A few questions & theories about light:

[mAdD INDIAN]

..so I started thinking about light one day and thought about its speed (which is currently 300,000km/s).

Now what would happen if the speed of light was say, reduced to the speed of sound or even lower. How would our percieved "sight" changed. When we see images, would they slowly appear (like fading in??)? Is it also possible to somehow slow down the speed of light by making it go thru different mediums?

Also I have a question about the ability for an object to absorb light so it will cloak itself..but I'll add that later...

gotta go now.

 

[imgod2u]

Light does slow down in different mediums. Think of what you see when you're underwater, light is bend by the water and your view is distorted. All in all, you wouldn't see things "fading in" because photons are packeted, not gradual. Whether a photon hits the retina of your eye at 300 km/s or 20 m/s really doesn't matter, your retina will pick up the pattern accordingly. Of course, if light was that slow, everything you see will be in "the past" to a degree that you will notice it (this happens anyway but at 300 km/s, you really can't notice the difference). I.e. if someone moves, it'll take a while before you are able to see his/her movement and by the time you can react, he/she is probably somewhere else already. Would make baseball games interesting though.

 

[KenGr]

If light was so slow you could perceive the time difference between things within your range of vision, it would start to get interesting. You would start to see a type of "frequency shift" effect. In the same way a train horn changes in pitch as it approaches and passes you, moving objects would seem to speed up and slow down as they approached and passed us. It would require an adjustment to our perception when driving, for instance.

 

[rimshaker]

Some researchers announced a while back that they have actually 'frozen' light. Totally forgot the specifics, but it was really interesting.

 

[CTho9305]

Originally posted by: rimshaker
Some researchers announced a while back that they have actually 'frozen' light. Totally forgot the specifics, but it was really interesting.

Yup, while it is in some medium. It is also slowed in certain media. However, once it leaves that material, it returns to near c (whatever the speed of light is through air / the liquid in your eyeball)

 

[PowerMacG5]

Here is the story you are talking about with "freezing" light:
http://www.nature.com/nsu/010125/010125-3.html#1
http://www.sciencenews.org/20010127/fob1.asp
http://www.cipi.ulaval.ca/news/sci_news/2001/01/22-1_cipi.shtml

 

[Agent004]

Originally posted by: mAdD INDIAN


...Also I have a question about the ability for an object to absorb light so it will cloak itself..but I'll add that later...

Black objects obsorbs lights (more correctly, the reason it was black or viewed as black is because it absorbs the visible spectrum of light which we see)

 

[Sahakiel]

Easiest way to cloak: Probably bending light around you with an EM field or something similar. A gravity well would probably do as well, assuming it has the right characteristics, but it's not hard to see the problems associated with that.

Biggest problem with cloaking : How are you gonna see where you're walking?

 

[Geniere]

Actually, the speed of light passing through a medium such as water is the same as it is in a vacuum. The exit photon is not the same photon that entered. Each entrance photon will be absorbed by an atom?s electron which moves to a higher energy level then falls back to a lower energy level and in doing so emits a photon. Eventually a photon exits, but it is not the original one. There is a propagation delay, but any photon traveling between atoms (which is after all a vacuum) moves at the speed of light (C). The propagation delay is perceived by an observer to be a reduction in the speed of light. Photons must always travel at the speed of light.

Regards

 

[dejitaru]

Originally posted by: mAdD INDIAN
..so I started thinking about light one day and thought about its speed (which is currently 300,000km/s).

Now what would happen if the speed of light was say, reduced to the speed of sound or even lower. How would our percieved "sight" changed. When we see images, would they slowly appear (like fading in??)? Is it also possible to somehow slow down the speed of light by making it go thru different mediums?

E=mc^2, so the act of walking would be difficult.
There's other perceptual stuff I can't process now.

Also I have a question about the ability for an object to absorb light so it will cloak itself..but I'll add that later...

If the light were absorbed, the object would appear black; it would have to transmit the light undistorted such as to reveal what is behind the object. There are some devices which use fiber optics to do this.

