# Can light REALLY slow down?

Once again I have contradictory impressions from PF postings and other web-sites with regards to slowing the speed of light. I am seeking a solid, definitive answer if possible.

Can the speed of light actually decrease, or, is this an "artifact" of delayed emission after absorption; such as going through crystals or the BOSE condensate?

Danger
Gold Member
This isn't definitive, but sort of boils it down. The speed of light is the 'artifact' that depends upon the medium; the speed of an individual photon is always 'c'.

the speed of an individual photon is always 'c'.

That's my take as well, so why is there confusion from others? I would argue that the so-called "slowing down" is the result of a delayed reaction on emission/re-emission while c is maintained.

Yet, I'm not sure... hence my post.

Try reading posts here on physics forums...you can observe apparently different speeds when viewing light via coordinate frames of reference...and red and blue shifting of light,say cosmic background radiation, for example, due to the expansion of space also can be confusing to us novices. When light is viewed in gravitational fields from a distance the curvature also seems to yield different observations. And the definitions of "accelerating" observers also appears different in current physics than when Einstein discussed the need for inertial frame observations....

I'm also still trying to sort through incorrect postings here since not only is the phenomena in different situations unlcear, gravity and non gravity and different frames of reference, accelerating and non accelerating, but distance also plays a part in observations and nomenclature and conventional terminology subject to interpretation.

Try reading the " Lightspeed" thread under Special and general relativity section of the Forums and also the other thread I reference there in post # 16....I have been torturing DrGreg with questions ...still ongoing...and I am now believe HE understands....I am not at all sure yet that I do as nomenclature in books I read appears a bit different than that accepted here....
I think it's safe to say that as you view lightspeed you observe "c" as long as space is flat, but the conditions on that are somewhat detailed...and someone here will surely disagree. One liners from many posters just aren't reliable as the context is usually obscure.

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Cthugha
That's my take as well, so why is there confusion from others? I would argue that the so-called "slowing down" is the result of a delayed reaction on emission/re-emission while c is maintained.

Yet, I'm not sure... hence my post.

The slowing down is not really due to absorption and emission processes in the usual sense. See for example the forums FAQ https://www.physicsforums.com/showpost.php?p=899393&postcount=4 [Broken] for good reasons why, especially the final part, although I think the wording ("does not absorb this photon and it is re-emitted but with a very slight delay") is a bit strange. Usually the wave picture gives an easy and good description of the problem in most materials. The incident light field produces some oscillating polarization, for example oscillations in the electron gas inside the sample, which in turn produce some em field. Now the superposition of the original and the field caused by the polarization behaves like an usual em field with a different speed of light.

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rcgldr
Homework Helper
I posted this in GR section, but no responses so far. Doesn't gravity affect the speed of light by accelerating or decelerating it? Isn't that the reason given for why light can't escape from a black hole?

Danger
Gold Member
I thought that gravity just flattens light; ie: stretches it to a longer wavelength.

LURCH
I posted this in GR section, but no responses so far. Doesn't gravity affect the speed of light by accelerating or decelerating it? Isn't that the reason given for why light can't escape from a black hole?

No, the light doesn't "slow down," it bends. From inside the event horizon, all "futures" end at the center of the BH. So, once emitted, light (along with anything else that is moving) travels toward the center. But it still travels at lightspeed (within the local reference frame, of course).

It might be helpful to consider the cases where the speed of light can be considered to be other than c. I can only think of four: 1) Propagating through a media, 2) Microwaves propagating down a wave guide, 3) Light in a laser cavity, 4) the non-local velocity as measured in another inertial frame due to gravitational effects. Any others?

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Time is relative, so couldn't our observation of light make it appear as if it slowed down, but it actually is just our perspective? In a nutshell..?

Time is relative, so couldn't our observation of light make it appear as if it slowed down, but it actually is just our perspective? In a nutshell..?

You've sort of switched the argument around. Relativity begins with the premise that the speed of light is constant. From this, and few other assumptions, elapsed time is relative to the observer.

Right. Maybe i'm just confused, or wording this incorrect.

Speed of light is constant, but an observer's relative point of view may apparently distort it. Ex. Light is traveling along *la ta da* but a person who is affected by some strange gravitational force or otherwise, it observing it much slower than it actually is traveling?

I think i'm off kilter here. Lol. I though I had something :/

This is quite interesting. Everyone's reponses are very helpful.

ZapperZ
Staff Emeritus
This is quite interesting. Everyone's reponses are very helpful.

I'm not sure how it can be. You asked for a specific situation in which light is slowed down ("Can the speed of light actually decrease, or, is this an "artifact" of delayed emission after absorption; such as going through crystals or the BOSE condensate?"), and you're getting responses coming in from all directions (gravitational force, observers view, etc), not just specific to your context. I'd say this simply adds to the confusion.

This issue has been tackled in our FAQ because it has been asked so many times. For the specific case of light moving in a medium, there is zero ambiguity. Photons always moved at c, and the mechanism that causes the apparent slowdown is quite clear. The question on what is being measured in such situation is what needs to be examined, since in practically ALL situations, when one says "the speed of light", one inevitably measures the group velocity of light! When one realizes this, then a lot of things falls into place, including the apparent FTL group velocity measured in the NEC experiment in anomalous dispersive medium.

Zz.

