Talk:Photon/Photons and Mass Debate4

message: Hi all,
Someone asked at the Physics Wikiproject to clarify the relationship of photons and mass.  I'll do my best! :)  I hope everyone will be patient with my explanations.
First, photons do contribute to inertia.  That's stated explicitly by Einstein in his 1909 article, The Development of Our Views on the Composition and Essence of Radiation, which I translated recently.  You can find the original German text on the web, but I'm not allowed to post it at the German Wikisource because of copyright issues.
Second, photons do gravitate.  Since they have energy, they contribute to the stress-energy tensor and, thus, to the curvature of space time.  Hence, they act gravitationally on other objects.  Conversely, they respond to a gravitational field of other objects; light's path is curved in a gravitational field, as shown (for example) by gravitational lensing.  
The "weighing" of a box of photons was used by Einstein in a famous thought-experiment to refute the Heisenberg uncertainty principle, \Delta E \Delta t \geq \hbar/2.  His idea was that a hole in a box of photons could be opened for an arbitrarily short time, allowing one photon to escape.  The energy of that photon could be measured to arbitrary accuracy by weighing the box before and after the hole opened.  Thus, it seemed that the uncertainty in time and energy could be made as small as possible.  Bohr's refutation (delivered the next morning) was a watershed moment for quantum mechanics.
All that being said, light does not have rest mass (to the best of our present knowledge).  That's not a problem, though; the rest mass of an object is its mass in a coordinate frame where the object is at rest.  There is no such frame for light moving through the vacuum; it moves at c in all reference frames.  
Hope this helps, Willow 12:06, 9 August 2006 (UTC)
replies: text:  COMMENT
It would help if you'd clarify. A photon's inertia is undefined (because it's energy is undefined) and cannot be measured quantitatively, until it's part of a system. Same for its weight. These two constitute a mass of E/c^2, which a photon adds to a system in which it is trapped (and in reference to which its energy is E, which now CAN be defined because we have defined a reference frame). And similarly a photon subtracts E/c^2 invariant mass from a system (seen in the COM frame) which emits it. SBHarris 15:28, 9 August 2006 (UTC)
replies: text:  Another comment written concurrently
As you can see by Ati's reply below, it doesn't help much.  What you've stated is pretty much what we all know and agree on.  What's left is the question of whether adding inertia and gravitation to an object is the same as adding to its mass.  Most of us think it is.  We all agree that light does not have rest mass.  Can you address his response below in terms of how it relates to standard modern physics as you know it? Dicklyon 15:32, 9 August 2006 (UTC)
replies: text: His response has nothing to do with physics as we know it, so there's not much place to compromise on the issue. If you increase the energy, inertia, and weight of a resting compound object, whether by heating it or adding one or more photons to it (which of course can be the same process-- try an infrared broiler), you have increased its proper mass, invariant mass, rest mass, and just plain old everyday mass as we and modern physicists use the word. The end.  It's not complicated. Only cranks believe otherwise. SBHarris 15:50, 9 August 2006 (UTC)
thread_title: Photons do not have rest mass, but they contribute to inertia and gravitate

