# What happens to relativity when light is slowed down?

#### Bartholomew

I know that light slows down when it goes through certain substances. Does that mean that relativistic effects would be more visible in, say, water than in space? Or would it be possible to travel faster than light in those circumstances, though still less than c?

I don't understand relativity very well, so please excuse me if I sound like an idiot.

Related Other Physics Topics News on Phys.org

#### Ambitwistor

Relativistic effects are related to the speed of light in vacuum (c), so you won't see them more easily merely by slowing light down in a medium. It is possible for matter to travel faster than light in a medium, though less than c: when charged particles do so, they produce what is known as Cherenkov radiation (kind of a "sonic boom for light").

#### russ_watters

Mentor
Originally posted by Bartholomew
I know that light slows down when it goes through certain substances. Does that mean that relativistic effects would be more visible in, say, water than in space? Or would it be possible to travel faster than light in those circumstances, though still less than c?

I don't understand relativity very well, so please excuse me if I sound like an idiot.
You aren't an idiot, its just a trick!!

Light does not ever slow down. It always travels at exactly C.

Unfortunately high school physics teaches that light slows down in a medium. That is true only as an oversimplification and leads to a lot of problems for people who go on to learn more about physics (such as relativity). Personally, I think that the basics of relativity must be taught when the behavior of light is discussed.

Anyway, how refraction REALLY works is that light hits the PARTICLES in the medium and gets absorbed and then re-emitted. The time delay between absorption and re-emission causes the AVERAGE speed of transit through the medium to be slower than C even though when the light exists as light it is traveling at C.

#### Ambitwistor

Re: Re: What happens to relativity when light is slowed down?

Light slowing down in a medium is a perfectly viable -- and relativistically correct -- concept in both classical and quantum electrodynamics, if by the "speed of light" you mean the phase velocity of the wave (or wavefunction).

#### wuyh

Re: Re: What happens to relativity when light is slowed down?

Originally posted by russ_watters
You aren't an idiot, its just a trick!!

Light does not ever slow down. It always travels at exactly C.

Unfortunately high school physics teaches that light slows down in a medium. That is true only as an oversimplification and leads to a lot of problems for people who go on to learn more about physics (such as relativity). Personally, I think that the basics of relativity must be taught when the behavior of light is discussed.

Anyway, how refraction REALLY works is that light hits the PARTICLES in the medium and gets absorbed and then re-emitted. The time delay between absorption and re-emission causes the AVERAGE speed of transit through the medium to be slower than C even though when the light exists as light it is traveling at C.
the index of a media is derived from ¦Å and ¦Ì. They are factors which reduce/enhance the effect of the electric field and magnetic field, so does it to the transmiting of radio wave.

What if light is absorbed and emitted in medium? Emission is hard to maitain a uniform direction, how could u explain the direction of light in water?

As we all know, a material can only absorb a photon which has certain frequency match with the energy gap between to energy step. how could u explain all light with different frequency can transmit in water, if the transmission mechanism is as what you've stated.

:)

#### Integral

Staff Emeritus
Gold Member
The direction of emitted photons is random, with a perhaps smidge of varition caused by the initial adsorption. Reflections appear where they do simply because it is the shortest time path between your eye and that object. I would recommend a book called QED By Richard Fynman. He addresses this topic prety well in laymen terms.

For a photon to be adsorbed by a atom or molecule there only has to be a energy band corresponding to the energy of the photon. Water is a large molecule and has MANY availble orbitals for the electrons to move to.

#### Bartholomew

So, what actually causes cherenkov radiation?

#### Ambitwistor

Originally posted by Bartholomew
So, what actually causes cherenkov radiation?
See, for instance,

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/cherenkov.html
http://www.shef.ac.uk/physics/teaching/phy311/coherent.html [Broken]
http://rd11.web.cern.ch/RD11/rkb/PH14pp/node26.html
http://www.cakes.mcmail.com/cerenkov/cerenkov.htm [Broken]

Summary: a charged particle passing through the medium will polarize molecules in the medium as it travels, which depolarize after it has passed. The depolarization emits light waves. If the charged particle travels faster than light in the medium, then the waves emitted by depolarization form a coherent "shock" wavefront, like a sonic boom.

Last edited by a moderator:

#### MathematicalPhysicist

Gold Member
Re: Re: What happens to relativity when light is slowed down?

