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
It was intended to show contradiction
s, or at least a lack of completeness
in quantum theory
which could only be resolved by adding additional hidden variable
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 photon
s 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.