…in our experiment the “exact position” of the particle is a mere speculative concept, like the gender, color or happiness of the particle. These degrees of freedom may also exist but if they cannot be measured then we should not include them in the description of the physical world.
Nonequilibrium Many-Body Theory of Quantum Systems: A Modern Introduction.
By: Gianluca Stefanucci
Schr¨odinger on Quantum Mechanics
Erwin Schr¨odinger wrote in 1935:
One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along
with the following device (which must be secured against direct interference by the cat):
in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in
the course of the hour one of the atoms decays, but also, with equal probability, perhaps
none; if it happens, the counter tube discharges and through a relay releases a hammer
which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself
for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The
psi-function of the entire system would express this by having in it the living and dead cat
(pardon the expression) mixed or smeared out in equal parts.
It is typical of these cases that an indeterminacy originally restricted to the atomic domain
becomes transformed into macroscopic indeterminacy, which can then be resolved by direct
observation. That prevents us from so naively accepting as valid a “blurred model” for representing reality. In itself it would not embody anything unclear or contradictory. There
is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and
Die gegenw¨artige Situation in der Quantenmechanik.
Translated by J.D. Trimmer
Feynman on Quantum Mechanics
Feynman said the following:
Does this mean that my observations become real only when I observe an observer observing
something as it happens? This is a horrible viewpoint. Do you seriously entertain the
thought that without observer there is no reality? Which observer? Any observer? Is a fly an
observer? Is a star an observer? Was there no reality before 109 B.C. before life began? Or
are you the observer? Then there is no reality to the world after you are dead? I know a number
of otherwise respectable physicists who have bought life insurance. By what philosophy
will the universe without man be understood?
Lecture Notes on Gravitation
Bell on Quantum Mechanics
According to John S. Bell:
It would seem that the theory is exclusively concerned about “results of measurement”,
and has nothing to say about anything else. What exactly qualifies some physical systems
to play the role of “measurer”? Was the wavefunction of the world waiting to jump for
thousands of years until a single-celled living creature appeared? Or did it have to wait a
little longer, for some better qualified system [. . . ] with a Ph.D.? If the theory is to apply to
anything but highly idealized laboratory operations, are we not obliged to admit that more
or less “measurement-like” processes are going on more or less all the time, more or less
everywhere? Do we not have jumping then all the time?
Einstein on Measurements:
But it is in principle quite false to base a theory solely on observable quantities. Since, in
fact, it is the other way around. It is the theory that decides what we can observe.
Albert Einstein, cited by Werner Heisenberg