# The Speed Of Light

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 249029.  Tue Dec 25, 2007 7:49 pm I have only just got around to reading the first QI book, of general ignorance, where it mentions the speed of light not being constant (3*10^8 m/s), but how it changes in different materials. While this is "sort of" true, it also isn't. One thing to consider is the difference between speed and velocity (the velocity of light *is* a constant). As is stated in the book, most of an atom is nothing, or a vacuum, which we know the light will travel through at the well known velocity. So why does it take longer for light to travel through a meter of glass rather than a meter of vacuum? The answer lies in what happens to the light when it's in matter. Light doesn't just pass through the matter, but it is absorbed by electrons in the atoms shells and then re-emitted. This process takes time, and causes the "slowing" of the light. One way to imagine it is a man walking down a street. He could walk the distance of the street in, say, 5 minutes. But if he is stopped during his travel and talks to people on his way down the street, it will take longer to walk down the road, even though he walks at the same speed all the way down. If he talks to more people, he will be slowed further. Sorry if this has been brought up (I did do a search, but found mostly threads on time travel). Also, my source is my lecturer, but I'm sure I could find a journal on it if need be.

 249042.  Tue Dec 25, 2007 9:52 pm But surely the velocity is only constant at the time of movement, if you're saying it is absorbed by electrons and re-emitted, then it's not in motion at that moment (or has a different rate of velocity). Speed is the mean rate of velocity when taking a specific entry point and a specific exit point and the time it has taken to pass between the two. In that case, I would support the Qi statement that speed is not constant.

 249127.  Wed Dec 26, 2007 1:17 pm It is fair to say that the speed is not constant, but when I found out why light "slows down" I thought it was amazingly interesting, which is the main reason I posted this. Also, saying that light slows down in matter does imply that it is moving slower, with less velocity, when really photons are stopping and starting rather than actually slowing down. Had you never wondered what actually caused the light to slow down?

 249138.  Wed Dec 26, 2007 2:53 pm I just thought it was chilling out, talking on the mobile

249148.  Wed Dec 26, 2007 3:21 pm

 Poncho wrote: I have only just got around to reading the first QI book, of general ignorance, where it mentions the speed of light not being constant (3*10^8 m/s), but how it changes in different materials.

I'm afraid this shows a profound ignorance of what the word "constant" means when applied to the speed of light. When we say we say the speed of light is a constant, we mean that it is constant in a vacuum and that it is constant in a vacuum, or, in terms of special relativity, that it is a constant in any inertial frame.

The fact that light travels at different speeds in different media does not mean that it isn't a constant.

http://www.weburbia.demon.co.uk/physics/speed_of_light.html

 249169.  Wed Dec 26, 2007 4:24 pm I think it's clear from the post that Poncho knows that djgordy, and the entry in GI makes it clear too. Thanks and welcome Poncho, I hadn't thought of that, or more likely I've forgotten it. It has upset me a little though, because once again the more I learn the less I understand. i.e. Why, if the light is interacting with electrons, does the QM wave function not collapse? i.e. if we do the double slit experiment with single photons in a non vacuum, would we get an interference pattern? And... why is the phase of the wave before and after the interaction maintained anyway? Why doesn't isn't the photon sent in a different direction? Conservation maintained by changing the values of the electron? Edit: typoLast edited by Davini994 on Wed Dec 26, 2007 4:34 pm; edited 1 time in total

 249170.  Wed Dec 26, 2007 4:27 pm Err, ask me another. Preferably about football. ;) (I do find all this relativity QI, just slightly tricky to comprehend. That may be because I'm crap at science, but then scientists seem to struggle with it too, so it's not all my bad.)

 249176.  Wed Dec 26, 2007 4:37 pm Why are England so appalling? How on earth is John Terry worth 120k per week? Why do people who have lived all their lives in West Bromwich not understand that I don't care when they laugh at me for being a Leeds fan? Would you like another physics question instead Samivel? ;)

