I don’t believe I am explaining MWI instead of arguing against it… whatever has this site done to me? Anyway, grossly simplified, you can think of the matter as being conserved because the “total” mass is the sum of masses in all worlds weighted by the probability of each world. So, if you had, say, 1kg of matter before a “50/50 split”, you still have 1kg = 0.5*1kg+0.5*1kg after. But, since each of the two of you after the split has no access to the other world, this 50% prior probability is 100% posterior probability.
Also note that there is no universal law of conservation of matter (or even energy) to begin with, not even in a single universe. It’s just an approximation given certain assumptions, like time-independence of the laws describing the system of interest.
Disagree on the conservation of energy though. Every interaction conserves energy (unless you know of time-dependent laws?). Though nothing alters it, we only experience worlds with a nontrivial distribution of energies (otherwise nothing would ever happen) (and this is true whether you use MWI or not)
I don’t know enough of the underlying physics to conclusively comment one way or another, but it seems to me defining “total mass” as the integral of “local mass” over all worlds wrt the world probability measure implies that an object in one world might be able to mysteriously (wrt that world) gain mass by reducing its mass in some set of worlds with non-zero measure.
We don’t actually see that in e.g. particle scattering, right?
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.
Defining total energy as the integral of energy over space implies that an object in one part of space might be able to mysteriously gain energy by reducing energy in other parts of space.
Do we see this in the real world? How useful is the word “mysterious” here?
Ordinary energy conservation laws are local: they do not just state that total energy is conserved, but that any change in energy in a finite region of any size is balanced by a flux of energy over the boundary of that region. I don’t think any such laws exist in “multi-world-space”, which even accepting MWI is basically a metaphor, not a precise concept.
There’s no mysterious quantum motion for the same reason there’s no mysterious energetic motion—because energy / mass / quantum amplitude has to come from somewhere to go somewhere, it requires an interaction to happen. An interaction like electromagnetism, or the strong force. You know, those ubiquitous, important, but extremely well-studied and only-somewhat-mysterious things. And once you study this thing and make it part of what you call “energy,” what would otherwise be a mysterious appearance of energy just becomes “oh, the energy gets stored in the strong force.” (From a pure quantum perspective at least. Gravity makes things too tricky for me)
The best way for a force to “hide” is for it to be super duper complicated. Like if there was some kind of extra law of gravity that only turned on when the planets of our solar system were aligned. But for whatever reason, the universe doesn’t seem to have super complicated laws.
Is there any plausible argument for why our universe doesn’t have super-complicated laws?
The only thing I can think of is that laws are somehow made from small components so that short laws are more likely than long laws.
Another possibility is that if some behavior of the universe is complicated, we don’t call that a law, and we keep looking for something simpler—though that doesn’t explain why we keep finding simple laws.
So I know you said you were simplifying, but what if the worlds interfere? You don’t necessarily get the same amount of mass before “collapse” (that is, decoherence) and after, because you may have destructive interference beforehand which by construction you can’t get afterwards.
As an aside, in amplitude analysis of three-body decays, it used to be the custom to give the “fit fractions” of the two-body isobar components, defined as the integral across the Dalitz plot of each resonance squared, divided by the integral of the total amplitude squared. Naturally this doesn’t always add to 100%, in fact it usually doesn’t, due to interference. So now we usually give the complex amplitude instead.
A) If they’re able to interfere, you shouldn’t have called them separate worlds in the first place.
B) That’s not how interference works. The worlds are constructed to be orthogonal. Therefore, any negative interference in one place will be balanced by positive interference elsewhere, and so you don’t end up with less or more than you started with. You don’t even need to look at worlds to figure this out—time progression is unitary by the general form of the Schrodinger Equation and the real-valuedness of energy.
I don’t believe I am explaining MWI instead of arguing against it… whatever has this site done to me? Anyway, grossly simplified, you can think of the matter as being conserved because the “total” mass is the sum of masses in all worlds weighted by the probability of each world. So, if you had, say, 1kg of matter before a “50/50 split”, you still have 1kg = 0.5*1kg+0.5*1kg after. But, since each of the two of you after the split has no access to the other world, this 50% prior probability is 100% posterior probability.
