Talk:Bucket argument

Link broken

Nothing philosophical here, but the link to Andrew Motte's translation of the Principia doesn't work anymore. The pdf file can be found here, and a streaming version (quicker to load) there. I don't know which one is better ( maybe both : the stream version inside the article and the pdf for download in the references ?), so i haven't changed anything in the article. (Feel free to delete this comment from the talk page once the link is fixed !) — Preceding unsigned comment added by 82.120.215.231 (talk) 09:49, 5 February 2012 (UTC)

Absolute vs Relative

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.

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)

Category; Classical mechanics

Sebastian (talk) had added [[Category:Kinematics]] to the article, commenting:

maybe should be dynamics, but there is no such cat as of now Sebastian (talk) 10:11, 2005 May 20 (UTC)

The current article on Kinematics opens with:

In physics, kinematics is the branch of mechanics concerned with the motions of objects without being concerned with the forces that cause the motion. In this latter respect it differs from dynamics, which is concerned with the forces that affect motion.

So the category 'Kinematics' is certainly inappropriate for the bucket argument. The bucket argument is about the philosophy of Classical mechanics. Newton is justifying his concept of laws of motion, he is very much concerned with the forces that are at work. Therefore I have removed the 'Kinematics' category, and replaced it with [[Category:Classical mechanics]]
--Cleon Teunissen | Talk 15:00, 20 July 2005 (UTC)

"Here Newton tacitly assumes the weight of the globes is known." Shouldn't that be mass??--Hobx 05:36, 3 September 2005 (UTC)

Something wrong?

This page is profoundly confusing, utterly useless for a beginner. I found an older version much more useful to understand things.(http://en.wikipedia.org/w/index.php?title=Bucket_argument&oldid=24025492) Please don't make things complicated just to give them the appearance of 'scientific rigour'. 59.163.146.5 09:04, 20 February 2006 (UTC)

I wrote the version that you prefer. My version was replaced, with the comment: 'amateur viewpoint removed'.

I have no trouble understanding the current version, being a physicist, and I know the relevant history of physics reasonably well. But I agree with your criticism that the current version is only understandable for people who already have a solid grasp of the subject matter; the current version assumes a lot of prior knowledge in the reader. The current version of the article is written for people who won't look up the article, since they already know.

In my version I focused exclusively on Newton's thinking and on translating concepts from newton's time to present day concepts, to make it an article that newbies to the subject can learn from. Maybe a merger should be attempted. Newton was very keen to debunk Descartes. --Cleonis | Talk 11:17, 20 February 2006 (UTC)

A few of that version are IMO misreadings, eventhough I base myself on the same translation of Newton. In particular, I think that he did not claim that motion under the influence of a force is true motion.

Neverheless there is much of value in that version, and reinserting some of it in the article may improve it for sure - as it is, the article is rather short anyway, so it will be a welcome complement. Harald88 22:35, 7 September 2006 (UTC)

The version of article that I wrote does not attribute to Newton a claim that 'motion under the influence of a force coincides with true motion'.

What I intended to convey in the version that I wrote is the following: 'true motion (as Newton conceived it) can be thought of in terms two components that add up to a single true motion. The two components are: a true uniform motion, that is unknowable, and accelerated motion (for example: orbiting around the Sun), that is observable.' --Cleonis | Talk 09:03, 10 September 2006 (UTC)

Good! Formulated like that, it is much clearer. Harald88 09:11, 10 September 2006 (UTC)

I agree. This is MUCH clearer. I am in no way interested in the unrelated explanation for why water takes on a parabola in a a spinning bucket. This needs to be restored or added to the first section of the article.Wmcleod (talk) 16:38, 3 November 2010 (UTC)

A newtonian style defence of absolute space

I'd like to present a line of reasoning, in spirit similar to the rotating bucket and the orbiting spheres. The purpose of presenting this line of reasoning is to show how I think Newton and his contemporaries were thinking.

A mill is built above a little stream of water, the water drives a waterwheel. The miller cannot directly see the stream from his living quarters, and he has build a device for monitoring the flow of water. An airtight sac is floating on the water, the sac has enough buoyancy to support a long pole. The pole extends through the floor of the miller's living quarters, and from the height of the pole the miller can see whether it is worthwile at all to go down and try to get some milling done.

If the pole does not move at all, then presumably the stream is dry. If water is flowing, then fluctuations in the flow of water will make the pole go gently up and down. The miller cannot see the water itself, what he can observe by watching the pole is change in the flow of water. In order for a thing to be changable, the thing itself must exist; water is flowing.

