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Newton's bucket absolute space?

  1. Oct 10, 2006 #1
    Newton's bucket...absolute space????

    As many of you know, Newton used a thought experiment to prove the existence of absolut space. He argued that the curvature of the water inside a bucket while rotating, demonstrates the the true motion of the water in reference of what must be absolute space. (he used a similar thought experiment using two spheres conected by an ideal rope in an empty universe. If these spheres rotate, a tension force is created on the rope, which indicates the "real movement" in reference to absolute space).

    Mach argued that curvature of the water was due to the rotatino in reference of the fixed stars, and even further, he argued that if we rotate the "fixed" stars around the bucket we will obtain the same result, concluding that space is relative.

    Of course we will never be able to rotate the whole universe around the bucket to prove mach's theory.

    I wonder if anyone knows better arguments, specially under the light of recent scientific evidence, to disprove Newton's absolute space. Please provide links if research papers are available.
  2. jcsd
  3. Oct 11, 2006 #2

    Mach's Principle has never been fully absorbed by General Relativity (in spite of Einstein's initial goal). At most we can say that inertia and gravitation (uniform fields) are equivalent, capable to produce the same physical effects (no known experience can make the difference).

    It is tempting now to adopt a stronger version of the Equivalence Principle and argue that inertia and gravitation have the same nature (Mach's Principle being a variation) but I don't think we have now sufficient reasons for this in the absence of a successful theory of quantum gravitation (in spite of some claims on the web that this is the case I am not yet aware of a strong enough argument clearly invalidating Newton's reasoning).

    In my view the claim that inertia and gravitation have the same nature is too strong for the moment, we still cannot make an obvious connection between them (among others we know that gravitational fields can be both homogeneous and inhomogeneous or inertial fields appear always homogeneous). Of course this does amount to say that things will ever be the same, the path is fully open here.

    But I don't think we need such a strong argumentation for considering Newton's reasoning as being at least unsound (such a hypothesis does not have any sort of epistemological privilege currently). After all it is quite a claim to say that the inertial effcts would remain the same in a world devoid of all matter (of course beyond the bucket, the water inside and all what is needed for the experiment), the current absence of a strong explanation for inertia does not make Newton's argument automatically sound (though neither can we consider it invalid).

    If we could somehow show clearly that inertia and gravitation have the same nature (GR being at least approximately true) then I'd say that we would have sufficient reasons to consider Newton's argument invalid.
    Last edited: Oct 11, 2006
  4. Oct 12, 2006 #3
    Here is an excellent article about the bucket's experiment... still doesn't explain clearly the sphere's thought experiment...

  5. Oct 30, 2006 #4
    Regarding this water bucket thought experiment, I've always been amazed about how Newton could have been so confused about something he postulated himself. His first law of motion says: ...object in uniform motion tends to stay in uniform motion unless acted upon by a net external force...

    I am sure he understood quite clearly, that a rock that is being rotated on a rope causes so-called "centrifugal force" simply because the rock tends to stay in uniform motion, but is acted upon by an external force (the pull on the rope).

    How come it was so hard for him to see this is exactly what the water is doing? It tends to continue along in a straight line but is acted upon by the bucket, and thus the end resut is that the water is pushed against the walls of the bucket. I cannot believe he missed this...

    So yes, what we can say is that acceleration is an "absolute" effect; if you accelerate away from another object, it is you who is accelerating alone, not the other object, and most definitely you are also the only one feeling the effects of the acceleration.

    It is also plain to see that any rotating object is a case of all of its elements attempting to continue in a straight line but since they are constrained to each others, they are forced to accelerate all the time. The rotational speed does not accelerate, but the elements feel inertial acceleration because they really are changing direction all the time.
  6. Jan 9, 2007 #5
    This is quite trivial and has brought some confusion.
    But the only valid explenation is that rotation is acceleration and therefore a noticable difference. The speed is changing, because we rotate (you need to think in vectors, it is the vector itself that changes, although it's length remains the same).

    To see it, please consider the same experiment, but then set up the bucket so that it stands on an axis. The axis is connected to a platform with walls, that can rotate. The bucket is standing on the axis.
    Now we rotate the floor and walls and we see the bucket (from our point of view) rotating. But the water level now remains fixed.
    Instead,we feel now the pressure of the wall we are leaning against.
  7. Jan 26, 2007 #6
    I searched on the internet but I could not find any satisfying information about Einstein's response to bucket argument. Could someone may help me?
  8. Jan 26, 2007 #7
    During his lifetime Einstein changed his mind several times on that.
  9. Apr 14, 2007 #8
    Modern Bucket Thought Experiment

    The following is a thought problem.

    Imagine a very rapidly rotating cylinder. For example, experimental variable speed motors have achieved; rotating in a vacuum; using magnetic bearing; rpm’s in excess of 100,000 rpm.

