Light Photons and Mass

[Bill Thmpson]

OK, here is the deal. since light, uh, travels at the speed of light, I cannot see how it could possibly have mass when I apply my understanding of the theory of relativity and the fact that inertia increases as something approaches the speed of light.

I am currently arguing with a co-worker, who insists that light has mass. Since I am getting paid to work and not argue, I would like to bring this "discussion" to an end. Please answer this question. Do photons have mass?

I will say this. I heard about a year ago that there was a discovery that photons were discovered to change some charasteristics and scientists were thinking that this was only possible if photons have mass. Can anyone elaborate?

Also, what about this whole thing about light being both particles and energy and Einstein being upset about this and some physicist proving that it is half and half in a strange way.

[I edited because of embarassing spelling errors]

[Eta C]

In general relativity, anything moving at c must have a rest mass of 0. This includes photons and, until recent discoveries proved otherwise, neutrinos. Remember, however, that mass and energy are not different (E=mc2). So photons, despite being massless, are effected by gravity.

Some fringe theories do posit a possible mass for the photon, but if true would require some massive rebuilding of theories.

As to wave, particle duality it's more of a semantic problem. If asked if light is a particle or wave the correct response is neither. Rather it is something that we then decribe as having wave properties when appropriate (diffraction, interference) and particle properties (photoelectric effect) when appropriate. Remember that this duality applies to everything, electrons, protons and other particles can also display "wave" properties. So in short, the old, classical, definitions of wave and particle do not apply to the world as described by quantum mechanics.

[patrioticamerican]

Photons have no mass - if they had mass they couldn't travel at the speed of light! They do exhibit what is called wave-particle duality, meaning that in certain experiments they exhibit the properties of waves, such as interference patterns, and in others particles, such as the photoelectric effect.

I believe what you're referring to is Einstein's disagreement with Heisenberg's Uncertainty Principle, from which comes his famous quote, "God does not play dice with the Universe".

[Laser Jock]

I will say this. I heard about a year ago that there was a discovery that photons were discovered to change some charasteristics and scientists were thinking that this was only possible if photos have mass. Can anyone elaborate?

You're confusing photons with neutrinos. There is some evidence that some types of neutrinos have mass, but photons do not.

[JS Princeton]

Photons do have an effective mass, though. One can see this by considering an experiment of a photon in a box bouncing back and forth. The box itself will expereince a change in momentum which will be equivalent to h{nu}/c^2.
That's the "effective" mass of a photon.

The rest mass of the photon is zero, but the effective mass does contribute to the gravitational field, interestingly enough.

[snowcelt]

JS Priinceton wrote that "(t)he rest mass of the photon is zero, but the effective mass does contribute to the gravitational field, inerestingly enough". Incredible! I would be interested in knowing where this mass fits in; in the total mass of the universe. Thankyou

[JS Princeton]

The "total mass" of the universe is a highly subjective measurement. Rather, what is generally discussed is an energy-density of the universe. Indeed, the photon energy density is a component of this, and was at earlier times an extremely important component. That was the so-called "radiation dominated" epoch of the universe. We then passed into matter domination and are now into dark-energy domination. The photon energy density is so low that it really doesn't contribute a substantial amount to shape the present universe.

[snowcelt]

If we have passed into a "dark-energy" epoch, does this mean that we may eventually enter some other epoch in the far future? A place where photons will have no place?

[DStahl]

Begorrah, a photon has kinetic energy, that's for certain--it spins those little thingamabobs with the vanes painted black on one side and white on the other, enclosed in glass globes. How can something without mass still have kinetic energy, your co-worker might ask? Usually when people say "That particle has mass" they mean "rest mass" and not "relativistic mass"--which, if am understanding JS Princeton and the others correctly, is what a photon has.

But in a table of particle characteristics, a photon is listed as massless: it has no mass in the way that particles of matter have mass. The so-far-unobserved graviton is also massless, and it is unclear to me whether all neutrinos are now thought to have a tiny mass or whether there is room in neutrino theory for a massless neutrino. The even more hypothetical and unobserved tachyon is supposed to have a mathematically imaginary mass, related to the square root of negative one. Sheesh!

[JS Princeton]

Begorrah, a photon has kinetic energy, that's for certain--it spins those little thingamabobs with the vanes painted black on one side and white on the other, enclosed in glass globes.

"Kinetic" energy is really poorly defined for photons. Better to say that a photon has momentum

How can something without mass still have kinetic energy, your co-worker might ask? Usually when people say "That particle has mass" they mean "rest mass" and not "relativistic mass"--which, if am understanding JS Princeton and the others correctly, is what a photon has.

No, a photon has a zero restmass. That's part of the reason it can never be at rest, for then it would have zero energy and no longer be anything at all!

and it is unclear to me whether all neutrinos are now thought to have a tiny mass or whether there is room in neutrino theory for a massless neutrino.

