3 improved formatting edit approved Jul 28 '14 at 8:45 Groo 10355 bronze badges  So another answer to your question is the following: people prefer  So another answer to your question is the following: people prefer to talk about an active link because they cannot accept the subjective interpretation of probability and the wave function. There is a lot of current research studying quantum measurement as an actual physical process involving thermodynamic limits of unstable negative temperature systems (bubble chambers etc.). To put this another way: alternative 1 implicitly assumes that in the combined system there are 'two particles', but this is probably a fallacy: quantum mechanics does not really recognise any precise notion of particle. As in thermodynamic limits, the notion of 'particle' is a useful approximation within a certain range of set-ups, and loses validity and leads to paradoxes if you attempt to use it outside the limits of its validity. Alternative 2 implicitly assumes that if something such as the wave function can only be approximately measured, it is somehow not 'physical', but this is unduly simplistic and troubles people because of the seeming necessity of dragging in the subjective Bayesian point of view. Alternative 3 is at least so open ended that one cannot find fault with it but neither is there a shred of experimental evidence for it. The only problems with QM are logical, not experimental. Therefore alternative 1 implicitly assumes that in the combined system there are 'two particles', but this is probably a fallacy: quantum mechanics does not really recognize any precise notion of particle. As in thermodynamic limits, the notion of 'particle' is a useful approximation within a certain range of set-ups, and loses validity and leads to paradoxes if you attempt to use it outside the limits of its validity. Alternative 2 implicitly assumes that if something such as the wave function can only be approximately measured, it is somehow not 'physical', but this is unduly simplistic and troubles people because of the seeming necessity of dragging in the subjective Bayesian point of view. Alternative 3 is at least so open ended that one cannot find fault with it but neither is there a shred of experimental evidence for it. The only problems with QM are logical, not experimental. Therefore if one questions the implicit assumptions made about the careless use of concepts such as 'particle', 'system', and 'probability', there are many more alternatives and the final answer is not in.  So another answer to your question is the following: people prefer  to talk about an active link because they cannot accept the subjective interpretation of probability and the wave function. There is a lot of current research studying quantum measurement as an actual physical process involving thermodynamic limits of unstable negative temperature systems (bubble chambers etc.). To put this another way: alternative 1 implicitly assumes that in the combined system there are 'two particles', but this is probably a fallacy: quantum mechanics does not really recognise any precise notion of particle. As in thermodynamic limits, the notion of 'particle' is a useful approximation within a certain range of set-ups, and loses validity and leads to paradoxes if you attempt to use it outside the limits of its validity. Alternative 2 implicitly assumes that if something such as the wave function can only be approximately measured, it is somehow not 'physical', but this is unduly simplistic and troubles people because of the seeming necessity of dragging in the subjective Bayesian point of view. Alternative 3 is at least so open ended that one cannot find fault with it but neither is there a shred of experimental evidence for it. The only problems with QM are logical, not experimental. Therefore if one questions the implicit assumptions made about the careless use of concepts such as 'particle', 'system', and 'probability', there are many more alternatives and the final answer is not in. So another answer to your question is the following: people prefer to talk about an active link because they cannot accept the subjective interpretation of probability and the wave function. There is a lot of current research studying quantum measurement as an actual physical process involving thermodynamic limits of unstable negative temperature systems (bubble chambers etc.). To put this another way: alternative 1 implicitly assumes that in the combined system there are 'two particles', but this is probably a fallacy: quantum mechanics does not really recognize any precise notion of particle. As in thermodynamic limits, the notion of 'particle' is a useful approximation within a certain range of set-ups, and loses validity and leads to paradoxes if you attempt to use it outside the limits of its validity. Alternative 2 implicitly assumes that if something such as the wave function can only be approximately measured, it is somehow not 'physical', but this is unduly simplistic and troubles people because of the seeming necessity of dragging in the subjective Bayesian point of view. Alternative 3 is at least so open ended that one cannot find fault with it but neither is there a shred of experimental evidence for it. The only problems with QM are logical, not experimental. Therefore if one questions the implicit assumptions made about the careless use of concepts such as 'particle', 'system', and 'probability', there are many more alternatives and the final answer is not in. 2 typos edited Dec 6 '11 at 12:16 joseph f. johnson 5,6512222 silver badges3838 bronze badges It is not really clear that cases 1,2, and 3 are exhaustive. Discussions about this phenomenon use a lot of terms which are not precisely defined. For example, 'particle' and 'system'. If there is entanglement, then there is one combined system, and it is misleading to call that one combined system 'two