I think that when you start reasoning about quantum foundations it should be remembered that you’re leaving the boundary of testable physics. This is to say that even if you’ve concluded that many-worlds is most likely to be correct with your current information, that there should remain a pretty high degree of uncertainty in your conclusion.
It has been shown experimentally long ago that MWI requires full Quantum Gravity, not just Quantum Mechanics (plus Newtonian gravity or General Relativity, or even semi-classical gravity).
EDIT: provided an alternate link (paywalled, sorry).
Not sure what you are asking, but Everett and many other MWIers certainly thought/think that “the wave function of the universe” is all one needs to know.
What does that mean? You can have MWI without Quantum Gravity. It just won’t have any gravity.
If I had to guess, I’d say that you mean that you won’t be able to get general relativity working just by doing quantum physics on a non-flat spacetime. You have to have the spacetime metric itself vary along different universes. This is true, and it seems pretty obvious. If you didn’t do that, then gravity would have to be the same in all universes. But there’s another universe where Earth is somewhere else, so the gravitational field obviously has to be moved.
I don’t understand. GR describes the metric tensor through the Einstein’s equations, relating (the) energy (tensor) and the metric tensor. If you grab yourself an empty universe, then put some stuff in it, then do the incredibly hard math (this step usually goes wrong) out you get a metric tensor. In QM the energy is given in terms of the wave-function. You claim that the observation that the earth’s gravity pulls us in the general direction of the earth is inconsistent with the idea of putting the full wavefunction’s energy into this equation?
If you look at Schroedinger’s equation for one particle, it’s easy to generalize it so that the particle is in curved spacetime. The problem is when you get entanglement involved. Normally, for n particles, you do Schroedinger’s equation in R^3n, and each triplet corresponds to the coordinates of one particle. You could generalize that to M^n where M is an arbitrary manifold, but that means you’d have to use that manifold for every universe.
You could try running Einstein’s field equations on the 3n+1-dimensional configuration space (+1 being time, not an extra space dimension) and running Schroedinger’s equation on that. I don’t know if that would work. If it does, you didn’t get MWI without quantum gravity. You discovered quantum gravity.
That link doesn’t work for me. Is there somewhere else to get whatever it’s intended to link to, or a summary, or something?
I find it very difficult to imagine what could possibly constitute an experimental demonstration that Everettian QM requires full quantum gravity and not QM + some semi-classical treatment of gravity. This isn’t code for “I don’t believe you”—just a remark that what you’re claiming is really startling, at least to me.
(Well. In some sense any understanding of QM requires full quantum gravity, in that without it we know we don’t have a theory that actually describes the real world. But that’s as true of any other theory as it is of Everett’s.)
I can summarise the basic gist of the paper in relatively non-technical language (other people, please comment if you disagree with what I’m saying here):
Einstein’s equation says that the curvature (read geometry) of spacetime is equal to the stress-energy tensor, which basically measures how much mass/energy/momentum there is in a place at a time. However, in quantum mechanics, the universe is in a superposition of states with different distributions of mass. A theory of quantum gravity would therefore say that the universe must therefore be in a superposition of states with different geometries. The alternative is to have semi-classical gravity, where there is only one geometry of spacetime.
The most obvious way to construct a theory of semi-classical gravity is to say that the geometry of spacetime is actually related to the average distribution of mass of all the Everett branches (if you’re an Everettian). To test this, you can take a large mass, and put the universe into a superposition of two states: one where you move the large mass to the left, and one where you move the large mass to the right. You then see if your mass is attracted to the mass in the other Everett branch. If semi-classical gravity and the Everett interpretation are right, then both masses should curve spacetime, and that curvature of spacetime should be felt by both of them, so each mass should be attracted to the one in the other Everett branch. If semi-classical gravity or the Everett interpretation are wrong, then the masses shouldn’t feel attracted to the ones in the other Everett branch.
The people who wrote this paper did something that was essentially equivalent to the experiment described above, and discovered that the mass was not attracted to the one in the other Everett branch, meaning that semi-classical gravity and the Evererett interpretation can’t both be true. They also argue that semi-classical gravity implies the Everett interpretation, and that therefore semi-classical gravity can’t be true, although I am suspicious of this argument.
Nice! I find myself wanting to say “no, surely that just means that they refuted one particular sort of semiclassical gravity” but I’m not sure what other sort there might be.
Still, for me the main conclusion is: Yup, semiclassical gravity is wrong, just as we already knew it to be. More specifically, surely no one expects semiclassical gravity to be a good enough approximation in situations where the distribution of mass is made appreciably “different in different branches” (I don’t mean to presuppose Everett here, it’s just the easiest way to say it). So this experiment is finding that semiclassical gravity isn’t a good approximation in situations it was never expected to work well in; blaming that specifically on the Everett interpretation seems perverse.
I think that when you start reasoning about quantum foundations it should be remembered that you’re leaving the boundary of testable physics. This is to say that even if you’ve concluded that many-worlds is most likely to be correct with your current information, that there should remain a pretty high degree of uncertainty in your conclusion.
