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cosmocrazy
2008-Apr-21, 10:32 PM
A couple of questions i have been pondering for some time now are -
Where does light get its velocity from? when light enters a medium it slows down, does it instantly accelerate back to light speed on leaving the medium and if so how does it do so?
I realize that some of the answers will depend on SR & GR so i.ll ask one more. If i was a photon of light say:lol: , based on SR, would anywhere i traveled be no distance at all in no time at all relative to me due to length contraction and time dilation? but possibly billions of light years relative to an observer stationary to me? If so does that mean for a photon of light there is no apparent distance traveled? So no velocity needed? It totally confuses me!

Help!!:confused:

Occams Ghost
2008-Apr-21, 10:46 PM
Ive heard Penrose say that the speed of light is constant because of time dilation. not the other way around.

alainprice
2008-Apr-21, 10:47 PM
It seems that light does not get velocity at all. It is born with it, all of it.

When it enters a medium, it slows down because it's a matter of time before the photon hits something, gets absorbed, and then is emitted a short time later to carry on. Light doesn't actually slow down or speed up. It is incapable of acceleration.

CodeSlinger
2008-Apr-21, 10:53 PM
Ive heard Penrose say that the speed of light is constant because of time dilation. not the other way around.

Citation please (paper, journal article, book + page number, etc.).

Occams Ghost
2008-Apr-21, 10:57 PM
It was a book by Penrose. I am sorry. I do not remember what book it was. I only read an excerpt.

Neverfly
2008-Apr-21, 10:59 PM
It seems that light does not get velocity at all. It is born with it, all of it.

When it enters a medium, it slows down because it's a matter of time before the photon hits something, gets absorbed, and then is emitted a short time later to carry on. Light doesn't actually slow down or speed up. It is incapable of acceleration.

This is the guy you want to listen to;)


what gives light its velocity?
A couple of questions i have been pondering for some time now are -
Where does light get its velocity from? when light enters a medium it slows down, does it instantly accelerate back to light speed on leaving the medium and if so how does it do so?
Alainprince answered this one.
Light does not accelerate or decelerate. The reason it 'slows down' in a medium is interaction. But it will return to its natural state pon exiting the medium-c.

I realize that some of the answers will depend on SR & GR so i.ll ask one more. If i was a photon of light say , based on SR, would anywhere i traveled be no distance at all in no time at all relative to me due to length contraction and time dilation? but possibly billions of light years relative to an observer stationary to me? If so does that mean for a photon of light there is no apparent distance traveled? So no velocity needed? It totally confuses me!

Help!!
Correct;)

cosmocrazy
2008-Apr-21, 11:04 PM
This is the guy you want to listen to;)


Alainprince answered this one.
Light does not accelerate or decelerate. The reason it 'slows down' in a medium is interaction. But it will return to its natural state pon exiting the medium-c.

Right, i understand that bit now! its all a bit relative! :lol:

Occams Ghost
2008-Apr-21, 11:13 PM
However, two German scientists where able to make light go at superluminal speeds, so relativity isn't all what it seems. I can give anyone the citation of reference if they wish.

blueshift
2008-Apr-21, 11:16 PM
Photons have no potential energy. A photon only has kinetic energy. They cannot be accelerated up to speed c. For all practical purposes photons are in a free-fall. They use no effort and they are created when electrons lose energy. Although do not become confused. The photon is not held or let go of by the electron. It is the interaction between the field in the electron's vicinity and the electron that absorbs or emits the photon. The photon is not a thing one can put on some table. The photon is more of a quantized disturbance in the field near the charge.

All of the energy of a photon is directed through space. None is directed through time. Photons do not age.

When photons pass through glass, not all the ones that entered come out. There are many quantum annihilations that occur within the glass that lengthens the time for light to move through the glass. We are not in the photons frame of reference. We measure the time they take to come out as being longer than those that missed the glass. The air around us does not have as many quantum annihilations in the same volume of space that glass has in a matching volume of space. Diffraction in prism glass causes electron energy jumps that will differ in magnitude from electron energy falls, giving us a rainbow of colors coming out. In glass that is not a prism the energy jumps will match with the energy falls...

I hope this helps.

Neverfly
2008-Apr-21, 11:19 PM
Right, i understand that bit now! its all a bit relative! :lol:

Yes, but we're not related.;)


However, two German scientists where able to make light go at superluminal speeds, so relativity isn't all what it seems. I can give anyone the citation of reference if they wish.

I do.
I've heard this one before...:think:

grant hutchison
2008-Apr-21, 11:25 PM
However, two German scientists where able to make light go at superluminal speeds, so relativity isn't all what it seems. I can give anyone the citation of reference if they wish.This refers to the group velocity of a pulse of light, which can be induced to move superluminally.
Information transfer is still slower than lightspeed, so relativity is untroubled. See, for instance, Gauthier et al.'s Superluminal light pulses, subluminal information transmission (http://www.phy.duke.edu/research/photon/qelectron/pres/nlo-2004/NLO2004.pdf).

Grant Hutchison

Delvo
2008-Apr-21, 11:32 PM
"c" is not just simply the speed of light like Mach 1 is the speed of sound or 95 MPH is the speed of fastball. Those are circumstantial things are arbitrarily defined/measured by us, and routinely change with the circumstances such as air density without any effect on the rest of the workings of the universe. But c is a non-circumstantial different kind of "speed of", perhaps better described as the speed of the universe than the speed of light, the universe's own natural built-in speed, one of the basic fundamental parts of how the universe works, even more truly fundamental than distance and time themselves. It has applications other than as just the speed of light.

