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If people were to travel back in time, change something, and return to their original time, this would create a paradox for the time traveler.

To give a completely hypothetical example, consider this situation: I have been taught in school about the Galipoli campaigns in WW1 and ANZAC day where there was great killings for Australian and New Zealand troops. Say I was appalled by this and afterwards invented a time machine, went back to 1914 and managed to stop the ships from landing at all, thereby avoiding the catastrophe that was ANZAC day. Now when I am at school, I won't be taught about this, because it never happened and because I did this after I was taught, then I won't go back in time to stop it, therefore the tragic events of ANZAC day would occur, making a paradox as you can see. Hence, my question is, how does Time Travel actually work, because all I can see is that it would result in paradoxes for the time traveler.

This situation is similar to this question on SciFi and Fantasy Stack Exchange, but different in that I am asking about these sort of examples where the events changed could not logically change the memories of the time traveler. So my question is what is the solution to this apparent paradox?

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    There's no solution to a paradox rising from a logical and actual impossibility! That's ABC of logic! – infatuated Mar 13 '14 at 9:28
  • @infatuated - does this mean that the fundamental principle of Time Travel stories (where one character changes something in the past) are logically flawed? – Often Right Mar 13 '14 at 9:30
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    Exactly! Time Travel is just a fiction nothing more. – infatuated Mar 13 '14 at 9:32
  • @infatuated Not all notions of Time Travel (why are we capitalising this?) are logically inconsistent. This need not be logically inconsistent either - one could propose that there are two "timelines", one where the intervention happens and one where it does not, and that they "cross-over". I have no reason to think that there are these multiple timelines, but establishing their (non) existence is not an exercise in logic. – Lucas Mar 13 '14 at 10:44
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    See in SEP the entries on Time Travel , Time Travel and Modern Physics and Time Machines : "There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is logically impossible." – Mauro ALLEGRANZA Mar 13 '14 at 11:47
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First let's recall that, whatever what is time, we are all time traveler, but humans, as far as I know, only travel in one way.

Now if you give yourself the hypothesis of backward time travel, that is going into a context where your perceptions would give you the expected inputs of a world matching your representation of a past world, then there would be two theory I'm aware of.

The first one stands that this world and the future from which you came from are binded, so you would not be able to make anything that would change the future in a way preventing you from going back. In other words, the world as integrated consistency rules that would prevent anything like you killing your ancestor before you was born.

The second one stands that this world and the future from which you came from are evolving independently and what you will change here won't change the world you came from. In other words you can think this more like a travel into a parallel world that happen to be the same as the one you came from with a time-shift : how would you make the difference? Even if you would "return to the future", how would you make the difference between actually going to the changed future and going to a parallel world where things seems like your point of depart with differences expected from your actions "in the past"?

Those said, assuming you have no bidirectionnal time machine nor parallel world exploration device, you will probably make a better use of your ability to plan by trying to avoid future disasters. Of course you don't know the future, but the point of studying past disasters is to avoid their repetition in your lifetime: a very hard challenge, but probably still far easier than trying to avoid all past tragedies of mankind.

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Maybe the solution to the problem is to say, there is no problem in the first place.

  1. Time does not exist
  2. You cannot travel in something that does not exist.

What does exist however is change. Can you travel in change? No, because you yourself are change, part of change.

See it as a guitar - string that has been plucked and resonates or an echo chamber. But with the only difference that the echo or sound does not get muted but stays the same power, but only changes frequency and the echo chamber changing shape as a response to the frequency.

Could you go back in time then to a place where there was a certain mixture of frequencies and a certain shape of the echo chamber? Theoretically yes. All you'd need was a substantially good sound-system and some means of measuring the actual and desired sound and shape characteristics inside the echo chamber without having the sound system and yourself inside the chamber to begin with, otherwise it probably would not be the same, unless you could cloak yourself or make yourself and the system acoustically transparent.

The final thing you'd need would be some computer or device that could compute the necessary output frequency mixtures (properly positioned in dimensional space, no less), to get to where you want to be. Of course this needs to be transparent or non existent as well.

Really simple when you think about it. Just use some non existing equipment and change history. :P

  • hehe, that's why the notion of time travel seems ridiculous on its face. Yes, really simple just at first thought. – infatuated Mar 20 '14 at 8:21
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What if time is not actually a fixed, solid state, but in a self-correcting form of dynamic equilibrium? In the Ozone Layer, for example, ozone is created with high-energy UV light (absorbed by ozone) hitting oxygen molecules, and destroyed by ozone molecules colliding and breaking down back into oxygen. (I know it's more complicated than that, but that's enough to illustrate this concept.) If the ozone levels drop, ozone will be created faster and destroyed slower, bringing the levels back up, and vice versa.

If time works the same way (although much more complicated), then a paradox would automatically be corrected by events being reconfigured, so that the paradox would be effectively removed. It would be reasonable to assume that this would happen with the least amount of change to events, in the same way water breaks through a dam at the weakest point, say. Time would not be intelligent or alive, but it would be a feedback loop which avoided paradox by changing itself.

For the much quoted Grandfather paradox, where you shoot your own Grandfather and erase your own existence, making it impossible to shoot your grandfather, suppose you time travelled to two years before your father was born, shot your grandfather and killed him, then returned to the present. There you could find that time had rewritten itself to make your father having been born 15 months earlier (and is now 15 months older) with a story of how his father was killed before he was born. Therefore, no paradox. (Of course, you could easily be hit by a car, have a heart attack or brain embolism, or some other fatal 'accident' which stopped you before you killed your grandfather, so no paradox.)

In your ANZAC paradox, how would you stop the ships from landing? Telling the generals/soldiers? They probably wouldn't believe you, so no paradox. If you tried to sabotage the ships, a sniper might spot you and blow your head off, so no paradox. Or a bomb might go off as you were standing next to it, so no paradox. Any simple coincidence could make you fail, and no paradox.

I don't know of any proof for or against this 'dynamic equilibrium' theory of time, but I think it's interesting.

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The multiple parallel universe theory is the one that makes the most sense to me.

  • There is a universe where the event occurred as we know from history.

  • There is a universe where you appeared from "the future" and prevented the event. In this history, the event did not occur, or occurred differently.

If you travel "back" to the future, which universe will you appear in? I don't really think it matters. You might even appear in a third, different universe.

It can be useful to think of these different universes as forking from a common point. So they were in fact the same universe, up until the difference occurred (the moment when you arrived, or did not arrive), and only then they split into two.

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This is partly a physics question. If time travel is impossible, then the problem doesn't arise and whether it is possible is a matter of active debate among physicists.

If it is possible, then the question is "What happens when you travel back in time?" One possibility is that there would be some effect that prevents you from making changes that would affect the future to produce inconsistencies. Nobody has proposed any way of doing this.

Physicist David Deutsch has suggested, in the context of the many world interpretation of quantum mechanics, that if you travelled back in time and changed the past you would end up in a new universe in which events happen differently from the universe you came from. So in your example if you changed events so that Anzac happened then any people in school in that universe would not be taught about Anzac. If there was a version of you in school in that universe he would not be taught about Anzac, but you would still know about it since you had been taught about it in a different universe in which it actually happened.

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