How to measure the mass of photon?

[poker]

In the gauge field theory photon is represented by a gauge vector field which is massless. But how can we measure the mass of the photon to very small value to convince ourselves that photon is definitely massless? Somebody say that as the universe is just about 13.7 billion years old from the big bang to now, so if we measure the mass of photon through any quantum effects, we will ultimately arrive at a threshold under which we can not push on. In other words, we have no method to measure the absolute zero mass of photon. What are your opinions?

[Cap'n Refsmmat]

There's one very simple way: if photons had mass, they could not reach the speed of light, according to relativity.

[ydoaPs]

i tried to derive the mass of a photon here a while back.

[poker]

But how to define the speed of light experimentally? And how to make sure that this method will lead to high precision result?

 

There's one very simple way: if photons had mass, they could not reach the speed of light, according to relativity.

[silkworm]

Just a small note.

 

If a photon had mass, whatever was emitting the photon would have to lose mass by doing so.

[m4rc]

Actually atoms do lose mass when they emit a photon http://www.newscientist.com/channel/fundamentals/mg18925411.300.html . If something absorbes a photon, it will gain a small amount of mass. Mass and energy always go together. If something has energy it has mass, if something has mass then it has energy. The two are related by E=mc^2.

 

When people say that photons have no mass, they are refering to its rest mass. You can measure a photon's relativistic mass by measuring the difference in mass of an atom after it emits a photon, but "that's not a huge change: detecting the difference is equivalent to detecting a hair's breadth change in the distance from New York to Los Angeles. Weighing an ion to that accuracy requires more than your standard set of kitchen scales. In fact, it requires quantum scales.

 

David Pritchard started building his quantum scales at the Massachusetts Institute of Technology in 1985. Back then he wasn't interested in verifying Einstein's equation."

[silkworm]

Do jumping jacks on a scale and tell me what happens.

 

Although your mass doesn't change it will appear to because you're jumping on the scale. Hell, I've never spent a day over 150 and I can get the scale over 300 by jumping on it.

 

To a lesser degree, if you stand on a scale and wobble around a bit the needle will jump all over the place.

 

I wonder how we controlled against this phenomenon.

[ydoaPs]

Do jumping jacks on a scale and tell me what happens.

 

Although your mass doesn't change it will appear to because you're jumping on the scale. Hell' date=' I've never spent a day over 150 and I can get the scale over 300 by jumping on it.

 

To a lesser degree, if you stand on a scale and wobble around a bit the needle will jump all over the place.

 

I wonder how we controlled against this phenomenon.[/quote']

the bouncing is because of the spring....simple harmonic motion(not quite, becuase it stops, though).

[Meir Achuz]

In the gauge field theory photon is represented by a gauge vector field which is massless. But how can we measure the mass of the photon to very small value to convince ourselves that photon is definitely massless? Somebody say that as the universe is just about 13.7 billion years old from the big bang to now, so if we measure the mass of photon through any quantum effects, we will ultimately arrive at a threshold under which we can not push on. In other words, we have no method to measure the absolute zero mass of photon. What are your opinions?

There are two ways to measure the mass of a photon.

(Actually to put limits on it.)

1. If the photon had mass, photons of different energies would have different velocities. Any (speculative) photon mass is ceertainly so small that this is a difficult measurement. Observing photons from a supernova

would give one limit, but probably not a very low one. This method was tried for the neutrinos from SN1987a (If I remember it right), with mixed interpretations.

 

2. Since one photon exchange is responsible for the Coulomb force (using QED), any variation of Coulomb's law at large distance from 1/r^2 would indicate a mass of the photon. This gilves the most sensitive limit on the photon mass. From this type of experiment, an upper limit of

mass(photon)< 2X10^{-16} eV has been set.

[silkworm]

the bouncing is because of the spring....simple harmonic motion(not quite, becuase it stops, though).

 

I know. I was referring to m4rc's post. If you're saying that an object that gains mass by picking up a photon and then loses that mass by emitting it, how do you know that your measurement is from a change in mass and not a change in energy, vibration.

