How exactly does FTL travel cause reverse time travel?

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Discussion Overview

The discussion revolves around the implications of faster-than-light (FTL) travel, particularly focusing on whether it could lead to reverse time travel. Participants explore theoretical frameworks, including Minkowski diagrams and Lorentz transformations, to understand the relationship between FTL travel and time manipulation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about how Minkowski diagrams imply FTL travel and its connection to time travel, suggesting that they can only visualize movement along the x-axis or forward in time.
  • Another participant argues that the Lorentz transformations indicate that if speed exceeds the speed of light, time intervals could become negative, suggesting a reversal of time, while acknowledging the contradiction with relativity.
  • A different participant defines an FTL jump drive as a method of instantaneous travel that is unaffected by relativistic effects, questioning the implications of such a drive on time travel.
  • One participant explains that as velocity approaches the speed of light, time effectively stops, leading to a state where no time passes for the traveler, and speculates that exceeding the speed of light would result in negative time intervals.
  • Another participant challenges the mathematical reasoning presented, pointing out that the square root of a negative number results in an imaginary number, thus questioning the validity of the earlier claims about time travel.
  • One participant introduces an analogy using sound to explain potential visual effects and causality issues that could arise from FTL travel, such as Cherenkov radiation and the concept of sonic images.
  • A participant revisits their earlier post to clarify their intent, stating that they do not see how FTL travel would lead to reverse time travel, using an example involving a spaceship and the moon to illustrate their point.
  • Another participant presents a theorem suggesting that if FTL travel is possible in one frame, it implies instantaneous travel in another frame, which could lead to back-in-time travel.

Areas of Agreement / Disagreement

Participants express a range of views on the implications of FTL travel, with some supporting the idea that it could lead to reverse time travel while others challenge this notion. There is no consensus on the validity of the claims made, and the discussion remains unresolved.

Contextual Notes

Participants reference mathematical concepts and transformations that may depend on specific assumptions about the nature of space and time. The discussion includes unresolved mathematical steps and varying interpretations of relativistic effects.

  • #31
It doesn't matter if you talk about travel, information transfer or any kind of causal relation.
Observers can not agree on the chronological order of space-like separated events, so a common sense interpretation is that they can not be causally related.
If you allow some space-like separated events to be causally related, you must by extension allow all of them to be causally related, in either direction, because there's nothing special about any pair of space-like separated events.
If you allow all space-like separated events to be causally related, you must allow time-like separated events to be causally related in reverse chronological order, by means of a third proxy event which is space-like separated from both.
 
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  • #32
georgir said:
It doesn't matter if you talk about travel, information transfer or any kind of causal relation.
Observers can not agree on the chronological order of space-like separated events, so a common sense interpretation is that they can not be causally related.
If you allow some space-like separated events to be causally related, you must by extension allow all of them to be causally related, in either direction, because there's nothing special about any pair of space-like separated events.
If you allow all space-like separated events to be causally related, you must allow time-like separated events to be causally related in reverse chronological order, by means of a third proxy event which is space-like separated from both.
georgir said:
It doesn't matter if you talk about travel, information transfer or any kind of causal relation.
That was part of my point - and the reason I was free to substitute information transfer for actual material travel.
georgir said:
Observers can not agree on the chronological order of space-like separated events, so a common sense interpretation is that they can not be causally related.
But for the purpose of the exercise, I presumed that one caused the other. To be specific what we are calling "cause" is the original source of the information. Since we are addressing a "what if" question, I wasn't going to challenge whether the experiment was actually possible.
georgir said:
If you allow some space-like separated events to be causally related, you must by extension allow all of them to be causally related, in either direction, because there's nothing special about any pair of space-like separated events.
Okay, but for this exercise we are presuming that certain ones have been created by our FTL transmitter - so they are special.
 
  • #33
.Scott said:
But for the purpose of the exercise...
My post was just answering the OP, don't take it personal ;) I completely don't mind and don't care what exercises you do in your own head...

I do see an interesting point in your reply, inadvertent or not... A single "special" pair of causally connected space-like events, or even a whole class of such pairs matching certain restrictions does not lead to reversely-connected time-like separated events.

The exact restrictions are interesting... the pairs will have to be parallel, or one-directional, or something else that does not allow them to form a "proxy".
 

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