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*From*: santarel@andromeda.rutgers.edu (Vincent Santarelli)*Date*: Tue, 2 Apr 1996 16:26:42 -0400

Dear Stanley,

Let b=v/c = 0.95 and a=1/sqrt(1-b^2) = 3.2026 and l = 200 m, the proper

length of the ship.

Then there are two ways to get the right answer:

Method A: simple physics in the earth frame

Method B: the Lorentz transformations.

Method A: A spaceship of length l'=l/a travels a distance vt while the

light beam travels a distance ct = vt + l'. Solve for t.

Method B: x = a(x'+ vt') and t = a(t'+ vx'/c^2) are the Lorentz

transformations from the ship frame to the earth frame. Let the tail of the

ship be at x'=0 at t'=0. and interpret the above equations as space and

time *intervals*. Accordingly set x' = 200 and t' = l/c, and solve for t.

Both methods give 4.16 micoseconds. You're right, the book is wrong.

Vince

Vince Santarelli santarel@andromeda.rutgers.edu

Physics Department, Smith Hall, Rm.366 OFFICE: (201) 648-5072

Rutgers University, Newark Campus DEPT: (201) 648-5250

Newark, NJ 07102 FAX: (201) 648-1434

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