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Comment by Jerry2 | Hacker News Reader

Sean Carroll, the author, is a huge proponent and believer [1][1.1] in Many-Worlds Interpretation (MWI) interpretation of QM. MWI's basic idea is that the wavefunction is real and that there is no wavefunction collapse. Each time there's a decoherence, there's a universe split. Some physicists think this is pseudoscience or 'crackpottery' at best [2] since it can never be proven. A lot of physicists also don't see any value in MW's interpretation since it offers no predictive value (if you assume it to be true, there's nothing that this belief 'buys' you).

So when Carroll talks about 'physicists not understanding QM', he's just unhappy that most physicists don't share his view about QM and he's been known for trying to shame physicists for not sharing his view too. [3][3.1]

[1] http://www.preposterousuniverse.com/blog/2014/06/30/why-the-...

[1.1] https://www.youtube.com/watch?v=HxgGYJ3OjM8

[2] https://motls.blogspot.com/2014/07/many-worlds-pseudoscience...

[3] http://www.preposterousuniverse.com/blog/2013/01/17/the-most...

[3.1] https://www.youtube.com/watch?v=ZacggH9wB7Y

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> So when Carroll talks about 'physicists not understanding QM', he's just unhappy that most physicists don't share his view about QM and he's been known for trying to shame physicists for not sharing his view too.

This is incorrect and bad faith representation of Carrol's position.

While Carrol is proponent of MWI, he is calling more research on foundations of quantum theory in general. If you follow his podcast it's clear what his position is. Some of these so called interpretations are "legitimately distinct physical theories, with potentially new experimental consequences". In this article he specially mentions Ghirardi–Rimini–Weber theory. https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%...

I don't believe any of the possible interpretations of QM have a clear path to being proven, so are they pseudoscience as well?

If Hawking Radiation can never be observed till the heat death of the universe, is that pseudoscience, or just really hard to test?

At this regime, all of physics is very hard to design an experiment for on ranges we have access to.

> I don't believe any of the possible interpretations of QM have a clear path to being proven, so are they pseudoscience as well?

The characterization is a little bit harsh, but yes. The Copenhagen interpretation is non-falsifiable, (relative to MWI) so while I don't know what it is, I know it isn't science.

There's a reason most reputable scientific journals don't accept papers dealing with interpretations of quantum mechanics. There's no "there" there.

Hawking radiation is interesting because a proof of lack of Hawking radiation is a proof of the falsehood of quantum mechanics. Which is really interesting; any time you can falsify something like quantum mechanics every physicist should be paying attention. Also, if certain models of string theory are correct, particle accelerators like the LHC should be able to produce micro black holes, which will then evaporate in detectable ways. So Hawking radiation is of observational interest as of 2010 as a possible way to provide evidence to support string theory. (spoiler alert: we didn't find any)

It feels like you are way too quick to kick things out that aren't on a path to being proven. String theory forever has had the problem that it didn't make experimentally verifiable conclusions. A quick web search finds recent articles discussing apparently discussing this. Aren't people still working on string theory?

Or why is mwi impossible to verify.

The difference is, string theorists are trying. Everyone knows that it is a major problem that we cannot currently design experiments to falsify ST or other quantum gravity theories, and everyone is working towards it in the hope that one day we may. When it comes to interpretations of QM, there is not only nothing on the horizon, people don't even speak in these terms. No one is insisting on falsifiablity, and indeed every new interpretation seems completely hell-bent on just providing a "story" around the equations in a way that does not - and fundamentally cannot - make any predictions that contradict basic QM. I can count on one hand the number of interpretations I can think of that make experimentally testable assertions, and for those I have plenty of respect[1]. But the majority of interpretations and their proponents seem to deliberately stay within the comfort zone of non-falsifiablity, writing paper after paper whose actual impact on either physics or philosophy can be summed up with "yeah well, that's just like, your opinion, man". I did my MSC in physics under a professor who is a well-known proponent of MWI (though my master's thesis was not directly related to the subject). Two years of working side by side with him proved to me that the detractors of work on interpretations are, by and large, correct in their assertions about the proponents of them.

[1] There is this one theory whose name I cannot remember, where each particle in the universe gets their wavefunction multiplied by a delta function or very thin gaussian at random intervals, and whose sudden localization also causes a sort of chain reaction and localizes all particles it is entangled with. This theory is interesting because it helps set a (statistical) limit between microscopic and macroscopic interactions; basically, when you go past a certain number of entangled particles, everything will be well-localized virtually all of the time, but for small numbers of particles we'll see quantum phenomena. Unfortunately I recall this theory having too many holes, and perhaps it has already been falsified, but the point is that at least it made an effort to address inherent problems in QM (namely, wavefunction collapse) in a way that physicists actually should; by making testable claims.

