# What happens if you make an observation of the spins of two entangled, far apart electrons at exactly the same time? [closed]

If we consider the spins of two, far apart, entangled electrons, what will happen if we make a measurement on both spins at exactly the same time (let's assume time is not discrete)? I see it as a problem because if you measure one spin the other one pops instantaneously in the opposite spin state, depending on what the observed spin of the other electron is. But that's the same the other way round. Isn't the system in some state of not to "know" (I don't know how to phrase it differently) which spin pair will emerge? Is this a paradox? Do the measurements adapt to each other?

I can't see a problem if you consider the wavefunction as a pilot wave, in which case the particles have well-defined positions, energies, and spins (which are always opposite and have already before the measurement well defined opposite values). By the way, I see that the uncertainty principle has nothing to do with this (as I presumed in the earlier formulation of this question). The locations of the two particles are not spooky connected.

## closed as unclear what you're asking by ZeroTheHero, Yashas, Jon Custer, sammy gerbil, Michael SeifertJun 9 '17 at 14:01

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• In principle, it could be done. But what is the exciting part about that? – ABCD Jun 7 '17 at 10:33
• I mean, which of the measurements is responsible for the change in the total wave function? If you measure the wave function of the spins at the same time, will that have the same effect as doing one spin measurement on one electron? The moment you make a measurement on one spin will decide the final combination of the two spins, but if you make a measurement on both spins at the same time, won't this have an effect on the measurement of the other spin, and vice-versa? You can't say you make a measurement of a combination of spins the electrons are already in. Isn't there some kind of "loop"? – descheleschilder Jun 7 '17 at 11:50
• You also misunderstand energy-time uncertainty. The $\Delta t$ in the energy-time uncertainty inequality refers to the time it takes for an individual energy-transferring process to take place, not the time between two measurements. – probably_someone Jun 7 '17 at 15:30
• Bipartite entanglement is the statement that the whole is in a definite state whereas a part is not. So, in principle, you cannot assign a state vector(or a rep in pos.space which one calls the wavefunction) to either of these. Also, entanglement has nothing to do with the measurement process in the way you have put in. Please read Nielsen and Chuang's book for more details on the basics of entanglement. – ABCD Jun 7 '17 at 15:48
• This question as worded is doesn't make much sense: 1. measuring spins has nothing to do with time-energy uncertainty, 2. time-energy uncertainty at best requires careful definition of what is $\Delta t$ (see physics.stackexchange.com/q/53802/36194), something not done (or doable?) in the question. – ZeroTheHero Jun 7 '17 at 15:57