This would manifest as non-conservation of energy-momentum in scattering, and, as far as I know, nothing like that has been seen since neutrino was predicted by Pauli to remedy the apparent non-conservation of energy in radioactive decay. If we assume non-interacting worlds, then one should not expect to see such violations. Gravity might be an oddball, however, since different worlds are likely to have difference spacetime geometry and even topology potentially affecting each other. But this is highly speculative, as there is no adequate microscopic (quantum) gravity model out there. I have seen some wild speculations that dark energy or even dark matter could be a weak gravity-only remnant of the incomplete decoherence stopped at the Planck scale.
I don’t see why differing spacetime geometries or topologies would impact other worlds. What makes gravity/geometry leak through when nothing else can?
Standard QFT is a fixed background spacetime theory, so if you have multiple non-interacting blobs of probability density in the same spacetime, they will all cooperatively curve it, hence the leakage. If you assert that the spacetime itself splits, you better provide a viable quantum gravity model to show how it happens.
Well, QFT can be also be safely done on a curved spacetime background, but you are right, you don’t get dynamic gravitational effects from it. What I implicitly assumed is QFT+semiclassical GR, where one uses semiclassical probability-weighted stress-energy tensor as a source.
What do you mean by “obviously wrong”? Because it would be evidence against MWI? Maybe it is, I don’t recall people trying to formalize it. Or maybe it limits the divergence of the worlds. Anyway, if it is not a good model, does this mean that we need a full QG theory to make MWI tenable?
Obviously wrong in that if you hold pure QFT + semiclassical GR to be complete and correct, then you end up with Cavendish experiments being totally unworkable because the density of the mass you put there is vanishingly small.
does this mean that we need a full QG theory to make MWI tenable?
I’m willing to state outright that MWI relies on the existence of gravity also being quantum outright, not semiclassical in nature. This does not seem like much of a concession to me.
Hmm, I don’t follow your argument re the Cavendish experiment. The original one was performed with fairly heavy lead balls.
I’m willing to state outright that MWI relies on the existence of gravity also being quantum outright, not semiclassical in nature. This does not seem like much of a concession to me.
That semiclassical gravity does not work in the weak-filed regime is a fairly strong statement. Widely accepted models like the Hawking and Unruh radiation are done in that regime.
A rigorous argument that semiclassical gravity is incompatible with MWI would probably be worth publishing.
Even if you start with an initial state where there is a well defined Cavendish-experimenter-man (which if you’re going with no objective collapse is a rather peculiar initial state) MWI has him all over the room, performing experiments at different times, with the weights at different displacements. They’d be pulling one way and the other, and his readings would make no sense whatsoever.
Semiclassical gravity is a perfectly fine approximation, but to say it’s real? Heh.
I meant something more limited than this, like a small cantilever in an unstable equilibrium getting entangled with a particle which may or may not push it over the edge with 50% probability, and measuring its gravitational force on some detector.
Oh. Well, then, it’s no longer ‘obviously false’ as far as that goes (i.e. we haven’t done that experiment but I would be shocked at anything but one particular outcome), but the whole point of MWI is to not restrain QM to applying to the tiny. Unless something happens between there and macro to get rid of those other branches, stuff gonna break hard. So, yeah. As an approximation, go ahead, but don’t push it. And don’t try to use an approximation in arguments over ontology.
This would manifest as non-conservation of energy-momentum in scattering, and, as far as I know, nothing like that has been seen since neutrino was predicted by Pauli to remedy the apparent non-conservation of energy in radioactive decay. If we assume non-interacting worlds, then one should not expect to see such violations. Gravity might be an oddball, however, since different worlds are likely to have difference spacetime geometry and even topology potentially affecting each other. But this is highly speculative, as there is no adequate microscopic (quantum) gravity model out there. I have seen some wild speculations that dark energy or even dark matter could be a weak gravity-only remnant of the incomplete decoherence stopped at the Planck scale.
I don’t see why differing spacetime geometries or topologies would impact other worlds. What makes gravity/geometry leak through when nothing else can?
Standard QFT is a fixed background spacetime theory, so if you have multiple non-interacting blobs of probability density in the same spacetime, they will all cooperatively curve it, hence the leakage. If you assert that the spacetime itself splits, you better provide a viable quantum gravity model to show how it happens.
Provide one? No. Call for one? Yes.
Sure, call for one. After acknowledging that in the standard QFT you get inter-world gravitational interaction by default....
In usual flat space QFT, you don’t have gravity at all, so no!
Well, QFT can be also be safely done on a curved spacetime background, but you are right, you don’t get dynamic gravitational effects from it. What I implicitly assumed is QFT+semiclassical GR, where one uses semiclassical probability-weighted stress-energy tensor as a source.
If that were true, MWI would have inter-world gravitational interactions. But it happens to be obviously wrong.
What do you mean by “obviously wrong”? Because it would be evidence against MWI? Maybe it is, I don’t recall people trying to formalize it. Or maybe it limits the divergence of the worlds. Anyway, if it is not a good model, does this mean that we need a full QG theory to make MWI tenable?
Obviously wrong in that if you hold pure QFT + semiclassical GR to be complete and correct, then you end up with Cavendish experiments being totally unworkable because the density of the mass you put there is vanishingly small.
I’m willing to state outright that MWI relies on the existence of gravity also being quantum outright, not semiclassical in nature. This does not seem like much of a concession to me.
Hmm, I don’t follow your argument re the Cavendish experiment. The original one was performed with fairly heavy lead balls.
That semiclassical gravity does not work in the weak-filed regime is a fairly strong statement. Widely accepted models like the Hawking and Unruh radiation are done in that regime.
A rigorous argument that semiclassical gravity is incompatible with MWI would probably be worth publishing.
Nawww, how could that be publishable?
Even if you start with an initial state where there is a well defined Cavendish-experimenter-man (which if you’re going with no objective collapse is a rather peculiar initial state) MWI has him all over the room, performing experiments at different times, with the weights at different displacements. They’d be pulling one way and the other, and his readings would make no sense whatsoever.
Semiclassical gravity is a perfectly fine approximation, but to say it’s real? Heh.
I meant something more limited than this, like a small cantilever in an unstable equilibrium getting entangled with a particle which may or may not push it over the edge with 50% probability, and measuring its gravitational force on some detector.
Oh. Well, then, it’s no longer ‘obviously false’ as far as that goes (i.e. we haven’t done that experiment but I would be shocked at anything but one particular outcome), but the whole point of MWI is to not restrain QM to applying to the tiny. Unless something happens between there and macro to get rid of those other branches, stuff gonna break hard. So, yeah. As an approximation, go ahead, but don’t push it. And don’t try to use an approximation in arguments over ontology.
Sorry, I forgot for a moment that the notion was designed to be untestable. Never mind.
What? All you need to do is falsify QM, and MWI is dead dead DEAD.
As I said, you identify QM with MWI. This is not the only option.
What is it, then?
Either the branches we don’t experience exist, or they don’t.
If they don’t, then what made us exist and them not?
Not this discussion again. Disengaging.
It’s never this discussion, since it never gets discussed, but OK!