It pops up in various ways in the formulas and calculations of physics, including the relationship between mass and energy and the relationships between some other basic constants of the universe that you might not expect to have anything to do with distance or time or speed, such as Planck's electric charge and temperature. It's also the speed around which relativity revolves; defining any and all other speeds by fractions of c is the only way to make mathematical sense out of the distortions of space and time with powerful gravity or acceleration in relativity. Other forces also move at that speed, not just the electromagnetic force, which makes it so much the standard universal speed of everything that the real question is how matter can possibly move at any other speed. (The answer, for a smooth and continuous universe, is essentially that there is only one possible rate of travel, including for matter, so everything goes that speed, but matter just skews the direction of the vector off of the space/distance axis and toward the time axis.)

If, instead of being smooth and continuous, space and time are broken up into tiny discrete chunks like the picture on your computer screen is broken up into pixels, then c is the speed of one "pixel" of space per one "pixel" of time, one tick of the universe's clock... and that distance, the Planck length, is also the minimum distance at which the laws of particle physics and quantum dynamics can possibly even theoretically have any meaning. In such a quantized universe, the only way to have an average speed that isn't c would be to sit still for some ticks of the universe's clock, like skipping turns in a game.

It's not that light just happens to be set at some speed; it's that there's a basic constant, one of the few original bits of information behind how everything works in the universe, which shows up in several different ways that we know of... and one of those is the universe's built-in relationship between distance and time for all things, so light just has to move at that speed because there aren't really any other speeds for it to move at.

cosmocrazy
2008-Apr-21, 11:48 PM
cheers guys! plenty to think about there. some great answers and its definitely helped my understanding!

Occams Ghost
2008-Apr-21, 11:53 PM
Still wanting that link? Grants basically explained it.

Occams Ghost
2008-Apr-21, 11:54 PM
But he never mentioned quantum tunelling. This was the cause in a nut shell.

cosmocrazy
2008-Apr-21, 11:58 PM
But he never mentioned quantum tunneling. This was the cause in a nut shell.

Any link would be appreciated, quantum tunneling? i have heard this mentioned before but have no idea what it is?:confused:

Occams Ghost
2008-Apr-22, 12:04 AM
I'll just post the article here...


''Public release date: 16-Aug-2007



Contact: Henry Gomm
henry.gomm@rbi.co.uk
020-761-11206
New Scientist

Light seems to defy its own speed limit
IT'S a speed record that is supposed to be impossible to break. Yet two physicists are now claiming they have propelled photons faster than the speed of light. This would be in direct violation of a key tenet of Einstein's special theory of relativity that states that nothing, under any circumstance, can exceed the speed of light.



THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 18 AUGUST 2007.

EDITOR’S NOTE: PRIOR PERMISSION IS REQUIRED BEFORE ANY REPRODUCTION OF THIS STORY IN FULL

IF REPORTING ON THIS STORY, PLEASE MENTION NEW SCIENTIST AS THE SOURCE AND, IF REPORTING ONLINE, PLEASE CARRY A LINK TO: http://www.newscientist.com/

UK CONTACT - Claire Bowles, New Scientist Press Office, London:
Tel: +44(0)20 7611 1210 or email claire.bowles@rbi.co.uk

US CONTACT – New Scientist Boston office:
Tel: +1 617 386 2190 or email jill.heselton@reedbusiness.com

Occams Ghost
2008-Apr-22, 12:05 AM
I just read the copy right... :)

cosmocrazy
2008-Apr-22, 12:14 AM
Interesting stuff!

Occams Ghost
2008-Apr-22, 12:16 AM
Yes, it is very interesting.

Steve Limpus
2008-Apr-22, 12:17 AM
"c" is not just simply the speed of light...

Nice post Delvo.

cosmocrazy
2008-Apr-22, 12:24 AM
Nice post Delvo.

I have just read through the posts again and yes i have to agree Delvo's post is a good one. It gives me a new perspective on things and helps me to understand the other implications involved. I like to keep an open mind on most theories and like to look at things in all perspectives. Grant has offered a good example too and some great comments & links from other posters. Cheers guys keep em coming!! :)

Gsquare
2008-Apr-22, 12:24 AM
A couple of questions i have been pondering for some time now are -
Where does light get its velocity from?




I am very surprised no one answered this question directly .

The simple (Correct) answer is.....

The velocity of light is determined by Maxwell's electrodynamic equations and in a vacuum its value is a direct result of the permittivity, e(0), and permeability, u(o), of the free space vacuum

The simple equation derived by James Clerk Maxwell is;

c = 1/sq.rt.[e(0) * u(0)]

This was the crowning achievement of Maxwell..--->deriving the value of c strictly from electromagnetic properties of the vacuum.

Relativity theory makes no attempt to determine its value, but only states it is constant (in any particular inertial frame). Maxwell's equations set the value.



G^2 :D

"The Works of the Lord are great, studied by all who have pleasure therein" - Inscibed in door of the the Archway leading to James Clerk Maxwell's Cavendish Laboratory - Cambridge

grant hutchison
2008-Apr-22, 12:24 AM
Interesting stuff!Nimtz & Stahlhofen's suggestion that they've observed a violation of special relativity has been fairly hotly contested. See, for instance, Winful's Comment on "Macroscopic Violation of Special Relativity" by Nimtz and Stahlhofen (http://arxiv.org/ftp/arxiv/papers/0709/0709.2736.pdf).
A recent paper by G. Nimtz and A. A. Stahlhofen [arXiv:0708.0681v1] makes the following claims: (1) that the authors have observed a macroscopic violation of special relativity, (2) that they have demonstrated the quantum mechanical behavior of evanescent modes on a meter-length scale, and (3) that barriers are crossed in zero time, implying superluminal (faster than light), and indeed, infinite tunneling velocity. Here I suggest that all these claims are erroneous and are based on a misinterpretation of a purely classical measurement accurately described by Maxwell’s equations.Grant Hutchison

cosmocrazy
2008-Apr-22, 12:27 AM
I am very surprised no one answered this question directly .