Originally posted by Ambitwistor
when charged particles do so, they produce what is known as Cherenkov radiation (kind of a "sonic boom for light").
what is their speed in this radiation?

#### Ambitwistor

Re: Re: Re: What happens to relativity when light is slowed down?

Originally posted by loop quantum gravity
what is their speed in this radiation?
What does "their" refer to?

A charged particle that produces Cherenkov radiation travels at some speed between the speed of light in the medium, and the speed of light in vacuum. The Cherenkov radiation itself travels at the speed of light in the medium, as long as it is in the medium, as you would expect.

#### russ_watters

Mentor
Re: Re: Re: What happens to relativity when light is slowed down?

Originally posted by wuyh
What if light is absorbed and emitted in medium? Emission is hard to maitain a uniform direction, how could u explain the direction of light in water?
Conservation of linear momentum.
As we all know, a material can only absorb a photon which has certain frequency match with the energy gap between to energy step. how could u explain all light with different frequency can transmit in water, if the transmission mechanism is as what you've stated.
You just described the mechanism. If something can't absorb a photon and hold its energy, it is unstable and quickly re-emits the photon. But it does absorb and re-emit the photon. This is part of the same oversimplification that I described before. Simply saying that a medium absorbs or transmits light is incomplete and leads to incorrect conclusions about what that means.

#### wuyh

Re: Re: Re: Re: What happens to relativity when light is slowed down?

Originally posted by russ_watters
Conservation of linear momentum.
if the photons are obsorbed and re-emitted, can we consider this process as a two dimension collision case?

if so, conservation of linear momentum does not guarantee the output particle has the same direction of input paticle. Part of energy of input particle(photon) may be shifted to molecule of media, and the freqency of photon may changed for it's energy is reduced..

then the color of the light is changed...

could u give me a clear/detailed explain?
thanks..

#### Integral

Staff Emeritus
Gold Member
Re: Re: Re: Re: Re: What happens to relativity when light is slowed down?

Originally posted by wuyh
if the photons are obsorbed and re-emitted, can we consider this process as a two dimension collision case?

if so, conservation of linear momentum does not guarantee the output particle has the same direction of input paticle. Part of energy of input particle(photon) may be shifted to molecule of media, and the freqency of photon may changed for it's energy is reduced..

then the color of the light is changed...

could u give me a clear/detailed explain?
thanks..
Why do you suppose objects have a characteristic color?

Emitted photons travel in nearly random directions. The emitted photons may or may not be of the same frequency as the adsorped, it depends on the decay rate and if there is a favored decay path for the electron. Again this is the mechanism which detemines the color of things.

#### russ_watters

Mentor
Re: Re: Re: Re: Re: What happens to relativity when light is slowed down?

Originally posted by wuyh
if the photons are obsorbed and re-emitted, can we consider this process as a two dimension collision case?

if so, conservation of linear momentum does not guarantee the output particle has the same direction of input paticle. Part of energy of input particle(photon) may be shifted to molecule of media, and the freqency of photon may changed for it's energy is reduced..

then the color of the light is changed...

could u give me a clear/detailed explain?
thanks..
What you are describing (for the most part) does happen some times. In the yellow lens on your turn signals for example. If the output particle (not exactly an accurate characterization of light, but I'll let it go) has a different energy then that means some of the energy was absorbed by the particle. Extra energy becomes heat.

And certainly light can be relfected or refracted in random directions. For example, on a white piece of paper.

#### kartiksg

Hi,

Im a little confused here... what exactly DOES happen when photons stike an interface between 2 media? People mentioned that it is absorbed and transmitted and hence the slow down. But why the particular direction change... and how can we explain Snell's law on the basis of collisions.. and what is 'shortest time path'? Can I have a formal definition of that plzz?

You just described the mechanism. If something can't absorb a photon and hold its energy, it is unstable and quickly re-emits the photon. But it does absorb and re-emit the photon.
I thought the electron never gets excited at all... it only gets excited if it has a energy band. Plz correct me if im wrong.

Also, how exactly is colour produced? For example.. I have been taught that in co-ordination compounds of d-block elements, the coloured ion arises from d-d electron transistions because of the splitting of the energy level of the d-orbitals under the effect of the attached ligands. I wonder.. if the colour arises due to preferential absorbtion of some wavelengths of light by the electron transition between d orbitals, then shouldnt the same wavelength be emitted when the electron falls back to the lower d orbital? Why arnt we able to see those photons?

