Saturday, August 12, 2017

Mach's principle

Mach's principle is fascinating. If General Relativity does obey it then GR is telling us something much more profound than just adding gravity and bendy space-time to special relativity.

Mach's principle is that our idea of what a rest coordinate frame is comes from the motion or shape of the bodies in the universe. So if all of the universe was rotating, our rest coordinate frame would also be rotating and it would be equivalent to the whole universe being at rest, as we see it now. Equivalently, if you spin around in space, your arms only separate with centrifugal force if there are stars that you are spinning relative to.

This is a wonderful idea that is telling us something more than Special Relativity. It is saying that space-time itself is only a product of mass-energy. It is not just that mass-energy warps space-time, space-time exists because of a distribution of mass. So flat (Minkowski) space-time does not result from the absence of mass, it is the approximation of space-time when mass is fixed and very far away. To put it a third way, matter doesn't sit on an infinite fabric, the fabric exists only in relating the motion of matter.

The only problem is that, even 100 years after GR, people can't seem to agree whether GR really is 'Machian'. However, on inspection it seems to me that it is:

Wheeler-Einstein-Mach spacetimes

An important paper is Wheeler-Einstein-Mach spacetimes:

"We define the Wheeler-Einstein-Mach (WEM) spacetimes to be those which contain a closed Cauchy surface, are inextendible, and satisfy field equations with a well-posed Cauchy problem, We show that a WEM spacetime can be reconstructed from the "York data" on any given closed (constant mean curvature) hypersurface contained in that spacetime. This result is the strongest and most precise statement to date of Wheeler's version of Mach's principle."

While the Machian view is that the inertial frame can be reconstructed by the distribution of mass-energy in the universe, the authors don't argue over exactly what this means:

"To decide, within these prescribed limits, exactly what should be labeled "mass energy" and used as the Machian input, we eschew philosophical considerations and choose what gives us a nice theorem, i.e., we choose the York data"

they just use the York data:
"Define the "York data" Q, of a given spacelike hypersurface S is composed of the conformal intrinsic geometry, the conformal (transverse traceless) extrinsic geometry, the conformal nongravitational and the mean extrinsic curvature scalar"  (paraphrased)

So based on this, they have found that this large class of GR space-times show no rotation of their universe from their preferred (rest) coordinates. Which is consistent with the idea that your coordinates rotate with the universe:

"Absolute rotation: None of the WEM spacetimes has an absolute net rotation in any well-defined sense."

People have cited the Godel universe as a counter-example, but this paper refutes it clearly:

"Local vorticity (of the sort found in the non-WEM Godel universe and also in the WEM Ryan spacetimes) does not constitute net rotation of the universe in any sense."

The idea that the Godel universe is local vortices is backed up by the wikipedia article on the Van Stockum dust solution:
"Note that unlike the Gödel dust solution, in the van Stockum dust the dust particles are rotating about a geometrically distinguished axis"

Frame Dragging

In the Machian view, the coordinates would rotate with a galaxy if there were no other in the universe. However, since there are others far away, it makes sense that coordinates would rotate partially with the galaxy, and to a lesser degree as one moves out towards the rest of the universe. This does happen and is called frame dragging or the Lens-Thirring effect. Again supporting the Machian idea. 

The linear acceleration equivalent is described in this very clearly written paper, again supporting the Machian principle.

Rotating Mass Shell

Also there is this paper: Induction of correct centrifugal force in a rotating mass shell
It shows that for a correctly shaped shell over fixed Minkowski space, the flat space-time inside the shell will rotate with the shell:

"Mach’s idea of relativity of rotation is confirmed for a shell-type model of the universe by showing that flat geometry in rotating coordinates, realising correct Coriolis and centrifugal forces, can be continuously connected through a rotating mass shell with not exactly spherical shape and latitude-dependent mass density to an asymptotically Minkowskian outside metric. The corresponding solutions of Einstein’s field equations are given to second order in the angular velocity w but it is plausible that the problem has a solution to any order of w"

I find this a very clear vindication of the principle. Even though it requires a specific shell shape, I expect that requirement becomes less sensitive if you replace it with a more disperse and distant universe.

Ozsvath-Schucking space-times

This metric is also cited as a refutation of Mach's principle. The space-time seems to be parallel gravitational waves. It is said to be an anti-mach metric because it contains no mass but is not just flat Minkowski space. This seems like a weird reason to claim to be non-Machian, for three reasons:
  1. Mach's principle doesn't claim that a universe lacking matter should be Minkowski, since that would preference inertial (rather than nonlinear) coordinates. It claims that a universe lacking matter has completely unconstrained rest coordinates.
  2. As discussed in the WEM spacetime paper above, the problem with infinite space-times is that they exclude a light-cone at infinity, and this may well assert a constraint on the rest frame. Hence the WEM spacetimes are compact.
  3. Mach's principle that the rest frame is derivable from the mass distribution alone was before anyone knew about gravity waves, it seems OK to me to include the dynamics of these waves in resolving the universe rest frame. Besides, gravity waves are a form of non-localised energy. The principle can remain, that if there is no mass or energy (including gravitational waves) then there can be no preferred rest frame (linear or otherwise).


