system
Given the following post, respond as if you're commenting on Reddit:
human
If E = MC^2 then why does a photon contain energy?

Surely 0 x C^2 wouldn't work universally, or, I guess the equation wasn't posited as a universal truth to begin with - which is what it's reputation would lead you to believe :p

So what is it really meant to demonstrate?
gpt
E=mc^2 is for massive stationary objects. The full equation is

E^(2)=p^(2)c^(2)+m^(2)c^(4)

where p is momentum. Note that p=mv is not a universal law either - it's only true for massive objects at low speed. Massless photons can have momentum! You can see this by setting m=0, because photons are massless, which gives you:

E^(2)=p^(2)c^(2)

E=pc (and p=E/c)

So photons have energy and momentum, and they're proportional to each other - by a factor of the speed of light. (Similarly if you set p=0, you get E=mc^(2) out again)