Wikipedia:Reference desk/Archives/Science/2015 October 27

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October 27

What is photon/quantum of energy?

It is said photons are massless particles that are force carriers. They are quantum of electromagnetic energy. If photons don't have mass, why are they called particles? The standard definition of particle is that particles are smallest units that constitute matter. Could anyone please explain in simple language how we conceptualize energy particles? What is quantum of energy? --IEditEncyclopedia (talk) 16:03, 27 October 2015 (UTC)

Someone can try to break it down for you in simple language, but in the meantime, see Elementary_particle and Wave–particle_duality. Also photon, which explains that they have zero rest mass, but positive relativistic mass. This is explained a bit in the section Mass–energy_equivalence#Massless_particles. SemanticMantis (talk) 16:12, 27 October 2015 (UTC)

(editconflict):I see SemanticMantis has mentioned Wave–particle duality already: in some circumstances, light behaves as waves, in others it behaves as particles: Interference for example is a wave property, the Photoelectric effect can be explained when we consider light as particles. It's similar to electrons, these are seen as particles, but will also behave as waves (will also show interference in a double slit experiment).

I'm not sure if "particle as the smallest unit of matter" is a standard definition, as the particle article says, it's rather general and would be further defined depending on the scientific field in which it is used. Elementary particle and List of particles gives more info: there are two fundamental classes of particles, the fermions and the bosons. Fermions are "matter" particles and bosons are "force" particles. It's quantum mechanics, and I think someone once said, if you're not shocked then you haven't understood it. Ssscienccce (talk) 16:30, 27 October 2015 (UTC)

Here it is written that photons are energy packets. These energy packets flow in a stream. What does it mean by energy packet? What is the distance between two energy packets. If light consists of a stream of photons, then there must be a difference between two photons. What is that distance? --IEditEncyclopedia (talk) 16:34, 27 October 2015 (UTC)

Not necessarily. Photons are bosons, which means that they can have the same exact set of quantum numbers. That means that you can literally have multiple photons occupy the exact same location in space and time. It's important to remember that photons are the way we model light behavior when it behaves as particles would (for example, during the photoelectric effect or during certain kinds of nuclear decay, or in the Bohr model of the atom). That is, sometimes light behaves in a way that a particle would, and when it does, we model that behavior as a photon, which is like a little ball or packet. Except this ball is not like a ball in all ways, just in the ways we need it to to represent what is happening during phenomena when light behaves like particles. --Jayron32 16:41, 27 October 2015 (UTC)

There's no particularly good reason photons are called "particles". I wouldn't read anything into it. Photons aren't really even "things". It's more accurate to say that light can only be emitted or absorbed in discrete amounts, at a single point, but when it's not being emitted or absorbed it obeys Maxwell's equations. Between emission and absorption, you don't have N "pieces" of light, you just have light. -- BenRG (talk) 08:25, 28 October 2015 (UTC)

BenRG is on to the most important thing to understand about this sort of physics. All we have, empirically, is data. We know that if you perform X experiment, you get Y results. Everything else we create to explain those results is theory, in the sense of Scientific theory, which is an experimentally verifiable explanation of events. Which is to say, that light does what light does. It isn't a particle, it isn't a wave, it's light, and its behavior is not governed by our expectations of what it should do. That we have those expectations is merely a product of our stubborn psychology, and not an issue with physical phenomenon. Back at the idea of a photon. A photon is a model of light, and a model which doesn't work all the time. That's what meant by wave-particle duality. That sometimes the photon model of light is necessary to explain a behavior (that is, to explain an observation we make about light) and sometimes the wave model of light is necessary. "But what is it really?" many people say. The answer is "light". It's just what it is. To fit its behavior into our paradigm of the world, that is into the set of expectations we have about how the world should work, we've invented these concepts like photons as "particles" of light. But, like any model, all models are wrong, but some are useful. The photon, it turns out, has been quite useful for explaining many of the observations we have made about light's behavior. --Jayron32 11:10, 28 October 2015 (UTC)

Photons do have mass - they carry energy, and mass and energy are two sides of the same coin. The complicating factor is relativity. Photons travel at the speed of light. If an ordinary piece of matter (a rock, say) were to travel at the speed of light, you'd need an infinite amount of energy to get it moving that quickly, and it would have infinite mass. Any number for the mass of the rock when stationary would be multiplied by infinity and would result in infinite mass. So if the photon could be stopped dead in it's tracks, what mass would it have? That's called it's "rest mass". What its mass would be if it weren't moving. The answer would have to be zero - since that's the only number you can multiply by infinity and still get a non-infinite answer. Zero times infinity can be any number - and so the photon has a nice, reasonable mass when it's zipping along at the speed of light - and zero mass if it were ever to be stationary. Weird, eh? SteveBaker (talk) 16:52, 27 October 2015 (UTC)

@IEditEncyclopedia: Steve's answer is a decent intuitive way to think about it, but be a little careful. What Steve is calling "mass" is technically relativistic mass, a concept that I am given to understand is somewhat out of fashion with physicists.