So the way I think of it is that space-time stretches between the masses and so it is impossible to notice any overall motion of the full set of masses. For example if all of the stars in the universe were actually oscillating laterally all at the same time with an amplitude of one metre, then space-time would also oscillate with it, and so the motion would be unobservable, and this goes for any motion. Note that you don't have to invoke magic for this, you need to attach rockets to all the stars and they will give positive and negative pressure values in the stress-energy tensor which bend space-time correctly to physically produce the oscillation.

So while Newton's relativity and Special Relativity both have no privileged inertial frame (linear velocity), in GR there is no privileged coordinate frame at all, linear or otherwise. Nature is not preferring linear motions, only geodesic ones. And ultimately our idea of what it means to be at rest comes down to a sort of average of the stars around us. Everything is indeed relative.

The main problem that seems to prevent people from accepting the Machian principle is that Minkowski spacetime prefers zero angular velocity and acceleration. Einstein understood this to be a problem with the metric at infinity, and that it is required to be constant. The issue with Minkowski spacetime is also expressed in this paper, quoting:

"The necessity of introducing an extended model of the Minkowski spacetime, in which a globally empty space is supplied with a cosmic mass shell with radius equal to its own Schwarzschild radius, in order to extend the principle of relativity to accelerated and rotational motion, is made clear."

"In the general theory of relativity the significance of the Minkowski spacetime is that it is used as the asymptotic metric outside a localized mass distribution, for example in the Kerr spacetime. This means that absolute rotation is introduced into the general theory of relativity through this choice of boundary condition when solving Einstein’s field equations."

and here supports the idea that there will be perfect inertial dragging for our universe:

"A meaningful boundary condition for flat spacetime is to introduce a massive shell that represents the cosmic mass inside the shell. As shown in the previous section the mass inside the lookback distance of our universe has a Schwarzschild radius equal to the lookback distance. Hence, it is natural to impose the boundary condition that the asymptotically empty spacetime is replaced by the boundary condition that there is a mass shell at the lookback distance with radius equal to its own Schwarzschild radius and mass equal to the cosmic mass inside the lookback distance."


Another point of contention is how Mach's principle effects the inertia or mass of objects. This is worth investigating, by looking at how linear and angular motion changes as the mass of the remaining universe drops down towards zero. In each case we can either have the background universe exclude the observer's mass (a) or include it (b).

Linear oscillation

In this case we want to oscillate by applying a sinusoidal force of fixed amplitude. The observer is pictured as the large sphere and the remaining (background) universe is simplified to just a single small sphere.
(a) In this case, as the observer applies its sinusoidal force (e.g. by emitting radiation, of zero mass), its large mass compared to the rest of the universe causes substantial frame dragging and the universe mainly follows the same sine wave. Consequently, the observed motion relative to the background universe has reduced.  

The rest point in this set-up is I think roughly at the centre of mass of the two bodies, which is much closer two the observer body. As such, the motion of the two bodies relative to the centre of mass looks like (with amplitudes scaled up):
(b) Relative to this fixed point, the motion is also smaller than for an infinite mass universe. 

In both cases the motion is less than expected and the observer may think that his mass has increased. For zero background mass, the lateral forces cause no motion.

Angular motion

(a) For a low mass background universe any angular torque will cause the background universe to rotate somewhat with the observer, so a larger torque is needed to attain a target angular velocity. However, the frame at the observer will rotate mostly with the observer, so the proportion of this relative angular velocity that causes centrifugal acceleration on the observer reduces.

(b) We define the rest frame as being the point where frame dragging of observer and universe cancel out (see pale blue lines). In coordinates where this cancel point is at rest, the torque required to achieve an angular velocity relative to this rest frame increases with decreasing universe mass. In this case, the rest point gets closer to the observer as the background universe mass decreases. So once you achieve the required angular velocity relative to the whole universe, your centrifugal acceleration would probably be similar as the background mass decreased. 


While it may appear to the observer that its mass and/or inertial moment increases with a diminishing background universe, this is not really the case. What is happening is that the effective mass/inertia are changing because these properties are relative to the universe's mass/inertia. As long as effective mass/inertia and absolute mass/inertia are conceptually distinguished the discussion of inertia remains clear.
In both choices of rest coordinate (a) and (b) the effective mass increases because forces applied to the observer act increasingly and oppositely on the universe rather than the observer as the universe mass diminishes.

Both examples demonstrate Mach's idea, that inertia only exists in the presence of a background universe.

The centrifugal force question is a bit more subtle. You certainly would feel less centrifugal force as the star masses decreased if you measured your rotational velocity relative to the stars, but if you measured it relative to the rotation of the universe including yourself, then centrifugal acceleration may well stay constant. What reduces is your ability to achieve that angular velocity, in other words your effective moment of inertia increases. 

Scale Dragging and Dark Energy

Is it possible that the expansion producing negative energy termed dark energy is in fact the apparent opposite action on the universe due to contractive frame dragging?

Imagine that the universe is a scattering of stars with no initial velocity, then they will contract under gravity. But if the entire universe is contracting then I would expect frame dragging to be contracting the reference frame as well. Therefore, relative to this reference frame the universe should have no net contraction. In fact, if the galaxies contract less than clusters due to their spinning, then the rest frame is contracting faster than the actual contraction of the galaxies, and in the rest frame we will see the universe as expanding. The process known as dark energy. 

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