Lorentz Contraction Circular Motion

[Eldgar]

Does Acceleration affect Lorentz contraction?

Suppose their was a Circle spinning around its center then its outer edges would decrease in length.

looking somewhat similar to a saw blade or something.

This doesn't make sense though because then then object would change shape depending on what frame of reference you are looking at.

However all the examples of Lorentz contraction i have seen involve objects moving in a straight line.

Circular motion could be viewed as an object constantly accelerating around a point, which is what lead me to the question, does acceleration affect Lorentz contraction.

 

[A.T.]

This known as the Ehrenfest paradox:
http://en.wikipedia.org/wiki/Ehrenfest_paradox

Here my take on it:
https://www.physicsforums.com/showthread.php?p=2417528

This doesn't make sense though because then then object would change shape depending on what frame of reference you are looking at.

The spatial geometry in the rotating frame is non-Euclidean (distorted)
http://www.phys.uu.nl/igg/dieks/rotation.pdf (page 10, chapter 6)

 

[bcrowell]

Here's my own way of understanding it: http://www.lightandmatter.com/html_books/genrel/ch03/ch03.html#Section3.4

It occurs to me for the first time that some aspects of this problem are very similar to Bell's spaceship paradox, http://math.ucr.edu/home/baez/physics/Relativity/SR/spaceship_puzzle.html . Essentially Ehrenfest's paradox is just the same paradox, but wrapped around into a circle.

 

[Fredrik]

Short answer: It isn't possible to increase the angular velocity of a solid disc without forcefully stretching it. If the circumference remains the same, it's because the material has been forcefully streched exactly by a factor of \gamma, so that it exactly compensates for the Lorentz contraction.

I would be surprised if the radius remains the same. The rotating disc would try to restore itself from the forceful stretching. (It's like it consists of a bunch of rubber bands of different sizes, all stretched to longer lengths that their equilibrium lengths). The sum of those forces on an (arbitrary) atom in the disc would be toward the center. But there's also a centrifugal force pulling the atom away from the center. These two forces would have to exactly cancel for the disc to keep its shape.

I agree that it's essentially the same as Bell's spaceship paradox.

I wouldn't say that the geometry is non-euclidean in the rotating frame, since the rotating frame agrees with the non-rotating frame about which hypersurface of spacetime to call "space, at time t". That hypersurface is flat, and flatness is a coordinate independent property.

 

[A.T.]

I wouldn't say that the geometry is non-euclidean in the rotating frame,

Do you agree that a ruler at rest in the rotating frame will measure a circumference greater than 2*PI ?
http://img688.imageshack.us/img688/4590/circleruler.png

If yes, how can the spatial geometry be euclidean in the rotating frame? A circumference greater than 2*PI usually implies negative curvature.

 

[Ich]

I wouldn't say that the geometry is non-euclidean in the rotating frame, since the rotating frame agrees with the non-rotating frame about which hypersurface of spacetime to call "space, at time t". That hypersurface is flat, and flatness is a coordinate independent property.

But you would certainly expect that space is orthogonal to time, which is a bit of a problem in the rotating frame.

 

[Fredrik]

Do you agree that a ruler at rest in the rotating frame will measure a circumference greater than 2*PI ?
http://img688.imageshack.us/img688/4590/circleruler.png

Yes.

If yes, how can the spatial geometry be euclidean in the rotating frame? A circumference greater than 2*PI usually implies negative curvature.

The spatial geometry of the rotating frame depends only on which subset of spacetime the rotating frame considers "space" at a given time t. This is the set of points that are assigned time coordinate t by the rotating frame, and it's the same set of points that are assigned time coordinate t by an inertial frame that's co-moving with the point at the center.

The measurements you describe don't have anything to do with the spatial geometry in the rotating frame. Those measurements will just tell you the lengths of the segments in a bunch of different inertial frames. (Someone should slap Brian Greene with a fish )

 

[bcrowell]

Yes, this is a little subtle. Correct me if I'm wrong, but here is what I think is the situation.

In the nonrotating frame, we have
<br /> d s^2=d t^2 - d r^2 - r^2d \theta^2<br />
With the transformation \theta&#039;=\theta-\omega t, we get
<br /> d s^2=(1-\omega^2 r^2)d t^2 - d r^2 - r^2d \theta&#039;^2 - 2\omega r^2d \theta&#039;d t<br />
The presence of the cross-term means that there is no clean separation between the time and spatial coordinates, so there's no clean way to separate the metric into parts that you can identify as temporal and spatial. You can complete the square,
<br /> d s^2=(1-\omega^2 r^2)\left[d t+\frac{\omega r^2}{1-\omega^2 r^2}d \theta&#039;\right]^2 - d r^2 - \frac{r^2}{1-\omega^2r^2}d \theta&#039;^2<br />
The thing in square brackets is not quite the total differential of T = t + \frac{\omega r^2}{1-\omega^2 r^2}\theta&#039;. I need to think about this a little more. I think you can handle this with Rindler coordinates or something...? Anyway, if you restrict yourself to synchronizing clocks at the circumference of the disk, where r is constant, then you don't have to worry about the fact that the the thing in square brackets isn't the total differential of T. Admittedly there is something not quite right here, but passing over that point:
<br /> d s^2=(1-\omega^2 r^2)dT^2 - d r^2 - \frac{r^2}{1-\omega^2r^2}d \theta&#039;^2<br />
This separates cleanly into parts that can be identified as temporal and spatial. The coordinate T isn't quite what you want as a synchronized time coordinate, because it has a discontinuity as a function of \theta&#039;. You can't synchronize clocks in a rotating coordinate system. This metric is still flat, because you got it by doing a coordinate transformation on a flat metric. However, if you now write down just the spatial part,
<br /> d s^2= d r^2 + \frac{r^2}{1-\omega^2r^2}d \theta&#039;^2<br />
it's non-Euclidean.

