Time Travel

[pendell]

let's skip the clickbait and go directly to the nature article

Here we show that, while in nature the complex conjugation needed for time reversal may appear exponentially improbable, one can design a quantum algorithm that includes complex conjugation and thus reverses a given quantum state. Using this algorithm on an IBM quantum computer enables us to experimentally demonstrate a backward time dynamics for an electron scattered on a two-level impurity.

More reading at your convenience.
Granted we're only talking about an electron .. but if time travel is possible at ANY scale does that mean ftl is plausible?

Respectfully,

Brian P.

[Korith]

FTL in terms of velocity is theoretically possible, just not by means of conventionally accelerating a particle with a sublight velocity and a mass.

Of course systemic time travel would be a very effective way of unconventionally accelerating a particle (or object) to FTL velocities. Simply launch it on Monday, have it arrive 0.99 light days away on Tuesday and time travel back to Monday.

That said, conflating reversing a quantum state with time travel seems to be pushing what's happening a bit far. It seems what they've done is more akin to unboiling a quantum egg. There's still the argument about whether it's the actual past egg, or merely a restoration of a prior state, and I'm leaning on the side of a restoration of a prior state.

[Lvl 2 Expert]

If I'm reading that correctly (man, they use a lot of fancy words, even for me) they don't claim actually reversing time, they claim an exception to the second law of thermodynamics, where a closed system with no energy being added will never see a decrease in total entropy. So they claim that in their electron sized system they've observed entropy going down without energy being added, which means the system develops as if going backwards in time. But it's not to how it was. If we imagine this happening on a human scale you'd technically grow younger while the rest of the world was aging around you, but you're not becoming the person you was yesterday and there is in fact no telling whether you're actually growing into a baby. You're just becoming anything that could theoretically develop into a full grown human ones entropy starts behaving correctly again.

Now, technically they could be measuring time travel I suppose, at this scale it's probably really hard to distinguish between an exception in the laws of entropy and an electron actually reversing the clock on itself. But physically speaking that would be a lot more complex, there are a lot more weird things you're have to explain about that for it to make sense. So they've probably just measured weird entropy behavior. This would still be scientifically massive if it indeed pans out and isn't some sort of measurement fluke (for one thing energy production could become not revolutionized but actually completely obsolete), but it would not be time travel.

[NichG]

Time reversal and time travel are not the same thing. The wave equation for small ripples on a water surface is time reversal symmetric - it runs the same forwards as backwards - as are Maxwell's equations for electromagnetic fields. So this kind of thing has nothing to do with FTL. Quantum mechanics behaves a little differently under time reversal, and that may have some relationship with the way in which statistical mechanics shows that you can take a microscopically time-reversible system and render it irreversible at large scales, so the article seems to be making a subtle technical point about that and the ability to construct reversals in QM systems.

[Radar]

Time reversal and time travel are not the same thing. The wave equation for small ripples on a water surface is time reversal symmetric - it runs the same forwards as backwards - as are Maxwell's equations for electromagnetic fields. So this kind of thing has nothing to do with FTL. Quantum mechanics behaves a little differently under time reversal, and that may have some relationship with the way in which statistical mechanics shows that you can take a microscopically time-reversible system and render it irreversible at large scales, so the article seems to be making a subtle technical point about that and the ability to construct reversals in QM systems.

... and that it does not require insane ammounts of luck and precision to actually get that effect.

In theory, you could kick every atom of a broken glass so that it would un-break and be a whole glass again. The usual problem is, you would have to be really precise in kicking every atom just right and any deviations would make you go from broken glass to a different arrangement of broken glass. An un-borken glass is just such a small target ih the vast sea of glass shard configurations.

Here they demonstrated that properly prepared system can show this kind of effect repeatedly. In of itself it has a lot of good applications in quantum computation - especially in protecting against decoherence. What else could be done with this trick? No clue yet, but I am actually excited, since someone somewhere will think of something truly neat.

[Bohandas]

A true exception to either law of thermodynamics would imply the possibility of perpetual motion, which is arguably even more groundbreaking than time travel