Quantum mechanics was the greatest revolution in physics in the last century. It’s considered as the most successful theory and the best achievement of human intellect. This is perhaps the reason why many people these days use the “quantum” word to make their theories look legitimate.
While the popular idea of quantum physics is mostly deviant from the actual discourse, quantum physics is famous not for nothing. It has so many paradoxes within it that are still to be resolved or explained. One of the most important paradoxes is the EPR ( Einstein-Podolsky-Rosen ) paradox.
The paradox is named after the people who discovered it.
Suppose two particles that once interacted are now apart and no longer interact. Since they do not interact, a measurement on one particle does not affect the other particle. But since they interacted in the past, a measurement on one particle may be an indirect measurement on the other particle. For example, we can measure the position of a particle indirectly. This is famously known as entanglement.
For clarity, consider an electron-positron pair that are entangled. They move in opposite directions from the point of their separation. Now both particles can have a position and a momentum. But since these are quantum particles, their position or momentum can be known only after a measurement and prior to it, the particle is in a quantum superposition of all possible outcomes of the measurement. Also from the uncertainty principle, we cannot measure both position and momentum with 100 per cent accuracy, simultaneously. So if one chooses to measure the momentum of the electron, let’s say, then we can immediately say that the momentum of the positron is the negative of this value, since total momentum is zero. Now if another person measures the position of the positron, and then both of the people meet, the first person knows the exact value of positron’s momentum while the second person knows the exact value of its position. This is a contradiction to the uncertainty principle that we cannot know the exact value of both position and momentum of a quantum particle simultaneously.
Now if the second person measured momentum of the positron in the first place, he has to get the negative value of what the first person got as the value of the electron’s momentum. This implies that although there is no communication between particle, there is somehow a connection that makes sure that the values are complementary. This is again a problem since, by quantum mechanical postulates, a particle collapses into a particular value of momentum only when measured and till then it is in a superposition. In this case, when the momentum of the electron is measured it immediately gives a value and even though no measurement has been made on positron, we are sure of it’s momentum value. And this happens immediately, which cannot happen by relativistic principles of the locality. ( by special relativity no information can travel faster than light).
So Einstein, Podolsky and Rosen suggested that the quantum theory is not complete and many people since then have tried to resolve this paradox by assuming that particles carry some hidden values with them. But Bell has shown that this is not possible and if it is then quantum theory turns out to be wrong, which it isn’t.
The paradox is still unresolved and has inspired many new hypotheses till today.
-Adarsha
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