 249224.  Wed Dec 26, 2007 5:43 pm I think it's a not unreasonable treatment of the idea for the QI format. The programme does occasionally give scientific concepts a very slight garbling, although were I an elf I'd probably call it "a light dusting with the glitter of nuance". I think it can be excused for the sake of being interesting. It certainly doesn't compare to the raging gibbercopters espoused by certain journalists in the national press! This does not of course preclude us from returning them to the rectilinear here :p C is a constant by definition and by experiment. No photon ever goes slower than 1.8 terafurlongs per fortnight, although when considered as a propagating waveform, light can be said to slow down and speed up. The time taken for a photon to get from one point to another can, of course, vary, without it having to break this rule. Here's how for those who might be interested: There are two ways. One, the path length can be cunningly varied, as shown cunningly in the experiment mentioned in the link below. [url] http://news.bbc.co.uk/1/hi/sci/tech/3308109.stm [/url] Alternatively, as in the case of absorption and re-emission by atoms, the photon which set out may not be the same as the photon which arrives! This is what happens in translucent and transparent materials. Photons are absorbed by the material and re-emitted at the same frequency, but after a brief delay. Sometimes, after just such a short and refereshing sojourn, a photon is re-emitted at a different frequency. This property is called fluorescence, and you will doubtless be familiar with its effects from your local kebabists or brothelhouse. Sometimes the re-emission occurs a long time later, even after the original source of photons has been removed. This effect is known as phosphorescence -just ask your friendly village jellyfish for a demonstration. Above all however, it's probably worth remembering that, as Arthur of Camelot's horse, Patsy, once said of Camelot: "It's only a model!"

249226.  Wed Dec 26, 2007 5:46 pm

 Quote: Why doesn't isn't the photon sent in a different direction? Conservation maintained by changing the values of the electron?

As the great Richard Feynman once said, "Nobody is are become understanding quantums grammar! Or am they???"

 249228.  Wed Dec 26, 2007 5:48 pm ORLY?

249254.  Wed Dec 26, 2007 7:22 pm

 djgordy wrote: I'm afraid this shows a profound ignorance of what the word "constant" means when applied to the speed of light. When we say we say the speed of light is a constant, we mean that it is constant in a vacuum and that it is constant in a vacuum, or, in terms of special relativity, that it is a constant in any inertial frame.

Yes, I did mention in my post that the velocity of light is constant, and that c did refer to the speed of light in a vacuum. I just wanted to attempt to explain what happens in materials to slow the light down. Obviously the Pentapod did a better job than me though, thanks!

249268.  Wed Dec 26, 2007 8:15 pm

 Davini994 wrote: Why are England so appalling? How on earth is John Terry worth 120k per week? Why do people who have lived all their lives in West Bromwich not understand that I don't care when they laugh at me for being a Leeds fan? Would you like another physics question instead Samivel? ;)

Yeah, fair point. Though if someone offered me a job at £120,000 a week, I'd say yes. And I wouldn't worry about whether I was worth the money or not.

:)

 249288.  Wed Dec 26, 2007 8:47 pm So pop quiz: how long does it take a photon, that exists at the centre of the sun, to get out*. *this is obviously an average, and based on calculations. I haven't tracked one.

 249309.  Wed Dec 26, 2007 10:24 pm You still fail to understand. A photon ceases to exist as soon as it is absorbed by another particle. Photons from the center of the sun never "get out". The factoid you're citing is that it "takes light a million years" to get out of the sun. This is only true for very particular meanings of the words "takes" and "light". The sun is not, fundamentally, a giant ball full of light. The first post is absolutely correct, and QI is wrong. When a photon is absorbed by an electron, it ceases to exist. A new photon is emitted. In that interval, there is no light, so counting that as propagation time is an error of perception. The error does not carry over into real physics. Just the layman's understanding of it. The "speed of light" is *not* the time it takes light to get from the source to your eye. It is the time it takes a photon to propagate from point A to point B. Photons do this, invariably, at 299792.458 km/s. Never faster, never slower. If you want to get cosmological, we can pretend that Hubble's law affects the speed of light at very large scales. In fact, it does not. It's space *itself* that is increasing in volume, which does not affect the velocity of signals through it. When the news talks about "scientists slow down light" that's a reduction in signal velocity, not in photon velocity. It would violate special relativity, thermodynamics, and Noether's theorem if the speed of light was not an absolute constant. It is. The phase velocity is irrelevant to this; it's allowed to be infinite. The group velocity is irrelevant to this; it's allowed to be infinite. Only the signal velocity is a physically real thing to measure. The signal velocity may be slower than the speed of light, but never faster. Information cannot be transmitted faster than 299792.458 km/s. Ever. That includes wave-form information. As already mentioned, photons are smaller than atoms, so even in a refractive medium, they move through quantum vacuum. Thus, the "speed of light in vacuum" is the only kind of speed of light there is. This isn't a multiple choice proposition.

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