If the neutrinos change flavor the way we think they do, all three of them have to have mass.

[JS Princeton]

If we have passed into a "dark-energy" epoch, does this mean that we may eventually enter some other epoch in the far future? A place where photons will have no place?

Photons really have no place in our present universe. Locally, they can do all sorts of damage, but looking at the universe as a whole there's really no difference between a universe with photons and a universe without photons at this point.

[DStahl]

JS Princeton--OK, momentum. That works for me.

I really love the conciseness of this: "No, a photon has a zero restmass. That's part of the reason it can never be at rest, for then it would have zero energy and no longer be anything at all!"

[DaveC]

Begorrah, a photon has kinetic energy, that's for certain--it spins those little thingamabobs with the vanes painted black on one side and white on the other, enclosed in glass globes.

This would be a Crookes radiometer, and the motion isn't really a result of photon momentum at all. The globe is partially evacuated and light falling on the dark side of the vane causes radiative heating of the air at the vane's surface. As the air heats and expands it exerts a tiny pressure differential across the vane causing the mechanism to rotate.
If the bulb were completely evacuated, and the bearing were frictionless, it is possible that photons bouncing off the white (or silver) side of the vanes could cause movement in the opposite direction - that is the light colored side of the vane would move away from the light source. I'd guess you'd need a powerful source of photons to induce movement in the real world where frictionless bearings dont exist.

[Bill Thmpson]

The neutrinos changing flavor is what what I think I heard about a year ago.

But, playing devils advocate -- and since I am pretty sure I am going to be asked this by my coworker -- how DO photons move those blades of those little toys that are painted black and white if they do not have a mass?

And, let's not forget those designs for a star ship using sails that reflect off of giant sails and push the craft slowly, over time, toward the speed of light. (maybe this is solar wind it is riding and this is somehow different) If this design relys on light pushing against it, does this not imply that photons have mass.

If Light does not have mass, how can it seemingly move things?

[JS Princeton]

Well, DaveC explained the answer to the Crookes radiometer well, but in a vacuum, light still has momentum and in principle the ability to "push" things around.

A photon must have momentum since it has energy and no mass. Perhaps you've seen the relativistic expression for energy?

E^2=m^2c^4 + p^2c^2

For a photon, m=0 (there is a zero restmass) so all the energy must be in "p" which is the symbol for momentum. In effect, for a photon where m=0

E/c=p

Divide the energy of your wave by the speed of light and you get a momentum.

Incidetally, the equation also works for a massive body at rest. In this case, the massive body has no momentum (p=0), but it has a mass (let's call it "m"). This means the equation reduces to:

E=mc^2

Look familiar?

One can convert from one form of energy to another, and in point of fact, one can convert radiation energy (in the form of light) to kinetic energy (in the form of some particle moving). A dramatic example of this is the so-called "photoelectric effect" which is explained well here:

http://www.colorado.edu/physics/2000...oelectric.html

In effect, the photon imparts a momentum on an electron in order to get it to move. In fact, sometimes you get reflected or scattered photons when you do the experiment that have a different energy. If you calculate the momentum difference associated with the change in energy ({delta}E/c) you will find that that's exactly the amount of momentum imparted on the electron. Thus the photons have the ability to exert a pressure (a force per unit area) on an electron, just like billiard balls.

More resources on the "mass" of light:

http://math.ucr.edu/home/baez/physic...ight_mass.html

[Bill Thmpson]

Very interesting.

I am impressed. This has been a very enlightening read!

Much thanks to everyone who contributed to this discussion !

[RickNZ]

Explain again to me please how is it that photons are effected by gravity?

Hey another question while were on the subject. Is it the wave or the particle aspect of the photon that travels at c?

[DStahl]

DaveC: "This would be a Crookes radiometer, and the motion isn't really a result of photon momentum at all. The globe is partially evacuated and light falling on the dark side of the vane causes radiative heating of the air at the vane's surface. As the air heats and expands it exerts a tiny pressure differential across the vane causing the mechanism to rotate."

HO! Bad science on my part. Thanks for the correction. Let's see if I can put a mental post-it note on that one so I remember it next time!

[JS Princeton]

Explain again to me please how is it that photons are effected by gravity?

Photons will always travel in geodesics which are paths that depend on the curvature of space (that's what gravity is due to). This is much like how planes take Great Circles when going from point A to point B. In effect, photons will travel the shortest distance between two points (principle of least action).

Hey another question while were on the subject. Is it the wave or the particle aspect of the photon that travels at c?

The wave and the particle both travel at the speed of light. The wave and particle are indistinguishable features.

[RickNZ]

Hey another question while were on the subject. Is it the wave or the particle aspect of the photon that travels at c?

The wave and the particle both travel at the speed of light. The wave and particle are indistinguishable features.

Lets just assume im a pleb here If the wave travels at c then would that not mean the photon particles need go much faster than c to travel in a wave pattern?

