Do EPR Tests Require Simultaneous Measurements?

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Discussion Overview

The discussion revolves around the concept of simultaneous measurements in quantum mechanics, particularly in the context of EPR (Einstein-Podolsky-Rosen) experiments. Participants explore the implications of measuring momentum in a particle that is not in a momentum eigenstate and whether such measurements can yield consistent results when performed simultaneously.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant questions whether two simultaneous measurements of momentum on the same particle must agree, citing a professor's assertion that quantum mechanics suggests one measurement must precede the other.
  • Another participant argues that the act of measurement collapses the wave function, implying that a second measurement cannot occur without the first collapsing the state, thus questioning the validity of simultaneous measurements.
  • Some participants discuss the implications of energy/time uncertainty and how it might relate to the feasibility of simultaneous measurements.
  • One participant asserts that performing simultaneous measurements on two entangled particles can yield consistent results due to their correlated properties, referencing conservation of angular momentum.
  • There is a challenge regarding the definition of "simultaneous" measurements, particularly in relation to different reference frames and the precision required to consider measurements as simultaneous.

Areas of Agreement / Disagreement

Participants express differing views on the possibility and implications of simultaneous measurements in quantum mechanics. There is no consensus on whether such measurements can be validly performed or how they should be interpreted.

Contextual Notes

Participants highlight the complexity of defining simultaneous measurements, particularly in the context of reference frames and the precision required for such definitions. The discussion reflects ongoing uncertainties and assumptions inherent in quantum measurement theory.

eep
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Suppose you have a particle which is not in a momentum eigenstate. You then measure the momentum (say, in the x-direction) by two separate apperati simultaneously. Do the results have to agree? I asked my professor about this and he said that quantum mechanics will tell you that you can't perform such an experiment, as one of your measurements is going to have to occur before the other one. He also said that this isn't a very satisfying answer, which is isn't. Can anyone elaborate further or is this just one of the things you have to accept about QM.
 
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According to what i think is the most common belief, the very act of doing a measurement collapses the wave function to an eigenstate of what you measure. So as soon as you have a measurement, the second measurement is forced to yield the same result. As far as trying to measure twice at the same time, i think that is a bit non-sensical. Measuring once you collapse the wavefunction, therefore there is no way to make a second measurement. No such thing as "exact" same time, as I'm sure you are aware, because of the uncertainty principle. You could hope to "clone" your particle and make many copies and then measure each of them, but it is a theorem of quantum mechanics that you can not make a clone without collapsing the wave function. (Making the clone is a sort of measurement by itself)
 
Ah, of course. We hardly ever talk about energy/time uncertainty so I hadn't considered it. That fixes everything!
 
Yes, and now comes the big joke. Of course you cannot perform a simultaneous measurement twice at the same moment on the same particle. In fact, if it is the same measurement, at the same time, on the same particle, then it is just one single measurement !

But, but, people tried to find a way around it. They set up systems of 2 particles, of which they KNEW that the measurement on one fixed also the outcome of the other. For instance, take conservation of angular momentum. If you start from a state with angular momentum zero, and you produce 2 particles that way, you know that they have OPPOSITE angular momentum.
And now the trick was to find out of this still works, because now we can do a simultaneous measurement on BOTH particles, which comes down to 'the same measurement' because we know that if we have one measurement, the other is fixed.

Well, lo and behold, THIS WORKS. These are called EPR experiments, for Einstein Podolsky and Rosen, who first thought up this kind of experiment (in fact, to show that it couldn't work! But it does...)
 
is that so?

eep said:
Ah, of course. We hardly ever talk about energy/time uncertainty so I hadn't considered it. That fixes everything!



How exactly do the so-improperly-called energy/time uncertainty fix THIS?
 
vanesch said:
Yes, and now comes the big joke. Of course you cannot perform a simultaneous measurement twice at the same moment on the same particle. In fact, if it is the same measurement, at the same time, on the same particle, then it is just one single measurement !

But, but, people tried to find a way around it. They set up systems of 2 particles, of which they KNEW that the measurement on one fixed also the outcome of the other. For instance, take conservation of angular momentum. If you start from a state with angular momentum zero, and you produce 2 particles that way, you know that they have OPPOSITE angular momentum.
And now the trick was to find out of this still works, because now we can do a simultaneous measurement on BOTH particles, which comes down to 'the same measurement' because we know that if we have one measurement, the other is fixed.

Well, lo and behold, THIS WORKS. These are called EPR experiments, for Einstein Podolsky and Rosen, who first thought up this kind of experiment (in fact, to show that it couldn't work! But it does...)



Oh, can we?. Simultaneous measurement with respect to which reference frame?
 
Athe said:
Oh, can we?. Simultaneous measurement with respect to which reference frame?

EPR tests can be performed in the same reference frame in many ways, up to and including performing the measurement at very nearly the same location. All you need is some fiber cabling and you can pretty well place the entangled photons anywhere (and therefore "anywhen" since you can control the length of the fiber) you want. At that point you are left with defining what you mean by "simultaneous". I.e. to what level of precision is it considered simultaneous.
 

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