Integral pointed to QED by Feynman... I would like to know if there is some good online resource on QED as well.... possibly free

Thanks for any help

Kartik

#### cucumber

Re: Re: What happens to relativity when light is slowed down?

Originally posted by russ_watters

Anyway, how refraction REALLY works is that light hits the PARTICLES in the medium and gets absorbed and then re-emitted.
just wondering: what governs whether a photon is emitted by a substance or absorbed (ie why does visible light pass through glass, but not through aluminium)

oh, and how come light travels through glass in a straight line, if it constantly gets absorbed and reemitted?? why doesn't it get scattered all over the place??

thanks.
cucumber.

#### yogi

Why light is slowed in a medium is a question that has arisen before on these boards - and never answered with any proof other than assertions re absorption and re-emission. This notion may not be correct - it would seem that the individual atoms would appear to radiate at different frequences and in different directions - something not generally observed in most optical media. The mechanism may be entirely different - photons may be slowed as they pass near the fields of the electrons and nuclei - rather than being absorbed and re-emitted - it might even be related to close gravitational encounter with the particles that comprise the medium -while we don't think of G effects as being very strong on the scale of atoms - they could be if the photon passes very close because of the inverse dependence - sort of a Shapiro effect on a small scale multiplied by the billions of occurrances that would be experienced in passing through a solid or liquid, i.e., the photon path gets effectively lengthened as it passes by the many perturbing influences of the nearby atoms

#### Integral

Staff Emeritus
Gold Member
Originally posted by yogi
Why light is slowed in a medium is a question that has arisen before on these boards - and never answered with any proof other than assertions re absorption and re-emission. This notion may not be correct - it would seem that the individual atoms would appear to radiate at different frequences and in different directions - something not generally observed in most optical media. The mechanism may be entirely different - photons may be slowed as they pass near the fields of the electrons and nuclei - rather than being absorbed and re-emitted - it might even be related to close gravitational encounter with the particles that comprise the medium -while we don't think of G effects as being very strong on the scale of atoms - they could be if the photon passes very close because of the inverse dependence - sort of a Shapiro effect on a small scale multiplied by the billions of occurrances that would be experienced in passing through a solid or liquid, i.e., the photon path gets effectively lengthened as it passes by the many perturbing influences of the nearby atoms
Whatever.

Do you expect to be given a graduate level course in QM on these boards? What sort of "proof" do you require? If you had even a basic understanding of these matters you would see that all who are educated in QM say exactly the same thing. Your personal lack of understanding is not necessarily shared by the world at large. Perhaps you should be reading closely the posts of Chroot, Ambitwistor, Tom, Marcus and others who have knowledge of these things. What you have posted above is not based on anything other then your personal believe. I for one do not much like seeing this sort of thing posted in place of Current Physical theory. Please restrict your replies to that realm. Your beliefs can be posted in Theory Development.

#### yogi

Integral - your posts reveal a total lack of appreciation for anything other than unfounded dogma - there are no predictive theories that explain the magnitude of the slowing based upon the idea of absorption and re-emission... if you cant tolerate new ideas you should have been a preacher - I have read many of your posts in the past - they are often plain wrong - you used to post information about some accident you had that affected your abilities - are you sure you have fully recovered??? If Einstein after many years of pondering the nature of the photon did not understand its nature - how do you claim such a talent???

#### Ambitwistor

Originally posted by yogi
Why light is slowed in a medium is a question that has arisen before on these boards - and never answered with any proof other than assertions re absorption and re-emission.
As Integral said, nobody is going to give you a graduate education here, especially if you're rude to them.

This notion may not be correct - it would seem that the individual atoms would appear to radiate at different frequences and in different directions - something not generally observed in most optical media.
Atoms in a background electromagnetic field radiate quite differently than free atoms. In particular, if you put an electromagnetic wave through them, they radiate preferentially in the direction of propagation of the wave.

