5 The Incredible Quantum Mechanics
97
cat. Remember, this is only a thought experiment, and not intended to be
carried out! 11
After one hour, when the experimenter opens the box there are two
possibilities:
(1) The radioactive source has not emitted a particle, the cyanide has not
been released, and the cat is alive and well;
(2) The radioactive source has emitted a particle, the cyanide has been
released, and the cat is dead.
From a quantum perspective, one might say that it is the observation
process of opening the box that collapses the wave function and reveals which
of the two possibilities is real. Before the box is opened, both possibilities exist
and the cat is in a mixture, or superposition, of two states, one in which it is
alive and one in which it is dead.
From a classical physics viewpoint, one would say that the cat is alive or
dead, but until we open the box and take a look, we do not know which. This
is akin to Einstein’s Hidden Variables theory that we discussed in the previous
Section. So under what conditions are the classical theories of physics accurate, and when do they fail, so that we must turn to QM? This is the dilemma
that Schrödinger’s Cat has highlighted for us.
Macroscopic objects, such as pebbles, balls, cats, and even us, contain
countless billions of molecules. 12 The wave function associated with these
large objects is a linear superposition of the wave functions associated with
the atoms of which they are constructed. For two waves to interfere, there
must be a stable phase relationship between them. This means that the peaks
and valleys of the waves are not randomly distributed. In this case, the waves
are said to be coherent . In a large body, the wave functions associated with
the billions of molecules are jumbled, and so do not add up coherently. In
this case, the superposition of the wave functions produces physical effects
that are just averages of the individual effects. This is akin to the classically
predicted behaviour.
As an example, the light emitted by a tungsten filament lamp is incoherent,
and so will not normally produce interference effects. 13 On the other hand,
light from a laser is produced by stimulating the atoms in the laser to all
11 Our satirical spoof, Doppelbelcher and the OOO, which opens Chap. 12, illustrates some of the
disastrous consequences of trying to implement Schrödinger’s thought experiment in the laboratory.
12 In the vicinity of 10 23 molecules per kilogram.
13 Interference effects, e.g. Young’s experiment, can be carried out with an incandescent filament lamp
if a narrow slit is placed in front of the filament to block out most of the light. Only light from a
relatively small number of atoms is then allowed to pass through.
97
cat. Remember, this is only a thought experiment, and not intended to be
carried out! 11
After one hour, when the experimenter opens the box there are two
possibilities:
(1) The radioactive source has not emitted a particle, the cyanide has not
been released, and the cat is alive and well;
(2) The radioactive source has emitted a particle, the cyanide has been
released, and the cat is dead.
From a quantum perspective, one might say that it is the observation
process of opening the box that collapses the wave function and reveals which
of the two possibilities is real. Before the box is opened, both possibilities exist
and the cat is in a mixture, or superposition, of two states, one in which it is
alive and one in which it is dead.
From a classical physics viewpoint, one would say that the cat is alive or
dead, but until we open the box and take a look, we do not know which. This
is akin to Einstein’s Hidden Variables theory that we discussed in the previous
Section. So under what conditions are the classical theories of physics accurate, and when do they fail, so that we must turn to QM? This is the dilemma
that Schrödinger’s Cat has highlighted for us.
Macroscopic objects, such as pebbles, balls, cats, and even us, contain
countless billions of molecules. 12 The wave function associated with these
large objects is a linear superposition of the wave functions associated with
the atoms of which they are constructed. For two waves to interfere, there
must be a stable phase relationship between them. This means that the peaks
and valleys of the waves are not randomly distributed. In this case, the waves
are said to be coherent . In a large body, the wave functions associated with
the billions of molecules are jumbled, and so do not add up coherently. In
this case, the superposition of the wave functions produces physical effects
that are just averages of the individual effects. This is akin to the classically
predicted behaviour.
As an example, the light emitted by a tungsten filament lamp is incoherent,
and so will not normally produce interference effects. 13 On the other hand,
light from a laser is produced by stimulating the atoms in the laser to all
11 Our satirical spoof, Doppelbelcher and the OOO, which opens Chap. 12, illustrates some of the
disastrous consequences of trying to implement Schrödinger’s thought experiment in the laboratory.
12 In the vicinity of 10 23 molecules per kilogram.
13 Interference effects, e.g. Young’s experiment, can be carried out with an incandescent filament lamp
if a narrow slit is placed in front of the filament to block out most of the light. Only light from a
relatively small number of atoms is then allowed to pass through.