4 ms·
Reversible dynamics with closed time-like curves and freedom of choice (2020)
- danbruc 6y agoDiscussion from 5 days ago [1], 208 comments. [1] https://news.ycombinator.com/item?id=25855367 https://news.ycombinator.com/item?id=25855367
- cletus 6y agoPrevious discussion: https://news.ycombinator.com/item?id=25855367 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.
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- at_a_remove 6y agoMy 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.
- cletus 6y agoAnother 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.
- at_a_remove 6y agoI whole heartedly agree. As a sci-fi nut who has consumed vast reams of the stuff from the 1940s onward, reality pales. 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.
- cletus 6y agoYou may also like Alastair Reynolds’ Revelation Space series.
- at_a_remove 6y agoI have been all over that, read all of them, even the short stories.
- jasperry 6y agoYour 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.
- jbay808 6y 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 https://en.m.wikipedia.org/wiki/Negative_temperature
- cletus 6y agoThat is interesting. But it's worth pointing out that it's a slightly different definition of temperature that is "negative".
- jbay808 6y agoTime dilation also applies to a slightly different time than Newton's time. It turns out this is the more foundational definition of temperature, and the other one (average kinetic energy) is mostly a useful heuristic that matches well under certain conditions (like when the distribution of energy is close to the Boltzmann distribution), but not others (a very fast-moving baseball).
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