# Finally photons got a mass, what now? [closed]

Finally photons have got a mass, and what we have got is a new state of matter. Most of you must have heard about this till now, for those who don't, visit this page.

I am not a pro or something, but the circumstances seems interesting to me. Anything and everything we have learnt till now, about photon of course, is based on a speculation that the photos are mass less. What about the theories, are they still valid?

• There is nothing in the article you refer to, which says that photons got a mass. – Trimok Sep 29 '13 at 13:58
• The slowing down of light in materials is not anything new, it happens in all transparent materials like water or glass.This does not imply that it has a mass - it is only an apparent effect. What appears to be new is that two photons seem to be able to interact with each other, violating linear superposition. But this is also only apparent as it is the non linearities in the material which is facilitating this 'interaction' - two photons do not directly interact by any means - this observation does not cause any changes to be made to the standard model or to quantum electrodynamics. Hence, – guru Sep 29 '13 at 14:03
• there is no new physics involved. – guru Sep 29 '13 at 14:04
• Shoo, popular news agencyies. – Abhimanyu Pallavi Sudhir Sep 29 '13 at 14:11
• Actually I don't agree with the downvoting of this question. To be sure, Abhishek held misconceptions, but we had a chance to show what those miscoceptions were. He will now be more knowledgeble. Surely this small diffusion of knowledge is a win for science? I should add that, in certain circles of optical physicists, "slow light" and "massive photons" are commonly conveyed notions - even though I don't like them - and that I am often thought of as being pedantic for not wanting to call the "stuff" in these experiments "light": I like to keep firmly in mind that we're dealing with ... – Selene Routley Sep 30 '13 at 3:08

I shouldn't be too hasty in claiming this kind of thing has much to say about true photons, i.e. the fundamental particle.

The article you point to is talking about light interacting with matter, and therefore forming quantum superpositions of free photons and excited matter states - these superpositions are quasi-particles known as (depending on exactly what photon / matter state superposition we're talking about) polaritons, plasmons, excitons. If they've found a fundamentally different kind of superposition, they may even coin a new name for their quasiparticle: who's betting on "sabreon"? Now I'm not meaning to detract from this kind of research - it is VERY exciting - but if one speaks of photons with mass in this field one means the quasi-particles. So we're not talking about the fundamental photon's having mass. A good analogy for what we are talking about here is a sysme of coupled harmonic oscillators - masses on springs with tethers transferring energy between them. The quantum light field is a collection of quantum harmomic oscillators. When we put matter in, the atoms and molecules can often be described by QHOs too: now the light QHOs and the matter QHOs are coupled together. Just as with coupled classical harmonic oscillators, you can find the normal modes of coupled QHOs: these eigenmodes of the coupled system are superpositions of the constituent system eigenstates and they have different eigenfrequencies from the constituent system eigenfrequencies. You can do a quantum field theory with these composite system eigenmodes replacing the light and matter QHOs, and it looks pretty much the same as the original, pure light one, only now the dispersion relationships turn out differently. So these composite eigenmodes behave as though they are the particles of the system. But they are not pure photons.

Since these quasiparticles move much slower than $c$, they will behave as though they have a mass - even a very large one. They will have complicated dispersion relations, maybe analogous to those coming from something like the massive particle Dirac equation (although these particles still have spin 1).

These quasiparticles in your article have interactions, as further explained in Martin's answer.

After having read parts of the paper (I'm no experimentalist and not in the field of quantum optics, so I can't understand all details of the paper by just reading it once), it's not exciting in the way it is made out in the article. Photons still continue to not have mass, as far as we know. What the scientists have done is that they have created some material, inside which photons behave as if they had mass. That's like the case with the magnetic monopoles that are states that behave as though they were magnetic monopoles (although they aren't). The picture of massive photons that act like in a lightsaber is just an effective one - similar to the Cooper-pair in superconductivity. If you look closer, you'll still see massless photons that just interact with a lot of stuff.

As far as I see, what they do is the following: They have a so-called "nonlinear medium", which is a device often used in quantum optics, where they send small pulses of light through. In addition to the nonlinear medium (which makes it possible for the light to be very slow), they have "Rydberg atoms", which are highly excited atoms that are also often used in quantum optics, and the light interacts with these Rydberg atoms. Due to this interaction and the interaction between the Rydberg atoms, it looks like we have bound states with small amounts of quanta. This effect (bound states with small amounts of quanta) is typical for "strongly interaction quantum field theories", i.e. you can - effectively - interpret the result as if you had strongly interacting photons, i.e. what you need for lightsabers.

That's what is actually new, the fact that you can make photons interact (not directly, though, but via the Rydberg atoms) not the "massive photon" part. From what I know it seems that this is indeed exciting for the quantum optics people because of potential applications in quantum information and quantum optics, however it is not what the paper makes of it.

So, sorry, no theories overthrown. (EDIT: And you most probably won't get anything near lightsabers from there, unless you actually live inside their matter)