The simple answer is.....
The velocity of light is determined by Maxwell's electrodynamic equations and in a vacuum its value is a direct result of the permittivity, e(0), and permeability, u(o), of the free space vacuum

The simple equation derived by James Clerk Maxwell is;

c = 1/sq.rt.[e(0) * u(0)]

This was the crowning achievement of Maxwell..--->deriving the value of c strictly from electromagnetic properties.



G^2 :D

"The Works of the Lord are great, studied by all who have pleasure therein" - Inscribed in door of the the Archway leading to James Clerk Maxwell's Cavendish Laboratory - Cambridge

Thanks Gsquare I was not aware of this.

Van Rijn
2008-Apr-22, 12:31 AM
I just read the copy right... :)

You did? Did you miss this?


This story posted here is the EXACT text used in New Scientist magazine, therefore advance permission is required before any and every reproduction of each article in full.


It appears you reproduced it in full. I'd suggest cutting it down to a paragraph, and including the link:


http://www.eurekalert.org/pub_releases/2007-08/ns-lst081607.php

Steve Limpus
2008-Apr-22, 12:33 AM
Y'know... how the hell does c get to be c?

The primate in me can feel my knuckles draggin' on the ground, still. :confused:

:)

cosmocrazy
2008-Apr-22, 12:36 AM
Nimtz & Stahlhofen's suggestion that they've observed a violation of special relativity has been fairly hotly contested. See, for instance, Winful's Comment on "Macroscopic Violation of Special Relativity" by Nimtz and Stahlhofen (http://arxiv.org/ftp/arxiv/papers/0709/0709.2736.pdf).Grant Hutchison

cheers Grant good link. This is why i have joined this forum! debate by theory and science.

cosmocrazy
2008-Apr-22, 12:54 AM
Y'know... how the hell does c get to be c?

The primate in me can feel my knuckles draggin' on the ground, still. :confused:

:)

A fundermental question that no one knows. Calculating c and proving the constant of c has been shown many times why it is approx 186,000 mps relative to any observer is a good question? why not any number? i guess it was defined at the begining of the universe?:confused::confused:

Occams Ghost
2008-Apr-22, 01:00 AM
It was defined at the very beginning, yes.

Delvo
2008-Apr-22, 01:06 AM
why it is approx 186,000 mps relative to any observer is a good questionBecause that's the number you get when you measure in what we call miles and seconds; the human-imagined unit "miles per second" turns out to be about 1/186000 of c.

Steve Limpus
2008-Apr-22, 01:31 AM
Reminds me of this line:


"Do you know like we were saying, about the earth revolving? It's like when you're a kid, the first time they tell you that the world is turning and you just can't quite believe it 'cause everything looks like it's standing still. I can feel it...

...the turn of the earth. The ground beneath our feet is spinning at a thousand miles an hour. The entire planet is hurtling around the sun at sixty seven thousand miles an hour. And I can feel it. We're falling through space, you and me, clinging to the skin of this tiny little world. And, if we let go...

...That's who I am. Now forget me, Rose Tyler. Go home."

The Doctor

mugaliens
2008-Apr-22, 05:13 PM
A couple of questions i have been pondering for some time now are -
Where does light get its velocity from? when light enters a medium it slows down, does it instantly accelerate back to light speed on leaving the medium and if so how does it do so?
I realize that some of the answers will depend on SR & GR so i.ll ask one more. If i was a photon of light say:lol: , based on SR, would anywhere i traveled be no distance at all in no time at all relative to me due to length contraction and time dilation? but possibly billions of light years relative to an observer stationary to me? If so does that mean for a photon of light there is no apparent distance traveled? So no velocity needed? It totally confuses me!

Help!!:confused:

Light is some pretty weird stuff! It always appears to be travelling 186,000 mps, regardless of whether the receiver is moving towards or away from the source, and regardless of how great their relative velocities might be.

Strange, huh?

What changes is the observed frequency of the light. Assuming a single, constant frequency, if the source and observer are moving towards one another, it's blue-shifted. Moving away? Red-shifted.

It's not like sound, which is actually moving through the medium of air itself at some 700 mph.

Because of this anomaly, if you have two airplanes 11 miles apart, and they're both travelling 700 mph (11 miles a minute) in the same direction, and the lead plane sounds a very loud horn, it will reach the trailing aircraft in just 30 seconds, because the sound is travelling towards the trailing aircraft at the same velocity as the trailing aircraft is travelling towards the sound. Thus, they meet in the middle in half the time.

And since most importantly, the two doppler shifts cancel so whatever frequency was emitted is the same frequency that's heard.

Same thing with light.

Two spaceships doing the same thing, travelling at .99999 c, 1 light year in trail.

To an observer at a very great distance away, perpendicular to their course, he will observe this to be true when the lead spaceship emits a very bright burst of light.

But to the trailing spaceship, it appears as if the light suddenly appears out of nowhere, just as observed with the sound on the trailing aircraft.

So what's the difference?

The observed velocities of the sound and the light.

The aircraft observes the sound to be travelling 1,400 mph relative to him, but the spacecraft observes light to be travelling at the same 186,000 mph regardless of how fast the two spacecraft are travelling in trail.

That's what I find that's weird.

