UPDATE I caused great confusion by putting the word “multiverse” in the title, when there is a whole other theory called “the” multiverse. Yet I did this because Hossenfelder herself, mainly in her prior book, Lost in Math: How Beauty Leads Physics Astray
In my view, the whole interpretative difficulty of QM centres around the notion of state *purity* (and expansion of that pure state in different bases) - and entanglement is just a special case of this applied to a Hilbert space that is the 'tensor product' of two subspaces.
Feynman probably summed it up best when he proclaimed that all of the mysteries of QM are to be found within the two-slit experiment.
I'm not a fan of the MWI and when I was based for a while in Oxford I spent a bit of time arguing with David Deutsch about it. I never won the arguments, though, because he's far, far, smarter than me.
I'm something of a pragmatist when it comes to QM - I tend to use the Copenhagen Interpretation with its projection postulate when actually doing calculations because I find it easier to work that way. I get the 'right' answers even though I recognise the profound philosophical problems therein. The thing is, all of my calculations could be re-worked within the Everett framework and I'd still get the same answers. There's no *experimental* way that we can distinguish between the interpretations since (so far) all of the predictions are identical.
I don't think your criticisms here are valid - for one thing, in the case of a binary 'up' or 'down' measurement (like for spin of a spin-1/2 particle, for example) there does not exist 2 versions of the world prior to the measurement which reflect the two possible outcomes. At the point of 'measurement' (whatever that is) the world splits into the two possibilities in the MWI - and there is a branching point. How we distinguish this from the supposition that there were two pre-existing worlds before the measurement each with its own future specified by the subsequent measurement outcome is not clear - although in this latter case we then have all of the problems of preferred bias. How did our 'pre-existing' worlds "know" that we were going to measure spin in the x-direction, and not the z-direction, for example?
The issue with entanglement (as I'm sure you know and have simplified for the purposes of the article) is not that long range correlations can exist, it's that such a long-range correlation exists (for an entangled state) *whatever* spin axis we choose to measure along. Alice and Bob, each getting one of these entangled particles can choose, at the very last instant to change the orientation of their measurement devices. When the data is subsequently compared it will be found that whenever they chose the same alignment they got this perfect correlation.
The data comparison is crucial here because as far as Alice and Bob are *individually* concerned, all they see from their spin measurements is a sequence of truly random bits when measuring the spins of a singlet state.
The issue with the potential for the existence of pure quantum states is neatly highlighted when considering entanglement. In any classical description the entropies of the sub-systems must obey the inequality : total entropy is less than (or equal to) the sum of the entropies of the sub-systems. QM, with state purity, allows us to add in another : the magnitude of the difference between the sub-system entropies must be less than (or equal to) the total entropy. If the state of the two systems is described by a pure state then this total entropy is zero - but the individual subsystem entropies are not zero (and are, in fact, *equal*). This is a really radical departure from any 'classical' understanding and its consequence is that when you measure the strength of correlation by the information 'content' of that correlation then in a quantum description you can have *twice* as much correlation as that admitted by the equivalent classical description of the 2-particle state.
It's this 'extra' correlation that the experimental tests for violations of the Bell inequality are trying to demonstrate - and it's this 'extra' correlation that is the basis for the ability to perform quantum computations.
You're right that we don't understand 'entanglement' but, for me, it's not an additional feature of QM - it's the same difficulty in interpreting what a quantum state **is** just played out in a more complicated Hilbert space.
Looking forward to more of your writing on QM issues.
"... and after a long time, long enough that the distance is so great the particles cannot communicate with each other, the left is measured spin up, and, voila, the right is spin down. How?"
Hmmm, if the distance is so great, no one can measure them. Alternatively, if you can measure them, then the distance was not so great and you were the communication medium.
When the experts in this topic talk about worlds, do they mean simply a planet that revolves around a star, or do they mean a region of the Universe that contains a different set of physical laws, such as the big G of the equation of universal gravitation having a different value?
Scientists: We're much better than those theologians who ask how many Angels can dance on a head of a pin huwe huwe huwe...
