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Comment by ThalesX | Hacker News Reader

Having been passionate about this for awhile, I have a layman’s surface overview of the state of physics, or I like to believe so.

It amused me to no end when my elderly aunt came to ask me for a tl;dr; on Quantum Physics. She stopped me shortly asking how she can control the universe with her mind.

I tried to explain a bit about the famous interpretation (Copenhagen, not Von Neumann–Wigner) but she would have none of it because the business course she just attended had a segment on how to control the universe and proceeded to disregard and mock me.

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To be fair, the Copenhagen interpretation doesn't really fit today's world view. We have largely moved away from viewing humans or "consciousness" as something special, so the notion that an observer collapses the wave function just seems weird now.

"Many worlds" or "the wave function never collapses, you are in a superposition" both make much more sense with how we currently view the world

I’d say a good heuristic would be to include all mammals in any theory about consciousness, wave function collapse, quantum consciousness, etc. and then see if the theory still holds up. If it does, you are probably on to something.

I think scientifically we will look back on “humans are uniquely conscious” as a categorical differentiation instead of a gradient with other mammals to be as absurd as believing the earth is the center of the universe. “Unique consciousness” is a quasi-religious mechanism we use at a societal level to not run around all the time terrified of death.

No interpretation of quantum physics proposed by scientists ever required a conscious observer, that's more of a misinterpretation of what an observer is. It's an interaction, not a person or animal.

As for the theory of consciousness, of course we are not special, it's information-processing, and it looks like thermodynamics is responsible for the emergence of information-processing structures.

https://arxiv.org/abs/1203.3271

A mental trick that helped me understand the concept of “an observer” in particle physics is to imagine it like playing billiards in a pitch black room.

In normal human-scale billiards, there are immense numbers of photons flying around bouncing off everything. The photon interactions are far too small to affect the path of a moving billiard ball, but we can detect them easily with our eyes. So we can use photons to passively observe the balls rolling around.

But when you’re trying to observe a photon itself… there are no tiny photon-equivalents flying around. It’s like playing billiards in pitch black: the only way to know which direction a ball is rolling is to touch it. And you can’t touch a rolling billiard ball without changing its path somehow. Likewise, you can’t “observe” a single photon without interacting with it in some way.

I know all analogies are imperfect, but that is a really great one. It really captures the problem of observing so elegantly.

Observer effect while applicable is distinct from Quantum Weirdness aka Heisenberg Uncertainty

This is a great analogy, but also in experiments the presence or absence of the measuring device like a beamsplitter determines the outcome. The measuring device, the which-way detector is the observer, and it can be regarded as a quantum mechanical system. It's correlations with the rest of the system causes the particle behavior.

> No interpretation of quantum physics proposed by scientists ever required a conscious observer, that's more of a misinterpretation of what an observer is. It's an interaction, not a person or animal.

Are you saying that the Von Neumann–Wigner interpretation[1] does not explicitly postulate consciousness to be necessary for the completion of the process of quantum measurement?

1. https://en.wikipedia.org/wiki/Von_Neumann%E2%80%93Wigner_int...

Those guys are excused, they were early on the floor, had no idea what's going on. They grew up believing that nature is deterministic, but quantum physics complicated that picture a great deal.

I should've said no interpretation in the 21st century.

> As for the theory of consciousness, of course we are not special, it's information-processing, and it looks like thermodynamics is responsible for the emergence of information-processing structures.

That's a really interesting paper, thanks for that. It's also a bit depressing to consider that there's a good chance that everything we think that makes us special is really just an emergent property of a thermodynamics memory and prediction system.

> “Unique consciousness” is a quasi-religious mechanism we use at a societal level to not run around all the time terrified of death.

It also helps us justify eating other lifeforms, mammals included.

The Copenhagen interpretation never defined an observer that way. It's a non-interpretation basically, just shut up and calculate, which is still the most widely accepted approach among scientists, since there are no other falsifiable interpretations.