Of course, it's explained by time dilation and distance compression (look up Lorentz on Wikipedia), but that's another story...

Amber Robot
2008-Apr-22, 05:21 PM
When it enters a medium, it slows down because it's a matter of time before the photon hits something, gets absorbed, and then is emitted a short time later to carry on.

I've heard many people say this and I've always been bothered by this explanation. I'm not sure what you mean by "absorbed", but most absorptions I've dealt with result in the destruction of the photon, excitation of the material, and subsequent emission of a new photon. This kind of absorption does not preserve things like direction and phase of the photon, so it can't be responsible for the reduction of speed of light in a material. Perhaps there is a different kind of absorption to which you refer?

grant hutchison
2008-Apr-22, 05:27 PM
I've heard many people say this and I've always been bothered by this explanation. I'm not sure what you mean by "absorbed", but most absorptions I've dealt with result in the destruction of the photon, excitation of the material, and subsequent emission of a new photon. This kind of absorption does not preserve things like direction and phase of the photon, so it can't be responsible for the reduction of speed of light in a material. Perhaps there is a different kind of absorption to which you refer?The photon is said to hang up in the "dressed state" of the atom. If the photon has an energy close to an absorption line of the atom it's interacting with, it can "borrow" the necessary additional energy under the terms of Heisenberg's Uncertainty Principle, for a period of time inversely proportional to the amount of energy in the shortfall.
Most conventional transparent materials have absorption lines just "out of sight" in the UV. Since blue photons have to "borrow" less energy than red photons in order to match the energy of the UV absorption lines, blue photons can participate in the dressed state for longer than red photons: hence the speed of light is slower and the refractive index correspondingly higher for blue photons than red photons.

Grant Hutchison

John Mendenhall
2008-Apr-22, 08:04 PM
The aircraft observes the sound to be travelling 1,400 mph relative to him, but the spacecraft observes light to be travelling at the same 186,000 mph regardless of how fast the two spacecraft are travelling in trail.

That's what I find that's weird.

Of course, it's explained by time dilation and distance compression (look up Lorentz on Wikipedia), but that's another story...

Good job, Mugs. Interesting way to look at the physics.

Thanks, John M.

tommac
2008-Apr-22, 08:27 PM
maybe light is standing still? And everything around is moving.

speedfreek
2008-Apr-22, 09:06 PM
maybe light is standing still? And everything around is moving.

Try to build yourself a mathematical model where light seems to be moving in opposite directions from a single source, but where light is standing still and everything around is moving.

Acolyte
2008-Apr-22, 09:31 PM
From http://www.hno.harvard.edu/gazette/2001/01.24/01-stoplight.html

"Two years ago we slowed it down to 38 miles an hour; now we've been able to park it then bring it back up to full speed." Lene Hau isn't talking about a used motorbike, but about light – that ethereal, life-sustaining stuff that normally travels 93 million miles from the sun in about eight minutes.

Less than five years ago, the speed of light was considered one of the universe's great constants. Albert Einstein theorized that light cannot travel faster than 186,282 miles per second. No one has proved him wrong, but he never said that it couldn't go slower..."

I find it interesting stuff - what we find when we get into things...

mugaliens
2008-Apr-22, 09:38 PM
Good job, Mugs. Interesting way to look at the physics.

Thanks, John M.

Thank you, John! I appreciate the compliment!

mugaliens
2008-Apr-22, 09:59 PM
From http://www.hno.harvard.edu/gazette/2001/01.24/01-stoplight.html

"Two years ago we slowed it down to 38 miles an hour; now we've been able to park it then bring it back up to full speed." Lene Hau isn't talking about a used motorbike, but about light that ethereal, life-sustaining stuff that normally travels 93 million miles from the sun in about eight minutes.

Less than five years ago, the speed of light was considered one of the universe's great constants. Albert Einstein theorized that light cannot travel faster than 186,282 miles per second. No one has proved him wrong, but he never said that it couldn't go slower..."

I find it interesting stuff - what we find when we get into things...

Me too! This is incredible stuff!

However, given my background, I find the following comment by the reporter rather off-target, ill-informed, and otherwise... Well, just read it:


Financial and other information would be prodigiously safer with a quantum computer."

Glamorizing comments such as "Financial and other information would be prodigiously safer with a quantum computer" are pure bunk. We can encrypt data safely without fear of decryption right now, not for billions of years, but forever.

It's easy! Just use a keyspace larger than the dataset. That renders the solution set equal to or larger than all possible solutions.

Encryption 101, and has been known for at least half a century... Crikey!

What ignorant reporters won't do to sell their stories.

Still, actually stopping light... THAT's amazing!

The encryption aspect - pure bunk. We've had that problem solved for decades.

CodeSlinger
2008-Apr-23, 02:07 AM
It's easy! Just use a keyspace larger than the dataset. That renders the solution set equal to or larger than all possible solutions.

Can you expand on this? It doesn't sound right to me. The way I'm thinking about it, unless your keyspace is infinitely large, it will only take finite time to search the entire keyspace and find the correct decryption key.

FriedPhoton
2008-Apr-23, 02:34 AM
A fundermental question that no one knows. Calculating c and proving the constant of c has been shown many times why it is approx 186,000 mps relative to any observer is a good question? why not any number? i guess it was defined at the begining of the universe?:confused::confused:

Might it have something to do with the rate of universal expansion? For instance, the universe is expanding at a a given rate - c, and nothing within in it can exceed that limit without containing enough energy to push the expansion of the universe? I guess if that were true then there would be subtle variations in the speed of light at some number far to the right of the decimal point and what we see is the average speed.