Also note that there is no universal law of conservation of matter (or even energy) to begin with, not even in a single universe. It’s just an approximation given certain assumptions, like time-independence of the laws describing the system of interest.
LOL @ your position. Agree on most.
Disagree on the conservation of energy though. Every interaction conserves energy (unless you know of time-dependent laws?). Though nothing alters it, we only experience worlds with a nontrivial distribution of energies (otherwise nothing would ever happen) (and this is true whether you use MWI or not)
I don’t know enough of the underlying physics to conclusively comment one way or another, but it seems to me defining “total mass” as the integral of “local mass” over all worlds wrt the world probability measure implies that an object in one world might be able to mysteriously (wrt that world) gain mass by reducing its mass in some set of worlds with non-zero measure.
We don’t actually see that in e.g. particle scattering, right?
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!
Defining total energy as the integral of energy over space implies that an object in one part of space might be able to mysteriously gain energy by reducing energy in other parts of space.
Do we see this in the real world? How useful is the word “mysterious” here?
Ordinary energy conservation laws are local: they do not just state that total energy is conserved, but that any change in energy in a finite region of any size is balanced by a flux of energy over the boundary of that region. I don’t think any such laws exist in “multi-world-space”, which even accepting MWI is basically a metaphor, not a precise concept.
So are there mysterious fluxes that move energy from one part of space to another?
Umm, yes ? They’re quite ubiquitous.
Those look more like boring, physical-law-abiding (non-mysterious) fluxes that move energy form one part of space to another.
Not mysterious ones, no—only the ordinary ones that Plasmon mentions.
“Mysterious” here means “via an otherwise unexplained-in-a-single-world mechanism.”
There’s no mysterious quantum motion for the same reason there’s no mysterious energetic motion—because energy / mass / quantum amplitude has to come from somewhere to go somewhere, it requires an interaction to happen. An interaction like electromagnetism, or the strong force. You know, those ubiquitous, important, but extremely well-studied and only-somewhat-mysterious things. And once you study this thing and make it part of what you call “energy,” what would otherwise be a mysterious appearance of energy just becomes “oh, the energy gets stored in the strong force.” (From a pure quantum perspective at least. Gravity makes things too tricky for me)
The best way for a force to “hide” is for it to be super duper complicated. Like if there was some kind of extra law of gravity that only turned on when the planets of our solar system were aligned. But for whatever reason, the universe doesn’t seem to have super complicated laws.
Is there any plausible argument for why our universe doesn’t have super-complicated laws?
The only thing I can think of is that laws are somehow made from small components so that short laws are more likely than long laws.
Another possibility is that if some behavior of the universe is complicated, we don’t call that a law, and we keep looking for something simpler—though that doesn’t explain why we keep finding simple laws.
“We looked, and we didn’t find any super-complicated laws.”
So I know you said you were simplifying, but what if the worlds interfere? You don’t necessarily get the same amount of mass before “collapse” (that is, decoherence) and after, because you may have destructive interference beforehand which by construction you can’t get afterwards.
As an aside, in amplitude analysis of three-body decays, it used to be the custom to give the “fit fractions” of the two-body isobar components, defined as the integral across the Dalitz plot of each resonance squared, divided by the integral of the total amplitude squared. Naturally this doesn’t always add to 100%, in fact it usually doesn’t, due to interference. So now we usually give the complex amplitude instead.
A) If they’re able to interfere, you shouldn’t have called them separate worlds in the first place.
B) That’s not how interference works. The worlds are constructed to be orthogonal. Therefore, any negative interference in one place will be balanced by positive interference elsewhere, and so you don’t end up with less or more than you started with. You don’t even need to look at worlds to figure this out—time progression is unitary by the general form of the Schrodinger Equation and the real-valuedness of energy.