steve@sri-unix (08/01/82)
It is a common misunderstanding that acceleration is a relative
thing in Relativity theory. One aspect of the Special Theory is that there are
"real" frames of reference, and "artificial" frames of reference.
A "real" frame is one that is not accelerating. An "artificial"
frame *is* (no prize) accelerating.
In other words, velocity, position, time, and mass are all
relative, but acceleration is absolute. Measurements taken in an
artificial frame (which by the theory can be deduced by the
individuals living in that frame) can determine that it
*is* an artificial frame.
Were this not the case, there would be an ambiguity in the
classic "twin" problem: a pair of twins are born on Earth. One stays
home, and the other gets on a ship which accelerates to
short-of-light-speed out to a certain point, then returns.
When they meet again, which has aged more? Were acceleration
relative, the theory would be ambiguous. Since the twin in the
ship has accelerated (4 times) he (she?) is the one in the
artificial frame, and therefore he/she has the incorrect observation. In
other words, the theory unambiguously predicts that the one that stayed
at home ages and the one on the ship does not.
When they meet, the one who stays home has aged more.
It turns out that the twin on the ship can predict the
difference, but only by assuming that his (her?) observations
(except for the observations of acceleration) are fallacious.
In other words he (she?) computes the problem as the other twin
wuld observe it - thus coming up with the "true" answer. (Well,
close enough for government work, and a damn sight closer
than if based on observations from the ship!)
It turns out that by definition an object that is rotating
is accelerating, and therefore is in an artificial frame of reference.
Therefore, the ant on the phonograph cannot make Andromeda
move faster than light simply by rotating - because his (her?)
observations are fallacious. (Of course, in theory this invalidates
every observation of modern physics because the Earth rotates, but this
can be corrected for. Lucky for us!)
It is an aspect of the *General* theory, not the Special theory
that there is no way to differentiate between an gravitational
attraction and an acceleration.
(I am a hell of a lot fuzzier about this than I am about the
Special Theory, so don't quote m)
This is manifestly false, as I will demonstrate:
One can measure "down" from two points. This defines two
mathematical lines. If they intersect "below" the two points,
then the observers are in a gravitational
field. If they intersect "above" then they are in a centripetal
field (say, Niven's "Ringworld" or some such artifact, probably
much smaller). If they do not intersect at all, then they are in
an accelerating frame, and all of their measurements are
fallacious. (Further, since *all* accelerating frames are fallacious,
this particular one has the distinction of being one in which the
individuals cannot correct their observations.)
I once read a science fiction story in which the idea that
acceleration is absolute was espoused as proof of the falsity
of the Special Theory. As with most science fiction, the
author was not attempting to be scientifically correct, but
rather to come up with a plausible rationale for changing the
status quo for his (her?) story. No slur is intended on the
author: there is no mandate on science fiction authors to
stay within current capabilities - indeed there is a mandate on them to exceed such limits (otherwise
it wouldn't be science fiction. NO FLAMING!).
I hope this clears up the question of the ant on the turntable.
Steve Den Beste
Tektronix