This was linked above and sound like the theory your footnote describes: https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%...

That's the one! Thanks.

Maybe particles and black holes are the same thing. They behave differently due to quantum gravity mechanics.

https://en.wikipedia.org/wiki/Black_hole_electron

> If Hawking Radiation can never be observed till the heat death of the universe

That's a pretty big "if". As I suspect you know, how much Hawking radiation a black hole generates is inversely proportional to its size (it's probably an inverse square function to be precise, I forget).

A consequence of that is that supermassive black holes, the kind at the center of galaxies that we are most likely be able to indirectly (or even directly) observe, will generate almost none and whatever is generated will likely be outshone many orders of magnitude by the accretion disk anyway.

But... small black holes should shine extremely brightly, albeit briefly. A black hole resulting from a supernova will take a stupendous amount of time to lose enough mass (from Hawking radiation) to get this small and the window of time is so small we're unlikely to ever directly observe this.

But if we can ever figure out a way to lase gamma rays we can construct so-called Kugelblitz black holes (that some call the ultimate battery or even the ultimate computer).

Granted there's no theorized let alone practical way of lasing gamma rays, which is a problem.

But nevertheless, the theory of Hawking radiation makes testable predictions so yes, it's unlike Many Worlds (or even, honestly, string theory).

Yes that is accurate, but I feel doesn't detract from the main point, which is about whether to do theoretical physical at in these regimes.

We might decide a theory isn't testable even on principle, or discover that a theory we thought that wasn't testable actually is testable, but we'll never know until we do the work.

How would it make the ultimate computer?

I agree that deciding between MWI and other interpretations seems impossible, so it's not a problem that's considered important in physics, more of a good topic for a dinner conversation among physicists.

Our understanding of quantum measurements is much better now than it was at the advent of quantum mechanics, and it's understood that there's no such thing as an "abrupt" collapse of a wave function during measurement, it's instead a continous process involving entanglement and decoherence between two quantum systems (the system being measured and the measurement system). This understanding allows us to explain any measurement process using only "plain" quantum mechanics, i.e. unitary evolution of two coupled systems described by a Hamiltonian. If we just look at the Hamiltonian of the measured system it will seem that it shows wave function collapse because its evolution is non-unitary, but the evolution of the entire system is still unitary. From this angle I personally find any further invokation of a wave-function collapse mechanism unnecessary, and I think it overcomplicates quantum mechanics without giving us any benefits, so we might as well do away with it. Unfortunately, most courses on quantum mechanics still introduce the students to wave-function collapse when explaining measurement outcomes, which might be the reason that so many physicists have a bad understanding of the measurement process. Of course wave-function collapse is still a good shortctut for describing strong quantum measurements, I think it should be introduced as an approximation of a continuous measurement though.

Contra your first paragraph, isn't your second paragraph ultimately saying that MWI has won over Copenhagen as the more useful theory?

I'd say wave-function collapse is a good approximation for many strong quantum measurements, but isn't able to describe many experiments that involve weak quantum measurements or quantum feedback. As I said it might be good to start teaching quantum measurements using the entanglement+decoherence mechanism first and then introduce wave-function collapse as an approximation.

Motl is a zealot for his own interpretation and thinks everyone else is an idiot crackpot, including his colleagues.

Carroll does not mention his preference for many-worlds in the article because that's not what it's about. The fact that he has a preferred interpretation is not relevant to the point that physicists should pay closer attention to the lack of consensus on the interpretation of qm.

> when Carroll talks about 'physicists not understanding QM', he's just unhappy that most physicists don't share his view about QM

I don't see that in this particular article (which is not to say it isn't there in other articles Carroll has written). In this article, he is simply pointing out that QM is the only physical theory we have ever had that does not have a single generally accepted interpretation. Not believing the MWI is fine (I'm not a fan of it myself), but that just means you have to find some other interpretation that works, and nobody has done that; every interpretation of QM has issues that nobody has figured out how to resolve.

> Each time there's a decoherence, there's a universe split.

The universe is constantly splitting. Not just when there is decoherence.

Or more accurately. Every possible state of the universe has a probability amplitude right now. And those probability amplitudes are constantly evolving, diffusing into each other, interfereing with each other.

Every such state with a probability amplitude is a world of the many worlds.

In the article, there is a statement: "Empirically minded physicists have realized that the phenomenon of measurement can be directly probed by sufficiently subtle experiments."

Then please write about these probes, and not just the rant that "nobody understands."

