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How the Many-Worlds Theory of Hugh Everett Split the Universe

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Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#111

Post by jeremyjh »

platz wrote: post #109
jeremyjh wrote: post #108

Earlier quoted context omitted.

Maybe so, but this sentence: "interference pattern emerges as a property of the whole collection of individual photon strikes." Is incorrect and seems to miss the whole point of the experiment. We don't need a "whole collection" to get interference, we can see it with one photon. If that doesn't mean the wave function is real, how else can it be explained?

There is both the "interference pattern" on the target wall made by individual photons over time, which we can directly observe, and the interference of the photon's wavefunction with itself, which we cannot directly observe. I was referring to the former. We know the photon interferes with itself, via the wavefunction as you describe, but this interaction is entirely unobservable and invisible to us, and in that sen…

Sorry, what is it we cannot observe in the latter? We see the pattern on the screen. Do you mean we cannot observe the interference that leads to that pattern? That is quite right since the only way to see a photon is to bounce it off of something.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#112

Post by bonoboTP »

yters wrote: post #27
fallingfrog wrote: post #8

You can do a really simple thought experiment that provides a persuasive argument in favor of the many worlds theory. Or at least, it shows that wave function collapse is relative to an observer, not universal everywhere. Start with Schroedingers cat: the cat is in a box, it’s in a state of quantum superposition. It’s 50/50 alive or dead. Now a researcher opens the box: the cat is found to be either alive, or dead, a…

The problem with many worlds is why do I think I'm in a particular timeline? If many worlds is correct, there is no "I", there are many rapidly branching "I"s, which intuitively doesn't make sense. Of course, each "I" will have the perception of being the sole "I", but the overall view that there is no singular "I" is very counter to my self conception. I prefer to preserve intuitions over conjectures, so I count tha…

There is a singular you, the one that is your time-line. There are also different versions of that in other timelines. But as you don't expect to feel the consciousness of the guy spatially next to you, the other parallel universe has also no influence on your conscious feeling of self.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#113

Post by monktastic1 »

bonoboTP wrote: post #110
monktastic1 wrote: post #82

Earlier quoted context omitted.

This is also known as the Wigner's Friend paradox. Your explanation is not the standard understanding of Many Worlds today, however. The party line is that world splitting happens when decoherence has become "sufficiently irreversible" (for some arbitrary definition of "sufficiently", since there's no evidence of genuine irreversibility anywhere in physics), which in almost all real-world setups happens almost immedi…

Is there actually a "split"? I've always imagined it more like parallel strands that were always distinct but had overlaps. Like in the cat case, it's undetermined whether I'm on the cat dead or cat alive time-line because both are "overlapping" in the part of space-time where my brain is at the time of not knowing the cat's state. Then when I make a measurement, the two strands no longer overlap. With Wigners friend…

I'm not sure I've digested your whole comment, but the basic idea is this:

A "split" only means that the respective branches of the wave function cannot interfere with each other any longer. Then they will evolve completely separately from each other, and can be considered distinct. The only way you can get branches to interfere is if you have precise control over all particles involved in the entanglement, which is practically impossible for any macroscopic system.

Long before you entangle with the cat system, the two branches have decohered. While technically there is still an enormous superposition there, for all practical purposes it is indistinguishable from being a classical system, with classically indeterminate branches.

I should say that I'm not a fan of this explanation, possibly related to your points. Regardless of "practical purposes," QM does not allow me to say that there is a well-defined answer before it entangles with me (and that there is one afterward). I'm much more comfortable using the word "split" only there. But this skirts too close to solipsism for some, since "my world" (which I also know as "the world") only comes into being upon contact with me.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#115

Post by kgwgk »

vilhelm_s wrote: post #102
kgwgk wrote: post #101

Earlier quoted context omitted.

The initial state contains as much information as the current state if the evolution is unitary (only Schroedinger’s equation matters according to the MWI, right?). And surely the wavefunction of a system of 10^80 particles (or whatever) does contain some information about what happens... or do you think that “every book would get written” independently of the form of the universal wavefunction?

But the "current state" of the entire wavefunction doesn't necessarily contain very much information. You only get most of the complexity (solar flares, novels) if you look at subsystems of it. I think the situation is exactly analogous to the Library of Babel in Borges' story. If someone sends you the entire library in the mail, they have given you zero bits of information. If they only send you a particular room of…

> If we start out with the photon about to go through the half-silvered mirror, the state looks like |photon>⊗|me>,

But in the MWI what we start with is the universal wavefunction |Psi>. Say there are 10^80 particles in the universe and they may have a spin and could be in infinite locations. That makes for an ininite-dimensional Hilbert space. If we consider that the precision in the location variables is limited by Planck's length there are still around 10^180 possible locations in the universe for each particle. The Hilbert space where the quantum state of the universe is defined is huge.

Now, you say "the state looks like |photon>⊗|me>". To keep things simple and in the realm of Schroedinger's equation I will consider an electron instead of a photon, prepared in the state spin z=+1/2.

Is "|up>⊗|me>" shorthand for "(1 |up> + 0 |down>)⊗|me, the electron at that position, the rest of the universe>" which would be the "in-branch" quantum state of the universe?

If indeed |Psi> can be written as a superposition of that state and an inconceivable huge number of orthogonal states, are all those states (when their coefficient is non-zero) the multiple worlds?

But given |Psi> there are infinitely many (or at least a lot of) possible basis to express the pure state as a decomposition (including of course the one where the only non-zero "world" in the superposition is the one described by the wave function |Psi>). Do we count as "worlds" as well the states appearing in other basis?