So I think the story here is that the geometry in the rotating frame is non-Euclidean, in the sense that if you do the best job of clock synchronization that you can, in the most natural way, then the hypersurfaces of constant time have curvature. The time coordinate T is natural because it corresponds to putting n clocks around the circumference of the disk and synchronizing clock j with clock j+1 in the sense that an inertial observer who is momentarily coincides with the midpoint between the clocks will receive flashes of light from them simultaneously. You synchronize 1 with 2, 2 with 3, ... and end up with perfect synchronization except that there's a big discontinuity between clocks n and 1.

 

[A.T.]

The spatial geometry of the rotating frame depends only on which subset of spacetime the rotating frame considers "space" at a given time t.

You don't need to consider spacetime to measure spatial geometry in a frame. You just need a bunch of rulers at rest in that frame. And if you lay out that bunch of resting rulers in the rotating frame, you will see that the spatial geometry is non-euclidean there. The rulers are at rest, so time doesn't matter.

The measurements you describe don't have anything to do with the spatial geometry in the rotating frame.

The measurements I describe to determine spatial geometry is: using rulers a rest. What is wrong with that? You could use the same method to measure the non-euclidean spatial geometry in a Schwarzschild-spacetime.

Those measurements will just tell you the lengths of the segments in a bunch of different inertial frames.

A rotating frame is a "bunch of different inertial frames". When you lay out rulers in a Schwarzschild-spacetime. you also get just "lengths of the segments in a bunch of different inertial frames", because inertial frames are only local there. But you still call the measurement of these resting rulers which stretch across different inertial frames: "the spatial geometry".

This metric is still flat, because you got it by doing a coordinate transformation on a flat metric. However, if you now write down just the spatial part, it's non-Euclidean.

Maybe that is the core of the disagreement here: Fredrik is talking about the space-time metric, while I mean the purely spatial geometry.

 

[Fredrik]

You don't need to consider spacetime to measure spatial geometry in a frame.

The frame is a function from spacetime to \mathbb R^4, and until you have considered its definition, you don't even know what the word "spatial" refers to.

You just need a bunch of rulers at rest in that frame. And if you lay out that bunch of resting rulers in the rotating frame, you will see that the spatial geometry is non-euclidean there. The rulers are at rest, so time doesn't matter.

The measurements I describe to determine spatial geometry is: using rulers a rest. What is wrong with that?

What's right with it? I don't know why you think this has anything to do with the spatial geometry in the rotating frame. (I haven't completely ruled out that I have misunderstood something).

Fredrik is talking about the space-time metric, while I mean the purely spatial geometry.

I know what you mean, but the metric of space is induced by the metric of spacetime. More specifically, it's the inclusion function pullback of the spactime metric: Define I:S_t\rightarrow M, where S_t is space at time t and M is spacetime, by I(x)=x for all x in S_t. The metric on S_t is I^*g (where g is the metric on M). We clearly can't talk about the geometry of S_t until we know what S_t is.

So how do we find S_t? It's defined as the set of all points that are assigned time t by the rotating frame, so we need to know how the rotating frame is defined. It's defined by the substitution \varphi\rightarrow\varphi-\omega t in the usual polar coordinates

x=r\cos\varphi
y=r\sin\varphi

So the relationship between the inertial coordinates and the rotating coordinates is

t=t&#039;
x=r&#039;\cos(\varphi&#039;-\omega t&#039;)
y=r&#039;\sin(\varphi&#039;-\omega t)
z=z&#039;

where I have put primes on all the coordinates of the rotating frame to clearly distinguish them from the coordinates of the inertial frame. Since the rotating frame assigns time t to exactly those points that are assigned time t by the inertial frame, S_t is just the hypersurface that's assigned time t by the inertial frame. The inclusion function pullback of the Minkowski metric to such a hypersurface is clearly Euclidean.

 

[Ich]

Fredrik, you may of course call any 3-D-slice "space". But usually, you would like to have space orthogonal to the bundle of observer worldlines that defines it. That is not possible if rotation is involved, you run into the problems bcrowell describes: you have to slice the infinite helix into pieces, and those pieces are not euclidean, of course.
If you borrow the synchronization from the inertial frame - as it is done in the real world to define a global time - that's ok, but arguably not "space in the rotating frame".

 

[Fredrik]

Fredrik, you may of course call any 3-D-slice "space". But usually, you would like to have space orthogonal to the bundle of observer worldlines that defines it. That is not possible if rotation is involved, you run into the problems bcrowell describes: you have to slice the infinite helix into pieces, and those pieces are not euclidean, of course.
If you borrow the synchronization from the inertial frame - as it is done in the real world to define a global time - that's ok, but arguably not "space in the rotating frame".

I think my edit (which I made before I saw your post) answers that. How else would you define the rotating frame? Each segment of the world line of any tiny piece of the disc defines a local inertial frame, but I don't see why anyone would say that those world lines have anything to do with a "rotating frame".

 

[pervect]

You can't synchronize clocks in a rotating coordinate system.

This is *the* key point, in my opinion. One might add "using the Einstein convention" to the above remark.

There are enough papers on this problem to fill a book. "Relativity in rotating frames: relativistic physics in rotating reference frames", Guido Rizzi, Matteo Luca Ruggiero for instance. While this book is good in that it shows a large number of viewpoints, it might be less useful for teaching or learning.

One can't really talk about the geometry of space until one defines how to slice space from space-time, by determining the mechanism that defines the points of space-time that compromise the 3 dimensional "space" corresponding to some specific time instant t. The problem becomes even more acute when one tries to explain this point to a more general audience - I'm not convinced any attempt I've seen has really succeeded :-(.

 

[Ich]

but I don't see why anyone would say that those world lines have anything to do with a "rotating frame".

