Comment by dbvdh
20 hours ago
"This rather fanciful interpretation of Quantum Physics has now become accepted by many theoretical physicists as being more satisfactory than the Copenhagen interpretation, despite the fact that the mechanism which causes the splitting of the Universe is not explained in any more detail than wavefunction collapse. As you can probably tell, I am in the minority of physicists who finds the Many Worlds interpretation dificult to accept, and not particularly illuminating."
I think these lectures are a good intro to QM. This statement feels very emotional though. Many-worlds just states there is no such thing as wave function collapse. The many worlds are a logical conclusion following. Copenhagen simply refuses to elaborate to when wave-function collapse happens, which to me is not a complete theory.
You could have some critique of many-worlds in relation to statistics, although I think many people have given quite satisfactory explanations.
Many worlds is just bare bones QM.. If any other theory is proposed, it should be falsifiable (like pilot wave theory for example). Doesn't mean it won't be difficult to measure.
FWIW (and it's not worth a lot, since I'm a layperson), I think the interpretation that makes the most sense to me is Quantum Decoherence (https://en.wikipedia.org/wiki/Quantum_decoherence).
My interpretation of the interpretation is that the universe is a giant constraint resolution system. Particles are free to be anywhere randomly as long as they do not randomly interact with other particles. As soon as they interact they have fewer "degrees of freedom", since now they interacted. When they interact with anything macroscopic they have no freedom left. Makes sense to me anyway.
Decoherence isn't an interpretation, but an experimentally established fact. What makes interpretations interpretations is that they cannot be experimentally distinguished (or in some cases, can only be with great difficulty). That said, the study of decoherence has certainly made aspects of the measurement problem easier to come to terms with, which in turn has bearing on the intuitive draw of some interpretations over others. In particular, it helps answer when universes should be said to "split" in MWI (though, as many will rightly tell you, that's not a complete solution to the measurement problem, as decoherence is still unitary--i.e., there's no in-principle irreversibility there).
Decoherence is how you get from "Maybe the wave function never collapses" to "Maybe there are many parallel 'universes' in QM.".
Decoherence gets you a diagonal density matrix description of the world. It doesn't get you a single-state description of the world. Going from "There are many parallel universes" to "and we are in one of them" still takes a lot of handwaving.
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Decoherence only hides entanglement by having the entire world go into a complex superposition. It's entirely compatible with MWI. Decoherence does not resolve the measurement problem on its own. It's not a separate interpretation. It only provides us a mathematical reason for why the world appears classical.
Many-worlds is an interpretation and not a theory. It makes no additional predictions beyond what QM does. Believing in any interpretation (or shutting up and calculating) is purely emotional; it has no scientific content.
(Disclaimer: I'm nowhere near truly knowledgeable about any of this)
Quantum computing was explicitly proposed as a way to prove the truth of many worlds. As far as we can tell, quantum computing appears to work.
I don't think most people believe QC truly proves many-worlds, but I think it is considered some evidence in its favor, and historically it was I think one of the reasons for the development of QC originally.
I've heard this claim, but I don't think it holds up. The idea is that supposedly the calculation is done in parallel across the different universes which allows you to see an exponential speedup in some calculations. But the whole point of many-worlds is that after a branch event the various branched portions of the wavefunction no longer interact. Even if it were true that quantum computing was really a bunch of parallel classical computations (which it is not), there would be no way for any of the other universes to communicate the result back to the universe you observe it in.
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I don't think there is anything in quantum computing that is specific yo the many worlds interpretation.
But if what you want is understanding, you want a good explanation. (Or to quote TheOtherHobbes from elsewhere in these comments, a reasonable metaphor: https://news.ycombinator.com/item?id=50220700)
If you only like predictions and disparage explanations, I might start calling you names, specifically instrumentalist.
What do call someone who wants good "Just So" story explanations that don't explain anything, but posit the existence of meaningless things?
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It has mathematical and philosophical content. People can be convinced for reasons other than empirical without it being emotional. Sean Carroll makes logical arguments for why he's convinced MWI is correct.
I'm not Sean Carroll. I'm interested in logical arguments for why I should be convinced that something completely impossible to ever test, not even in theory, is correct.
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The quantum world involves states that are a complex linear superposition of all possible states. It isn't just that the world bifurcates into one where the cat is |alive> or |dead>, but also all c1 |alive> + c2 |dead>, where c1^2+c2^2 = 1 and c1 and c2 are complex. The MWI has no explanation for how this other infinity of possible states disappears into just two states. You still wind up with the collapse of the wave function--involving a lot of hand-waving or appealing to some nebulous property of consciousness.
>The MWI has no explanation for how this other infinity of possible states disappears into just two states. You still wind up with the collapse of the wave function--involving a lot of hand-waving or appealing to some nebulous property of consciousness.
The real issue here is the preferred basis problem, not an infinity of worlds: a single state vector can be decomposed many ways, and the bare math doesn't say which decomposition counts as "the branches." Decoherence picks it out: once the cat is entangled with its environment, the alive/dead decomposition becomes effectively unique and stable, through ordinary unitary dynamics. No collapse, no consciousness (that's von Neumann–Wigner, not Everett). MWI does have an open problem, but it's a different one: the Born rule, i.e. why |c|^2 should count as probability when every branch is real. Also, the normalization is |c1|^2 + |c2|^2 = 1.
