This is a major curiosity of mine, I've grown familiar with the notion of spacetime and while I know space and time aren't directly interchangeable I've still come across thoughts of changing direction in time and going backwards. My question then becomes if two observers moving in opposite directions of time, while I'm certain they may observe more trivial aspects of events occurring differently (For example I'm certain they'd argue events that we see occur last for us occurred first for them) would we still agree on identical laws of physics or is there some objective physical distinction between moving forward vs backwards in time. In less words it's there a principle of relativity for movement through time or are they distinguishable?
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4In mainstream physics there are no observers moving backward in time, so mainstream physics has nothing to say about what they would observe. In my opinion, this question is as off-topic as posts that start with “Assume an observer who is moving faster than light.” – Ghoster Mar 14 '23 at 23:39
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1@ghoster: there is nothing in relativity that would allow you to distinguish a traveler backward in time from a traveler forward in time so if you can't rule out the latter, you can't rule out the former. – WillO Mar 15 '23 at 03:58
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Have researchers managed to "reverse time"? If so, what does that mean for physics? – mmesser314 Mar 15 '23 at 05:09
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1Is anti-matter matter going backwards in time? – mmesser314 Mar 15 '23 at 05:11
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What is time, does it flow, and if so what defines its direction? – mmesser314 Mar 15 '23 at 05:12
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And for fun, Are time crystals real? – mmesser314 Mar 15 '23 at 05:14
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Also related (kind of): Question about a mirror Universe – PM 2Ring Mar 15 '23 at 07:26
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The OP isn't claiming that an observer can travel backwards in time, he's just asking what the laws of physics would look like to such an observer. So this is a mainstream question about time reversal symmetry. – PM 2Ring Mar 15 '23 at 07:39
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An observer moving backward in time with respect to what? Keep in mind "moving" is ordinarily defined as a change in something else with respect to time, so it's not obvious what it means to move in time, and it would help to define that clearly. – Sten Mar 15 '23 at 08:09
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2@Ghoster "moving backwards in time" is just T symmetry so the question is asking whether the laws of physics are invariant under T symmetry. This is most likely a duplicate, but it is certainly mainstream physics. – John Rennie Mar 15 '23 at 09:48
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Clearly no, the second law of thermodynamics does not work the same way if $t\to-t$. Also it is unclear how would work the un-collapse of the wavefunction in that scenario.
Mauricio
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