Locality and non- locality
You are spending the summer in Europe. Your mother calls you from California to tell you
that you have won the biggest lottery. A whole 70 million dollars. You are flying from
happiness.
What happened in San Francisco - where your mother lives - influenced you seven thousand
miles away. Your mum's voice - the cause of your pleasure - had to travel seven thousand
miles, and although it took only a tiny fraction of a
second to reach
your ears, yet it consumed "some" time. So the cause of your pleasure had to
travel through space for some time till it influences you. This is called
"locality".
On the other hand non-locality means that an event at one place
shall affect another event, far away from it, instantly. Although this is against
special relativity -which prohibits any signal to travel faster than light - it was proved
true in quantum mechanics.
The EPR (Einstein-Podolsky-Rosen) Paradox introduces a class of experiments, which turn
out to involve some of the strangest consequences of quantum mechanics. This experiment
involves a pair of particles, or physical systems, which interact and then move apart.
Quantum theory shows that the results of measurements on one particle enable us to predict
the results of corresponding measurements on the other particle.
That is because both particles were "one" physical system. Now if we perform a
measurement on one particle, the wavefunction shall jump to assume the value of the
measurement on this particle. But what about the second particle, since this system was
"one" system, this means that a measurement (or jumping) at particle 1 implies
an instantaneous measurement (or jumping) at particle 2.
Because the experiment involves some advanced physical properties of particles (spin, polarization…), we designed an analogous experiment using colors so the concept of non-locality can be understood easily. (This experiment is not real.)
Suppose that we have a
white particle. This particle was then split into two particles: a green particle and a
magenta particle. Now imagine that the two particles moved in opposite directions at light
speed for 10 years, so that they eventually were 20 light years apart. Now according to
quantum mechanics, any measurement (trying to determine the color of a particle) on
particle 1 shall determine the outcome of measurement on particle 2.
So if we examined the color of particle 1 and found it to be green, then the other
particle is automatically magenta.
Now suppose you decided
to inspect or measure the color of particle 1 in a red light chamber, and found it yellow
(green + red). According to quantum mechanics, at the exact same time, the second particle
has turned blue, so that the sum of the colors of the two particles remains white.
Now how did particle 2 "know" about particle 1 measurement and how come it was
affected by it?