Question

The positive kaon (K+) which is an elementary particle has on average a lifetime of 0.1237µs when measured in the rest frame of the kaon. Kaons moving with a speed of 0 .990c relative to the laboratory reference frame are produced.
a) Which frame has the proper time?
b) How far can the kaons travel in the laboratory frame during their lifetime?

Answer 1

a) No frame has the proper time. The time depends on the reference frame.

b) The distance is

$$d=vt=0.99ct=36.71\text{ m}.$$