Have a look at http://books.google.com/books?id=IyJhCHAryuUC&pg=PA90#v=onepage&q=&f=false". (I hope the link works)
I'm sure I've read a more exhaustive paper by Gron, but I can't find it right now.
--But I see that pervect joined the discussion, IIRC he gave me the link back then.

 

[yuiop]

I think my edit (which I made before I saw your post) answers that. How else would you define the rotating frame? Each segment of the world line of any tiny piece of the disc defines a local inertial frame, but I don't see why anyone would say that those world lines have anything to do with a "rotating frame".

I question if a tiny piece of a rotating disc can be defined as a local inertial frame. An observer located at any point would feel and measure acceleration and even locally it would seem difficult to transform that fact away. Perhaps you mean local inertial frames in the sense of momentarily co-moving frames or perhaps I misunderstanding what you are saying?

For example can we claim somebody standing on the surface of the Earth or in the more extreme standing on a neutron star is in a local inertial frame?

 

[A.T.]

The measurements I describe to determine spatial geometry is: using rulers a rest. What is wrong with that?

What's right with it? I don't know why you think this has anything to do with the spatial geometry in the rotating frame.

For me "spatial geometry" in a frame means the spatial metric in that frame, determined by spatial distances which are measured with rulers at rest in that frame.

Do you agree that the spatial metric around a big mass is non-Euclidean, because the circumference at a distance r is different from 2*PI*r, when both lengths are measured with rulers at rest? The same applies to rulers at rest in a rotating frame.

 

[yuiop]

You can't synchronize clocks in a rotating coordinate system...

This is *the* key point, in my opinion. One might add "using the Einstein convention" to the above remark.

You can synchronise clocks in rotating frame using the Einstein convention as long as you restrict yourself to a small portion of the circumference. It is only impossible to synchronise clocks if you try to synchronise the clocks all the way around.

 

[A.T.]

You can't synchronize clocks in a rotating coordinate system

This is *the* key point, in my opinion.

How is that preventing you from measuring spatial geometry with rulers? You can't synchronize clocks around a massive object either, but you can still say that the spatial geometry is non-Euclidean there. What is different in a rotating frame?

One can't really talk about the geometry of space until one defines how to slice space from space-time, by determining the mechanism that defines the points of space-time that compromise the 3 dimensional "space" corresponding to some specific time instant t.

Why do you have to consider the time dimension at all? You have rulers placed at rest in the rotating frame. What they measure doesn't change with time.

 

[bcrowell]

I'm sure I've read a more exhaustive paper by Gron, but I can't find it right now.

Gron, Relativistic description of a rotating disk, Am. J. Phys. 43 (1975) 869

I question if a tiny piece of a rotating disc can be defined as a local inertial frame. An observer located at any point would feel and measure acceleration and even locally it would seem difficult to transform that fact away. Perhaps you mean local inertial frames in the sense of momentarily co-moving frames or perhaps I misunderstanding what you are saying?

For example can we claim somebody standing on the surface of the Earth or in the more extreme standing on a neutron star is in a local inertial frame?

The Gron paper discusses this in excruciating detail. He considers three different observers: S is at rest with respect to the disk's axis; Sk is an observer in an inertial frame whose position and velocity instantaneously coincide with a point on the edge of the disk; S' is an observer rotating with the disk.

In SR, there is a notion of an inertial frame. S and Sk are inertial in the SR sense.

In GR the notion of an inertial frame is not useful. What's useful is the notion of a free-falling frame, i.e., the frame of an observer who is not subject to any nongravitational forces.

The historical interest of the example comes from the fact that Einstein used it as a bridge from SR to GR. He figured out that the spatial geometry was non-Euclidean, as measured by rulers at rest with respect to the disk (Gron's observer S'). By the equivalence principle, S' can describe the motion of Sk as arising from a gravitational field. S' says, in agreement with all the other observers, that the local four-dimensional spacetime is flat. However, you can have a flat spacetime that is still permeated with a gravitational field, according to one observer's coordinates.

S' says, "There's an outward gravitational field in this region of flat space. By hanging on to the disk, I can keep from falling. Sk is falling. Sk's frame is a free-falling frame, and he agrees with me on that. I consider my frame to be inertial and Sk's to be noninertial, but Sk says it's the other way around."

Sk says, "There's no gravitational field in this region of flat space. S' is accelerating due to the force from the disk. My frame is a free-falling frame, and S' agrees with me on that. I consider my frame to be inertial and the frame of S' to be noninertial, but S' says it's the other way around."

 

[bcrowell]

How is that preventing you from measuring spatial geometry with rulers? You can't synchronize clocks around a massive object either, but you can still say that the spatial geometry is non-Euclidean there. What is different in a rotating frame?

In the four-dimensional space around a massive object, you can measure the Riemann tensor, which is local and coordinate-independent. Therefore the issue doesn't arise.

Here we're talking about measuring the curvature of a three-dimensional subspace defined by a constant t. Unless you can define t, you can't define what subspace you're talking about. Once the subspace is defined, then you can measure the three-dimensional Riemann tensor within that subspace.

 

[A.T.]

How is that(desynchronized clocks) preventing you from measuring spatial geometry with rulers?

In the four-dimensional space around a massive object, you can measure the Riemann tensor, which is local and coordinate-independent. Therefore the issue doesn't arise.

Under measuring the spatial metric I understand placing rulers at rest around the massive object, to determine the distances. I don't see how the desynchronized clocks stop me from doing that. Neither around a mass nor in the rotating frame I see any issue with that measurement, because I don't use any clocks. And in both cases the measurement will reveal a non-Euclidean spatial metric.

 

[Fredrik]

Have a look at http://books.google.com/books?id=IyJhCHAryuUC&pg=PA90#v=onepage&q=&f=false".

