# Whether light has mass? if yes what is the reason behind it?

by vidyanaik
Tags: light, mass
 P: 2 whether light has mass?if yes what is the reason behind it?
 P: 602 Light (photons) has no rest mass. But it has relativistic mass and momentum. More details for questions can be seen in this link. http://crib.corepower.com:8080/~relfaq/light_mass.html
 P: 93 To my knowledge, a photon does not have mass. If it had mass it could not travel at the speed of light. This is because it would take an infinite amount of energy to accelerate it to that velocity. And if a massive object traveled at the speed of light it would have an infinite amount of mass.
 P: 122 Whether light has mass? if yes what is the reason behind it? "Light (photons) has no rest mass. But it has relativistic mass and momentum. More details for questions can be seen in this link. http://crib.corepower.com:8080/~relfaq/light_mass.html" This paper is wrong, photons are massles bosons and momentum cannot be applied nor can be describe as having momentum. You could say they have magnitude but not momentum.
 P: 104 Light can be used to cause objects to move. In other words, they impart momentum to objects. Thus, if we are to believe the law of conservation of momentum, we have to assume that the initial momentum of the photon was non-zero.
 P: 280 Particles in QFT are irreducible representations of the proper orthocronous Lorentz group. The momentum $$p^{\mu}$$ is the generator of space-time translations. Proper orthocronous Lorents transformations leave invariant the square $$p^{2}=p^{\mu}p_{\mu}$$ and the sign of energy $$p^{0}$$. Only representations with $$p^{2}\geq 0$$ exist in Nature (as far as I know). The ones with $$p^{2}=m^{2}>0$$ are massive particles, the ones with $$p^{2}=0$$ are massless particles, like photons. For theese, $$p^{\mu}=(\hbar\omega,\hbar\mathbf{k})$$, and the relation $$p^{2}=0$$ implies $$\omega=|\mathbf{k}|$$, that is, photons travel at the speed of light c (=1 in my units). Things that go at c cannot have mass.
 P: 2 light is wave ,it has no mass
P: 280
 Quote by shuidi light is wave ,it has no mass
The electron is also a wave, and it has mass.
P: 30
 Quote by threadmark "Light (photons) has no rest mass. But it has relativistic mass and momentum. More details for questions can be seen in this link. http://crib.corepower.com:8080/~relfaq/light_mass.html" This paper is wrong, photons are massles bosons and momentum cannot be applied nor can be describe as having momentum. You could say they have magnitude but not momentum.
Why photons can not be described as having momentum?
 P: 5,632 Photons DO have momentum. Lots of insights here: http://www.physicsforums.com/showthr...=mass+of+light
P: 2
 Quote by filegraphy To my knowledge, a photon does not have mass. If it had mass it could not travel at the speed of light. This is because it would take an infinite amount of energy to accelerate it to that velocity. And if a massive object traveled at the speed of light it would have an infinite amount of mass.
but what would you like to say about the photoelectric effect?Light there acts as an impulse.....i suppose impulse is created by only those bodies with mass
P: 5
 Quote by vidyanaik but what would you like to say about the photoelectric effect?Light there acts as an impulse.....i suppose impulse is created by only those bodies with mass
Light is energy, and as E=mc2 says, energy and mass each carry a "punch"

Also, light has no mass, but does have momentum. Momentum for zero-mass particles relates to their energy, not mass. Energy of a massless particle equals planck's constant x frequency.
Momentum of a massless particle equals that same (Planck,s constant x frequency) divided by speed of light.
p=(hf)/c
 Mentor P: 11,878 Light has no rest mass, but it does have energy. I'm not sure whether that energy is in the form of magnitude, or momentum, or whatever. Whats the difference in magnitude and momentum in this context anyways?
P: 1,345
In classical electrodynamics light carries momentum and energy. You can't really talk about photons because the theory produces waves however it is still a field theory and relevant to QM.

In this sense, light has no rest mass, waves don't have a mass in classical electrodynamics, however light can carry energy and momentum. As a result of light waves carrying momentum we get radiation pressure. Radiation pressure is something that can physically be measured supports momentum carried by light.

 Quote by Drakkith Whats the difference in magnitude and momentum in this context anyways?
It's very important. Which way is the momentum "flowing"? The direction is actually given by a tensor, horribly named the Maxwell stress tensor.
Mentor
P: 11,878
 Quote by Feldoh It's very important. Which way is the momentum "flowing"? The direction is actually given by a tensor, horribly named the Maxwell stress tensor.
I'm not sure i understand what your saying. Is the momentum of the wave the amount of energy it carries? Is the magnitude the direction?
P: 5
 Quote by Drakkith I'm not sure i understand what your saying. Is the momentum of the wave the amount of energy it carries? Is the magnitude the direction?
The energy an EM wave carries is Planck's constant x the wave's frequency E=hf

So yes, the momentum derives from the energy of the wave. Momentum p =(hf)/c
or momentum = wave's energy divided by light speed.

I am no scientist, but do love physics. All I could find of magnitude is that it is a scalar value (or so I understood it).
 P: 5 So when you say, is the magnitude the direction, I think (?) an EM wave exists in all directions until it interacts. If so, I don't know if "direction" applies here?
 P: 104 Magnitude and direction are the two quantities that define a vector. So no, the magnitude is NOT the direction.

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