goring
Lv 6
goring asked in Science & MathematicsPhysics · 9 months ago

# What is the proof that light particles have no mass?

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• Anonymous
9 months ago

There is no such proof because there's proof of the opposite.  Light particles (i.e., photons) have been proven to have mass (an extremely small amount of mass) by observing how it bends towards gravity, like when passing a star.  Light particles having mass is also what makes black holes black -- the gravity is so intense that light particles, because of their mass, cannot escape.

• 9 months ago

Neb...you must have loved the answers in this group....argh.

So, goring, from special relativity we can derive the total energy equation (TE)^2 = (Mc^2)^2 = (mc^2)^2 + (Mvc)^2; where M = m/sqrt(1 - (v/c)^2) = relativistic inertia of a particle with rest mass m going v speed.  Mvc = kinetic energy of that particle going v speed, mc^2 is the rest mass equivalent energy and Mc^2 is the total energy.

If we can agree that photons travel at the speed of light, then we can set v = c in the total energy equation.  So that becomes (Mc^2)^2 = (mc^2)^2 + (Mcc)^2 = (mc^2)^2 + (Mc^2)^2; so we have (Mc^2)^2 - (Mc^2)^2 = 0 =(mc^2)^2.

And there we are.  (mc^2)^2 = 0 reduces to mc^2 = 0.  And since c > 0 that means that m must = 0.  The rest mast of a photon must be zero if it's going light speed.  QED.

• D g
Lv 7
9 months ago

photons have NO REST MASS

they  are born moving at the speed of light  and  never  go at 0 m/s  relative to anything..

they do  have  RELATIVISTIC MASS ... that is because  the  light is moving at  a velocity

it comes from the  energy formulas  for  light   in quantum physics

• 9 months ago

Photons move at the speed of light. Nothing with mass can do that.

Photons are bosons, so many can be in the same place at the same time. Nothing with mass can do that.

• 9 months ago

In science there is never proof.  There are philosophical positions at work.  During my time light was assigned a mass called a "relativistic mass" which was the mass of the energy contained.  Using E = mc^2

But such a view would also mean that the mass of a particle that had kinetic energy would vary from one frame of reference to another.

So that if mass is to be an invariant property of an object we cannot consider that the mass energy is part of its mass.

Which leads to many other philosophical points.

Basically we redefined mass, energy and momentum to fit this model.

Remembering that the only real test of a model is if it works and if it makes valid predictions.  If this model works then it is as good a model as any other.  Perhaps better than some.  Yet it remains a model and that may be torn down in the future with some other model that is "better".

• 9 months ago

You are talking photons, a sub atomic energy front. A wave front and narrow part of the EM spectrum.

There is no evidence to show they are totally without mass, but Crooks radiometer would suggest the do have some level of mass in order to exert force.

Gravity can bend the path of photons also suggesting a mass. Very small, albeit, but not massless.

• Anonymous
9 months ago

The fact that they travel at the speed of light.

• 9 months ago

Because of its speed you don't have to wait!

• 9 months ago

Everything has mass.