Does absolute space exist?

[blike]

For those of you who enjoyed "The Elegant Universe", Brian Greene has a new book out. From the short moments I spent with it tonight, it seems to be more philosophical.

 

Anyhow, as I was scanning through the first few chapters, I noticed an interesting question posed by newton that I have never considered.

 

I'm going to relay it to you on memory, so someone correct me if I make little errors here and there.

 

Imagine you have a bucket which is nearly full of water suspended from the ceiling by a rope that has been twisted so as soon as you let go (of the rope) the bucket will begin spinning. Neither the water or the bucket are in motion until you let go of the rope. After you let go, the bucket begins to spin as the string unwinds. At first, the bucket begins spinning around the water, and the water does not move. Shortly after, the water will begin to spin along with the bucket. However, as the water spins, it starts to bevel downward in the center as the water crawls up the sides.

 

Newton asked himself: "Why does the water bevel?" To most of us, this is easily answered by experience: Because it's spinning! But Newton was asking something else. What does it mean to be spinning? Spinning relative to what?

 

At first, we may answer that it is the bucket which spinning relative to the water which causes the bevel. But this is obviously not the case, as the water does not immediately bevel when the bucket first begins to spin. It is also not the relative acceleration of the water. Imagine a ring of people standing around the bucket. The water will still bevel when it is spinning relative to the people. Yet if the people were to start moving in a uniform, circular manner about the bucket, the water does not experience any bevelling.

 

To what is the absolute reference point? Newton claimed that it was space itself which is a fixed reference point. Yet, if space were a fixed reference point, would we not be able to recognize motion (assuming no acceleration) even if we were in a pitch black absolute vacuum?

 

I read up until Greene introduced a man named Mach who argued that without other matter as a fixed reference point, spinning would not be felt. He argued (from what I remember) that if the universe was just a giant void, that spinning would not produce any detectable effects.

 

I believe Greene intended to further address the issue and ultimately answer the question, as he sets this up very early on and then abandons the topic abruptly.

[Pinch Paxton]

Yet if the people were to start moving in a uniform, circular manner about the bucket, the water does not experience any bevelling.

 

Never heard this before, can someone please explain this. I have an answer, but first I need to get all of the facts.

 

Pincho.

[alt_f13]

Yah, but the water is moving about itself as well.

 

But if there was only one string in a void, would it be possible for it to move?

 

 

 

[edit]

 

This book sounds awesome already.  What was it called?

[blike]

It's called The Fabric of the Cosmos : Space, Time, and the Texture of Reality

[Radical Edward]

surely the issue is that there is an acceleration on the water.

[blike]

What about when the water and the bucket are in equillibrius, uniform motion? No acceleration acting on the bucket or the water? It seems obvious that "objects in motion tend to stay in motion", and this is why we experience centripetal acceleration. Yet, objects in motion relative to what? If you're in space spinning uniformly with the bucket of water, neither the water or the bucket will appear to have any motion.

 

When you're travelling in a car, and it makes a sudden turn, you are pressed against the seat because of a change in velocity which results in acceleration. In this situation, the reference point is clear: the car around you. Yet in a dark area of space devoid of any surrounding matter, would you feel the change in velocity? Would you feel acceleration?

 

Mach argues that your only reference frame in this situation is the sum of all the mass in the universe. He says that inertial effects are not innate in a body with mass, but are dictated by its relationship with all of the other mass in the universe. From what I gather, Einstein agrees with Mach in that he says since all of the mass in the universe dictates the geometry of spacetime at any given point, inertial effects are related to the mass of the entire universe. Einstein named Mach's idea "Mach's Principle".

 

From encyclopedia.com: "According to Mach's principle, a body experiences no inertial forces when it is at rest or in uniform motion with respect to the center of mass of the entire universe. "

[blike]

Pinch Paxton said in post # :

 

Never heard this before, can someone please explain this. I have an answer, but first I need to get all of the facts.

 

Pincho.

 

I was pointing out that relative acceleration does not cause the beveling effect.

[JaKiri]

blike said in post # :

and this is why we experience centripetal acceleration.

 

Whut?

[JaKiri]

blike said in post # :

Yet in a dark area of space devoid of any surrounding matter, would you feel the change in velocity? Would you feel acceleration?

 

Yes. See: GR and SR.

[Pinch Paxton]

If something is spinning in the opposite direction to the bucket the bevel dissapears? I still can't see how that happens. Weight must be a factor. A feather spinning in the opposite direction would not remove the bevel? I still need more information.

 

Pincho.

[JaKiri]

The bevel itself is a product of gravity, duder. It wouldn't happen if you spun it in micro/zero gravity.

[Pinch Paxton]

Right gravity, ah yes, now I understand. I need to think about it for awhile.

 

Pincho.

[blike]

MrL_JaKiri said in post #8 :

 

Whut?

 

I meant centripetal force.

 

 

MrL_JaKiri said in post #9 :

 

Yes. See: GR and SR.

 

The mach principle, which einstein tried to incorporate into GR, holds that inertial forces are due to the quantity and distribution of all matter in the universe. If there is a universe containing only one object, that object would have 0 inertia.

 

MrL_JaKiri said in post #11 :

The bevel itself is a product of gravity, duder. It wouldn't happen if you spun it in micro/zero gravity.

