If a tree falls in a forest

"If a tree falls in a forest and no one is around to hear it, does it make a sound?" is a philosophical thought experiment that raises questions regarding observation and perception.

Philosopher George Berkeley, in his work, A Treatise Concerning the Principles of Human Knowledge (1710), proposes, "But, say you, surely there is nothing easier than for me to imagine trees, for instance, in a park [...] and nobody by to perceive them. [...] The objects of sense exist only when they are perceived; the trees therefore are in the garden [...] no longer than while there is somebody by to perceive them." (It is worth noting that the quote from section 45 is arguably a statement of an objection to Berkeley's view, and not a proclamation of it.) Nevertheless, Berkeley never actually wrote about the question.

Some years later, a similar question is posed. It is unknown whether the source of this question is Berkeley or not. In June 1883 in the magazine The Chautauquan, the question was put, "If a tree were to fall on an island where there were no human beings would there be any sound?" They then went on to answer the query with, "No. Sound is the sensation excited in the ear when the air or other medium is set in motion." This seems to imply that the question is posed not from a philosophical viewpoint, but from a purely scientific one. The magazine Scientific American corroborated the technical aspect of this question, while leaving out the philosophic side, a year later when they asked the question slightly reworded, "If a tree were to fall on an uninhabited island, would there be any sound?" And gave a more technical answer, "Sound is vibration, transmitted to our senses through the mechanism of the ear, and recognized as sound only at our nerve centers. The falling of the tree or any other disturbance will produce vibration of the air. If there be no ears to hear, there will be no sound."

Albert Einstein is reported to have asked his fellow physicist and friend Niels Bohr, one of the founding fathers of quantum mechanics, whether he realistically believed that 'the moon does not exist if nobody is looking at it.' To this Bohr replied that however hard he (Einstein) may try, he would not be able to prove that it does, thus giving the entire riddle the status of a kind of an infallible conjecture—one that cannot be either proved or disproved.

...
Wikipedia