Saturday, February 13, 2010

a bucketful of problems

while others played spin the bottle, Newton preferred spin the bucket

Canto: I think we should tackle the biggies, space and time, and such, since we've just seen a doco on time and whether it really did begin at the big bang.
Jacinta: We'll only be wading into the shallows, but hey we're here to educate ourselves as best we can, with whatever paltry capacities we have.
Canto: These are not only scientific questions but arguably metaphysical ones. Did space and time begin with the big bang, and if not, then do we go back infinitely somehow, with no first cause? Is the big bang a cause or an effect, or do we have to rethink our language?
Jacinta: The danger here, it seems to me, is to imagine you can come up with the right answer just by thinking about it. That way lies metaphysics, definitely, as well as theology.
Canto: Yes, it's all about basing your thinking on evidence, but in the field of cosmology - or in some areas of cosmology anyway - evidence is very hard to come by. A cosmologist or a string theorist spoke in the doco about his very firm conviction that time didn't begin with the big bang, that it was caused, I think by the clashing of two universe-planes which would in general circumstances remain unconnected, as parallel universes. Now what would constitute evidence for this? How can the theory be tested? As well as all the other theories floating around in cosmology?
Jacinta: It's not exactly metaphysics in the sense of dreamed-up noumenal worlds or the constructions in Plato's Timeaus, it's more mathematically based, and quite likely calculated to extreme degrees of precision, and maybe even testable in terms of some kind of internal consistency, and even coherence with theories already tested and accepted, and yet ...
Canto: So let's stick with the tested theories, and current understandings. What is space?
Jacinta: It's probably something that everyone has a 'common-sense view' about - for example, it's simply what separates discrete objects in the universe. Or it's everything that lies outside of the earth's atmosphere. In  The Fabric of the cosmos, by Brian Greene, the first three chapters - all I've read so far - look at this question, as well as the question of time, as it was wrestled with mainly by three thinkers; Newton, Mach and Einstein. It starts with a bucket experiment described by Newton. A bucket of water is tied by a rope which is twisted around and then let go, so that the bucket spins. The water in the bucket stays still and flat at first, but then, due to friction, the water starts to spin around too, and the surface becomes concave.
Canto: The water's being forced to the edge of the bucket, away from the centre, by the bucket's spinning motion. But how?
Jacinta: The water is being forced around the edge of the bucket, it can't move in a straight line because of the barrier of the bucket's edge. The spinning is the result of containment. The real issue, of course, is motion itself. When the bucket starts to spin, the water doesn't spin. The bucket is spinning relative to the water [and it's key to understand the relativity of all motion]. Then the water starts to spin relative to the bucket - the bucket's relative motion decreases. And the water's surface takes on its concave shape. Interestingly, though, when the bucket's rope untwists and starts twisting the other way, the bucket decelerates, while, for a time, the water continues to accelerate, increasing the relative motion between them once again. The surface of the water remains concave, more concave than ever.
Canto: So you're saying that the relative motion between water and bucket isn't itself responsible for the water's shape. Surely it's about force. The spinning bucket has imparted a force to the water, a force which continues to increase as the bucket accelerates. Even as the bucket slows down, it's still going in the same direction [it's responding to the force of the rope], imparting the same-wise force to the water. Only when the bucket has stopped does it cease to impart any force. The water carries on spinning - and that's where we have to introduce the concept of momentum?

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