# Backwards Time Travel

• B
• MikeeMiracle
MikeeMiracle
TL;DR Summary
Trying to get my head around travelling backwards in time / affecting my causual past.
I've got a good conceptual understanding of relativity (not maths based,) but the one thing which I can't quite get my head around is being able to influcence my casual past. I can picture being able to leave my light cone with faster than light travel, but not to the point where I could go under my hypersurface of the present.

Is it because if I could travel multiple times faster than light I could in effect leave / break out of my "light cone" and travel "backwards" parallel to the outside of my light cone and then re-enter my "past" light cone. I assume this would need me to be traveling a significant amount of times faster than light to achieve if this is possible.

Any light cone / spacetime diagrams explaining this would be much appreciated.

Mentor
If you have mass then you cannot go faster than light.

MikeeMiracle
I know that I nor anything else can go faster than light. I am trying to get my head around the concept if you could travel FTL. One of the contributers here on the relativity forums wrote a while back the following.

"If you could travel faster than light then you could affect events in your casual past."

I don't understand how that is possible theoretically, hence the post to try and get an understanding of that theroretical statement. Unless you are stating that the statement was wrong to begin with?

I understand that for events outside my light cone we can't even agree if an event happened in the past, present or future. Hence why for me to affect my casual past, I believe the only way would be to leave my light cone and re-enter it or send a light signal that can re-enter my past light cone.

Mentor
I don't understand how that is possible theoretically,

MikeeMiracle and jbriggs444
I generated this image to show how you could get to your causal past if you could travel faster than light. It may be a little bit misleading, because it assumes that the laws of relativity apply, except you can go faster than light, which is a logical contradiction. But I think it shows why people say, "If you could travel faster than light then you could affect events in your casual past."

MikeeMiracle
Gold Member
I generated this image to show how you could get to your causal past if you could travel faster than light. It may be a little bit misleading, because it assumes that the laws of relativity apply, except you can go faster than light, which is a logical contradiction. But I think it shows why people say, "If you could travel faster than light then you could affect events in your casual past."View attachment 264732
Sorry, there's a slight error in your diagram. The line labelled "A's 4-velocity" should be tilted clockwise from vertical, not anti-clockwise.

MikeeMiracle
Thanks phyzguy, that diagram helps. I have seen the triangle to the left of your number 2 before but it was on a single diagram. Drawing it in 2 separate diagram makes more sense now.

Sorry, there's a slight error in your diagram. The line labelled "A's 4-velocity" should be tilted clockwise from vertical, not anti-clockwise.
I don't understand your comment. Doesn't it just depend whether A is moving toward +X or -X?

Gold Member
I don't understand your comment. Doesn't it just depend whether A is moving toward +X or -X?
If A's 4-velocity is as you drew it, then "A's surfaces of constant time" should be tilted the other way, i.e. clockwise from horizontal.

2022 Award
I don't understand your comment. Doesn't it just depend whether A is moving toward +X or -X?
Your axes are rotated in a Euclidean sense, not "scissored" in the Minkowski sense. It captures the effect, but the details aren't right.

Your axes are rotated in a Euclidean sense, not "scissored" in the Minkowski sense. It captures the effect, but the details aren't right.
I see what you mean now. Let me try and fix it.

Thanks to @DrGreg and @Ibix for pointing out the error in my diagram. I think this is now correct (let me know if not!). The conclusion is the same as before.

DrGreg and Ibix
MikeeMiracle
Thanks all, the whole concept seems even more far-fetched than before now with Steps 1 & 3 featuring some kind of instanteneous "jump." Doesn't this instanteneous jump mess about with Relativity of Simularity? This seems to be getting too theoretical even for theoretical physics :)

2022 Award
Thanks all, the whole concept seems even more far-fetched than before now with Steps 1 & 3 featuring some kind of instanteneous "jump."
Any motion faster than ##c## is instantaneous travel in some frame. In other frames, it can be even faster.
Doesn't this instanteneous jump mess about with Relativity of Simularity?
That's kind of the point, yes. But the anti-telephone effect works for any super-luminal motion. It's just easiest to illustrate in the "instantaneous travel in some frame" case.

Mentor
Doesn't this instanteneous jump mess about with Relativity of Simularity?
Any faster than light travel does. Because of relativity of simultaneity , the beginning and end of a FTL journey will be simultaneous in some frame.

Mentor
Thanks all, the whole concept seems even more far-fetched than before now with Steps 1 & 3 featuring some kind of instanteneous "jump." Doesn't this instanteneous jump mess about with Relativity of Simularity? This seems to be getting too theoretical even for theoretical physics :)
In post 2 I clearly said that it was impossible. So of course it is far fetched. The instantaneous jumps are not even remotely the main problem. The actual problem is that something with mass cannot go faster than c. It seems the height of silliness to be told that something is impossible, demand to discuss it anyway, and then complain about the most benign part of the whole farce.

MikeeMiracle

I agree the whole thing is far-fetched. But there are lots of discussions of things like wormholes or Alcubierre drives that allow you to move to a point outside of your light cone without ever locally exceeding the speed of light. So I think the point is that if you somehow could do this, it would allow you to travel to your past light cone. To me this makes it seem that such things (wormholes or Alcubierre drives) will never be possible in our universe.

Mentor
there are lots of discussions of things like wormholes or Alcubierre drives that allow you to move to a point outside of your light cone without ever locally exceeding the speed of light.

You can't move to a point outside of your light cone without locally exceeding the speed of light. Wormholes and Alcubierre drives do not involve anything moving on spacelike worldlines. They involve spacetime geometries that violate certain conditions that we intuitively think should never be violated.

Mentor

Done.