Comment by smoyer | Hacker News Reader

Comment by smoyer | Hacker News Reader

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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.

> The value of c is specific to the medium.

c is the speed of light in vacuum, a universal constant that obviously doesn't depend on the medium.

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.

https://en.wikipedia.org/wiki/Photon#Photons_in_matter

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.

[1] https://en.wikipedia.org/wiki/Refractive_index

> The speed of light in a medium varies according to the refractive index of the medium.

Sure, as he said "Speed of light through glass is ~2e5 km/s"

> Water has an RI of ~1.33, meaning photons move through water at about 1/1.33,

Not really. Something moves at this lower speed, but it's not the photons.

That's not correct, but a widely held view. The speed differs to the properties of the EM field in these materials, not due to absorption.

How do you explain the properties of the EM field in different materials in terms of photons, electrons, etc?

How do you explain the magnetic and dielectric constants of free space?

With great difficulty.

cf. https://en.wikipedia.org/wiki/Permittivity

Edit: maybe I didn't understand what is the "incorrect, but widely held view" you talked about.

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.

Ok, probably you will agree with the "Speed of light through glass is ~2e5 km/s, but photons are still travelling at 3e5 km/s."

I guess you object to the "The speed of light, that is the speed of a photon, is a constant" part, where he identifies "light" with "photons".

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.

To be fair, the speed of a free photon is c but when the photon is interacting it may not make sense to keep talking about its intrinsic speed.

In any case, I feel objections to what you said should come from a “better” QED description and not from a “lower-level” classical description.

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.

No, they don't.

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.

> electrons don't move through conductors at the speed-of-light either

The drift velocity of electrons through conductors is many orders of magnitude lower than the speed of light (~mm/s).

Speed of light != Speed of photons (in general)

Speed of a photon = Speed of light in vacuum

The Feynman Lectures on Physics, Vol I, 31–1 The index of refraction:

http://www.feynmanlectures.caltech.edu/I_31.html

"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."

Acquiring mass is interesting. It means that in the course of this process they must be interacting with the Higgs field.

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.

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