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xbck1
2004-Jun-24, 02:25 AM
My brother has been bugging me with all manner of physics questions lately. The latest one was basically, "Why does light travel at that speed, and why doesn't it go at another one?" I wasn't sure how to answer him, so I just said "I dunno." I keep bugging him to just join the BABB, but he doesn't want to.

So, why does light go at that speed? :-s

wedgebert
2004-Jun-24, 02:33 AM
The speed of light is c because that is the solution to the wave equation. If you like math, the here is the derevation (http://people.ccmr.cornell.edu/~muchomas/P214/Notes/OtherWaves/node18.html)

Celestial Mechanic
2004-Jun-24, 04:25 AM
So, why does light go at that speed? :-s

Because it can.
On a more serious note, I'm not sure if any reason can be given for why the fundamental constants and particle masses are what they are. If we had any first principles that enabled us to calculate these things, chances are that there would be some other level of even more fundamental constants and we would then have a whole new set of mysteries to solve.

Normandy6644
2004-Jun-24, 05:10 AM
So, why does light go at that speed? :-s

Because it can.
On a more serious note, I'm not sure if any reason can be given for why the fundamental constants and particle masses are what they are. If we had any first principles that enabled us to calculate these things, chances are that there would be some other level of even more fundamental constants and we would then have a whole new set of mysteries to solve.

Interestingly enough, I was reading a bit about String/M-theory and how it might be possible (eventually) to explain why the fundamental constants have the values that they do, and even further, how other membranes (universes) could have different values for the same thing, i.e., a universe where particle physics is different and the electron weighs nearly as much as a proton! Impossible here, but elsewhere, you never know.

rohitbd
2004-Jun-24, 08:33 AM
IMO, the (value of) speed of light is relative in the sense that it has been derived using known (or perhaps assumed) constants. Had these constants been different, the (value of) speed of light would be different, even if the actual speed is the same in any case. For example, we consider the unit of time for all measurement purposes as 1 second. Had it been 1 minute (or if the value of 1 second were different), the value of the speed would have been different...it's just a case of calling a "tree" a "tree"...by all means, call it by any other name, it still doesn't change - it remains what it is.

Swift
2004-Jun-24, 12:51 PM
Keep in mind that c is a "speed limit". Its the speed of light in a vacuum. A photon can go slower in other media, it just can't go any faster.

Ut
2004-Jun-24, 02:14 PM
IMO, the (value of) speed of light is relative in the sense that it has been derived using known (or perhaps assumed) constants. Had these constants been different, the (value of) speed of light would be different, even if the actual speed is the same in any case. For example, we consider the unit of time for all measurement purposes as 1 second. Had it been 1 minute (or if the value of 1 second were different), the value of the speed would have been different...it's just a case of calling a "tree" a "tree"...by all means, call it by any other name, it still doesn't change - it remains what it is.

Well, you can give the numerical value of c any number you want, simply by measuring it in any units you wish. This is true for all constants. It's true for anything. 299792458 is entirely dependant on the length of the metre.

Science only answeres questions such as "How?" and "What?". "Why?" is the realm of philosophy and metaphysics.

ToSeek
2004-Jun-24, 02:22 PM
It has been suggested that light (or any electromagnetic radiation) is slowed by interactions with the "quantum foam" of virtual particles. If that foam could somehow be suppressed (as with the Casimir Effect (http://physicsweb.org/article/world/15/9/6)), light might actually go faster than the "speed of light."

Ricimer
2004-Jun-24, 03:41 PM
a brief description of the physics behind the derivation:

Light is an electromagnetic wave.

A changing electric field creates a changing magnetic field, and vice versa.

The electric field changes, creating a magnetic field, which causes the electic field to change...and it perpetuates itself.

The rate at which these fields can change, are determined by that equation, and are found to be based upon the two fundamental constants of electricity and magneticims (mu and epsilon). These two constants have their smallest value (thus minimum retardation due to the relationships in the equation) when measured in a vacuum, and are called Epsilon-naught (Eo) where the o is subscripted, and Mu-naught.

