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Although I am familiar with quantum physics and know about only being able to calculate the probability of an electron's position in space, I do not quite understand how this one works. If two electrons with coherent spins and all are traveling in opposite directions and you do some research and you find whether one is spinning up, you have to know, faster than the speed of light that the other electron spins down. My question is basically, people say that before you know the particle's spin, you have the probability of its spin, and so it spins both up and down, but this is confusing, because knowing the probability should not mean it is in both states, it should only mean that you have not found out for sure which state it is in. This is the definition of probability, right? If you role a die, but do not look at it, you can figure it has a 1/6 chance for landing on each side, but this does not mean it has landed on all six sides. Please help me understand.


This is a fun and fascinating, but very confusing and complicated subject indeed. I think the thing that might be confusing you is the nature of the particle *before* you measure its property. It is not quite right to say the particle has a 'probability' of having a property -- that implies that the property is already set, but that you just do not know it. Instead, prior to measuring it, the particle is a mix of all the possible properties. It is all of them at once, and only becomes one once you measure it. This is known as 'quantum superposition'. The particle would be in a limbo of all of the possibilities until it 'collapses' into a single possible state when you measure it. After the measurement, you still get a distribution of states, and you can calculate probabilities based on that, but that distribution only takes shape after the measurement -- before that each particle is a smear of all the possibilities. This sounds counter-intuitive, but experiments have been performed that confirm this strange behavior.

This concept is very confusing, and perhaps unsatisfying. In fact, many scientists have historically disliked this explanation too. A famous thought experiment was proposed by Erwin Schroedinger (of the Schroedinger equation) where a cat would be both alive and dead at the same time until a separate measurement would be made, and then the cat would 'collapse' into being either dead or alive. (this is a *thought* experiment only -- no cats were harmed!) The question Schroedinger tried to ask is *when* and *why* and *how* a particle transitions from all the possibilities into just one result. Unfortunately, those details are still very much being worked out.

Another interesting aspect is known as quantum entanglement -- where the state of two particles are related, not independent. By measuring the property of one particle, you constrain the other particle to a smaller set of possibilities.

So this is just a taste of this fascinating (or perhaps frustrating) field of study. I recommend you start reading on the following topics: quantum entanglement, quantum superposition, Copenhagen interpretation, and the EPR Paradox. Once you have these in mind, you can start reading about the experiments that led to them.

Hope this helps,

Burr Zimmerman


Two factors of modern physics are needed to understand this situation.

First, fast velocity: When an object appears to move close to the speed of light, distances and times shift in such a way that the object's velocity measures to always be less than the speed of light. If Albert and Barney can move in opposite directions, both at three-fourths of the speed of light. If Albert measures Barney's velocity, it will be less than the speed of light as Albert sees the universe. This is called Relativity.

Second, states of a particle: Spin-up and spin-down are the only two measurable states of an electron, but not the only states. When the spin of an electron is actually measured, it becomes either spin-up or spin-down, pointing along the direction that spin is actually measured. If you measure vertical spin, then the spin becomes either up or down in the vertical direction. If you measure spin along the north/south direction, then the spin becomes either toward the north or toward the south. Measurement does not show you what the state is.

Measurement forces the particle to become one of the measurable states.

The act of measuring something can affect what you are measuring.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College

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