I don't see anything that disagrees with what I said. The surface that he says has constant negative curvature isn't "space" as defined by the rotating coordinate system. It's an entirely different surface. Two distinct points on the spiral drawn in the figure aren't simultaneous.

Perhaps you mean local inertial frames in the sense of momentarily co-moving frames

I do.

For example can we claim somebody standing on the surface of the Earth or in the more extreme standing on a neutron star is in a local inertial frame?

No, but the tangent to that observer's world line is a geodesic, which can be taken as the time axis of a local inertial frame.

For me "spatial geometry" in a frame means the spatial metric in that frame, determined by spatial distances which are measured with rulers at rest in that frame.

I know what I mean by a "frame", but I don't know what you mean. You seem to think of it as a physical object rather than as a coordinate system.

Do you agree that the spatial metric around a big mass is non-Euclidean, because the circumference at a distance r is different from 2*PI*r, when both lengths are measured with rulers at rest? The same applies to rulers at rest in a rotating frame.

I agree that it's non-euclidean, but I don't know if it can be justified that way. The example we're talking about here suggests that it can't.

 

[yuiop]

In GR the notion of an inertial frame is not useful. What's useful is the notion of a free-falling frame, i.e., the frame of an observer who is not subject to any nongravitational forces.

An inertial frame is equivalent to a free falling frame. In fact we can go further and say a free falling frame is an inertial frame. What the SR and GR definitions of an inertial frame have in common is that the observers in those frames do not feel any acceleration, or put another way, an observer is in an inertial frame if an accelerometer carried by the observer reads zero.

..
S' says, "There's an outward gravitational field in this region of flat space. By hanging on to the disk, I can keep from falling. Sk is falling. Sk's frame is a free-falling frame, and he agrees with me on that. I consider my frame to be inertial and Sk's to be noninertial, but Sk says it's the other way around."

An accelerometer carried by S' would not indicate zero acceleration, so S' can not consider himself to be in an inertial frame by the above argument.

 

[A.T.]

For me "spatial geometry" in a frame means the spatial metric in that frame, determined by spatial distances which are measured with rulers at rest in that frame.

I know what I mean by a "frame", but I don't know what you mean.

And rulers cannot be at rest in what you call "frame"? Or what is the problem with the quoted text of mine?

Do you agree that the spatial metric around a big mass is non-Euclidean, because the circumference at a distance r is different from 2*PI*r, when both lengths are measured with rulers at rest?

I agree that it's non-euclidean, but I don't know if it can be justified that way.

That is the only justification I would accept, for the claim that space is non-Euclidean: You have to be able to measure the non-Euclidean metric using rulers.

 

[bcrowell]

An inertial frame is equivalent to a free falling frame. In fact we can go further and say a free falling frame is an inertial frame. What the SR and GR definitions of an inertial frame have in common is that the observers in those frames do not feel any acceleration, or put another way, an observer is in an inertial frame if an accelerometer carried by the observer reads zero.

You can define "inertial frame" and "free-falling frame" to be the same thing if you like. However, that does contradict the Newtonian definition of an inertial frame. An accelerometer on my desk right now will read 9.8 m/s2. According to your definition, that means my desk is not an inertial frame. But in Newtonian mechanics my desk would be considered an inertial frame. So you just need to be keep in mind that using "inertial frame" that way may cause confusion if you don't explain that you're not using it in a way that's consistent with Newtonian mechanics.

An accelerometer carried by S' would not indicate zero acceleration, so S' can not consider himself to be in an inertial frame by the above argument.

S' is not in a free-falling frame. If S' learns Newtonian mechanics and does local experiments, he will consider himself to be in an inertial frame in the Newtonian sense, and immersed in a gravitational field. Similarly, students who learn Newtonian mechanics on the surface of the Earth consider themselves to be in an inertial frame in the Newtonian sense, and immersed in a gravitational field.

 

[bcrowell]

Under measuring the spatial metric I understand placing rulers at rest around the massive object, to determine the distances. I don't see how the desynchronized clocks stop me from doing that. Neither around a mass nor in the rotating frame I see any issue with that measurement, because I don't use any clocks. And in both cases the measurement will reveal a non-Euclidean spatial metric.

I think the issue here is that although you can use your definition in terms of laying down rulers, it's not quite as absolute and natural as it might seem at first. Ich has been pointing out that by your definition, space and time are not orthogonal. What I understand him to mean by that is that if you use your definition to get a three-dimensional spatial metric d\ell^2=g_{ij}x^ix^j, where i and j are spatial indices, then the four-space metric is not simply ds^2=dt^2-d\ell^2; you will also have some cross-terms of the form tx^i. Suppose that you have two cars with odometers. I think the odometers match up with your notion of d\ell defined by laying down rulers. The cars also have clocks on their dashboards. You send one car out around the disk in the clockwise direction, and the other in the counterclockwise direction. When the cars meet up on the far side of the disk, their clocks will be out of sync due to the d\theta&#039; dt term in the metric, even though they've traveled an equal distance at an equal speed. You could just accept this, but it's uncomfortable, because it leaves you wondering where the funny asymmetry comes from. Someone who doesn't like your laying-down-rulers definition can say, "See? I told you that definition would lead to no good!"

 

[Fredrik]

And rulers cannot be at rest in what you call "frame"? Or what is the problem with the quoted text of mine?

A metric is a mathematical concept. A ruler is a physical object. You don't define mathematical concepts in terms of physical objects, so you need to be more clear about what you mean.

That this the only justification I would accept, for the claim that space is non-Euclidean: You have to be able to measure the non-Euclidean metric using rulers.

That sounds quite bizarre to me. So conditions like "Riemann tensor"=0 is unacceptable to you? (I'm not sure if that's sufficient to imply that the metric is Euclidean, but it's at least a start).

 

[A.T.]

For me "spatial geometry" in a frame means the spatial metric in that frame, determined by spatial distances which are measured with rulers at rest in that frame.

