Università di Firenze, November, 30, 2021, h. 16.30-17.30 CET

If a tree falls in a wood, and no one’s around, does it make a sound? I start by arguing that the answer to this classic question is most definitely ‘no’ in a very important sense: we should draw a very sharp distinction between the longitudinal compression waves travelling through the particles in the air, and the sound of crashing and snapping we hear if we are standing in the wood as the tree falls. Similarly, there was complete silence on the early Earth when a volcano erupted, even if there were lots of compression waves in the air that would many years later be called ‘sound waves’ by human beings. Transferring the discussion to the nature of light, I argue that it is just as certain that ‘red light’ is not coloured red – instead it is merely the case that we experience the colour red when such ‘red light’ enters our eye and is processed (via the optic nerve and the brain). I then take the story into more unfamiliar territory. First of all, the sun doesn’t shine – again, that’s just how we experience the abundance of electromagnetic radiation flooding out of the sun. But why think, then, that the universe is ‘lit up’ at all? Thus it is argued that the universe is completely dark. This is intended as something close to a scientific fact, as opposed to a piece of speculative metaphysics.

Audio