[ydoaPs]

crookes wheel(i think that is the name of it) tried to show that photons have mass. it turned out that light was just unevenly heating it.

 

the thought behind the wheel is that photons hit the plate and if they have mass, it will turn the wheel. it did turn, but, as i said before, not for that reason.

[Klaynos]

crookes wheel(i think that is the name of it) tried to show that photons have mass. it turned out that light was just unevenly heating it.

 

the thought behind the wheel is that photons hit the plate and if they have mass' date=' it will turn the wheel. it did turn, but, as i said before, not for that reason.[/quote']

 

Surely that would be momentum not mass?

 

And photons do have momentum.

 

E=mc² is not teh full form of the equation the one in my signiture is.

 

imo the best method for measuring the mass of a chaged particles is to push it through a magnetic filed and see how much it deviates from it's known path... (mass spectrometer style)

[ydoaPs]

Surely that would be momentum not mass?

 

And photons do have momentum.

it was before they knew that momentum wasn't dependant on mass.

 

imo the best method for measuring the mass of a chaged particles is to push it through a magnetic filed and see how much it deviates from it's known path... (mass spectrometer style)

1)i thought that was used for measuring momentum.

2)photons aren't charged.

[Klaynos]

it was before they knew that momentum wasn't dependant on mass.

 

 

1)i thought that was used for measuring momentum.

2)photons aren't charged.

 

1) You pass the particle through a known E field and work out it's speed bassed on it's charge.

2) I was refering to the "weighing" of an ion refered to by m4rc

[m4rc]

The measurement of the mass of an ion before and after emitting a photon has been done by David Pritchard at MIT.

 

"The mass difference was measured by comparing the cyclotron orbit frequencies of two single molecules trapped in a strong magnetic field for several weeks"( http://web.mit.edu/newsoffice/2005/emc2.html ) . In the case of you affecting your weight by jumping on your bathroom scales, you will not get an accurate measurement while you are accelerating. So for the ion, you need to measure the mass of the ion before and after it emits the photon, but not during the short time in which it is emitting the photon (a few nanoseconds). I am certain that the researcher took the recoil of the ion into consideration when doing their experiment.

[Severian]

You can see from here, that the most constraining limit on the photon mass is by Ryutov et al, using observations of the magnetohydrodynamics of the solar wind.

 

http://pdg.lbl.gov/2005/listings/s000.pdf

[Meir Achuz]

You can see from here' date=' that the most constraining limit on the photon mass is by Ryutov et al, using observations of the magnetohydrodynamics of the solar wind.

 

http://pdg.lbl.gov/2005/listings/s000.pdf[/quote']

Thank you. I just took a quick look at my "wallet card" version, which was from a few years ago. Also, I left out a number of types of experiment. However Ryutov et al. is a highly theoretical measurement. In any event, it is clear that there is no experimental evidence or theoretical suggestion

(except maybe in this forum) for any mass of the photon.

[J.C.MacSwell]

Just a small note.

 

If a photon had mass' date=' whatever was emitting the photon would have to lose mass by doing so.[/quote']

 

I think it does lose mass.

[5614]

Well yes; if energy is mass and it loses energy then you can say it must also lose mass.

[Ragib]

may i but in..

Cap'n Refsmmat said - There's one very simple way: if photons had mass, they could not reach the speed of light, according to relativity.

 

poker said - But how to define the speed of light experimentally? And how to make sure that this method will lead to high precision result?

 

Well, its a photon, the speed it goes at is the speed of light. A photon is light...basically, if you belive in Relativity (which you probably should for the next 20 years at least, before string theory finds something), by definition a photon will always travel at the same speed, nothing can slow it down, therefore it has no rest mass.

[Rocket Man]

you can weigh a photon by taking it's frequency, multiplying it by plancs constant(?) to measure it's energy, and running it through E=MC^2

 

now supposing you had a single photon into which you impart 90 penta joules. it would weigh a respectable 1kg.

 

The only thing i would question is how does it react when it strikes something? (momentum wise) would you be able to work out the resultant motion using newtonian dynamics and treating the photon as a mass?

and would it have a gravitational feild? or is that something reserved for matter?

