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# Speed of Light

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Posted (edited)

Do photons ever reach "light speed" i.e. the max theoretical limit? I ask because the speed of light is defined in a vacuum but as we know a true vacuum is not possible even in deep space, reasons being that there is no absolute zero out there and space is full of rays/particles etc.

Alan

Edited by Alien 13

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29 minutes ago, Alien 13 said:

Do photons ever reach "light speed" i.e. the max theoretical limit? I ask because the speed of light is defined in a vacuum but as we know a true vacuum is not possible even in deep space, reasons being that there is no absolute zero out there and space is full of rays/particles etc.

Alan

That is a very good question and I think that answer could be:

Photons travel at the speed of the light "most of the time"

Or rather, there are two different ways of thinking about it - first we need to "understand" why light slows down in medium other than vacuum. Common, but in my view rather wrong explanation is that light is "bouncing" off the stuff, and also that photons get absorbed and then re emitted by atoms. I would describe process as follows: light wave function is propagating at "light speed", but when it "hits obstacles" secondary wave (sort of reflection) gets superimposed with primary light wave making total wave propagating at slower rate. Here we are talking about wavefunction rather than classical wave.

So in principle, in outer space where density of matter that can interact with light is so low, light wave travels at the "speed of light" most of the time, occasionally "slowing down" (such a small drop in speed that we would not be able to measure it, but it in principle happens).

Another way of thinking about it would be in terms of Feynman diagrams, where ever more complex interaction contributes less and less to final outcome (increasingly smaller probability). In medium there is a big probability associated with scattering events and probability that photon will "arrive" late is great - we detect most of photons arriving late, or what we interpret as photons traveling at speed lower than "speed of light" (or rather causality speed). In outer space - very near vacuum, probability of photon arriving later is tiny, very tiny, and most of photons arrive "on time". So in principle, given enough photons, we would measure some of them as being a bit late, but most of time they would be on time.

Btw, what I've described above is just my way of rationalizing what is going on - by no means does it have solid foundation in theory. I would also like to hear more theoretically backed / accurate answer.

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13 minutes ago, vlaiv said:

In outer space - very near vacuum, probability of photon arriving later is tiny, very tiny, and most of photons arrive "on time". So in principle, given enough photons, we would measure some of them as being a bit late, but most of time they would be on time.

Thank you so much for your informative answer but it does raise the question of the probabilities of the light slowing when its been travelling through space for 13 billion years or so and the accuracy of far object distance measurements.

Alan

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its the speed of dark im worried about ie its too light at night  sorry i couldnt help myself. charl.

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2 minutes ago, Alien 13 said:

Thank you so much for your informative answer but it does raise the question of the probabilities of the light slowing when its been travelling through space for 13 billion years or so and the accuracy of far object distance measurements.

Alan

I'm not sure that we measure anything in terms of distance from actual "time of flight". Most measurements are based on red shifts rather than time it took for light to reach us. Even if we do use light speed, I think that slowing down of light in interstellar medium is something like couple of days per 13by at most - very tiny percentage.

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I'm sat outside with the scope at the moment. Its either pitch black and freezing cold or as it is tonight, pleasant but too bright. Damn moon will be here soon as well!

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3 minutes ago, xtreemchaos said:

its the speed of dark im worried about ie its too light at night  sorry i couldnt help myself. charl.

Absolute darkness fascinates me, I doubt there is any place in the universe where you could not detect something even in a box made of "vanta black"

Alan

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This is indeed a good question. You are quite right a true vacuum even if it exists could not be observed as observing it would require energy and stop it being a vacuum!

However, there a regions where the interaction of light with the medium is so low the speed is effectively c.

The interaction of light with matter is very complex and required QED to explain it. But a classical explanation of how light interacts with a transparent medium works well.

As it is transparent no absorption takes place. As an em wave enters the medium its oscillating electric field causes the electron to oscillate in sympathy . They absorb some energy from the em field but re-radiate it slightly out of phase with the exciting field with the net result that the is speed is reduced.

Regards Andrew

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All good and all, but whats the speed of dark?

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2 minutes ago, MarsG76 said:

All good and all, but whats the speed of dark?

It's imaginary = ic

(or not as it's dark)

Regards Andrew

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15 minutes ago, andrew s said:

It's imaginary = ic

(or not as it's dark)

Regards Andrew

I disagree that it's imaginary.. my baby boy makes a lot of dark matter and fast....

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18 minutes ago, MarsG76 said:

All good and all, but whats the speed of dark?

This is another of my crazy thoughts, if for example you have a light shining at a distance of 10 miles from an observer then switch it off is the time delay before its detected the same as it would be at the speed of light?

A more in depth and a bit off topic thought I have had is what would happen if the Sun suddenly vanished, would space-time bounce and would the effect be instantaneous or again at the speed of light?

Alan

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4 minutes ago, Alien 13 said:

This is another of my crazy thoughts, if for example you have a light shining at a distance of 10 miles from an observer then switch it off is the time delay before its detected the same as it would be at the speed of light?

A more in depth and a bit off topic thought I have had is what would happen if the Sun suddenly vanished, would space-time bounce and would the effect be instantaneous or again at the speed of light?

Alan

Yes to the first question.

Ignoring the sun just vanishing but in that spirit gravitational changes travel at the speed of light. There is a proof that if there is a speed limit (I.e. c) then there can only be one such limit.

Regards Andrew

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16 minutes ago, MarsG76 said:

I disagree that it's imaginary.. my baby boy makes a lot of dark matter and fast....

I have seen boys from the black stuff but not the other way around. But then we had girls.

Regards Andrew

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8 hours ago, andrew s said:

Regards Andrew

Very sensible.

Olly

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