It has been shown experimentally long ago that MWI requires full Quantum Gravity, not just Quantum Mechanics (plus Newtonian gravity or General Relativity, or even semi-classical gravity).
EDIT: provided an alternate link (paywalled, sorry).
Who out there, MWI-adherent or not, seriously thinks that QM is a fundamental rule of nature for everything BUT gravity?
My position here is Knightian uncertainty—I have no idea whether that’s true AND I have no idea what are the chances of it being true.
That’s, umm, nice, but I don’t see how it helps answer the question since I suspect the number of ‘not me!’ would be enormous.
Not sure what you are asking, but Everett and many other MWIers certainly thought/think that “the wave function of the universe” is all one needs to know.
What does that mean? You can have MWI without Quantum Gravity. It just won’t have any gravity.
If I had to guess, I’d say that you mean that you won’t be able to get general relativity working just by doing quantum physics on a non-flat spacetime. You have to have the spacetime metric itself vary along different universes. This is true, and it seems pretty obvious. If you didn’t do that, then gravity would have to be the same in all universes. But there’s another universe where Earth is somewhere else, so the gravitational field obviously has to be moved.
I don’t understand. GR describes the metric tensor through the Einstein’s equations, relating (the) energy (tensor) and the metric tensor. If you grab yourself an empty universe, then put some stuff in it, then do the incredibly hard math (this step usually goes wrong) out you get a metric tensor. In QM the energy is given in terms of the wave-function. You claim that the observation that the earth’s gravity pulls us in the general direction of the earth is inconsistent with the idea of putting the full wavefunction’s energy into this equation?
If you look at Schroedinger’s equation for one particle, it’s easy to generalize it so that the particle is in curved spacetime. The problem is when you get entanglement involved. Normally, for n particles, you do Schroedinger’s equation in R^3n, and each triplet corresponds to the coordinates of one particle. You could generalize that to M^n where M is an arbitrary manifold, but that means you’d have to use that manifold for every universe.
You could try running Einstein’s field equations on the 3n+1-dimensional configuration space (+1 being time, not an extra space dimension) and running Schroedinger’s equation on that. I don’t know if that would work. If it does, you didn’t get MWI without quantum gravity. You discovered quantum gravity.
That link doesn’t work for me. Is there somewhere else to get whatever it’s intended to link to, or a summary, or something?
I find it very difficult to imagine what could possibly constitute an experimental demonstration that Everettian QM requires full quantum gravity and not QM + some semi-classical treatment of gravity. This isn’t code for “I don’t believe you”—just a remark that what you’re claiming is really startling, at least to me.
(Well. In some sense any understanding of QM requires full quantum gravity, in that without it we know we don’t have a theory that actually describes the real world. But that’s as true of any other theory as it is of Everett’s.)
I can summarise the basic gist of the paper in relatively non-technical language (other people, please comment if you disagree with what I’m saying here):
Einstein’s equation says that the curvature (read geometry) of spacetime is equal to the stress-energy tensor, which basically measures how much mass/energy/momentum there is in a place at a time. However, in quantum mechanics, the universe is in a superposition of states with different distributions of mass. A theory of quantum gravity would therefore say that the universe must therefore be in a superposition of states with different geometries. The alternative is to have semi-classical gravity, where there is only one geometry of spacetime.
The most obvious way to construct a theory of semi-classical gravity is to say that the geometry of spacetime is actually related to the average distribution of mass of all the Everett branches (if you’re an Everettian). To test this, you can take a large mass, and put the universe into a superposition of two states: one where you move the large mass to the left, and one where you move the large mass to the right. You then see if your mass is attracted to the mass in the other Everett branch. If semi-classical gravity and the Everett interpretation are right, then both masses should curve spacetime, and that curvature of spacetime should be felt by both of them, so each mass should be attracted to the one in the other Everett branch. If semi-classical gravity or the Everett interpretation are wrong, then the masses shouldn’t feel attracted to the ones in the other Everett branch.
The people who wrote this paper did something that was essentially equivalent to the experiment described above, and discovered that the mass was not attracted to the one in the other Everett branch, meaning that semi-classical gravity and the Evererett interpretation can’t both be true. They also argue that semi-classical gravity implies the Everett interpretation, and that therefore semi-classical gravity can’t be true, although I am suspicious of this argument.
Note that it is possible to do tests for semi-classical gravity that don’t rely on the Everett interpretation, although these don’t seem to have been done yet (see http://physics.anu.edu.au/projects/project.php?ProjectID=31).
Nice! I find myself wanting to say “no, surely that just means that they refuted one particular sort of semiclassical gravity” but I’m not sure what other sort there might be.
Still, for me the main conclusion is: Yup, semiclassical gravity is wrong, just as we already knew it to be. More specifically, surely no one expects semiclassical gravity to be a good enough approximation in situations where the distribution of mass is made appreciably “different in different branches” (I don’t mean to presuppose Everett here, it’s just the easiest way to say it). So this experiment is finding that semiclassical gravity isn’t a good approximation in situations it was never expected to work well in; blaming that specifically on the Everett interpretation seems perverse.