Is this slicing of |Psi> into other states just a mathematical trick or does it have a physical meaning? Even if we can redo the operation at different times on the evolving |Psi(t)>, how are the "emergent substates" at different times related?

The answer to all these questions is far from trivial.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#116

Post by yters »

faissaloo wrote: post #106
yters wrote: post #100

Earlier quoted context omitted.

I guess my question is if the conscious observers are themselves part of the universe, what caused their waveform to collapse? And what is the special 'observation' part of their material properties such that it can cause collapse? Why don't other material configurations cause collapse?

I'm inclined more towards a dualistic explanation than a material one. Perhaps a better word to describe these concious and proto-concious entities would be 'collapsing agents', 'collapsants' or something of the sort.

Interesting idea, where would the dualist agents exist?

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#117

Post by faissaloo »

yters wrote: post #116
faissaloo wrote: post #106

Earlier quoted context omitted.

I'm inclined more towards a dualistic explanation than a material one. Perhaps a better word to describe these concious and proto-concious entities would be 'collapsing agents', 'collapsants' or something of the sort.

Interesting idea, where would the dualist agents exist?

I'd speculate that they exist as some unobservable component of reality, considering we see no hard evidence for it with our existing tools. I'm no expert on string theory, but as I understand it predicts a large number of dimensions, if we think of these as arrangements of data in memory, perhaps this information could be encoded there?

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#118

Post by vilhelm_s »

kgwgk wrote: post #115
vilhelm_s wrote: post #102

Earlier quoted context omitted.

But the "current state" of the entire wavefunction doesn't necessarily contain very much information. You only get most of the complexity (solar flares, novels) if you look at subsystems of it. I think the situation is exactly analogous to the Library of Babel in Borges' story. If someone sends you the entire library in the mail, they have given you zero bits of information. If they only send you a particular room of…

> If we start out with the photon about to go through the half-silvered mirror, the state looks like |photon>⊗|me>, But in the MWI what we start with is the universal wavefunction |Psi>. Say there are 10^80 particles in the universe and they may have a spin and could be in infinite locations. That makes for an ininite-dimensional Hilbert space. If we consider that the precision in the location variables is limited by…

Indeed not, but is any of this relevant to the original quote, that deterministic dynamics means that the initial state of the universe must have lots of information hidden in the far-off decimals? I would hope that one could consider a simplified setting for that and then extrapolate back to the continuous case.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#119

Post by ncallaway »

lisper wrote: post #50
ncallaway wrote: post #41

Earlier quoted context omitted.

Sure, but I would think every one of those advocates would also agree with what I wrote. That the "world" described by many worlds is a convenient short-hand for a single position in a massively entangled system of particles in a complex-superposition. It's the "Many Worlds" is common moniker, and the rhetoric does use the concept of worlds all the time, but whenever you get down to the specifics of the theory there…

> I would think every one of those advocates would also agree with what I wrote. No, they wouldn't. They don't. See my response to lmm here: https://news.ycombinator.com/item?id=20941803 > "MWI insists that experiments have multiple outcomes in point of metaphysical fact." It's a fairly moot point at this point since that statement was removed. I'll stand by that with one minor modification: contemporary advocates of…

Sorry for the slow response. Been a busy couple of days :)

> No, they wouldn't. They don't. See my response to lmm here

I don't think lmm and I disagree.

I said "[The underlying principle of Many Worlds] is that there is a singular wave function, which is the true reality". lmm says: "The only thing that MWI demands we take as metaphysically real is the wavefunction".

I think this is expressing the same thing in different terms.

I said: "The things we refer to as "worlds" are massively-complicated entanglements and superpositions within that wave function". lmm says: "Given a wavefunction like 1/sqrt2(|experimenter who observed a live cat> + |experimenter who observed a dead cat>) ...". Again, I think our descriptions are in fairly close agreement.

I think lmm's description is a good summary of how the "worlds" in Many Worlds are viewed by Many Worlds advocates: "in the same way that people who hear a particular pattern of sound might find it convenient to think of it as a chord of two or three notes". The "worlds" aren't actually fundamental to the theory, but rather an emergent phenomena from them.

The reason for the label "Many Worlds" is not because they are the fundamental concept. It's because it's the most... interesting element from a pop-sci marketing angle.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#120

Post by kgwgk »

vilhelm_s wrote: post #118
kgwgk wrote: post #115

Earlier quoted context omitted.

> If we start out with the photon about to go through the half-silvered mirror, the state looks like |photon>⊗|me>, But in the MWI what we start with is the universal wavefunction |Psi>. Say there are 10^80 particles in the universe and they may have a spin and could be in infinite locations. That makes for an ininite-dimensional Hilbert space. If we consider that the precision in the location variables is limited by…

Indeed not, but is any of this relevant to the original quote, that deterministic dynamics means that the initial state of the universe must have lots of information hidden in the far-off decimals? I would hope that one could consider a simplified setting for that and then extrapolate back to the continuous case.

I agree the original quote doesn’t present a very strong argument against the MWI. In the same way that arguments against determinism in classical physics are not very convincing. But seemed worth pointing out.

In the MWI case it’s not clear how sensitive the “worlds” are to the in initial state. It’s not even clear how sensitive they are to the current state! Better understanding of how (and how many) worlds “emerge” would be needed to say if the “problem” is or not as bad as in the classical case.

Is the number of worlds stable? Is it increasing? In any case, those worlds are the inevitable consequence of the initial conditions even though we can’t even estimate how much fine-tuning is needed. I don’t think “randomness generated along the way” is a satisfactory counter-argument.

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