When I say "out of fashion", I don't mean wrong. You can define it precisely, predict how it behaves, and express your equations in terms of relativistic mass, and it works just fine. You can also express your equations without ever using relativistic mass, and it also works just fine. Physicists seem to have decided that the latter approach is, I don't know, more convenient, more perspicuous, a "better" conceptualization in some regard. I don't know whether they're right or wrong; I see some downsides to their preferred approach, but then I'm a bystander and don't really get to "vote". The important thing is that you need to be aware that when a physicist says "photons have zero mass" and Steve says "photons do have (presumably nonzero) mass", they're not really contradicting each other, just using different terminology. --Trovatore (talk) 17:15, 27 October 2015 (UTC)

One big problem with "relativistic mass" is that we already have a perfectly good name for it: energy. There's no good reason to give different names to the same quantity in different units, and relativistic mass is just another (redundant) name for the energy when it's divided by c^2 to put it into units of mass. --Amble (talk) 20:29, 27 October 2015 (UTC)

Well, it behaves like mass in a number of ways, and it is additive, whereas invariant mass is not. In several ways it more closely matches our intuitions about mass than either rest mass or invariant mass. There may be no need to have more than one name for the same thing, but there's also no great harm, and there are times it comes in handy. But in any case our personal preferences are of secondary importance here; the "facts on the ground" are that readers may encounter both systems of nomenclature, and need to avoid getting confused by what boils down to just two different bookkeeping methodologies. --Trovatore (talk) 21:02, 27 October 2015 (UTC)

I'm not describing my personal preference, I'm trying to help you understand the reasons why physicists generally do not use "relativistic mass" and instead simply talk about "mass" and "energy". Relativistic mass doesn't behave at all like mass in Newtonian physics: it changes with velocity, whereas invariant mass is a property of the object. (And neither is additive in GR). These aren't even two different bookkeeping methodologies, just a needlessly confusing historical terminology that has no currency in the field but for some reason continues to be brought out to dazzle the uninitiated by making relativity seem more bizarre than it really is. --Amble (talk) 22:04, 27 October 2015 (UTC)

I already understand their reasons. I don't necessarily find them compelling. But it doesn't matter whether I do or I don't. The terminology exists, and that's not going to change, so we need to help "the uninitiated" navigate the possibly-apparently-contradictory things that they may encounter. --Trovatore (talk) 22:06, 27 October 2015 (UTC)

That's the thing, the terminology "relativistic mass" scarcely does exist except in a historical context and in needlessly confusing explanations given to laymen. It's as though every time someone asked about how a thermostat works we started by telling them all about the galvanic and caloric fluids. We don't usually keep outdated terminology around just for the purpose of confusing people, and there's no good reason why we should do it here. --Amble (talk) 22:27, 27 October 2015 (UTC)

No, sorry, I don't agree. It may be true that the concept is not really used in present-day research physics. But it has a plenty big footprint in writings that are still relevant and useful for learners. The thing to do is just explain what's going on, not try to excise Tolman because he doesn't follow your preferred language reform. I see that as "tidy-mindedness" as I think the British say (it's not a compliment). --Trovatore (talk) 23:49, 27 October 2015 (UTC)

I might be way off, but I find "relativistic mass" to be a semi-intuitive term, and I'm fully happy to have similar things have different names, especially when their units and scaling are different. The main drawback in my opinion is that you also have to talk about "mass energy equivalence", which was easy enough for me to mention in my first response. But maybe Trovatore and I share some biases - I think sometimes it's useful to talk about antiderivatives, and sometimes it's useful to talk about indefinite integrals :) SemanticMantis (talk) 00:59, 28 October 2015 (UTC)

I think we should usually follow the relatively clear terminology used in physics research and education. If that makes me a tidy-minded language reformer, so be it. Talking about relativistic mass is fine where there's an actual use for it, but there's no good reason why it should be presented to learners as the mass in relativity. --Amble (talk) 01:07, 28 October 2015 (UTC)

Certainly you can rearrange words and concepts to avoid talking about the relativistic mass of the photon - but then you get into deeper complications with explaining that light has momentum...which classically, is mass times velocity. It is certainly convenient to separate out the concepts of rest mass from relativistic mass. But there are times when that's less convenient. SteveBaker (talk) 15:46, 28 October 2015 (UTC)

It remains a concept that is difficult to understand, even for physicists: see the 2008 book The Nature of Light: What is a Photon?. A photon is the smallest energy packet that can be measured or emitted for a given wavelength/color of light. In that sense it is somewhat similar to an atom being the smallest amount of any given element. However, the trouble with photons is that you cannot really measure them without destroying them, so you can't really 'follow' their behavior. - Lindert (talk) 16:55, 27 October 2015 (UTC)

ruthenium -4 compound129.132.90.77 (talk) 16:15, 27 October 2015 (UTC)

Today I found that Ruthenium has oxidation state -4. Who knows where is Ru(-4) published?129.132.90.77 (talk) 16:15, 27 October 2015 (UTC)

This mentions the existence of Ru^-2 in Ru(CO)₄^-2 complex ions as the lowest known oxidation state of Ruthenium. Several other sites note the existence of this complex ion. I can't find any confirmation of a -4 Ruthenium, at least as yet. --Jayron32 16:34, 27 October 2015 (UTC)

doi:10.1016/j.solidstatesciences.2007.12.001 seems to have Ru as –4. I would be interested to know where the questioner found it noted, in case there are other leads there for references (or other interesting info). DMacks (talk) 02:04, 28 October 2015 (UTC)

List of oxidation states of the elements gives Ru(−4) in metal-rich compounds containing the octahedral complex [RuIn_6−xSn_x] (the Fe and Os homologues are also known). The two sources given are [1] and [2]. Double sharp (talk) 14:58, 29 October 2015 (UTC)

Those are the same two links, and also the same ref as the one I posted. The List article does have a second ref for Ru, but it does not appear to be about the –4 oxidation state. DMacks (talk) 15:33, 29 October 2015 (UTC)