When any two quantum objects, such as electrons or photons,
come into contact with each other, their quantum states (the mathematical information
describing their properties) combine, or become entangled. Thereafter, their
fates remain intertwined, however far apart they move in the future. This bit
is not so strange, perhaps, since it is easy enough to believe that, having a
shared past means two entities will have affected each other's properties in
some way at the time of their interaction. The effect of this interaction can
still be seen when we check the particles afterward.
However, entanglement becomes much stranger than that! In the
quantum world, entities can exhibit two or more conflicting characteristics
simultaneously, such as spinning in opposite directions at the same time. This is
called "superposition." Now, if a photon, say, is entangled with
another it can "infect" it with its superposition so that they are
both in superpositions. However, once we look at one of them, this constitutes
a measurement, and we force the photon to decide which way it is spinning. But,
because it is entangled with its distant partner, we also force the other photon
to make the same choice. This happens instantaneously, even if the two photons
are now millions of miles apart.
by Jim Al-Khalili,
30-Second Theory
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