The solidity of
the world seems totally indisputable. As a fixed thing that you can see and
touch, your body is also reassuring the solid. But beginning with
Einstein, modern physics have assured us that this solidity is a mirage. Quantum
physics tells us that reality is far beyond human perception and intuition. In
other words, our rational mind and common sense are just not capable of
understanding the true nature of reality. And, therefore why particles can
experience the full weirdness of quantum mechanics, whereas we evidently
cannot. "In short, how does the well-behaved, everyday classical world
emerge from the schizophrenic quantum realm.

The great Richard Feynman once said that all of Quantm physics can be understood through the double-slit experiment. For the last few weeks I have watched and re-watched the 1979 Douglas Robb Memorial Lectures with Feynman discussing photons – Corpuscles of light.

Trying to make sense of single-photon behaviour
in relation to reflection of multi surfaces. A topic, it seems infinitely more
subtle then the high-school treatment of Newtonian optics. So when one starts to think they are coming
to terms with the counter-intuitive nature of light, photons behave even more
strangely when pushed through a double-slit experiment.

So
photons (particles for that matter) do not have a particular location and velocity;
they merely have probabilities of location and velocity meaning a particle is
all the possible futures it can have Viz-a-viz the Copenhagen interpretation suggesting that quantum mechanics does
not yield a description of an objective reality but deals only with
probabilities of observing, or measuring, various aspects of energy quanta.

So all
but one of these futures collapses when it is observed. Meanwhile, these
futures of the same particle can interfere with each other. Quantum probability
is not a mere description of where a particle could be found and could be going,
a mere mathematical abstraction, it is an actual property.

The
photon in the experiment is all the possible paths it can take, some through
one slit, some through the other, and it is interfering with itself. For each
photon, measuring devices record one possible outcome of this
self-interference.

Then comes the
quantum eraser, proposed by Scully and DrÃ¼hl in 1982. Given the basic principle
of complementarity (for each degree of freedom, the dynamical variables are a
pair of complementary observables) the precise knowledge of one implies
complete unpredictability of the other.

For example,
precise knowledge of a particle position implies complete unpredictability of
its momentum. Because it’s generally assumed that the mechanism responsible for
the loss of the interference pattern is the uncertainty principle, the
“which-way” information of the particles is found without disturbing their wave-function.

The reason of the
interference loss is the quantum information contained in the measuring
apparatus, by means of the entanglement correlations between the particles and
the path detectors. The experiment shows that if such quantum information is
afterwards erased from the system, then the interference reappears (which would
be impossible in the case of a perturbation).

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