# Experiment on mass of a photon

1. Mar 1, 2013

### Slait

Ive been trying to find if anyone has done a simple experiment to test relativistic mass of a photon. I was hoping if anyone has found documentation or results of the following experiment:

In a vacuum fire a stream of photons at different frequencies at a pressure plate and measure the pressure exerted on the plate from the photons. Also if the same experiment was done with a reflective pressure plate for a different result.

So far i haven't found anything with google, but it seems like a simple experiment that someone out there has tried.

2. Mar 1, 2013

### sophiecentaur

That experiment would show the Momentum of photons and has been done, in various forms, many times.
Simply finding momentum will not tell you mass.

3. Mar 1, 2013

4. Mar 1, 2013

### sophiecentaur

"As of July 2007, their reported (upper) limit on the photon mass is 6×10−17 eV/c2"
Pretty small,huh? Compared with about 5.5e-11 eV/c2 for the electron. This limit has decreased by a factor of about a hundred over the last twenty years or so. It is still falling fast . . . .

5. Mar 1, 2013

### bcrowell

Staff Emeritus
Goldhaber and Nieto, "Terrestrial and Extraterrestrial Limits on The Photon Mass," Rev. Mod. Phys. 43 (1971) 277–296

R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters , 1998, 80, 1826-1829, http://silver.neep.wisc.edu/~lakes/mu.html

Luo et al., “New Experimental Limit on the Photon Rest Mass with a Rotating Torsion Balance”, Phys. Rev. Lett, 90, no. 8, 081801 (2003)

The tighter limits are model-dependent.

6. Mar 1, 2013

### Staff: Mentor

I think you meant 5.11e5 here.

The http://pdglive.lbl.gov/popupblockdata.brl?nodein=S000M&inscript=Y&fsizein=1 [Broken] has a large collection of photon mass measurements. The best limits come from astronomy, but there are lab experiments as well.

Last edited by a moderator: May 6, 2017
7. Mar 1, 2013

### sophiecentaur

The mass is about 5.5MeV. When you divide this by csquared, don't you get the same answer as I did? I was only using(trying to use) the same units as were used in that quote - I wouldn't have done that voluntarily in a million years. Did I get my sum wrong?
Less than one millionth of the mass of an electron for the upper limit for the mass of a photon seemed ok to me and pretty damn near zero.

8. Mar 1, 2013

### Staff: Mentor

The mass of an electron is 511keV in a unit system where c=1. If you want to use SI-units, the mass is 511keV/c^2.
The neutrino mass searches are at a precision of roughly 1 eV now, and the (model-independent) upper limit on the photon mass is ~10-24 times the electron mass.

9. Mar 1, 2013

### sophiecentaur

Thanks - the units in that first reference confused me. The situation is way more extreme then.

10. Mar 1, 2013

### Staff: Mentor

Hi sophiecentaur, the units are a little weird, but it is a common unit in particle physics. The eV is a unit of energy (1.6E-19 J), and by E=mc² you can have a unit of mass which is eV/c². The eV/c² is already a unit of mass, so you don't need to manually divide by (299792458 m/s)² to get the mass. So an electron has a rest-energy of 511 keV and therefore a mass of 511 keV/c².

So a 6E-17 eV/c² limit is not just one millionth of the mass of an electron, but closer to 1E-22, or a ten-billion-trillionth of the mass of an electron. At a certain point, numbers just become unfathomable. To put it in scale, that is approximately the mass ratio between a typical human and mars. And that isn't the tightest limit on the mass of the photon.

11. Mar 1, 2013

### sophiecentaur

Is you sayin I is fat?

12. Mar 1, 2013

### Slait

the only one that sounds like it might be close to what im talking about is the most recent experiment but i dont know how to access the documents for the details. The reason I'm searching is I'm skeptical about light having mass. I want to see an experiment of light actually pushing a physical object. All the other experiments seems like a math calc using the observed energy and not the effect on other matter

13. Mar 2, 2013

### sophiecentaur

I think you are still missing the point that it is the Momentum of the light that does the 'pushing' (hundreds of examples of this, all over the place). The momentum of a photon does not involve the photon having mass. Indeed,a very 'simple' outcome of Maxwell's equations, applied to waves, predicts light pressure (the correct amount) without involving the use of photons at all. Any advanced EM book will show how this is done.

14. Mar 2, 2013

### DrGreg

Last edited by a moderator: May 6, 2017
15. Mar 2, 2013

### bcrowell

Staff Emeritus
The FAQ doesn't address the question of how we know that the rest mass of the photon is zero (or smaller than some empirical upper bound).

Last edited by a moderator: May 6, 2017
16. Mar 3, 2013

### Slait

Ok, sorry, momentum without mass...i think something is wrong there. Anyway you mentioned hundreds of examples, do you have a link to any of those examples? because that's what I'm looking for

17. Mar 3, 2013

### Staff: Mentor

No, nothing is wrong. Energy, mass, and momentum are related as follows: $c^2 m^2 = E^2/c^2 - p^2$. For p=0 that reduces to the familiar E=mc², and for m=0 it reduces to E=pc.

18. Mar 3, 2013

### Slait

Again, that equation is based on the energy. I want to see light pushing an object. I don't believe that it can. Science isn't a religion, something isn't true just because a book says so.

The only thing that interferes with my learning is my education. -Albert Einstein

19. Mar 3, 2013

### ZapperZ

Staff Emeritus
So you don't buy into (i) Compton scattering (ii) the RF structures that are used in many particle accelerators to accelerate particles?

Zz.

20. Mar 3, 2013

### Mentz114

Einstein demonstrated in a 1917 paper that 'light quanta' exchange momentum with atoms when emitted or absorbed. This exchange of momentum is used to slow down bosons to cool them. This experiment won a Nobel prize for the physicists involved.
http://www.nobelprize.org/nobel_prizes/physics/laureates/1997/illpres/index.html and http://physics.aps.org/story/v21/st11

Einstein's 1917 paper is called "On the Quantum Theory of Radiation", Physik, Z. 18, p 121

Last edited: Mar 3, 2013