Evidence that space and time has equal footing?

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Discussion Overview

The discussion centers around the relationship between space and time, exploring whether they can be considered equivalent or if their differences are significant. Participants examine theoretical implications, particularly in the context of general relativity and concepts like time travel, black holes, and the nature of spacetime.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that space and time are treated similarly in physical equations, questioning whether this indicates they are fundamentally the same.
  • Others argue that space and time are not the same due to their different signatures in the metric tensor.
  • A participant suggests that the behavior of time and space inside black holes, where time can become space and vice versa, serves as evidence for their similarity, though this is contingent on specific coordinate systems.
  • Concerns are raised about the concept of time travel, with some asserting it lacks evidence and may lead to paradoxes, while others mention theoretical frameworks that could allow for time travel under certain conditions.
  • One participant notes that the constancy of the speed of light leads to necessary adjustments in measurements of time and space, implying a deeper connection between the two.

Areas of Agreement / Disagreement

Participants express differing views on the nature of space and time, with no consensus reached on whether they are fundamentally the same or distinct entities. The discussion includes both supportive and critical perspectives on the implications of time travel.

Contextual Notes

Participants highlight the dependence on specific coordinate systems in general relativity, indicating that interpretations may vary based on the chosen framework. The discussion also reflects unresolved questions about the implications of time travel and the nature of causality.

Gerenuk
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Space-time is supposed to be one unity making time in a way equivalent to spatial dimensions. Of course in the metric for time has a different sign though.

But what if it's just a mathematical accident that space and time take very similar positions in physical equations?

What is the best evidence that time and space are somehow the same?

The point is, if they weren't, then time-travel and all similar ideas with past/future would be a senseless idea.
 
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Gerenuk said:
Space-time is supposed to be one unity making time in a way equivalent to spatial dimensions. Of course in the metric for time has a different sign though.

But what if it's just a mathematical accident that space and time take very similar positions in physical equations?

What is the best evidence that time and space are somehow the same?

The point is, if they weren't, then time-travel and all similar ideas with past/future would be a senseless idea.
What do you mean by "equal footing". If "space and time take very similar positions in physical equations" isn't that itself "equal footing"?

But then you ask for evidence that time and space are "somehow the same". They aren't. For one thing, space and time coordinates have different "signatures" in the metric tensor.
 
Gerenuk said:
What is the best evidence that time and space are somehow the same?

I guess the best evidence that time and space are somehow the same is the conventional interpretation of the inside of a black hole where time becomes space and space becomes time. That is worth exploring in this context ;)


Gerenuk said:
The point is, if they weren't, then time-travel and all similar ideas with past/future would be a senseless idea.

Well time-travel probably IS a senseless idea. There is absolutely no evidence for it outside of holywood movies. If time travel is ever destined to be invented millions of years in the future, why is there no evidence of time tourists now? There is also Steven Hawking's "grandfather paradox" which basically implies that time travel would make a mess of causality.
 
kev said:
I guess the best evidence that time and space are somehow the same is the conventional interpretation of the inside of a black hole where time becomes space and space becomes time. That is worth exploring in this context ;)
Only in certain coordinate systems which are badly-behaved at the horizon like Schwarzschild coordinates, there is nothing physically significant about this. You could design badly-behaved coordinate systems around the Earth where one coordinate switched from being spacelike to timelike when you crossed the boundary of the upper atmosphere, and another coordinate switched from being timelike to spacelike. In GR you are basically free to pick any crazy coordinate system you can think of, thanks to the principle of "diffeomorphism invariance".
kev said:
Well time-travel probably IS a senseless idea. There is absolutely no evidence for it outside of holywood movies. If time travel is ever destined to be invented millions of years in the future, why is there no evidence of time tourists now? There is also Steven Hawking's "grandfather paradox" which basically implies that time travel would make a mess of causality.
The grandfather paradox predates Hawking by quite a lot, and even if time travel were possible, the paradox could be avoided using something like the Novikov self-consistency principle. Certain spacetime solutions in GR do allow time travel in the form of closed timelike curves, although many physicists have put forward arguments suggesting that in a theory of quantum gravity which integrates general relativity with quantum mechanics, these loopholes would likely be cut off (as suggested by Hawking's chronology protection conjecture, which may have been what you were thinking of)
 
Gerenuk said:
But what if it's just a mathematical accident that space and time take very similar positions in physical equations?
It's not so much of an accident but rather a consequence of the fact that two observers flying past each other will measure the same velocities. If the velocities are the same, and they have different measurements of space, then measurements of time will have to change to get them the same velocity.

Once you start with the premise that the speed of light is constant in all reference frames, then a *lot* of stuff follows.
 

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