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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

#101

Post by kgwgk »

vilhelm_s wrote: post #98
kgwgk wrote: post #90

Earlier quoted context omitted.

There is no "randomness generated along the way" in the MWI. Schroedinger's equation is deterministic and according to many-worlders is the only thing that "makes things happen". Everything that happens in every "world" was bound to happen since the beginning of the evolution of the "multiverse".

Right, but my point is that this doesn't mean that the initial state contains very much information. Maybe a different example makes this clearer. Suppose you use a quantum random number generator to choose letters from A to Z, and write out a full book this way. If you consider the entire (multiple) world, it's basically Borges' Library of Babel: it has non-zero amplitudes for every possible book. So one possible "t…

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?

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

#102

Post by vilhelm_s »

kgwgk wrote: post #101
vilhelm_s wrote: post #98

Earlier quoted context omitted.

Right, but my point is that this doesn't mean that the initial state contains very much information. Maybe a different example makes this clearer. Suppose you use a quantum random number generator to choose letters from A to Z, and write out a full book this way. If you consider the entire (multiple) world, it's basically Borges' Library of Babel: it has non-zero amplitudes for every possible book. So one possible "t…

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 it you get more information, and narrowing it down to sending a particular book gives you more bits still.

It's the same with the multiverse. If we start out with the photon about to go through the half-silvered mirror, the state looks like |photon>⊗|me>, and then over time it evolves into

(1/√2)|photon went left>⊗|me observing left> + (1/√2)|photon went right>⊗|me observing right>

and the latter state contains no more information than the former one (you can run the time evolution backwards to recover the original state). However, if you restrict your attention and only consider one of the branches, e.g. |photon went right>⊗|me observing right>, then you have added (one more bit) of information. And most macroscopic processes can be explained "one branch at a time", since in practice decoherence means that there are no noticable interactions between branches. I think in the MWI this is the source of most complexity in the universe: not detailed information encoded in the initial state, and not objective collapse, but the bits of information you implicitly create if you only consider one branch of an experimental outcome.

I imagine the same thing happens in general: we start with some uniform soup (with ~zero information), in a bunch different MWI branches it self-gravitates slightly differently so we get a different set of galaxies and stars in each branch, then for each branch that has a planet, different quantum randomness may cause different intelligent life to develop, and eventually they may write different books. Of course, it's also possible that some small initial state gets magnified, as a separate mechanism, but that's the same in objective-collapse and MWI. The random bits created by objective collapse are the same same the ones you get by looking at particular branches in MWI.

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

#103

Post by jeremyjh »

platz wrote: post #70
gizmo686 wrote: post #67

Earlier quoted context omitted.

Haven't we tested precisly this? The fact that we observe interference patterns shows that the wavefunctions are real. As far as I can tell, Copenhagen posits that there is a scale at which wave functions stop being real, and instead collapse down to a single "macroscopic" state. Convieniently, this scale happens to be around the scale where our experiments stop being able to test for it in practice. Put another way,…

> The fact that we observe interference patterns shows that the wavefunctions are real. well, not quite - remember that what we actually observe for a given particular event is something that looks like particle that is localized in space. If you're referring to the double slit experiment, the interference pattern emerges as a property of the whole collection of individual photon strikes. Yes, you need to use the wav…

No, in a particle model there could be no interference pattern. That is the whole point. You can do the experiment - and it has been done - with a single photon and you still get interference.

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

#104

Post by platz »

jeremyjh wrote: post #103
platz wrote: post #70

Earlier quoted context omitted.

> The fact that we observe interference patterns shows that the wavefunctions are real. well, not quite - remember that what we actually observe for a given particular event is something that looks like particle that is localized in space. If you're referring to the double slit experiment, the interference pattern emerges as a property of the whole collection of individual photon strikes. Yes, you need to use the wav…

No, in a particle model there could be no interference pattern. That is the whole point. You can do the experiment - and it has been done - with a single photon and you still get interference.

You've misunderstood my point.

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

#105

Post by lmm »

TheOtherHobbes wrote: post #75
lmm wrote: post #57

Earlier quoted context omitted.