A metric is a mathematical concept. A ruler is a physical object. You don't define mathematical concepts in terms of physical objects,

I define a metric in terms of distances which are measured using rulers. See my quote above again.

so you need to be more clear about what you mean.

I mean what is described here in more detail (last paragraph of page 91):
http://books.google.com/books?id=Iy...=Let us now see how the non-Euclidean&f=false

 

[heldervelez]

Do you agree that a ruler at rest in the rotating frame will measure a circumference greater than 2*PI ?
http://img688.imageshack.us/img688/4590/circleruler.png

I do not agree, and I will explain why it must be equal to 2*PI.

If the ruler shrink then all elements of the world of that frame have to shrink accordingly.

------------- graphical explanation ------------------------
artwork (in a linear fashion to ease the representation)

H -- ruler
HHHHHHHHHH -- Object with 10*H
---------------------------------------
I -- shorter ruler
IIIIIIIIII -- shorter object with 10*I
-------------

---- the answer yes means this: --------------
I -- shorter ruler
IIIIIIIIIIIIIIIIII -- Object with 18*I the object is greater
-------------
this is not length contraction but length dilation and is an obvious error in spite of the original and overwelming authority of Einstein
-------------

https://www.physicsforums.com/showthread.php?t=363853"

 

[A.T.]

Do you agree that a ruler at rest in the rotating frame will measure a circumference greater than 2*PI ?
http://img688.imageshack.us/img688/4590/circleruler.png

---- the answer yes means this: --------------
I -- shorter ruler
IIIIIIIIIIIIIIIIII -- Object with 18*I the object is greater
-------------

Sure:

The length of moving objects measured with a co-moving ruler is greater than measured with a ruler at rest.

this is not length contraction but length dilation

No, this is still length contraction just stated the other way around. My above statement is equivalent to:

The length of moving objects measured with ruler at rest is less than measured with a co-moving ruler.

There you have the "contraction".

 

[Fredrik]

I define a metric in terms of distances which are measured using rulers. See my quote above again.

Still seems to me that you're trying to define a mathematical object in terms of physical objects. (The book you're quoting isn't).

I mean what is described here in more detail (last paragraph of page 91):
http://books.google.com/books?id=Iy...=Let us now see how the non-Euclidean&f=false

I find it pretty strange that they're using the term "spatial geometry" when they're not even talking about "space". They've made it obvious that "space" is flat, and that the hypersurface with negative curvature that they're considering doesn't consist of simultaneous events, and therefore doesn't deserve to be called "space". Hmm...now I see that they're doing something funny on page 90. They're not considering space at all. They're considering an entirely different concept which they call "rest space". If I understand them correctly, it's the metric of "rest space" that says the circumference is 2πγr (because they're really calculating the length of a spiral), while the metric of "space" says the circumference is 2πr (because now we're dealing with the length of a circle).

 

[pervect]

How is that preventing you from measuring spatial geometry with rulers?

If you want to talk about the spatial geometry of a set of points, you do need to specify the set of points that you're talking about, if you expect to get an answer that everyone agrees with.

For instance, you can slice up the space-time around a massive body with Schwarzschild coordinates in which the spatial slices are not flat - or in Painleve coordinates, in which the spatial slices are flat. So in general, it doesn't make sense to talk about a spatial geometry unless you also specify the manner in which you create the time slices.

Now, you specify "with rulers", so perhaps you aren't taking the same basic approach that I am of considering a geometry as something that applies to a set of points. *If* you regard a ruler as measuring the distance between worldlines, I believe you can get a well -defined answer for the circumference of a rotating disk. (You have to make some basic assumptions that the distance between worldlines is the shortest worldline connecting them, and that this distance is static because the geometry is static, and that you take the limit for closely space worldlines). Note that what happens is that you start at one point on one worldline to measure the circumference, and trace out some path through space-time. This path ends on the same worldline it started from, by definition, but it does NOT end at the same point on that worldline - it ends on the same worldline at a different time than when it started out.

This defines a circumference, but it's not clear that this approach actually defines a "geometry". The "circumference" defined by this means is not a closed curve!

I'm not aware of anyone using this particular approach in the literature - though there may be someone, I'm not familiar with all of the literature on the topic by any means, it's quite large.

You can't synchronize clocks around a massive object either, but you can still say that the spatial geometry is non-Euclidean there. What is different in a rotating frame?

You can syncyhronzize clocks around an equipotential surface of a massive non rotating object, I'm not sure why you think you can't?

 

[heldervelez]

...
The length of moving objects measured with ruler at rest is less than measured with a co-moving ruler.
...
.

I see your point. My error, because I did not saw the 'cross-polinization'

But each observer, with each respective ruler, will have to measure the same value.
As we can not find a single object in the whole Universe that has an extension of 2*Pi, I will measure in a physical way:
Suppose I've a ruler made of 1 atoms of Hidrogen at rest and the circular perimeter by suposition, measures 100 Hidrogen atoms, then when in motion I'will have to measure 100 atoms (contracted) with my ruler (also 1 H contracted).
But the the object, does not have two distinct realities, but only one and it will be perceived with distinct shapes.

 

[A.T.]

I think the issue here is that although you can use your definition in terms of laying down rulers, it's not quite as absolute and natural as it might seem at first.

It is 'absolute' in the sense that it is frame invariant: Everyone will agree what the rulers will measure.

When the cars meet up on the far side of the disk, their clocks will be out of sync due to the d\theta&#039; dt term in the metric, even though they've traveled an equal distance at an equal speed. You could just accept this, but it's uncomfortable, because it leaves you wondering where the funny asymmetry comes from.

