Quantum Probability
Most recent answer: 06/20/2008
Q:
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 don’t 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 shouldn’t mean it’s in both states, it should only mean that you haven’t found out for sure which state it’s in. This is the definition of probability, right? If you role a dice, but don’t look at it, you can figure it has a 1/6 chance for landing on each side, but this doesn’t mean it has landed on all six sides. Please help me understand if I’m wrong.
Ben Knutson
- Ben Knutson (age 15)
Omaha, NE : United States
- Ben Knutson (age 15)
Omaha, NE : United States
A:
Great question.
Ordinary probabilities are, as you say, concerned with facts which are unknown. In quantum mechanics (at least most interpretations) before the measurement there simply is no fact as to whether the spin is up or down. There is a beautiful argument, based on something called the Bell Inequalities, to show that any theory (other than weird conspiracies or non-local theories) which assumes that there are prior facts disagrees with quantum mechanics- and more importantly, disagrees with experiment. "Non-local" theories are ones where facts (such as that the spins are opposite) are distributed through all of space.
Mike W.
Ordinary probabilities are, as you say, concerned with facts which are unknown. In quantum mechanics (at least most interpretations) before the measurement there simply is no fact as to whether the spin is up or down. There is a beautiful argument, based on something called the Bell Inequalities, to show that any theory (other than weird conspiracies or non-local theories) which assumes that there are prior facts disagrees with quantum mechanics- and more importantly, disagrees with experiment. "Non-local" theories are ones where facts (such as that the spins are opposite) are distributed through all of space.
Mike W.
(published on 06/20/2008)