I Is Chronology Protection Still Relevant in Modern Physics?

[ErikZorkin]

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?

 

[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?

See my https://link.springer.com/article/10.1007/s10702-006-0516-5
It also explains why the Novikov's principle is not artificial at all.

 

[ErikZorkin]

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.

 

[ErikZorkin]

Yes.

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

 

[Demystifier]

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

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".

The above was a mathematical argument, but physicists sometimes object that it constrains the initial conditions, which they find physically unacceptable. But why do they find it unacceptable? Because that contradicts the "fact" that humans have "free will" - the ability to choose any initial condition they like. But if one accepts that "free will" is just an illusion (which many physicists do accept), then the constraint on initial conditions is not longer a problem. Nature will choose only consistent solutions (because inconsistent ones are not solutions at all), while a human trapped in such a solution may have an illusion that the corresponding initial condition was "her own free choice".

 

[ErikZorkin]

The above was a mathematical argument, but physicists sometimes object that it constrains the initial conditions, which they find physically unacceptable.

I also heard time travel enabled such things as indefinite energy magnification, hypercomputation and stuff like that. These are hard to call physical