Many Worlds vs Modalism

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There seems to be as many branches of Everettian QM as there are branches in Everettian QM. One of the things that never seem to be clarified by anyone is whether MWI is really just Modalism or not. I.E. whether *all worlds* happen *all the time*, or if there are specific conditions necessary. I re-read a thread I was part of regarding improbability in MWI and noticed a stark contrast between the participants.

One school of thought seems to advocate for modalism I.E. that everything happens all the time from all branches, and others indicate there are parameters.

To put this into perspective: everyone seems to agree that if MWI is true then a quantum measurement branches the world into all non-zero probabilities, making them actual. However, some say that since the entire universe is fundamentally made up of subatomic particles this means that right now on this branch we inhabit reading this sentence will branch further onto at least 1 where the sun spontaneously goes supernova or turns into a giant pumpkin. The latter seems absurd, but isn't it just a natural consequence of taking MWI *really* seriously?
 

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  • #2
PeterDonis
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One of the things that never seem to be clarified by anyone is whether MWI is really just Modalism or not.
What is "modalism"? Do you have a reference?
 
  • #4
PeterDonis
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re-read a thread I was part of regarding improbability in MWI
Which thread? Please give a link.
 
  • #5
PeterDonis
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everyone seems to agree that if MWI is true then a quantum measurement branches the world into all non-zero probabilities, making them actual
No, that's not what MWI says. MWI says that the wave function is real. The wave function already contains all the "branches"; nothing has to be "split" when a measurement takes place. All that happens during a measurement, according to the MWI, is a unitary evolution that entangles subsystems. Nothing is "made" actual that wasn't already.
 
  • #6
PeterDonis
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some say that since the entire universe is fundamentally made up of subatomic particles this means that right now on this branch we inhabit reading this sentence will branch further onto at least 1 where the sun spontaneously goes supernova or turns into a giant pumpkin.
There might be a small nonzero amplitude for the first (sun spontaneously going supernova) in the wave function, and if so, yes, MWI says the unitary evolution of the wave function would include it.

There is no amplitude for the second (sun turning into a giant pumpkin) since it violates conservation laws. So the MWI does not say the unitary evolution of the wave function would include it.
 
  • #7
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There might be a small nonzero amplitude for the first (sun spontaneously going supernova) in the wave function, and if so, yes, MWI says the unitary evolution of the wave function would include it.

There is no amplitude for the second (sun turning into a giant pumpkin) since it violates conservation laws. So the MWI does not say the unitary evolution of the wave function would include it.
Why though? I fail to see how they are physically different on a fundamental level. While the pumpkin sounds more ludicrous, it's just a reconfiguration of subatomic particles, just like a sun going supernova?
 
  • #8
PeterDonis
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While the pumpkin sounds more ludicrous, it's just a reconfiguration of subatomic particles
No, it isn't, it's a huge change in chemical composition which will violate conservation laws.
 
  • #9
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No, it isn't, it's a huge change in chemical composition which will violate conservation laws.
And a stable sun going supernova in the split of a second isn't?
 
  • #10
PeterDonis
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a stable sun going supernova in the split of a second isn't?
If it can go supernova spontaneously, it's not stable.

In any case, a supernova, while it certainly induces reactions that change the chemical composition, does not do so in a way that violates any conservation laws.
 
  • #11
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If it can go supernova spontaneously, it's not stable.

In any case, a supernova, while it certainly induces reactions that change the chemical composition, does not do so in a way that violates any conservation laws.
No, but I mean we know right now that the sun is stable, so for it to become unstable right this second branching off from this current branch, I don't see how that is inducing less radical changes than a pumpkin would?
 
  • #12
PeterDonis
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we know right now that the sun is stable
If we actually know that with certainty, then there is zero amplitude in the wave function for it to go supernova spontaneously. That means it can't "become unstable".

If there is in fact a nonzero amplitude in the wave function for the sun to spontaneously go supernova, then we are mistaken if we currently believe the sun is stable.

Since we don't know the exact wave function of the sun, we don't know for sure which of the two alternatives above is in fact the case.

In the case of the sun turning into a pumpkin, however, we do know with certainty that the first of the two alternatives above (zero amplitude in the wave function) is the case. We know that not because we know the exact wave function of the sun, but because we know that turning the sun into a pumpkin would violate conservation laws, and the wave function cannot contain nonzero amplitude for any process that violates conservation laws.
 