Because they can oscilate fastest at these values, they go as fast as they can in a vacuum.

In a medium Epsilon and Mu are altered, and is one reason why light goes slower in a medium. The other reason, light bounces around so it takes longer to get through.

milli360
2004-Jun-24, 03:58 PM
Ut:
Well, you can give the numerical value of c any number you want, simply by measuring it in any units you wish. This is true for all constants. It's true for anything. 299792458 is entirely dependant on the length of the metre.
At the present time, it's sorta the other way around. The meter is defined in terms of the speed of light.

Science only answeres questions such as "How?" and "What?". "Why?" is the realm of philosophy and metaphysics.
Up to a point. For instance, when Maxwell formulated his famous equations and derived the possibility of electromagnetic waves, and found that their computed speed was the same as what had been measured for light, that suddenly opened the doors to a lot more speculation about the fundamentals. A lot of Why? questions were answered--although the answers always seem to raise other Why? questions.

ToSeek
2004-Jun-24, 05:52 PM
Science only answeres questions such as "How?" and "What?". "Why?" is the realm of philosophy and metaphysics.
Up to a point. For instance, when Maxwell formulated his famous equations and derived the possibility of electromagnetic waves, and found that their computed speed was the same as what had been measured for light, that suddenly opened the doors to a lot more speculation about the fundamentals. A lot of Why? questions were answered--although the answers always seem to raise other Why? questions.

Science is always answering "why" questions, but each answer brings up another "why" question (or ten).

milli360
2004-Jun-24, 06:08 PM
ToSeek:
Science is always answering "why" questions, but each answer brings up another "why" question (or ten).
Yeah, and why is that?

ToSeek
2004-Jun-24, 06:39 PM
ToSeek:
Science is always answering "why" questions, but each answer brings up another "why" question (or ten).
Yeah, and why is that?

Well, because if the answer is x, the next question is why x or how x or what is x, exactly.

George
2004-Jun-24, 06:53 PM
ToSeek:
Science is always answering "why" questions, but each answer brings up another "why" question (or ten).
Yeah, and why is that?

Why not? :wink:

Tobin Dax
2004-Jun-24, 07:07 PM
ToSeek:
Science is always answering "why" questions, but each answer brings up another "why" question (or ten).
Yeah, and why is that?

Why not? :wink:

Or better yet, y_naught. :D 8-[ *hides*

George
2004-Jun-24, 07:16 PM
Since speed is time dependent, would time itself not possibly dominate c?

In Brian Greene's book "Fabric of Space", he stated a person sitting still is traveling through time at the speed of light. Can time be considered as some sort of flowing dimension regulating at least the three we know?

Or better yet, y_naught. *hides*
Ya know, ya aught naught say y_naught (unless your in a vacuum). :wink:

milli360
2004-Jun-24, 07:47 PM
George:
In Brian Greene's book "Fabric of Space", he stated a person sitting still is traveling through time at the speed of light.
I'm picking this book up this afternoon, on reserve at the library. Can you give me a page number? Thanks.

George
2004-Jun-24, 09:28 PM
George:
In Brian Greene's book "Fabric of Space", he stated a person sitting still is traveling through time at the speed of light.
I'm picking this book up this afternoon, on reserve at the library. Can you give me a page number? Thanks.

Shoot, I'm not 100% sure it wasn't in "Elegant Universe" instead as I bought it when it first came out. I'll try and find it tonite at home. Since then, I bought "Elegant Universe" and like the former a little better.

George
2004-Jun-25, 12:05 AM
Can you give me a page number? Thanks.

Page 48 of "The Fabric of the Cosmos...". I had the title wrong, of course. I plan to re-read it (starting with the title :) )

wedgebert
2004-Jun-25, 12:55 AM
The way I was explained it is this:

Picture a 2d graph. Your velocity though the the physical dimesions is represented by the X axis. You velocity though time is represented via the Y axis. X^2 + Y^2 always equal c^2, or the square root of (X^2 + Y^2) = speed of light.

