# Going back in space vs Going back in time

## Main Question or Discussion Point

Watch from 34:50 till Joseph Lykken speaks.

"In space I can go from New York to Chicago and then I can change my mind and go from Chicago to New York", he said.

I don't understand how that means moving back in space...Everything is moving, the earth is moving, the galaxy, the universe is expanding so how come that you can go back to the same point you came from? maybe he can go back to the city he came from but that doesn't mean that he went back to the same point in space. That New York he went back to is not in the same place as the New York he left.

What am I missing?

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## Answers and Replies

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DaveC426913
Gold Member
Watch from 34:50 till Joseph Lykken speaks.

"In space I can go from New York to Chicago and then I can change my mind and go from Chicago to New York", he said.

I don't understand how that means moving back in space...Everything is moving, the earth is moving, the galaxy, the universe is expanding so how come that you can go back to the same point you came from? maybe he can go back to the city he came from but that doesn't mean that he went back to the same point in space. That New York he went back to is not in the same place as the New York he left.

What am I missing?
You get to choose your frame of reference. The FoR he's chosen is the surface of the Earth.

In that FoR, he's gone from A to B and back to A.

There is no FoR in which he can do this with time.

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You get to choose your frame of reference. The FoR he's chosen is the surface of the Earth.

In that FoR, he's gone from A to B and back to A.

There is no FoR in which he can do this with time.
so he is not talking "seriously" in a sense.. I mean, you can't separate time from space unless you are going at the speed of light and it's only then that time stops.
so why don't we consider that moving BACK in space is exactly like moving BACK in time?

Edit:
by my last question i mean that they are both not doable. (or at least the same hard to do)

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DaveC426913
Gold Member
so he is not talking "seriously" in a sense.. I mean, you can't separate time from space unless you are going at the speed of light and it's only then that time stops.
so why don't we consider that moving BACK in space is exactly like moving BACK in time?

Edit:
by my last question i mean that they are both not doable. (or at least the same hard to do)
I don't understand your confusion. You can move back in space, you cannot move back in time. We do not have control over our movement through time.

phinds
Gold Member
2019 Award
"Back" is a very amorphous term in this context. For someone starting at A and going to B, then going from B to A, we use sloppy English to say he's going "back". For someone STARTING at B and going to A, he's NOT going "back". It is, as Dave said, a matter of reference frames and NEITHER of those travelers can move in a way that goes along the arrow of time in a "reverse" direction since it only points one way.

phinds
Gold Member
2019 Award
Edit:
by my last question i mean that they are both not doable. (or at least the same hard to do)
No, "hard to do" is not a correct description of travel in the reverse direction along the arrow of time. It is IMPOSSIBLE to do. You were correct in the first part of your statement. They are not both doable.

I don't understand your confusion. You can move back in space, you cannot move back in time. We do not have control over our movement through time.
What I understand from your post is that we are making some kind of a thought experiment in which we imagine the non existence of time so we can say that we can move back in space (move back to the same point we started moving from).

My confusion is about the concept of space-time continuum.. Space and time are connected.. To my brain it makes no sense that we can assume something about space without considering time..

Let me rephrase the question according to your answer.
You can move back in space, you cannot move back in time.
What do I do to move back in space?

phinds
Gold Member
2019 Award
What I understand from your post is that we are making some kind of a thought experiment in which we imagine the non existence of time so we can say that we can move back in space (move back to the same point we started moving from).

My confusion is about the concept of space-time continuum.. Space and time are connected.. To my brain it makes no sense that we can assume something about space without considering time..

Let me rephrase the question according to your answer.

What do I do to move back in space?
You are getting hung up on English words. USE MATH. You can go for point A to point B or from point B to point A and you can CALL then whatever you want as long as the math is right.

If you are talking aout the fact that point B has some different characteristics when you get "back" to it, then that's a different issue.

DaveC426913
Gold Member
What I understand from your post is that we are making some kind of a thought experiment in which we imagine the non existence of time so we can say that we can move back in space (move back to the same point we started moving from).
Well, when we say return the same "place", we usually implicitly talk about the spatial dimensions only.

But it is true, we cannot return to the same coordinates in 4D space time.

But it is true, we cannot return to the same coordinates in 4D space time.
That answers my question.
Thank you..... and sorry for not expressing myself well enough as English is not my native language.

Thank you, phinds.

DaveC426913
Gold Member
That answers my question.
Thank you.....

Thank you, phinds.
:grumpy: That was me!

phinds is always stealing my glory! :runs into bedroom, throws self on bed and cries:

phinds
Gold Member
2019 Award
:grumpy: That was me!

phinds is always stealing my glory! :runs into bedroom, throws self on bed and cries:
wimp :rofl:

I lol'd :rofl:

Matterwave
Gold Member
If you consider a set of space-like hypersurfaces which we call a plane of simultaneity (even though, these hypersurfaces are not planes) meaning that in our preferred coordinates, these planes all have t=constant on them, we may move along world lines in such a away as to "cut through" these hypersurfaces (we leave a dot in each) in a way such that the t-coordinate on them increases. We cannot move along world lines in such a way that we cut through these hypersurfaces in a way that the t-coordinate on them decreases.

However, we can freely move to the left or to the right. This is the lack of symmetry between being able to move in any direction you want in space, but only in 1 direction in time.

Think of sheets of paper being stacked up. These sheets can represent our "hypersurfaces" with t increasing going up. We can draw a world-line that goes from the bottom sheet to the top sheet in any random (as long as we don't make our lines space-like) fashion we want; however, we cannot draw a world line going from the top sheet to the bottom sheet, nor can our world line ever start to move down towards the bottom sheet.

I think the question would then be, why can't I trace the same world-line, but going in the opposite direction?

But it is true, we cannot return to the same coordinates in 4D space time.
This becomes almost painfully clear when you stop trying to visualize things happening in 4D space time. For something to happen requires both a space and a time for it to happen in. If you wrap time and space into spacetime, then for anything to happen or move in spacetime requires the invention of a new time co-ordinate.

Suppose you have a bunch of 2 dimensional creatures living in a 2 dimensional universe + time. They have 3D spacetime. If you broke their time up into frames like a movie and stacked them on top of each other in sequence you would have essentially turned their time dimension into one of our space dimensions. We see them as 3D objects which are static and do not move. If our down is their future and one of them asks "Why cant we move up?" we see the question as silly because we see them as 3D objects that have height, where as they see themselves as 2D objects that have duration.

I'm probably doing a very poor job of explaining this. If I have successfully communicated my thought then great, if not let me know and I'll try again.