Physics Help Forum When is Now?

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 Sep 9th 2017, 04:22 AM #1 Senior Member     Join Date: Jun 2016 Location: England Posts: 786 When is Now? In a recent post on entanglement I stated that when the superposition of states of one entangled particle is collapsed, the superposition of states of it's partner also collapses at the same instant. Is this correct? If so it implies that the two particles share the same "now". However I have come across articles on relativity that indicate that "now" is a relative term that will be disagreed upon depending on the relative motion of the observers. Is this correct? If both are correct, how can this be reconciled? __________________ ~\o/~
 Sep 9th 2017, 12:35 PM #2 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 550 Good point! See this video from about 22 minutes. __________________ Burn those raisin muffins. Burn 'em all I say.
Sep 12th 2017, 02:12 PM   #3
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 Originally Posted by Woody In a recent post on entanglement I stated that when the superposition of states of one entangled particle is collapsed, the superposition of states of it's partner also collapses at the same instant. Is this correct?
Not exactly. The system consists of two particles. When a measurement is made on one of the particles it is a measurement on the system itself so the system collapses, not just one of the particles. In addition to that you may be wondering when a particle obtains its specific state such as spin up or spin down. To answer this simply remember two things in this example (1) a particle is not in a specific state until a measurement is made to make that determination and (2) for two electrons which are entangled their spins will always be in opposite states.

The main thing to take away from this is to keep in mind that you can only speak about what is measured in QM. In this case its the entire quantum mechanical system which collapses because that term is a mathematical one and does not have a direct physical meaning.

Note: The term "measurement" in quantum mechanics means an interaction of a subatomic system with a macroscopic one.

 Originally Posted by Woody However I have come across articles on relativity that indicate that "now" is a relative term that will be disagreed upon depending on the relative motion of the observers. Is this correct?
While not always true for spatially separated events its most certainly true in general in that events which have a spatial separation parallel to the direction of motion of the two frames its true.

 Originally Posted by Woody If both are correct, how can this be reconciled?
Exactly how I explained it above. I.e. in QM if you're not talking about the results of measurements then you're not talking about physics, you're then talking about metaphysics.

Last edited by Pmb; Sep 13th 2017 at 12:22 AM.

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