Explanation required: photons

[D H]

So, at least as far as science is concerned the mass of a photon is exactly zero?

I just seemed to suggest that the mass was very low, on the order of

.000000000000000000000003 unit(s) as compared to zero.

You are misreading the article. It very clearly says that 10^-14eV is an upper bound on the mass of the photon.

 

What the article does not explicitly say (and probably should say) is that a mass of zero is completely consistent with those experimental results. There is always some amount of error associated with every scientific experiment. The theoretical value of zero is well within those experimental bounds.

 

What units are we supposed to use for mass?

In this case, eV, or electron-volts. Strictly speaking, an electron volt is a measure of energy. Since mass is a form of energy, units of energy can be converted to units of mass.

 

1 eV is about 1.60×10^-19 joules. In terms of mass units,that's 1.78×10^-36 kilograms or 1.78×10^-33 grams.

The 10^-14 eV upper limit on the photon mass cited in that article is equivalent to 1.78×10^-47 grams.

[Mr Skeptic]

You mean this statement?: "The fact that no such effects are seen implies an upper bound on the photon mass of m < 3 × 10^-27 eV"

 

Electron volts are a unit of mass-energy, the energy an electron gains moving across one volt of potential. 1 eV = 1.8 × 10^-36 kg. So according to that statement, the mass of a photon is less than 5 × 10^-63 kg. Compare to the mass of the milky way galaxy at 10^42 kg, or the mass of an electron at 9 × 10^-31 kg. And remember, that number is a maximum.

[PaulS1950]

OK, I think I have an understanding now.

The math says that the mass of a photon has to be zero.

We can be sure that it is, at most, very close to zero, at less than

.00000000000000000000000000000000000000000000000000005 grams

or

.00000000000000000000000000000000000000000000000000000005 Kg

 

That is something I can agree with being very near zero at most and in computations it is likely to be close enough to call it zero.

Thank you both - it really helps.

[D H]

It's not just close enough to being zero with regard to computations. Those small masses are upper bounds. Do you understand what that means?

[PaulS1950]

Yes, I believe I do. It means that it would be impossible for the mass to be higher than that, period. That the "actual" mass would be well below that point.

Essentially it is a small enough value, even at the upper limit, that the actual "evidentury" mass is zero.

 

Thank you for being sure.

[D H]

Essentially it is a small enough value, even at the upper limit, that the actual "evidentury" mass is zero.

It is a lot more than that. Suppose experimentation had been able to confirm a positive lower bound for the mass of the photon. No matter how small that lower bound, that experiment would mean that photons truly do have a non-zero mass. That non-zero mass would have extremely large repercussions on theoretical physics. That is not what happened; no experiment to date has been able place a positive lower bound on the photon's mass.

[PaulS1950]

Ok, I think we are dancing differently to the same music.

All evidence says that the mass of a photon is zero. Right?

No evidence has ever been available that produces a mass greater than zero. Right?

My use of language seems less accute than you prefer but I think I have the understanding that the mass of a photon is never >0. (all evidence considered)

That would be as close to an absolute fact as there is.

(I resist absolutes under normal conditions but this seems to be as close to an absolute as there is) Photons are massless. Got it.

[sr.vinay]

I still don't understand how some theories consider spin quantum numbers of photons as +1 and -1, when photons are massless. How can massless particles be so stable as to move at 'c' and spin at the same time?

Aren't photons more of an energy field or plane than a single particle?

[D H]

It's a mistake to think of the spin quantum number as meaning that a particle truly is spinning. You aren't the only one to make that mistake, sr.vinay. Professional physicists made that mistake in the initial formulations of quantum physics. Dirac provided a better explanation with the Dirac equation, but the term 'spin' stuck.

 

Particles aren't really spinning. While they do have an intrinsic quantity that looks a lot like angular momentum in classical physics, that particles have a (quantized) angular momentum does not mean the particles truly are spinning. The pre-Dirac explanations of angular momentum assumed particles truly were spinning arose from those early quantum physicists not quite overcoming their classical physics mentality.

Edited April 20, 2010 by D H
Grammar