Saturday, June 25, 2011
Paging a Relativity Specialist
I'm having big trouble with an idea in Phil Dowe's otherwise quite exemplary analytical philosophy survey of contemporary theories of causation.
He uses an example—he's trying to describe things that aren't really physically “real” but “pseudo” in some sense (a distinction I dispute somewhat).
Here's the scenario: I've changed it a little for clarity. Dowe says “laser beam” and I say “laser pointer.”
He imagines a laser pointer. Then he imagines a gigantic surface. The laser pointer is very powerful. The surface is very far away.
You hit the surface with the laser pointer. Then you wiggle it back and forth. (Imagine doing this on your wall, not so hard to do.)
His claim is that the speed of the wiggling dot of laser light is faster than light.
Having read two books by Einstein and two other books on relativity theory this makes no sense to me. You just can't add to the speed of light.
Then there's the issue of the gigantic surface. It's not whether building such a thing is feasible—say you can. It's the fact that light would take so long to hit the surface and return (let's say it could, the laser is phenomenally powerful), that the experiment would in effect take hundreds of millions of years to set up for any measurable effect to occur.
Maybe this is just a failure of my imagination but I think we're in an arche-fossil like situation in which you'd have to put the surface way past the light cone to wiggle it in a way that would give (for me the illusion of) faster than light speed. Which is absurd, because such an event could never be observed (it's outside your light cone).
Scale it back a little. If you hit the moon with a laser pointer, wiggling it would still not add to the speed of light, right? So even if you had a very very accurate device that could measure the whole setup, you wouldn't see that the laser dot was faster than light. Right?
Could someone with more experience in relativity theory than me please help me out? I think Dowe's argument is very spurious.