Now I'm not sure if the actual equations work out that simple, although the equation sqrt(1 - (v^2/c^2)) where v your velocity does come to mind. So it might not be exact, but it is close and is a good analogy.

milli360
2004-Jun-25, 01:10 AM
George:
Page 48 of "The Fabric of the Cosmos...". I had the title wrong, of course. I plan to re-read it (starting with the title :) )
:) Thanks, George. Turns out they had Elegant Universe for me, and Fabric later. O well.

Sam5
2004-Jun-25, 02:32 AM
The speed of light is c because that is the solution to the wave equation. If you like math, the here is the derevation (http://people.ccmr.cornell.edu/~muchomas/P214/Notes/OtherWaves/node18.html)

“Amazingly, the constants (e) and (u) of ordinary E&amp;M laboratory experiments involving charges, currents and electric and magnetic fields are precisely what are needed to understand the seemingly completely different phenomenon of light!”

The laboratory was located at the surface of the earth, so the results are geocentric. This reveals the speed of light at the surface of the earth.

wedgebert
2004-Jun-25, 03:08 AM
No, what that means it that light is a form of EM radidation. The experiments mentioned were not used to determine the speed of light, but rather of examples of how things that seemed unrelated at the time could be used to undertsand light.

rohitbd
2004-Jun-25, 07:27 AM
Pardon me if this is a rather silly question...but if light is a form of EM radiation (I assume EM means "electromagnetic") then why is it that no radio (or electromagnetic) instrument is affected by it? I mean there surely must be atleast something that would be affected by light in the manner radio waves affect radio antennas...

milli360
2004-Jun-25, 07:51 AM
Pardon me if this is a rather silly question...but if light is a form of EM radiation (I assume EM means "electromagnetic") then why is it that no radio (or electromagnetic) instrument is affected by it? I mean there surely must be atleast something that would be affected by light in the manner radio waves affect radio antennas...
Radio antennas are typically about the size of the radio wavelength--a few, sometimes many, meters. The wavelength of light is less than a micron. The distance between the centers of the photoreceptor cones of your eyes is just about three times that.

rohitbd
2004-Jun-25, 07:57 AM
Ahh yes!! Wavelength!! I had totally overlooked that. Thanks.

Sam5
2004-Jun-25, 11:51 AM
Pardon me if this is a rather silly question...but if light is a form of EM radiation (I assume EM means "electromagnetic") then why is it that no radio (or electromagnetic) instrument is affected by it? I mean there surely must be atleast something that would be affected by light in the manner radio waves affect radio antennas...

The key is "resonation". Radio antennas resonate if they are the same length as a radio wave, or some even fraction, like 1/2, 1/4, etc.

An electromagnetic instrument is affected by light if it is small enough, such as the rods and cones in your eyes that act like EM antennas and are affected by light. The same with grains of film and the tiny pixels of electronic cameras. That is why your rods and cones are so small, and there are so many of them because they act like “pixels”. They don’t react to radio waves because they are too small. Radio antennas don’t react to visible light because they are too long. Bugs tend to be attracted to high frequency blue light because their eye “antennae” are smaller than ours.

rohitbd
2004-Jun-25, 12:03 PM
This is getting a little confusing...is it the same for a solar cell or a photovoltaic device...? I thought that photo devices worked by absorbing energy from light rather than by electro-magnetic induction as in a typical antenna...

Sam5
2004-Jun-25, 12:22 PM
This is getting a little confusing...is it the same for a solar cell or a photovoltaic device...? I thought that photo devices worked by absorbing energy from light rather than by electro-magnetic induction as in a typical antenna...

The absorption of energy from light often works by means of the light causing certain miscropic chemical molecules or atoms to “resonate”. For example, in photographic film, certain light frequencies can cause resonation in the film grains on the molecular level and can cause silver halides to break apart into free silver and free halide. The halide goes off as a gas, leaving the free silver. In a black and white photo print, the black is essentially tarnished free silver. In some cases, such as with infra-red light heating things up, resonation can cause molecular vibration which produces heat energy.

