Why Einstein Proposed That Speed Of Light Is Invariable....

On May 15, 4:37*am, Tom Roberts wrote in
sci.physics.relativity:
Mike wrote:
Mike wrote:
Either the bug is dead smashed by the river or not
dead.
http://hyperphysics.phy-astr.gsu.edu.../bugrivet.html

OK. That is a rather bad gedanken, because a) it assumes seriously
impossible properties of the rivet, and b) there is no inertial frame in
which the rivet REMAINS at rest. I have no desire to discuss it because
it is so bad. That's probably why I had forgotten it.

If you want to discuss the pole-barn paradox, be sure to use the version
that has equal-length barn and pole (in their respective rest frames),
with barn door 1 initially closed and door 2 initially open. The pole
enters through door 2, and immediately after the back of the pole passes
door 2 it is closed and then door 1 is opened (VERY short delay, VERY
fast doors). So the pole sails through without ever touching either
door, yet there was an instant in the barn frame when both doors were
closed and the (shortened) pole was between them. In the pole frame, of
course, this is described as door 1 opening before the front of the pole
reaches it, and there is a period of time during which both barn doors
are open and the (unshortened) pole slides through the (shortened) barn
with both doors open; after the back of the pole passes door 1 it closes
and the pole continues out of the barn.

My point is: some things are reasonable to assume in a gedanken, and
some are not. It is reasonable to assume that doors can open and close
arbitrarily quickly, because they need not really be physical doors. But
it is not reasonable to assume a rivet is prefectly rigid, because that
is inconsistent with SR (the speed of sound cannot exceed the speed of
light, which makes a perfectly rigid object impossible). And it is not
reasonable in a gedanken to expect the student to wrestle with
accelerating frames (such as that of the rigid rivet after its head
stops by hitting the wall).

* * * * Of course in the bug-and-rivet gedanken, if one does not
* * * * assume infinitely-rigid rivet and wall, the bug is
* * * * always crushed as the rivet and wall disintegrate upon
* * * * impact. A 10-gram rivet traveling at 0.9 c would have a
* * * * kinetic energy comparable to that of a small atomic bomb.

Tom Roberts

Roberts Roberts you feel so good in Einstein zombie world don't you.
Zombies would never consider a very thin "bug" and, accordingly, a
lower, non-disintegrating speed. Yet some time ago an exception
occurred: a zombie calling himself "Dono" sudenly discovered that,
although Divine Albert's Divine theory predicts that a 80m long pole
can gloriously be trapped inside a 40m long barn, there is still
something awkward about this prediction:

http://www.math.ucr.edu/home/baez/ph...barn_pole.html
"These are the props. You own a barn, 40m long, with automatic doors
at either end, that can be opened and closed simultaneously by a
switch. You also have a pole, 80m long, which of course won't fit in
the barn....So, as the pole passes through the barn, there is an
instant when it is completely within the barn. At that instant, you
close both doors simultaneously, with your switch. Of course, you open
them again pretty quickly, but at least momentarily you had the
contracted pole shut up in your barn."

Who cares about zombie Dono's discovery now? Nobody. Even zombie Dono
himself has forgotten it and is fighting even more fiercely against
those who claim that Divine Albert's Divine Theory is an
inconsistency. Nice place Einstein zombie world isn't it Roberts
Roberts.

Pentcho Valev