Jim Baggott explores how quantum theory mirrors the question, "If a tree falls in the forest, and there’s nobody around to hear, does it make a sound?":

Philosophers have long argued that sound, colour, taste, smell and touch are all secondary qualities which exist only in our minds. We have no basis for our common-sense assumption that these secondary qualities reflect or represent reality as it really is. So, if we interpret the word ‘sound’ to mean a human experience rather than a physical phenomenon, then when there is nobody around there is a sense in which the falling tree makes no sound at all.

This business about the distinction between ‘things-in-themselves’ and ‘things-as-they-appear’ has troubled philosophers for as long as the subject has existed, but what does it have to do with modern physics, specifically the story of quantum theory? In fact, such questions have dogged the theory almost from the moment of its inception in the 1920s. Ever since it was discovered that atomic and sub-atomic particles exhibit both localised, particle-like properties and delocalised, wave-like properties physicists have become ravelled in a debate about what we can and can’t know about the ‘true’ nature of physical reality.

He concludes:

[W]e can no longer assume that the properties we measure necessarily reflect or represent the properties of the particles as they really are. These properties are like secondary qualities – they exist only in relation to our measuring devices. This does not mean that quantum particles are not real. What it does mean is that we can ascribe to them only an empirical reality, a reality that depends on our method of questioning.

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