[HN Gopher] Quantum particles can feel the effects of gravitatio...
___________________________________________________________________
Quantum particles can feel the effects of gravitational fields
Author : elorant
Score : 53 points
Date : 2022-06-05 12:51 UTC (1 days ago)
(HTM) web link (www.scienceinter.com)
(TXT) w3m dump (www.scienceinter.com)
| yarg wrote:
| It's not that they feel that which they never touch - the
| probabilistic waveform will experience gravity according to its
| distribution.
|
| It means that (like with relativity) you end up dealing with some
| rather nasty intractable calculus, and in order to make sense of
| things you'll be looking to sound enough approximations of what's
| actually going on.
|
| This consequences of this will be particularly amusing in near
| proximity to event horizons.
| antognini wrote:
| Detecting the Aharonov-Bohm effect for gravity is a super
| impressive experimental achievement, though it's not as "weird"
| as the effect for magnetic fields.
|
| One way of interpreting the normal Aharonov-Bohm effect is that
| it implies that the electromagnetic potential is more "real" than
| the electromagnetic field. We tend to think of the
| electromagnetic field as what's "really" out there because if you
| stick a charge in an electric field you can measure the field
| strength by measuring the force the charge feels or you can
| measure the force on a moving charge and calculate the magnetic
| field. By contrast the electromagnetic potential is not uniquely
| defined which makes it feel like a mathematical construct. You
| can add any function with zero divergence to the electric
| potential and any function with zero curl to the magnetic
| potential and have a potential that behaves identically for any
| experiment you could do (these are different gauge choices in the
| parlance).
|
| The classic Aharonov-Bohm effect is that you take a particle and
| pass it around a magnetic solenoid in a superposition of two
| states --- say one state goes to the left of the solenoid, the
| other goes to the right. In a sense the particle takes both paths
| simultaneously. The magnetic field is zero along both paths the
| particle takes. If you turn the solenoid on and off you would
| think it would make no difference because no matter what you do,
| the magnetic field the particle encounters is zero. The key,
| though, is that the magnetic potential is _not_ zero along the
| particle 's path, and the magnetic potential affects the
| particle's phase in measurable way when the two states recombine.
|
| Sometimes I see interpretations that this means that there is a
| small but non-zero chance that the particle is found inside the
| solenoid where the magnetic field is non-zero, but I don't think
| that's right. You can construct the experiment in a way that the
| chance of this happening is negligible. The real implication
| seems to be that the electromagnetic potential is the more
| physically fundamental quantity even though it is not generally
| directly observable.
|
| All the math for the Aharonov-Bohm effect applies for
| gravitational fields as well. But here it's not quite as "weird"
| a result because gravitational fields can't be shielded the same
| way magnetic fields can be. The particle always encounters the
| gravitational field along both paths, so it's not so spooky that
| the field has a measurable effect.
| jmakov wrote:
| What gravitational fields? Thought gravity isn't a dorce nor a
| field.
| tus666 wrote:
| > A quantum effect reveals that even if subatomic particles are
| not subject to gravitational forces, they can feel the effects of
| warping.
|
| Eh? That warping _is_ the gravitational force.
| cryptonector wrote:
| That's badly phrased. They're referring to the gravitational
| equivalent to an experiment where magnetic fields not evident
| in a particle's path affect it anyways due to the uncertainty
| principle meaning that the particle can be found, with some
| probability, to be in the area where those magnetic fields can
| be observed.
| ianai wrote:
| Seems like gravity would work the same way on a wavefunction.
| Slight bias in the direction of mass or something.
| alpineidyll3 wrote:
| This is the straightforwards Aharonov-Bohm effect. There never
| could have been any doubt that any massive particle would
| experience it. Related, even weirder experiments were the stuff
| of niche journals, a long time ago.
| https://journals.aps.org/pra/abstract/10.1103/PhysRevA.48.17...
|
| We just live in an age of hype.
| JanisErdmanis wrote:
| Perhaps would be an interesting read, but SciHub have not yet
| included the paper in the database :( Can anyone help out?
| heavyset_go wrote:
| I've been noticing less and less being available on SciHub,
| unfortunately.
| MilStdJunkie wrote:
| Huh. I wonder if you had, like, an entangled system of X and Y,
| if something comes under the gravitational influence of that
| system X-Y, what directions would the gravitation acceleration
| go? Would it go towards the middle of the two? Would the
| "detection" of the gravitation acceleration make it collapse
| towards X/Y? If you didn't detect the gravity acceleration, would
| the third body enter into the entangled system via gravity?
|
| I am about as far from an expert as it's possible to be, but I
| always wondered what would happen if an entangled system exerted
| its gravity on something else.
