The following two things seem to be true:
(1) The universe is massively entangled because the wave function that describes the entire universe has measure 1 of being entangled. Further, given how entanglement spreads, it seems likely that the universe is massively entangled.
(2) On a canonical presentation, entanglement of properties implies that one cannot specify the property of one particle without referring to the properties of the others entangled within the system. the example is the EPR state. One can't talk about the spin of either particle without referring to the spin of the other.
But it seems like we are able to talk about the spin of particles independently in the lab. After a measurement we may say that one particle is x-spin up without reference to any other particles. But if that's true, how is the truth of (1) and (2) maintained? How do we manage to talk about the state of a particle as if it wasn't entangled with the rest of the universe? How is that even possible given that entanglement implies that we must refer to the rest of the universe in our discussion of that property? Thanks for any answers! I'm sure I'm just really confused about something…
Best Answer
Massively entangled - yes, completely entangled - no. Photons are being created all the time and the ones that are in transit that have not directly interacted with anything yet are not yet entangled with anything.
The measurement of a pair of entangled particles is a (futile) attempt to full identify its spin state by e.g. measuring both its vertical component and its horizontal component simultaneously. When we measure the spin state of a single particle by passing it though a Stern Gerlach device we could detect whether it is spin up or spin down (let's say it was up), but we have no idea of its horizontal component. We can try to determine its horizontal component by passing it through a second SG device that is orthogonal to the first and for example determine its horizontal component is left spin. Now we think we have a good idea that the particle has both up spin and and left spin, but we are wrong. This can be demonstrated by passing it through yet another vertical SG spin analyser and 50% of the time it will tell us the the particle is spin down! This means measuring the horizontal component altered the vertical component so each time it passes through an analyser its orientation is changed. This also infers that when we passed the particle through the first vertical analyser we altered its left/right spin component so we all we know about the original spin state of the particle is that it had a vertical component and an unknown horizontal component.
We can also demonstrate that measurement alters the state of the particle when analysing polarisation of photons in a simple experiment that you can even do at home. Get some polarising filters and place them at 90 degrees to each other. Almost all the light is blocked. This is because if the first filter lets photons with a greater vertical than horizontal component pass through, then they do not pass through the second filter. Now if we insert a third filter between the first two, that is orientated at 45 degrees to both of them, significantly more light passes through. This is because some of the light that had a greater vertical component passing through the first filter now has a greater horizontal component after passing through the middle filter. The filters measuring the polarisation, also alter the polarisation.
This means the statement "One can't talk about the spin of either particle without referring to the spin of the other." is more accurately stated as "One can't talk about the (exact state of) spin of either particle
without(even when) referring to the spin of the other."When a particle like a photon is emitted from an atom, it born in a coherent and not entangled with the universe. When it interacts with other particles and imparts energy to them it decoheres and its entanglement spreads out to the particles it interacts with. What constitutes an interaction? Normally bouncing off a mirror or passing through a polariser in e.g. an interferometer does not count as an (measurement) interaction. If we were to mount the mirrors in such a way that we could could measure the recoil of a photon bouncing off it in order to determine "which way" path information, that would constitute an measurement interaction and it would decohere and spoil the interference pattern. Same goes if we try to mount to mount the source in such a way as to measure the recoil to try to determine the time of emission. In fact, until the photon interacts with other particles in such a way that its location can be determined, there is no way to determine the photon even exists. Once detected we can infer approximately when it was emitted and even which path it took, but the more information we have about it, the more it decoheres and more entangled it gets with the rest of the universe. Of course we never gain complete information about the particle such as its exact polarisation or spin as this is forbidden by Quantum Mechanics and in particular, the Heisenberg uncertainty principle.