Change of velocity is physically manifest. When a carriage suddenly moves faster, the passengers feel the lunge. Change of velocity exists, we observe it. In order for a thing to be changable, the thing itself must exist. This proves the existence of velocity with respect to absolute space.

As I announced: the above depicts a line of reasoning that I think Newton or a contemporary might have offered.

It is sometimes suggested that Newton was naive in asserting absolute space. But it is the other way round: it is a sign of ignorance to accuse Newton of underestimating the range of options. --Cleonis | Talk 09:58, 10 September 2006 (UTC)

I was also impressed by Newton's reasoning. Note that we can only include in the article what has been published. Still, I think that Newton himself made his line of reasoning sufficiently clear, it's just a matter of highlighting and clearly paraphrasing the pertinent passages of his "Scholium". Harald88 10:45, 10 September 2006 (UTC)

Of course, I presented the line of reasoning only to offer a guideline of highlighting and paraphrasing pertinent passages of the 'Scholium'.

In the version of the article that I wrote it ends with a section that switches to a modern point of view. In the definitions (definition III) Newton defines a concept of 'vis inertia' which today would be called 'reactive inertial force'. If a force is impressed on an object, then due to inertia there is a reactive inertial force that is exerted by object. Newton follows the habit of his time (a habit that is just as current today) to describe inertia as a property of the object in and of itself. In Newton's time the usual name is 'vis insita' the 'life within' the object. The modern point of view is that Newtonian absolute space must be conceived of as acting upon objects, opposing change of velocity. In this view the assumed absolute space is conceived of as the underlying cause of the phenomenon of inertia. It seems it cannot be ascertained for sure whether Newton thought of inertia as an innate property of objects, or as a consequence of absolute space acting upon matter. --Cleonis | Talk 11:30, 10 September 2006 (UTC)

Statement of Descartes' position not understandable

The article proposes:

that space is nothing other than the extension of matter, and that the true motion of a body consists in its transference from the vicinity of bodies immediately surrounding it to the vicinity of other bodies.

The use of "it" and "its" which may refer to any of a number of different nouns occurring earlier in the sentence render the meaning obscure. Here are some options:

that space is nothing other than the extension of matter, and that the true motion of a body consists in the body's transference from the vicinity of bodies immediately surrounding the body to the vicinity of other bodies.

that space is nothing other than the extension of matter, and that the true motion of a body consists in the transference of motion from the vicinity of bodies immediately surrounding the body to the vicinity of other bodies.

that space is nothing other than the extension of matter, and that the true motion of a body consists in transference of space from the vicinity of bodies immediately surrounding the space of one body to the vicinity of other bodies.

And so on. Can this sentence be cleared up? It also would be helpful later on to have Descartes' theory as applied to the bucket, so we can see that in fact the bucket experiment contradicts his theory. Brews ohare (talk) 14:11, 12 May 2008 (UTC)

Traditional version of experiment has some problems that may slow understanding

The text currently has:

Presumably, the concavity of the water shows rotation relative to something else: absolute space. This argument is incomplete, as it limits the participants relevant to the experiment to only the pail and the water, which has not been established. In fact, the concavity of the water clearly involves gravitational attraction, and by implication the Earth also is a participant.

The discussion gets unnecessarily complicated when the traditional setting of the thought experiment is used. It doesn't help that the "which has not been established" part above does not really make sense.

The earth and its gravity would participate in how the experiment would look -- if run on earth. And the twisted rope would not work unless it were connected to something at the top and being stretched by the earth and the bucket of water from the bottom. But those factors are just involved because the early writers wanted to make something that could be understood well enough to grasp the essentials.

I've seen a drawing in which the bucket is suspended from a disembodied "hand of God" in an empty universe, but it is presented as a cartoon, not as an attempt to explore the operational factors of the experiment.

The better experiment would put a spaceship through a wormhole into an empty universe (so there is no gravity field from distant stellar objects). Is the spaceship moving? Is it still? There is no meaning to that question. If two spaceships come through and they end up close to each other they may detect that they have a closing velocity with regard to each other. But that gets into another area of the question about whether space is a substance.

With the one ship it is possible to do experiments with acceleration because the ship still has its rockets. But to make things a little more graphic, let's assume that the rocket engines have gone dead, but the ship has hull-mounted projectile weapons. They aim four equally spaced weapons tangentially to the circumference of the ship. Everything we know about the mysterious "thing" called inertia tells us that crewpersons floating at the center of the ship will see the walls start to spin around them. But somebody grabbing a handhold on the inner circumference of the hull will experience the same forces that one experiences riding in a merry-go-round or what the cat in the clothes dryer experiences. (Even one of Schroedinger's. Fame provides no exceptions to inertia.)

