<P> 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 . </P> <P> The current phrasing appears to have originated in the 1910 book Physics by Charles Riborg Mann and George Ransom Twiss . The question "When a tree falls in a lonely forest, and no animal is near by to hear it, does it make a sound? Why?" is posed along with many other questions to quiz readers on the contents of the chapter, and as such, is posed from a purely physical point of view . </P> <P> Can something exist without being perceived?--e.g. "is sound only sound if a person hears it?" The most immediate philosophical topic that the riddle introduces involves the existence of the tree (and the sound it produces) outside of human perception . If no one is around to see, hear, touch or smell the tree, how could it be said to exist? What is it to say that it exists when such an existence is unknown? Of course, from a scientific viewpoint, it exists . It is human beings that are able to perceive it . George Berkeley in the 18th century developed subjective idealism, a metaphysical theory to respond to these questions, coined famously as "to be is to be perceived". Today meta - physicists are split . According to substance theory, a substance is distinct from its properties, while according to bundle theory, an object is merely its sense data . The definition of sound, simplified, is a hearable noise . The tree will make a sound, even if nobody heard it . The definition states that sound is a hearable noise . So the tree could have been heard, though nobody was around to do so . </P> <P> Can we assume the unobserved world functions the same as the observed world?--e.g., "does observation affect outcome?" A similar question does not involve whether or not an unobserved event occurs predictably, like it occurs when it is observed . The anthropic principle suggests that the observer, just in its existence, may impose on the reality observed . However, most people, as well as scientists, assume that the observer doesn't change whether the tree - fall causes a sound or not, but this is an impossible claim to prove . However, many scientists would argue as follows, "A truly unobserved event is one which realises no effect (imparts no information) on any other (where' other' might be e.g., human, sound - recorder or rock), it therefore can have no legacy in the present (or ongoing) wider physical universe . It may then be recognized that the unobserved event was absolutely identical to an event which did not occur at all ." (this apparent quote has no attribution or reference and none can be found online with reasonable effort). Of course, the fact that the tree is known to have changed state from' upright' to' fallen' implies that the event must be observed to ask the question at all--even if only by the supposed deaf onlooker . The British philosopher of science Roy Bhaskar, credited with developing critical realism has argued, in apparent reference to this riddle, that: </P>

If a tree falls in the forest and no one is there does it make a sound