particles'. The comment about realism and approximation is also inaccurate: all positions and data in classical physics are approximate too, this has nothing to do with the difference between classical and quantum or the difference betweeen using a Hamiltonian system whose states are points given by momentum and position coordinates and using a Hamiltonian system whose points are rays in a Hilbert Space. The comment about entanglement only originating from contiguity in the past is inaccurate and even if true, proves nothing if the Big Bang is true, then nothing prevents every part of the universe from being entangled, and it probably is entangled, but in a way that has no practical importance. People's comments here touch on the important issue of whether the wave function is objective or subjective. The view that probabilities represent our knowledge is called the 'Bayesian' view, it is the 'Bayesian'Bayesian or subjective interpretation of probability, as contrasted to the 'objective view' which has some problems. But the Bayesian view has problems as well, since you wind up linking quantum mechanics with consciousness instead of with material measuring apparati such as geigerGeiger counters and bubble chambers.  So another answer to your question is the following: people prefer  to talk about an active link because they cannot accept the subjective interpretation of probability and the wave function. There is a lot of current research studying quantum measurement as an actual physical process involving thermodynamic limits of unstable negative temperature systems (bubble chambers etc.). To put this another way: alternative 1 implicitly assumes that in the combined system there are 'two particles', but this is probably a fallacy: quantum mechanics does not really recognise any precise notion of particle. As in thermodynamic limits, the notion of 'particle' is a useful approximation within a certain range of set-ups, and loses validity and leads to paradoxes if you attempt to use it outside the limits of its validity. Alternative 2 implicitly assumes that if something such as the wave function can only be approximately measured, it is somehow not 'physical', but this is unduly simplistic and troubles people because of the seeming necessity of dragging in the subjective Bayesian point of view. Alternative 3 is at least so open ended that one cannot find fault with it but neither is there a shred of experimental evidence for it. The only problems with QM are logical, not experimental. Therefore if one questions the implicit assumptions made about the careless use of concepts such as 'particle', 'system', and 'probabity''probability', there are many more alternatives and the final answer is not in. It is not really clear that cases 1,2, and 3 are exhaustive. Discussions about this phenomenon use a lot of terms which are not precisely defined. For example, 'particle' and 'system'. If there is entanglement, then there is one combined system, and it is misleading to call that one combined system 'two particles'. The comment about realism and approximation is also inaccurate: all positions and data in classical physics are approximate too, this has nothing to do with the difference between classical and quantum or the difference betweeen using a Hamiltonian system whose states are points given by momentum and position coordinates and using a Hamiltonian system whose points are rays in a Hilbert Space. The comment about entanglement only originating from contiguity in the past is inaccurate and even if true, proves nothing if the Big Bang is true, then nothing prevents every part of the universe from being entangled, and it probably is entangled, but in a way that has no practical importance. People's comments here touch on the important issue of whether the wave function is objective or subjective. The view that probabilities represent our knowledge is called the 'Bayesian' view, it is the 'Bayesian' or subjective interpretation of probability, as contrasted to the 'objective view' which has some problems. But the Bayesian view has problems as well, since you wind up linking quantum mechanics with consciousness instead of with material measuring apparati such as geiger counters and bubble chambers.  So another answer to your question is the following: people prefer  to talk about an active link because they cannot accept the subjective interpretation of probability and the wave function. There is a lot of current research studying quantum measurement as an actual physical process involving thermodynamic limits of unstable negative temperature systems (bubble chambers etc.). To put this another way: alternative 1 implicitly assumes that in the combined system there are 'two particles', but this is probably a fallacy: quantum mechanics does not really recognise any precise notion of particle. As in thermodynamic limits, the notion of 'particle' is a useful approximation within a certain range of set-ups, and loses validity and leads to paradoxes if you attempt to use it outside the limits of its validity. Alternative 2 implicitly assumes that if something such as the wave function can only be approximately measured, it is somehow not 'physical', but this is unduly simplistic and troubles people because of the seeming necessity of dragging in the subjective Bayesian point of view Alternative 3 is at least so open ended that one cannot find fault with it but neither is there a shred of experimental evidence for it. The only problems with QM are logical, not experimental. Therefore if one questions the implicit assumptions made about the careless use of concepts such as 'particle', 'system', and 'probabity', there are many more alternatives and the final answer is not in. It is not really clear that cases 1,2, and 3 are exhaustive. Discussions about this phenomenon use a lot of terms which are not precisely defined. For example, 'particle' and 'system'. If