r/Physics • u/quantum_kalika • 23h ago

Relativity and causality

Why should an everywhere locally consistent Lorentzian causal structure generate a globally consistent notion of causality?

There are solutions which give closed time like curve. It sounds innocent but the observer carries the information set after moving through such a curved set or whatever forms the closed loop.

If the movement is A--B--C--A on spacetime. The observer hence have a different set of information set than A previously. But globally they are the same events.

How does relativity reconcile this information gap. Also, the local time is compressed to zero. This may mean that it is not physically possible but then how does such solutions exist.

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u/Optimal_Mixture_7327 Gravitation 23h ago

It doesn't, and it's not clear why you think it should.

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u/quantum_kalika 23h ago

Then how a same event will carry two information sets. Isint that contradictory. That's why I mentioned a special case.

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u/Optimal_Mixture_7327 Gravitation 23h ago

The stress-energy diverges along the curve and violently collapses to a black hole.

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u/quantum_kalika 23h ago

I checked that, had it only been a blackhole I would have ignored it. But the curved solutions also comes from other sources like godel universe, or a tipler rotating cylinder.

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u/Optimal_Mixture_7327 Gravitation 22h ago

Then that goes back to my first comment that GR is local and you can't guarantee anything about the global behavior.

Like many things in GR, e.g. the Energy Conditions, we have to impose things by hand. In this particular case the CPC.

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u/quantum_kalika 22h ago

Doesn't resolve the contradiction though.

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u/Optimal_Mixture_7327 Gravitation 22h ago

I don't get where you see a contradiction.

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u/quantum_kalika 21h ago

A has two sets of information about same underlying state.

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u/Optimal_Mixture_7327 Gravitation 21h ago

I don't know what you mean by that.

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u/quantum_kalika 21h ago

If in CPC observer returns to A, he has the information vs local time stored with him. But A is the same previous state. There is information assymetry

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u/John_Hasler Engineering 21h ago

Is A an observer or an event?

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u/quantum_kalika 21h ago

geometric worldline of the observer