Homework Help: Special relativity - tachyons

1. Sep 15, 2008

Shukie

1. The problem statement, all variables and given/known data
Hypothetically we could wonder what happens when two observers communicate with one
another using so-called tachyons: faster-than-light particles.

a: Draw a spacetime diagram of two observers, Bob and Boba, both at rest. Draw a tachyon
signal with a speed v = 5c from Bob to Boba.
b: Do Bob and Boba both agree that Bob sent the signal before Boba received it?
c: Now Alice passes both Bob and Boba on her way to infinity. Is it possible for Alice to
pick a speed such that for her both the emission by Bob as well as the reception of the signal by Boba are simultaneous?
d: Draw another observer for which the reception of the signal in fact happens prior to the
emission by Bob.

3. The attempt at a solution

a: No problem.
b: Yes, only another observer could observe Boba receiving the signal before Bob sent it?
c: Just a guess, (1/5)c? I have no idea why.
d: This I can do by adding another observer who is also whizzing by Bob and Boba and who receives the tachyon signal from Alice?

I attempted some solutions, but I'm feeling really lost and all I'm doing is guessing. Could someone guide me through b, c and d in a simple manner? I'm only in the second week of my first year as an undergraduate, so I really need an explanation that doesn't assume too much background yet.