Talk:Photon/Photons and Mass Debate1

---

I have deleted the generally in the first sentence. Some notes about that:

• Electro-magnetic radiation can travel over unlimited distances. In gauge theories infinite range of a interaction boson is associated with the property of it having mass zero. There are actually several experiments confirming this and giving thery small upper bounds for the photon mass. (cf. Review of Particle Physics, publisherd by CERN, for an overview)

• In high-energy physics it is custumary to call mass' the rest mass of a particle and energy its total energy (rest mass (= rest energy) and kinetic energy).

• So a photon's moving mass is of course not zero because a photon carries energy, and energy is mass according to E=mc^2. Hence a photon is e.g. subject to gravitation (cf. gravitational lensing). But this is usually called the photons energy not its mass. Note especially, that this mass/energy is not always tiny, but can be quite substancial.

• Of course, a photon always travels exactly with the speed of light, per definitionem: A photon is light, hence its speed is the speed of light.

What you might have been thinking about is that a photon in matter travels at a speed smaller than the speed of light in vacuum. But that's then still the speed of light. And strictly speaking, a photon within matter is not a photon anyway, but a polariton, as correctly remarked at the end of the article.

-- User:sanders_muc (a physicist)

---

A Photon is a particle of very little mass that likes to travel at the speed of light. After all that it is what light is, photons. These particles appear to be everywhere. They carry energy with them when they are let loose. Photons can come from such places as combusting fuel (gas). In order to see fire, photons are given off. How exactly this mechanism takes place, we do not know. There are many theories on how such occurrences take place, but until you can prove something it is nothing more than a bedtime story.

Science is all bedtime stories, eh? In a main article like photon?

As a result of their size photons can pass through several different types of matter (with certain structures, one being glass) without even seeing it.

This isn't true. Photons, even photons of visible light, interact with glass - you can tell because they move slower through it, are refracted by it, and so forth. And size has nothing to do with it, as all subatomic particles are more than small enough to slip through glass.

---

The photon is a 0 mass boson responsible for transmitting the electromagnetic force and producing visible light. While virtual photons, or photons that have a probability to exist due to the uncertainty principle carry the E&M forces, a changing electromagnetic field creates an electromagnetic wave, which when quantized, can be broken up into particles, which are real photons. Because of their massless nature, photons travel at a speed of C and changes in energy are manifested only as changes in frequency, the higher the frequency, the greater the energy. Frequencies falling between a certain range detectable by the human eye are called visible light.

But on to some other points brought up in previous entries.

According to GR, gravity can be described as curved space-time, and objects that "fall" in a gravitational field are simply taking streight paths through curved space, known as geodesics. Photons, although they have no mass, and therefore should not alter the geodesics themselves, still travel along a special type of geodesic, called a null geodesic, which is basically a path of zero distance through space-time.

At first glance, this may not seem to make sense, since we can clearly see that light moves from one place to another. But notice that I didn't say the distance through space is 0, but rather the distance traveled through the 4 dimensional space-time continuum. Due to the odd sort of geometry used in relativity, the total distance of any particle through space-time is equal to x^2 - y^2 - z^2 - t^2, where x,y,z, and t are all in the same units. "How can this be?" you ask, "since x,y, and z are spacial coordinates and t is a time coordinate?" Don't forget that a fundimental principle of relativity is that time is simply another dimension of space, and can therefore be expressed in spacial units, with c, (the speed of light) as the conversion factor, c units of space per one unit of time (note this means we could also write the above equation as x^2 - y^2 - z^2 - ct^2).

As for two photons having mass, I believe this is possible in a scenario where the two have momenta in opposite directions, creating a standing wave. Since E^2 = p^2c^2 + m^2c^4, or if you preffer, for each individual photon E = pc, but since p2 (momentum of the second particle) can be described as -p1, the momentum of the entire system is equal to zero. However, there is still energy present, which is equal to 2hf, (where hf is planks constant times the frequency, which equals the energy of one photon), so(2hf)^2 = 0^2 + m^2c^4, which reduced to 2hf = mc^2, and m = 2hf/c^2. -- Mark Palenik

---

Could you someone write this for someone who has never taken Physics in college? quatum? excitation? superposition? -- Taku 18:58, 21 Sep 2003 (UTC)

---

I think there was some kind of controversy about whether light was a photon or a wave. Now the currently held belief I think is that you look at it however it makes the most sense for the experiment you are doing. Like, maybe there's really no difference between a massless particle and a wave? A short wave is a high frequency, and packs more punch in a shorter amount of time so it's a high energy photon. Is there such a thing as a high-intensity low-frequency beam? Or does it have to be higher frequency to fit more energy in a narrower beam? --Luke Parrish 18:48, 21 September 2003 (-0600)