 

[OrionAntares]

I've heard that they've been able to both stop light and speed it up past the "speed of light." I remeber seeing the later on a news program maybe a year or so ago.

Originally posted by: mAdD INDIANAlso I have a question about the ability for an object to absorb light so it will cloak itself..but I'll add that later...

As for this, the "color" black is the absense of reflected light. Since color is produced by colored light reflecting off an object and being interpreted by your eyes and brain. To cloak an object you would need to bend the light around it so that you would be able to, for all practical purposes, be albe to see "through" it, but it would still be there inside of the light around it.

 

[heliomphalodon]

Originally posted by: Geniere
Actually, the speed of light passing through a medium such as water is the same as it is in a vacuum. The exit photon is not the same photon that entered. Each entrance photon will be absorbed by an atom?s electron which moves to a higher energy level then falls back to a lower energy level and in doing so emits a photon. Eventually a photon exits, but it is not the original one. There is a propagation delay, but any photon traveling between atoms (which is after all a vacuum) moves at the speed of light (C). The propagation delay is perceived by an observer to be a reduction in the speed of light. Photons must always travel at the speed of light.

Hmmm... By your reasoning, light never travels through any medium at all. I think you're wrong on this. Photons that are absorbed by the electrons orbiting the atoms of the medium, then re-emitted, would be scattered.

In any case, the time required for a light pulse to traverse a given distance in a medium such as water is greater than the time required for a light pulse to traverse the same distance in vacuum. This is what is meant when one says that "light slows down" in water. The ratio of these times is the refractive index of the medium.

Cherenkov radiation is produced in the water around an atomic pile, as a result of electrons traveling through the water at a speed greater than that of light in water...

 

[Geniere]

Originally posted by: heliomphalodon

Hmmm... By your reasoning, light never travels through any medium at all. I think you're wrong on this. Photons that are absorbed by the electrons orbiting the atoms of the medium, then re-emitted, would be scattered.

In any case, the time required for a light pulse to traverse a given distance in a medium such as water is greater than the time required for a light pulse to traverse the same distance in vacuum. This is what is meant when one says that "light slows down" in water. The ratio of these times is the refractive index of the medium.

Cherenkov radiation is produced in the water around an atomic pile, as a result of electrons traveling through the water at a speed greater than that of light in water...

Let me try to clarify.

A light pulse consisting of 1 million photons traveling in a vacuum strikes a 1cm thick piece of glass. Eectrons in the glass pane will absorb each of the 1 million photons. The electrons will then re-emit 1 or more photons and return to the original energy state. If it re-emits more than one photon returning to a lower energy state, those photons will be of lower energy (different color) than the incident photon. Many of the photons are absorbed in the glass and the effect will be measurable as a rise in temperature of the glass. Some of the re-emitted electrons are as you say scattered and may not be visible to an observer on the other side of the glass pane. So out of the 1 million entrance photons, all interact with electrons, perhaps 1/4th are absorbed or scattered by the glass. What the observer perceives is a decrease in light intensity, color distortion, and a propagation delay. Imagine what would happen in a 100-meter thick pane. All information carried by the incident photons would be lost.

Above assumes photons in the visible spectrum. Different phenomena occur at higher or lower photon energy levels but every photon moving in any medium travels at speed =C.

As stated in a physics forum:

"It is a process of adsorption and reemission. A photon "Hits" an atom and causes an electron to become excited to a higher energy state. By its very nature this new state is unstable and after a short period of time photons will be readmitted, only to be adsorbed by a neighboring atom. It is this time between adsorption and reemission which determines the speed of light in a medium."

One post describes an experiment in which a photon was ?frozen?. The photon itself was not ?frozen? and was long gone. What was frozen was information. The info was used to generate another identical photon at a later time.

Regards

 

[heliomphalodon]

Geniere, that's a very interesting interpretation. I think I understand, now, what you're saying -- in a certain precise sense, light does not travel through any medium other than vacuum. When light is "passing through" any other medium, the photons themselves are either traversing the vacuum between atoms of the medium, or they do not exist as photons at all, but rather their energy has been imparted to electrons orbiting atoms of the medium.

