# Light mass

1. Aug 14, 2011

### bodhi

please tell me if a photon of light is considered massless,then it should penetrate even walls,but that does not happen,then how can you consider light as massless????????????

2. Aug 14, 2011

### Staff: Mentor

Though massless, photons still have energy and momentum. But why in the world do you think that being massless implies penetrating walls?

3. Aug 14, 2011

### ZapperZ

Staff Emeritus
"massless" and "penetrate walls" are two properties that have NOT been shown to correlate with each other.

Unless you are able to show such correlation, then your question is moot and is based on a false premise. I'm sure you'll understand that it is impossible to try to explain something based on a false premise.

Zz.

4. Aug 14, 2011

### bodhi

thats because if you have no mass then nothing can block you.
also if a particle is showing characteristics it cant be zero,i dont think there is any thing in world which has characteristics and still carry no mass.

Last edited by a moderator: Aug 16, 2011
5. Aug 14, 2011

### A.T.

According to what? Pokemon battle rules?

Last edited by a moderator: Aug 16, 2011
6. Aug 14, 2011

### ZapperZ

Staff Emeritus
You appear to be making up your own rules and physical laws.

Zz.

7. Aug 16, 2011

### bodhi

well i have observed a basketball go through the net but it cant go through the hole used in golf,whereas a marble can easily go through the golf hole.obviously the dimensions matters
now why dosent this apply to light if mass is zero it musthave dimensions equal to hence even the smallest hole is big enough for it ,hence it must pass through it,but that does not happen,hence i think photon might have some mass.
please correct me if i am wrong.

8. Aug 16, 2011

### LostConjugate

If light had no mass (no energy) it would pass through objects sure. But it does have mass. You read that it can be treated as a particle with zero rest mass, this would mean that if it were at relative rest it would have no mass. As soon as it has any relative velocity it has energy and therefor is treated as a massive particle with momentum. Though most photons have such little mass (energy) that it does pass through walls.

Think of it this way, the more times it oscillates as it passes through the wall the less probability it has of making it to the other side. The energy (mass) is proportional to the rate of oscillation.

9. Aug 16, 2011

### Staff: Mentor

bodhi and LostConjugate, I would recommend that you both read the following FAQ entries:
http://www.desy.de/user/projects/Physics/ParticleAndNuclear/photon_mass.html

Passing through objects has nothing to do with mass. It has to do with interaction. Neutrinos have more mass than photons but penetrate much deeper through ordinary matter because they interact only very weakly.

Last edited by a moderator: May 5, 2017
10. Aug 16, 2011

### LostConjugate

A neutrino has more total energy than a photon of say visible wavelength?

Last edited by a moderator: May 5, 2017
11. Aug 16, 2011

### ZapperZ

Staff Emeritus
This is wrong.

Light has momentum, but no mass, as in invariant mass that we are familiar with. Do not make things up like this.

Passing through walls have nothing to do with having a mass or not. Neutrinos have mass, but they pass through the earth with very little interaction!

Zz.

12. Aug 16, 2011

### LostConjugate

True, invariant mass doesn't have anything to do with passing through objects.

However even though bodhi may not be saying it the proper way. He is not wrong by thinking of mass as the limitation of passing through objects. The lower your energy the higher the probability of passing through an object without disturbing it.

13. Aug 16, 2011

### Staff: Mentor

It certainly can depending on its speed, but regardless of the total energy it always has more mass.

14. Aug 16, 2011

### ZapperZ

Staff Emeritus
This is incorrect as well.

I can show you a band-pass filter in which a lower frequency of light will NOT pass through.

You are making things up.

Zz.

15. Aug 16, 2011

### Staff: Mentor

He is wrong, and the statement that the lower the energy the higher the probability of passing through is not true in general. If it were true then absorption spectra would be monotonically decreasing with increasing wavelength instead of displaying peaks.

EDIT: ZZ is faster on the draw!

16. Aug 16, 2011

### LostConjugate

Ok ok, I stand corrected. I was thinking on a more fundamental level I suppose.

A simple potential barrier and a particle. The probability of passing through is proportional to a Gaussian of the energy.

Also was thinking of EM waves. A radio wave happily passes through the walls of your house while the EM wave coming from your lamp does not, and a X-ray tears it's way through. All of this of course the expectation value of a great number of photons meeting a great number of different potentials.

Edit: so maybe I don't stand corrected?? I dont know.

17. Aug 16, 2011

### ZapperZ

Staff Emeritus
If you don't know, let me give you a strong advise: don't make things up and offer them as answers. Your original response was given with such certainty as if you know these things intimately. Instead, you're guessing!

People look up to this forum because it has a higher signal-to-noise ratio than other forums. Do NOT add to the noise.

Zz.

18. Aug 16, 2011

### Staff: Mentor

Again, what is important is the interaction. Suppose you have a thick block of scintillator crystal, then the x-ray will be strongly absorbed, the visible light somewhat absorbed, and the radio wave not absorbed very much, but instead suppose you have a large but very thin sheet of metal (or a wire mesh) then the radio wave will be completely absorbed, the visible light somewhat absorbed, and the x-ray will not be attenuated much at all. Completely opposite behavior because the interactions are different.

Last edited: Aug 16, 2011
19. Aug 16, 2011

### LostConjugate

Ok was not planning on making anything up.