The public doesn't have to worry (and we don't have to read) about the "interpretations" until there is a specific proposition of how to do such probe an the experiment can be constructed and performed.

He writes "students who demonstrate an interest in the topic are gently but firmly — maybe not so gently — steered away, sometimes with an admonishment to “Shut up and calculate!”."

My answer to this article would be, to theoretical physicists: "shut up and construct an experiment that can be performed." As long as there's no practical difference (in the sense that any experiment that can be imagined can demonstrate it) there's just no physics there.

So yes... "shut up and calculate."

It's either that or "shut up and meditate"

A thought experiment called Quantum Suicide [1] is an interesting read in this context.

[1] https://thoughtexperiments.net/quantum-suicide/

All these people saying many worlds and Copenhagen have identical predictions are totally ignoring this. If many worlds implies quantum immortality and Copenhagen doesn't, that's a clearly detectable physical difference.

Detectable how precisely?

By not dieing.

So if you do it and you don't die would you say that you proved something?

If someone does it and doesn't die would you think that did prove anything?

If one thousand people do it and one of them is still alive after 10 coin flips does that prove anything?

What is the detectable physical difference? How do you carry on an experiment to detect that physical difference? What is the protocol to determine that the physical difference has been detected?

Please let me know how should we do to not totally ignore this.

QI predicts you’ll still be alive in 1M years with 100% probability. Not-QI puts that probability lower.

So if you are still alive in 1M years that’s evidence of QI. Basic Bayesian reasoning.

Or you are just lucky. It all depends on the prior probability, even if you are eager to misapply Bayesian reasoning.

I'm not sure what point you are trying to make. It's still a different physical prediction and evidence of QI over non-QI.

You can always just be lucky; maybe gravity causes the Earth to move randomly and we've just been lucky to see if orbiting the sun for the past few years.

So the physical prediction is that I will never die, unlike the alternative physical prediction that I will die at some point. And therefore every second I remain alive is evidence for the MWI. If not having died before makes you think you're immortal more power to you. I don't find that reasonable.

>> Each time there's a decoherence, there's a universe split.

Or, the universe was split before and that quantum uncertainty is really us perceiving those already-existing other universes. The collapsed wave is really just us measuring the only particle, the one in our universe, capable of stronger interactions. Under that interpretation the rest of the electron shell is a cloud of other electrons existing across universes but unable to interact strongly in ours, literal ghost particles.

Every claim you are making requires justification why it explains reality better than the model you are arguing against. Without that this reply paints you as a crank.

I, and more than a few others, find the ongoing existence of multiple universes easier to swallow than the concept of new universes being created whenever a wave function collapses. The pre-existence of an infinity is of greater likelihood than the continuing creation of multiple infinities as time moves forward.

This is testable, if not practical atm. If something like an electron exists in ghost forms across multiple universes then they may be linked in those other universes more strongly than ours, a concept that may offer explanations for a variety of quantum behaviors. Proof of this would be the observation of a reliable link between particles not linked in our universe.

IMO, I have heard no explanation that is more consistent and coherent than this.

---

Consider the famous Schroedinger's Cat.

The decayed particle has two states: decayed and not decayed.

The cat has two states: dead and alive.

The researcher that opens the box has two states: sad and happy.

The waveform does not collaprse. The particle, the cat, and the researcher each have a uncollapsed waveform; i.e. are in both states.

However, these waveforms are not independent; they are entangled. The decayed particle is coincident with the dead cat and the unhappy researcher. And the undecayed particle with the alive cat and the happy researcher.

All states "exist" simultaneously, but not independently. This bifurcation in the wave function magnitudes is a "split" in the universe: every future waveform interaction will be in the context of decayed particle/dead cat/unhappy researcher and undecayed particle/alive cat/happy researcher.

"Many worlds" sounds nonsensical, but it is really just a colloquial expression for waveform entanglement.

---

Now this does present a philosophical question on how consciousness functions in light of the split waveforms.

I maintain that science -- quantum mechanics or otherwise -- has no explanation or even language to express "consciousness". It's like asking science an ethics question. And this inability applies to all science, not just quantum mechanics.

Many Worlds is not "a colloquial expression for wavefunction entanglement." Entangled states are part of the mathematics of QM, not a feature of an individual interpretation.

MWI distinguishes itself by omitting (as you say) any notion of "collapse" from its axioms. This is appealing for simplicity but the problem is how it connects to experimental results:

1. How does the Born rule arise? If the universe split into two but the measure of cat-dead is 1/pi, why do I find myself in the cat-alive universe more often?

2. More generally, how does randomness arise? MWI contains only deterministic axioms, so how can it explain the experimental observation that measurements of similarly prepared states result in different values?

