The result of a
thought experiment invented by
Albert Einstein,
Boris Podolsky and
Nathan Rosen
and publised under the title
Can quantum-mechanical description of physical reality be considered complete?
in issue 41 of
Physical Review in 1935.
It was intended to show
contradictions, or at least a lack of
completeness in
quantum theory,
which could only be resolved by adding additional
hidden variables.
The experiment works like this:
- Take two physical systems that initially interact with each other in such a way that both are
described by a single Schrödinger wave equation. This allows you to infer information
about one system from an observation made on the other.
- Separate the systems
- Make a precise measurement in one system that allows you to infer information about one in a set
of noncommuting observables in the second system.
- Make a precise measurement of one of the other observables of the set in the second system.
- Et voila! You have just violated the uncertainty principle in that system.
- Watch the universe evaporate in a cloud of logic.
To be more specific: in step 1, take two
photons that emerged from the
decay of a neutral
pion
at rest. The law of
conservation of momentum says that they must have exactly opposite
momentum,
since the original particle had none. In step 3, measure the momentum of the first photon. You now know
the momentum of the second one without having actually touched (measured) it. In step 4, measure the
position of the second photon, which the uncertainty principle says you cannot know at the same time
as the momentum.
It counts as a thought experiment because current technology is unable to create completely isolated
physical systems, let alone separate and move apart two that contain particles moving at the
speed of light. Basically, any straightforward setup would fuck up the measurements many times over.
However, in 1964 John Bell formulated a derived formula (now called Bell's Inequality Principle)
which would show the existence of the above-mentioned hidden variables and could be tested for in
a variety of far more manageable circumstances. Results were negative, but many physicists argued that
there are subtle flaws in Bell's reasoning or that the experimental setups were not adequate. The
issue remains unresolved.