General Entanglement and Measurement

[Vinncent]

I apologize in advance if this has been asked many before, but I didn't see anything that answered my question on the first page of threads.

Take, for example, an isolated subatomic particle which breaks down into two entangled particles.

From what I've read, each of these two particles are in a state of both having an "up" and "down" spin state simultaneously. They lose this property when one is measured, and one will take on either a definite "up" or "down" spin state, and the other particle will take on the opposite spin state of the first.

My question revolves around the idea of "having both up and down" spin before measurement.

How are we able to detect that a particle shows signs of having both an "up and down" spin state, without directly measuring the particle in such a way that it takes on a definite spin state? Similarly, how do we know one of those particles wasn't -always- in an "up" spin state, rather than some sort of "up and down state at the same time, until measured"?

 

[Nugatory]

How are we able to detect that a particle shows signs of having both an "up and down" spin state, without directly measuring the particle in such a way that it takes on a definite spin state? Similarly, how do we know one of those particles wasn't -always- in an "up" spin state, rather than some sort of "up and down state at the same time, until measured"?

Google around for "Bell's theorem". You'll find a pretty decent Scientific American article in the first page or so of hits, and a bunch of other explanations. The basic idea is that there is no way of preassigning values for the spins at three different angles (0, 120, and 240 degrees) such that if you measure both particles at the same angle you always get the opposite spins and also get the QM-predicted probability of the same/opposite spins (because of the angle, either are possible) if measuring at different angles. Measure enough pairs with the detectors at various angles and we can see the actual distribution, see if it matches the QM prediction... And so far it has.

 

[Bill_K]

how do we know one of those particles wasn't -always- in an "up" spin state, rather than some sort of "up and down state at the same time, until measured"?

You can measure the spin projection of a particle along any axis you like, and you'll always find one of just two values: +ħ/2 or -ħ/2. (This is for a spin-½ particle like an electron.) If the explanation was that the electron had already been in that state even before you measured it, it would have to guess in advance which axis you were going to use!

 

[Crazymechanic]

Well in simple terms we have the two possible outcomes before the measurement because we don't know and can't know before we measure and also when we measure we get to know because measurement is an interaction in the same time in QM and an interaction leaves the particle at a certain state not at many different ones.
But this doesn't mean that before the measurement the particle was both "up" and "down" at the same time.

Even though there is a slightly different touch to it there is a thread which is similar to your question that relates to quantum states and superposition (particle at many different states before measurement)

https://www.physicsforums.com/showthread.php?t=682271

You can check it out.