I'm not offering this as an answer posed as a question, it is a question posed as a question. And it is also blind conjecture. :) Go easy on me... I'm an acknowledged moron when it comes to physics... although I am learning more here than I have anywhere else. :dance:

Eta C
2008-Apr-23, 03:21 AM
A couple of observations and clarifications

1) The speed of electromagnetic radiation can be derived from Maxwell's equations. One of the great outcomes of Maxwell's synthesis was discovering that the speed term in the wave equation derived from his theory matched the measured speed of light in vacuum. This led to the identification of light as electromagnetic radiation and means that it is no different, in principle, from radio waves or x-rays.

2) One problem with Maxwell's equations is that they are not invariant to simple Galilean transforms of coordinate systems as are, say, Newton's laws. This would mean that the laws of physics would be different for different observers. Lorentz discovered the coordinate transformations that do leave Maxwell's equations invariant, the so-called Lorentz transformations.

3) A problem with Lorentz transforms is that they do not allow for the existence of absolute time and space. One of Einstein's great leaps was to establish that the invariance of physical laws is more important than the existence of absolute time and space. By claiming this (the so-called principal of relativity) as his basis and assuming that the speed of light (in vacuum) is the same for all observers he was able to derive the Lorentz transforms separate from Maxwell's equations. This is the Special Theory of Relativity as described in his seminal 1905 paper. This is the fundamenatal reason why the speed of light in vacuum is the same regardless of the motion of the source or observer. The invariance of physical laws requires it. Absolute time and space do not.

4) Note that the constancy of c (the speed of light) only applies in vacuum. In a medium, such as air, glass, water, etc., the speed of light will be lower than c. The index of refraction gives the change (v = n*c if n is the index of refraction). I'll leave it to others to debate whether this is due to the electromagnetic properties of the medium or to the constant absorbtion and re-emission of photons. The practical matter is that as n goes to zero the speed of light in a medium goes to zero as well.

As many here know, I am a physicist. I hope this clarifies things. If not, I can only recommend that people here get their education from a good physics course or textbook and not from posts on an internet bulletin board from people who may or may not have any understanding of the topic (with exceptions made for this post I hope). :)

George
2008-Apr-23, 12:17 PM
The photon is said to hang up in the "dressed state" of the atom. If the photon has an energy close to an absorption line of the atom it's interacting with, it can "borrow" the necessary additional energy under the terms of Heisenberg's Uncertainty Principle, for a period of time inversely proportional to the amount of energy in the shortfall.
Most conventional transparent materials have absorption lines just "out of sight" in the UV. Since blue photons have to "borrow" less energy than red photons in order to match the energy of the UV absorption lines, blue photons can participate in the dressed state for longer than red photons: hence the speed of light is slower and the refractive index correspondingly higher for blue photons than red photons.
Wow, what an elegant explanation for refraction. Very helpful, Grant. Thanks.


One of Einstein's great leaps was to establish that the invariance of physical laws is more important than the existence of absolute time and space. It is interesting that he had prefered to call his theory the Invariance Theory, but the media, I think, popularized Relativity, instead.

trinitree88
2008-Apr-23, 02:14 PM
A couple of observations and clarifications

1) The speed of electromagnetic radiation can be derived from Maxwell's equations. One of the great outcomes of Maxwell's synthesis was discovering that the speed term in the wave equation derived from his theory matched the measured speed of light in vacuum. This led to the identification of light as electromagnetic radiation and means that it is no different, in principle, from radio waves or x-rays.

2) One problem with Maxwell's equations is that they are not invariant to simple Galilean transforms of coordinate systems as are, say, Newton's laws. This would mean that the laws of physics would be different for different observers. Lorentz discovered the coordinate transformations that do leave Maxwell's equations invariant, the so-called Lorentz transformations.

3) A problem with Lorentz transforms is that they do not allow for the existence of absolute time and space. One of Einstein's great leaps was to establish that the invariance of physical laws is more important than the existence of absolute time and space. By claiming this (the so-called principal of relativity) as his basis and assuming that the speed of light (in vacuum) is the same for all observers he was able to derive the Lorentz transforms separate from Maxwell's equations. This is the Special Theory of Relativity as described in his seminal 1905 paper. This is the fundamenatal reason why the speed of light in vacuum is the same regardless of the motion of the source or observer. The invariance of physical laws requires it. Absolute time and space do not.

4) Note that the constancy of c (the speed of light) only applies in vacuum. In a medium, such as air, glass, water, etc., the speed of light will be lower than c. The index of refraction gives the change (v = n*c if n is the index of refraction). I'll leave it to others to debate whether this is due to the electromagnetic properties of the medium or to the constant absorbtion and re-emission of photons. The practical matter is that as n goes to zero the speed of light in a medium goes to zero as well.

As many here know, I am a physicist. I hope this clarifies things. If not, I can only recommend that people here get their education from a good physics course or textbook and not from posts on an internet bulletin board from people who may or may not have any understanding of the topic (with exceptions made for this post I hope). :)

Eta C. little typo here. n=c/v...not v/c...you've got the inverse here. Glass with an index of ~1.33 (depending on composition)... has light traveling slower in it hence c, in vacuum divided by c in glass yields a refractive index larger than 1. pete

I make a few boo-boos too.

tommac
2008-Apr-23, 03:13 PM
Try to build yourself a mathematical model where light seems to be moving in opposite directions from a single source, but where light is standing still and everything around is moving.


Well I am not sure if I can ... however ... can you answer this:

Lets say that we have a 4 dimensional space time. We shine a light in one direction ( or two directions ... I am not sure if it matters ).

We take pictures at very fast stage. With a camera that is impossibly fast.

