Talk:Bucket argument

so... is this true then or what? does this prove there is an absolute frame of reference? i have been wondering this since i was like 12 so it would be nice to know why nobody ever discusses it... -plasticlax

It appears that rotation is absolute, but translation is still relative. pstudier 08:12, 2004 Apr 1 (UTC)

(William M. Connolley 21:42, 18 Oct 2004 (UTC)) I think the variant is due to Ernst Mach. But OTOH the variant I'm used to is an otherwise empty universe featuring two spheres joined together by a rope. If you measure the tension in the rope you can find out if they are spinning. Mach's principle says that there is no tension. I think.

so... is this true then or what? does this prove there is an absolute frame of reference? i have been wondering this since i was like 12 so it would be nice to know why nobody ever discusses it... -plasticlax

It appears that rotation is absolute, but translation is still relative. pstudier 08:12, 2004 Apr 1 (UTC)

Einstein's theory of general relativity does not need the assumption of absolute space. Of course, Einsteins theory of gravity and motion does need to provide a mechanism that is telling matter whether it is accelerating or not, the bucket thought experiment proves that.

According to the theory of general relativity there is a universal inertial field. This universal inertial field is transparant to velocity, so all velocities are indistinguishable, but whenever an object accelerates, there is interaction with the universal inertial field, opposing (but not preventing) the acceleration, hence the formula of proportion: F=ma. In a universe without matter the inertial field would be identical to minkowsky space-time everywhere. But matter deforms the inertial field in its neighbourhood, and wherever the universal inertial field is deformed only limited volumes of space are effectively indistinguishable from minkowski space-time. The part of this deformation of space-time geometry that counts the most is the gravitational time dilation. When matter moves through deformed space-time geometry, the line of travel is seen to be curved when looked at from a sufficient distance. Unlike electromagnetic interaction, that is mediated by a "carrier" that travels in space, gravitational interaction is mediated by deformation of the very fabric of space-time itself. Deformation of the gravito-inertial field and deformation of the space-time geometry are one and the same thing in the theory of general relativity.

Rotation is not absolute, but according to general relativity it is exceedingly rare to see significant rotation of local space-time geometry with respect to the universe. Hence rotation measured against the locar space-time geometry and rotation with respect to distant stars are invariably seen to match. General relativity does predict under what (extreme) circumstances significant local rotation of space-time geometry with respect to the Universe will occur.

When two spaceships are co-accelerating, then their relative velocity is zero. However, when they want to communicate, for example by radio signals, they observe the signals are distorted. Their interaction with space-time affects the signals. It is only when both space-ships are moving inertially that the laws of special relativity apply. When the spaceships are both accelerating they must each take their individual interaction with the universal inertial field into account. --Cleon Teunissen | Talk 18:19, 19 Mar 2005 (UTC)

(William M. Connolley 21:44, 19 Mar 2005 (UTC)) The bucket argument proves nothing. Its something to ponder, but no-one has drawn any defendable physical theory from it.

Well, Einstein did draw a conclusion from the thought experiment. To Einstein it showed a severe condition that any theory of motion must meet. Einstein agreed with Mach's objection against newtonian absolute space. Newtonian absolute space acts on matter but it is not acted upon, wich, argued Einstein, is profoundly unsatisfacty. Einstein's physics intuition told him that a theory of physics that wants to really represent Nature should describe Nature as interactions between physical things that act upon each other and that are being acted upon.
To illustrate the condition that any theory of motion must meet Einstein devised the rotating liquid spheres thought experiment. Einstein argued that this thought experiment proves that there must be an interaction of local matter with distant matter. The problem of taking account of inertia is a problem of information. Independent of the choice of coordinate system, either rotating with respect to the liquid sphere floating almost alone in space, or a coordinate system that has zero rotation with respect to the liquid sphere floating in space, it must be recognized that some mediator is providing the information that determines exactly how much the liquid sphere is bulging at the equator.

Any theory that lays claim to being a good representation of Nature must describe a mechanism that relays that information. Einstein believed that general relativity achieves this aim, and the scientific consensus in the community of physicists is that general relativity indeed achieves that aim. Mach's original proposal was that any theory of motion and in particular inertia should not refer in any way to space, Mach argued that it should be about interactions of matter only. Einstein did not follow that proposal of Mach. --Cleon Teunissen | Talk 09:13, 20 Mar 2005 (UTC)

The bucket and the floating globes (new article)

I wrote a new article, after reading Newton's Scholium.

I concentrated on the physics, and I avoided the philosophy. Newton's rotating bucket is not situated in empty space; the two globes, connected by a cord under tension had been situated by Newton in otherwise empty space. The bucket argument is not a thought experiment, it is an inference from something seen in daily life; the two-globes-connected-by-a-cord is a thought experiment. It is understandable that the two have been merged into a bucket-with-water-in-empty-space, but I feel the article should be historically correct.

The globes-and-cord thought experiment is by far the most interesting line of thought, it is wider in scope than the rotating bucket argument.

I decided not to write about Mach's philosophical objections against the tacit assumptions in newtonian dyanamics. --Cleon Teunissen | Talk 18:45, 20 Mar 2005 (UTC)