Also scientists: So, the Multiverse...
Ok. Nothing is measurable and finagling that produces a measurement skews the measurement and is false.
It’s times like this I am glad my wife is an astrophysicist, which just saved me a lot of time reading today.
In my view, the whole interpretative difficulty of QM centres around the notion of state *purity* (and expansion of that pure state in different bases) - and entanglement is just a special case of this applied to a Hilbert space that is the 'tensor product' of two subspaces.
Feynman probably summed it up best when he proclaimed that all of the mysteries of QM are to be found within the two-slit experiment.
I'm not a fan of the MWI and when I was based for a while in Oxford I spent a bit of time arguing with David Deutsch about it. I never won the arguments, though, because he's far, far, smarter than me.
I'm something of a pragmatist when it comes to QM - I tend to use the Copenhagen Interpretation with its projection postulate when actually doing calculations because I find it easier to work that way. I get the 'right' answers even though I recognise the profound philosophical problems therein. The thing is, all of my calculations could be re-worked within the Everett framework and I'd still get the same answers. There's no *experimental* way that we can distinguish between the interpretations since (so far) all of the predictions are identical.
I don't think your criticisms here are valid - for one thing, in the case of a binary 'up' or 'down' measurement (like for spin of a spin-1/2 particle, for example) there does not exist 2 versions of the world prior to the measurement which reflect the two possible outcomes. At the point of 'measurement' (whatever that is) the world splits into the two possibilities in the MWI - and there is a branching point. How we distinguish this from the supposition that there were two pre-existing worlds before the measurement each with its own future specified by the subsequent measurement outcome is not clear - although in this latter case we then have all of the problems of preferred bias. How did our 'pre-existing' worlds "know" that we were going to measure spin in the x-direction, and not the z-direction, for example?
The issue with entanglement (as I'm sure you know and have simplified for the purposes of the article) is not that long range correlations can exist, it's that such a long-range correlation exists (for an entangled state) *whatever* spin axis we choose to measure along. Alice and Bob, each getting one of these entangled particles can choose, at the very last instant to change the orientation of their measurement devices. When the data is subsequently compared it will be found that whenever they chose the same alignment they got this perfect correlation.
The data comparison is crucial here because as far as Alice and Bob are *individually* concerned, all they see from their spin measurements is a sequence of truly random bits when measuring the spins of a singlet state.
The issue with the potential for the existence of pure quantum states is neatly highlighted when considering entanglement. In any classical description the entropies of the sub-systems must obey the inequality : total entropy is less than (or equal to) the sum of the entropies of the sub-systems. QM, with state purity, allows us to add in another : the magnitude of the difference between the sub-system entropies must be less than (or equal to) the total entropy. If the state of the two systems is described by a pure state then this total entropy is zero - but the individual subsystem entropies are not zero (and are, in fact, *equal*). This is a really radical departure from any 'classical' understanding and its consequence is that when you measure the strength of correlation by the information 'content' of that correlation then in a quantum description you can have *twice* as much correlation as that admitted by the equivalent classical description of the 2-particle state.
It's this 'extra' correlation that the experimental tests for violations of the Bell inequality are trying to demonstrate - and it's this 'extra' correlation that is the basis for the ability to perform quantum computations.
You're right that we don't understand 'entanglement' but, for me, it's not an additional feature of QM - it's the same difficulty in interpreting what a quantum state **is** just played out in a more complicated Hilbert space.
Looking forward to more of your writing on QM issues.
"... and after a long time, long enough that the distance is so great the particles cannot communicate with each other, the left is measured spin up, and, voila, the right is spin down. How?"
Hmmm, if the distance is so great, no one can measure them. Alternatively, if you can measure them, then the distance was not so great and you were the communication medium.
I request a clarification, please.
When the experts in this topic talk about worlds, do they mean simply a planet that revolves around a star, or do they mean a region of the Universe that contains a different set of physical laws, such as the big G of the equation of universal gravitation having a different value?