How is Energy Defined for a Massless Particle?

AI Thread Summary
For massless particles, energy is defined as E=pc, where p is momentum and c is the speed of light. The equation E^2 = (pc)^2 + (mc^2)^2 simplifies to E=pc when mass (m) is zero, as the term (mc^2)^2 becomes irrelevant. The participant expresses confusion regarding the relationship between momentum (p) and the Lorentz factor (γ) for massless particles, realizing that the standard definition of p does not apply. The discussion highlights the importance of understanding that for massless particles, the Lorentz factor approaches infinity, complicating the interpretation of momentum. Ultimately, the clarification resolves the confusion about the application of these equations to massless particles.
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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.
 
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p = γmu does not apply to massless particles.
 
Ahhh simple as that lol, thanks.
 
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