Such an interpretation gives rise to several questions:

1) If the medium to be traversed is very thin, do some of the photons pass through it without interacting with the atoms of the medium?
2) Are photons that are absorbed then re-emitted (but not scattered), absorbed on one side of an atom and emitted on the other side?
3) If the photons don't actually traverse the medium, but rather are passed through in "bucket-brigade" fashion, then how do lasers work?
4) Since the energy levels of electrons in the atoms of the medium are quantized, and the photons traversing the medium are quanta, is the medium opaque to all but certain specific "colors" of light?

I invite RossGr to comment on this, please.

 

[f95toli]

Originally posted by: heliomphalodon
Geniere, that's a very interesting interpretation. I think I understand, now, what you're saying -- in a certain precise sense, light does not travel through any medium other than vacuum. When light is "passing through" any other medium, the photons themselves are either traversing the vacuum between atoms of the medium, or they do not exist as photons at all, but rather their energy has been imparted to electrons orbiting atoms of the medium. Such an interpretation gives rise to several questions: 1) If the medium to be traversed is very thin, do some of the photons pass through it without interacting with the atoms of the medium? 2) Are photons that are absorbed then re-emitted (but not scattered), absorbed on one side of an atom and emitted on the other side? 3) If the photons don't actually traverse the medium, but rather are passed through in "bucket-brigade" fashion, then how do lasers work? 4) Since the energy levels of electrons in the atoms of the medium are quantized, and the photons traversing the medium are quanta, is the medium opaque to all but certain specific "colors" of light? I invite RossGr to comment on this, please.

1) Yes. But it has to be very thin.

2) No, not really. I don't know how to explain this using the photon-picture (it is easier to exaplain using electric dipoles)

3 and 4) A photon can only be absorbed if there are some suitable energy levels available in for example the atoms. That is, a photon with the energy 1 eV can only be absorbed if there are two energy levels in the medium with a difference in energy of 1 eV. This means that some frequencies (since the frequency is directly proportional to the energy of the photon) will simply pass through the medium without any interaction. Pure elements have a fairly simple distribution of energy levels (or spectra as it is called) but once they form molecules it becomes more complicated since the position of the energy levels changes due the binding energy, hence you get rotational spectra, vibrational spectra etc.

I will give you an example: Microwaves (lets say 10-1000 GHz) hardly interact at all with glass, this means that regular glass lenses do not work, instead you can use lenses made out of teflon to focus the microwaves (another good material is magnesium-oxide).

 

[Geniere]

Heliomphalodon, with apologies to mAdD INDIAN for taking over his post:

Your questions indicate a sharp learning curve. I hope I can keep up.

f95toli has answered as well as I can. Your question 2 is difficult to answer. I can say that most of the re-emitted photons tend to move in the same direction as the incident photons.

As far as the workings of a Laser, I believe the original Ruby Laser was a cylinder of ruby crystal with a mirror on one end and a half silvered mirror on the other end. The length of the ruby is a multiple of the wavelength of the emitted photons. Lasing action is initiated by strobe lamp flash. Only photons with the exact energy needed (dictated by quantum mechanics) are adsorbed by electrons bound to the crystalline structure of the ruby. Those electrons; now at a higher energy state (I believe it?s called a population inversion) re-emit a photon at a precise frequency when they return to a lower energy state. The first of the re-emitted photons stimulates other excited electrons to do likewise and initiates a chain reaction. The mirrors increase efficiency and tend to bunch the photons.

Truly, no one fully understands electro-magnetic radiation. All accepted theories assume the speed of light is inviolate. All calculations are based on it. Experimental data validates it to many decimal places. If it were found not to be so, many physicists would spend the rest of their days in a loony bin.

Regards

 

[RossGr]

Reemited photons are totally random, there is no such thing as angle of incidence and reflectlion for photons. The equal angle thing comes from the existance of a mimimum time path. It is the random nature that explains partial reflections.

