Why is the speed of light constant?

[Oldfart]

Well, I don't mean to duck the question, I'm trying to explain why its a bad question, and not doing to well, I admit. The short answer is yes, its fundamentally impossible. As a number, the speed of light is not a "god number", because its numerical value depends on your chosen unit of length and your chosen unit of time. .

Rap, perhaps we remain disconnected because of my misuse of the term "god number." I have no interest in defending or defining a particular "numerical value" for c,

Lets go back a few thousand years, Zog the caveman, who has no notion of meters or seconds, cleverly observes that light is very fast and wonders if someday science will be able to explain why it goes at the fastness that it does. What do you tell him?

 

[Rap]

Rap, perhaps we remain disconnected because of my misuse of the term "god number." I have no interest in defending or defining a particular "numerical value" for c,

I fully understand.

Lets go back a few thousand years, Zog the caveman, who has no notion of meters or seconds, cleverly observes that light is very fast and wonders if someday science will be able to explain why it goes at the fastness that it does. What do you tell him?

I would say "tell me, how fast does it go?" What would he respond?

 

[Oldfart]

I would say "tell me, how fast does it go?" What would he respond?

"Zog unhappy you impolitely answer his question with a question...but anyway, Zog not know, wonders if Mr. Smartguy here from advanced civilization can explain how to reckon its fastness."

I don't think that asking Zog will be much help here, as he (and I) is here to ask, not explain stuff.

 

[Drakkith]

I'm sorry, I'm going to have to stop that assumption right there. That is exactly what does NOT progress physics. That claim can be compared to people that have said in the past "everything that has been invented, has already been invented." It is simply your limited perspective.

Learning something such as why light travels at such a magnificent speed would ultimately help push physics further as a science. it would help us unveil the laws of physics as it is. Because truly, we don't really understand much at all.

There's nothing wrong with trying to figure out HOW light travels so fast. Currently, it is simply a physical constant that we cannot explain. When we answer the question by saying that "It's just the way it is" doesn't imply that we will NEVER figure it out, but only that at this time we have no way of explaining it.

 

[Delta2]

I can give a scientific answer why light has a finite speed. If it had infinite speed all the wave phenomena related to it would break down and our world would be much different.

But i can't answer why it is c and not 2c or 10c or pi*c, seems if it was faster like 10c it would be good for telecommunications but don't know what other implications this could had.

 

[jhmar]

Assuming that gravitons are real, then experiments on the speed of light can be explained by assuming that light travels throught a given number of particles in a given unit of time.

 

[Drakkith]

Assuming that gravitons are real, then experiments on the speed of light can be explained by assuming that light travels throught a given number of particles in a given unit of time.

How so? Light doesn't travel slower or faster when gravity differs, but merely changes frequency.

 

[Brandon_R]

https://www.youtube.com/watch?v=wMFPe-DwULM

 

[PhysDrew]

Haha nice

 

[Rap]

"Zog unhappy you impolitely answer his question with a question...but anyway, Zog not know, wonders if Mr. Smartguy here from advanced civilization can explain how to reckon its fastness."

I don't think that asking Zog will be much help here, as he (and I) is here to ask, not explain stuff.

See Brandon's Feynman answer - it's getting to the core of things. Of course, its Feynman :).

The thing is, Zog cannot conceptualize a velocity as something isolated and alone, without reference to something else. A dog or a cat can deal with velocity, by reacting to it, but only Zog and Mr. Smartass can conceptualize it. Mr. Smartass needs to know how Zog conceptualized it in order to answer.

Suppose the speed of light were constantly varying, but Zogs idea of length and time were constantly varying too, in such a way that length divided by time made it look like the speed of light were constant. Zog and Mr. Smartass would never know it, never be able to prove that the speed of light was varying. You cannot talk about the speed of light without referencing it to something, another speed, or a length and a time. So it makes no sense to ask why the speed of light is what it is. It only makes sense to ask why it is so much faster than Zog can walk, or why it covers 300 billion of Zogs arm spans (i.e meters) divided by the time between his heart beats (i.e seconds). You cannot conceptualize the speed of light without some reference points, even if they are very primitive (i.e. it moves VERY fast compared to speeds I encounter in every day life). Mr. Smartass needs to know how Zog has come upon his idea of "fastness" of the speed of light in order to explain why it is so fast.