[Klaynos]

you can weigh a photon by taking it's frequency' date=' multiplying it by plancs constant(?) to measure it's energy, and running it through E=MC^2

 

now supposing you had a single photon into which you impart 90 penta joules. it would weigh a respectable 1kg.

 

The only thing i would question is how does it react when it strikes something? (momentum wise) would you be able to work out the resultant motion using newtonian dynamics and treating the photon as a mass?

and would it have a gravitational feild? or is that something reserved for matter?[/quote']

 

 

in the equation you have stated, E=mc² the m is rest mass, as a photon cannot be at rest this is 0. This is the rest mass energy equation. A photon when you measure it's energy will be moving, this means you need the total energy equation:

 

E²=(m₀c²)²+(pc)²

 

This simplifis in the case of a photon to:

 

E=pc=hf

 

Which has no mass in it...

[Tom Mattson]

Well' date=' its a photon, the speed it goes at is the speed of light. A photon is light...basically, if you belive in Relativity (which you probably should for the next 20 years at least, before string theory finds something), by definition a photon will always travel at the same speed, nothing can slow it down, therefore it has no rest mass.[/quote']

 

I think you've fallen for a common misconception. Special relativity (SR) calls for an invariant speed. This speed was called the "speed of light" because SR was first developed by taking a close look at Maxwell's electrodynamics. But that is just an accident of history. You can derive the Lorentz transforms apart from Maxwell's EM theory, and if you do then it becomes clear that SR merely demands an invariant speed, not some special one. If it were one day discovered that light does have mass and that it really does not travel at the invariant speed, it wouldn't affect the theory of relativity in the slightest.

[Tom Mattson]

In the gauge field theory photon is represented by a gauge vector field which is massless. But how can we measure the mass of the photon to very small value to convince ourselves that photon is definitely massless?

 

Since you mention field theory' date=' let me offer a suggestion. Let's pretend the photon [b']does[/b] have a mass [imath]\mu[/imath]. In that case we can write down the Klein-Gordon equation to describe the dynamics of this massive spin-1 field as follows.

 

[math](\nabla^2-m^2)\phi=-\frac{\partial^2\phi}{\partial\phi^2}[/math]

 

If we look at the static limit then the time derivative goes to zero, and we have the following.

 

[math](\nabla^2-m^2)\phi=0[/math]

 

For a spherically symmetric point charge we have the following solution.

 

[math]\phi=C\frac{e^{-kmr}}{r}[/math]

 

I don't remember the exact combination of constants in the exponent, so I put the "k" in there to absorb any missing factors. It may well be that k=1. Anyway that's not the important part. What is important is that the above is the classical potential for an interaction mediated by a massive spin-1 field. If m=0 then we recover the old familiar Coulomb potential. So you could take the negative gradient of this potential and derive what the electrostatic force would be if the photon were massive. Then you could in principle perform sensitive tests on violations of the inverse square force law to try and find the photon mass.

[Rocket Man]

in the equation you have stated' date=' E=mc[sup']2[/sup] the m is rest mass, as a photon cannot be at rest this is 0. This is the rest mass energy equation. A photon when you measure it's energy will be moving, this means you need the total energy equation:

 

E²=(m₀c²)²+(pc)²

 

This simplifis in the case of a photon to:

 

E=pc=hf

 

Which has no mass in it...

 

i agree that a photon has no rest mass, it's pure energy and the energy is stored in the speed, but a photon is energy, energy has mass and it has a distorted form of momentum, so a photon with 90penta-joules would impart a lot of momentum, 90 penta joules =1kg, otherwise it would violate momentum laws so a photon must have mass when it's moving.

you are right to say a photon has no rest mass but the energy in it weighs something

take away the speed, you take away the energy, you take away the mass.

 

 

this site discusses it

http://en.wikipedia.org/wiki/Photon

also this

http://en.wikipedia.org/wiki/Photon_gas

if it has pressure, the individual particles must have momentum, momentum is a property directly related to mass