| whatshisface wrote:
| Nonrelativistic quantum mechanics offers an answer to your
| question. You can write a Hamiltonian involving a Newtonian
| gravitational potential and the answer you will get out is that
| your third particle, Z, will enter into a superposition of two
| states, one being attracted to point X and the other to point
| Y.
| monktastic1 wrote:
| Indeed. And one result of _that_ is that we can get
| decoherence even if the objects are otherwise entirely
| shielded: https://physics.stackexchange.com/questions/34993/r
| eversing-...
| cwillu wrote:
| Worth noting that this is a question by Scott Aaronson
| jerf wrote:
| While not commonly expressed this way as it is usually phrased
| in terms of much flashier black holes, this is one of the
| questions that we need a (correct) theory of quantum
| gravitation to deal with. I think the best answer is nobody
| knows. I think you could make a decent handwavy case that this
| probably produces some sort of gravity entanglement that will
| collapse (or whatever, insert your favorite theory here) later,
| as it would ultimately be _very_ strange if gravity is not
| itself "quantum" w.r.t. also ending up with imaginary
| probabilities and the quantum effects that naturally follow
| from that, but this has neither been successfully formalized
| into math nor experimentally verified.
| zmgsabst wrote:
| > as it would ultimately be very strange if gravity is not
| itself "quantum" w.r.t. also ending up with imaginary
| probabilities and the quantum effects that naturally follow
| from that
|
| Is there an argument for this rather than the other way?
|
| I'm probably misremembering, but I thought I'd seen something
| about spacetime being continuous and particles being braids
| in that aether -- but did that blow up when string theory
| didn't happen at the LHC?
| kadoban wrote:
| > but did that blow up when string theory didn't happen at
| the LHC?
|
| String theory isn't really possible to rule out via any
| experiment I've ever seen. At best you can rule out some
| versions of it, but even that is hard AFAIK.
| gpderetta wrote:
| AFAIK non-stringy quantum gravity theories have been
| proposed.
| MilStdJunkie wrote:
| Oh interesting! That makes sense! I guess measuring the
| gravitational attraction of entangled systems is probably a
| few dozen orders of magnitude beyond any sensor that exists,
| unless it's entangled black hole stuff. Even if you cranked
| up a proton to 90% C, that's still just adding maybe an order
| of magnitude, then you have to entangle it with another
| relativistic proton, and _then_ shoot another particle so it
| can maybe enter the two entangled relativistic protons '
| gravitational field(s?). This sounds like a great
| experimental design to prove a whole lotta nothin'.
| riskable wrote:
| Couldn't you do an experiment where you entangle two
| particles at different altitudes and measure to see which one
| drops faster? If they're entangled and influenced by their
| local gravity then you'd expect both to fall at some half-way
| point (their average) in terms of gravitational influence
| since they'd each be influenced by the other's gravity
| (equally?).
|
| If they both drop at the same exact speed (and it's not what
| it should be from a locality standpoint) it would confirm
| that local gravity is having an impact on both particles at
| the same time. In other words, if you drop a particle from 10
| meters at sea level it would fall at a certain rate. However,
| if you entangle it with a particle that's 6,000 meters up
| that particle at sea level might drop at a rate equivalent to
| what it would at around 3,000 meters (assuming gravity
| influences both particles equally).
|
| If that's how it worked then in theory you could entangle a
| large mass of particles here on earth with some out in space
| and you'd only need a fraction of the amount of energy for
| the particles here on earth to reach escape velocity.
| jerf wrote:
| "Couldn't you do an experiment where you entangle two
| particles at different altitudes and measure to see which
| one drops faster?"
|
| Yes. The experiment is easy to run. The equipment that can
| detect the factor-of-10^-50 percent difference between them
| is not so easy to come by. The problem with quantum gravity
| is the lack of data brought on by the fact gravity
| effectively doesn't exist on Earth at what we typically
| call "quantum" scales, compared to the overwhelming
| influence of electromagnetism and the nuclear forces.
|
| (The inhabitants of the Dragon's Egg in the science fiction
| novel of the same name, which are creatures made out of
| matter on the surface of a neutron star, presumably did not
| have such troubles making a theory of quantum gravity,
| because they could gather data directly. We creatures made
| of atoms in an environment that permits that are not so
| lucky!)
|
| It's not that hard to come up with situations where you get
| less-than-Planck time expected differences in behavior.
| This is another frontier in science nobody knows the answer
| to. For as small as Planck time is, there are ways you can
| theoretically detect if the universe was "rounding" at that
| scale, and so far they have all consistently come up as
| "nope, the universe isn't rounding"... yet it is also
| almost inconceivable with modern physics how it couldn't be
| in one way or another. (The most interesting one was trying
| to pick up various effects of such quantization on photons
| travelling so far across the universe that the differences
| would be detectable.) There are still many mysteries!
___________________________________________________________________
(page generated 2022-06-06 23:01 UTC)