3 ms·

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) theo

by 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.
- jerf 4y ago
  "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!

by jerf 4y ago

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.

jerf 4y ago
"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!