# Causality in current physics

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## Main Question or Discussion Point

What is currently the common opinion on Chronology Protection manifested by Hawking almost 30 years ago?

There does not seem to be any fully accepted no-go result for Thornian time machines. Energy conditions can be violated in QFT, semi-classical results suffer from counter-examples, Novikov's and multi-world statements sound too artificial and only try to come around the real issue.

I also didn't see any discussion on the 2nd law of thermodynamics in the works by Visser etc. Does it somehow relate to the problem of causality though?

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Demystifier
I also didn't see any discussion on the 2nd law of thermodynamics in the works by Visser etc. Does it somehow relate to the problem of causality though?
It also explains why the Novikov's principle is not artificial at all.

It also explains why the Novikov's principle is not artificial at all.
How exactly? You also say that the thermodynamic arrow of time prevents paradoxes on the macroscopic level. So, do you actually allow time machines, but forbid them to generate paradoxes a-la Novikov?

Demystifier
So, do you actually allow time machines, but forbid them to generate paradoxes a-la Novikov?
Yes.

Yes.
...a-and how is this not "artificial"?

Demystifier
The Novikov's principle says that multivalued solutions should be discarded. One may think that it is artificial, but it is not. The multivalued solution is in fact the same as an inconsistent solution, which is best viewed as something that is not a solution at all. If you have a differential equation for a function $f(t)$, then a map that gives two different values of $f$ for the same $t$ is, by definition, not a mathematical function. So by requiring that the solution of the differential equation must be a function, one automatically discards those multivalued "solutions".