Is anyone else annoyed by the fact that while MIT news is good at creating interest in their stories, they are annoyingly watered down and never link back to the source paper they are paraphrasing? In this instance, they never mention the paper title even once.
And they tend to over-generalize - "While photons normally have no mass and travel at 300,000 kilometers per second (the speed of light)". The maximum speed of a photon is (currently?) around 300,000 kps but they often travel slower than this. Our fiber optics systems are coursing with photons that travel approximately 200,000 kps. The key results of this experiment is that the photons are interacting with each other ... and that they acquire mass.
The flashlight analogy is actually one that I not only described to people, but built a unique product around in the '90s. CyberFiber (yeah ... it was an tongue-in-cheek name for an engineering project that escaped into the marketing and sales department) was a duplex communication platform that used the same wave-length of light for both directions. It was a great marketing tool but required that your optical connections had very low reflectivity (and nobody likes to put coupling gel in their connectors).
Photons travelling through fiber are still traveling at 300,000 km/s. It's just that they bounce of walls making their path longer and much more importantly than that, the medium is denser. Speed of light through glass is ~2e5 km/s, but photons are still travelling at 3e5 km/s. It's just that they hit atoms, energy gets absorbed and re emitted and so on. But photons always move at 3e5 km/s. The speed of light, that is the speed of a photon, is a constant.
Well ... no. The value of c is specific to the medium. In "empty" space, c is the constant we know and love, and is related to the permittivity and dielectric constant of the vacuum.
Specifically with a classical E-M model, when you are in a material different than vacuum, the differing dielectric and permittivity of the material are (in a macroscopic sense) what lead to the reduced c*. This is encapsulated in the form of the index of refraction, which is effectively the ratio of velocities between vacuum and in-bulk.
At a microscopic E-M quantum level, you can argue about vacuum state differences between physical vacuum and in-bulk. Indeed, in a physical vacuum, you have virtual pair creation/destruction, virtual photons, etc. In bulk, you may have that, but you also have the bulk electronic and physical structure to work with, as well as different fields and field structures.
Sometimes people write c with subscripts, so I've seen the notation he's using before. It sounds reasonable enough to me.
Edit: To be specific, in this context c was used for the speed of light in the medium and c_0 for the speed of light in vacuum. (To match epsilon and epsilon_0).
Have you seen c being used to refer to something else than the speed of light in vacuum in a discussion about the interaction of photons with matter?
Edit: yes, I know. But using c in that way is absolutely non-standard in physics (v is used instead). Actually he later used c* for the speed of light in a medium, so he knows what c is.
It seems your opinion is not very popular around here, looking at how many classical counter-arguments are given to your (necessarily imperfect) quantum mechanics description.
Light that travels through transparent matter does so at a lower speed than c, the speed of light in a vacuum. [...] The factor by which the speed is decreased is called the refractive index of the material.
In a classical wave picture, the slowing can be explained by the light inducing electric polarization in the matter, the polarized matter radiating new light, and that new light interfering with the original light wave to form a delayed wave.
In a particle picture, the slowing can instead be described as a blending of the photon with quantum excitations of the matter to produce quasi-particles known as polariton (other quasi-particles are phonons and excitons); this polariton has a nonzero effective mass, which means that it cannot travel at c.
Path length differs from velocity. The speed of light in a medium varies according to the refractive index of the medium. [1]
Free space (vacuum) has an RI of 1.0. Photons move fastest in free space, about 300,000km/sec.
Water has an RI of ~1.33, meaning photons move through water at about 1/1.33, or about 75% of the free space velocity of light.
RI of optical fiber is often ~1.4. So while there may be path length increase due to off-axis propagation and bouncing, the actual photon speed in the medium is 1/1.4, or about 71% of the 300,000km/sec speed of light in vacuum.
Yes, the electronic structure of a crystalline material.
That is one dimension of the "optical dielectric function" which is to do with the directionality of variation.
Light is the motion of magnetic and electric fields, the "ease" of this motion is affected by a variety of factors.
The speed of light depends not only on electric, but also magnetic field formation. And of each kind, there are a variety factors.
The vacuum itself resits electromagnetic motion to some degree. I was observing that it isnt the case that light simply has one speed, the field in which it travels itself is damped.
Wow, I've been afk for a day and you answered so many comments, thank you, I've learned something. And yes, I should have left the speed of light out of the last sentence, my bad.
Nope. See https://en.wikipedia.org/wiki/Permittivity. But basically the material resists the formation of an electric field, which means the photons actually travel slower.
You might want to look into the theory behind fiber optics a bit ... I left out the deviation due to path length (bouncing) as that obviously has a lot of variations (including wavelength). I've also ignored SBS and other unrelated sources of scatter.
It's pretty common to hear people refer to the "speed-of-light in a vacuum". If the speed of light were a constant, why would you need to specify the vacuum? It's also worth noting that electrons don't move through conductors at the speed-of-light either - in the cable television industry, the best hard-line coaxial cable was about 0.85.
"We have said before that light goes slower in water than in air, and slower, slightly, in air than in vacuum. This effect is described by the index of refraction $n$. Now we would like to understand how such a slower velocity could come about. In particular, we should try to see what the relation is to some physical assumptions, or statements, we made earlier, which were the following:
That the total electric field in any physical circumstance can always be represented by the sum of the fields from all the charges in the universe.
