wrote:
You can make the end of a light beam sweep along a screen at any
arbitrary speed though - but nothing physical is actually moving faster
than the speed of light. Same with a very narrow angle pair of scissors
the crossing point can be made to advance at c (at least in
principle).

Er, no it can't. The classic mistake is to apply non-relativistic
equations of motion.

Yes. It can. Nothing physical has to move faster than the speed of
light to acheive it. The scissors do have some technical difficulties,
but they can be avoided by using a French guillotine style setup with a
suitably shallow angle of cut. A 1m wide blade with a 1/100 radian
slope moving down at c/10 will do quite well enough to make the point.

Physical geometry requires that the crossing point advances 100x faster
than the closing speed of the blades. There is no conflcit with
relativity here. Nothing physical moves faster than light and the
mechanism cannot be used to send a signal FTL. See the Relativity
Physics FAQ for details:

http://jcbmac.chem.brown.edu/scissor...lScissors.html
(see the caveat at the bottom for details of how superluminal scissors
can be made to work)

This URL also deals in part with some practicalities relating to the
question of the OP.

Regards,
Martin Brown