The general energy-momentum relation for a massless particle is $E=cp$.
I know that for a photon we have $E=hf$. Is this relation valid for any massless particle?
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$\begingroup$ Why wouldn't it be? $E^2=p^2c^2+m^2c^4=p^2c^2\big|_{m=0}$, doesn't it? $\endgroup$– controlgroupCommented Nov 23 at 21:51
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$\begingroup$ What is the general energy expresion for a masless particle? In terms of what other quantities? $\endgroup$– GhosterCommented Nov 23 at 21:58
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The Planck relation $$E=hf$$ is valid for all particles, massive $(m\ne 0)$ and massless $(m=0)$.
The energy-momentum relation $$E^2=(cp)^2+(mc^2)^2$$ is valid for all particles. The relation $$E=cp$$ is just a special case of this, valid only for $m=0$.
answered Nov 23 at 22:02
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$\begingroup$ Thank you Thomas. I read the link Planck relation. It is good to know that it was de Broglie who postulated the general validity of Planck's relation. $\endgroup$– facenianCommented Nov 23 at 22:57