The first paragraph seems to be pretty much all misleading or wrong:
It’s one of the oddest tenets of quantum theory: a particle can be in two places at once—yet we only ever see it here or there. Textbooks state that the act of observing the particle “collapses” it, such that it appears at random in only one of its two locations. But physicists quarrel over why that would happen, if indeed it does. Now, one of the most plausible mechanisms for quantum collapse—gravity—has suffered a setback.
As soon as it says "here or there" the writer is clearly trying to make it binary again. It isn't "here or there" but a function of probability between those two "places." It's an infinite number of possible "locations," but we narrow it down to a large number of decimal places because otherwise we wouldn't be able to make use of it at all. If things were as written here, you could do all quantum calculations with a straight up coin toss (although some might argue that can with an adequately tiny coin, but that still only gets at the most basic understanding of "spin" and only becomes an accurate analogy if the coin is flipped randomly in zero-g).
Our higher level concept of gravity (at least in the classical sense) has been known to break down to the point that it doesn't apply to quantum mechanics for quite a long time now. The writer should just google "quantum gravity" to discover what a complicated subject that becomes.
Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics. Unfortunately this is also a sizable fraction of all physicists. Thus we have articles like this :/
Basically all the work in getting large scale quantum computers to work lies in these fields. They will continue to grow.
I am not a physicist but isn't it an instance of "if you think you understand quantum mechanics, you don't understand quantum mechanics"?
The measurement problem, which deals with collapse is still unsolved. It is also the reason why there are so many weird interpretations of quantum mechanics and that even top physicists can't agree on one. In fact the same physicists tend to sweep the problem under the rug and instead focus on the equations that, to be fair, did a lot more to science and technology than trying to solve the measurement problem.
There is no measurement problem. It so happens that most measurement apparatus are macroscopic and at a temperature much higher than the quantum gap of the system being probed. Measurements in qm imply a rotation between systems, and mixture with an incoherent thermal state implies a quantum superposition will lose coherence. Full stop. This is all understood in full, grab any text on non equilibrium quantum dynamics. One can even simulate the collapse process in full detail, and obtain superior agreement with experiments.
If the measurement apparatus is coherent, measurements can be performed without collapse. This was well thought out even within wigner and einstein's lifetimes. c.f. the vaidman bomb detector.
This is certainly my view as a physicist, although I'm not the greatest physicist by any measure. QM can give us a perfectly clear picture of "collapse", although it's not a trivial matter and it's hard to explain to laypeople. Especially when physics fans have these ideas about ~conscious observers collapsing the wavefunction~, or something.
It depends on what one calls the measurement problem.
This solves the "consistency/small problem", i.e. treating the macroscopic apparatus as boolean is justified.
It doesn't resolve the "outcome problem", i.e. which outcome is selected. Of course if you accept the world is not deterministic this isn't really a problem.
One could argue even classical mechanics isn't deterministic as we think of it because of chaos, which has fascinating connections with QM. W Hoover (of the Nose-Hoover thermostat fame) did some great work with reversible thermostats exploring the instability of Newtons equations of motion.
I think that's different. Chaos still uses classical probability and the randomness is just ignorance of underlying initial conditions. This is very different from QM.
Measurement is a transfer of a state information from a subsystem 'being measured' to a 'measurement device' caused by an interaction. (ie: a quantum bus is a measurement) Just like in a classical situation, further dynamics can depend on the final state in the measurement device. The only difference is that all measurement states will be involved in the quantum measurement device, and all possibilities of dynamics conditioned on the measurement are explored. At any point in the future the whole chain of events can be collapsed, if the measurement device interacts with a classical incoherent object.
Decoherence doesn't really solve these issues. It gives you an approximately diagonal density matrix for the macroscopic degrees of freedom, but:
(a) Not exactly diagonal
(b) There isn't a unique decomposition of the macroscopic density matrix even after decoherence thus it cannot be taken as simply ignorance of some set of macrostates.
You need something stronger, namely superselection or irreversibility.
