Reversible dynamics with closed time-like curves and freedom of choice (2020)

https://iopscience.iop.org/article/10.1088/1361-6382/aba4bc

by 9dev • 5 years ago

14 11 5 years ago

iopscience.iop.org

danbruc• 5 years ago

Discussion from 5 days ago [1], 208 comments.

[1] https://news.ycombinator.com/item?id=25855367

cletus4• 5 years ago

Previous discussion: https://news.ycombinator.com/item?id=25855367

I'm not sure if this paper falls into the category of saying X is possible because they plug a value outside of the domain of a function into that function where X can be wormholes, FTL drives and, yes, time travel.

Typical examples: velocities > c, negative energy, negative mass.

Garbage in, garbage out.

Probably the best way I can illustrate this point is with temperature. Imagine what happens when we start plugging temperatures below absolute zero into various formulae. But temperatures < 0K make no sense. I also feel that velocities outside [0, c] make no sense.

I also think there's pretty good evidence of the inability to time travel: and that is the apparent absence of any observed time travelers.

at_a_remove3• 5 years ago

My physics version of GIGO has been, once you allow one impossible thing to happen ...

Just as violating conservation of charge can violate conservation of mass-energy, I think a similar principle applies. And if we have "exotic matter," we can create warp drives.

cletus4• 5 years ago

Another common physics-breaking trope is inertial dampening. You've right about the prevalence of "exotic matter" too.

I get the psychological desire for this: that space isn't the massive tyranny of distance it appears to be. But all the evidence we have thus far is that there is a universal speed limit, as depressing as that might be.

Which, as an aside, is what gives me the most confidence we don't have aliens in UFOs on Earth.

jasperry• 5 years ago

Your translation of GIGO to physics gave me an "Aha" moment. It made me think of logic, where if you have an inconsistent set of axioms you can prove anything. I guess that's the formal equivalent of GIGO.

jbay8082• 5 years ago

But temperatures < 0K make no sense.

Actually, they do! They behave very differently than you might expect. The reason is that temperature in thermodynamics is related to energy and entropy by dQ/dS = T, so if you can prepare a system that releases energy via decreasing in entropy, it has negative temperature. And, it turns out, you can actually do this. But the result is the complete opposite of cold.

https://en.m.wikipedia.org/wiki/Negative_temperature

cletus4• 5 years ago

That is interesting. But it's worth pointing out that it's a slightly different definition of temperature that is "negative".

Reversible dynamics with closed time-like curves and freedom of choice (2020)

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