Reversible dynamics with closed time-like curves and freedom of choice (2020)
iopscience.iop.org
iopscience.iop.org
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.
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.
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.
Firefly, I thought, rather neatly sidestepped the FTL issue by putting everything in one vast solar system wherein even the moons had been terraformed (although that itself is a technology that is equally ... unlikely, although for different reasons). I've read not too many series wherein the absence of FTL was acknowledged.
Sometimes reality just doesn't live up to the hype.
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.