(xviii)


“The laws of mechanics presuppose the existence of an inertial frame.”

But what was an inertial frame?

A shoebox, perhaps. A little laboratory, floating in empty space. You needn’t look outside, it should all be local. You’d throw rocks around inside, three of them, at right angles, and the lines would have to come out straight.

It never pays to get fancy, in gendankenexperiment. A shoebox.

But what makes the lines straight?

You’d suppose you could adopt some different convention, a different sort of metric, a different definition of “straight”. And then. But if the lines curved round and met, of course, that would be different, that would be topological. But your topology might be different too. Or is that the point? No, you’d just make the shoebox smaller, so small the lines didn’t run together; and this was part of what you meant by “local”, that your laboratory could be as small as you needed.

But then you’d have to worry about the uncertainty principle. Too small a shoebox would have an intrinsic jiggle.

A shoebox.

And what had any of this to do with that enigmatic Machian prescription, “relative to the fixed stars”?

But roughly at this point I notice I am pissing on my foot.