  • #13
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If we actually know that with certainty, then there is zero amplitude in the wave function for it to go supernova spontaneously. That means it can't "become unstable".

If there is in fact a nonzero amplitude in the wave function for the sun to spontaneously go supernova, then we are mistaken if we currently believe the sun is stable.

Since we don't know the exact wave function of the sun, we don't know for sure which of the two alternatives above is in fact the case.

In the case of the sun turning into a pumpkin, however, we do know with certainty that the first of the two alternatives above (zero amplitude in the wave function) is the case. We know that not because we know the exact wave function of the sun, but because we know that turning the sun into a pumpkin would violate conservation laws, and the wave function cannot contain nonzero amplitude for any process that violates conservation laws.
So then going back to the example Michael Price gave: how would he become president in at least one branch? That whole argument was based around the particles reconfiguring spontaneously and putting him in the white house while also reconfiguring the particles making up the brains of everyone to produce false memories of him indeed as president?
 
  • #14
PeterDonis
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going back to the example Michael Price gave
In what thread? As I asked before, please give a link.
 
  • #16
PeterDonis
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Thanks for the link.

That whole argument was based around the particles reconfiguring spontaneously and putting him in the white house while also reconfiguring the particles making up the brains of everyone to produce false memories of him indeed as president?
I'm not entirely sure what process he was thinking of or whether such a process would actually have a nonzero amplitude in the wave function. My basic objection to such scenarios is precisely that: instead of actually trying to figure out what the relevant wave function is and what processes have nonzero amplitudes, people just wave their hands and assume that any process they can imagine will have a nonzero amplitude in the wave function, with no justification whatever for that assumption.
 
  • #17
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The sun may be suddenly swallowed by a black hole passing by, and then the cosmic rays may well synthesize a pumpkin.
 
  • #18
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The sun may be suddenly swallowed by a black hole passing by, and then the cosmic rays may well synthesize a pumpkin.
Wouldn't this require that said black hole already preexisted on the branch? Or are you saying that it has a non-zero probability of just suddenly "happening"?
 
  • #19
PeterDonis
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The sun may be suddenly swallowed by a black hole passing by, and then the cosmic rays may well synthesize a pumpkin.
Please show me the wave function that has a nonzero amplitude for this process.
 
  • #20
Demystifier
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And a stable sun going supernova in the split of a second isn't?
There are two types of physical laws:
A) Exact, always valid laws.
B) Statistical laws, the violation of which is very unlikely.

An example of an A-law is energy conservation. An example of a B-law is the 2nd law of thermodynamics. The Sun suddenly transforming into a supernova is forbidden by a B-law, not by an A-law.
 
  • #21
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The MWI is distinct from modal realism. A modal realist concerns themself with a broader kripkean notion of 'world'. For example:
To put this into perspective: everyone seems to agree that if MWI is true then a quantum measurement branches the world into all non-zero probabilities
A modal realist might also consider a world where the MWI is false to be just as actual as a world where it is true.

Or a world where everything is made up of classical billiard balls.

Or a world where there are no regularities to be codified by laws of physics.
 
  • #22
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There are two types of physical laws:
A) Exact, always valid laws.
B) Statistical laws, the violation of which is very unlikely.

An example of an A-law is energy conservation. An example of a B-law is the 2nd law of thermodynamics. The Sun suddenly transforming into a supernova is forbidden by a B-law, not by an A-law.
I am still struggling a bit with this. While I am of course well aware of the laws of physics in general, I still struggle to see how the usual understanding of them don't go out the window with MWI. Naturally anything that violates energy conservation is impossible, like a perpetuum mobile, but I don't see why rearrangement of the mass of the sun into another state of matter violates it.

Let's use another example:

Take this tweet by Sean Carroll


Given that the brain (based on all known physics at the moment) is a heat bath which kills all decoherence too fast, and the fact that one nucleus decaying would not be sufficient to alter behavior: why is this possible? I.E. it would take millions of atoms all decaying at once for his claim to be real. Sure, this is statistical, but why wouldn't the sun suddenly changing into a pumpkin (it surely has enough energy) also just be statistical?
 
  • #23
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Naturally anything that violates energy conservation is impossible, like a perpetuum mobile, but I don't see why rearrangement of the mass of the sun into another state of matter violates it.
Who said that such an rearrangement violates the conservation of energy? It doesn't.
 

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