nice article and explanation of the history of locality/realism (or lack thereof) in qm! what i find (especially) interesting about entanglement is the way it truly transcends concepts of causality, a point that i didn't fully appreciate for a long time. if the measurements of each entangled partner are made sufficiently far in space and sufficiently close in time (space-like separation) then you can find a frame of reference in which either member of the pair is measured first! hence it is truly impossible to say that the measurement of one 'causes' the other to assume a particular state, even 'instantaneously causes'. causality is meaningless if the events can happen equally well in either order. my belief is that entanglement is a hallmark of a deeper theory/set of physical principles that doesn't include spacetime as a basic ingredient, and that is why its properties defy spacetime. after all, a qm-style theory can be easily formulated without any reference to spacetime; it's an extremely general framework.
Yes, I agree that that is a very interesting part of it. I certainly tend to think of entanglement as imposing rules of consistency on the outcomes of measurements of the entangled pair rather than as a measurement of one causing a change in the other, for the reason you say.
On the topic of quantum mechanics and physics beyond spacetime, have you read Sean Carroll's book "Something Deeply Hidden"? It is about the many-worlds interpretation of quantum mechanics and explores the idea of spacetime as an emergent phenomenon, especially with regards to how this could help to address the preferred basis problem which I understand to still be the primary technical challenge to that interpretation.
The apparent collapse of the wavefunction is explained by decoherence - the combination of the enormous number of particles in the measurement device with random phases drowns out the phase of the individual quantum particle, so that the two dimensional quantum space is projected onto a classical 1D space. But decoherence doesn't explain in what basis this projection should happen - that's the preferred basis problem. For example, decoherence can explain why Schrodinger's cat doesn't stay in superposition, but not why the basis it collapses to is {alive, dead} instead of any other linear combinations of alive and dead.
hmm, that's interesting. i now remember i have actually heard about that problem. when i first read about this i read about that problem as being commonly addressed with 'environmentally-induced superselection' (i'll link the article at the bottom) - basically, the measuring device (i.e. the 'environment') determines the preferred basis. a device designed to measure in a certain spin basis will cause the system to collapse onto that spin basis (by definition). i think of the logic here as, if the set-up is such that a certain basis is singled out that has a big effect of the long-term evolution of the system, then the system will project onto that basis (precisely because different states in that basis lead to large-scale, observable changes to the universe).
would be curious as to how emergent spacetime could address the same problem (presumably in a complementary, rather than an alternative, way, since environmentally induced superselection at the very least seems natural and unavoidable)
nice article and explanation of the history of locality/realism (or lack thereof) in qm! what i find (especially) interesting about entanglement is the way it truly transcends concepts of causality, a point that i didn't fully appreciate for a long time. if the measurements of each entangled partner are made sufficiently far in space and sufficiently close in time (space-like separation) then you can find a frame of reference in which either member of the pair is measured first! hence it is truly impossible to say that the measurement of one 'causes' the other to assume a particular state, even 'instantaneously causes'. causality is meaningless if the events can happen equally well in either order. my belief is that entanglement is a hallmark of a deeper theory/set of physical principles that doesn't include spacetime as a basic ingredient, and that is why its properties defy spacetime. after all, a qm-style theory can be easily formulated without any reference to spacetime; it's an extremely general framework.
Yes, I agree that that is a very interesting part of it. I certainly tend to think of entanglement as imposing rules of consistency on the outcomes of measurements of the entangled pair rather than as a measurement of one causing a change in the other, for the reason you say.
On the topic of quantum mechanics and physics beyond spacetime, have you read Sean Carroll's book "Something Deeply Hidden"? It is about the many-worlds interpretation of quantum mechanics and explores the idea of spacetime as an emergent phenomenon, especially with regards to how this could help to address the preferred basis problem which I understand to still be the primary technical challenge to that interpretation.
i haven't read that book! what is the preferred basis problem?
The apparent collapse of the wavefunction is explained by decoherence - the combination of the enormous number of particles in the measurement device with random phases drowns out the phase of the individual quantum particle, so that the two dimensional quantum space is projected onto a classical 1D space. But decoherence doesn't explain in what basis this projection should happen - that's the preferred basis problem. For example, decoherence can explain why Schrodinger's cat doesn't stay in superposition, but not why the basis it collapses to is {alive, dead} instead of any other linear combinations of alive and dead.
hmm, that's interesting. i now remember i have actually heard about that problem. when i first read about this i read about that problem as being commonly addressed with 'environmentally-induced superselection' (i'll link the article at the bottom) - basically, the measuring device (i.e. the 'environment') determines the preferred basis. a device designed to measure in a certain spin basis will cause the system to collapse onto that spin basis (by definition). i think of the logic here as, if the set-up is such that a certain basis is singled out that has a big effect of the long-term evolution of the system, then the system will project onto that basis (precisely because different states in that basis lead to large-scale, observable changes to the universe).
https://arxiv.org/abs/quant-ph/0312059v4
would be curious as to how emergent spacetime could address the same problem (presumably in a complementary, rather than an alternative, way, since environmentally induced superselection at the very least seems natural and unavoidable)