Question: Explain in full what the per paradox is

Sam Geen answered on 21 Jun 2013:

The EPR paradox tries to disprove the Heisenberg Uncertainty Principle, which says that you can’t know your position and speed perfectly at the same time (see here for a long explanation about why: /speedj13-zone/2013/06/20/explain-non-commuting-variables-and-what-they-mean-for-the-uncertainty-principle/)

The EPR paradox says this – suppose you have 2 particles that are moving along, and then at some point they separate. You measure the speed of the first particle and the position of the second one. By conservation of momentum, you can find out the speed and position of both! The paradox underlines how we don’t know what a “measurement” in quantum mechanics actually is.

There are two ways to think about this. One is the “Copenhagen interpretation”, which was named after the group in Denmark that Niels Bohr founded. This says that quantum mechanics is completely random, and that measuring something affects our ability to measure new things, not the particles themselves. Another idea bout how it works is the “Many Worlds” interpretation, where measuring a particle causes the universe to split into many different universes, each with a different measurement result. This interpretation is popular with people who make quantum computers (like Rob, I think, but I guess he can say more). But we’re still unsure as to why quantum mechanics does what it does, and what “measuring” something actually means.