Time travel to the past is not possible

Great Debates
1

Here’s a thought I had a couple of days ago regarding time travel. Most likely it has been thought of by others, but I’m curious to see what Dopers think the holes in the reasoning might be, if any.

Basically, it seems to me that there can be only one 1995. Not two. You can’t have a 1995 “before” some event happened, and a 1995 “after” that event happened.

The reason is that “before” and “after” only make sense for things that operate within time. They do not make sense for time (or space-time) itself.

So, if you invent a time machine and go back to 1995 and visit yourself, you will have changed 1995, since at the moment, you don’t remember 1995 as a time when you met your future self.

If time travel to the past were possible, there would be two 1995’s. The “first one”, “before” you invented your time machine, in which you went through the whole year without meeting your future self, and the “second one”, “after” you invented your time machine.

If you look at space-time as an “object” that simply exists (it does not change, since change would imply time passing, and time does not pass for the space-time “object” itself), then space-time can contain only one 1995. Since you know that you never met yourself in 1995, you simply cannot travel back to that year and meet yourself.

This relates to a pet-peeve of mine, and that is when people say that something “moves through spacetime”. You cannot move through spacetime. You have a trajectory through spacetime, and that trajectory describes your motion through space, as a function of time.

Anyway, the point is that there is no “later” of “after” for spacetime. So there is no new 1995 “after” you invent your time machine. So, you can’t travel back.

Any thoughts? Any holes in the above reasoning?

2

I believe what you are describing is the theory of parallel universes.

3

What? No he isn’t. He’s explicitly disavowing such thinking; he’s describing the theory of just one universe.

To the OP, I share your basic perspective, including your pet peeve, but I disagree with the conclusion that time travel to 1995 is not possible.

It remains perfectly possible for me to go to 1995 and open up a pie shop in Chicago… as long as, in fact, I did pop in Chicago in 1995 and open up a pie shop. (Of course, if I did not pop up in Chicago in 1995 and open up a pie shop, well, then, I can’t use my time machine to do so.)

So, sure, I can’t go back and talk to my ten year old self, because I remember that never happening. But that doesn’t mean it’s impossible for anyone else to, at age 10, be visited by their future self, and then at age 20 go back in time and make it happen.

4

Sorry, it’s late. :slight_smile: What I meant was, I believe his theory is best opposed by the theory of parallel universes.

I don’t know how it was relevant, really. I suppose it’s saying “yes, someone thought about that, and came up with this theory.”

5

There are a lot of things you did not do in 1995, so if time travel to 1995 were possible, you could go back to 1995, but could not do any of those things.

But how would that be possible? You find yourself back in 1995 but something is somehow preventing you from opening a pie shop or meeting your former self? How would that work? You would be physically prevented by some force from visiting a park your former self happened to be in on a certain date?

I don’t think this would be possible. It is more reasonable that you simply can’t go back.

1995 is what it is. Whatever happened in that year happened. It can never change.

6

I’d be prevented by the force of logic, same as prevents me from both opening and not opening a pie shop in 2016; this doesn’t seem a markedly unsatisfactory preventive force to some people, but it seems like the most fundamental imaginable impediment to me.

“1995 is what it is. Whatever happened in that year happened. It can never change.” I agree. The only thing is, maybe interesting things happened in that year which correlate in the appropriate ways with my stepping into a fancy machine tomorrow.

7

The logic here does not prevent time travel into the past - it prevents time travel into the past that’s already happened. In other words, it does not preclude, say, a time machine that can only travel back in the past as far as the time in which it was invented. It just hasn’t been invented yet. Maybe the time machine will be invented tomorrow, and at the same time we’ll be inundated with hordes of time tourists from the future. Who will, of course, not have free will since they traveled to the past because they had to - because in their past, history shows that they did travel back.

8

Er, I meant to say “this seems a markedly unsatisfactory preventive force to some people, …”

9

With absolutely no background in physics, I feel comfortable saying that time travel into the past is likely to be impossible simply by observing that we haven’t seen anyone from the future.

Also, isn’t there some kind of law preventing the creation of matter/energy? Wouldn’t you be creating matter if you went back in time? Seems like the universe would have about 215 extra pounds of stuff that didn’t exist before, if a future version of myself showed up.

10

In point of fact, time travel is explicitly possible through relativistic theory, as I understand things. Not horribly simple, but possible.

First, take two ends of a singularity. Have them in roughly the same place. Say, Hicksville.

Second, place one end in a rocket ship. Fly at relativistic speeds to a location a few hundred light years away.

Fly back.

The resulting time dilation will result in the singularity on board having experienced somewhat less time passage than the singularity that remained in place. As the two ends are hooked in a one-to-one ratio, entering the end that stayed on Earth will send you back in time.

(Of course, the question of entering the singularity through a weak zone and surviving is… dubious, but it might be technically possible to send photons through it.)

At any rate, the original post seems to assume the consequences, and then state them as fact. If it is possible that time can not be changed, then time can not be changed. On the other hand, perhaps it can.