Yes, "funny things" happen with clocks in non-inertial frames, but the good news is that you don't have to use clocks to measure spatial geometry.

it's the metric of "rest space" that says the circumference is 2πγr (because they're really calculating the length of a spiral),

They are calculating what http://img688.imageshack.us/img688/4590/circleruler.png" would measure, when placed at rest in the rotating frame. And this ruler is a circle, not a spiral.

You can't synchronize clocks around a massive object either, but you can still say that the spatial geometry is non-Euclidean there. What is different in a rotating frame?

You can syncyhronzize clocks around an equipotential surface of a massive non rotating object, I'm not sure why you think you can't?

Now you added a restriction, which also applies to clocks in a rotating frame: You can synchronize clocks which are equidistant to the rotation axis in a rotating frame.

So my above question still stands: Why is it correct to call the spatial geometry around a massive object non-Euclidean, but "controversial" to say the same about the spatial geometry in a rotating frame? Clock synchronization issues arise in both cases, so what is the difference?

 

[Fredrik]

They are calculating what http://img688.imageshack.us/img688/4590/circleruler.png" would measure, when placed at rest in the rotating frame. And this ruler is a circle, not a spiral.

Yes, but they're doing it by calculating the length of a spiral in spacetime.

So my above question still stands: Why is it correct to call the spatial geometry around a massive object non-Euclidean, but "controversial" to say the same about the spatial geometry in a rotating frame? Clock synchronization issues arise in both cases, so what is the difference?

The controversial part is to use the term "spatial geometry" about the geometry of a surface that isn't "space".

 

[A.T.]

They are calculating what http://img688.imageshack.us/img688/4590/circleruler.png" would measure, when placed at rest in the rotating frame. And this ruler is a circle, not a spiral.

Yes, but they're doing it by calculating the length of a spiral in spacetime.

That is what they do in chapter 5.1. While in chapter 5.2 they arrive at the non-Euclidean spatial geometry just trough Lorentz contraction. You don't need to consider spirals in space-time to predict what the ruler will measure. In space the ruler is just a circle and it measures spatial distances, which determine the spatial geometry in the ruler's rest frame.

The controversial part is to use the term "spatial geometry" about the geometry of a surface that isn't "space".

So in your opinion, rulers at rest in the rotating frame don't measure "spatial geometry" in that frame ? Fine, we can use the term "proper spatial geometry" for what these co-rotating rulers measure, in analogy to "proper length" which is measured by a co-moving ruler.

For me this "proper spatial geometry" is the physically relevant spatial geometry:

If I want to build a huge structure near a massive object, I have to the take the non-Euclidean spatial geometry around the mass into account, when calculating the lengths of the structure's segements.

Analogously:

If I want to build a fast rotating structure, I have to the take the non-Euclidean "proper spatial geometry" in the rest frame of the structure into account, when calculating the lengths of the structure's segements.

 

[bcrowell]

This paper http://www.phys.uu.nl/igg/dieks/rotation.pdf gives a very nice treatment that I think is both more transparent than Gron's and more directly related to the question that Fredrik and A.T. are debating. There are problems with applying ruler measurements directly to this case, because the rulers are subject to Coriolis and centrifugal forces. You might say that this is no big deal, because we can just use nice, rigid rulers. However, there is a relativistic limit to how rigid the rulers can be. (If they were perfectly rigid, then vibrations would propagate along them at v>c.) If you bring your rulers out to r=c/\omega, then their velocity relative to the axis equals the speed of light, which is impossible. Physically, they must be torn apart by centrifugal forces before they get there, even if they are as rigid as relativity allows any material object to be. Even supposing maximum-rigidity rulers, you are going to get dynamical effects at smaller values of r, and therefore you can't use rulers to measure the spatial geometry quite as directly as A.T. is claiming, or as Einstein believed in 1912. The way to get around all these issues is to use radar measurements to establish the spatial geometry, and that requires clock synchronization.

Another way of getting at this is suggested by Wald, near p. 119. When we want to split the metric into separate spatial and temporal parts, with the form (\ldots)dt^2-(\ldots)dx^\mu dx^\nu, that means we're claiming the spacetime can be put in what's technically known as static (as opposed to stationary) form. Static is more strict than stationary. All static metrics have to have time-reversal symmetry. An example that Wald gives is that a rotating fluid can't have a static metric applied to it. If you want to put material objects like rulers on the disk, they're analogous to the rotating fluid. They have their own stress-energy tensors, etc. You clearly don't have time-reversal symmetry, and therefore you can't measure a static metric using material objects. Again, this can be sidestepped by not using material objects to measure the geometry, but then you have to do clock synchronization.

 

[A.T.]

This paper http://www.phys.uu.nl/igg/dieks/rotation.pdf gives a very nice treatment that I think is both more transparent than Gron's and more directly related to the question that Fredrik and A.T. are debating.

That's the reference from post #2. It also describes the spatial geometry in the rotating frame as non-Euclidean (Chapter 6) :

The spatial geometry defined by the line element (5) is non-Euclidean, with a
negative r-dependent curvature

And if you do a search on "non-Euclidean spatial geometry rotating frame" you find a lot of references using this interpretation, way back to Einstein:
http://books.google.de/books?id=DH7...idean spatial geometry rotating frame&f=false

There are problems with applying ruler measurements directly to this case, because the rulers are subject to Coriolis and centrifugal forces.

The rulers are supposed to be Born rigid as described here: http://books.google.com/books?id=Iy...q=Let us now see how the non-Euclidean&f=true
Which I guess means they are not subject to inertial forces?

BTW: Coriolis force for rulers at rest in the rotating frame?

The way to get around all these issues is to use radar measurements to establish the spatial geometry, and that requires clock synchronization.

Can you synchronize rotating clocks along the same r-coordiante and then radar-measure the circumference at r?

