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My support for the hypothesis that reliable time travel will never be invented (or that humanity will be wiped out in entirety shortly afterwards):

If time travel ever will be invented, then it will, presumably, exist as long as our species maintains that technological state (or any other species we might come into contact with around that time will have it). Therefore, every being that lives, from that point on, will have the potential of travelling to any time. Given the truly mind-boggling number of beings this would entail, and the likelihood that some would return to other time periods, either for the purpose of researching history, during said history, or for artifact looting, or for any number of other reasons, this would mean that, say, in any given century, one person comes back to our time. Assuming that humanity exists for 1,000,000 years after this, that would mean that 10,000 people currently on earth (and, moreover, most likely at *any* given time) are from some point in the future.
In addition to this, there is the point that, at any point in the future, they might decide to add our time to their cooperative. If any in 1,000 generations has the option to do it to us, eventually, it becomes highly likely that such would occur. Indeed, even with the strictest guidelines governing use of time travel at any given time, the numbers of people visiting one time, from all other times, would probably make it somewhat difficult to miss, if they tried to hide it at all.

Thoughts? comments?

Jesse

^ That's sort of like the Fermi paradox, but with time travellers. However, the theory of time travel via wormholes has an answer to this--it would be impossible to travel to a time before the wormhole was created, and large traversable wormholes are unlikely to be a naturally-occurring phenomenon. So, unless aliens have already created some, we may not see any time travellers because none of them can get back to these early days before the first traversable wormhole was constructed.

NZAmoeba

Another fallacy with sci fi time travel is going back in time with the intention to change the past...

Say you want to go back in time 1 week to make sure your wife didn't get hit by a car or something. So you go back, save her, and return to the future... but its a future in which you had no need to go back in time, because your wife never died... so you don't go back in time... so your wife re-dies, so you go back to fix it, but then you don't bother because she's alive now, and re-dies as a result... and it brings the entire universe into an endless loop, dooming everything to unsuspectingly repeat the same week for all eternity. woo!

HeatherD

I also think that time travel is impossible, well maybe not truly impossible but totally useless.

I don't have time to go into the specifics as it's late and I have class early in the morning. I will get back to it later but one major part of my "thought experiment" relates to the fact that matter is moving in 4 dimensions. Unless your "time machine" can reverse (or accelerate) the motion through time of all the matter in the universe, time travel would be pointless. If you travel through time in reverse, the universe will be forward of you, if you travel forward ahead of the universe, you'll have to slow down to wait for it to catch up with you.

I'll have more when I have time

P.S. While the theory of relativity appears to give us the possibility of time travel, I don't think it truly qualifies as traveling forward in time. It is just that, compared to the rest of the universe, time is slowed for you.

Jesse

"Motion through time" doesn't even make sense as a concept, unless you postulate some sort of meta-time. Better to think in terms of frozen paths through spacetime, or "worldlines" as they're called in relativity.

Thinking about physics in terms of ordinary language is often misleading--the language of physics is math. And the math of general relativity suggests it is possible to have "closed timelike curves" in some cases, ie weird paths through spacetime which curve back on themselves and end up in a region occupied by an earlier section of the path. It is certainly possible that these unusual cases are ones where general relativity gives inaccurate predictions (just like Newtonian physics gives inaccurate predictions in cases where relativistic or quantum effects become significant), but thought-experiments alone are unlikely to demonstrate any internal inconsistencies in the theory itself.

Tom Sawyer

I think that if it were possible to come back into the past and change the future, we'd already know about it because someone would have mentioned it. Since they haven't, we can assume that they have not shown up. No matter what safeguards or "Prime Directive" type restrictions they put in place, eventually someone will break them and we'd have heard of it.

A black man who's angry about how his people were treated centuries ago could have come back and saved Martin Luther King. A Jew could have come back and shot Adolf Hitler when he was a child. A tyrant could have conquered the world while we were trying to fight back with muskets. A Christian could have had Jesus mention quantum physics during his sermon on the mount to shut us atheists up. Since none of these things has happened, it's easy to assume that the ability to make them happen cannot ever happen.

That's not to say that time travel itself cannot happen. All it means is that it cannot happen in a way that allows the time traveller to affect the future. It may be that the quantum thoery that every action creates its own reality is correct and by coming back in time, the time traveller creates a different reality that he can affect, but this will have no affect on our own reality. So by going back in time, he would immediately enter a different universe and never be able to return to the one he came from. Even if he didn't affect anything and went back to the future to see the people he left, they would all be waiting for him in the original reality, to which he would never return since he would travel into the future of the new reality and not the original, even though the two may be identical.

Stiletto One

Well, that can be self-consistent (multiple/branching universes)—the act of jumping back destroys (disconnects?) the timeline you came from, but allows a 'new' branch in which your wife didn't die.

On the other hand, almost all sci-fis seem to completely ignore the fact that the time traveller also existed in the time he was jumping back to. As in, they ignore the phenomenon of doubles.

My brain hurts.

Wounded King

I don't think most sci-fi ignores this at all, in fact it is central to a number of sci-fi plots. 'Twelve Monkeys' springs to mind as an obvious example of a recent film involving time-travel which had a character present as both an older and younger self. The 'Back to the Future' films similarly had many instances of a character being present more than once in the same time period.

So which sci-fi was it you were thinking of that ignores the issue?

Majestyk

If, time is a property of existence as opposed to a medium in which existence resides then, "time travel" would seem to be a misconception. Maybe we do not travel thru time but, are a manifestation of time. A wholly inadequate anology would be to say that we are not travelling down a river but, that we are the river. At leat that's the way I imagine it based on my very limited understanding.

Am I completely inaccurate?

trekbette

I just read a very good sci-fi book in which a sort of time travel was discovered--The Light of Other Days, by Arthur C. Clarke and Stephen Baxter.

They discovered how to open worm holes in other places for instantaneous communication (the "straight line is the shortest distance" idea). Someone accidentily dialed up the right place, but the wrong time and past time viewing was born. The characters could look at the past like it was a movie. The concept of privacy completely went away.

Haven't worm holes been theorized to actually exist, right now, in real life?