David Bowman's excellent answer to Tom Wayburn's questions leaves
one thing to be said which David takes for granted, but which is
not understood by many who ask questions in this group. I think
it deserves emphasis, so I'll ride this particular torch-bearing
hobby horse a bit farther into the gloom in the hope that I can
cast light into the thoughts of many without stifling any active
imaginations out there.
The problem that frequently arises, when I get students fired up
about some aspect of Nature at the fringes of observability, is
that of hypothetical unphysicality. A student will ask, in the
excitement of the moment, "What would happen if..." This is
followed by some inaccessible situation like "...the Sun suddenly
vanished" or "...we were to look at this from a billion light
years outside the universe" or "...if I observed this while
moving at the speed of light." It is unreasonable to expect
anyone, and least of all a physicist, to answer such questions by
employing physical principles when the questions themselves posit
unphysical conditions.
Now Einstein is said to have been led to his special theory of
relativity by pursuing just such a line of thinking, asking
himself what would happen as he looked into a mirror while moving
at the speed of light in the direction of his gaze. Evidently he
never did answer this question satisfactorily; it has an
unphysical hypothesis, of course. It was the recognition that the
question was itself unphysical that led to relativity, not the
answer to that question.
I always try not to discourage a student from asking questions;
questions are an essential ingredient even of my lectures to 200
or more students. After all, if the element of interactivity is
removed from the lecture, how does the lecture differ from a much
cheaper, slicker, potentially error-free presentation on
videotape? Nonetheless, when I get a question like those I
describe here I always require it to be recast with physically
respectable Gedankenconditions. It may make me come across as a
picky old fellow, but it is necessary that the student understand
why I require it.
Leigh
(My attitude to Star Dreck has always been negative. I guess
being asked to suspend disbelief in several different ways each
episode convinced me that the writers were lazy hacks who felt
not at all constrained by physics. Suspension of disbelief in
superluminal travel, local nonconservation of matter, and time
travel in most episodes, enhanced by telepathy and other ad hoc
atrocities as suit the writers in the moment is just too much for
my physical sensibilities. Roadrunner has greater claim to
verisimilitude than Star Dreck!)