6 ms·

Quantum particles can feel the effects of gravitational fields

80 points by elorant 4y ago

MilStdJunkie 4y ago
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.
- yarg 4y ago
  Not necessarily the middle. It's gravity (approximately an inverse square law) operating across the probabilistic distribution of the locations of the system.
  - yarg 4y ago
    Just to expand on this, the solution is the problem is a probabilistic integral across infinitely many three-body problems. https://en.wikipedia.org/wiki/Three-body_problem https://en.wikipedia.org/wiki/Three-body_problem Actually, the gradient function is the probabilistic integral across the gradient functions of infinitely many three-body problems. The actual solution is the integral of that.
- jerf 4y ago
  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.
  - MilStdJunkie 4y ago
    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'.
  - zmgsabst 4y ago
    > 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 4y ago
      > 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 4y ago
      AFAIK non-stringy quantum gravity theories have been proposed.
  - riskable 4y ago
    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.
- whatshisface 4y ago
  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 4y ago
    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/reversing-gravitational-decoherence https://physics.stackexchange.com/questions/34993/reversing-...
    - cwillu 4y ago
      Worth noting that this is a question by Scott Aaronson
JanisErdmanis 4y ago
Perhaps would be an interesting read, but SciHub have not yet included the paper in the database :( Can anyone help out?
- heavyset_go 4y ago
  I've been noticing less and less being available on SciHub, unfortunately.
jmakov 4y ago
What gravitational fields? Thought gravity isn't a dorce nor a field.
tus666 4y ago
> 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 4y ago
  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 4y ago
    Seems like gravity would work the same way on a wavefunction. Slight bias in the direction of mass or something.
alpineidyll3 4y ago
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.48.1799 https://journals.aps.org/pra/abstract/10.1103/PhysRevA.48.17... We just live in an age of hype.
yarg 4y ago
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 4y ago
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.
- hammock 4y ago
  Is magnetic fields not being real kind of like centrifugal force not being real?
8bitsrule 4y ago
Good to see some appreciation for (Pennsylvania-born) physicist Bohm, who was rendered unemployable in the US in 1951.
- deleted 4y ago
  [deleted]
- Agamus 4y ago
  In all of my amateur studies on physics, the most challenging statement I have encountered was from Bohm, in this clip: https://youtu.be/BFvJOZ51tmc?t=1862 https://youtu.be/BFvJOZ51tmc?t=1862 "The world is... not analyzable into parts". If there are no parts, what are numbers counting?
  - deleted 4y ago
    [deleted]
Scienceinter 4y ago
Even if you keep your distance, a black cat in your path is bad luck if you're superstitious. Similarly, in quantum physics, particles can sense the influence of magnetic fields with which they have no direct contact. Scientists have now demonstrated that this strange quantum effect applies not only to magnetic fields but also to gravity — and it's not a myth.

80 points by elorant 4y ago

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.

yarg 4y ago
Not necessarily the middle. It's gravity (approximately an inverse square law) operating across the probabilistic distribution of the locations of the system.
- yarg 4y ago
  Just to expand on this, the solution is the problem is a probabilistic integral across infinitely many three-body problems. https://en.wikipedia.org/wiki/Three-body_problem https://en.wikipedia.org/wiki/Three-body_problem Actually, the gradient function is the probabilistic integral across the gradient functions of infinitely many three-body problems. The actual solution is the integral of that.
jerf 4y ago
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.
- MilStdJunkie 4y ago
  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'.
- zmgsabst 4y ago
  > 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 4y ago
    > 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 4y ago
    AFAIK non-stringy quantum gravity theories have been proposed.
- riskable 4y ago
  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.
whatshisface 4y ago
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 4y ago
  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/reversing-gravitational-decoherence https://physics.stackexchange.com/questions/34993/reversing-...
  - cwillu 4y ago
    Worth noting that this is a question by Scott Aaronson

JanisErdmanis 4y ago

Perhaps would be an interesting read, but SciHub have not yet included the paper in the database :( Can anyone help out?

heavyset_go 4y ago
I've been noticing less and less being available on SciHub, unfortunately.

jmakov 4y ago

What gravitational fields? Thought gravity isn't a dorce nor a field.

tus666 4y ago

> 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 4y ago
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 4y ago
  Seems like gravity would work the same way on a wavefunction. Slight bias in the direction of mass or something.

alpineidyll3 4y ago

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.48.1799 https://journals.aps.org/pra/abstract/10.1103/PhysRevA.48.17... We just live in an age of hype.

yarg 4y ago

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 4y ago

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.

hammock 4y ago
Is magnetic fields not being real kind of like centrifugal force not being real?

8bitsrule 4y ago

Good to see some appreciation for (Pennsylvania-born) physicist Bohm, who was rendered unemployable in the US in 1951.

deleted 4y ago
[deleted]
Agamus 4y ago
In all of my amateur studies on physics, the most challenging statement I have encountered was from Bohm, in this clip: https://youtu.be/BFvJOZ51tmc?t=1862 https://youtu.be/BFvJOZ51tmc?t=1862 "The world is... not analyzable into parts". If there are no parts, what are numbers counting?
- deleted 4y ago
  [deleted]

Scienceinter 4y ago

Even if you keep your distance, a black cat in your path is bad luck if you're superstitious. Similarly, in quantum physics, particles can sense the influence of magnetic fields with which they have no direct contact. Scientists have now demonstrated that this strange quantum effect applies not only to magnetic fields but also to gravity — and it's not a myth.