    Assuming a 2 cm shaft, at 100,000 rpm, the velocity at the edge of the shaft is 2*pi*R/T where T is 0.0006 sec. V = 200 m/s. The centripetal acceleration = V^2/R = 2193245 m^2/sec or dividing by 9.81 m2/s = approx. 200,000 g.

    Place a highly radioactive substrate on the surface of the shaft.

    As per general relativity's principal of equivalence, time slows down for the substrate at the edge of the shaft. The rate of radioactive decay slows down as one moves from the centre of the shaft to the outer edge.

    From the physical world view, there is and must be a physical reason, to explain why the radioactive decay slows down. From the physical world view there is a physical reason for every phenomena change. Time is a concept, not part of physical space.

    How do you see this thought experiment from Einstein’s world view?
  10. Apr 14, 2007 #9
    In GR nothing really slows down, it just takes more time for consecutive signals on that moving surface to reach a far away observer.
  11. Apr 16, 2007 #10
    Clocks in centrifuges? Yeah, well if by "Einstein's world view" you are referring to a relativistic spacetime, then let's see... it would probably be easiest to understand it in terms of a 2D spacetime diagram where the clocks are moving back and forth like attached to pistons.

    I.e. so that the worldlines of those clocks draw waves with same frequency but different amplitudes onto the diagram.

    This is much like looking at the centrifuge directly from the side, and it should be a good enough approximation to understand how this is modelled in relativity. We can actually just use special relativity at this point, can we not! For one, the clock at the outer part of the centrifuge is covering larger distances in spacetime, and it is experiencing larger acceleration. Its simultaneity plane tilts more violently in the lab-frame, etc...

    So it works in Einstein's view as well.

    But what this means ontologically, a different matter entirely :)

    Whaat? Please explain what you mean by this statement.

  12. Apr 16, 2007 #11
    Well, just what it says. :smile:

    You have to realize that two observers can travel different paths between two intervals. If we compare their clocks we can deduce the lenghts of each path by looking how long each clock has ticked.

    A clock that slows down is simply a bad clock!

    Also you should realize that there is a difference between observers and signals between observers. When an observer receives delayed signals from another observer due to relative motion or geodesic divergence then that does obviously not mean that something at the origin is slowing down.
    Last edited: Apr 16, 2007
  13. Apr 16, 2007 #12


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    I don't see why you think Newton is confused. By looking into the bucket and checking if the water is flat or not, he argues that we can determine absolutely whether or not we're in a rotating frame of reference.
  14. Apr 17, 2007 #13
    So, you meant to comment that in GR the physical phenomena doesn't slow down in a mundane sense, but the clock does suffer time dilation? I was (and still am :) very confused by your phrasing "in GR nothing slows down" since "slows down" is often referring to the relativistic time dilation as well :)

    Well sure there's a difference, but according to GR the "substrate at the edge of the shaft" will suffer a greater time dilation than the substrate closer to the center. I'm wondering how does "it takes more time for consecutive signals on that moving surface to reach a far away observer" say something about this thought experiment?

  15. Apr 17, 2007 #14
    Well sure if that's the only thing he thought the rotating bucket determines. But it seems like he used it to try and prove absolute space (that space is independent from matter and there exists such a thing as absolute motion against that space).

    Maybe this is just a common misconception about what Newton thought, I don't know. In any case, after having established his first law of motion: "An object in motion will remain in motion unless acted upon by a net force", it should be quite clear that the water in the bucket is also a case of something "trying to remain in motion", regardless of the existence of absolute space or not. And likewise it should become clear that "rotating frame of reference" is just a mental concept and not something that objects create (i.e. not something the bucket creates for the water). What he can arrive to from the first law of motion directly is that "straight lines" exists, and still he could not have told if the straight lines are properties of "space" or defined by matter... Should not have been too complicated to him, seems to me...

  16. Apr 17, 2007 #15
    The situation in general relativity is not much different than in the Newtonian case.

    Consider a universe with two fluid balls, one rotates and the other one does not. The equatorial bulge easily gives away which one actually rotates and which one not. :smile:
  17. Nov 19, 2008 #16
    Re: Newton's bucket...absolute space????

  18. Nov 19, 2008 #17
    Re: Newton's bucket...absolute space????

    Yes, I'm quite sure that the curvature of the water is tautologous to Newton's own first law of motion. Certainly doesn't seem like too hard to figure that out.

    Took another look at couple of quotes from Newton, and despite Hurkyl's comment above, it still seems to me that Newton was indeed confused about that; he did talk about absolute space as an explanation to this.

    As to why so many clever people have missed the obvious, well, we come to think about reality with certain concepts, they are hard to let go, and some concepts can mislead you to wrong paths.

