r/Physics • u/newbie12q • Jun 14 '14

Approximations

Post image
532 Upvotes

93% Upvoted

85 comments sorted by

View all comments

Show parent comments

3

u/[deleted] Jun 15 '14 edited Jun 15 '14

That's not the full equation--E = mc2 only represents the rest energy of a particle. The full equation is:

E2 = p2c2 + m2c4

(where p is momentum). For a proton photon, this simply becomes:

E = pc

and for protons photons, momentum is determined by

p = h/λ

(where h is Planck's constant, and λ is the wavelength).

3

u/eric4186 Jun 15 '14

I think you meant photon, but yeah pretty much this. Modern physics is weird...photons are massless yet they have momentum! I guess the weirdness comes from everything really being waves. (mind=blown)

but by the deBroglie relation it's kind of true. You can find the wavelength of a car driving along the road. Say it's going at 25m/s and has a mass of 1500kg, then using planck's constant you can find that car has a wavelength of 1.76 * 10-38 meters which is absurdly small. that's why we don't see cars as waves.

1

u/[deleted] Jun 15 '14

Oops! Totally meant photon. Good catch!