Take another situation - two scientists are communicating from different parts of the universe, and they wonder if the speed of light is the same for both of them - how do they do it? They cannot do it without agreeing on a unit of length and time, like the radius of a hydrogen atom, and the time between the peaks of the radiation from the Balmer line of hydrogen. But then they will wonder if maybe the hydrogen atom is bigger for one of them than for the other. Is the time between the peaks the same? IT WILL NEVER REALLY BE SETTLED. And so, the constancy of the speed of light will never be really settled until one of them meets the other and verifies that the hydrogen atom from one's part of the universe is the same size as the atom from the other, etc. What they absolutely CAN settle on is the value of the dimensionless fine structure constant. By measuring the speed of light in whatever units (the units don't have to agree between the two physicists), measuring Planck's constant in whatever units they want, etc, and calculating the fine structure constant, they will be able to absolutely decide whether the fine structure constant is the same.

When Zog talks about the speed of light, whether he knows it or not, he is referencing it to some speed that he is familiar with. The only question we can really answer is why the ratio of the speed of light to his reference velocity is what it is. Thats why the question "why is light so fast" makes sense, but the question "can we ever derive the speed of light" does not.

 

[gmax137]

Sorry, Rap, I just don't follow this argument that velocity is somehow related to the units we use. If I ask you (or better, the guys at Chevrolet) "how fast will the new Corvette go?" - there is an answer that can be calculated from the aerodynamic coefficient and the motor horsepower. Why is that possible for the Corvette but not for light?

 

[jhmar]

How so? Light doesn't travel slower or faster when gravity differs, but merely changes frequency.

Surely photons slow down as the get closer to a black hole? are not frequency changes caused by compression or decompression? We observe the same speed because the measuring instrument is subject to the same gravitational pressure as the experiment hence regardless of the direction of observation or the speed of the observer, the observer sees light passing through the same number of particles in the same unit of time. How else does gravity alter time? Is there any alternative practical explanation of the Michelson-Morley experiment?

 

[Buckleymanor]

Surely photons slow down as the get closer to a black hole? are not frequency changes caused by compression or decompression? We observe the same speed because the measuring instrument is subject to the same gravitational pressure as the experiment hence regardless of the direction of observation or the speed of the observer, the observer sees light passing through the same number of particles in the same unit of time. How else does gravity alter time? Is there any alternative practical explanation of the Michelson-Morley experiment?

There is an alternative practicle eplanation of the M.M experiment which has not been refuted .Though it might have been and I have not nowticed.
Simply put any gain in the velocity of light in one direction is exactly canceled out when the light is traveling in the opposite direction.

 

[Rap]

Sorry, Rap, I just don't follow this argument that velocity is somehow related to the units we use. If I ask you (or better, the guys at Chevrolet) "how fast will the new Corvette go?" - there is an answer that can be calculated from the aerodynamic coefficient and the motor horsepower. Why is that possible for the Corvette but not for light?

A Corvette can be broken down into pieces, and their connections can be calculated, but a photon is an elementary particle, so unless future physics can break them down somehow, the analogy won't work. Even then, the speed of light is different than the speed of a Corvette, its a universal speed, always the same. I don't understand the workings o a Corvette, but if you ask me how fast will it go, I know it will never go faster than the speed of light.