I'm reminded of learning about the grandfather paradox as a kid. I recall hearing shit about scientists saying if you were to travel back in time, you would be compelled by a universal force to do things that don't alter the future and create a paradox. Even as a kid, that sounded so idiotic. In retrospect it was almost certainly the reporting that was wrong.

Is that really so weird? That's a very common version of time travel in SF, perhaps the most common. You can travel back in time, and you think you're changing history, but through a series of unlikely coincidences you end up being the person who created the history you were trying to change.

Come to think of it, maybe that's more common in short stories, where the "gotcha!" format works. It's probably harder to spin that unchangeable-history gimmick out to novel length, whereas "you can change history and it has endless weird side effects" can work well in long form. I can't think off-hand of any time travel novels with unchangeable history.

It's not weird. It's fun.

But it's stupid as a serious time travel proposal. Things like "travel in back in time 5 seconds unless you see your future self is already there" create a hard paradox that this logic cannot solve. Time loops are easily broken by anyone interested in testing them.

You sound as confident as the protagonist of many a time travel short story!

Edit to add: your comment reminded me specifically of a Ted Chiang story, "What's Expected Of Us": https://www.nature.com/articles/436150a

I found that link via a blog post that excoriates it for its supposed logical fallacies: https://loopingworld.com/2019/07/13/debunking-ted-chiang-rec.... I don't necessarily agree or disagree with that, but I do disagree that it's "obviously" logically wrong or inconsistent.

It's really just on them for trying to alter the future in large ways before conducting small experiments to understand the mechanics of the universe imo.

Time loops are only a problem if you insist on being able to simulate reality by computing the next time step from previous ones, without discarding any. Or if you for some reason insist on total free will.

> Or if you for some reason insist on total free will.

Like, the free will to do something other than what you saw your future self do? Yeah, you do have that. You have that ability even as a completely deterministic brain.

Heck you could send a machine to go back in time 5 seconds that displays a number 1 greater than the number it just saw its future self display from the previous travel and your time loop has infinitely increasing state with no human in the process. Or the mundane, send a bomb back in time to destroy the time machine and your past self. It's very easy to think of examples.

> Time loops are only a problem if you insist on being able to simulate reality by computing the next time step from previous ones, without discarding any

yes?

If you frame the system as "the universe conspires to create a series of coincidences such that no paradox exists", then it makes perfect sense that nobody chooses to run that experiment, because the simplest coincidence that prevents the paradox is for the characters in question to simply not think to run that particular experiment. This raises the obvious question of how the time machine got invented in the first place, which seems like a great story.

Sure sure. And it's fun fiction. But it's obviously not a plausible physics explanation.

>I can't think off-hand of any time travel novels with unchangeable history.

Harry Potter has this as a plot point in one of the books. He essentially teaches/inspires himself to a spell to save the day.

Ah, yeah, good one. (Although more of a "novel that happens to have time travel in it" than "novel about time travel".)

Netflix's Dark is a good 3 seasons of a single nearly stable time loop.

You're thinking of the Von Neumann–Wigner interpretation, not Copenhagen.

I also prefer the Many Worlds interpretation and I do think that it is gaining popularity in relation to Copenhagen, but I'm not sure what you mean by "today's world view". And who is this "we" that you write about?

I prefer Many Worlds because it is a simpler explanation, so I follow Occam's razor. That's it.

Consciousness and the existence of qualia remains a fundamental mystery. There is something definitely special about it, in the sense that it currently does not fit any scientific model. It is also a deep philosophical problem that started being considered millennia ago and dates at least all the way back to Plato vs Aristotle.

In my experience, people who think that consciousness is somehow a settled matter simply haven't thought about it enough and are perhaps a bit naive on the many ramifications of the issue that have been explored do far.

What surprises me is not that many people think consciousness is a settled matter; but rather, people who, when faced with the claim that "consciousness" isn't settled, are so often tempted to assert that it is that they rapidly provide another example of how it isn't.