message: The views that the photon contributes E/c^2 to the inertia of a system are not the view of modern physics. While it is true that Einstein demonstrated in september 1905 [1] that a body radiating em would loose a mass \Delta m where that math is related to the energy variation \Delta E=\Delta m *c^2 , the converse is paradoxically not true, the naive view that adding a photon to a system would increase its mass by E/c^2 is refuted by modern experiments. One can only tell that the total energy of the system has increased by E and its momentum has increased by p (proper mass is not additive in relativity, the so called "relativistic" mass is, but that is an obsolete term ). Turns out that the photon has zero mass as proven by modern experiments. Modern experiments [2] look for a violation of the Maxwell's laws predicted by A. Proca in his 1937 seminal paper. The violation is of the order of mu_g=m_g*c/\hbar where m_g is the "photon mass". Any mu_g >0  would result into an observable phenomenon , confirming indirectly that m_g is greater than zero. This would bring about a violation of gauge invariance. As it can be seen from the many other experiments [3]-[11], this is not the case.Ati3414 14:05, 9 August 2006 (UTC)
replies: text: COMMENT
You actually write: 
The views that the photon contributes E/c^2 to the inertia of a system are not the view of modern physics. While it is true that Einstein demonstrated in september 1905 [1] that a body radiating em would loose a mass Δm where that math is related to the energy variation ΔE = Δm * c2 , the converse is paradoxically not true...
I wonder if you know how completely crazy that idea is?  You're suggesting conservation of energy and mass in one direction for a process, but not the other direction. The same process. Give it up. You've lost this one.
And by the way, if your (true) arguments that photons have no rest mass were relevant here (which they aren't), a system wouldn't lose mass when emiting a photon, either. So you shot yourself in the foot there by even admiting that. But I understand why you don't want to call Einstein wrong, since they you'd be claiming he said something incorrect a century ago, and nobody caught him for a century until YOU came along. LOL.  SBHarris 15:34, 9 August 2006 (UTC)
replies: text: What else is new, you obviously can't read (or you twist whatever you are reading). Keep on reading : "One can only tell that the total energy of the system has increased by E and its momentum has increased by p (proper mass is NOT additive in relativity, the so called "relativistic" mass IS, but that is an OBSOLETE term)." This was the argument from the beginning but it doen't register with you. Try reading the modern papers and the modern way of teaching the subject, I put them on the website for you. Maybe you learn and you stop "manufacturing" mass from gedank experiments.Ati3414 15:43, 9 August 2006 (UTC)
replies: text:  Actually, proper mass (aka invariant mass) is additive in special circumstances, so long as you measure mass in the COM inertial frame for each object or submass, and measure the total mass in a common COM frame for the whole. Put two boxes on a scale in relativity and you get the summed mass of two boxes. Put one box on a scale and keep it closed, and its mass will stay the same no matter what goes on inside it. You could generate heat, have a nuclear reaction, blow up a bomb-- even an atomic bomb if the box was superstrong. So long as you let nothing out of the box, all the heat, light, and mass of whatever was inside, would continue to have the same mass and weight, before, during and after the reaction. SBHarris 16:08, 9 August 2006 (UTC)
thread_title:  Photons have energy, momentum and spin but no mass 

message: ::: If we could keep things off the repeated personal attack, the only thing I have been saying all along is that you cannot infer that "For example, a mirrored box containing a gas of photons, or even a single photon, with total energy E will have greater rest mass (by Δm = E/c²)" because it conveys the notion of photon having mass. And this comes to odds with the whole rest of the wiki page (and with mainstream physics). Try to accept this: since there is no "rest" vs. "relativistic" mass for the photon, it makes no sense to talk about the "photon mass contribution". It also makes no sense to talk about "rest/relativistic mass" anymore. The modern teachers have moved on. That's allAti3414 17:36, 9 August 2006 (UTC)
replies: text: No, try and accept this (it's quite simple):there is no photon "rest mass", there IS a photon "relativistic mass" .  Ok?  --Michael C. Price talk 17:50, 9 August 2006 (UTC)
replies: text: Just a quibble: There is a photon rest mass; it just happens to be zero. (Dilbert: "I've got a personality." Dogbert: "Let's not have that 'is zero a number' argument again.") --Trovatore 17:56, 9 August 2006 (UTC)
replies: text: Just a quibble-quibble: Debatable, since the photon can never be at rest... :-)  --Michael C. Price talk 17:59, 9 August 2006 (UTC)
replies: text: OK, but the serious part of my point is that we should always phrase things in terms of whether things "have nonzero (foo) mass" rather than whether they "have (foo) mass". --Trovatore 18:01, 9 August 2006 (UTC)

text:  There is no more such notion of "rest/relativistic" mass. Neither of them makes any sense in the case of the photon and both have been banished from modern teaching of physics. Here it is, the same explanation again:
http://www.lassp.cornell.edu/~cew2/P209/part11.pdf
and here:
http://www.courses.fas.harvard.edu/~phys16/Textbook/ch11.pdf Have a look at chapter 11.8.
Ati3414 18:15, 9 August 2006 (UTC)
replies: text: That POV is already explained at relativistic mass.  Go there.  --Michael C. Price talk 18:33, 9 August 2006 (UTC)
replies: text: It is very poorly explained there. The photon is an exception, you cannot apply to it the notions of "relativistic/rest  mass" . I have been trying to get the new way of teaching physics in, would you please have a look at the referenced curses from Cornell and Harvard. I know the subject very well, I do not need more refences to the way wiki botches it. This is not only my POV , it is the POV of leading universities in the US:
http://www.lassp.cornell.edu/~cew2/P209/part11.pdf
and here:
http://www.courses.fas.harvard.edu/~phys16/Textbook/ch11.pdf Have a look at chapter 11.8.
Ati3414 18:52, 9 August 2006 (UTC)
replies: text: What part of "relativistic mass" = E/c^2 don't you understand?  --Michael C. Price talk 19:04, 9 August 2006 (UTC)
replies: text:  The same one that you seem unwilling to read on, the one that is no longer used in teaching physics because of the confusion it generates, the one that does not apply to photon:
http://www.lassp.cornell.edu/~cew2/P209/part11.pdf
and here:
http://www.courses.fas.harvard.edu/~phys16/Textbook/ch11.pdf Have a look at chapter 11.8.
Ati3414 19:09, 9 August 2006 (UTC)

