# What is the solution to Newton's bucket problem? [duplicate]

What is the solution to Newton's bucket problem?

The Wikipedia page does not give me any finality. After reading, I felt as if the problem implied either one of these two things

1. There is a special frame (the frame of distant stars)

2. There is a special frame which is determined by coordinates (position).

My Background

Pardon me if I have terribly misunderstood the article, but I know high school math and science fairly well and I have participated in a Science Olympiad. So I would like explanations to be made assuming this.

## marked as duplicate by knzhou, Buzz, Kyle Kanos, John Rennie general-relativity StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Dec 12 '18 at 13:00

• Would you read this previous answer, see what you think . physics.stackexchange.com/q/3986 – user146020 Feb 23 '17 at 17:46
• I am afraid that I had previously gone through that answer and I felt that it was not answering my specific question(in terms of clarity and basic concepts). So I would request not to mark this as duplicate. Thanks – Agile_Eagle Feb 23 '17 at 17:49
• Yeah, that's what I thought too , I would say that explicitly in your post though, as comments get deleted. Best of luck with it. – user146020 Feb 23 '17 at 17:52
• Thanks for your support. Excited as this has been my first question.(Dont know if its appropriate to write this in comments) – Agile_Eagle Feb 23 '17 at 17:54

If we want to know what the solution is, we first have to figure out what the problem is.

Say we have a bucket of water in the middle of space, with nothing else in the universe. If the bucket isn't spinning, the water will rest in the bucket. If the bucket is spinning, the water will be pushed up against the edge.

Here is the problem: who is to say whether the bucket is spinning or not? It seems as though if there is absolutely nothing else in the universe, the two situations are completely the same. That is the "problem."

But in a certain sense, there is no problem. There are a certain set of special frames in the universe called "inertial frames" in which there are no fictitious forces. If we look at the bucket when we are in an inertial frame, and it spinning, we can deduce that the water should be pushed to the sides. If the bucket is not spinning when we are in an inertial frame, then we can deduce that the water should not be pushed to the sides.

No single inertial reference frame is privileged. If we move at a constant velocity with respect to an inertial frame, we are ourselves also at the center of some other inertial frame. However, if we rotate a frame by a time dependent angle, we will no longer be in an inertial reference frame, and it will take fictitious forces to explain the phenomena we see.

This is the "solution" to the "problem," but perhaps it isn't satisfying. You might object that there is nothing physically different happening if the bucket is spinning or not. I would counter, "who are you to demand what is physical or not?"

Einstein was bothered by the bucket problem. He felt as though a theory of space, time, and gravity would solve it. Somehow, he imagined, perhaps a set of stationary stars define what an "inertial" frame is, but if there were no stars very far away, the water would never be pushed to the sides.

While this was one of his motivations for developing general relativity, the actual theory he came up with doesn't actually work the way he hoped. The problem is that the distant stars really don't have an effect on the bucket. They're irrelevant to the problem at hand.

However, Einstein's formulation of general relativity does "explain" the bucket problem in its own way.

Say we take space to be flat, described by the regular Minkowski metric in an inertial frame, and a non-spinning bucket is sitting there in space. We can change coordinates to a rotating frame of reference. In this new frame of reference, with the new coordinates we use, the Minkowski metric will look different. Obviously, all we did is choose different coordinates, but if we ourselves are now rotating with respect to the original inertial frame, these are the coordinates we would use.

With our new metric, things will move on geodesics/straight lines if they are not acted on by forces. (This includes the water particles in the bucket, which would leave the bucket if the bucket's walls were not forcing them to stay inside.) However, these "straight lines" would not look straight to us when we are spinning. So the water particles, which are sitting still in the inertial frame, are now moving in circles.

I should note that this isn't really that different from the Newtonian explanation. I've just changed "moving without fictitious forces" with "moving in a straight line/geodesic." But that is what general relativity has to say about it.

Now that I've explained that GR doesn't really have anything new to say about the bucket problem, I should mention that there's actually more to the story.

It's not enough to have a few "distant stars" out at infinity, but say instead you have a lot of distant stars evenly distributed throughout space. Furthermore, say that the entire mass of stars are orbiting around some central point for no reason, all with the exact same angular velocity. (Who knows why they're doing that, just imagine that they are.) If we then use Einstein's equations to solve for the spacetime metric, we find that there actually is a centrifugal force that would act on a bucket placed in the middle of the space time! In other words, if we placed a non-spinning bucket of water in the middle of this spinning universe, the water actually would be forced to the edge! This is called the "Lens Thirring" effect.

Aha! It seems as though general relativity then DOES solve the Newton bucket problem! Well... not quite. This centrifugal force depends on the mass density of the distant, evenly distributed stars. From a Newtonian analysis, we know that the centrifugal force on the water from changing frames should have absolutely nothing to do with the density of distant stars. Furthermore, the centrifugal force produced by the Lens Thirring effect will always be less than the centrifugal force you would normally expect from changing reference frames.

There is some confusion about the Lens Thirring effect. I have seen some physicists claim that it solves the Newton bucket problem. While it might seem like it does at first glance, it actually doesn't. There is a fundamental difference between the universe where all the distant stars are stationary as described by a rotating coordinate system and the universe where all the distant stars are somehow rotating together in an inertial frame.

The general consensus is that Newton's laws are defined such that they hold true for the inertial reference frame, which could be described as a constant displacement in euclidean space. This seems to work for the frame of the fixed stars relative to earth. (by experiment) Absolute space is a mathematical constraint invented to allow Newton's laws to work properly. It's almost circular reasoning if it weren't for the accuracy of the predictions of his laws of motion in most any situation if taken from this 'frame'.

I would say that it's entirely possible to hold Newton's law's of motion as good within a rotating frame if you introduce new force fields. For example the centrifugal and coriolis forces. We can't on the other hand have something like an elliptical frame, unless velocity is defined as sum of displacements from two points. Incidentally, there is one other frame for which we can keep the laws of motion. an object accelerating at a constant rate will be able to assume it's frame if it simply introduces a new force which acts constantly from some arbitrary direction. One frame might explain forces more generally than another, but no rest frame is safe from question. Even the one Newton described breaks down for objects travelling at a (high) velocity due to special relativity.

• "unless velocity is defined as sum of displacements from two points" I don't get this part. – Mockingbird Feb 24 '17 at 1:30
• "an object accelerating at a constant rate will be able to assume it's frame if it simply introduces a new force which acts constantly from some arbitrary direction" and this part... – Mockingbird Feb 24 '17 at 1:32
• There's no way to be at rest if you're undergoing elliptical motion, because you are always accelerating with respect to any point within the ellipse. There's no force field you can invent in the elliptical frame that fixes this problem for every observer. Basically, Newton's first law is violated because an object accelerates without a force. That is, unless we define 'velocity' to be the rate of change in the sum of the displacements from two foci. – lucky-guess Feb 24 '17 at 10:40
• Imagine being in a closed elevator under constant acceleration. We could assume that the elevator is at rest with a field pulling everything towards the floor. The law's of motion will work for anything inside the elevator, even though it was treated as a rest frame – lucky-guess Feb 24 '17 at 10:42
• But, as soon as the rotating frame change it's angular velocity or something, the invented laws within that frame are broken and the observers inside will be able to deduce that they are rotating in a euclidean coordinate system. This is the crux of Newton's argument of 'absolute' space. The explanation does not necessary require absolute space to be a physical entity as he claimed. But it does prove that there is a preferred set of reference frames, for which his laws of motion and gravitational force predicts all motion. – lucky-guess Feb 24 '17 at 11:06