Exactly. A related deep millennia old observation is that no one has actually seen the real world. Everything we’ve ever experienced has been filtered through our minds. Every experiment, every measuring device, every meticulously crafted model of reality… it is all inescapably limited by what we are able to experience.

One could and many have argued that the reality we’ve been observing and operating in is consciousness. The “real” world could be entirely different and largely inaccessible.

These kinds of thought exercises don’t have much practical utility, apart from one very important feature; they humble you. At the end of the day we don’t fundamentally know anything, and should always recognize that at its most basic root level, everything we do is an educated guess. A fundamentally skeptical and curious outlook that acknowledges our perceptual limitations is how we got all of the sophisticated models of very difficult to observe phenomenon in the first place. If we want to continue to get the best understanding of whatever it is we’re experiencing, I think it’s very important to stay humble and ensure our knowledge is treated as a hard earned set of well reasoned guesses rather than unquestionable dogma. 99.9% of objections to well established ideas and models might be a waste of time entertaining, but you never know what might turn out to be the seeds of a whole new universe of understanding that invalidates huge swaths of our existing corpus of knowledge.

Most of what I’m saying here is probably obvious to a lot of readers, and I don’t think anyone in this thread is being particularly arrogant or dogmatic, but I think it’s worth reiterating. If people who understand the limits of knowledge aren’t constantly emphasizing the fact that we don’t know what we don’t know, that creates fertile ground for both dogmatic assertions and unreasonable skepticism, and I think a huge amount of dysfunction in the culture at large is explained by insufficient well calibrated humility amongst otherwise very intelligent people who set an example for others.

> I prefer Many Worlds because it is a simpler explanation, so I follow Occam's razor. That's it.

Zillions of new invisible universes being created every moment is the simplest explanation you found?

MWI is exactly as simple mathematically as CI, so not sure what you mean by "simpler".

MWI still postulates the equivalent of wave function collapse, but instead of it happening only for the quantum system being measured, it is happening in the mind of the observer, as each "version" of that mind gets entangled with a single "version" of the outcome.

Even if you were to accept that this process is more natural (so not an "assumption") than wave-function collapse in principle, that simplicity completely falls apart when you then need to recover the relationship between the probability of observing a certain outcome and the amplitude of that outcome in the wave-function.

CI just says "when a quantum system described by a wave-function interacts with a measurement apparatus that measures in a certain basis, the wave-function gets updated to one of the values of its decomposition in that basis, with a probability equal to the modulus of the square root of its amplitude in that basis". Of course "measurement apparatus prepared in a certain basis" does a lot of work here, as we don't know how to define this in terms of a quantum system.

To make a similar quantitative prediction, MWI needs to define something like "the number of worlds", so that it can then say something like "when a quantum system interacts with a measurement apparatus prepared in a certain basis, the measurement apparatus becomes entangled with the quantum system such that for each value of that basis state there is a number of worlds proportional to the square root of the amplitude of each value of the decomposition in which the apparatus sees that particular value; if we were to compute the probability that we happen to live in a world where the apparatus is showing the value X, that probability would naturally be higher the more worlds there are where it shows this value X". So, the MWI has to actually introduce extra elements (the worlds and their number, and the observer wanting to compute a probability) to explain the actual measured results of quantum experiments.

> you then need to recover the relationship between the probability of observing a certain outcome and the amplitude of that outcome in the wave-function.

I have never understood how that is a strong objection. We've experimentally determined that the state you are more likely to find yourself in is based on the squared amplitude. How is this different from CI but with probability of observing given state - which was also determined empirically?

> Of course "measurement apparatus prepared in a certain basis" does a lot of work here, as we don't know how to define this in terms of a quantum system.

Yes, this is where the Occam's razor bit comes in.

> So, the MWI has to actually introduce extra elements (the worlds and their number, and the observer wanting to compute a probability) to explain the actual measured results of quantum experiments.

