Fizika

Does the moon exist when nobody is looking at it? Realists vs Antirrealists (I)

Although the title could seem a little presumptuous, the uncertainty actually can bring us to a farther knowledgment (either can lose us definitively ), both intentions are contained in this space. Let’s deal now with a discussion performed at the border between Physics and Philosophy.

Not so recently, one of Einstein’s young friends, Abraham Pais, reported that round about 1950 Einstein had asked him if he really believed that the moon existed only if he looked at it.
Einstein himself had no doubts as to the answer. In his view the commonsense belief is correct. The moon does exist in objective reality whether or not anyone is observing it.
So why did he ask the question?
He did so because he had long disagreed with a lot of the most important and influential physicists of his time, about the interpretation of that area of physics known as quantum
physics that deals with the behaviour of objects in the microphysical, subatomic, world. Many of these physicists 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

Einstein himself was a realist. He believed that there is a real world that exists independently of the human mind.
Many quantum physicists were, and still are, antirealists. Many believed, and many still believe, that there is no such thing as an objective reality.
Following, often unknowingly, in the footsteps of certain influential “Idealist” philosophers like the eighteenth century philosopher, Bishop Berkeley, antirealists hold
that what we call “reality” is merely a mental construct and hence that things like the moon exist only in so far as human beings are observing them.
Among these idealist-minded physicists were some of the greatest physicists of the 20th century. Two of them – Niels Bohr and Werner Heisenberg – had contributed, along
with Max Planck and Einstein himself, to establishing the foundations of quantum physics.

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 can’t be done?

The Antirealist Interpretation
Bohr and Heisenberg gave an explanation that has come to be known as the “Copenhagen Interpretation” according to which each of the following claims is true:

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

In Heisenberg’s words:
The hope that new experiments will lead us back to objective events in time and space is about as well-founded as the hope of discovering the end of the world in the
unexplored regions of the Antarctic.
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 stones or trees exist independently
of whether we observe them. 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. Indeed, one may say that its only mode of
being is to be observed; for the notion of an atom existing with uniquely definable properties of its own even when it is not interacting with a piece of observing apparatus,
is meaningless within the framework of this point of view.

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

Thus Heisenberg claimed:
The concept of the path of an electron between two successive measurements is meaningless. Likewise another physicist, Percy Bridgman, put it:
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:Modern physics has achieved its greatest successes be applying the methodological principle that concepts which refer to distinctions beyond possible experience have no
physical meaning and ought to be eliminated.

-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.

This claim was made by early defenders of the Copenhagen interpretation and is still being made by a number of its current supporters.
Thus, 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. On the contrary, he claimed,
they can be inferred only if one adopts certain indefensible philosophical principles for the interpretation of the data. His antirealist opponents, he would claim, may have done
good physics, but have been lured into doing bad philosophy.

(na španskom jeziku) je na španskoj verziji bloga: http://nohaycloacas.wordpress.com/2014/12/02/existe-la-luna-cuando-nadie-la-mira-realistas-vs-antirrealistas-i/

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

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