Special Relativity - Speed of Photon

A π⁰ meson (rest-mass energy 135 MeV) is moving with velocity 0.8c  in the laboratory rest frame when it decays into two photons γ₁ and γ₂. In the π⁰ rest frame, γ₁ is emitted forward and γ₂ is emitted backward relative to the direction of flight. The velocity of γ₂ in the laboratory rest frame is

A. −1.0c 
B. −0.2c 
C. +0.8c 
D. +1.0c 
E. +1.8c 

(GR9277 #37)

Solution:

(B), (C), (E) are FALSE
Photon moves with the speed of light (1.0c)

Since γ₂ is emitted backward, the velocity of γ₂ is −1.0c

Answer: A