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You certainly know it, but sharing it for the ones who don’t:

The full equation is E² = (pc)² + (m₀c²)² where E, p, c, m₀ are respectively: energy, momentum, the speed of light, inertial mass. The (pc)² part is kind of a big deal, because otherwise massless particles (like photons) would have no energy.

If p = 0, then the equation simplifies into that famous E=mc².

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Karyoplasma@discuss.tchncs.de ⁨1⁩ ⁨day⁩ ago
What I think is funny is that if you assumed E=mc^2 for all particles, then that would mean that red light is lighter than blue light.

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- lvxferre@mander.xyz ⁨1⁩ ⁨day⁩ ago
  This, too: if E = hc/λ and E = mc², then we can combine both as hc/λ = mc². Rearranging this shit you get m = h/λc. Since h/c = 2.2*10⁻⁴²kg*m, you’d get:
  
  Photon colour wavelength (in nm) “Weight” (in 10⁻³⁶kg = 10⁻³qg)
  
  red 650 3.4
  
  yellow 600 3.7
  
  green 550 4.0
  
  blue 450 4.9
  
  …BLOODY ULTRAVIOLET IS MAKING ME FAT!
  
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Photon colour	wavelength (in nm)	“Weight” (in 10⁻³⁶kg = 10⁻³qg)
red	650	3.4
yellow	600	3.7
green	550	4.0
blue	450	4.9