 

The bucket of water is the original thought experiment Newton worked with. You can apply the same principle to an astronaut spinning inside a giant bucket in space. He will experience a force towards the walls perpendicular to the axis of rotation.

[Giles]

The Mach principle is a rubbish explanation for inertia since (i) it wouldn't explain inertia and (ii) the idea of explaining fundamental concepts in natural laws is silly.

 

The astronaut will not experience a force towards the walls perpendicular to the axis of rotation. If he is already in contact with the bucket, directly or indirectly, then he will experience the same acceleration towards the axis as anything else. If not, he feels no force.

 

The idea of acceleration in a universe devoid of any source for the force producing the acceleration is pretty useless as a thought experiment.

[JaKiri]

blike said in post # :

 

I meant centripetal force.

 

I know. I've no idea why people think it's an upshot of rotation (as you appear to be assuming) rather than a prerequisite of orbital motion.

[BrainMan]

This was posted elsewhere, and I found one response especially helpful in understanding what is going on here. I will just post the response in full (but I can't garauntee that I am able to answer questions about it should they arise:

 

actually, this "bucket experiment" has been around since Newton, who invented it (in part) to refute Descartes' vortex cosmology. Newton also used it as an indirect argument in favor of absolute space, wrt which the bucket was supposed to be rotating. Ernst Mach (and later Dennis Sciama) argued that such absolute space is not properly verifiable, since absolute space is essentially defined by the presence of inertial motion; and one could not do an experiment to determine whether objects not moving "absolutely" do not experience inertial forces. i.e., if there's no other way to observe absolute space, then one might as well blame otherwise unobservable ghosts for the inertial forces. Mach actually thought that the distribution of the matter in the rest of the universe was somehow responsible for the inertial forces experienced in the rotating reference frame, since one can only properly verify that the bucket is rotating relative to the rest of the matter in the universe. Einstein sought to incorporate Mach's ideas into his new theory of gravity, but he ended up with something else entirely: matter does influence the structure of spacetime in such a way that one can determine experimentally whether an object is not on a geodesic path (i.e. experiencing inertial forces); but one can do this because there is an "absolute" structure to spacetime. i.e., the curvature of spacetime is invariant wrt general coordinate transformations.

 

so, in a sense, Newton was right all along. he really was a smart son-of-a-bitch.

 

it's interesting...if you go back to Newton's own writings on the subject (i did this once when i was an undergrad), you find that Newton didn't really intend his bucket experiment to be the only argument for the existence of absolute space. he argued that space and time must have some kind of existence -- independently of objects which exist "in" space and time -- in order for a mathematical theory of mechanics to be coherent. to date, no one has yet succeeded in carrying out Mach's program of eliminating reference to "space" and "time" (as separate entities) from physics.

 

if memory serves, there's a very interesting article by Richard Laymon (?) entitled "Newton's Bucket Experiment" (or something similar to that). i really recommend it for anyone interested in delving into this interesting topic.

 

From: [link]http://pub138.ezboard.com/fponderersguildfrm9.showMessageRange?topicID=734.topic&start=1&stop=20[/link]

[BrainMan]

Oops. That was: http://pub138.ezboard.com/fponderersguildfrm9.showMessageRange?topicID=734.topic&start=1&stop=20

 

 

[alt_f13]

QUESTION!

 

If something moving past you at near light speed is the same as you moving past something at near light speed, then how come it takes so much more energy for the thing that originally accellerated to accellerate further? There has to be a universal reference point.

 

So what if there was one just one piece of mass in the entire universe? Would it be possible for that to accellerate? In reference to what? Would there then be a cosmic speed limit?

 

Am I correct in assuming that beyond the vacuum there is nothing? How can something be more nothing-like than nothing?

 

 

I can almost visualise absolute nothingness as a flat surface on the membrane. Like a flatline on a heart monitor. It's the strangest feeling.

[Sayonara]

alt_f13 said in post # :

If something moving past you at near light speed is the same as you moving past something at near light speed, then how come it takes so much more energy for the thing that originally accellerated to accellerate further?

e=mc²

That ² in there makes it an exponential relationship (or thereabouts). Think graphed curves.

 

There has to be a universal reference point.

Isn't that "c"?

[alt_f13]

I think you missed my point.

 

In reference to each other they are both moving near light speed.

 

I am only backing up the statements made by blike with my first comments, that there is a universal spacial reference point that applies to all matter and energies.

 

I am also suggesting that it would not be possible for a lone piece of matter in the universe to accellerate because it has nothing else with which to compare velocity.

[Sayonara]

Oh, you mean a positional universal reference.

 

With a lone object accelerating you can compare its origin rest frame with its terminal rest frame, can't you?

[alt_f13]

I suppose so, but when a mass constitues the entire mass of the universe, who's to say it moves at all?

 

It does have a start and finish point (for our purposes those terms will suffice ) but unless you have something to compare even those to, it may as well have stayed put.

 

I suppose too that there would most likely be a propellant involved, but without a central positional reference point, the exhaust and the original mass would be moving with opposite velocities.

 

Now, as I mentioned before, there must be a universal positional reference point or moving at near light speed and accellerating would be exactly the same as moving slowly and watching a near light speed object accellerate past you. Clearly that is not the case as it takes immense ammounts of energy to accellerate the near light speed object.

 

As blike said before, energy and mass shape timespace, so I imagine the universal positional reference point must be the center of mass for the universe.