[jeffreyH]

I am going to say something else on this matter. Say you had a lattice of photons that you decided were going to be your fixed background. How can you determine that the whole lattice is not moving in a particular direction? That the paths of the photons are in fact not those of an absolutely stationary frame. If the local frame is inertial then it is not possible to tell since the laws of physics are the same for all inertial frames of reference.

[Bill S]

I am going to say something else on this matter. Say you had a lattice of photons that you decided were going to be your fixed background. How can you determine that the whole lattice is not moving in a particular direction? That the paths of the photons are in fact not those of an absolutely stationary frame. If the local frame is inertial then it is not possible to tell since the laws of physics are the same for all inertial frames of reference.

Agreed, but if your lattice of photons is in an infinite void, or any infinite "space-like entity", each photon must be perceived as occupying a central position, so differentiating between them, in terms of position, is possible only within your artificially constructed IRF. Beyond that, how is it possible to determine motion if you cannot determine position?

[evan_au]

How about still in the sense that the tennisball observes no net redshift or blueshift of the cosmic microwave background radiation?

There is a slight asymmetry in the CMBR, equivalent to a Doppler-shift of Earth traveling about 600 km/second through space. This could be due to motion of our galaxy within the local galaxy cluster.

So I guess that you could use today's very precise measurements of the CMBR as a "common" frame of reference. Not that it would make any difference to the results of any experiments that you (or anyone else) does in their laboratory - as Einstein showed, all inertial frames are equivalent.
 
See: https://en.wikipedia.org/wiki/Cosmic_microwave_background#CMBR_dipole_anisotropy

[guest4091]

If motion is relative, then stillness must also be relative.
I maintain the idea of abs. rest is a result of the definitions by Newton using an  over-simplified system of two valued logic, or complementary attributes, eg. hot-cold, light-dark, etc.
In relativity, two objects can be at relative rest, if both have the same velocity.
It seems contradictory, they are moving and at rest in simultaneous observations,  (depending on who is looking).
That would be my revised definition of rest, i.e. everything is in motion in a dynamic universe.
As in fuzzy logic, when measuring one attribute, you are indirectly measuring the complementary one.
Motion can be measured over a range of values, stillness is its complement.
Abs. rest is a special case of zero motion. You can have one or the other, but not both, a case of mutual exclusion. With a container, there is fullness and emptiness, by degree, i.e. complementarity. Full and empty are mutually exclusive.

[Bogie_smiles]

I would like to weigh in on this one. No, as I understand the universe, nothing can be still in space.

If we could strip space clean, and have emptiness, I think I would call that stillness, but that is not the same as something being still in space, is it.

[jeffreyH]

I am going to say something else on this matter. Say you had a lattice of photons that you decided were going to be your fixed background. How can you determine that the whole lattice is not moving in a particular direction? That the paths of the photons are in fact not those of an absolutely stationary frame. If the local frame is inertial then it is not possible to tell since the laws of physics are the same for all inertial frames of reference.

Agreed, but if your lattice of photons is in an infinite void, or any infinite "space-like entity", each photon must be perceived as occupying a central position, so differentiating between them, in terms of position, is possible only within your artificially constructed IRF. Beyond that, how is it possible to determine motion if you cannot determine position?

Exactly! Position, or more correctly displacement, has to be relative. Therefore, you can never define an absolute position. Your position relative to what? Otherwise the constancy of the speed of light in vacuum would be violated.

[jeffreyH]

Let's say we have a position P in spacetime that we know with absolute certainty is stationary with respect to the rest of the universe. We travel past P for one light second and mark another absolute position to show how far light travels for us in 1 second. Other travellers do the same. Their marks are all at different distances to ours since their velocities differed as they passed point P. Since quantum mechanics shows position to be a continuum, these points form an infinite series. Thus we see an infinite amount of variation in the perceived speed of light. The traveller who is stationary at point P would see an instantaneous speed for light. Since he has not moved. However, in relativity it is not possible to tell you are moving or not in an inertial frame of reference. So it can easily be seen that relativity breaks down if we can define a fixed background.

[Bill S]

Great response; even I can follow that!

Would it be reasonable to say that it might be possible for an object, or person, to be absolutely still; but the conditions that would make it possible would be atypical and there would be no way to perceive it, or the resulting "stillness"; so one might as well say that it cannot happen?

 WYSIWYG - WYDSIWYDG. - or something!

[jeffreyH]

No because everything is relative. I have wrestled with the notion of an absolute frame for quite a while now. I have convinced myself through study that it cannot exist. There may still be a frame for which the stationary observer above would hold and his speed of light would be the 'true' speed of light. However it will be impossible to detect it. It would also have more than one but not all points in spacetime.

P.S. You are then talking about the interface between dark energy and gravity.

[Bill S]

P.S. You are then talking about the interface between dark energy and gravity.

I need a bit more, there, please.  I don't see the connection. 

[jeffreyH]

If you think about it gravity is trying to pull everything together while dark energy is trying to push it all apart. At some points in spacetime these forces should exactly cancel. It is a bit like the lagrange points in the gravitational field. The problem is these points of cancellation are themselves likely to be moving position. If so then that complicates things.

[jeffreyH]

Maybe those interfaces are what protect the galaxies themselves from being expanded.