Sure, the MWI still has an open problem which is why it looks like there is a collapse of the wave function.
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AI comment
Given we haven’t bridged quantum with gravity yet, my bet is the function collapses right at the boundary when gravity starts to play a meaningful role.
That’s a more physically plausible interpretation than ever “tick” of the universe actually creates infinitely more universes because every fundamental particle is quantum and each one is constantly interacting under the wave function every plank second. And the infinite number of possible events that a single particle would experience is multiplied by the joint combination of all particles in the entire universe. It doesn’t seem plausible.
How would you define exactly ‘when gravity starts playing a role’. It always plays a role, from photons to planets.
And on the ‘every tick creates more universes’: we are completely fine with superposition in nanoscopic systems. It’s just uneasy to follow that conclusion to the logical end. Just my two cents. I think just because the conclusion seems hard to grasp, the reasoning to get there is completely sound..
“Starts to play a role” means when the effect of gravity becomes significant to the quantum equations which currently don’t have any placeholder for gravity.
Just like Newton’s equations are accurate to describe gravity and motion but only because the full equations Einstein worked simplify to them at small speeds.
So similarly it would make more sense that quantum equations and relativity are two sides of the same coin and there’s some missing mathematical bridge where at small particle sizes it simplifies to quantum and at large particle scales gravity matters and we get Newton and then further relativity.
Using Occam’s razor that’s at least my prior vs some undefined mechanism of action where all possible interactions of every particle actually creates an infinite superposition of universes where that did happen and you get an infinite number of such superpositions ad infinitum. Much easier to believe the huge gaping hole between two very accurate theories explains how quantum mechanics turns into classical mechanics, especially since we know gravity exists, what it is, and quantum mechanics can’t explain gravity.
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This is more or less Roger Penrose's position: https://web.archive.org/web/20150327124054id_/http://image.s...
He argues that the wavefunction collapses on a timescale of h-bar / E_G, where E_G is the gravitational self-energy of the quantum system.
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When the future has multiple instances of you, it becomes problematic to define what the Born rule means.
For example, "Many Worlds, the Born Rule, and Self-Locating Uncertainty" notices that the rule doesn't mean anything, but it is rational to behave like it does. It's not a very satisfying position, I'd say.
Is many worlds falsifiable?
Yes. Many worlds is just the Schrödinger equation without the wave function collapse. It makes less assumptions. Falsify the Schrödinger equation, and you have falsified many worlds. However, is the wave function collapse falsifiable? It's one postulate which many worlds doesn't make, and it cannot be confirmed or falsified by experiment.
This is not what falsifiability means in the Popperian sense. You are confusing falsifiability of a theory’s predictions with falsifiability of an additional claim of what those theories mean. We might as well say that the simulation hypothesis is falsifiable because the laws of physics govern our simulated universe, so if you can falsify those laws you can falsify the simulation hypothesis. That being said, the concept of falsifiability is not without its weaknesses.
> Many worlds is just the Schrödinger equation without the wave function collapse.
It is not, it is the Schrödinger equation plus an extra rule that says that classical observers are only certain kinds of quantum states. There's no explanation in the MWI for why a device never reads out that a particle is 1/sqrt(2) |here> + 1/sqrt(2)|there>, even though that is exactly as probable a state as being |here>. And it turns out that this is exactly equivalent to positing wave function collapse / the Born rule.
And no, decoherence has nothing to do with this. Decoherence explains why there is no observable interference at the classical level between these states, not why you can only observe some states.
This is known as the preferred basis problem, for anyone who wants to read more about it.
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Exactly.
Any good scientific theory must create falsifiable hypotheses.
Otherwise it’s just a nice story.
>Any good scientific theory must create falsifiable hypotheses.
For any behavior inside the world, this is good.
But for the world itself and what harbor's it, asking for falsifiability is like measuring the expansion of space using a ruler that inhabits the same space.
Even science, that works in terms of cause and affect, which implies space and time, cannot be used to describe space and time itself.
Vanilla "scientists" have been trying to do that, but that is a dead end.
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So time extending into the future is not a useful hypothesis? Space continuing beyond the cosmic horizon is not a useful hypothesis? The past entropy of the past being arbitrarily low (vs what is essentially last tuesdayism) is not a useful hypothesis?
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The Copenhagen interpretation doesn't even deserve to sit with the other interpretations because it's not well defined. It's protoscience, like alchemy.
It's "rules of thumb for a working physicist".
"Many-worlds just states there is no such thing as wave function collapse" is the emotional statement. The contempt for many-worlds is both principled and rational.
Many worlds as real ontology are just as wild as saying their is a God. Physicists are thus making claims that just as wild as the theologians .. I find this fascinating.
Just like the AI nerds and the AGI singularity or Musk saying we are in simulation...
ie so many crazy ideas.. for the rationalist, its like being in an insane asylum and trusting one mental patient or another..
I suppose at some level its just not rational why we exist. We must accept some wild, fundamental metaphysical assumption
I think a common mistake when approaching MWI is reading too much into it. These Other Worlds "exist" in similar sense to the number i. MWI does _not_ claim anything about there being infinite alternative equivalent planes of existence where these physically manifest or anything like that.
You make an extraordinary claim.
Where in MWI do you believe the preferred unique macroscopic reality is defined?
This has never not been true. There are always hand wavy axioms at the bottom.