The biologists are currently studying this phenomenon with plants. They speak of plant “antennas” and “resonation” of plant chemical compounds.

“Here, we investigate the regulation in plants by studying the chlorophyll fluorescence response to harmonically modulated irradiance. We propose that modulation frequency may be tuned to resonate with internal regulatory mechanisms ranging from nonphotochemical quenching to circadian rhythms.”
SOURCE (http://www.biophysj.org/cgi/content/full/83/4/2180)

“One chlorophyll a molecule - a PRIMARY PIGMENT – acts as a reaction centre for the photosystem, while the other molecules ‘catch’ the photons of light and resonate, transferring the energy to the central molecule. These are ACCESSORY PIGMENTS and act as an antenna complex for light.”
SOURCE (http://66.102.7.104/search?q=cache:d7I8KmkiYWoJ:www.ukphoneshop.com/motorsport/PHOTOSYNTHESIS%2520LDS%2520LIS.ppt+resonate+photos ynthetic+light+antenna+&amp;hl=en&amp;ie=UTF-8)

rohitbd
2004-Jun-25, 12:34 PM
:o WOW!! This is interesting... :o

Thanks a lot for the links...

Sam5
2004-Jun-25, 12:34 PM
“The retina is composed of tiny antennae that resonate to incoming light (rods and cones) • There are three types of cones-each sensitive to a different frequency range and allow us to see color.”

SOURCE (http://216.239.57.104/search?q=cache:L5Jc4yvtChcJ:www.winona.edu/physics/physics115/chap%2520notes%2520new%2520book/chap26notes.htm+resonate+eye+cones&amp;hl=en&amp;ie=UTF-8)

Sam5
2004-Jun-25, 12:37 PM
“The most basic collector is the human eye. The retina at the back of the eye is covered with tiny antennae, called rods and cones, that resonate with incoming light. Resonance with visible electromagnetic radiation stimulates nerve endings, which send messages to the brain that are interpreted as visual images. Cones in the retina are sensitive to the colors of the visible spectrum, while the rods are most sensitive to black and white.”

rohitbd
2004-Jun-25, 12:37 PM
Just a thought...what if we could, by some means, genetic or otherwise, increase the bandwidth of the photoreceptors in our retina...? Wonder what colour infra-red or ultra-violet would appear to be...

Sam5
2004-Jun-25, 12:54 PM
Just a thought...what if we could, by some means, genetic or otherwise, increase the bandwidth of the photoreceptors in our retina...? Wonder what colour infra-red or ultra-violet would appear to be...

I don’t know. I’ve often wondered that myself. What “color” would x-rays be, and what “color” would infra-red and radio waves be?

ToSeek
2004-Jun-25, 02:45 PM
Just a thought...what if we could, by some means, genetic or otherwise, increase the bandwidth of the photoreceptors in our retina...? Wonder what colour infra-red or ultra-violet would appear to be...

People with artificial lenses or that have had their lenses removed can see ultraviolet light (http://education.guardian.co.uk/higher/medicalscience/story/0,9837,724257,00.html). Sounds as if it comes through as blue.

Tensor
2004-Jun-25, 03:50 PM
This is getting a little confusing...is it the same for a solar cell or a photovoltaic device...? I thought that photo devices worked by absorbing energy from light rather than by electro-magnetic induction as in a typical antenna...

They do. No induction.

TravisM
2004-Jun-25, 03:59 PM
For that matter is the blue in my brain the same blue in yours? I know the wavelength is fixed but our brains' wiring isn't. (I bet it's close, but who knows? Maybe you see my pink as your blue...)
Also, describe 'color' to someone who's been blind from birth...
Alternately, have the genetically modified uv/ir fella tell you what 'color' radio is... He'll just say, it's radio...

