kali_fornicationit's a long distance relationship
To be fair, the “change one” part is wrong. Two particles that are quantum entangled maintain the same quantum state when separated. But if you change the quantum state of one it doesn’t propogate. They are just in sync.
The analogy that makes most sense to me so far, is this:
You rip a photograph in half and put both halves into envelopes. Now you send one of the envelopes to your friend in Australia. You open the other envelope. Boom! Instantaneous knowledge of what’s in the envelope in Australia. Faster than light!!!
In quantum terms, you “rip a photograph in half” by somehow producing two quanta, which are known to have correlated properties. For example, you can produce two quanta, where one has a positive spin and the other a negative spin, and you know those to be equally strong. If you now measure the spin of the first quantum, you know that the other has the opposite spin.
The important distinction here (and I get it, analogies are always imperfect) is that the photograph analogy has “hidden variables”. That is, each half is fixed at the moment of their separation and you just don’t know what’s in the envelopes until you open one. That’s not how entangled particles work though, and which “half” is which is not determined until the instant of measurement, at which point the state of both are known and fixed.
I’m open for counterarguments, but I always felt this was a silly way of looking at things. You cannot measure stuff at the quantum level without significantly altering what you measured. (You can never measure without altering what you measured, since we typically blast stuff with photons from a light source to be able to look at it, but for stuff that’s significantly larger than photons, the photons are rather insignificant.)
As such, you can look at measuring quanta in two ways:
Well, and isn’t quantum entanglement evidence for 1.? You entangle these quanta, then you measure one of them. At this point, you already know what the other one will give as a result for its measurement, even though you have not measured/altered it yet.
You can do the measurement quite a bit later and still get the result that you deduced from measuring the entangled quantum. (So long as nothing else altered the property you want to measure, of course…)
BjörnSomething something Bell’s Theorem. I don’t really understand it but that one was supposed to be counterevidence to hidden variables.
“it can’t be hidden variables because they’re not as even as this math says they should be!” really just seems to be the whole QM field agreeing to stop arguing about spooky action at a distance.
The distinction between wave-functions as real things that collapse at superluminal speed and the same as mere mathematical placeholders for deterministic local effects which occur without subjective time seems to be a semantic and philosophical one, similar to the “multiple realities” explanation of quantum uncertainty or the “11 dimensions” explanation for why gravity is weaker.
As a practical matter, the only thing that students and non-physicts should remember is that wavefunction collapse allows superluminal coordination but not superluminal communication.
MinnesotaGoddamokay so if i understand this right, if i take half of schroedingers box and open it up, by observing the half of the cat i have i will instantly know if the half of the box the other guy’s got has got half of an alive cat in it? and i’ll be able to tell if his half of an alive cat is purring and void or garfield and shit is my stupid analogy right?
You want to cut Schrödinger’s box in half? This kills the cat, unless the box is big enough for the cat to avoid the blade, in which case you’ve opened the box and the cat is probably going to need some convincing to get out from under whatever furniture it can find.
no this is a quantum box and a quantum cat, you can do things like that
schroedinger’s cat is an intentionally absurd metaphor from when QM dorks were still arguing about spooky action at a distance.
Both the cat, the box, the vial of poison, and the cesium atom itself are all observers as far as a real QM wavefunction would care. But as i understand it, getting any utility out of the idea of real collapsing wave-functions requires treating at least the atom as if it wasn’t, and once we start including atomic scale things we might as well just include everything up to and including the cat.
also schroedinger was an awful person so having him associated with a terrible metaphor is kind of great
pcalau12iThe point that Bell tried to point out in his “Against ‘Measurement’” article is that when you say “we start including atomic scale things we might as well just include everything up to and including the cat,” but you have to place the line somewhere, sometimes called the “Heisenberg cut,” and where you place the line has empirically different implications, so wherever you choose to draw the line must necessarily constitute a different theory.
Deutsch also published a paper “Quantum theory as a universal physical theory” where he proves that drawing a line at all must constitute a different theory from quantum mechanics because it will necessarily make different empirical predictions than orthodox quantum theory.
A simple analogy is, let’s say, I claim the vial counts as an observer. The file is simple enough that I might be able to fully model it in quantum mechanics. A complete quantum mechanical model would consist of a quantum state in Hilbert space that can only evolve through physical interactions that are all described by unitary operators, and all unitary operators are reversible. So there is no possible interaction between the atom and the vial that could possibly lead to a non-reversible “collapse.”
Hence, if I genuinely had a complete model of the vial and could isolate it, I could subject it to an interaction with the cesium atom, and orthodox quantum mechanics would describe this using reversible unitary operators. If you claim it is an observer that causes a collapse, then the interaction would not be reversible. So I could then follow it up with an interaction corresponding to the Hermitian transpose of the operator describing the first interaction, which is should reverse it.
Orthodox quantum theory would predict that the reversal should succeed while your theory with observer-vials would not, and so it would ultimately predict a different statistical distribution if I tried to measure it after that interaction.
Quantum mechanics is more weird than that. It’s not accurate to say things can be in two states at once, like a cat that is both dead and alive at the same time, or a qubit that is both 0 and 1 at the same time. If that were true, then the qubit’s mathematical description when in a superposition of states would be |0>+|1>, but it is not, it is a|0>+b|1> where the coefficients are neither 0 or 1, and the coefficients cannot just be ignored if one were to give a physical interpretation as they are necessary for the system’s dynamics.
You talk about it being “half” a cat, so you might tihnk the coefficient should be interpreted as proportions, but proportions are such that 0≤x≤1 and ∑x=1. But in quantum mechanics, the coefficients can be negative and even imaginary, and do not have to sum to 1. You can have 1/√2|0>-i/√2|1> as a valid superposition of states for a qubit. It does not make sense to interpret -i/√2 as a “half,” so you cannot meaningfully interpret the coefficients as a proportion.
Trying to actually interpret these quantum states ontologically is a nightmare and personally I recommend against even trying, as you will just confuse yourself, and any time you think you come up with something that makes sense, you will later find that it is wrong.
Sure,but if i open one of the doors and show you the goat’s not there, do you change your answer?
Aw I wanted the goat. I guess so
MaxThe whole idea is that the quantum particle can’t have had the state you’re measuring all along. If it did, then measuring a particular set of outcomes would be improbable. If you run an experiment millions of times, you have a choice in how you do the final measurement each time. What you find with quantum particles is that the measurements of the two different particles are more correlated than they should be able to if they had determined an answer (state) in advance.
You can resolve this 3 ways:
1: you got extremely unlucky with your choice of measurement in each experiment lining up with the hidden/fixed state of each particle in such a way as to screw with your results. If you do the experiment millions of times, the probability of this happening randomly can be made arbitrarily small. So then, the universe must be colluding to give you a non uniform distribution of hidden states that perfectly mess with your currently chosen experiment
2: the particles transfer information to each other faster than the speed of light
3: there is no hidden state that the particle has that determines how it will be measured in any particular experiment
See quantamagazine.org/how-bells-theorem-proved-spook… for a short explanation of what ‘more correlated than expected’ means
This is pretty conclusively addressed by the Bell Inequalities and empirically tested. It’s absolutely counter-intuitive and feels “wrong” but it is definitely how our universe operates.
The photograph example has local hidden variables. The quantum version doesn’t, but nothing has ruled out non-local hidden variables. Locality is a nice property we want the universe to have, but the universe doesn’t have to obey our desires.