Light Sails Won't Work?

In article ,
Paul Blay wrote:
... (It is not meaningful to speak of whether it is "really"
accelerated or not; all such statements must be made relative to a
specific observer.)

Er, didn't Einstein have something to say on that subject?
/Velocity/ is only meaningful relative to a particular observer, but
"is this accelerating / not accelerating" is a meaningful question.

Actually, what Einstein had to say on the subject is precisely the
opposite of what you're suggesting. In Newtonian mechanics, acceleration
is absolute: you can work in an accelerated frame of reference only by
introducing "fictitious forces" (like centrifugal force) to fudge the
details. But General Relativity removes the absoluteness of acceleration,
by telling you that those fictitious forces could be real gravitational
forces instead, and there is no way to tell the difference. Relative
acceleration between yourself and the mass of the universe produces the
same measurable effects, in GR, regardless of who you say is "really"
moving.

Dealing with
non-inertial frames of reference gets messy.

Quite.

And *that* is unchanged in General Relativity. :-)
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

OK, those who claim that a Crook's radiometer won't work in a vacuum,
please explain why. Every time I try to argue from first principles
and symmetry, I get that they will, in principle (of course, reversed
in direction from those in a low vacuum.)

Yes, in a vacuum it will turn due to radiation pressure, in the
opposite direction from the "regular" Crooke's radiometer. But the
forces involved are smaller so you need to cut down on things like
friction. For example:

A PSSC filmloop called "The Pressure of Light" shows some experiments
such as shooting a bb at a suspended metal bar to show that a
rebounding particle imparts more impulse than a particle that sticks
(they used grease to catch the bb). They also show a Crooke's
Radiaometer. It turned the "wrong" way. By refining the experiment
with an ultra high vacuum pump and hanging the vane from a quartz
fiber they were able to show the correct effect of light
pressure.
http://www.physics.brown.edu/Studies...emo/4d2010.htm