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If light has mass, then why don't we have light particles accumulated all over the earth?

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  • $\begingroup$ Suppose you leave a heavy lead weight and a lot of ping-pong balls alone in a room. Why doesn't the gravity of the lead weight attract all the ping-pong balls, so that they cluster around the lead weight? $\endgroup$ – Peter Shor Oct 31 at 10:36
  • $\begingroup$ Because that's not how gravity works, lol. $\endgroup$ – Nephew of Stackoverflow Oct 31 at 11:19
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Photons have momentum, but no rest mass.

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  • $\begingroup$ So does the mass vary? $\endgroup$ – Nephew of Stackoverflow Oct 31 at 9:05
  • $\begingroup$ No, it's always 0. Momentum does vary depending on wavelength. $\endgroup$ – pavel Oct 31 at 9:10
  • $\begingroup$ So how does it exist if it's 0? $\endgroup$ – Nephew of Stackoverflow Oct 31 at 9:16
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    $\begingroup$ Mass is not a necessary requirement for something to 'exist'. Photons have both energy and momentum, but no rest mass. $\endgroup$ – pavel Oct 31 at 9:24
  • $\begingroup$ This is true only for a single free photon or for photons flying in the same direction. Any system of two or more photons at an angle to each other has a rest mass. Also photons confined inside a mirror box also have a rest mass. $\endgroup$ – safesphere Oct 31 at 15:19

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