Pligget

As far as we're aware, we're surrounded by them. Wherever you look you see light and the things it illuminates. You hear sounds coming from all around you. Our eyes, ears and brain tell us they're all out there. We experience light and sound as things that exist out there, separate from us.

Not true. There's only one place where light and sound exist. In your head. They're your invention.

Think about the night sky on a clear night. Ignoring the stars for a minute, the only source of light up there is the moon. The surrounding sky is black. But we all know the moon's not a light source, it's just reflecting sunlight. For the moon to be able to reflect sunlight, it has to be in the sunlight's path. This means the night sky, which is effectively black, has to be full of what we think of as "light". Just think about that for a minute. The black night sky is full of sunlight. The only reason we don't "see" it is that it's going away from us and not into our eyes. The moon scatters the sun's light in all directions, including into our eyes, and that's when we see it.

So light is only "light" when it enters our eyes. It only exists in our heads. For a blind person whose eyes aren't functioning, it doesn't exist at all.

What we call light is just that part of the electromagnetic spectrum that the chemicals in the cells at the back of our eyes are sensitive to. The full EM spectrum includes radio waves, infra-red, ultra-violet, microwaves, X-rays, gamma rays etc. The only reason we don't see those as well is not that they're any different in nature from "light" - they're just at wavelengths that our eyes' chemicals don't respond to.

The same goes for sound. Compression waves in the air, or water, or whatever medium, ripple out in all directions from things that happen - in much the same way that pond ripples are generated by a thrown stone. These compression waves only become sound when they cause our eardrums to move, setting up a chain reaction in our middle and inner ears that cause us to perceive it as sound. Sound doesn't exist for people whose ears don't work, and it doesn't exist for hearing humans when the vibrations are at frequencies above about 20,000 Hz (ripples per second). It's no longer "sound" when it's at 30,000 Hz, although dogs would disagree with that. Also, there's no sound if there's no medium to carry the vibrations to your ear - like in the vacuum of space.

So the next time someone poses that old chestnut about whether a tree falling deep in the forest makes a sound, you know the answer. Of course it doesn't. It only makes a sound if there's something there with ears to hear it.

The great thing about light and sound being your own invention is imagining what form everyone else's inventions take. I know how I experience the colour blue, or the sound of a violin, and I'd love to know how those things are perceived by other people.

Have you ever considered that there is no light or sound out there?