The worlds and their number are equivalent to the states & probability of CI without having to introduce the "measurement apparatus" that is distinct from the quantum system.

> How is this different from CI but with probability of observing given state - which was also determined empirically?

It's not different, that's the point. For both interpretations, the relationship between the probability of observing a certain outcome and the amplitude of that outcome in the wave function are an extra postulate, the exact same extra postulate in fact, the Born rule.

> The worlds and their number are equivalent to the states & probability of CI without having to introduce the "measurement apparatus" that is distinct from the quantum system.

The measurement apparatus as a separate thing from the quantum system was actually partially explained by decoherence (ironically discovered by MWI proponents), which needs this separation for the same reasons as CI: we need some way to explain why quantum phenomena don't happen at our scale. Now, MWI makes this concept relative to an observer, whereas in CI it is often assumed to be absolute.

Basically, we can deduce from the Schrodinger equation alone that after the interaction of a quantum system with the environment, coherence is lost, and the different "states" of the wavefunction can no longer interfere with each other.

CI postulates that, as a result of this interaction, only one of the states will remain, with the Born rule probability. This is the most direct way of interpreting our experimental results.

MWI says that nothing changes after this interaction. It postulates though that, if we were to ask how likely we are to be in the same "state" as a particular result, we should expect that to be the Born rule probability. This explanation takes the theoretical description of the wave function to be more real in some sense than the actual observations we make.

> The measurement apparatus as a separate thing from the quantum system was actually partially explained by decoherence

I disagree that the decoherence at all partially explains a different measurement apparatus separate from the system. After observation, we still have the the system is in a superposition of multiple states. How this system "collapses" to one state in the measurement apparatus is unexplained.

Decoherence explains why each state of the observer can't tell that the other states also exist.

> This is the most direct way of interpreting our experimental results.

Sure, except we have to invent an entirely new non-unitary transformation of "collapse" despite all observations and predictions of quantum mechanics showing that unitary evolution of the wavefunction continues even for large macro-scale objects.

> Decoherence explains why each state of the observer can't tell that the other states also exist.

Sure, but, again, decoherence doesn't explain (1) why the wave function is split in the classical states (position, momentum etc.) and not other states (linear combinations of position and momentum and spin etc.); and (2) doesn't explain why different states have the precise probabilities of being observed that they do. You still need to postulate these features.

> Sure, except we have to invent an entirely new non-unitary transformation of "collapse" despite all observations and predictions of quantum mechanics showing that unitary evolution of the wavefunction continues even for large macro-scale objects.

You're confusing the map for the territory. What we can see, plain as day, is that unitary evolution of the wavefunction does not continue for macro-scale objects. MWI explains this through the framework of the universal wavefunction and its mutually un-interacting "branches", CI explains it through wavefunction collapse. No one has ever successfully put a macroscopic object in superposition, so the idea that the universe itself could be in superposition remains highly theoretical at best.

Note also that movement in the classical world (and in GR) is often highly non-linear/non-unitary (such as the movement of a free pendulum, or many kinds of orbits). While far from perfect for this, the Born rule at least allows us to "sneak in" non-unitary evolution through this collapse; but, in the MWI, we predict that the movement of a free pendulum should in fact be non-chaotic. This is rarely discussed, but is an interesting observation that could be experimentally tested as we get better at creating larger and larger systems that remain coherent for longer and longer periods of time.

> Sure, but, again, decoherence doesn't explain (1) why the wave function is split in the classical states (position, momentum etc.) and not other states (linear combinations of position and momentum and spin etc.); and (2) doesn't explain why different states have the precise probabilities of being observed that they do. You still need to postulate these features.

I agree that (1) is a strong objection that I do not have an easy solution to. (2) I think is addressable in the same way that it is in CI - our theory has to explain our observed experience and the born rule (whether CI-flavored or MWI-flavored) is how we do so.

> No one has ever successfully put a macroscopic object in superposition

But getting larger and larger. https://www.nature.com/articles/s41567-019-0663-9

Thanks for an informative discussion!