For and execellent description of these phenomena find a copy of Richard Fynmans QED.

We really have a very good handle on the nature and behavior of light and its interaction with the electron structure of atoms. The last atomic pysicsists retired years ago, this is no longer even a active area of research, it is pretty well done.

 

[heliomphalodon]

I can see that my understanding of these phenomena is much weaker than I thought!
Thanks to everyone for their patience.

Time to get a copy of QED! -- after all, Feynman is a god in my personal pantheon...

 

[heliomphalodon]

One last (from me, anyway) comment on the original topic -- check out the "Mr. Tompkins" books by George Gamow. They're centered around the adventures of a guy in worlds where quantum effects are macroscopic or the speed of light is 100MPH, etc.

 

[RossGr]

Actually, in Mr Tompkins in Wonderland the speed of light is 15mph. The effects are explored while Mr. Tompkins rides his bike around town. Good books.

 

[KF]

I thought I understood this a bit before I read all this. Now I know I don't.

>For an excellent description of these phenomena find a copy of Richard Feynmans QED.

From what I recall, the path is explained by the existence of one path among all paths where things do not cancel. It is the physical interpretation of a mathematical treatment, wherein the solution is given by finding a minimum.

It is interesting that "the angle of incidence equals the angle of reflection" makes light follow the shortest distance between two points, given that it touches the mirror. In fact if the object were where it appears to be in the mirror, it would be a straight line.

In a similar way, the offset apparent position of an object in the water, making a stick appear to bend as it enters the water, is the result of light traveling the path which takes the least time from the object to the eye.

It's as if light knew where it was going before it got there.

It was guessed that light was an electromagnetic wave when James Clerk Maxwell formulated the electromagnetic equations, and deduced that the speed of an electromagnetic wave would be the ratio of two known quantities which give the strength of magnetism and of electrostatic attraction. The number came out to be the speed of light, which was also known through astronomical observation of the moons of Jupiter. It must have been amazing.

Einstein is supposed to have devised the theory of relativity when he did thought experiments imagining what the world would look like riding along with a light beam and saw things that were impossibilities.

Light is supposed to move slower in a gravitational field. I suppose the speed of light is always the same where it is, but since we may not be located there, it is different to us.

 

[RossGr]

From what I recall, the path is explained by the existence of one path among all paths where things do not cancel. It is the physical interpretation of a mathematical treatment, wherein the solution is given by finding a minimum.

This is correct, but the equal angle law, depends on our point of view. Note that even though our eye moves the angle between our eye, the mirror, and the object change so as to maintain AR = AI, the only way this could possibly happen is that there are photons taking EVERY POSSIBLE PATH. The angle which a photon takes when it leavea an atom is totally random, it is a quantum event after all.

It's as if light knew where it was going before it got there.

There is more to this statement then meets the eye! (pardon the pun)

Since a photon travels at the speed of light according to Special Relativity photons do not experiance time, or distance. So yes, as far as the photon is concerned it is adsorped the instant it is emitted, sort of like it knew where it was going. But then since length is contracted to zero, all points in the universe are adjacent so it goes everywere.

The number came out to be the speed of light, which was also known through astronomical observation of the moons of Jupiter. It must have been amazing.

This must have been a gut wrenching experiance, to compute this number and find himself looking at the speed of light, do you suppose he fully understood all the implications and turmoil which would follow his work? With that computation he pretty well buried Newtonion Physics, Physicsts spend the next 50yrs arguing about how to repair the rift caused.

Light is supposed to move slower in a gravitational field. I suppose the speed of light is always the same where it is, but since we may not be located there, it is different to us.

I think the reason light could be said to travel slower in Gravitational fields is that since space is curved and light follows space time geodesics it must take a path that is a bit longer then we precive. Most of our math assumes a nice flat Euclidiean space, to correctly compute the speed of light in a Gravitional field one must consider the underlying surface, which is not Eucledian. When the correct metic tensor is used, the speed of light remains constant.