Why Time Travel Into the Past is Impossible

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Many people fantasize about a "time machine" being invented one day where you could just hop in and go anywhere in time, like say go back to the Civil War, watch that for a bit, then go back to the present. I, in fact, as a child fantasized about BEING the inventor of it!!

But I realize now that time travel into the past is impossible. Why? Because if time travel into the past were possible, it would become possible to travel into the past, kill the person who would invent the time machine or stop him/her from being born, then the person would not have invented the time machine at all, but since the time travel had to occur for the time machine's inventor's existence to be negated, time travel would have both occurred and not occurred at the same time, which is a contradiction. So time travel into the past is impossible.

It doesn't even matter if everyone's careful not to let this happen, because it's still POSSIBLE for a contradiction to occur if we could time-travel into the past.

A sort of "time travel" into the future is possible due to relativity, but it's a one-way ticket to the future. You cannot go back, or else it's the aforementioned impossible time travel into the past.

depends on how time functions.

mind you i dont really think time travel as depicted in fiction is feasible in any way shape or form, even relativistic timetravel requires far more energy than we have mustered througout our existence, i am also agnostic about the many worlds theory, it is interesting but utterly untestable with todays technology.

that said, if time follows the many worlds theory then in essence every choice ever made exists in all its permutations, this means there would be a virtually endless amount of parallel universes, so when you time travel you are in essence jumping to another past, almost identical to yours, if you killed the inventor of time travel there then only that universe would be without time travel, since your universe still exists as you left it no paradox occurs.

other cultures far older than our own, believe that all that ever happened in time, and all that is to manifest in time as a 'happened" event, already exist preserved for all time.

With the grandfather paradox making one kind of time travel into the past impossible, and other theories on time travel so far out of our reach, is it possible that the past still exists? That it's out there somewhere and not just gone forever?

I would so like to travel back in time millions of years to meet the first bi-pedal apes and early hominids.
Forget about traveling to the future. We're going there anyway, right?

Not if you keep going...

So basically what you're saying is if I invent a time machine (proving my original post wrong in the process), go into the past, and kill my past self, then the time/decision trajectory where I DON'T kill my past self still exists, and I can return to the time/decision point where I was.

But this would also mean that I can time-travel "sideways" to a point at this SAME TIME, but a different universe, where instead of, say, calmly sitting at the computer at 1:30 AM posting about time travel on Wrong Planet, I had instead decided to, say, drive to a bar, get drunk and then attempt to drive home, and gotten busted for DUI or even collided head-on with a car loaded with kids. Basically the reverse of how the machine's use for mistake correction would go - travel sideways from a bad decision to a good decision.

This many worlds theory is very interesting, but we're far from sure that it's real at this point...

Wouldn't that be a sideways paradox? The universes would no longer be parallel if a change occurs on one. There would have to be an infinite number of universes. One created at every point in time that a change caused the universes to diverge.

There is a great story, I don't know the author's name, about time travel. It's called 'A Walk in the Park with Mozart.'
A Walk in the Park with Mozart

It can be read at the above link and goes into minute detail from a UW professor about time travel.

that is esentially what the many worlds theory is,

it supposes that any choice splits the universe into copies whenever a choice is taken.

as said it is far out by any measure.

there is also ways to fold space time where the very concept of time ceases to exist, that theory can even explain how the big bang started, i dont know it well enough to explain it or convey its full meaning.


@OP

i dont know if one would be able to travel into another choice, since your very choice of travelling in time in essence is the choice that seeds the split,
it also changes shroedingers paradox at a very fundemental level, renders it mute so to speak, since the cat is quite literally alive and dead at the same time, only in different timelines, perhaps ones reality is chosen by consensus with a major overlap between timelines, so in one universe you open the box and since you expect the cat to be alive and there is a cat that is alive the two congregate to form a solid reality, perhaps the inherent instability of the universe is just that, all of the last part here is nothing but mastubatory speculation though.

if nothing else it would make a killer novel.

**edit** strike that, perspective is irrelevant if the entire universe inclduing the people are copied.

It largely depends on the nature of space and time. Barring the multiverse theory for a moment, there is a theoretical way of traveling backwards in time and that would be through the use of wormholes (provided they actually exist). The problem though is that you would need to create one that terminated at a precise point in space and time that you wanted to travel to. Needless to say, this would be difficult since both space and time are relative and it would be hard to pinpoint the spot you wanted to come out at in the past. You could end up in the middle of a star because you calculated for a point on a planet that has moved due to the movement of the star system both within a galaxy and also due to the movement of the galaxy itself.

I would also postulate that were it possible, would we not see time travelers today? Then again, how do we know that they have not already interacted with us? In the end then it all comes down to faith and what you choose to believe since we do not know everything that has or will happen. This is one of those areas within science where science and religion start to blend since there is no way of absolutely determining the facts, only what we cannot do today.

Time travelers? They could be lurking on WP.

The "grandfather paradox" can be resolved by assuming time (and causality) is not completely linear. If there were only one time line then backward time travel would imply a paradox. But suppose there is an entire manifold of "time lines". Then going back to an event and altering it would produce an alternate path in the time manifold.

ruveyn

So there would be an alternate path in a time manifold, as apposed to infinite copies of universes as is implied by the many worlds theory. That would certainly save a lot of space in the grand scheme of things.

What about the idea of going back in time, through a wormhole or whatever, and rather than the choice splitting the universe into another copy, your presence there would create a superimposed reality bubble of just that particular time and space and you can interact within the environment in the bubble, which would cease to exist after you leave, leaving everything unchanged.

I would call this the alternate reality bubble theory.

All speculation aside, retrograde time travel is not possible because it would violate the principle of causality: An effect must occur after its cause(s). This is the same reason why faster-than-light (FTL) travel is not possible -- you can not arrive at your destination at the same time or sooner than would a photon sent toward your destination at the exact moment of your departure.

Teleportation? Sure ... ... for you, the trip would be (nearly) instantaneous; but for an outside observer, your trip would take at least as long to get to your destination as you would have if you had been traveling at (or nearly at) the speed of light.

Anything else -- parallel universes, divergent timelines, et cetera -- are merely speculative at scales above the Planck or Quantum level.

@ Fnord:
"Women must remain silent..." -- The Apostle Paul, a Fundamentalist Christian.

^^^^^ What do you mean by this?????

I have seen so many shows about time travel. I am surprised no one has mentioned the Flux capacitor/BTTF triolgy. But in all of these movies there are different ways the characters start to do time travel. In one movie they go into a telephone booth and can do it. A common concept in most of these is that you can't let your past self see you, their future self;

But if there is time travel in the future, why haven't those people traveled back into the past to stop world wars/holocaust from happening? If they did do that, would all the people who died from those events be magically alive again? What would they be thinking?