Deutsch may believe that. (Heck, I believe it). It's a natural interpretation of the MWI wavefunction. But it is an interpretation, a layer on top of the physical claims, a model - not a mandatory part of MWI. Deutsch may interpret a given wavefunction as describing a particular ensemble of 'tangible' and 'shadow' objects, but someone who interprets that same wavefunction a different way (but agrees on the same wavef…

That makes MWI a "no true parallel Scotsman" interpretation. Either the other versions of reality are tangible, or they're not. If they're just abstractions, you have't solved the problem - you've taken a long way around a circular argument that starts with the wavefunction and ends with it doing something that results in a measurement, maybe. Unfortunately explaining how that happens is the problem you're trying to…

> Either the other versions of reality are tangible, or they're not. If they're just abstractions, you have't solved the problem - you've taken a long way around a circular argument that starts with the wavefunction and ends with it doing something that results in a measurement, maybe.

The wavefunction is physically real. Whatever you interpret it as is your business. We've solved a real problem: we can explain the evolution of the wavefunction and the results of experiments without needing some weird collapse/decoherence concept that breaks all the usual rules. The Born Rule is still something that remains to be explained, agreed, but it's a much smaller something than the full Copenhagen style collapse.

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

#106

Post by faissaloo »

yters wrote: post #100
faissaloo wrote: post #96

Earlier quoted context omitted.

Observation causes collapse, so perhaps because the effect it has is negligible to the system of the universe there's simply no point 'rendering'/wave collapsing. Perhaps there is some other collapse mechanism to explain life before conscious observers, or perhaps quantum behavior emerged as time went on after the big bang, or perhaps there exists some kind of proto-conciousness.

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.

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

#107

Post by lmm »

TheOtherHobbes wrote: post #73
lmm wrote: post #17

Earlier quoted context omitted.

Conversely, Copenhagen-like explanations need to explain (or postulate) a decoherence/collapse operation that violates all the usual rules of QM: it would be non-time-symmetric, non-unitary, likely non-local (propagating faster than light), ...

Non-locality is practically axiomatic if you understand Bell's Inequalities, and doesn't require true FTL signalling. MWI doesn't really solve either the locality or the FTL problem. If you're going to believe in MWI you have to explain how an entire universe appears instantaneously, identical in every way to another universe, except for one almost insignificant difference in exactly one quantum interaction. That's a…

> Non-locality is practically axiomatic if you understand Bell's Inequalities, and doesn't require true FTL signalling.

My complaint is exactly that the Copenhagen interpretation introduces that unnecessary FTL signalling requirement. Under the MWI, nothing weird is going on in the EPR thought experiment. But if you believe that a collapse occurs when we measure system A, then that collapse has to somehow occur non-locally and/or be FTL-transmitted between A and B.

> If you're going to believe in MWI you have to explain how an entire universe appears instantaneously, identical in every way to another universe, except for one almost insignificant difference in exactly one quantum interaction.

You don't; to talk about it in those terms is putting the cart before the horse. You have to believe that a wavefunction evolves in the usual way and represents the thing that it usually represents. You have to believe that when a pure system interacts with a system in superposition then the result is two systems that are entangled and in superposition - something you presumably already believe if you believe in any kind of QM at all. You only have to believe that a new universe appears instantaneously if your interpretation of putting a particle into superposition is that a new particle appears instantaneously, which is not an approach I've ever heard advocated.

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

#108

Post by jeremyjh »

platz wrote: post #104
jeremyjh wrote: post #103

Earlier quoted context omitted.

No, in a particle model there could be no interference pattern. That is the whole point. You can do the experiment - and it has been done - with a single photon and you still get interference.

You've misunderstood my point.

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?

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

#109

Post by platz »

jeremyjh wrote: post #108
platz wrote: post #104

Earlier quoted context omitted.

You've misunderstood my point.

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 sense, we don't understand the ontological status of the wavefunction and what it's physical status is.

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

#110

Post by bonoboTP »

monktastic1 wrote: post #82
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…

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 there are also these two branches all along, but they overlap where the friend hasn't yet looked in the box, they then go apart in the part where the friend makes an observation, overlap where Wigner hasn't looked at his friend yet, but then they are distinct again where Wigner has looked at the friend's state. Similarly to how particles and light take all possible trajectories at the same time (path integral formulation), we could think that in the space-time places where the branches overlap, all compatible pasts are equally valid, at other space-time locations the particular strand extends back through the past to just one of the cat states, so the state seems collapsed, but the stand also led through the overlap area, so you remember the time where you didn't yet have the "which branch you're on" information.

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