text: Can't you even figure out the indent thing?  BTW I have read those links, as I've already told you on my talk page.  Their POV is explained at relativistic mass.  --Michael C. Price talk 19:14, 9 August 2006 (UTC)

text:  Sorry, this is a nonsense argument. There is no "rest mass" for the kinetic part of the energy of a particle (I'm talking about JUST the kinetic energy). However, despite that, the kinetic energy of particles in a system (2 particles or more) DOES HAVE MASS. And it can be rest mass, if the system is at rest. This kinetic energy ADDS mass to all systems which contain the moving particles. Not only their rest mass, but their kinetic energy shows up as mass in the COM frame. Kinetic energy is part of the mass of "heat." It's half of heat in solids, and it's ALL of heat in ideal gases. And yet this kinetic energy has no rest mass when you look at the particles one at a time. So where is the mass it represents? Well, the mass of the kinetic energy of particles is part of the system of particles, even though you cannot connect it with any particular individual particle (indeed it hops around from place to place as you change your reference frame). That confuses some people when they study relativity, but our job in these articles is dig out this confusion and confront it, not bury it and pretend it doesn't exist. Kinetic energy has no rest mass for single particles but it DOES have rest mass for SYSTEMS of particles when THEY are at "rest" (rest in systems means we are looking at the system from the COM frame). Get used to it. Get over it. It's modern physics.  Photons are merely a special case of the same problem, since in a sense they're ALL kinetic energy and nothing else. None has ANY definable rest mass when viewed one at a time (you can make their energy anything you like by choice of reference frame), any more than massive particles have any particular mass associated with THEIR kinetic energy, when viewed one at at time (again, you can make it anything you like). BUT the kinetic energy of particles AND photons (which for photons we conventionally call merely their energy) adds mass to any system they're part of.  All of this is ordinary mass. It has inertia, gravity, weight. It's not only relavistic mass, but also proper and invariant mass in the COM frame. We count it as heat whether it's kinetic energy or photons. And as heat, it HAS mass and ADDS mass, no matter which kind of energy it is. SBHarris 19:55, 9 August 2006 (UTC)
thread_title: Mass of systems of photons

message: It does seem like more than one person here could use a nap. ;)  Let's all remain true to the physics!
The last remaining difficulty — which seems perfectly valid — is pedagogical, i.e., not with the physics per se, but with how to explain it to lay people (i.e., non-physicists), which will include most people reading Wikipedia.  There will naturally be at least a few readers who will be confused by the apparent paradox that the photon is massless and yet is able to gravitate and to increase the inertial mass of an object that absorbs a photon.  As Ati seems to be pointing out, we should try to explain to these readers as carefully as possible why this is not contradictory.
The two web-pages cited by Ati do not seem to contradict what I said earlier.  The second one (in its section 11.8) argues that physics teachers should replace the term "rest mass" with "mass" (and the symbol m_{0} with m) for pedagogical reasons, since inexpert students might try to take over other elements of Newtonian physics that are not valid relativistically.  That's a valid viewpoint, but it is not universal in the physics community, since there are other confusions that can arise from using just the term "mass".  Working physicists, however, do generally stick with the briefer term "mass" when speaking among themselves.  Regardless of the teaching method, the essential physics has not changed, merely its nomenclature and symbols.  
I'm rather pre-occupied now with family matters, but I can try to find a compromise wording, if everyone is patient enough with me.  I was thinking of working more on photons, anyway; I'm in the middle of translating Planck's 1900 paper and Einstein's 1905 "photoelectric" effect paper for Wikisource.
I'm kind of busy for the rest of today, though. Willow 18:42, 9 August 2006 (UTC)
replies: text: I'm against "compromising" with Ati3414; that will not improve the article. We can't allow articles to be skewed just because editors with, shall we say, unusual enthusiasms show up and make a lot of noise. --Trovatore 18:47, 9 August 2006 (UTC)
replies: text: I absolutely agree.  You can't comprise with a crank either on the talk pages or in the article.  His beef is with conventional terminology -- it has no physical insight or merit.  He simply can't accept that "relativistic mass" = E/c^2.  That's his problem, not ours, and articles should not suffer because of it.  --Michael C. Price talk 18:53, 9 August 2006 (UTC)
replies: text:  Sure, let's leave things as they are. Ignorance is bliss. Ati3414 18:54, 9 August 2006 (UTC)