That's correct, But there are numerous good methods to propogate irreversible dynamics in qm. Many of these are exact in the limit of a noninteracting bath of bilinearly coupled oscillators which is sufficient to describe a measurement collapse. There's no mystery or inexact proscription to such a simulation of a collapse process. It's just complicated.
You are really missing the point - the details of how a measurement happens with specific instruments is not what the measurement problem is about.
The issue is linear evolution means the measurement of a superposition leads to a superposition of measurement devices. If the quantum state is real that gives you many worlds.
If you are suggesting there is nonlinear evolution, well a) it must be non-local and b) the theoretical research suggests it would be inconsistent. QM is a very rigid theory - “an island in theory space”. It isn’t easy to slightly modify.
What is the physical evidence that collapse actually happens?
Flash news. Nobody has ever produced any.
To the contrast we have lots of lines of evidence that an observer described by quantum mechanics should, upon observing a quantum experiment, be thrown into a superposition of observers. Each of which appears to have observed collapse. The notion is utterly repugnant to our biases so many reject the idea out of hand.
But as we create ever more complex but controlled systems, we can perform ever more elaborate experiments verifying that quantum mechanics works exactly as predicted. At some point if we take seriously the idea that the most successful scientific theory of all time is an accurate description of ourselves, then we have to accept that perhaps there is no collapse after all.
I happen to agree with you, the observer is a quantum system must get entangled with the quantum system, but that still doesn't explain probabilities. If you prepare a system - say sqrt(1/3) spin-down + sqrt(2/3) spin-up, and then observe it, repeatedly, your subjective experience is that you saw spin-down 1/3 of the time, and spin-up 2/3 of the time. I don't understand how purely unitary evolution can explain this. Does it?
> I don't understand how purely unitary evolution can explain this. Does it?
What's the alternative? Assuming unitary evolution and some fairly common-sense axioms about how we'd expect subjective experience to behave (things like: we never experience being in a branch that has amplitude zero; if we experience being in a given branch then we continue to be in that branch), the Born probabilities are the only model anyone's ever come up with for how our subjective experience should go. So what's there to explain?
The alternative is non-MWI theories, which typically introduce the Born rule via new axioms.
Regarding what's to explain, it's quantum randomness (which distills the Born rule objection). Our subjective experience is that we see spin-down 1/3rd of the time, and our theories say the result is otherwise impossible to predict, even in principle. But a deterministic theory cannot produce a random outcome, even a subjective one.
To paraphrase you, how do we get from probability amplitude to observed frequencies if there is no collapse?
This is were we have to invoke philosophy. Specifically how does consciousness interact with time? The common-sense thinking is that our soul is tied to our body and is traveling forward through time with it. Another way of thinking is that the soul is tied to a given position of the space-time-probability. It does not travel. You today is not the same as you tomorrow or yesterday. The you that observes spin up is not the same you as the one that observes spin down. Your soul is perceiving reality from a randomly chosen vantage point among all the possibilities with which have a compatible body. If we condition on those bodies belonging to experimenters who have observed frequencies, then we get the distribution.
No it can't. There have been many attempts and they don't work. The Born rule is independent of unitary evolution. The closest one can get is to declare that the quantum state is fundamentally a statistical object (i.e. the only information in it is observation probabilities) and then with certain assumptions about the size of the state space you can show that the Born rule is the only possible rule for connecting the state to statistics consistent with the unitary dynamics.
So under the assumption that the state encodes probabilities, state space assumptions and consistency with unitary evolution you get the Born rule. However this is not the same as the Born rule arising dynamically from unitary evolution alone.
Isn't your subjective experience just one probabilistic eigenvalue of a particular combination of operators corresponding to your observation? How does unitary evolution break down here?
It's not unitary evolution breaking down, just that the Born rule isn't a consequence of unitary evolution. They're separate independent hypotheses. In most derivations of QM from an axiomatic basis they're consequences of separate combinations of axioms.
Well in the most common family of interpretations "collapse" isn't an actual physical process, just Bayesian updating. So you wouldn't expect to find physical evidence of it in that sense.
