96
R. Barrett and P. P. Delsanto
journey. Our act of measurement collapses the wave function of the first electron, resulting in its angular momentum becoming precisely defined (i.e.,
no longer described by a probability distribution). However, because the
two particles are entangled, the wave function of the second electron must
also simultaneously collapse, and its angular momentum becomes precisely
specified, no matter how far away the second electron has moved from the first.
This aspect of QM was not accepted by Einstein, who maintained that this
“spooky action at a distance ” violated the Theory of Relativity, which indeed
it does (as we shall see in Chap. 6). He claimed that QM must therefore be
incomplete, and that the electron spins were indeed well defined, although
hidden from experimenters until they had carried out their experiments. This
interpretation was known as the “hidden variables” theory. A major contribution by the Irish physicist, John Stewart Bell, was to show that no hidden
variables theory can ever reproduce all the predictions of QM.
The first experimental test of the Einstein versus Bell disagreement was
performed in 1972, with the evidence strongly in favour of Bell [8]. Since
then, many tests have been made to close possible loopholes in the methodology. As time has passed, the experimental evidence in favour of entanglement has accumulated, and now few physicists doubt its existence. Many
research projects are underway to harness its capabilities in such diverse fields
as Quantum Computing, cryptography, ultra-precise clocks and biology.
5.9 Schrödinger, and His Long-Suffering Cat
No Chapter on QM would be complete without a mention of Schrödinger’s
cat. The story has been told so often that it has become hackneyed, so we
apologise in advance if the reader is already familiar with it. However, it
is a thought experiment that raises important questions about the boundaries between quantum and classical physics, and is worth presenting for that
reason. There are several variants to this experiment, but the one below is a
typical example.
Imagine that we have a box containing a cat and a weakly radioactive
source that has a probability of 50% of emitting one radioactive particle per
hour. Also in the box we have a Geiger counter to detect the decay product
from the source. The Geiger counter is wired so that when it registers a
detection, it releases a hammer which swings down and smashes a phial of
cyanide gas, thereby releasing poisonous fumes into the box and killing the
R. Barrett and P. P. Delsanto
journey. Our act of measurement collapses the wave function of the first electron, resulting in its angular momentum becoming precisely defined (i.e.,
no longer described by a probability distribution). However, because the
two particles are entangled, the wave function of the second electron must
also simultaneously collapse, and its angular momentum becomes precisely
specified, no matter how far away the second electron has moved from the first.
This aspect of QM was not accepted by Einstein, who maintained that this
“spooky action at a distance ” violated the Theory of Relativity, which indeed
it does (as we shall see in Chap. 6). He claimed that QM must therefore be
incomplete, and that the electron spins were indeed well defined, although
hidden from experimenters until they had carried out their experiments. This
interpretation was known as the “hidden variables” theory. A major contribution by the Irish physicist, John Stewart Bell, was to show that no hidden
variables theory can ever reproduce all the predictions of QM.
The first experimental test of the Einstein versus Bell disagreement was
performed in 1972, with the evidence strongly in favour of Bell [8]. Since
then, many tests have been made to close possible loopholes in the methodology. As time has passed, the experimental evidence in favour of entanglement has accumulated, and now few physicists doubt its existence. Many
research projects are underway to harness its capabilities in such diverse fields
as Quantum Computing, cryptography, ultra-precise clocks and biology.
5.9 Schrödinger, and His Long-Suffering Cat
No Chapter on QM would be complete without a mention of Schrödinger’s
cat. The story has been told so often that it has become hackneyed, so we
apologise in advance if the reader is already familiar with it. However, it
is a thought experiment that raises important questions about the boundaries between quantum and classical physics, and is worth presenting for that
reason. There are several variants to this experiment, but the one below is a
typical example.
Imagine that we have a box containing a cat and a weakly radioactive
source that has a probability of 50% of emitting one radioactive particle per
hour. Also in the box we have a Geiger counter to detect the decay product
from the source. The Geiger counter is wired so that when it registers a
detection, it releases a hammer which swings down and smashes a phial of
cyanide gas, thereby releasing poisonous fumes into the box and killing the