The mechanism may be entirely different - photons may be slowed as they pass near the fields of the electrons and nuclei - rather than being absorbed and re-emitted - it might even be related to close gravitational encounter with the particles that comprise the medium -
Quantum electrodynamics has a perfectly adequate description of the propagation of light through a medium. If you choose to look at it from a field perspective, then the phase velocity of the wavefunction is slowed down quite analogously to the classical case. If you choose to look at it from the particle perspective, then it is described by photon absorption and emission, as others have said. Atoms take a finite time to drop down from an excited state, so this slows down the effective propagation speed of light, even though real photons propagate at c.

while we don't think of G effects as being very strong on the scale of atoms - they could be if the photon passes very close because of the inverse dependence
Feel free to calculate the gravitational cross-section of an electron as seen by a photon. I think you will find it is totally neglibible compared to the electromagnetic cross-section.

there are no predictive theories that explain the magnitude of the slowing based upon the idea of absorption and re-emission...
I shouldn't need to point this out, but: your personal ignorance of modern physics does not constitute an inability on the part of modern physics to describe physical phenomena.

If you want to learn how to do the calculation, you need to learn quantum optics. I have heard that Siegman's book on lasers is good. I haven't read it myself, so I don't know if it contains this particular calculation. But you should spend some time trying to learn some actual physics before you make such statements.

Last edited:

#### yogi

Another example of sensitivity to new concepts - whatever the mechanism of slow down - if you can show me how to calculate the delays based upon sound methods - fine - but at present this cannot be done without a number of fiat(ly) introduced factors. Moreover - where did I say the photon was effected by the electron(s) - I said its path might be influenced by the mass of the nucleous - and if you are within 10^-15 meters from the atomic center, the G forces are quite large - much larger than the force acting on a photon as it grazes the Sun - and we know that the Sun bends light - so if the paths of the photons are bent at the interatomic level - they will be longer - and therefore the time to traverse the material will be larger -

and don't jump on my case for blasting integral - he started the insults - read his post

#### Janus

Staff Emeritus
Gold Member
Originally posted by yogi
and if you are within 10^-15 meters from the atomic center, the G forces are quite large - much larger than the force acting on a photon as it grazes the Sun -
Where did you get that idea? Given the mass and radius of the Sun, you get a value of 276 m/sec² for the acceleration due to gravity at the surface of the sun.

Given the the mass and radius of a silicon atom, you get a value of .0000003466 m /sec², a much smaller value.

Last edited:

#### Ambitwistor

Originally posted by yogi
if you can show me how to calculate the delays based upon sound methods - fine - but at present this cannot be done without a number of fiat(ly) introduced factors.
What factors? Do you even know how to do the calculation? If so, please summarize it here.

Moreover - where did I say the photon was effected by the electron(s) - I said its path might be influenced by the mass of the nucleous
You spoke of both the electrons and the nucleus. If you're talking about the nucleus, it's even more unlikely for the photon to interact.

- and if you are within 10^-15 meters from the atomic center, the G forces are quite large - much larger than the force acting on a photon as it grazes the Sun -
Are they?

Anyway, the electromagnetic forces are vastly larger, no matter how close you are. They will always dominate.

so if the paths of the photons are bent at the interatomic level - they will be longer - and therefore the time to traverse the material will be larger
Like I said, go ahead and calculate the magnitude of the effect, if you can. Then calculate the probability of a photon actually coming close enough for the magnitude to be right. I think you will be surprised.

and don't jump on my case for blasting integral - he started the insults - read his post
I didn't see any insults. I did see that he correctly noted that you have a lack of understanding of the quantum mechanical explanation of light propagation -- in fact, you claim that there isn't a correct quantum mechanical explanation involving photon absorption and re-emission. That is simply wrong, which is why I referred you to a quantum optics text.

Last edited:

#### yogi

Depending upon the size you assign to the photon and the number of nucleon encounters in a given length of a medium - you may or may not have a gravitational influence - yes - I know the electrical fields are many orders of magnitude greater than the G fields (10^42 if you consider the electron-positron electrical to gravitational force ratio)- so it may well be that the electrical properties are determinative of the velocity characteristics (I don't have any problem with that), and I never said they were not - but what are the effects of the electromagnetic fields on photons - these are less known than the affect of mass on light. The velocity of propagation of both light and radio waves is determined by the permeability and permittivity (admittedly electrical properties) - what I don't buy is the absorption and release as a physical explanation - take a one meter radio wave photon - how can it get stored in some kind of atomic configuration and released - yet its velocity will be measured to be slower in a physical medium. The reason why the G field appeared worth considering was because photons are influenced thereby. Whatever a photon is - and whatever part of it is acted upon by matter - it is not unreasonable to investigate whether the forces it experiences as it passes through the atomic field may be determinative of its velocity

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving