How is Energy Defined for a Massless Particle?

[khkwang]

Homework Statement

Show that if mass = 0, then E=pc and u=c.

Homework Equations

E^2 = (pc)^2 + (mc^2)^2
B = u/c = pc/E

The Attempt at a Solution

I understand that if m=0, then E^2=(pc)^2 => E=pc.

But isn't p = Ymu? then:
E^2 = (Ymuc)^2 + (mc^2)^2

Plugging in m=0 sets E=0...

It seems strange to me that we can remove (mc^2)^2 from the equation due to m=0, but we can leave (Ymuc)^2 in.

I'm thinking that this definition of p is not the same as Ymu?

EDIT: wait... Y would equal infinity. Which multiplied with 0 makes ? I'm just plain confused now.

 

[kuruman]

p = γmu does not apply to massless particles.

 

[khkwang]

Ahhh simple as that lol, thanks.