That the field from a single charge is given by its acceleration evaluated with a retardation at the speed $c$, always (for the radiation field).
But, for a piece of glass, you might think: “Oh, no, you should modify all this. You should say it is retarded at the speed $c/n$.” That, however, is not right, and we have to understand why it is not."
If you know what data you sent, and if the same reflections happen every time, could you subtract them out as you read? (I'm imagining convolving the just-sent data with the recorded reflections of a test pulse.)
Yes ... we could do the equivalent to echo cancellation to a very small degree. The problem is that there will be more than one echo and, with the processing power available at the time, it was too difficult to cancel the reflected signal. I should also have noted that we were able to send analog signals over these devices. Cancelling the reflected signal would have been much easier with a digital signal.
Yeah, 'photons interacting' means nonlinear optics. That's not a new field, to put it mildly. MIT's PR people are not helping science (or the institute) gain credibility.
The bury the real interaction (mediated via the mass in atoms not photon-photon) but the paragraph is in there,
>If another photon is simultaneously traveling through the cloud, it can also spend some time on a rubidium atom, forming a polariton — a hybrid that is part photon, part atom. Then two polaritons can interact with each other via their atomic component. At the edge of the cloud, the atoms remain where they are, while the photons exit, still bound together. The researchers found that this same phenomenon can occur with three photons, forming an even stronger bond than the interactions between two photons.
> While photons normally have no mass and travel at 300,000 kilometers per second (the speed of light), the researchers found that the bound photons actually acquired a fraction of an electron’s mass. These newly weighed-down light particles were also relatively sluggish, traveling about 100,000 times slower than normal noninteracting photons.
300,000 / 100,000 = 3
"Relatively sluggish?" That seems more like profoundly sluggish, for light.
An index of refraction of 3 is relatively high (like silicon carbide). Glass and a lot of optically transparent materials are only ~1.5.
But, if you look at IR transparent materials like silicon, gallium arsenide, or germanium, you can get up to and over 4.
More to the point, diode pumped solid state lasers are a common thing these days so photons interacting just isn't news... (edit) however if (and I can't tell without reading the original paper) it is actually the effect of photon mass (eg due to light density) on the photons itself, then it's a significant technical achievement! Just not how I'd have phrased it to a technical audience.
Relatively is a modifier that here means "compared to something else". When someone says "relatively X", they are saying it is not actually "X" but it seems so in comparison. So they are saying that the combined particle isn't sluggish (because 3km/s is very much faster than a slug), but in comparison to light it _is_ sluggish. Hence, relatively sluggish.
Please clarify the model that describe the mechanism of the observations. Is it
A) a photon travels through a medium and changes the state of an atom in some way. Another photon travels through the medium and interacts with the same ‘excited’ atom. The second photon is then changed in a way directly related to the first photon
Or
B) a photon interacts with an atom but instead of being absorbed and retransmitted, it is just absorbed and captured. Only when another photon is captured does the atom release them both.
In A the information from the first photon is stored in the atom (flipped spin of an electron or proton for example) but the total energy of the atom is not changed much.
In B, the energy of the photon is contained in the atom (jump of an electron to a higher energy orbital) and the atom is truly in an excited state- this would be the mechanism of laser. An incident photon would then push the excited atom to quantum orgasm where it would spew out its photons.
I tend to favor B because it might make me famous as the inventor of the ‘quantum orgasm’.
Yeah, there were some serious faulty elements of the article. That was just one. Apart from that, I'm not aware of experiments that measure absolute phase, only relative phase differences.
But I've been out of physics for 20+ years, so mebbe they've gone and done it.
Just trying to wrap my head around this. Photon energies are distributed across quantised states. We can obtain the single photon energy for a given wave-length by the Planck-Einstein relation. We then assume the photons become "entangled", and decrease their speed. Is this to conserve momentum/energy, assuming the frequency is unaltered. I assume this frequency stays the same on the premise of constructive interference, and hence not altering the phase or frequency.
No, no changes to Maxell's equation, but I think it's easy to be mislead by the bad press coverage and title.
They only made a weird material where photons interact in a strange way, the photons "like" to be bound in triplets in that material.
There are other unusual materials, for example one special crystal can split a green photon into two infrared photons. Or the inverse, pick two photons and create a green photon with them. https://en.wikipedia.org/wiki/Spontaneous_parametric_down-co...
The process in the article is different because it's temporal and it's only inside the material, but the split/unsplit make new photons than can live forever outside the material. (There are other differences ...)
Maxwell's Equation is only valid in vacuum, or when the material doesn't do anything interesting. The technical term is probably "linear materials". In nonlinear materials, weird things can happened and you can no long use the Maxwell's Equations.
I came here to say that. I'm no expert in optics, much less quantum optics, but they seem to have created some kind of fancy material with unusual optical properties.
I'm very disappointed that MIT News themselves are so misleading. If I couldn't pirate the paper, I'd be left with the impression that they said they broke some kind of fundamental physics rule. They should leave heavy editorialization and sensationalisation for the news websites further down the chain.
It's also only valid classically. Quantum-mechanically we expect to generate photon-photon interaction terms (the leading piece is light-by-light). Essentially, two photons have an interaction moderated by the virtual charged particles in the vacuum.
I never got into nonlinear optics deeply. But I always thought that the nonlinear effects like frequency shifting work with the ordinary Maxwell equations, but with nonlinear material equations that have higher orders of susceptibility and permittivity in them.