 

[bcrowell]

Re #38 by A.T. -- Ah, thanks for pointing out that you'd already posted a reference to the Dieks paper. I wish I'd paid more attention to you and read it earler :-) I agree with you that the spatial geometry is non-Euclidean -- were you under the impression that I disagreed on that point? Unfortunately the books.google.com links aren't helping me much; their software is blocking me from seeing the relevant parts of the Rizzi anthology and the Gron book, presumably because they want to make sure I don't see too much through the keyhole without paying money. (Well, the Rizzi anthology is only $359 on amazon; maybe we should all buy copies.) Because of that I'm not able to make much of this: "The rulers are supposed to be Born rigid as described here: [...] Which I guess means they are not subject to inertial forces?"

Re Born rigidity, the information I have available is in the Gron paper and the WP article, http://en.wikipedia.org/wiki/Born_rigidity . The thing to realize is that Born rigidity isn't a physically possible attribute of real objects. E.g., the Gron Am. J. Phys. paper (p. 872) says:

"By definition a Born rigid motion of a body leaves lengths unchanged, when measured in the body's proper frame. As made clear by Cavallieti and Spinelli, and by Newburgh, a Born rigid motion is not a material property of abody, but the result of a specific program of forces designed to set the body in motion without introducing stresses. The result of the analysis given above shows that a transition of the disk from rest to rotational motion, while it satisfies Born's definition of rigidity, is a kinematic impossibility. This is the kinematic resolution of Ehrenfest's paradox."

I think you may be under the impression that I'm taking sides with Fredrik in the debate you two have been having. Actually there are some points where I agree with you, and some points where I agree with Fredrik. I agree with you about the non-Euclidean spatial geometry, and that mathematical descriptions need operational definitions to tie them to physical reality.

BTW: Coriolis force for rulers at rest in the rotating frame?

I think Coriolis forces are at least potentially relevant here. If you want to form an operational definition of non-Euclidean geometry in this situation, using rulers, then you have to have some way of comparing radial and azimuthal distances. This requires rotating rulers, and then Coriolis forces will compress or expand the rulers, depending on whether you rotate them in the same direction as the disk's rotation or the opposite direction. If you rotate them slowly enough, you're probably okay, but this is an example of how you really can't get away with ignoring the dynamics of the rulers. The Coriolis force is also what prevents you from transporting a ruler past r=c/\omega along a radial line.

 

[Fredrik]

...and that mathematical descriptions need operational definitions to tie them to physical reality.

You're saying this as if it's something you expect that I'd disagree with. If there's anything in what I've said that suggests that I would, it was 100% unintentional. I was just objecting to the idea that mathematical objects can be defined by statements about physical objects. That's what A.T. seemed to be doing.

An operational definition is something else entirely. It's actually a poorly stated axiom of a theory of physics. For example, the statement "time is what you measure with a clock" is often described as an operational definition of "time", but it isn't really an attempt to define a term. It's an attempt to explain how something in the real world corresponds to something in the mathematical model. To really do that, we at least have to be precise about what mathematical quantity we have in mind. This is how I would say it: "A clock measures the proper time of the curve in spacetime that represents its motion". It's misleading to characterize this statement as a "definition". It's an axiom of a theory of physics.

The statement I used as an example is one of the axioms of special relativity. We clearly need a similar axiom about length measurements, but it's surprisingly hard to state such an axiom in a satisfying way. It's hard enough to write down an axiom that's valid for measuring devices doing inertial motion, and I have no idea what an axiom that's valid for measuring devices in an arbitrary state of motion would look like. I definitely haven't seen one.

That last part is the main reason why I don't like A.T.'s approach. He talks about this stuff as if it's trivial, and it certainly isn't. When we use a method that neither of us understands, the result is likely to be wrong. The standard axioms are however perfectly clear. The geometry of a set of simultaneous events (i.e. "space" at some time t) is Euclidean. I really don't see the point of defining a hypersurface that consists of a bunch of spirals in spacetime and call it "rest space", just so we can describe its geometry as "spatial geometry".

 

[A.T.]

I agree with you that the spatial geometry is non-Euclidean -- were you under the impression that I disagreed on that point?

No I didn't want to imply this. I just wanted to point out that it is something I read in several sources.

The thing to realize is that Born rigidity isn't a physically possible attribute of real objects.

The way I understand it: It is an idealized ruler. When measuring with a real ruler you would have to account for the elastic deformations, to calculate the result of the idealized ruler.

Gron Am. J. Phys. paper (p. 872) says:
"The result of the analysis given above shows that a transition of the disk from rest to rotational motion, while it satisfies Born's definition of rigidity, is a kinematic impossibility.
"

Two points on this:

1) Do we have to use a solid disk? I proposed a http://img688.imageshack.us/img688/4590/circleruler.png" , which can change it's proper circumference without introducing any tangential stresses. I think it could satisfy Born's rigidity (in the tangential direction) without a kinematic impossibility.

2) Do we have to care about the transition from rest to rotational motion? We could build the rotating ruler from small Born's rigid parts in the rotating state already.

This requires rotating rulers, and then Coriolis forces will compress or expand the rulers, depending on whether you rotate them in the same direction as the disk's rotation or the opposite direction.

Okay, that is an issue during transport of the rulers. But once they are at rest in the rotating frame the only problem I see is the centrifugal force. But who says that the centrifugal force has to be countered by the rigidness of the ruler? You could support the structure with small rocket engines facing outwards. The rigid parts from point (2) above don't even have to be fixed to each other, they just use their rockets to form a rotating circle. Then you measure the circumference by counting how many of them you needed.

Another idea: making r very large and omega small (while still keeping a relativistic tangential velocity), should make the centrifugal force sufficiently small.

But all this thinking about how to deal with inertial forces on the rulers seems a bit weird: Don't we have to assume massless rulers anyway? Because otherwise the ruler would also produce gravitation, that could counter the centrifugal force. But we don't want our measuring device to change the scenario and introduce non-Euclidean geometry by it's own mass.

 

[A.T.]