Can you prove to me that a photon of light moving at the speed of light. Will not be in many time-locations ( a point in space time , a particular location at a particular time i.e 1,2,3,7 ) at the same time?

remember the photon has wave and particle characteristics and for youngs experiment it goes through both slits at the same time.

My point is that somehow light is timeless and thusly really doesnt move at least continuously. Again this is probably way ATM.

tommac
2008-Apr-23, 03:35 PM
Can it be proved that light does not travel at infinite speed in a pure vacuum that has no gravitational effects? Could the speed of light be DEFINED by the warping of space-time? Basically a gravitational field compresses space and time and a graviational field is infinite ( am I right here? ) (although negligable after a certain point ).

So light in a pure vacuum ( which I would argue does not exist ) light would travel from point a to point b on 0 time or it would exist at both point a and point b and at all points inbetween at the same time. It would be a pure wave at this point. It would have 0 amplitude and infinite wavelength.

As space-time curves. Somehow light will partially take the path of space-time curvature. The properties of light change slightly making it become more particle-like. The greater the graviational field the more warped the light wave becomes. The greater the gravitational force the more particle like it appears. The less the move wave-like it appears. So in any gravitational field it will appear to travel at c which would then equal the amount of particle-likeness that light exibits.


Now ... can all particles become wavelike in certain extreme or impossible conditions I havent through through.




Might it have something to do with the rate of universal expansion? For instance, the universe is expanding at a a given rate - c, and nothing within in it can exceed that limit without containing enough energy to push the expansion of the universe? I guess if that were true then there would be subtle variations in the speed of light at some number far to the right of the decimal point and what we see is the average speed.

I'm not offering this as an answer posed as a question, it is a question posed as a question. And it is also blind conjecture. :) Go easy on me... I'm an acknowledged moron when it comes to physics... although I am learning more here than I have anywhere else. :dance:

tommac
2008-Apr-23, 03:37 PM
Can one photon of light be in two places at the same time?



A couple of observations and clarifications

1) The speed of electromagnetic radiation can be derived from Maxwell's equations. One of the great outcomes of Maxwell's synthesis was discovering that the speed term in the wave equation derived from his theory matched the measured speed of light in vacuum. This led to the identification of light as electromagnetic radiation and means that it is no different, in principle, from radio waves or x-rays.

2) One problem with Maxwell's equations is that they are not invariant to simple Galilean transforms of coordinate systems as are, say, Newton's laws. This would mean that the laws of physics would be different for different observers. Lorentz discovered the coordinate transformations that do leave Maxwell's equations invariant, the so-called Lorentz transformations.

3) A problem with Lorentz transforms is that they do not allow for the existence of absolute time and space. One of Einstein's great leaps was to establish that the invariance of physical laws is more important than the existence of absolute time and space. By claiming this (the so-called principal of relativity) as his basis and assuming that the speed of light (in vacuum) is the same for all observers he was able to derive the Lorentz transforms separate from Maxwell's equations. This is the Special Theory of Relativity as described in his seminal 1905 paper. This is the fundamenatal reason why the speed of light in vacuum is the same regardless of the motion of the source or observer. The invariance of physical laws requires it. Absolute time and space do not.

4) Note that the constancy of c (the speed of light) only applies in vacuum. In a medium, such as air, glass, water, etc., the speed of light will be lower than c. The index of refraction gives the change (v = n*c if n is the index of refraction). I'll leave it to others to debate whether this is due to the electromagnetic properties of the medium or to the constant absorbtion and re-emission of photons. The practical matter is that as n goes to zero the speed of light in a medium goes to zero as well.

As many here know, I am a physicist. I hope this clarifies things. If not, I can only recommend that people here get their education from a good physics course or textbook and not from posts on an internet bulletin board from people who may or may not have any understanding of the topic (with exceptions made for this post I hope). :)

Eta C
2008-Apr-23, 06:28 PM
Tommac,
Your question (can a photon be in two places at once) is meaningless. Given the Heisenberg uncertainty principal one can't establish a definite location for any particle, including photons.

Trinitree, You are right of course, about the standard definiton of index of refraction. You will note, however, that I was consistent in my error in that I had n going to zero instead of infinity as the speed of light went to zero. Maybe I should agitate for a new convention. :)

mugaliens
2008-Apr-23, 07:00 PM
I've heard many people say this and I've always been bothered by this explanation. I'm not sure what you mean by "absorbed", but most absorptions I've dealt with result in the destruction of the photon, excitation of the material, and subsequent emission of a new photon. This kind of absorption does not preserve things like direction and phase of the photon, so it can't be responsible for the reduction of speed of light in a material. Perhaps there is a different kind of absorption to which you refer?

Whether the information (phase and direction) are preserved depends on two factors - the nature of the material, and the frequency of the photon.

Humid air is transparent to long-wave radiation (radio waves go right through clouds, unabated) but opaque to shorter wave radiation such as the visible spectrum of light.

Same thing is true of glass. It lets visible light through, but certain ingrediants in modern glass can block nearly 100% of UV radiation.

Often, the phase is preserved, but some materials can change the phase. This is what a polarizer does. Light reflecting off of some materials, such as a mirror, retain it's phase. But reflect it off of glass, and that polarizes it as well.

How much the light is slowed depends on the material, too, and the refractive index is directly proportional to the velocity of light through the medium. Diamonds, for example, have one of the highest refractive indices of all natural materials (I think it may be the highest).

mugaliens
2008-Apr-23, 07:11 PM
Can you expand on this? It doesn't sound right to me. The way I'm thinking about it, unless your keyspace is infinitely large, it will only take finite time to search the entire keyspace and find the correct decryption key.