    I.e., if someone thinks about this experiment in terms of rotating frames, as many people intuitively do, they will have to conclude there exists one special rotation frame where things are at rest.

    The error there was assume that the mental concept/definition of "rotation" has got some meaning to reality. No one forced you to ever invoke the idea of rotation at all. You could choose to comprehend reality - and that bucket - in terms of tiny sphere-symmetrical elements in linear motion/acceleration. It would just be very complicated to understand some situations like that. That is to say, rotation and rotating reference frames are very handy mental concepts that allow for simpler understanding of reality, as you don't have to think about tiny things that keep accelerating, you can just think about objects that "rotate".

    It's very common mistake that people confuse their mental concepts with reality itself. I just see people doing that very stubbornly and very frequently.

    Btw, another interesting example of very clever people having trouble letting go of old notions is the EPR-paradox, which is trivially explained in terms of relativistic spacetime (just as an arbitrary example)

    I'm right now having an interesting conversation elsewhere, about laws of physics being tautologous to few relatively simple symmetries, that always exist in the mapping of any unknown data patterns. But there too most people seem to have troubles grasping the idea, because it goes somewhat against the specific concepts they happen to use to map/comprehend reality (like, say, "relativistic spacetime"). It's kind of the same issue as banging your head onto "rotating reference frames" thinking there's some ontology behind them when there just isn't.

    Anyway, what are you reading then? Note that some people talk about the idea of "whole universe" creating its own rotating frame (like Mach), but that's somewhat moot because:
    1. By definition there are no defined entities outside of "whole universe" that the universe could be rotating around.
    2. Curvature of the water is in any case tautologous to Newton's first law of motion. Oops.

  19. Nov 20, 2008 #18
    Re: Newton's bucket...absolute space????

    Thanks for the reply. I've only just joined the forum, so it was nice to have such a quick reply. I can appreciate that what we see as obvious might not have been at the time, we are standing on the shoulders of giants after all.

    I guess the thing that is bugging me is the bucket experiment still seems to be used in discussions about absolute space, when as you say it doesn't say anything about absolute space. I guess the books are just telling the history, but I do find it confusing. I'm currently reading "The Fabric of the Cosmos" by Brian Greene. Haven't finished it, so I probably shouldn't be posting until I have, maybe all will be made clear. I've read quite a few pop-science books over the years, but this is the first time I've come across the bucket experiment. I really know very little about physics, I just like getting my head around the high level concepts.

    I have a very simple way of looking at things (almost certainly wrong, but it seems to work). I like to quantize things, make them discrete. I'm a computer programmer by trade to I like too think of everything as particles. The way that I like to think about the water is to imagine water molecules being pushed around with friction. They hit the side of the bucket, get a force applied to them, head away from the bucket with a force perpendicular to the radius, obviously the bucket stops them flying out, but there is still a force there pushing them out, so more modecules congregate at the edges of the bucket, so the water is higher there.

    Similarly for the person spinning and their arms going up. There isn't any upward force, but because of the force causing the rotation there is an outward force, and because our arms are hinged at the shoulders they go up. I'm probably being over-simplistic here. But what I'm getting at is it's the force causing the rotation that causes an outward force, not a motion relative to absolute space, not the universe, or the rest of the matter in the universe. Is this correct?

    I really didn't understand how Mach came up with his idea on the rest of the matter in the universe causing this "strange and unexplained" pushing out force. He didn't even give any explaination on how or why that force would happen.

    I've always been bothered by rotation, so its really good to hear you say that rotation has no special meaning to reality. There is no rotation force, just forces changing direction over time.

    So Newton explained it as rotation against absolute space, Einstein extended this and said it was absolute space-time. But I still don't see that either of them were right. It isn't a well formed question. How can you ask a question about rotation when rotation itself has no meaning. And as space-time warps to cause gravity even that isn't absolute. Is there anything absolute? Is the word absolute just a human invention so that we can compare something against something else. When you get right down to it, there isn't a "something else" outside the universe that we know that we can compare things to?

    - Stewart
  20. Nov 20, 2008 #19
    Re: Newton's bucket...absolute space????

    Well, it's a logical consequence of Newton's first law of motion. If you hold the first law as valid, you must also expect your hands to rise. Otherwise your worldview is not self-coherent.

    That depends on what "right" means :)
    We are juggling with mental concepts that somehow correspond to the behaviour of reality, and it's one thing to hold mental concepts that yield correct predictions, and another to actually understand what reality is like ontologically.

    That is why I said "tautologous to Newton's first law of motion" instead of "this is how it is in reality".

    Space can always be validly (predictionwise) defined in many different ways, so I don't really expect to see any ontology in any specific definitions. Here's some commentary about the issue, you might find it interesting;

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