Re: Relativity Allows Us To Measure Absolute Motion?


"Simon G Best" <simon.g.best@xxxxxxxxxxxxxx> wrote in message news:S9-dnbdeFpp3ltLanZ2dnUVZ8u-dnZ2d@xxxxxxxxx
Hello!

Thinking about the supposed lack of absolute motion one day, I thought:
Hang on, what about the cosmic microwave background radiation (CMBR)?
Isn't that a dead give-away?

The CMBR does a very good impression of being ideal black-body
radiation. What's more, there are only very slight variations in its
apparent temperature across the sky - except for a rather prominent bit
of Doppler shifting, that is. Measure its temperature in different
directions, and it's as if you can measure your own motion relative to
the source of the CMBR (the surface of last scattering, if I remember
correctly). But doesn't this look very much like detectable, even
measurable, absolute motion?

It puzzled me. Here we are in a universe which, we are told, does not
have absolute motion included. And yet, when we measure such things as
mean velocities of galaxy clusters, the apparent CMBR temperature in
different directions, and stuff like that, there really does seem to be
something very much like a special velocity. It's as if there is such a
thing as absolute rest.

How, in a universe free of such absolute motion, could we end up with
such a special, absolute-rest-like velocity emerging? It looks
suspiciously like the universe is blatantly contradicting the idea that
absolute motion's undetectable!

Motion relative to the frame in which the CMBR is locally isotropic
is detectable at every point in the universe but there is nothing necessarily
absolute about this. If you arrange to be in such a frame nothing special
happens (except of course the CMBR is isotropic).

Compare this with something which does seem to be absolute
such as acceleration. If you arrange to be in a non-accelerating
(inertial) frame, Newton's laws of motion take on a simpler form
that they would do in, say, a rotating (non-inertial) frame.



I also pondered the shape of the universe, and ended up considering a
simple, contrived scenario.

Imagine a flat space-time, with two dimensions of space, and one of
time. Imagine, also, that the space is toroidal, like the surface of a
ring doughnut, so that it's like the screen of that Asteroids video
arcade game from years ago. (The screen is rectangular, and when you go
off the left hand side, you come back on the right hand side, and when
you go off the top, you come back on the bottom. It wraps around in
those two ways.) Keeping it simple, this space-time is flat, Euclidean,
no gravity, no curvature. We can just use Special Relativity.

Note that the spacetime of SR is not Euclidean.

Now imagine laying down a line of measuring rods, all of equal length,
just as Einstein might. Eventually, because of the orientation of these
rods, we end up back where we started. We also place synchronised
clocks at the ends of these measuring rods, where they meet.

Now, next to that line of rods and clocks, we move at high speed
relative to them, in a parallel direction. As we move, we lay down rods
and clocks, like before, but moving with us. Eventually, we'll wrap
round, and get back to where we started.

Lorentz transformations and all that, and we find that we needed more of
the moving rods before we got back to where we started. Also, while all
the moving clocks were synchronised relative to the moving rods as we
laid them down, we find that the last clock and first clock are
unsynchronised when compared via the last rod. But, when comparing
those two clocks the long way round, we find they're synchronised.

So what?

Well, there's one, special velocity in that space-time at which the
minimum number of rods would be needed, and at which all such clocks
would be synchronised, regardless of which route along the rods they're
compared along.

While Special Relativity itself doesn't provide any such special
velocity, the combination of Special Relativity and a suitable
space-time does involve such a special velocity. It's as if Special
Relativity itself isn't enough to establish absolute motion, but the
combination of Special Relativity and a suitable space-time can
establish something that looks at least a bit like some kind of absolute
motion. What's more, Relativity enables us to measure velocities
relative to that special velocity.

Am I now a heretic? :-D

No, I think you are just confused about relativity, I am not sure where to start.


--
Martin Hogbin



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