In a rotating cylinder, coins ranged side by side around one "great circle" drawn on the hull could be kept in place until jostled or until the stacks got tall enough that there would be no more room for a full complement of coins and there was going to be an overlap. Sooner or later, there would be a stack crash. The dislodged coins would eventually come into contact with the hull at points ranging fore and aft of the original line — slower if in vacuum. As long as the coins and the hull were not frictionless, eventually they would all get accelerated by hull contact and there would be the inevitable "concavity" of the surface of the sea of coins.

In any experiment there would be a acceleration and there would be rotations (at different velocities) relative to some other things -- the propellant, i.e., the projectiles shot off to start the ship rotating. The words "presumably" and "something else, absolute space" do not have to enter here. Definitely the concavity of the water (at both ends of the water cargo, actually) would show rotation because rotation is acceleration as far as movable components are concerned. The real connection to "absolute space" is probably in the existence of the phenomenon called inertia.

Inertia seems to be tied in with a very general empirical generalization: Nothing happens unless some force is exerted. Stones do not jump off mountain peaks. They have to be pushed or dislodged. Asteroids heading straight for collision with earth do not suddenly shear off to avoid a disaster. They have to be pushed or pulled by something. My dirty clothes do not put themselves in the washing machine and flick the switch. No free lunch -- even in an otherwise empty universe, I suspect.

If only one spaceship goes through the wormhole into the empty universe, there is no indication of motion, or of inertia, as long as the spaceship is just floating there. If, however, the spaceship can split in the middle by firing explosive bolts that are holding two spring-loaded halves together, there is motion. It might turn out to be different, but everything in our experience in this universe tells us that the universe does not work in a magical way. Is inertia a function of space? Would what we know as inertia change at great distances if it turns out that the universe is not correctly described by Euclidean geometry? If inertia is primarily a characteristic of the universe, can the universe be given a meaningful account without including something called space? Can the universe and inertia be explained or given a full account if space is interpreted as being only a relationship or set of relationships? P0M (talk) 20:45, 2 July 2008 (UTC)

There is a good discussion by Kant, in the Inaugural Dissertation I think, about the mathematical description of left-handed and right-handed objects. There are some difficulties in giving full descriptions when chirality is involved. If I remember correctly, you have to start with some empirical example of a "left hand" or a "right hand" and then on that basis one can characterize other objects as handed -- but defining them requires reference to some standard. If space is "only a relation," then why are two hands (or two molecules) that are relationally the same different in all kinds of interactions? (Some molecules may be digestible or indigestible depending on whether they are "sinister" or not. ;-)

How abstract can knowledge be and still pertain to and adequately describe our universe? Can space be given an adequate description only in terms of the relations among objects in [it]? Or does space have some characteristic or characteristics that cannot be captured by specifying relationships?

The same kind of questions come up in regard to sequence of events and the direction of the sequence of events. Feynman points to the several kinds of atomic events that look different to us as long as we have the idea, "first there was this particle sitting there and then there was a couple of blasts of energy diverging from that place," and "First these two blasts of energy collided and then there was a particle sitting there," but then you realize that it is the "same thing" happening in different directions. If you get very abstract, there does not seem to be anything to account for the "arrow of time," and to account for that arrow of time it becomes necessary to look at the total probabilities of the events involved in the whole. It is rather unlikely that I might throw an eraser at the blackboard, that it would slot itself neatly into the chalk tray at the bottom and go barreling down to the other end of the blackboard where it hits a hammer resting there that some careless worker left. But happens, maybe at the rate of once in a thousand lifetimes. The probability that some workman hits a resting eraser at the end of the chalkboard, and that it goes toward the other end of the track, suddenly hops up and into contact with the blackboard, then bounces outward and directly into the hands of the instructor who has just turned away from the class and closes his fingers just at the right instant to trap the unexpected missile — well, if anybody wants to gamble I know which side I'll bet on.

With regard to the not entirely abstract characteristics of space, perhaps it will turn out that probability has a part to play. Or maybe not.

