Figments, Science:
Steven re "Ether"

.I thought I'd follow up a bit on the Ether topic from the Figments page since there was an interesting little debate about absolute space vs. relational space way back between Leibniz and Newton (via a surrogate). Leibniz thought that that spatial properties were all relative properties and that what defined a space was the relations between objects. Newton defended the more conventional view of space as a sort of container. A fair bit of abstract debate went on but a very cool thought experiment came out of this discussion.

Imagine a bucket about half-full with water; now start rotating the bucket. What you'll see is the water in the bucket get lower in the middle and creep up the sides due to centrifugal force. Of course this is markedly different from the way the water behaves when the bucket isn't rotating. If you have container space all this makes perfect sense and works normally. But if you only have relational space you've got a problem. If the only object in the world is the bucket and the water in it should the water be concave and creeping up the sides or flat? A relational space picture doesn't seem to have an answer since there's no way of distinguishing between a rotating and non-rotating bucket. And quite a few people took this as pretty convincing evidence in favor of the container space/ether picture of the world.

I know that Mach came up with the answer but I'm not sure of someone else got to it first. Mach simply said that in relational space if the bucket was the only thing there then the surface of the water would remain flat. In fact he went on to bite the bullet on this somewhat wacky theory by pointing out that what causes the creeping water effect in a rotating bucket is the bucket's motion relative to all the other matter in the universe and that if you slowly deleted the distant stars and everything else the surface of the water in a rotating bucket would grow more and more flat. I imagine that was pretty hard to swallow for a lot of scientists; after all it appears to require action at a distance without some kind of causal medium connecting the objects. Of course, we can see the germs of our current theories in what Mach said.

I think though that even relativity requires a substantivalist view of space, or at least that it gets horribly complex if you don't have a substantivalist view of space. God hasn't left the building yet apparently.

—Steven Palmer Peterson
July 2002

Thanks for the cool thought experiment.

The idea that a bucket half-full of water could be "the only object in the world" is an essentialist fallacy—based on the premise that it's meaningful for a bucket half full of water to be "an object" rather than a collection of particles, which exist relative to each other.

What does "substantivalist" mean?

—JoT
August 2002

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