How come presumed massless fotons made to equate their energy with Mc^2 which contains a Mass.?

Question

is there really such a thing as rest mass?nothing seems at rest evirithing muves?

Isnt E=hf imply that there is an angular momentum. light particles move in a circle
or in straight line?angular momentum applies only for circular motion?

Answer 1

E=MC^2 describes the relatinship between mass and energy (light is an energy, you know).

Answer 2

The "m" in the E=m*c^2 expression that everyone is familiar with refers to the rest mass, and is only valid for particles that are at rest. Photons have no rest mass and are never at rest, and so this equation cannot be used with photons.

The full relativistic expression for the energy of a particle is actually:

E^2 = p^2 * c^2 + m^2 * c^4

where:
c is the speed of light in a vacuum
m is the rest mass of the particle
p is the momentum of the particle.

This expression holds for both particles at rest and particles that are in motion. This equation reduces to the familiar E=mc^2 when the momentum of the particle is equal to zero (i.e., the particle is at rest). Massless particles like the photon, however, are never at rest.

Note that although photons are massless, they do have momentum. Letting m= 0 in the full energy expresion yields:

E = p*c

or

p = E/c for a photon (or other massless particle)

Furthermore, the Einstein-Plank equation relates the energy of a photon to its wavelength, L, (or frequency, f, because L*f = c):

E = h*F = h*c/L

where h is Planck's constant. Using this expression for E in the relativistic equation for the photon's momentum results in:

p = h*F/c = h/L, which relates the momentum of a photon to its wavelength.

Answer 3

"hfshaw" gave a good answer.

The "m" in E = mc^2 is the rest mass of the object.
Further more, E = mc^2 is not the complete equation, the object's momentum component is often omitted. Although photons have no rest mass, they do have momentum since they have energy (Per the Plank's equation, E = hv).

Photons, by their vary nature, are never at rest.
Photons are the particle nature of light, hence they ALWAYS are traveling at the speed of light. In ANY and EVERY frame of reference, the speed of light is ALWAYS measured to be the same.

Answer 4

For one nothing is every at rest that was proven after einstein also fotons have a mass.