Newton's bucket experiment, Why the debate?

[avicenna]

Newton's bucket experiment. Why the debate?

I just cannot see why there ever is any need of long, long debates about the experiment and people even invoke general relativity. As far as I can see, there is nothing to debate about.

We live in this "real" infinite universe of ours and we have far away stars and galaxies. So there is an "absolute" frame of reference for us - it is the frame of the fixed stars; you may say it with other words and terms, like the masses of the rest of the universe, but nothing matters. Inertial frames do not have rotation relative to our "absolute" fixed universe. So what is there to debate about. I just don't understand.

In videos of spacemen in our space laboratory being able to experience "freely floating" in space without gravity, there is a catch. It is so only because the space stations have been stabilized and do not have any rotation relative to the Earth centered frame. If the space station were to rotate about its center of mass at high speed, all the people in the space station would be vomiting and knocking themselves about the sides of the space station - there would not be any of them "floating" freely in space.

 

[sophiecentaur]

You have claimed there is a 'debate'. You need to quote some source for this so that we can make a comment. Your list of points is largely too vague.

It is so only because the space stations have been stabilized and do not have any rotation relative to the Earth centered frame.

The frame makes no difference, in principle. The craft in Earth orbit is still rotating once every 90 minutes or so, about its own axis. That imposes a centripetal acceleration on everything inside it. That contributes to the 'microgravity' that's experienced in every satellite.

 

[avicenna]

You have claimed there is a 'debate'. You need to quote some source for this so that we can make a comment. Your list of points is largely too vague.The frame makes no difference, in principle. The craft in Earth orbit is still rotating once every 90 minutes or so, about its own axis. That imposes a centripetal acceleration on everything inside it. That contributes to the 'microgravity' that's experienced in every satellite.

There is debate if you view internet replies in stack exchange, quora etc, even wikipedia. None gave a definite simple explanation without going tangentially to long, long stories, GR, etc

It is understood the space lab has no "significant" rotation, not absolute zero rotation.

 

[avicenna]

The Newton bucket experiment is not just a thought experiment; it can be demonstrated on our Earth laboratory.

There is no mystery about the experiment. It is all covered in the classical mechanics of Newton himself! What it demonstrates is that a rotating frame is not inertial - that's the "mystery". We can have a huge closed bucket with water and a platform above the water to station an observer. The observer would be thrown towards the bucket side when there is rotation; also a ball on the platform would roll off the platform.

So rotation is always relative to an inertial reference frame to start with. With the bucket experiment on the ground, the "inertial" frame is that of the fixed earth. In outer space, there is the universal inertial frame of the fixed stars.

 

[Paul Colby]

Even in GR there are always local frames where the metric is essentially Minkowski at a point with a non-vanishing curvature. I think the bucket experiment always works for a sufficiently small bucket. Still there are GR solutions where the distant stars rotate about you in your local non-rotating frame. Could this be the issue?

[Edit]

Actually, now that I think about it, this is very much the situation on Earth as shown by gravity probe B. The rotation of the distant star is very small, but exists if I understand frame dragging correctly.

 

[russ_watters]

There is no mystery about the experiment. It is all covered in the classical mechanics of Newton himself! What it demonstrates is that a rotating frame is not inertial...

Agreed.

So rotation is always relative to an inertial reference frame to start with. With the bucket experiment on the ground, the "inertial" frame is that of the fixed earth. In outer space, there is the universal inertial frame of the fixed stars.

This isn't clear. Earth is rotating, so it isn't inertial. Maybe you knew that, which is why you put "inertial" in quotes? If so, why even use the word there?

Then, "the universal inertial frame of the fixed stars"(or, rather, distant galaxies and the CMB) is a bit confusing or misleading because the word "universal" has two distinct definitions:
1. Of the universe: the CMB provides an inertial reference frame based on the structure of the universe.
2. Absolute/special/preferred: The CMB reference frame is not absolute/special/preferred over other inertial reference frames.

Anyway, you ask "why the debate" without referencing an actual debate or bringinging one here (unless you disagree with #2), so I'm not sure what the purpose of the thread is. Are you just looking for confirmation of the accepted position?

 

[avicenna]

This isn't clear. Earth is rotating, so it isn't inertial. Maybe you knew that, which is why you put "inertial" in quotes? If so, why even use the word there?

Anyway, you ask "why the debate" without referencing an actual debate or bringinging one here (unless you disagree with #2), so I'm not sure what the purpose of the thread is. Are you just looking for confirmation of the accepted position?

inertial frames is an ideal. It is well known and accepted that the fixed Earth is a near inertial frame which is an inertial frame.

Can you tell what is the accepted position - I can't figure it out from the various internet answers.

 

[Janus]

The Newton bucket experiment is not just a thought experiment; it can be demonstrated on our Earth laboratory.

There is no mystery about the experiment. It is all covered in the classical mechanics of Newton himself! What it demonstrates is that a rotating frame is not inertial - that's the "mystery". We can have a huge closed bucket with water and a platform above the water to station an observer. The observer would be thrown towards the bucket side when there is rotation; also a ball on the platform would roll off the platform.

So rotation is always relative to an inertial reference frame to start with. With the bucket experiment on the ground, the "inertial" frame is that of the fixed earth. In outer space, there is the universal inertial frame of the fixed stars.