I really don't see the point of defining a hypersurface that consists of a bunch of spirals in spacetime and call it "rest space", just so we can describe its geometry as "spatial geometry".

If you don't like using the name "spatial geometry" for it, then it is just about semantics. I didn't make this name up.

But I understand the pragmatic reason why this is considered the "spatial geometry in the rotating frame" by many authors, way back to Einstein: The physical consequences of this "rest space geometry" are the same as those in other cases that involve non-Euclidean spatial geometry (e.g. due to a massive object).

 

[bcrowell]

A.T. -- The thrust of your #41 is that you're proposing a variety of ways of handling the dynamics of the rulers. I would make the following general comments:

This is likely to be extremely difficult and complicated. E.g., the WP article on the Ehrenfest paradox has the following: "1981: Grøn notices that Hooke's law is not consistent with Lorentz transformations and introduces a relativistic generalization." You're going to run into lots and lots of issues like this. It seems like your original motivation for the treatment using rulers was that it seemed conceptually simple, but it looks to me like it is actually much more conceptually complicated than the treatment using light rays.

I have no doubt that it is possible, by picking crafty approximations, to make the dynamical treatment of the rulers work, to some approximation, and that within this approximation you will get the non-Euclidean spatial geometry that we've all been convinced was right ever since Einstein first thought about the example in 1912.

I suspect that the ruler method can achieve either one or the other, but not both, of the following: (1) an exact result, or (2) a method that avoids clock synchronization. The reason is that material rulers can't be used at r&gt;c/\omega, so it takes quite a leap of faith to imagine that they could be made to work perfectly at r=(.999)c/\omega. I suspect that if you wanted to use rulers at (.999)c/\omega, they'd be under so much strain that in order to correct for all the dynamical effects you'd need an explicit model of their behavior in terms of relativistic quantum mechanics. But it seems unlikely to me that a QED model of a ruler can be carried out without dealing with time as a variable, which would obviate the goal of avoiding clock synchronization.

Re Born rigidity, do you have access to the Gron Am. J. Phys. paper? The main point of the paper is that Born rigidity is a kinematical impossibility. E.g., when you say, "You could support the structure with small rocket engines facing outwards," this is exactly the kind of thing that Gron is proving is kinematically impossible, and it's kinematically impossible because of issues relating to clock synchronization.

 

[atyy]

Doesn't the ruler method does involve clock synchronization to define the radial direction, ie. the radial line is the line along which clocks can be synchronized?

 

[bcrowell]

Doesn't the ruler method does involve clock synchronization to define the radial direction, ie. the radial line is the line along which clocks can be synchronized?

You can synchronize clocks along a non-straight curve that connects the axis to an off-axis point. You just can't do a global synchronization without discontinuities.

I think the ruler method can be used to define a radial line as the shortest curve connecting the axis with an off-axis point. This does assume that you can locate the axis using nothing but static ruler measurements, but I think that is possible. The Ricci scalar curvature of the spatial metric (which I guess is probably some constant multiple of the Gaussian curvature?) is R=6/(r^2-2r^2+1), where \omega=1. So since you can determine R with static ruler measurements, I think you can locate the axis by looking for where R has a local minimum value of R=6.

 

[A.T.]

The thrust of your #41 is that you're proposing a variety of ways of handling the dynamics of the rulers.

Yes but I end with:

Don't we have to assume massless rulers anyway? Because otherwise the ruler would also produce gravitation. But we don't want our measuring device to change the scenario and introduce non-Euclidean geometry by it's own mass.

Doesn't this make all discussions about problems with inertial forces acting on the rulers kind of pointless?

The reason is that material rulers can't be used at r&gt;c/\omega, so it takes quite a leap of faith to imagine that they could be made to work perfectly at r=(.999)c/\omega.

To determine that the spatial geometry is non-Eclidean it would suffice if they worked at r=(0.1)c/\omega.

Re Born rigidity, do you have access to the Gron Am. J. Phys. paper? The main point of the paper is that Born rigidity is a kinematical impossibility. E.g., when you say, "You could support the structure with small rocket engines facing outwards," this is exactly the kind of thing that Gron is proving is kinematically impossible, and it's kinematically impossible because of issues relating to clock synchronization.

I will have a look at that. What is the exact title? Or can you summarize his argument?

BTW, This chapter is by Gron as well:
http://books.google.de/books?id=DH7...idean spatial geometry rotating frame&f=false
It deals also with methods to synchronize clocks along a circumference in the rot. frame.

 

[atyy]

Won't massless rulers travel at the speed of light?

 

[atyy]

You can synchronize clocks along a non-straight curve that connects the axis to an off-axis point.

Interesting!

I think the ruler method can be used to define a radial line as the shortest curve connecting the axis with an off-axis point. This does assume that you can locate the axis using nothing but static ruler measurements, but I think that is possible. The Ricci scalar curvature of the spatial metric (which I guess is probably some constant multiple of the Gaussian curvature?) is R=6/(r^2-2r^2+1), where \omega=1. So since you can determine R with static ruler measurements, I think you can locate the axis by looking for where R has a local minimum value of R=6.

Operationally, how is a particular "off-axis point" identified?

 

[A.T.]

Won't massless rulers travel at the speed of light?

I'm just pointing out, that if placing rulers at rest in the rotating frame is seen as problematic due to the rulers' inertia, you could just as well make a problem of the rulers own gravitation curving spacetime. And measuring distances with light doesn't help, because the energy of the light beam curves spacetime as well.

 

[atyy]

I'm just pointing out, that if placing rulers at rest in the rotating frame is seen as problematic due to the rulers' inertia, you could just as well make a problem of the rulers own gravitation curving spacetime. And measuring distances with light doesn't help, because the energy of the light beam curves spacetime as well.

If we assume special relativity then gravity disappears. Not sure about inertial forces though.