---

This is especially for late editors of this article, namely TakuyaMurata and Pizza Puzzle. Thanks for editing. But please make changes that are consistent. If you want to bring in front some facts you judge more important then adjust the article for them, especially remove these informations in other places so that the article has some natural flow and obeys a structure. Also try to avoid contradictions. The present state of this article is currently the worst that can come out from cooperative edition: things are obviously a heap of independent editors, repeating and tellings things and their converse. To the reader the result is complete confusion. If the previous text didn't make sense to non-physicist, now opening as 'a relatively massless particle; which generally travels at the speed of light' it doesn't make sense even to physisists. Particle is vivid picture for the layman but actually very poor concept to a physicist. Even on Wikipedia there is no decent article for this. Quanta of excitation (we could say energy) with a proper link for quanta, is not only exactly true, I think it is also very understandable by anybody. Generally travelling at speed of light is confusing (and actually false, if it doesn't travel at the speed of light it is not a true photon anymore). Relatively massless is also confusing. May be we should think of splitting this article in two, one vivid, particle, massless, light picture for layman, and a proper scientific article covering this issue at depth. I am not sure this can be done in a single article since this would need to assert false or at best vague all the previous material for non-scientific audience. I understand the previous form was not adequate but its present form I find worst. Thanks to discuss and I hope we improve this important article tackling an important concept.

Photons carry mass?

Photons do not have mass. Photons have energy. Some claim that E=mc² means that they are the same thing, but this is not a widely accepted theory. Gravity simply couples energy, of which mass is a type. Photons do not have that type of energy. It is interesting to note that two photons can have mass, even though one does not.--BlackGriffen

Photons don't even "have" energy. They have the property that they can transfer a certain amount of angular momentum from one charged particle to another in a certain amount of time. True, this is a fine distinction in semantics, but one I think should be discussed. --D. Estenson II July 8, 2005 09:08 (UTC)

---

Does it matter if one or both of them is Catholic?

--- BlackGriffen: you write "It is interesting to note that two photons can have mass, even though one does not." I'd be very interested to know how this works. -- SJK

In relativity mass and energy are the same thing. It is customary to separate out kinetic energy, which stems from the motion of the system in question as a whole, and invariant mass, which stems from its simple existance. In composite systems, though, there can be binding energy which raises the mass relative to the sum of the components. Actually, though, I'm not sure how photons can get stuck together like that... --- My bad, guys, I think. But it's nice to see that Wikipedia is working in removing the (accidental!) piece of bullshit that I put in. Any chance of expanding upon why photons don't have mass, and why they're affected by gravity, which only affects mass-carrying particles if they don't?

And is invariant mass just a better, non-flawed description for rest mass, or are they something subtly different? -- Same thing, but invariant mass is the preferred term, as rest mass carries the strong connotation of the mass at rest and that doesn't make any sense for a photon.

With thanks, Dave McKee.

---

I'm not sure how two photons having mass arises. I recall two things a theoretical physicist said at a colloquium: 1, two photons can have mass; 2, it has something to do with the physical arrangement. My guess would be that when two photons are moving in opposite directions through the same space, they can create a standing wave. At least, two hertzian waves can. In order to be standing and still have energy, it must have mass. Don't quote me yet, though.

"Any chance of expanding upon why photons don't have mass, and why they're affected by gravity, which only affects mass-carrying particles if they don't?"

Certainly. I don't recall the experimental evidence, but here is the theoretical background:

p = γm_ov
E = γm_oc²
γ = 1/√(1-(v/c)²)

Photons travel at c, right? Well, sticking that in the above equations, which all particles (including photons) must obey, yields an infinite γ. If the photons have any finite non-zero mass, they would have an infinte energy and momentum! Last time I was knocked over by a photon... ;) In all seriousness, the reason they are affected by gravity is because gravity doesn't couple with mass, it couples with energy (of which mass is a type).

The idea that energy and mass are the same comes from one of these two fallacies:

mv = γm_ov
mc² = γm_oc²

leading to:

m = γm_o

Essentially, an attempt to redefine mass so that the old equation for momentum works, the idea that gravity couples to mass still holds, and that it was consistent with the new rest mass energy formula (sticking in v = 0). The only thing Einstein said was that the total energy of the system is:

E = γmc²

So you might say that in order to measure mass you need to bring the system being measured to rest. Already the idea of a photon having mass runs in to trouble since a photon can't be brought to rest (the easiest way to imagine trying to stop a photon is by trying to move your reference frame at c in the same direction [never mind the practical problems] as the photon. The dopler effect and other relativistic spatial transformations will reduce the frequency [and thus the energy] to zero).--BlackGriffen

---

Hopefully recent edits and links clarify the situtation. -- Beland 06:27, 14 August 2005 (UTC)

Coherence edit and adding simpler examples

This is probably not the right form of address:

In layman's terms, photons are the building blocks of electromagnetic radiation: that is, a photon is a "particle" of light, although, according to quantum mechanics, all particles, including the photon, also have some of the properties of a wave.

Nowadays we simply say "mass", meaning either of the "invariant mass" or "rest mass", since the old-style "relativistic mass" is deprecated because it is likely to cause confusion.

For overall consistency, bolds are for vectors, and not required to define symbol usage.

-Guest April 23, 2005