But there is no "universal inertial frame of the fixed stars".
There is no absolute rest frame against which you can measure inertial motion. As long as you are in an inertial frame, there is no way to absolutely measure your velocity. Put an observer in a small room moving inertially, and there is no test he can make to tell if he is moving or not. Knock him out and accelerate the box to some new velocity. When he wakes up and performs the same tests, he will not be able to make any distinction between now and before he was accelerated.
But if he was awake during the acceleration, he could measure the acceleration. He can detect the transition between the two states, even though he can't detect any difference between the two states.
And that is what raises the interesting question: Why does there appear to be an absolute frame for non-inertial motion, while there isn't one for inertial motion.
In your Newton bucket experiment done on the ground, the Earth isn't really an inertial frame, it has its own spin. If you were to perform this experiment at one of the Poles, spinning the bucket is one direction( realtive to the Earth) would give a slightly different result than spinning it in the other. In one case you are spinning it in the same direction as the Earth spins, adding your spin to that, and in the other you will be subtracting the Earth's spin. Due to the small angular velocity of the Earth, the effect will be small, but it will be real.

And as far as the "fixed" stars go, they really aren't "fixed". Not only do they have their own proper motions, but they are part of a galaxy that rotates, which is part of a local cluster of galaxies orbiting each other...
The other question them becomes, with all these individual parts of the universe in motion with respect to each other, what exactly determines the absolute frame against which we measure non-inertial motion?

 

[russ_watters]

inertial frames is an ideal. It is well known and accepted that the fixed Earth is a near inertial frame which is an inertial frame.

That depends on what you use it for. For most purposes on Earth, I would agree, but Foucault would certainly disagree that Earth is inertial.

Can you tell what is the accepted position - I can't figure it out from the various internet answers.

Haven't we already covered it? What part are you not clear on?

 

[avicenna]

But there is no "universal inertial frame of the fixed stars".
There is no absolute rest frame against which you can measure inertial motion. As long as you are in an inertial frame, there is no way to absolutely measure your velocity.
...

The frame of the fixed star universe is an inertial frame; there is no absolute inertial frame.

Inertial frames has nothing to do with the ability of an observer within the frame to detect his velocity or "motion". It is about the 1st law of motion; if an object with no impress force, it will be at rest or have rectilinear motion. This has been confirmed with our experience within non-rotating space stations - we can balance a small steel ball to float freely at rest. So such space stations are "inertial", but only if they have no significant rotation.

In your Newton bucket experiment done on the ground, the Earth isn't really an inertial frame, it has its own spin. If you were to perform this experiment at one of the Poles, spinning the bucket is one direction( realtive to the Earth) would give a slightly different result than spinning it in the other. In one case you are spinning it in the same direction as the Earth spins, adding your spin to that, and in the other you will be subtracting the Earth's spin. Due to the small angular velocity of the Earth, the effect will be small, but it will be real.

We have to take the small rotation of the Earth to be insignificant as compared to the rotating bucket otherwise there is nothing much to discuss.

And as far as the "fixed" stars go, they really aren't "fixed". Not only do they have their own proper motions, but they are part of a galaxy that rotates, which is part of a local cluster of galaxies orbiting each other...
The other question them becomes, with all these individual parts of the universe in motion with respect to each other, what exactly determines the absolute frame against which we measure non-inertial motion?

The fixed star universe is almost not moving and this is what we mean when we say "fixed". In a closed world in outer space where that world rotates relative to the fixed stars, it could be detected as non-inertial just as with the concavity of the bucket experiment.

 

[Nugatory]

Newton's bucket experiment. Why the debate?

I'm still unclear on exactly what you're asking here. There's much discussion of the bucket experiment, but a "debate" is a discussion involving two or more disagreeing positions and I'm not sure what opposing positions you think you're seeing. Perhaps if you could state what these positions are then it might be easier to answer your question.

I agree that there is much discussion of this experiment. The explanation for the indisputable experimental fact that the surface of the water curves tells us much about whichever theory of gravitation (Newton or Einstein) we choose. However, neither explanation relies much on the possibility that the fixed stars (or in more modern terminology, the distribution of mass in the rest of the universe) is an essential element of the explanation.

 

[FactChecker]

Many years ago, I read about a calculation that was done applying GR to a bucket of water where the bucket was not rotating but the rest of the universe was rotating in a shell. It said that there was a gravitational effect which pulled the water outward. I mentioned this on another thread and someone asked for a reference, which I could not supply. Does anyone know of such a calculation and reference?

 

[avicenna]

I say there is debate as I seem to read about Mach disagreeing with Newton about something - I am not even sure what. What I understand is the concavity of the water surface is a trivial case of classical mechanics, that of rotation of the water within the inertial frame of the fixed earth.

[Mentors' note: This post has been edited to remove a quotation of and response to another member's post that had been removed as a violation of the forum rules.]

 

[Nugatory]

I say there is debate as I seem to read about Mach disagreeing with Newton about something - I am not even sure what.

That's not a good starting point for a discussion. This thread is closed; as with all thread closures, if there is more that needs to be said, PM any mentor to ask that the thread be reopened for your contribution.