there is entanglement, then there is one combined system, and it is misleading to call that one combined system 'two particles'. The comment about realism and approximation is also inaccurate: all positions and data in classical physics are approximate too, this has nothing to do with the difference between classical and quantum or the difference betweeen using a Hamiltonian system whose states are points given by momentum and position coordinates and using a Hamiltonian system whose points are rays in a Hilbert Space. The comment about entanglement only originating from contiguity in the past is inaccurate and even if true, proves nothing if the Big Bang is true, then nothing prevents every part of the universe from being entangled, and it probably is entangled, but in a way that has no practical importance. People's comments here touch on the important issue of whether the wave function is objective or subjective. The view that probabilities represent our knowledge is called the 'Bayesian' view, it is the Bayesian or subjective interpretation of probability, as contrasted to the 'objective view' which has some problems. But the Bayesian view has problems as well, since you wind up linking quantum mechanics with consciousness instead of with material measuring apparati such as Geiger counters and bubble chambers.  So another answer to your question is the following: people prefer  to talk about an active link because they cannot accept the subjective interpretation of probability and the wave function. There is a lot of current research studying quantum measurement as an actual physical process involving thermodynamic limits of unstable negative temperature systems (bubble chambers etc.). To put this another way: alternative 1 implicitly assumes that in the combined system there are 'two particles', but this is probably a fallacy: quantum mechanics does not really recognise any precise notion of particle. As in thermodynamic limits, the notion of 'particle' is a useful approximation within a certain range of set-ups, and loses validity and leads to paradoxes if you attempt to use it outside the limits of its validity. Alternative 2 implicitly assumes that if something such as the wave function can only be approximately measured, it is somehow not 'physical', but this is unduly simplistic and troubles people because of the seeming necessity of dragging in the subjective Bayesian point of view. Alternative 3 is at least so open ended that one cannot find fault with it but neither is there a shred of experimental evidence for it. The only problems with QM are logical, not experimental. Therefore if one questions the implicit assumptions made about the careless use of concepts such as 'particle', 'system', and 'probability', there are many more alternatives and the final answer is not in. 1 answered Dec 1 '11 at 5:30 joseph f. johnson 5,6512222 silver badges3838 bronze badges It is not really clear that cases 1,2, and 3 are exhaustive. Discussions about this phenomenon use a lot of terms which are not precisely defined. For example, 'particle' and 'system'. If there is entanglement, then there is one combined system, and it is misleading to call that one combined system 'two particles'. The comment about realism and approximation is also inaccurate: all positions and data in classical physics are approximate too, this has nothing to do with the difference between classical and quantum or the difference betweeen using a Hamiltonian system whose states are points given by momentum and position coordinates and using a Hamiltonian system whose points are rays in a Hilbert Space. The comment about entanglement only originating from contiguity in the past is inaccurate and even if true, proves nothing if the Big Bang is true, then nothing prevents every part of the universe from being entangled, and it probably is entangled, but in a way that has no practical importance. People's comments here touch on the important issue of whether the wave function is objective or subjective. The view that probabilities represent our knowledge is called the 'Bayesian' view, it is the 'Bayesian' or subjective interpretation of probability, as contrasted to the 'objective view' which has some problems. But the Bayesian view has problems as well, since you wind up linking quantum mechanics with consciousness instead of with material measuring apparati such as geiger counters and bubble chambers.  So another answer to your question is the following: people prefer  to talk about an active link because they cannot accept the subjective interpretation of probability and the wave function. There is a lot of current research studying quantum measurement as an actual physical process involving thermodynamic limits of unstable negative temperature systems (bubble chambers etc.). To put this another way: alternative 1 implicitly assumes that in the combined system there are 'two particles', but this is probably a fallacy: quantum mechanics does not really recognise any precise notion of particle. As in thermodynamic limits, the notion of 'particle' is a useful approximation within a certain range of set-ups, and loses validity and leads to paradoxes if you attempt to use it outside the limits of its validity. Alternative 2 implicitly assumes that if something such as the wave function can only be approximately measured, it is somehow not 'physical', but this is unduly simplistic and troubles people because of the seeming necessity of dragging in the subjective Bayesian point of view Alternative 3 is at least so open ended that one cannot find fault with it but neither is there a shred of experimental evidence for it. The only problems with QM are logical, not experimental. Therefore if one questions the implicit assumptions made about the careless use of concepts such as 'particle', 'system', and 'probabity', there are many more alternatives and the final answer is not in.