It's true that from the perspective of an external superobserver the quantum state evolves to contain terms for each observer observation state. However since all interference observables turn out to be non-physical for macroscopic systems we get a superselection rule and so the probabilities for different macrostates are classical probabilities and thus reflect simple ignorance of the observer's post measurement state.
There's very little motivation for reading the quantum state "ontically" in the way you are doing.
But this doesn't answer the question. If you claim that all of these possible observers 'exist', how does this have a physical meaning?
This is what I never understood about MWI, in what physical sense can the many worlds be said to exist? Where are they in our universe? What direction would we have to travel to find them? Do they exert gravity on us? If not, then how can we claim that they exist in a physical sense?
No you're thinking of MWI all wrong. Your conception of the universe you exist in as being non-quantum is fundamentally flawed. The universe with the superposition of all the possible observers exists more purely in hilbert space. Sean Carroll has even started to put together a model for how spacetime could emerge from that hilbert space.
So the universes all exist in the same place, since they are the same universe. Your idea of what an observation is, is just an eigenvalue of that corresponding operator.
An object that moves far enough away from us is said to leave the observable universe, because with the continual expansion of space, it or anything it interacts with would have to travel faster than light back toward us in order to have any effect on us. Should we say that objects that leave the observable universe continue to exist? Should we amend our theories to include a new fall-off effect separate from gravity that says things stop existing when they exit our observable universe?
> "Existence isn't based on something affecting our world, obviously - that's just absurdly self-centered."
This is very unfair. This is a niche field with contested interpretations, don't make people feel stupid for asking fair questions.
It's obvious what the other person meant: what does 'our world' and 'other worlds' mean, and how do you know it's not just a figment of your imagination, as a scientific theory must be falsifiable -> i.e. measurable and provable / disprovable somehow.
You should at least point people to reading material before making fun of them.
In physical terms, we do generally define existence that way - for example, we say that time and space didn't exist 'before' the big bang, because there was nothing that could have a position or change. I was thinking of the same notion of existence and how it can be applied to MWI - essentially existence in the physical sense must mean that something is measurable, that it has some effect on the world (perhaps in the past or in the future).
Speaking as a barely informed enthusiast, we can say they exist in the Occam’s Razor sense that the maths is much less complicated when we assume they do.
I think there’s also an experimental setup, whose name I forget, but which is essentially nested Schrödinger's cat setups: Alice is in a box, Bob is in a box which contains Alice’s box, Carol is outside; Alice goes into superposition of |Alice+> and |Alice->, Bob opens the box and Carol can now demonstrate that Bob is in a superposition of |observing Alice+> and |observing Alice-> instead of the combination of 100%|observing> and a superposition of |Alice+> and |Alice->.
For the past several decades Penrose has been a fountain of nothing but frivolous fringe theories. Physicists are like investments: past performance is not indicative of future returns ;P
I agree. There are a (very) few experiments that show the effect of gravity in a system with strong quantum effects.
Fox example you can split a ray of neutrons, direct each beam through a different path with different height and then make them collide and see the interference pattern. The idea is that the split creates a superposition and each half has a different gravity potential, changing the orientation of the experiment produce different interference patterns. (The details are in the book of Sakurai "Modern Quantum Mechanics" pp127-129, with data from an experiment of Colella, Overhauser, Werner (1975).)
I don't understand why the old experiment was not enough to falsify this theory.
>Gravity was never a leading explanation for collapse; it was always a fringe idea.
Why not? Gravity is (probably) mediated by a particle, and therefore all matter will interact with all other matter ... so why shouldn't gravity therefore cause a collapse?
It’s one of the oddest tenets of quantum theory: a particle can be in two places at once—yet we only ever see it here or there. Textbooks state that the act of observing the particle “collapses” it, such that it appears at random in only one of its two locations. But physicists quarrel over why that would happen, if indeed it does. Now, one of the most plausible mechanisms for quantum collapse—gravity—has suffered a setback.