The important question here is how do you know you find yourself in the cat universe more often? How can that possibly be measured? It's a difficult question when it comes to conscious experience.

Imagine instead of a single experimenter observing a billion possible outcomes, a billion experimenters observing a single possible outcome.

I propose to measure this by writing down the results on a piece of paper, and then counting them.

The hope of MWI is to produce a simpler theory by eliminating an axiom. If this forces us to axiomatize "conscious observers" then it has already failed. Though I admit it may be fun to think about.

That's the funny thing, MWI works perfectly fine without conscious observers. It's predictions match other QM interpretations. The problem comes from introducing ourselves as a conscious observer and trying to understand what it means to be consciously aware of a single outcome.

Yeah, this is indeed the key question.

My guess would be that they are all consciously experienced, some more and some less. The alternative seems to be to say that they are so called "p-zombies". Very similiar to us, evolving according to the same laws, but not concious. But it's really up in the air.

Okay, let me rephrase. Many Worlds states that there is no collapse; the entanglement continue to propagate. Many Worlds is the familiar label for saying there's entanglement, and that's it.

> 1. How does the Born rule arise?

> 2. More generally, how does randomness arise?

As you said, with Many Worlds there is no randomness. The multiverse is entirely deterministic. Randomness is a Copenhagen-like concept.

There is a "universe" with an alive cat and a "universe" with a dead cat. These universes have a certain waveform distribution.

Neither of these universe are any more real or extant than the other (they have the same magnitude).*

* Though consciousness and its observations seems to follow only one path. This is a peculiarity outside the realm of physics.

>* Though consciousness and its observations seems to follow only one path. This is a peculiarity outside the realm of physics.

Of course physics can answer this: the human minds on the different branches can't physically interact. They can't share memories. Each one is going to remember experiencing only its own branch.

Sure, but why am "I" the one and not the other?

Whether this is a problem or not depends on what your conception of consciousness/identity is. I see a conflict or unanswered problem there; you may not.

There's multiple, and each of them have their own experiences since the split. The other versions of yourself are basically other people that just happen to share a part of your history.

Wondering why you're a specific one of them and not the other seems equivalent to me to wondering why you're you and not some another person in our own world. I think it's an interesting question, but I'm not sure what an answer to it could possibly look like (since any answer that says why you're you somehow needs to work out differently for other people to explain why they're themselves too) which implies the question is malformed somehow. (If your consciousness were ... someone else, or even switched people it was tracking somehow, it's not like anyone would notice.)

> seems equivalent to me to wondering why you're you and not some another person in our own world.

Indeed.

Randomness is an empirical observation. Identically prepared quantum states, when measured, produce different experimental results, distributed probabilistically.

MWI's solution to this is to say the experiments are wrong, and the results are actually the same. It rejects empiricism to simplify the theory. But that's not good science.

> Neither of these universe are any more real or extant than the other (they have the same magnitude)

What is meant by "magnitude?" It can't be the measure of the wavefunction because those can easily be different, e.g. if the particle's decay probability is not 50%. This is the Born rule problem again.

If the universe is deterministic, how can there be entanglement or branching at all? What need is there for a multiverse if you have determinism?

It’s as if non-determinists aren’t sure that 2 doesn’t act like a 3 sometimes because of decoherence and so 2 + 3 = 6 as a result.

Where am I going wrong in this line of thinking? How could randomness be injected into a system without superpositioning of quarks? Is that literally the only way?

Is Hawking radiation the smoking gun we think it is?

Edit: I admit I am tired when posting but this topic is perhaps one of the most fascinating of all. I can’t stay away.

The waveform is deterministic. It does not "collapse" to a randomly chosen value; it continues to be all the values in perptuity.

It's like matrix multiplication: yes you can wind up with more elements after the operation, but the result as a whole is entirely deterministic.

If you defined matrix multiple multiplication where you collapsed the result down to a single unspecified vector, then you would have "randomness" (according to the selection).

Many worlds says "don't choose"; keep the entire matrix, and the results will get larger and larger.

The universe is not fundamentally rolling dice, though it appears that way from your perspective since you can only see a single slice of the matrix.

> This is a peculiarity outside the realm of physics.

So the “realm of physics” is now disconnected from empirical reality? Great.

> empirical reality

Unfortunately, yes. Empirical reality is limited by the empiricist.

Sure. But I wouldn't say it's outside the realm of physics.

I think more concisely, we don't understand it yet.

