🚀 go-pugleaf

I am shocked by the idiocies Tom Roberts has written below. It turns
out Roberts is sillier than his zombies, much sillier than Dono in
particular. I should not have paid so much attention to zombie
Roberts' writings.

Pentcho Valev
pvalev@yahoo.com

Dono wrote in sci.physics.relativity: There is a large class of
thought experiments (the pole in the barn being one of them) that rely
on the misguided idea that length contraction allows larger objects
(like the pole) to fit inside smaller enclosures (like the barn) as a
byproduct ofrapid relative motion. This is not the case.

Tom Roberts wrote: I assume you intend "smaller" and "larger" to refer
to objects' PROPER length, along the axis of their relative motion.
Then you are wrong. The "pole and barn" gedanken is valid. Let me
phrase it without any accelerations -- a pole of length 2L can fit
inside a barn of length L, if it is moving faster than 0.866 c
relative to the barn, in this way: With pre-arranged timing in the
barn frame, one could start with the front door closed and the rear
door open, and then close the rear door before opening the front door
-- so in the barn frame both doors can be closed with the pole inside
(opening and closing must be VERY fast (:-)). In the pole frame, the
front door opens before the rear door closes, and the pole is never
wholly inside the barn (which appears to be shorter than the pole to
the pole observer). This example is every bit as much a demonstration
of the relativity of simultaneity as it is of length contraction.

Dono also said: if you close both doors simultaneously in the barn
frame, you have trapped the pole inside the barn. This can only happen
if the pole has become physically shorter than the barn.

Tom Roberts: This is confusing, if not wrong -- you are using a PUN on
"physically". Yes, the pole itself is unchanged by measurements
performed in the barn frame, or doors closing, etc. But the PHYSICAL
relationship between front end of pole and front door, and the
PHYSICAL relationship between rear end of pole and rear door, can be
such that in the barn frame the pole fits in between the two closed
doors for a (very) brief interval of time. At least in a gedanken.
Consider these two spatially-separated events: A) front door opens
with front end of pole still inside barn B) rear door closes with rear
end of pole already inside barn It is possible, given the conditions
of the gedanken, for B to precede A in the barn frame; in the pole
frame A precedes B.

Dono also said: the only way the pole would fit inside the barn is if
the pole got compressed.

Tom Roberts: This is not true. The way the pole can fit into the barn
when moving rapidly is that the time-ordering of spatially-separated
events is frame dependent: in the barn frame B precedes A, but in the
pole frame A precedes B. There's no need for the pole to be
"compressed". As I have often said around here, attempting to argue
via exhaustive enumeration nearly always fails. It did for you in this
case. TomRoberts