Wednesday, June 29, 2011

Isaac Watts on the size of the universe

I read through chapter X of Isaac Watts’ Philosophical Essays, where he talks about light from stars. My memory was bad: it is nothing to do with Olbers' paradox. The first section discusses whether space can be empty or not, and Watts argues it cannot. For light particles are passing through all parts of space in all directions. He gives a neat Fermi-like example. Imagine an auditorium containing a thousand plates, surrounded by an audience of a thousand people. Each person can see each plate. Thus, we can draw a line from any plate to every person. And similarly from any person to every plate. And (assuming light is particles) there is a constant emission of particles filling the air. The same must be true of space and the light emitted from stars. Thus, space must be full up of light particles. He wonders why the planets are not slowed down by all of this (an interesting question that I will leave to the experts).

The second section is on whether the universe is infininitely large or not. Watts argues not. For although the Earth was created by God 6,000 years ago (remember the time he was writing, in 1742), it is probable that the other solar systems were created long before that. In which case, if the light particles were being constantly emitted from the stars, the universe would be all used up and dark and dead, which it isn’t.

He mentions with apparent approval the opinion that when light particles are emitted from a star, gravity eventually draws them back to their source. Thus the universe will not be depleted by the constant emission of light, and it will have a finite size.

I was surprised to find he knows the speed of light (“one hundred and four score miles per second of the Minute”). I always imagined this was a nineteenth century discovery.

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