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[Phys-L] absolute acceleration and not velocity or position



I can hardly imagine a more appropriate topic for discussion
in this forum.

Furthermore, asking "what's the evidence" is a fine way to
approach the topic.

==============

When I chose the previous Subject: line -- absolute "motion" --
I could sense that something was wrong, so I put "motion" in
scare quotes. That was dumb. That violated some basic
pedagogical principles that I try to live by:
a) Don't be sloppy, especially when a perfectly good
non-sloppy approach is readily at hand.
b) Don't take questions too literally.

There was a question that asked about motion, but I should
not have taken it so literally. I should have transmuted
it into /three/ questions:
1) If you're asking about absolute position, there's no such
thing. This is a bedrock principle of physics, older even
than relativity.
2) If you're asking about absolute velocity, there's no such
thing. This is Galileo's principle of relativity (1632).
http://www.av8n.com/physics/spacetime-welcome.htm#sec-quote-galileo
3) If you're asking about absolute acceleration, fine, that
is important. That's a real thing.

Talking about position, velocity, and acceleration is in
every way better than talking about "motion". Putting
"motion" in scare quotes doesn't solve the problem. Sorry.

My first reply mentioned a Foucault pendulum and an inertial
navigation system. These are good answers to question (3).
They are entirely within the spirit of Galilean relativity,
because they can be done inside a moving ship, without using
any outside references.

Also, if you're asking about absolute acceleration, the
acceleration due to the daily rotation of the earth is as
good as any other acceleration.

The other answers involved annual parallax and/or annual
aberration of starlight. These are /partially/ in violation
of the idea of relativity, because they depend on an outside
reference, namely the stars. However, we can make sense of
these examples as follows: Note the following contrast:

1') A single observation of foreground stars relative to
background stars is a measurement of /relative/ position.

2') A single measurement of aberration is a measurement of
/relative/ velocity.

3') Meanwhile, the annual *variation* in the parallax or
aberration gives us an indirect measurement of the /absolute/
acceleration. With a tiny bit of effort, the proper motion
of the stars drops out of the analysis, leaving us with a
clean measurement of the absolute acceleration.

Note that the European Space Agency is getting set to launch
an amazingly accurate astrometry satellite, so the "best
evidence" for parallax is about to get a lot better.
http://en.wikipedia.org/wiki/Gaia_%28spacecraft%29