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According to Einstein $E= mc^2$ so $m = E/ c^2$. So energy present in an atom contributes to its mass. So if a photon is a carrier of energy shouldn't it have mass since energy contributes to mass of an atom?

Emilio Pisanty
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Spidey Rahul
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1 Answers1

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$E=mc^2$ is the formula for an object with 0 momentum. If an object has momentum then the formula is $E^2/c^2=m^2 c^2+p^2$. A photon always has momentum and that momentum is $p=E/c$ so $m=0$

Dale
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  • To me it always seems a stretch to incorporate or relate P to the other equation. P already has c in it. So when you add it to the other equation your adding c + c. It's redundant – Bill Alsept Oct 17 '18 at 05:56
  • Are you unaware of the four-momentum? – Dale Oct 18 '18 at 02:08
  • yes but that doesn’t explain why c was in the equation twice. It is redundant. Look at the equation and consider each time speed is involved. – Bill Alsept Oct 18 '18 at 02:51
  • It isn’t redundant. It is necessary both times for the units to work out. – Dale Oct 18 '18 at 02:57
  • I know, it like ad hoc. – Bill Alsept Oct 18 '18 at 03:00
  • But it isn’t ad hoc. It comes directly from the norm of the four-momentum. That is why I asked if you were familiar with it above. Your comments here make me think you are not aware of it. – Dale Oct 18 '18 at 03:02
  • I understand the four vectors and the reason for the calculation. Inserting c twice is still redundant and is a stretch based on the assumption that all the energy of a photon comes from linear motion. Its possible that most of the energy comes from something else related to frequency. – Bill Alsept Oct 18 '18 at 04:09
  • Let us continue this discussion in chat. – Dale Oct 18 '18 at 10:55