I am not a mathematician, however I have just started reading about quantum physics. I have a question. If two paired photons are travelling in opposite directions through the universe, one lands on a detector in a lab on Earth and has it's y axis "spin" measured, am I right in saying that if the other particle lands on an alien planet on a detector in a lab, it will not be able to have it's x axis "spin" measured? And if I have got this right, why should there only be two explanations for this; that there is either instant communication between the two, or hidden variables. Why is the third explanation not proposed which merely states that time and space are not the construct we imagine it to be and there is something outside of this world which links the two particles? (such as both being stored in the same set of variables in a super computer?)
First of all, you're a little off in your measurements. If we measure a particle created with zero spin and we get one spin up in our lab, the other lab has to get a spin down. On to your basic point, that is one of the major issues with the orthodox view of quantum mechanics is this thing which is called the EPR paradox. It's been proven that there would be a measureably difference in local hidden variable theories and quantum mechanics as it stands (look up Bell's inequality for more on that). Basically, the idea of local realism breaks down on the quantum level, and people are still debating what all this means. I'm not exactly what you would call up to speed on this research, perhaps someone else on the board knows more of an explaination for the EPR Paradox.
If you measure Sx in one lab and Sy in another that is perfectly possible. It is just that there would be no correlation. There are many ways of thinking of this and there was a publication a year or two ago that showed that hidden variables are not completely excluded. They just have to be more complicated that what Bell excluded. But I agree that the most promising explanation is that space and time are emergent properties of our world, not the fundamental core.
For the sake of their credibility, I would surely hope that they would both measure spin zero on a zero-spin particle. There is no "spin up" or "spin down" for photons. Bye! Crisp
Unfortunately there is no satisfactory answer for quantum entanglement.We have the following options: 1.The observed connections are 'nonclassical' not 'nonlocal',the wavefunction is only a mathematical tool (the point of view of Bohr and Heisenberg),'c' is the maximum speed so that there are no instant connections.This is the main view today among copenhagenists,trying to retain full coherence with SR (no preferred frame of reference).The drawback is that it offers no real explanation for the observed correlations (see for example Bohr's answer to EPR paradox,not satisfactory).Everett's many worlds interpretation also qualify here for it does not require nonlocality,c being also a limit. 2.There exist nonlocality,the wavefunction is real,stretched in space,so that the entangled particles still form a single system (superposition of states) even when far apart,allowing instant connections,the 'collapse' of the wavefunction is responsible for the complementary values of spin measured.Thus nature is somehow 'wired' by allowing instant connections but we cannot use this link to send information superluminally.This view is held by a minority of copenhagenists. 3.Superluminal connections exist,nature is somehow 'wired',though we cannot use those connection to send information superluminally.There exist non local hidden variables (it is widely accepted that Aspect's experiment rule out only the premise of locality) or advanced and retarded waves (as implied by the transactional interpretation of QM).In Bohm's interpretation the quantum potential (an effect of a subquantum unified domain) is responsible for the connection at distance observed.
Crisp, photons are spin-1 particles so they can exist in three different spin states. It is the initial pair of photons that is created in a total spin state of 0. This is called a singlet state. Whichever spin you measure for one of the photons, the other one will have the opposite value as long as you measure along the same axis.
This is not presented as an explanation but just food for thought. If as Prof Tryon claimed his calculations of the observable universe showed that the "Net Accumulative Energy of the Universe = Zero" or "Nothing" and we view the Big Bang as an ex nihilo event, further define "Nothingness" as being the "Absence of Time-Space". Does it not make sense that every particle is linked to its ex nihilo origin and hence by "Absence of time-space are linked at that singularity (via the vacuum foam)? Then would not particle entanglement make sense that particles communicate instantly since to do so no space is traveled and no time consumed? Now don't ask me to explain how that is possible but it suggests all creation is somehow connected to this common singularity of "Nothingness", its origin.
Crisp, that is incorrect. Photons are described by vector fields, and thus are invariant under the SO(3) group of transformations. Thus the photon field has 3 rotation degrees of freedom, i.e. 3 spin states, 1, 0 +1
Yes, my mind slipped over that one. The point I was trying to make was not about photons though, one of the posts said that a particle with zero spin could yield a spin up as a result.
photons are described by a 4-vector field, which has 4 components, and transforms covariantly (not invariantly) under not only the spatial rotation group SO(3), but the full spacetime rotation group SO(3,1) first of all, the photon, being massless, doesn't have a spin quantum number, since it has no rest frame. instead, you have the helicity quantum number. second of all, it has not 3 degrees of freedom, but rather 2 degrees of freedom, because it is massless. 2 of the 4 degrees of freedom in the 4-vector are gauge degrees of freedom, and therefore not physical.
And Lethe comes and puts it in an eleggant and rigorous form. Yes Lethe, that is correct, I agree with you totally. However, the seeming exception to gauge invariance is the Aharanov-Bohm effect, of which I remind you that I still have a lot of work to do to understand
The Bohm-Aharanov effect is gauge invariant. the difference in phase between two paths is the flux of the area bounded by the two paths. the field strength, and therefore also the flux, is gauge invariant. and i think i meant to say before, a photon has only 2 spin states: you can measure z-component spin +1 or -1 for a photon, but not 0. only massive particles can have z-component spin 0, so the same applies to the graviton. here i need the z-axis to be the direction of propagation of the photon. then the z-component of the spin is called helicity
Is this in reference to the two spinning bits they're always talking about with Quantum physics ? As you can see, I've only just recently read a book too ... If I'm right, the argument goes, that wherever you find these particles, they are ALWAYS in pairs that spin in opposite directions ? So if you move one to another planet and change the direction of spin of the one on Earth you have instant messaging ?? Sounds good. So, I noticed the other day that whenever you see shoes, they exist as a left & a right shoe. If I put one on Mars and one on Earth and then change the one on Earth from left to right, will the one on Mars change from right to left ? We could save millions on expensive research ... I'm going to buy shares in Clark's !!
Lethe, you seem to know something about photons, I have tried to figure out about how the three spin states of photons relate to the polarization state of the photon. I have seen some hints that +1 and minus 1 relate to polarization II or =. The word helicity seems to imply that there is a polarization along the photon path too. Does this relate to the 0 spin state?
the photon has only 2 spin states, because it is massless the photon has only transverse polarizations. helicity refers to the component of angular momentum along the axis of motion, which is nonzero, even though the photons polarization has no component along the direction of motion. so if the photon is travelling in the z direction, then the z-component of the vector field is zero, but the z-component of the angular momentum is not. the z-component is constrained to be +1 or -1. it cannot be 0 (unlike the massive case)
