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

1. Aug 5, 2010

### vidyanaik

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

2. Aug 5, 2010

### Rajini

Re: light

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 [Broken]

Last edited by a moderator: May 4, 2017
3. Aug 5, 2010

### filegraphy

Re: light

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.

4. Aug 5, 2010

### threadmark

Re: light

"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" [Broken]

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.

Last edited by a moderator: May 4, 2017
5. Aug 6, 2010

### ballzac

Re: light

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.

6. Aug 6, 2010

### Petr Mugver

Re: light

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.

7. Aug 6, 2010

### shuidi

Re: light

light is wave ,it has no mass

8. Aug 6, 2010

### Petr Mugver

Re: light

The electron is also a wave, and it has mass.

9. Aug 7, 2010

### blenx

Re: light

Why photons can not be described as having momentum?

Last edited by a moderator: May 4, 2017
10. Aug 7, 2010

### Naty1

11. Aug 27, 2010

### vidyanaik

Re: light

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

12. Sep 20, 2010

### laQuinta

Re: light

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

13. Sep 20, 2010

### Drakkith

Staff Emeritus
Re: light

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?

14. Sep 20, 2010

### Feldoh

Re: light

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.

It's very important. Which way is the momentum "flowing"? The direction is actually given by a tensor, horribly named the Maxwell stress tensor.

Last edited: Sep 20, 2010
15. Sep 20, 2010

### Drakkith

Staff Emeritus
Re: light

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?

16. Sep 21, 2010

### laQuinta

Re: light

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).

17. Sep 21, 2010

### laQuinta

Re: light

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?

18. Sep 21, 2010

### ballzac

Re: light

Magnitude and direction are the two quantities that define a vector. So no, the magnitude is NOT the direction.

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