Unless the wave aspects are from the medium light travels thru. Woohoo ether does exist! LoL

Sigh i feel thick.

[DStahl]

For a discussion of that, see this topic.

[RickNZ]

Cheers for that, im on the right path now.


BTW is it possible for light to be made up by more than one particle type?

[JS Princeton]

In as much as you can have different polarization of photons, light can be made up of two distinct kind of particles (photon polarizations). However, in order to be electromagnetic radiation, the fields that are coupled to each other (the E and B fields) are always the same "kind" of field. Therefore there is only one "kind" of photon.

[DoctorDon]

My understanding is that it's not actually photon impact that drives those little spinning black/white light bulb things. I sat down with one once and looked at it closely and found that it actually spins in the reverse direction of what you'd expect from photon momentum transfer (you'd expect it to spin away from the white side, because reflection transfers twice as much momentum as absorption, but it spins away from the black). I did some poking about, and read that what's really going on is that the impact of light causes the black material to shed molecules, and it's the momentum of the ejection of the molecules that causes the thing to spin.

Seemed to make sense. 'Course, I have no way to experimentally verify it.

Don

[kucharek]

The bulb with the light-mill isn't completely evacuated. The air above get's a little bit more warmed than the air over the silver side. Warm air gives more bumbs, so the mill begins to turn. Light-mills don't work in a perfect vacuum. Your first reasoning about your observation was correct, but not your conclusion.

Harald

[DoctorDon]

The bulb with the light-mill isn't completely evacuated. The air above get's a little bit more warmed than the air over the silver side. Warm air gives more bumbs, so the mill begins to turn. Light-mills don't work in a perfect vacuum. Your first reasoning about your observation was correct, but not your conclusion.

Well, that's another reasonable-sounding theory that I have no way to confirm or falsify. :-) If I had one, I could pump all the air out and see if it still worked. If not, that would falsify the "black enamel emits more molecules" theory, and lend support to the "hot air near black side has increased pressure" theoy. :-) But I have neither mill nor pump, alas, so I must remain uncertain.

And it wasn't *my* conclusion -- as I said, it was an explanation I read somewhere.

Don

[Reacher]

ok, 2 things. firstly, someone happened to mention that a photon at rest has no energy, and therefore is nothing. well, then, that recently invented machine( its in another strand) that slows down light, would cause a photon to lose energy... then the universe loses energy, etc. Secondly, i have read in one of my science books "the universe explained," that when a particle passes on a trajectory that eventually puts it on a rough "tangent" (ps. is this the correct word, considering that a black hole is spherical, or at least its event horison is, or something) with a black hole, then some electrons can be sheared off, and the universe looses mass, ore energy, or some such thing. well, by my understanding of what a black hole is - the singularity thing - then the black hole is a part of the universe, so the universe loses no mass, or energy.

[Reacher]

pps: i cant work the quote thingy.

[Bill Thmpson]

It seems my co-worker still insists that photons have mass. I have asked him to post his views here.

I hope he does.

Maybe he is shy.

ops:

:roll:

[JS Princeton]

ok, 2 things. firstly, someone happened to mention that a photon at rest has no energy, and therefore is nothing. well, then, that recently invented machine( its in another strand) that slows down light, would cause a photon to lose energy... then the universe loses energy, etc.

The "slowing down of light" can be described qualitatively as due to scattering interactions. In effect, what's going on is that the light travels at the speed of light and then has some "interaction time" with an atom or a molecule. This "interaction time" basically relies on changing the light's properties. We had a discussion earlier about whether one can say that the photon was "destroyed" and "recreated", but generally the rule is that when a photon is interacting it ceases to be a photon in the real sense of the word. Really a photon is only a photon when it isn't interacting. Thus it travels in its geodesic at the speed of light and doesn't really slow down in the machine you refer to with the high index of refraction. However, it appears to be that way due to the nature of scattering in a medium. The details are messy but we can go into them. The quick and dirty way to answer the question is that the rules about photon energy and propagation only apply when you are in a vacuum.

Secondly, i have read in one of my science books "the universe explained," that when a particle passes on a trajectory that eventually puts it on a rough "tangent" (ps. is this the correct word, considering that a black hole is spherical, or at least its event horison is, or something) with a black hole, then some electrons can be sheared off, and the universe looses mass, ore energy, or some such thing. well, by my understanding of what a black hole is - the singularity thing - then the black hole is a part of the universe, so the universe loses no mass, or energy.

Hmm, I don't exactly know what you are referring to here. It sounds to me like you are referring to a paraphrased version of Hawking Radiation, but the details of your description are quite different. A black hole really can't have electrons "sheared" off of it because there really can be no way of accelerating an electron past the event horizon. However, quantum mechanics has some interesting paradoxes, among them tunneling which allows for Hawking Radiation.