Really, the question of paradoxes inherent in the system depends very much on how the universe reacts to their creation. For example, it is equally possible for the entire matter to be one of perspective. Instead of attempting to view all of space and time at once, focus on the traveller, from 2010 to 1995. He is now in 1995, and 2010 is the future. He is no longer part of the future, simply a result of it. Thus, if he goes and kills himself, why would he vanish? He exists. The new 2010 that follows from his insertion point would not be identical, but the point of time travel is that is violates simple casuality.
Of course, the question is, what would happen if you killed your 1995-self, then returned through the singularity? I would suggest that you might re-enter your previous timeline, where you had not killed yourself. Fascinating.

On the other hand, it doesn’t do it in a fashion any more difficult than a for-do loop in programming. The result is a sum of all loops previous. The previous loops don’t have to exist after you pass it.

11

Just for interest, it’s well worth reading ‘The Time-Traveller’s Wife’ which deals entirely with this theory. Not only does the main character visit his future wife in the past, he also does visit himself - and the story holds together beautifully.

12

[QUOTE=Mosier]
With absolutely no background in physics, I feel comfortable saying that time travel into the past is likely to be impossible simply by observing that we haven’t seen anyone from the future.

How can you be 100% certain of that?

For all you know that guy who passed you in the street yesterday was in fact from 2010.

13

See what I mean - it’s simple!

14

Time travel to the past is indeed possible, and easy. I will demonstrate by travelling backwards in time a minute or so, then posting again in this thread.

15

you can load a previous saved game of The Sims anytime, but you’ll have to be a programmer to extract a copy of your sims character and itsert it into the old saved game. (as opposed to merely loading up 1995 without the time traveller)

16

The Time Patrol tries to keep the Global Standard Timeline standard. This way we don’t have any issues like that time the pyramids in Egypt needed to be eradicated (complicated story but it prevented something really bad in 2655) and everyone was going around saying “What’s that thing on the back of a one-dollar bill?”

If history makes sense, thank the Chrono-Guard. If it seems to not make sense, just write your observations down someplace permanent. They will be reviewed sometime and acted upon.

17

There’s various ways to get around that. For example, it might require something like a wormhole, which would only allow travel back to the creation of the wormhole. Or, everybody dies fairly soon, and there’s no one left to build anything. Or, it might be possible but fatal. Or, it might be possible but impractical, or so difficult that by the time our descendants can do so, they have no interest in showing themselves to us.

Or for more speculative ( alright, wildly speculative ) ideas : Time might be self-healing; travel back in time, do what you want, and when you leave the effects are erased. Or, time has a speed, and the effect and existence of a time traveller are therefore limited to a “travelling moment”, with the rest of time before and after being unaffected ( sort of a linear version of time travel creating a parallel universe ). Or, the past might not exist any more; you can travel back, put there’s nothing there ( probably unhealthy ).

You’d only produce the illusion of the creation of mass-energy. In the larger, four dimensional, time + space perspective, you’ve just moved it around some.

18

This can be answered if we go with the notion that you cannot travel back further than when the time machine was invented.

For instance, take E-Sabbath’s example of zooming around with a wormhole as a means of time travel. The end of the wormhole in the “past” is anchored at the moment it was created (actually moving forward through time itself as well…just slower than the “future” end). As such the would be timetraveler cannot go back to some arbitrary time in the past but only to when the wormhole was created.

Which, as alluded to, one way you may know you made a workable time machine will be the appearance of dozens of future people popping up to witness the moment of the creation of the first time machine. :slight_smile:

19

What if the universe reconciles itself after a while? Remember Back To The Future, when Marty McFly’s siblings started disappearing, then his hand started to fade? If there is a lag between the reconciliation, then it’s possible to change the past or future without resorting to parallel universes.

So, in your example, you don’t remember visiting yourself. You go back and visit yourself. Then, after a while, you remember visiting yourself.

20

The “Grandfather Paradox” is unusually invoked to show that Time Travel, as in the simple sort imagined in science fiction stories, can’t happen – otherwise, you;d be able to go back into the past to kill your own grandfather, thereby preventing you from building a time macine and going into the past in the first place. One way people wriggle out of this is to use a variation of the “Many Worlds” interpretation of Quantum Mechanics, and say that maybe if you go into the past and change things, you’ve essentially created or gone into a new branch, a new World line.

Robert Forward, physicist (and science fiction author) didn’t buy this. He said he’d believe the Grandfather Paradox if it could be proven or demonstrated in a scientific manner. He went so far as to write his own time travel novel, Time Master, in which his hero could go back in time, but that anything he did didn’t alter the past, because it was irrevocably part of the past already, and there’s only one past, without any of this man Worlds, Riemannian Manifold nonsense. Interesting book.
Other people have taken this point of view, before and after Forward – “You can’t change the past because it’s alreadt happened”, but it raises Hob with the concept of Free Will. So far as I know, nobody’s written a time travel story explicitly to make this philosophical discussion, but I might just not know about it.

Some of this stuff is in Larry Niven’s essay “The Theory and Practice of Time Travel”, in his collection All the Myriad Ways. Also, look up Time Travel in The Encyclopedia of Science Fiction.