When the keyspace is truly random, and of the same size as the message, the solution set encompasses all possible solutions. Encryption 101.

Amber Robot
2008-Apr-23, 07:19 PM
Whether the information (phase and direction) are preserved depends on two factors - the nature of the material, and the frequency of the photon.

Humid air is transparent to long-wave radiation (radio waves go right through clouds, unabated) but opaque to shorter wave radiation such as the visible spectrum of light.

Same thing is true of glass. It lets visible light through, but certain ingrediants in modern glass can block nearly 100% of UV radiation.

Often, the phase is preserved, but some materials can change the phase. This is what a polarizer does. Light reflecting off of some materials, such as a mirror, retain it's phase. But reflect it off of glass, and that polarizes it as well.

How much the light is slowed depends on the material, too, and the refractive index is directly proportional to the velocity of light through the medium. Diamonds, for example, have one of the highest refractive indices of all natural materials (I think it may be the highest).

I understand all of what you said, but little of it actually addresses my question. I am specifically referring to the slowing of the speed of light when it passes through materials. I have oft heard this explained as an "absorption and re-emission" using up time, thus slowing it down, i.e., it goes at "c" from absorption point to absorption point, but each absorption uses a small amount of time, so that the bulk speed is effectively slowed. My point is that I don't understand the nature of this "absorption". I wonder if the better explanation is that the EM-wave nature of the photon interacts with the EM-field generated by the material in such a way as to actually slow the propagation of the wave. Although the result may be the same, these two explanations are based on quite different physical models.

grant hutchison
2008-Apr-23, 07:23 PM
I wonder if the better explanation is that the EM-wave nature of the photon interacts with the EM-field generated by the material in such a way as to actually slow the propagation of the wave. Although the result may be the same, these two explanations are based on quite different physical models.Yes. As I tried to indicate in my previous post (http://www.bautforum.com/1224536-post35.html), the quantum wave-function of the photon interacts with the wave-function of the atoms of the transparent material in such a way as to retard the arrival of the photon at some detector on the far side of the material.

Grant Hutchison

Amber Robot
2008-Apr-23, 07:42 PM
Yes. As I tried to indicate in my previous post (http://www.bautforum.com/1224536-post35.html), the quantum wave-function of the photon interacts with the wave-function of the atoms of the transparent material in such a way as to retard the arrival of the photon at some detector on the far side of the material.

Yes, and what you state here makes more sense to me than your previous post in which you talked about "dressed states" and "borrowed energy".

Thanks.

tommac
2008-Apr-23, 08:11 PM
why meaningless? the answer is either yes it is possible or no it is not possible. hmmm i thought that the heisnenberg priciple stated that the more you focused on the location of a particle at a certain time the less you could know about the momentum of the particle. This has nothing to do with the method of detection but rather that things on the atomic level are based on purely on probability. I think ... but I am not sure ... that one photon of light can be in two places at the same time ( although not 100% ). However I am not sure. That was my question.


Tommac,
Your question (can a photon be in two places at once) is meaningless. Given the Heisenberg uncertainty principal one can't establish a definite location for any particle, including photons.

Trinitree, You are right of course, about the standard definiton of index of refraction. You will note, however, that I was consistent in my error in that I had n going to zero instead of infinity as the speed of light went to zero. Maybe I should agitate for a new convention. :)

grant hutchison
2008-Apr-23, 08:31 PM
Yes, and what you state here makes more sense to me than your previous post in which you talked about "dressed states" and "borrowed energy".Two versions of the same story, though. The first focuses on the picture of a single atom and a single photon, interacting under Heisenberg's ΔE.Δt uncertainty principle: that's the origin of the "absorbed/re-emitted" version of the story. The second pulls back to describe the cumulative outcome of all those individual interactions, producing an overall description of the wave-function of the photon: that comes out like your version of the story.

Grant Hutchison

CodeSlinger
2008-Apr-23, 08:40 PM
When the keyspace is truly random, and of the same size as the message, the solution set encompasses all possible solutions. Encryption 101.

I'll start a new thread for this.

cosmocrazy
2008-Apr-23, 08:42 PM
Might it have something to do with the rate of universal expansion? For instance, the universe is expanding at a a given rate - c, and nothing within in it can exceed that limit without containing enough energy to push the expansion of the universe? I guess if that were true then there would be subtle variations in the speed of light at some number far to the right of the decimal point and what we see is the average speed.

I'm not offering this as an answer posed as a question, it is a question posed as a question. And it is also blind conjecture. :) Go easy on me... I'm an acknowledged moron when it comes to physics... although I am learning more here than I have anywhere else. :dance:

I considered this possibility myself, but it has been mentioned about a mathematical model to show this effect. I cannot even begin to start to construct one. But if you think back to my OP i compared the observed distances traveled by a me as a photon:lol: and an observer static relative to me. Due too length contraction and time dilation i could measure no distance in no time, when the observer could measure billions of light years in time and distance. One could conclude that light does not travel at any speed to any distance but that everything else is moving relative to light. This does not seem to make any sense to me perhaps mostly because it has not been fully explained to me. But there are some very knowledgeable people on this forum with lots of great answers on this thread and i really appreciate everybody and their efforts!! :)

Amber Robot
2008-Apr-23, 09:19 PM
Two versions of the same story, though. The first focuses on the picture of a single atom and a single photon, interacting under Heisenberg's ΔE.Δt uncertainty principle: that's the origin of the "absorbed/re-emitted" version of the story. The second pulls back to describe the cumulative outcome of all those individual interactions, producing an overall description of the wave-function of the photon: that comes out like your version of the story.