How can the original passage quoted above be reformulated so that it does not seem like a "cut" at the idea of absolute space and yet make clear the problematical aspects of a full description of the rotating bucket experiment? "Absolute space," "inertia," etc., all seem to be terms that we might not be able to do without. What are the authoritative sources we can cite to help establish which if any of the traditional concepts can be tossed out as redundant? Can we say, "We don't need inertia because we have absolute space," or, "We don't need absolute space because we can explain everything with relations and/or inertia," or what? P0M (talk) 21:33, 2 July 2008 (UTC)

The effect of the shape of the bucket

The section headed Fluid mechanics says:

the parabolic shape applies only for surface regions sufficiently far from the bucket walls, and for water deep enough to keep the surface away from the bucket bottom

The approximate solution actually does not apply all the way to the bucket walls

For a real bucket and a real liquid, the solution near the bucket walls may become quite complicated

This disagrees with An Introduction to Fluid Mechanics & Heat Transfer by J.M.Kay, which says:

In the special case of the forced vortex the fluid is rotating as a solid object

In which case the surface will not be affected by the shape of the bucket. It will be a paraboloid right up to the edge of the water surface (apart from the meniscus caused by surface tension). So I suspect that the entire Fluid mechanics section of this article is incorrect.

Occultations (talk) 14:18, 20 April 2010 (UTC)

I have removed the entire Fluid mechanics section of this article, because I believe it is incorrect. I think I understand where the misunderstanding came from. In this part of the deleted section:

Lamb points out that the assumed velocity of the liquid has a non-zero curl, and hence cannot be realized in an ideal liquid that cannot support tangential forces. Another way to put this is that the velocity cannot be represented as the gradient of a velocity potential, it is not irrotational, a rather anomalous situation. In particular, it can be shown that any motion generated from rest by impulsive pressure only is necessarily irrotational. For example, if we begin with a stationary fluid with a flat surface, its velocity has zero curl. Then it can be shown that any subsequent velocity configuration obtained by gradually increasing the rotation of the water also will have zero curl.

the referenced document[1] talks about Euler's equation. But Euler's equation only applies to inviscid flow, ie. flow where there is no viscosity. And, indeed, if there is no viscosity then the fluid cannot be rotated up from stationary until the entire mass of fluid is rotating as a solid body. But this article says:

Newton discusses a bucket filled with water hung by a cord.

Eventually, as the cord continues to unwind, the surface of the water assumes a concave shape as it acquires the motion of the bucket spinning relative to the experimenter.

So it is clearly talking about a liquid with viscosity. The deleted section therefore does not apply.

Occultations (talk) 17:41, 27 April 2010 (UTC)

POV marking

This article is titled " bucket argument", which refers to Newton's argument for his point of view; at least it should fairly present that argument, that is, elaborate on it, citing secondary sources that defend it. Instead it only cites secondary sources that dispute that argument and even cites these sources as facts; more POV-pushing than that is hardly possible. Harald88 (talk) 08:23, 9 August 2010 (UTC)

For example Einstein's 1920 discussion is a good secondary source for Newton's bucket argument:

" the mechanical behaviour of a corporeal system hovering freely in empty space depends not only on relative positions (distances) and relative velocities, but also on its state of rotation, which physically may be taken as a characteristic not appertaining to the system in itself. In order to be able to look upon the rotation of the system, at least formally, as something real, Newton objectivises space. Since he classes his absolute space together with real things, for him rotation relative to an absolute space is also something real. Newton might no less well have called his absolute space ``Ether``; what is essential is merely that besides observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real. [emphasis mine]

It is true that Mach tried to avoid having to accept as real something which is not observable by endeavouring to substitute in mechanics a mean acceleration with reference to the totality of the masses in the universe in place of an acceleration with reference to absolute space. But inertial resistance opposed to relative acceleration of distant masses presupposes action at a distance; and as the modern physicist does not believe that he may accept this action at a distance, he comes back once more, if he follows Mach, to the ether, which has to serve as medium for the effects of inertia. But this conception of the ether to which we are led by Mach's way of thinking differs essentially from the ether as conceived by Newton, by Fresnel, and by Lorentz. Mach's ether not only conditions the behaviour of inert masses, but is also conditioned in its state by them. " - http://www.tu-harburg.de/rzt/rzt/it/Ether.html

Harald88 (talk) 08:43, 9 August 2010 (UTC)

.. I note that the POV marking has been removed without correcting the issue. Effectively the article contains 1 century old misinformation, as Mach's principle is not fully included in GR. I'll correct this now with a link to the Wikipedia article that clarifies and elaborates on that. Harald88 (talk) 13:52, 23 October 2016 (UTC)

Is This Actually Relative?

I don't believe that this example truly describes absolute motion. I beleive the water would be spinning, but it would be spinning relative to the bucket, wouldn't it? — Preceding unsigned comment added by MusicAndPhysics (talk • contribs) 01:26, 2 December 2012 (UTC)

..No. And you can try it at home. :-) Harald88 (talk) 13:53, 23 October 2016 (UTC)