Around about 1950 Einstein asked Abraham Pais, one of his young friends, if he really believed that the moon only existed if he looked at it. Einstein himself had no doubts as to the answer. The moon does exist in objective reality whether or not anyone is observing it.

moonSo, why did he ask the question?

He did so because he had long disagreed with the most important and influential physicists of his time about the interpretation of the quantum physics. They were committed to an interpretation from which it follows that nothing – the moon included – exists unless it is being observed. Einstein wanted to know whether Pais was on his side or theirs.

Realism versus Antirealism

Many quantum physicists, like Niels Bohr and Werner Heisenberg, are antirealists. They believe that there is no such thing as an objective reality. Following, often unknowingly, in the footsteps of certain influential “Idealist” philosophers like, Bishop Berkeley. Antirealists hold that what we call “reality” as merely a mental construct. In other words, moons exist only in so far as human beings are observing them.

Einstein agreed with Bohr and Heisenberg about the experimental data that had been obtained when they had tried to carry out simultaneous measurements of the position and momentum of subatomic particles like electrons. It simply couldn’t be done, for reasons having to do with Planck’s discovery that energy comes in multiples of little packets called “quanta”. But what was the significance of the fact that it could not be done?

The Antirealist Interpretation
Bohr and Heisenberg gave an explanation that has come to be known as the “Copenhagen Interpretation”:

-Antirealist claim 1:
Physical theories should restrict themselves to what can be observed or in some way measured.

Some physicists would prefer to come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as trees exist independently of whether we observe them or not. This, however, is impossible. As a later physicist, David Bohm, put it when describing the Copenhagen interpretation (with which he disagreed) “In the usual Copenhagen interpretation of the quantum theory, an atom has no properties at all when it is not observed”

His antirealist opponents, he would claim, may have done good physics, but have been lured into doing bad philosophy.

-Antirealist claim 2:
It is “meaningless” to talk about an object existing except when it is being

Likewise another physicist,  Percy Bridgman, said “Since an object never occurs naked but always in conjunction with an instrument of measurement or the means whereby we obtain knowledge of it, the concept of ‘object’ as something in and of itself, is an illegitimate one”. Max Born was fully aware that this claim derived from a philosophical decision to adopt a certain methodological principle for the interpretation of experimental results.

-Antirealist claim 3:
Since measurements can only be carried out by conscious human beings, and objects don’t exist except when they are being measured, objects can’t exist independently of human consciousness.
In his 1979 Scientific American article, Bernard d’Espagnat wrote “The doctrine that the world is made up of objects whose existence is independent of human consciousness turns out to be in conflict with quantum mechanics and with facts established by experience”. And the contemporary physicist, David Mermin, explicitly contradicted Einstein when, in 1981, he wrote “The moon is not there when nobody looks”.

In Einstein’s view, none of these antirealist claims can validly be inferred from the data yielded by experiments in the domain of quantum physics. His antirealist opponents, he would claim, may have done good physics, but have been lured into doing bad philosophy.

Ezequiel López López

(Original: Does the moon exists when nobody is looking at it? – Ray Bradley)

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