text: As pointed out above, the kinetic energy of particles has no "rest mass" and yet it contributes to the mass of resting systems full of such particles (like a can of heated gas). If you can deal with that paradox, you can deal with photons. Perhaps our problem is we didn't make people deal with it before they got to photons. Here, in an article on photons, it returns to bite us. Had it been thoroughly dealt with by the people arguing here in relativistic kinematics before they ever heard of photons, they'd have an easier time seeing photons as just "bare" kinetic energy. SBHarris 20:03, 9 August 2006 (UTC)
thread_title: Perhaps the problem is merely pedagogy

message: This article fails to mention that the zero photon state also has energy and that this gives rise to the well known Casimir effect. Another consequence is that the mass of an empty box with reflecting walls is not zero (plus the mass of the walls). Count Iblis 21:08, 9 August 2006 (UTC)
thread_title:  Mass of an empty box 

message: A while back, I finished up writing at the end of the photon in a box section "consistent with the system's total energy and mass at rest satisfying E = m c²."  I later took it out as redundant, but MichaelCPrice put it back modified as "each component of the composite system, including the total composite system itself, obeys E=mc², where m is the relativistic mass."  Now, I've been trying to be good and avoid the concept of relativistic mass.  Is it really correct here?  Is it to emphasize that in the system of the box, the photon can indeed be attributed a relativistic mass, as an independent component?  What are the components of the system?  Is kinetic energy a component?  Or it kinetic energy to associated with some unit of mass?  Does the notion of components really make sense? Are we sure we want to go this far?  Obviously Ati will object; anyone else? Dicklyon 04:31, 10 August 2006 (UTC)
replies: text: Look up "A simple exercise" entry above. Try getting the "photon mass contribution" from formula [5] Would be good to read the older controversy "Photons carry mass?" from further above as well. Looks like this group has been gone thru this before Ati3414 04:59, 10 August 2006 (UTC)
replies: text: It's not "muddling", it is the (most) general case you asked for, remember? You asked about the "components". I gave you the "components".  Now that you got an "inconvenient answer" you are trying to weasel out of the situation. How does the most general formula applies to the "photon(s)in a box" now? What is the "mass contribution" in the general case? Have you read the section "Photons carry mass?". Yes or no? Don't let all this "inconvenient questions" rattle your (mis)conceptions about what is really mainstream Ati3414 13:53, 10 August 2006 (UTC)
replies: text: I said I can accept it.  I'm wondering if anyone else does. Dicklyon 15:55, 10 August 2006 (UTC)
replies: text: Well, you can add relativistic masses and energies, so it's correct that the total relativistic mass and relativistic energy of the box is the sum of these things for the components. But that's one reason we don't use relativistic mass, because we do this job using energy, then just dividing by c^2 where we like (and after we're done). In the COM frame (as for a box) the rest mass and invariant mass happen to equal the relativistic energy and relativistic mass, SO that fact (in this special frame) allows you to get invariant mass of the box by adding up all the RELATIVISTIC energies. Which seems a bit weird to many people because you certainly can't get invariant mass by adding the invariant masses of components. SBHarris 20:38, 10 August 2006 (UTC)
thread_title: ''E'' = ''m c''²

message: The "odd" fact that relativistic energy can be added to get rest mass of systems, is what allows energies which have NO rest mass (like photons and the kinetic energies of moving particles) to be counted (indeed summed up to be) the regular mass (invariant mass) of bound systems. Energy with no rest mass DOES have rest mass in a system! We've made this point as many places as we could, in part because it seems to be widely misunderstood. It's a special property of systems which does not appear for individual particles. The kinetic energy of a (single) free gas molecule doesn't WEIGH anything, because as soon as you go to a frame where it can be weighed, it goes away. And if you trap the gas molecule, you're no longer talking about a single object, but a system of molecule plus can. The kinetic energy of a particle in a can of gas, DOES weigh something. Kinetic energy is a SYSTEM property. It doesn't reside in single objects, but in systems of two or more objects. The MASS associated with kinetic energy IS THE SAME KIND OF THING. It's not really "in" a single moving object, but rather is a property of a system of 2 or more objects in motion relative to each other. 
And much the same is true of photons. Single photons have no definable energy, anymore than massive objects have a definable kinetic energy--- it's all frame dependent. The kinetic energy of a single particle or a single photon can be anything you like, depending on your reference frame chosen. But get a system of two or more objects, or photons, and now you have some residual minimal energy which you cannot get rid of by choice of frame. THAT energy has mass. THAT energy contributes invariant mass to systems.  
When you get this, you'll be fine. SBHarris 20:36, 10 August 2006 (UTC)
thread_title: Worth repeating. Energy with no rest mass in single particles, DOES have rest mass in systems