who were around at one point and then never were.
If it were as simple as some people falling out of existence, there would be no paradox. But now ask, if you never existed, who was it who killed your father, and where did that killer come from? That’s where the paradox comes in.
How about this:
What we think of as the past *is *the sanitized version. There were vastly greater Evils perpetrated than we know about. Only small potato pipsqueaks like Hitler remain in our record. The time travelers have already fixed all the biggies.
Now *there’s *a sobering thought. Also makes me wonder who’s next to get the 'ol temporal eraser used on them. Is that somebody in our past or our future?
FTR I’m not making a serious proposal here. Just tossing out a wild conjecture for entertainment. Might make a good short story plot.
I came up with this theory from TNG Tapestry. I wondered, in Q’s little torture experiment on Picard, what happened to all the people whose lives were totally different in the time between Picard getting stabbed/not getting stabbed and the show’s present? Or the “time” (as it were) between the loss of the Enterprise C and the events of Yesterday’s Enterprise? Million, billions, of people live or die who did not before. So do their lives count for nothing? Does the universe not care about their hopes, dreams, adventures? Do they just disappear, poof!, never having been? It seems so unfair.
My theory is, there are not parallel universes, there is only one, and it is a the result of probability. There is no “correct” version of the universe. Every time a time travel change is made, the universe just shifts to this new setting.
There are no “souls” and no free will in this iterpretation. The universe, as it were, doesn’t care. We’re all just collapsed quantum waveforms, and the version where you get killed at birth, or become Hitler, are equally valid and equally possible and equally neutral.
The universe is a giant complex equation, and it is so complex that it rearranges itself and whole civilizations appear and disappear with no great impact. In this universe, there is no “time travel” as we understand it. The universe’s equation just bounces around a little.
We are not in the universe, we are the universe.
It’s a very depressing interpretation of the universe.
I haven’t read the whole thread, but I’m posting my thoughts anyway.
Assume space-time is a continuum. Since the Past has already happened, it can’t be changed. Since we’re in a continuum, the Future ‘already exists’ – like the number 1,276,598 exists in the continuum even though we’re living on 12. You can travel into the Future by slowing down your personal time (approaching the speed of light), but that’s about it.
Assume a temporal wormhole. If you’re in a single continuum, then the wormhole may deposit you in the Past or the Future, but nothing you do in the Past will change anything in the Present because whatever you did in the Past, you’ve already done. If you go forward, the Future is your new Present. If you hop forward a week and get the winning lottery numbers and then come back to the Present and play them, then of course you’ll win because you’ve already won in the Future.
But if there is a multiverse with an infinite number of possibilities, then you can ‘change the Past’. But in the universe you are now in, your changes already exist in that multiverse’s Future.
This will sound flippant, but not meant to be: If time travel becomes possible in the future, we would have heard of it by now.
Frankly, after decades of hearing various versions of this debate, pro and con, immutable vs. multiple-world, on and on, etc. etc, I’ve adopted a mantra given to me by an expert in the field:
“Wibbly wobbly. Timey wimey.”
Like I said - Heisenberg’s universe… or worse yet, Schrodinger’s Universe. All possibilities exist until you find the dead cat.
the 2002 film version of The Time Machine kind of addressed it that way:
[spoiler]Alexander invents the time machine after his fiancee was shot and killed by a mugger. He goes back and prevents the murder, but then she’s struck and killed by a carriage shortly thereafter. realizing no matter what he does she will still die at the same point in time, he travels around time to see if he can find a solution. accidentally traveling thousands of years into the future. the “Uber-Morlock” explains why there’s nothing he can do to save Emma:
“You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you.”[/spoiler]
Basically, Vonnegut’s depiction of time in Slaughterhouse 5. Free will is a human construct. Time/history are immutable, everything that will happen in the future is already set. So, even if you invent a time machine and go backward or forward, you will change nothing because everything will always be as it has always been, including your travels and any butterflies you crush. Want to take a machine gun to Calvary? Fine, but you won’t do anything with it, because a machine gun is never used at Calvary.
I like this because it is simple, and it comports to predictable mathematics. Once something is set in motion, it will not alter its course until it is acted upon by something else. Given the human mind is the product of the arrangement of physical things (neurons, chemicals, etc.), even human behavior ought to be predictable IF we know all the connections, what they mean, and what stimulus is coming–there will be no free will, the “mind” will respond according to its construction. The concept of entropy is just intellectual shorthand for, “I didn’t know all the variables and/or couldn’t work out the math.” There is no reason to suggest anything random happens in the universe, simply because there are too many moving parts for our simian noodles to keep track of. With no random actions, there are no multiple possibilities on the timeline.
Or deliberately, as Asimov did 62 years ago.
But since the time tourists would be definition be returning to a different universe than the one they came from, I doubt it would be too popular a trip.