You will find answers to 1 and 2 in https://arxiv.org/abs/quant-ph/0503009

> A lot of physicists also don't see any value in MW's interpretation since it offers no predictive value

David Deutsch (in many ways, the father of quantum computing) initially imagined quantum computing as a way to test MWI: if a problem could be solved that required more gates than there are particles in the universe, then it would (so the argument goes) prove that the problem was being solved in the multiverse.

Paraphrasing the title of the submission, “David Deutsch doesn’t understand Quantum Mechanics” (if what you said is true).

> 'physicists not understanding QM'

This is quite a popular claim among the Physics community. It goes even further saying: those who understand it don't understand it. ;-) Emphasizing how counter-intuitive the concepts are and also the fact the even Standard QM (with 1 or 2 particles) is still a research topic. Even Einstein didn't believe in QM as it stands now.

"he's just unhappy that most physicists don't share his view about QM" - Can you prove that? Are you just unhappy that Carroll doesn't share your view about QM?

>Each time there's a decoherence, there's a universe split.

Where does the energy come from to create the other universes?

Completely agree.

The article strikes me as crackpottery, but this statement

> A lot of physicists also don't see any value in MW's interpretation since it offers no predictive value

is not entirely correct either.

This is the probably best "explain like I am an undergraduate not in Physics" article on Everett's proposal.

https://plato.stanford.edu/entries/qm-everett/

To further summarize the value of Everett's proposal:

In the standard von Neumann-Dirac collapse formulation of QM, a collapse happens when we measure it. But what a measurement event looks like and conditions when it occurs are not described by the theory at all. This is one reason why QM cannot completely describe the universe, or approximate it. Note that even if we can describe what a collapse is, this+QM still fails to model parts of the world, only approximating it.

But one can try to propose a formulation of QM that agrees with the collapse formulation, that does not have that collapse, so that it is closed in the sense that for each system of observers observing a quantum system collapsing, that system can be modeled as quantum system itself. That is Everett's proposal. From the linked article,

> Everett’s solution to the problem was to drop the collapse postulate from the standard formulation of quantum mechanics then deduce the empirical predictions of the standard collapse theory as the subjective experiences of observers who were themselves modeled as physical systems in the theory. The result was his relative-state interpretation of pure wave mechanics.

The “no predictive power” is beyond what is predicted by standard QM, because the MWI is supposed to recover the same predictions. And it’s not clear how does it so, because if standard QM has an issue with “what a measurement event looks like and conditions when it occurs” then exactly the same problem remains in the MWI if the same predictions have to be recovered. Even though there is no collapse in the MWI it looks like there is a collapse, even though you don’t have to specify when the collapse happens you have to specify when does it look like a collapse does happen.

> even though you don’t have to specify when the collapse happens you have to specify when does it look like a collapse does happen.

In MWI there is no collapse event, because there are no observers in the traditional sense that may collapse the system. In MWI we resign and just say that observers are subject to QM, and when a "measurement" is made, what happens is that observers and the observed system get entangled so that certain combinations of properties of the observer-observed system are not possible. This entanglement of two projections of a composite system can be described in the language of QM.

> what is predicted by standard QM, because the MWI is supposed to recover the same predictions.

MWI recovers the same predictions. Where in standard QM if one says an observer observes that a system may collapse 40% in state a and 60% in state b, MWI says that in systems (of the observer and the observed) where the observer makes the observation in the same exact manner, the observer will record state a with the system in state a 40% and the observer will record state b with the system in state b 60%, and there are no states where the observer observes something and the subsystem is in the other state.

Of course, you can simply model the entire system of observer and observed in standard QM and recover the same relative predictions without invoking the absolute observation of an observer. In MWI we do away with the conceit that there is an absolute observation of an absolute, but ill-defined observer.

The point is that if standard QM has a problem to make predictions, then the MWI would face the same problem to make the same predictions starting from a universal wavefunction.

If standard QM has no problem to make predictions then I agree than in principle the MWI could make the same predictions (but it seems to me that in practice it can only make predictions by using standard QM and saying that those are the same predictions that could in principle be derived).

Can you clarify what you mean by standard QM? I had taken your usage of "standard QM" to mean, as you used it in context, to be collapse interpretations, but strictly speaking standard QM just concerns itself with quantum systems and is not incompatible with MWI.

Edit: it's too late to alter my original ancestor comment, but I think the way I wrote it may have yielded a misunderstanding. I didn't mean to suggest that MWI has value because it can predict phenomena. I meant to say that it has value because it is a metaphysically cleaner model of reality than the collapse interpretations.

By "standard QM" I mean the "textbook quantum mechanics" which is based in a set of postulates (like for example https://personal.utdallas.edu/~son051000/comp/postulates.pdf) including in some form von Neumann's projection postulate.

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