I'm not sure how it can be. You asked for a specific situation in which light is slowed down ("Can the speed of light actually decrease, or, is this an "artifact" of delayed emission after absorption; such as going through crystals or the BOSE condensate?"), and you're getting responses coming in from all directions (gravitational force, observers view, etc), not just specific to your context. I'd say this simply adds to the confusion.

Zz.

It did, Zapper, and I was fully aware of that. Sometimes I can be too polite.

What I think I understand is that individual photons move at "c" in a vacuum or a gravitational field; In the latter they don't always appear as "c". But this,too, maybe an oversimplification.

How does one find the FAQ on this subject....

I think you need to think about exactly what you mean by "the speed of light". If you mean the speed of individual photons: they always move at c, which is the speed of light in a vacuum.
The speed of light in a medium is usually slower than c (the group velocity of a lightwave can even be larger than c) but the individual photons still move at c! The 'problem' is that people do not generally mean the speed of the individual photons when they say the speed of light...

isnt group velocity the speed at which energy is transmitted?

rcgldr
Homework Helper
I posted this in GR section, but no responses so far. Doesn't gravity affect the speed of light by accelerating or decelerating it? Isn't that the reason given for why light can't escape from a black hole?

No, the light doesn't "slow down," it bends.
Is there an explanation for why gravity only accelerates light perpendicular to it's direction of travel, while never affecting any component in the direction of travel?

I think you need to think about exactly what you mean by "the speed of light". If you mean the speed of individual photons: they always move at c, which is the speed of light in a vacuum.

So is it safe to say that photons, under any condition and reference frame, travel at C? If this is the case, and we consider a light source such as a star and a completely reflective solar sail moving away from this star, the light emitted from the source will hit the sail and change velocity of the photons from C to -C accordig to SRT. However, the photons hitting the sail will cause a transfer in momentum of the sail pushing it away from the star and increasing its velocity. Since the sail is perfectly reflective the photons momentum must be conserved, yet momentum is being transfered to the sail. This would violate conservation of energy/momentum.

I've pondered this problem for a while and can not find what is wrong about my logic. If the speed of photons can not slow down under any circumstances how does this senario not violate conservation of momentum?

As for the black hole thing, I believe light doesn't travel directly into a black hole normal to its graviational field, but the gravity of the hole bends the light to such a degree it sort of "spirals" it in. But unlike water going down the drain, I think the light spiraling around the black hole does have a curl and therefor does have an angular acceleration due to the gravitational field.

So is it safe to say that photons, under any condition and reference frame, travel at C?

Yes and no!! , see Lightspeed thread under general and special relativity.....light always travels at c in a open space but will not always be observed to do so...only local observations in a free falling (inertial) frames see light at "c"...Accelerating observers only see "c" under special definition....when accompanied by a co moving (same speed) inertial observer observer.

As for the black hole thing, I believe light doesn't travel directly into a black hole normal to its graviational field, but the gravity of the hole bends the light to such a degree it sort of "spirals" it in.

A local free falling (inertial) observer sees light move normally (in normal time) and pass thru the event horizon; a distant observer sees light slow...and curve and it never reaches the event horizon.

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So is it safe to say that photons, under any condition and reference frame, travel at C? If this is the case, and we consider a light source such as a star and a completely reflective solar sail moving away from this star, the light emitted from the source will hit the sail and change velocity of the photons from C to -C accordig to SRT. However, the photons hitting the sail will cause a transfer in momentum of the sail pushing it away from the star and increasing its velocity. Since the sail is perfectly reflective the photons momentum must be conserved, yet momentum is being transfered to the sail. This would violate conservation of energy/momentum.

I've pondered this problem for a while and can not find what is wrong about my logic. If the speed of photons can not slow down under any circumstances how does this senario not violate conservation of momentum?

I am not that specialized on this subject, but I am pretty sure that the speed of a photon has very little to do with its momentum, at least not in the same way that speed matters in momentum of massive particles.
The momentum of a photon is $$p = \frac{h}{\lambda}$$ where h is planck's constant and lambda is the photon's wavelength.
I think what happens when a photon reflects off the solar sail you mention is that the wavelength of the photon changes, which changes it's momentum and thus conservation of energy/momentum still applies.

Of course you can write $$p = \frac{h}{\lambda} = \frac{hf}{c}$$ which again depends on the speed of the photon (c) but since that is always constant, the only parameter that can change is f, the frequency (and thus the wavelength).

Is it just your point of reference? If I am looking into a transparent chamber where the temp is close to absolute zero, for some molecules inside and an external laser is fired and emissions absorbed, by those molecules. The photons in reference to me from outside the chamber are moving slowly, while still moving at "c". If I am inside the chamber, at the temp of those molecules they are moving at "c", but would appear to be moving very quickly indeed. Theoretical of course, because my thought process is very slow near absolute zero. Just my 2 cents.

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rcgldr
Homework Helper
If the speed of photons can not slow down under any circumstances how does this senario not violate conservation of momentum?
The photons are reversing direction, their momentum has changed signs, and the momentum of the sail should be increased by twice the momentum of each reflected photon, in the direction of the photon relative to the sail. If photons are captured and not reflected, then the momentum should be increased by the photons momentum.

Here is a question I have and I'm probably in over my head here. If the gravity of a black hole prevents light from escaping would that not mean that gravity has an affect on the speed of light? Perhaps the gravity of a black hole is so massive that the gravity we experience here on earth has such a negligible affect? Take it easy on me guys.