# The reason why travelling back in time is logically impossible.

## Main Question or Discussion Point

If someone wanted to travel back to a certain moment in time, then logically every atom in the universe would have to be in exactly the same position in space as it was at that time, the energy level of the atoms would also have to be the same and the position of the electrons in the atoms would also have be the same. The problem with this is that the atoms that make up the person going back would not be in that position at that time so the person could not exist at that time, this also means that anything sent back in time (a camera for instance) could also not exist at that time.
If this is true then is any scientific program looking at time travel into the past a futile waste of time and money?

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mathman
Yes it would be a waste. However I am not aware of any such program.

I disagree. Your argument does not prove that it would be impossible to travel backwqrd through time, you just prove that it would be INEVITABLE that you travel through time

Relativity makes the idea of "At that time" rather complex. Two events being simultaneous depends on your point of view relative to those objects. I seem to recall descriptions of black holes that do describe objects within going back in time after they enter the event horizon, but they are impossible to view from outside the whole.

Is there actually a physical law that prevents a particle's timeline from looping back through the same event in time? Wouldn't it be impossible to detect if this happened because any particle is the same as any other of it's type?

then logically every atom in the universe would have to be in exactly the same position in space as it was at that time
Every atom actually is at its position at any time - with or without time travel. That's a matter of course.

The problem with this is that the atoms that make up the person going back would not be in that position at that time
No matter at what position the atom is - it is the position of that atom at that time. Therefore it is impossible that the atoms are not at their positions at that time.

I think your reason may be based on an unfounded assumption; but there are plenty of problems...

You're supposing that my sudden appearance in a past time is a problem because there was a certain correct configuration of things at that time, and my sudden presence there does not correspond or fit to that time's proper configuration... the particular past time I was supposed to be going to can't be the one I arrive in because I'm not supposed to have been there. My presence will have changed it. I'm displacing some space that would have been other wise occupied, and otherwise influencing things with heat and pressure and chemicals that would have not been there otherwise, not even to mention all the mischief my actions might provoke...

But is there really that otherwise uncontaminated proper past time?

If I did manage to go back in time, then maybe that "otherwise uncontaminated proper past time" never existed... my appearance is and always has been a proper part of that time. And wouldn't my sudden disappearance from the present when I begin my trip raise all the same concerns about the "otherwise uncontaminated proper present time" as my appearance in the past?

Once you begin to talk about changes in time ( the "otherwise uncontaminated proper past time" was altered when I went back so as to make a "new" version of past time), then conceptually you have to add another meta-time under which all the possible changes in time happen... and then you have to add another meta-meta-time layer to order the variations in the meta-time layer below... the whole thing stacks up and diverges wildly when you insist on changes in possible times.

So that leaves us back with me showing up by surprise in the past... do we need backward causality to account for this?

The backwards causality problem is that we attribute causality to our past; the time traveler has a more complex version of this problem... before his travel back in time, he has a past full of causes and memories, but at the point where he arrives in the past, all those causes are now in his future, but may still be in his mind.
If the traveler keeps his memory, he know his future when he arrives in the past, yet when he lived that span before traveling he did not know his future... this is another contradiction that requires another layer upon layer of meta-time hierarchy that diverges.

Concider time travel as a hole and meeting yourself in the future(for argument sake): - the time traveler meeting himself in the future+ the time traveler going back in time and waiting for the moment of seeing himself, meet himself..... The moment when the traveler going back in time has reached the future (without time travel this time) of experiencing the time travel event of seeing himself, there are suddenly three "me" at the same platform in spacetime. Thus not identical events, in an identical universe anymore.

Yet, this is all supposed to operate in the same universe. But it can't operate in the same universe because the future event, is no longer the future event, because it's no longer the same event.

This violates both scientific and logical principles.

there are suddenly three "me"
I just count two. The first went to the future by time travel and the second took the long way. Where does the third comes from?

I just count two. The first went to the future by time travel and the second took the long way. Where does the third comes from?

The third is the time traveler revisiting the future moment of seeing himself meeting his future self.

The third is the time traveler revisiting the future moment of seeing himself meeting his future self.
No, thats the second one. #1 goes to the future, meets #2 and returns to the past. Than he waits, becomes #2 and meets #1.

Who is the third one and where does he comes from?

No, thats the second one. #1 goes to the future, meets #2 and returns to the past. Than he waits, becomes #2 and meets #1.

Who is the third one and where does he comes from?

I suppose you mean that he will be physically predetermined to later go by time travel again.. and the event would unfold itself identically. It would probably result in a paradox, though.

I don't have time to explore it at the moment. My logic course is hard as it is!

I suppose you mean that he will be physically predetermined to later go by time travel again..
#1 travels to the future and returns. There is no need for additional time travels.

and the event would unfold itself identically.
The event actually is identical.

It would probably result in a paradox, though.
Only if you include a paradox from the beginning.

It sound like the problem posed is that of entropy. That the atoms and matter have moved to another state and the assumption is that entropy would have to "reverse" to it's original position "back in time"?