Tensor
2004-Jun-25, 04:04 PM
If we had more than a one octave range of sight, would it be posible to see 'chords', like we can hear in sound?

rohitbd
2004-Jun-28, 07:31 AM
Well, if we were able to "see" IR and UV as visible light, then I guess we simply would have referred to them by some more names...just as for all other "normally visible" colours...

MrObvious
2004-Jun-28, 09:45 AM
If we had more than a one octave range of sight, would it be posible to see 'chords', like we can hear in sound?

Thats what I thought art was, music for the eyes. Oil paintings etc....

Some people see IR and UV close to the visible spectrum. It's all to do with the sensitivity. Higher intensity near visable IR looks like a dull red, high intensity near visible UV looks blueish.

The real limit of course when seeing IR is that we can't see below the black body frequency of our eyes. The closer we get to that the worse the effect.
Bees (and some other insects) can see UV, if you place a high pass filter in front of your eyes and seal off any incoming visible light then your eyes also adjust to see a bit more UV because the pupils open up more and overcome the sensitivity problem for a wider range of wavelengths.

It's an expensive pair of goggles to make but can be done with some custom filtering techniques as used to make filters for telescopes. Not much fun either unless you consider bumping into things fun.....

swansont
2004-Jun-28, 10:20 AM
Bees (and some other insects) can see UV, if you place a high pass filter in front of your eyes and seal off any incoming visible light then your eyes also adjust to see a bit more UV because the pupils open up more and overcome the sensitivity problem for a wider range of wavelengths.

Good way to damage your eyes, though, if you did this a lot.

This points out a hazard of giving kids (or adults, for that matter) cheap plastic sunglasses - the plastic often does not filter UV, and by dimming the visible signal, the glasses cause the iris to open and allow more UV light in. This has the potential to cause damage to the retina.

swansont
2004-Jun-28, 10:25 AM
In some cases, such as with infra-red light heating things up, resonation can cause molecular vibration which produces heat energy.

Just want to make my usual note that IR is not heat. The statement is somewhat ambiguous, and could be interpreted that way - visible light can heat things up this way, too.

2004-Jun-28, 10:25 AM
Some people see IR and UV close to the visible spectrum. It's all to do with the sensitivity. Higher intensity near visable IR looks like a dull red, high intensity near visible UV looks blueish.

...

Would that be why some people claim to be able to see "auras"? :oops:

Ricimer
2004-Jun-29, 04:50 PM
a friend of mine is sensitive enough outside the "normal" spectrum of light that he's always the first to spot people in camoflauge, including (to our suprise) some military specialists in Gillie suits doing a demonstration at a military air show.

He grabbed my shoulder and pointed to...well, nothing really. 5 minutes later soldiers stood up in the field right in front of the crowd, ~20-30 yards away.

John Dlugosz
2004-Jun-29, 07:59 PM
a brief description of the physics behind the derivation:

Light is an electromagnetic wave.

A changing electric field creates a changing magnetic field, and vice versa.

The electric field changes, creating a magnetic field, which causes the electic field to change...and it perpetuates itself.

But what’s a field? Virtual photons! Photons are made of more photons—that's not as enlightening as I had hoped.

—John

John Dlugosz
2004-Jun-29, 08:13 PM
If we had more than a one octave range of sight, would it be posible to see 'chords', like we can hear in sound?

No. Our vision works with RGB primary sensors (the cones) but the image processing is done in a LAB-like color space; our perception of color is such that a "tristimulus value" or 3 numbers can mimic the appearance of any color. That is, pure light of frequency f will start out by tickling the three kinds of cones at relative amounts; give some mixture of other frequencies that together stimulate the three sensors by the same relative amount and we see the same color.

Mix two colors of light together, and you sum the tristimulus values and get a different color.

I suppose you could argue that a non-spectral color (e.g. magenta does not correspond to a frequency) is analogous to a "chord".

