Name: Kim
Status: student
Grade: n/a
Location: NH
Country: USA
Date: Winter 2012-2013

Question:
I am in 10th grade and have developed a real interest in Quantum Physics and my question has to do with the Copenhagen interpretation - which states that the act of measuring a quantum state causes the quantum wave function to collapse and it is not just that the scientist does not know which state it is in, but it is rather that the physical reality is not determined until the act of measurement takes place. How do you prove that something does not have a state until you observe or measure it in the first place? What follows is how I think one could explain this. Can you correct me where I am wrong? Ok, my explanation.....You have some fancy lab equipment that takes one particle with spin=0 and from this it creates two particles X and Y. One of these particles HAS to spin -1/2 and the other +1/2 but you do not know which one is which (until you look at one of them). If you measure X to be +1/2 then you know Y has to be -1/2. If you do not first look at or measure "X" and instead send it through a "particle flipping" device (which reverses whatever original spin it started with) then at this point you would think both particles should now have the same spin because you flipped one (and have not looked or measured yet)....but when you now measure flipped particle X, Y will have instantaneously changed to be the opposite of X (when you would of thought it would be the same after flipping the other), so in essence its state was undetermined before the flipping/measuring. So is this the correct way of explaining the Copenhagen interpretation? I am thinking even though YOU do not know what the particle flipper did, the particle flipper knows, so would this break down the function? Or does it have to be the photons entering your eyeball from the resulting measurement.I just cannot understand how scientists KNOW that something does not have a state before being observed. Thanks for any clarification.
Replies:
Hi Kim,

Thanks for the question. First let me say that a particle can exist in two states at the same time. This is called a superposition. Let A be a wave function for the first state and let B be a wave function for the second state. A wave function satisfies the SchrÃ¶dinger equation. A superposition exists because the sum A+B is also a solution to the SchrÃ¶dinger equation as the S.E. is a linear differential equation.

When you carry out a measurement on a superposition state (A+B) you will collapse the state into either A or B. The act of measurement forces the system to be in state A or state B as measurements will ONLY find so-called eigenstates--this is one of the postulates of quantum mechanics.

An example of the construction of two opposite spins (mentioned in your text below) is pair production in which a 1024 keV photon generates an electron and a positron.

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