

From: George Murphy <>
Date: Wed Sep 17 2008 - 17:23:09 EDT

1) The electrons don't see the pipe as being a few km long. If they're quickly accelerated close to c, it's a lot shorter for them.

2) "Longitudinal mass" is a measure of a particle's inertia - i.e., how hard it is to change its speed &/or direction - if it's to move in a straight line. "Transverse mass" is a measure of its inertia if you try to accelerate it at right angles to its motion - i.e., make it move in a circle. Of course in general the acceleration will have components both tangent to & perpendicular (transverse) to the particle's motion.

But again, that's a somewhat old fashioned approach.

3) I still haven't checked Ley's book. But while you wait, here's another good general physics problem for the list. If you've got a 2 stage rocket, do you separate & fire the 2d stage when the 1st has exhausted its fuel or when the combination has reached apogeee (the top of its trajectory)? & why? First pair of correct answers wins a table of even primes.

  ----- Original Message -----
  From: George Cooper
  Sent: Wednesday, September 17, 2008 12:39 PM

   Hi George,


  [George wrote] 1) Here's an amusing, though relativistically simple, question. (In fact I was asked it during my grad school oral comp.) Since the source for electrons at an accelerator like SLAC are emitted at random, & since the target is a few km away down a pipe only a few cm in diameter, how does any significant fraction of those electrons manage to make it to the target?


  I would assume the huge mag. field controls their vector, but that is too obvious an answer, so..?


  2) A slightly pedantic comment on "increase of mass with velocity": That is an old-fashioned, though not "wrong," way to speak about relativistic dynamics. But if it's to be used it's necessary to remember that in such a formulation the "longitudinal mass" of a particle differes from its "transverse mass." The former is mG^3 while the latter is mG, where m is the rest mass and G = 1/(1 - v^2/c^2)^(-1/2). Many popular discussions incorrectly use the transverse mass when talking about straight line motion.


  This is interesting. What is transverse mass? I can envision the contraction, but does it squash outward from the direction of travel? [Is the negative exponent correct?]


   3) On Burgy's earlier comment, I'm not sure that it was Clarke who first suggested multi-stage rockets. I need to dig out Willy Ley's book on the history.


  There seems to be a number of inventors dating as far back as the 14th century.


  I vaguely recall that it was not part of the original Von Braun program, but came in to solve the Apollo weight dilemma.






    ----- Original Message -----

    From: Don Winterstein


    Sent: Wednesday, September 17, 2008 4:44 AM



    As I recall, Larry Johnston also told us that the klystrons in the Stanford Linear Accelerator are made to operate as if the speed of electrons at injection is exactly the speed of light. If it were not, the particles would not be accelerated. "Accelerated" in this case does not mean increase in speed but in mass. In other words, the electron speed differs from the speed of light all along the 2-mile traverse by a negligible amount.




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Received on Wed Sep 17 17:24:16 2008

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