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Performing the famous double-slit experiment near a black hole will never work:
Don't try to do a quantum experiment near a black hole — its mere presence ruins all quantum states in its vicinity, researchers say.
The finding comes from a thought experiment that pits the rules of quantum mechanics and black holes against each other, physicists reported April 17 at a meeting of the American Physical Society. Any quantum experiment done near a black hole could set up a paradox, the researchers find, in which the black hole reveals information about its interior — something physics says is forbidden. The way around the paradox, the team reports, is if the black hole simply destroys any quantum states that come close.
That destruction could have implications for future theories of quantum gravity. These sought-after theories aim to unite quantum mechanics, the set of rules governing subatomic particles, and general relativity, which describes how mass moves on cosmic scales.
"The idea is to use properties of the [theories] that you understand, which [are] quantum mechanics and gravity, to probe aspects of the fundamental theory," which is quantum gravity, says theoretical physicist Gautam Satishchandran of Princeton University.
Here's how Satishchandran, along with theoretical physicists Daine Danielson and Robert Wald, both of the University of Chicago, did just that.
First the team imagined a person, call her Alice, performing the famous double-slit experiment in a lab orbiting a black hole (SN: 11/5/10). In this classic example of quantum physics, a scientist sends a particle, like an electron or a photon, toward a pair of slits in a solid barrier. If no one observes the particle's progress, an interference pattern typical of waves appears on a screen on the other side of the barrier, as if the particle went through both slits at once (SN: 5/3/19). But if someone, or some device, measures the particle's path, it will register as having gone through one slit or the other. The particle's quantum state of apparently being in two places at once collapses.
Then the team imagined another person, Bob, sitting just inside a black hole's event horizon — the boundary beyond which not even light can escape the black hole's gravity. Even though Bob is doomed, he can still make measurements (SN: 5/16/14). The laws of physics behave the same just inside the horizon as outside. "At the horizon, you wouldn't even know you fell in," Satishchandran says.
When Bob observes which slit Alice's particle went through, the particle's quantum state will collapse. That would also let Alice know Bob is there, messing up her experiment. But that's a paradox — nothing done inside a black hole should affect the outside. By the laws of physics, Bob should not be able to communicate with Alice at all.
"The paradox is that black holes are a one-way street," Satishchandran says. "Nothing done in the interior of a black hole can affect my experiment that I do in the exterior. But we just made up a scenario in which, definitely, the experiment will be affected."
The team then guessed at a possible solution to that paradox: The black hole itself forces the quantum state of Alice's particle to collapse, whether Bob is there or not. "It must be that there's an effect that no one has calculated in these theories that comes to the rescue," Danielson says.
The rescue came from the fact that charged particles radiate, or emit light, when shaken. No matter how carefully Alice sets up her experiment, her particle will always emit a tiny amount of radiation as she moves it, the physicists showed. That radiation will have a different electromagnetic field depending on which way Alice's particle went.
When the radiation crosses the black hole's event horizon, the black hole will register that difference, effectively observing enough about the original particle to destroy its quantum state.
"The horizon actually 'knows' which way the particle went," mathematically speaking, Satishchandran says. Alice blames the black hole for ruining her experiment, not Bob, and the paradox is resolved.
The team took the idea a step further. If Alice's particle is a graviton, a particle of gravity, the same thing happens as if it were an electron. And if the horizon in question is not a black hole, but the cosmic horizon marking the edge of the visible universe, then Alice's particle will still collapse, the team reported at the same meeting.
(Score: 2) by maxwell demon on Friday May 05, @04:27AM (5 children)
No. The interaction happens at Alice's side because that's where the double slit experiment happens. And the interaction by itself destroys the interference pattern, no observer needed.
I've done research in the field of quantum information. You can safely assume that I understand the fundamentals of that field.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 1) by khallow on Saturday May 06, @07:05PM (4 children)
An interaction happens there, sure. But it looks a lot different to Alice than it does to Bob!
Except, of course, when it doesn't do that.
Same.
(Score: 2) by maxwell demon on Sunday May 07, @07:52AM (3 children)
But the relevant question here is how it looks to Alice. In particular, if it looks different to Alice when Bob makes a measurement at his place than if he doesn't. The point is, it doesn't.
If it doesn't destroy the interference, then Bob can't obtain which-way information from the result of that interaction.
It may be noteworthy that this is not really an either-or; there can be interactions that merely reduce the interference pattern, and which then allow Bob to obtain partial which-way information (i.e., different probabilities that the particle went through the left of right slit). But again, whatever observation Bob makes won't change the interference pattern that Alice observes.
Now Bob communicating his results back to Alice might allow Alice to recover the interference pattern (so-called delayed choice), but that explicitly requires information from Bob reaching Alice, which won't happen if Bob is inside a black hole, and Alice is outside.
In that case I invite you to actually do the calculation. Then you'll see I'm right.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 1) by khallow on Sunday May 07, @09:30PM (2 children)
My bet is that we can set up an experiment where Alice can determine that information from the experiment had been sent in Bob's direction without observing what came of that information or collapsing the entanglement of that information. Yes, she won't know what Bob observed or even if he observed.
And it's not relevant how it looks to Alice without also considering how it looks to Bob.
Bob does destroy the interference from Bob's point of view. That's never been contested. My point is that he can do that without destroying the interference from Alice's point of view.
Indeed. Even if we suppose as is likely that black holes can evaporate, the information from Bob won't come out for a time.
Which interference do you speak of? The interference from Alice's point of view or the interference from Bob's point of view? Why are we to believe the two are the same when they no longer interact (or rather interact one way)?
What calculation is there? There's no information for Alice on Bob's collapsing of entanglement via said observation hence no collapse of entanglement for A.
(Score: 2) by maxwell demon on Monday May 08, @04:07AM (1 child)
Don't bet. Calculate. Quantum mechanics is an exact theory.
The interference from Alice's point of view, of course. Did you understand anything I wrote?
That question in particular makes me belief that your previous claim that you've done research in the field of quantum information simply is a lie.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 1) by khallow on Monday May 08, @05:44AM
You have a similar lack of calculations.
In the relativistic world QM isn't exact. They have yet to properly unite the two. And collapse of entanglement introduces irreversible processes which are poorly modeled in QM.
You keep using the term "the interference" without mentioning point of view. Relativity is very different from normal quantum mechanics in that it has built in limited and varied view points (via frames of reference and metrics). Much which is considered normal like an absolute chain of events break in general relativity. We are heavily abstracting from that to get the event horizon model of one-way information transfer.
If it doesn't destroy the interference from Bob's point of view. You have yet to explain why you're using Alice's viewpoint instead. It's different.
For a glaring example of the pointlessness of calculation, consider the case where both Alice and Bob are behind their own event horizons and completely isolated from each other. Then what happens in one point of view is completely independent of what happens in the second. We could do calculations, but that's what those calculations would determine.