On the other hand, the ear works by sensing the energy of many frequencies. Look at the "spectrum view" on a wave editor program rather than the amplitude view, and thats more like what are ears work with. Mix two frequencies together, and the ear senses that this is the case. Chords sound nice to us because it is similar to how real complex waveforms are factored into components frequences. But it's not an illusion: mix two frequences and you get a more complex waveform physically.

John Dlugosz
2004-Jun-29, 08:19 PM
a friend of mine is sensitive enough outside the "normal" spectrum of light that he's always the first to spot people in camoflauge, including (to our suprise) some military specialists in Gillie suits doing a demonstration at a military air show.

He grabbed my shoulder and pointed to...well, nothing really. 5 minutes later soldiers stood up in the field right in front of the crowd, ~20-30 yards away.

I postulate that this is due to a form of color blindness, not an extended frequency range. That is, green in plants is due to light in the green frequency. Green in printing is commonly due to a mixture of cyan and yellow pigments. More generally, the color designed to look the same is actually a different mix of frequencies, and anyone with non-standard tunings of the frequency curves or relative strengths of the cones in the eyes would detect the difference.

Tom Mazanec
2004-Jun-29, 09:38 PM
Gamow (IIRC) once wrote a story where the speed of light was about 10 Miles per Hour and you noticably foreshorten if you run real fast.

Chip
2004-Jun-29, 09:47 PM
...I'm not sure if any reason can be given for why the fundamental constants and particle masses are what they are. If we had any first principles that enabled us to calculate these things, chances are that there would be some other level of even more fundamental constants...

Yeah. If you could come up with a viable theory of the value of constants, you'd probably be up for a Nobel prize. 8)

But actually - considering the propagation of light in space moving at c: From the reference frame of light, time is zero. From this "point of view" of light (not our "point of view,") would not the constant of c be akin to being at rest since time does not pass?

In other words from the frame of light, everywhere is now, so EM radiation reaches a state matching the velocity of c when observed from our frame.

Wally
2004-Jun-30, 02:13 PM
Since speed is time dependent, would time itself not possibly dominate c?

I think I'd reverse the wording on this. Time is speed dependent. Therefore, speed dominates time.

Kebsis
2004-Jul-01, 01:08 AM
Keep in mind that c is a "speed limit". Its the speed of light in a vacuum. A photon can go slower in other media, it just can't go any faster.

Correct my misunderstanding, but I was under the impression that photons don't actually move slower in a medium. They are absorbed by an atom in the medium and then re-emitted. They go from moving at the speed of light, to stopped, to moving at the speed of light, so on and so forth. It gives the illusion that the light is moving slowly but it isn't really going at any speeds other than zero and C.

Is this correct?

Tensor
2004-Jul-01, 02:55 AM
If we had more than a one octave range of sight, would it be posible to see 'chords', like we can hear in sound?

No. Our vision works with RGB primary sensors (the cones) but the image processing is done in a LAB-like color space; our perception of

Snip....

But it's not an illusion: mix two frequences and you get a more complex waveform physically.

Thanks for the info John. Good explanation.

2004-Jul-01, 05:22 AM
Correct my misunderstanding, but I was under the impression that photons don't actually move slower in a medium. They are absorbed by an atom in the medium and then re-emitted. They go from moving at the speed of light, to stopped, to moving at the speed of light, so on and so forth. It gives the illusion that the light is moving slowly but it isn't really going at any speeds other than zero and C.

Is this correct?

This makes the most sense to me. The instantaneous speeds are always either zero and c, but the average velocity is lowered. If that's right, I don't really know.

milli360
2004-Jul-01, 10:06 AM
Kebsis:
Correct my misunderstanding, but I was under the impression that photons don't actually move slower in a medium. They are absorbed by an atom in the medium and then re-emitted. They go from moving at the speed of light, to stopped, to moving at the speed of light, so on and so forth. It gives the illusion that the light is moving slowly but it isn't really going at any speeds other than zero and C.

Is this correct?
It's much more complicated than that. :)

Look at the index of refraction for telescope glass. If each actual photon were being absorbed and just re-emitted, then we would not be able to see through the telescope.