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.
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.