I understand that everything with mass does have a gravitational pull no matter how small the object, which means even a grain of sand has a gravitational pull. But light is something that exists, which means it must be made out of something, which is Photons. And doesn't that mean that the photons must have at least a small area which contact can be made? And if that's true doesn't that mean it has some sort of mass? Do we just say light has no mass because it's so little that it's completely useless to recognize?


Of course light has a gravitational pull since photons carry the frequency-dependent energy $e=h\cdot f$, which is equivalent to mass via $m=e/c^2$. If you had a system where no radiation could get out, but in, and you'd pump a constant 100 Watt of light into the system, after 28 million years the system would have gained 1 kg (see example)

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  • $\begingroup$ This is true, though I think it's easier and potentially less confusing to just say that energy causes gravity, rather than converting it to the equivalent mass first. $\endgroup$ – David Z Feb 18 '15 at 2:54
  • $\begingroup$ The mass/energy-equivalence seemed to be the point where the thread-starter had his problems. $\endgroup$ – Gendergaga Feb 18 '15 at 3:19
  • $\begingroup$ You are completly wrong, $E=mc^2$,cannot be applied to photons,you must apply $E^2-p^2=m^2$,you will be correct if p=0,but OP is not talking about that. $\endgroup$ – Paul Feb 18 '15 at 4:15
  • $\begingroup$ Less confusing statement will be energy causes gravity. $\endgroup$ – Paul Feb 18 '15 at 4:18

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