# Local and non-local physics

# One of the weirdest things about quantum physics is non-locality.

A local interaction is one that involves direct contact, or uses an intermediary who is in direct contact. Examples are the forces we know, such as friction or gravity.

Three properties: the local interaction must have a speed of propagation less than the speed of light; use an intermediary such as electromagnetic waves, photons, gravitons; the force decreases with distance.

Well, if all the forces we know are local, what is left?

There remains the collapse of the wave function. It is “Spooky action at a distance”, a term given by Albert Einstein, to show that quantum physics was incomplete.

Einstein and collaborators Boris Podolsky and Nathan Rosen proposed an experiment, today called the EPR Paradox.

Let it be two entangled atoms, in a state like (| 00> + | 11>) / sqrt(2).

If one atom is measured as zero, the second is also measured as zero.

If an atom is measured as one, the second is also measured as one.

There is a 50% chance of measuring 0 or 1, and it is completely random.

There is nothing in the equations that says anything about the distance between atoms.

Take one of these atoms and place it in one side of the universe. Place the other atom on the other side of the universe. How do they “know” that the other atom has collapsed? Does the measurement, the collapse of the wave function, occur immediately? In that case, there would be a faster communication than light?

Einstein’s theory of relativity prohibits speeds above the speed of light. How to solve?

In short, the solution is to sweep the problem under the carpet. The collapse of the wave function is non-local, that is, it does not need an intermediary to interact. Thus, there is no wave that makes communication faster than light. An atom may be hundreds of billions of billions of light-years away from the other, yet they will be connected as if they are glued to each other …

Based on Explorations of Quantum Computation, Collin Williams.

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