Grant Hutchison

Yeah, I see that. I'll need to rethink your other version a bit -- see if I can get it to sink in. Normally I am a "light is a particle" kind of guy, so I'd prefer if I could reconcile it with a particle-like explanation rather than a wave-like one.

mugaliens
2008-Apr-25, 07:35 PM
I understand all of what you said, but little of it actually addresses my question. I am specifically referring to the slowing of the speed of light when it passes through materials. I have oft heard this explained as an "absorption and re-emission" using up time, thus slowing it down, i.e., it goes at "c" from absorption point to absorption point, but each absorption uses a small amount of time, so that the bulk speed is effectively slowed. My point is that I don't understand the nature of this "absorption". I wonder if the better explanation is that the EM-wave nature of the photon interacts with the EM-field generated by the material in such a way as to actually slow the propagation of the wave. Although the result may be the same, these two explanations are based on quite different physical models.

I, for one, am sick and tired of people asking ridiculously base questions on this message board. This is not what this is for, particularly when the answers to your questions are merely a click away, already having been compiled by multitudes who're experts in their fields.

ALL TAKE NOTE

Please follow the links for details. (If I take the time to look this up - please take the time to follow them. Better yet, start your search foir knowledge there, learning how to Wikipedia (http://en.wikipedia.org/wiki/Main_Page), Google (http://www.google.com/), and the dictionary (http://www.merriam-webster.com/), instead of asking us to do your work for you). In other words, read and learn, instead of asking inane (http://www.merriam-webster.com/dictionary/inane)questions here on BAUT.

When a photon (http://en.wikipedia.org/wiki/Photon)enters the orbital space (http://en.wikipedia.org/wiki/Atomic_orbital)of an atom (http://en.wikipedia.org/wiki/Atom), it is captured by the atom, and excites (http://en.wikipedia.org/wiki/Excited_state)an electron (http://en.wikipedia.org/wiki/Electron)to a higher orbital (http://en.wikipedia.org/wiki/Atomic_orbital). The higher orbital state isn't stable (http://en.wikipedia.org/wiki/Atomic_orbital#Electron_placement_and_the_periodic _table), but it does take some time to decay back to the lower orbital.

NOW - If I see a SINGLE post on this thread indicating that someone has NOT followed, AND READ the links I've included in the text above, instead of being LAZY and wasting my time, and the time of others, I WILL report you and your post to the MODs.

DON'T BE LAZY, PEOPLE!!! Learn something!

Amber Robot
2008-Apr-25, 07:52 PM
Nice. ;)

tommac
2008-Apr-25, 07:57 PM
instead of asking inane (http://www.merriam-webster.com/dictionary/inane)questions here on BAUT.


Inane in the membrane
Inane in the brain!
Inane in the membrane
Inane in the brain!

Amber Robot
2008-Apr-25, 08:06 PM
When a photon (http://en.wikipedia.org/wiki/Photon)enters the orbital space (http://en.wikipedia.org/wiki/Atomic_orbital)of an atom (http://en.wikipedia.org/wiki/Atom), it is captured by the atom, and excites (http://en.wikipedia.org/wiki/Excited_state)an electron (http://en.wikipedia.org/wiki/Electron)to a higher orbital (http://en.wikipedia.org/wiki/Atomic_orbital). The higher orbital state isn't stable (http://en.wikipedia.org/wiki/Atomic_orbital#Electron_placement_and_the_periodic _table), but it does take some time to decay back to the lower orbital.

In all seriousness, though, this process you described is called "resonance absorption", something I'm quite familiar with, and is not the process that slows photons on their travels through transparent materials.

mugaliens
2008-Apr-25, 08:26 PM
In all seriousness, though, this process you described is called "resonance absorption", something I'm quite familiar with, and is not the process that slows photons on their travels through transparent materials.

Wow! An intelligent response!

Ok, so what is the process that slows photonic travel through a medium such as water, glass, or diamond?

And links are perfectly ok with me! The more the merrier. That's how I learn.

Delvo
2008-Apr-25, 08:31 PM
If I see a SINGLE post on this thread indicating... I WILL report you and your post to the MODs.For what violation?

cosmocrazy
2008-Apr-25, 10:23 PM
I, for one, am sick and tired of people asking ridiculously base questions on this message board. This is not what this is for, particularly when the answers to your questions are merely a click away, already having been compiled by multitudes who're experts in their fields.

Some people ( like me ) have not been members long and are just beginning to find information about different theories along with scientific evidence available to support or falsify those theories. Some of the questions may seem a bit layman to others but we are all here to learn through discussion. Personally i will take note and try to read up on a subject before asking any questions from now on. Thanks for your patience.

Thanks for any links guys & gals it is most helpful! :)

grant hutchison
2008-Apr-25, 10:32 PM
Some people ( like me ) have not been members long and are just beginning to find information about different theories along with scientific evidence available to support or falsify those theories. Some of the questions may seem a bit layman to others but we are all here to learn through discussion. Personally i will take note and try to read up on a subject before asking any questions from now on. Thanks for your patience.Don't fret. Mugaliens was expressing a personal opinion. :)

Grant Hutchison

cosmocrazy
2008-Apr-25, 10:53 PM
Don't fret. Mugaliens was expressing a personal opinion. :)

Grant Hutchison

Thanks Grant, I have read lots of threads on Q&A and ATM on this forum and there are lots of very knowledgeable people out there ( including yourself ) i am a complete novice when it comes to physics and cosmology but like Tommac i have a zillion questions to ask! :) especially about gravity, speed of light and space-time! just wish i had the time to read up on it all!!

Cheers :)