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Comment by adekok | Hacker News Reader

The hard part for physicists to accept is that the theory requires the speed of light to change.

It would be interesting to see how the theory behind the Unruh radiation works with "The quantum vacuum as the origin of the speed of light" (http://arxiv.org/abs/1302.6165#)

Or also MOND (https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics) Which also has predictions based on very low accelerations.

It seems like the theories could be related.

(says an ex Nuclear physicist who's now doing computers).

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> The hard part for physicists to accept is that the theory requires the speed of light to change.

I'm a complete layman here, so I don't understand how the link you shared says that the speed of light is changing; it seems more like a change in the way we define the speed of light. Is that right?

If the actual speed of light is to change, how is that possible? We've experimentally verified it up one side and down the other, so this seems like a huuuuuge discovery.

So there's c, which is essentially a conversion factor between space and time, and also happens to be the speed at which massless particles move. If photons happen to not be massless, the speed of light might change, though 'c' wouldn't have to.

Sagnac effect shows that speed of light is not constant and can be changed.

[1] http://arxiv.org/pdf/physics/0609222.pdf

A large part of theoretical physics is getting to grips with the terminology -- not that you should imply that I'm in any way an expert. I think I'd go with the wording "it seems more like a change in the way we understand the speed of light". From that, it implies that, yes, its definition would change, but it's something much bigger!

So yes, this seems like a huge discovery. Pretty much all of our understanding of relativity is based on the fact that c is, well, immutable. Changing the way we understand c means that we'll need to completely change the way we grok the last century of theoretical physics.

doesn't refraction also require the speed of light to change? I'm guessing you mean something subtler.

In case of refraction it is a macroscopic illusion. If you look closely enough, you will see photons traveling at the speed of light between atoms, getting absorbed by atoms and reemitted later. The macroscopic speed of light is just the result of absorption and emission probabilities and the microscopic geometry of the material.

But there is another layer below but I am less sure about that, it may be (slightly) wrong. In quantum electro dynamics photon take, figuratively spoken, all possible paths from source to detector, also paths that require going slower and faster than the speed of light. All those possible paths interfere with each other and the net result is that the photon seems to move with the speed of light with very high probability. But really take that with a large grain of salt.

If you look closely enough, you will see photons traveling at the speed of light between atoms, getting absorbed by atoms and reemitted later.

That's a story we tell kids who ask too many awkward questions. If it were true we would see sharp changes in the observed speed of light at wavelengths depending on the absorption spectrum of the material in question.

My understanding is that the real answer involves photons gaining inertia via coupling with particles in the material they are passing through (cf. Higgs) ... but I'm probably mangling that explanation horribly.

> If it were true we would see sharp changes in the observed speed of light at wavelengths depending on the absorption spectrum of the material in question.

Uh, we do. The imaginary portion of the index of refraction near a resonance generally looks roughly like a gaussian, and the real portion does have jumps that look a bit like tanh.

I shouldn't have written »absorbed by atoms and reemitted later«, that is really a different thing, interacting with the atoms is better. The superposition of all possible paths and interactions results in a wave function that effectively travels slower than the speed of light.

IS this what is meant when people say the speed of light in x or y material is less than in a vacuum?

Yes, due to the interaction of the photon with the atoms of the material the photon takes longer to move through certain distance of a material than through the same distance in vacuum.

Do photons absorbed and reemitted follow the trajectory of the original photon?

Just to be clear, I am not a physicist, only a interested layman. As I understand it currently, photons don't travel any path. They are emitted somewhere at some point in time and detected somewhere else at another point in time, what they did in-between is not a meaningful question, at least in the classical sense of asking for a path the photon took. When we want to calculate the probability of observing the photon at a specific place at a specific point int time, then we consider all possible paths - including the photon traveling to the edge of the universe and returning from there, including the photon going into the future and coming back - and sum them up. There is a relatively simple formula for every possible path and those possibilities interfere when we sum them up, some path amplify each other, some cancel out, in the end you have a probability distribution in space and time telling you where it is likely and where and when it is unlikely to find the photon in case you tried to observe it there. What the photon did in-between, we don't know, is a meaningless question, ... at least I can't tell.

They often do. Momentum must be conserved (despite this article...)

Fun fact: Cherenkov light is emitted when particles move faster than the speed of light in a given medium. It can be seen in nuclear reactors in blue. Tho is of course, in line with current theory, only possible because the speed of light in a medium is lower than the speed of light in vacuum, which particles cannot reach.

"speed of light" normally means "speed of light in a vacuum" (eg see https://en.wikipedia.org/wiki/Speed_of_light)

vacuum is not empty?

So its speed of light in quantum foam?

¯\_(ツ)_/¯

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