4 ms·

I think the point being made is a non-local variable is just "instant communication" in a different guise. It's a variable that everyone can see, and once it'

I think the point being made is a non-local variable is just "instant communication" in a different guise. It's a variable that everyone can see, and once it's value is observed is instantly for all observers no matter how far apart.

bowsamic 4y ago
Well the idea of hidden variables is that they are totally inaccessible to current technology, so they aren't really observable. In the case of Bohmian mechanics the "hidden variable" is the configuration of all particles in the universe, which is clearly not something that can be observed by any observer. It's also not a quantity that is changed by observation which your line "once it's value is observed is instantly for all observers no matter how far apart" implies. It isn't a quantum observable: it is a normal classical kind of state. It's just that the physics for one particle depends on the configuration of all other particles, which is completely non-local.
- rstuart4133 4y ago
  To me all these references to Bohmian mechanics, hidden variables and faster than light communication makes hides more than it reveals. These connections always arise because of some invariant our universe preserves, eg charge is conserved, or spin is conserved, or whatever. The universe happens to let us create particles - an electron say, from pure energy. But the conservation of charge means you can't just create an electron, there has to be a positron too otherwise charge isn't conserved. They would instantly annihilate each other if they hung around together, so in order for us to see them they must be moving away from each other (with equal but opposite momentum, because that's conserved too). Until we measure it, we don't know what the charge of either of particles is. Quantum superposition means that until we measure it, the charge of both particles is effectively in a 3rd state: unknown (as opposed to positive or negative). Putting this in terms of hidden variables, there is a variable that is in one of three states: "particle-A: Unknown charge, particle-B: Unknown charge", "A: positive, B: negative", "A: negative, B: positive". It doesn't look to me like this this variable exists everywhere. It only describes the state of these two particles which by definition occupy a tiny positions in space-time. And no it isn't unobservable either: you can observe the variable by just measuring one of the particles. It does raise all sorts of interesting questions about how the universe operates. How does it preserve these invariants across space and why are they preserved? Does this quantum superposition look anything like it's being described here? I've seen other descriptions that make it arise quite naturally from the math of 2-norm probabilities and thus not require multi-verses or some mysterious "collapse" (which look to me to be about a useful as explaining what s going on as "Bohmian mechanics"). So these hidden variables are a hand wavy way of describing how conversation laws interact QM superposition. It's not really that complex - is it? Was there any need to introduce "hidden variables" at all? It seems to me a lot of the difficulty people have in describing QM is not in QM itself, but from the knots people get themselves into in their personal struggle to understand it. They write their knots down, we gumbies read it and say "bugger me that looks hard".
lamontcg 4y ago
QM nonlocality isn't useful for instant communication though. If it is a physical process, you still cant influence it on one end in order to communicate faster than the speed of light. And nothing is "observed instantly" since that phrase doesn't have meaning in SR. Two space-like separated observations, which cannot communicate with each other in SR, will have correlated measurements. Which measurement comes first or if they happen at the same time, will depend on your reference frame and motion. So once a single measurement has been made somewhere in the universe, all observers (light-like or space-like separated) will measure entangled values that agree with that measurement, but the entangled state was created indeterminately. If the observations are space-like separated you cannot say which one "caused" the collapse of the wave function. And like I said you can't use it for communication. The fact that you can't assign which observation was cause and which one was effect is probably tightly tied to the fact that you can't use it to communicate -- which side is the sender and which side is the receiver? That depends on the reference frame, which produces nonsense, so to avoid a paradox it is banned.

bowsamic 4y ago

Well the idea of hidden variables is that they are totally inaccessible to current technology, so they aren't really observable. In the case of Bohmian mechanics the "hidden variable" is the configuration of all particles in the universe, which is clearly not something that can be observed by any observer. It's also not a quantity that is changed by observation which your line "once it's value is observed is instantly for all observers no matter how far apart" implies. It isn't a quantum observable: it is a normal classical kind of state. It's just that the physics for one particle depends on the configuration of all other particles, which is completely non-local.

rstuart4133 4y ago
To me all these references to Bohmian mechanics, hidden variables and faster than light communication makes hides more than it reveals. These connections always arise because of some invariant our universe preserves, eg charge is conserved, or spin is conserved, or whatever. The universe happens to let us create particles - an electron say, from pure energy. But the conservation of charge means you can't just create an electron, there has to be a positron too otherwise charge isn't conserved. They would instantly annihilate each other if they hung around together, so in order for us to see them they must be moving away from each other (with equal but opposite momentum, because that's conserved too). Until we measure it, we don't know what the charge of either of particles is. Quantum superposition means that until we measure it, the charge of both particles is effectively in a 3rd state: unknown (as opposed to positive or negative). Putting this in terms of hidden variables, there is a variable that is in one of three states: "particle-A: Unknown charge, particle-B: Unknown charge", "A: positive, B: negative", "A: negative, B: positive". It doesn't look to me like this this variable exists everywhere. It only describes the state of these two particles which by definition occupy a tiny positions in space-time. And no it isn't unobservable either: you can observe the variable by just measuring one of the particles. It does raise all sorts of interesting questions about how the universe operates. How does it preserve these invariants across space and why are they preserved? Does this quantum superposition look anything like it's being described here? I've seen other descriptions that make it arise quite naturally from the math of 2-norm probabilities and thus not require multi-verses or some mysterious "collapse" (which look to me to be about a useful as explaining what s going on as "Bohmian mechanics"). So these hidden variables are a hand wavy way of describing how conversation laws interact QM superposition. It's not really that complex - is it? Was there any need to introduce "hidden variables" at all? It seems to me a lot of the difficulty people have in describing QM is not in QM itself, but from the knots people get themselves into in their personal struggle to understand it. They write their knots down, we gumbies read it and say "bugger me that looks hard".

lamontcg 4y ago

QM nonlocality isn't useful for instant communication though. If it is a physical process, you still cant influence it on one end in order to communicate faster than the speed of light. And nothing is "observed instantly" since that phrase doesn't have meaning in SR. Two space-like separated observations, which cannot communicate with each other in SR, will have correlated measurements. Which measurement comes first or if they happen at the same time, will depend on your reference frame and motion. So once a single measurement has been made somewhere in the universe, all observers (light-like or space-like separated) will measure entangled values that agree with that measurement, but the entangled state was created indeterminately. If the observations are space-like separated you cannot say which one "caused" the collapse of the wave function. And like I said you can't use it for communication. The fact that you can't assign which observation was cause and which one was effect is probably tightly tied to the fact that you can't use it to communicate -- which side is the sender and which side is the receiver? That depends on the reference frame, which produces nonsense, so to avoid a paradox it is banned.