> (2) I think is addressable in the same way that it is in CI - our theory has to explain our observed experience and the born rule (whether CI-flavored or MWI-flavored) is how we do so.

Absolutely, but I would still say that it means we are adding an extra postulate that can't be derived directly from the Schrodinger equation even in MWI. I'm not trying to claim that the MWI is wrong: just that it is not simpler than the CI (nor it is more complex though - I'm arguing it has the same number of postulates).

> But getting larger and larger.

Sure, and I'm really hoping we'll advance closer to macroscopic objects in my lifetime, so maybe some of these questions may be elucidated.

> Thanks for an informative discussion!

Thank you as well!

>MWI still postulates the equivalent of wave function collapse

It's not postulated, but deduced from the Schrodinger equation. MWI is simpler in a sense that it has one fewer axiom. But Occam's razor isn't really applicable here, because it selects from otherwise equal theories, which CI isn't. There are more important criteria to use before Occam's razor.

You can't deduce the Born rule from the Schrodinger equation. MWI can say "look at every basis separately", but you still have to postulate that the probability of finding yourself asking that basis vector (seeing that measurement outcome) is proportional to the square root of the amplitude.

> We have largely moved away from viewing humans or “consciousness” as something special.

Who is “we” in this comment? Christianity and Islam emphasize consciousness and the uniqueness of human experience, and their adherents still account more than 50% of the population.

As much popularity as the many worlds theory is gaining, the generic “we” almost certainly still doesn’t believe it, and many (most?) probably don’t even know about it.

The CI has absolutely nothing whatsoever to do with consciousness. The CI view is simply "when a quantum system interacts with a measurement device, the wave function of the quantum system changes to one of some subset of real values, with a probability given by the modulus of the amplitude of that value in the wave function". Exactly what constitutes a measurement apparatus is undefined, but it certainly doesn't involve a human.

In fact, it is precisely the MWI that requires human observers for its explanations, at least to some extent, as it makes the entire notion of a classical world a fiction that only exists inside your own head because you are the one that's getting entangled with a quantum system. Any probability you compute in MWI is relative to you personally, since in MWI any possible event happens with probability 1 when checking at the universe level.

What I don't understand is why we call it "observation" when it should really be "interaction". The quantum weirdness resulting from the collapse of probabilities has nothing to do with a conscious observer, just whether and at what point the phenomena in question interact with something else in an observable way.

> What I don't understand is why we call it "observation" when it should really be "interaction" […] at what point the phenomena in question interact with something else in an observable way.

That’s why. If it’s observable it could be observed by an observer in an observation.

Well, an interaction that leaves a persistent change in state in the "observer".

Because when an object "interacts" with another object, it goes into a quantum superposition with that object. It is only when we observe that we don't see the superposition.

The obvious implication that people don't like to talk about is that there is nothing special about observation, it is just that our own body goes into superposition and we only subjectively experience one of the quantum states.

> there is nothing special about observation [...] we only subjectively experience one of the quantum states

That subjective experience seem something special!

There is nothing special about quantum superpositions - they are pure quantum states like any other. They are superpositions when we consider them in a particular basis. How does the subjective experience project your body - and the rest of the universe - onto one element of the right basis?

I don't really understand much of quantum physics - I'd say as much as someone with a passing interest in chemistry would need to (and that's pretty focused on what groups of electrons get up to), but every once in awhile I'll read about the actual problems and experiments that the old Nobel laureates got up to and all of a sudden something will fall into place. A big key was reading about the ultraviolet catastrophe and how Max Planck basically just played around with equations and sort of hit on quantizing energy levels and almost accidentally invented quantum mechanics. If my education had just been a history of all the discoveries in physics from the end 19th to mid 20th centuries (and to be fair to my education that was mixed in a bit), I think I would have been served a lot better, but I also acknowledge that could be down to my individual way of learning.

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