I don't mean to imply that the speed of light is related to the units that we use. What I mean to say is that you don't conceptualize it as something isolated and alone. If you think of the speed of light, you are always thinking in terms of a distance and a time, or else relating it to some other velocity, something that you are familiar with. You have an arm span of about a meter, a heartbeat of about a second, and that gives you a familiar velocity, and then you look at the speed of light and say "its really fast". What if there were a huge organism, with an arm span of a light year, and a heart rate of once per year. It would take light a heartbeat to go from one hand to the next, and light would seem to travel much more slowly to the big organism. But still the velocity is the same. So you and the big organism would have to settle on a unit distance that you agree is not changing in order to talk about the speed of light. Like the radius of a hydrogen atom. But then it comes down to "why is the radius of the hydrogen atom what it is?" You would have to agree on a fixed time interval like the time between peaks of the radiation from some atom, like the time between peaks of radiation from some line in a Krypton atom. But then "why is that time interval what it is?" etc. etc. What you are really asking is " why does light travel n times the radius of a hydrogen atom in M times the time between peaks of the Krypton radiation? But that brings in other physical constants, like the Planck constant, the charge on an electron, etc. Its not a simple question. Ultimately you realize that the speed of light could be anything, the Planck constant could be anything, etc., etc., but as long as the dimensionless constants that you build up from these values are the same, you will never detect any difference.

 

[jhmar]

There is an alternative practicle eplanation of the M.M experiment which has not been refuted .Though it might have been and I have not nowticed.
Simply put any gain in the velocity of light in one direction is exactly canceled out when the light is traveling in the opposite direction.

Cannot find any reference to the 'explanation' you mention; do you have a reference?

 

[vish_al210]

but why is it not instantaneous because it is not actually matter so it does not have to overcome friction to move.

Well sound is also not matter by wave of energy. but it takes time to travel... actually slower than light.. as it needs to impart the energy to the next molecule to make it propagate the energy further.. (Just like electricity..through a solid conductor, the atom itself need not move as long as the electron transfer is sufficient to cause the electric current to flow.) If the sound intensity is higher than can be propagated by just vibration transfer, the propagating particle causes the other particle to move as well as vibrate.
Even light if we consider light in just the wave form.. It is not light that travels but the energy propagated by the particle onto the other that causes the light wave to flow(seemingly). And as we all know transfer of energy takes time (however little it may be) and hence speed. And oreover slow and fast is only relative..to something else. At the moment.. light is relatively faster than all other forms of transfers that we can observe. Our perception of light being slow or fast is limited by our reference of speed. Because one traveling does not care how fast or slow he/she/it is going.. but only try to go at their energy peak.

 

[Buckleymanor]

Cannot find any reference to the 'explanation' you mention; do you have a reference?

Sorry I don't have a reference.It was an explanation and interferometer that I was involved with twenty odd years ago. I remember most of the details, if you are interested I can post them.

 

[vish_al210]

A Corvette... ~||~ ...e same, you will never detect any difference.

Sorry.. lost you there.. somewhere... can't remember where...??

Well regardless of the units of reference, the speed of light remains the same. It is a ratio of distance and time.. even relatively speaking... It is another matter of understanding as to how fast or slow it is.

And as of now, as we do not have any reference of speed faster than light, I am made to believe that light is the fastest there is at present. Also ligh tin one medium may be slower than the same light in another medium. But still the fastest is light in space... Or so I was taught..

 

[alexam20]

maybe because photons are mass less particles.

 

[Ash Small]

I'd argue with this, I've tried to answer this question before and got into trouble with a moderator, but the answer I'd give this time is that it is not, as several people have said, a ratio of distance and time (or similar).

The answer is wavelength times frequency. (wavelength in metres, maybe, and frequency per second, maybe.)

Whenever you multiply wavelength by frequency you get the speed of light, in whatever units you choose to use, much the same as circumferance divided by diameter gives Pi.

It is the relationship between wavelength and frequency that we should be discussing.

(Maybe I'll get into trouble again?)

 

[jhmar]

Sorry I don't have a reference.It was an explanation and interferometer that I was involved with twenty odd years ago. I remember most of the details, if you are interested I can post them.

Very interested, please osp details.

 

[vish_al210]

I'd argue with this,

still don't get what u'd argue about... well may be u could argue about the wrding,.. but not what I expressed.. ;)
Still... enlighten me... well on a PM though, not on this thread.. I don't want all to laught out loud on my ignorance... just kidding ;o ...
take care...
Merry Christmas... and anyways a wonderful year ending...

 

[Ash Small]

still don't get what u'd argue about... well may be u could argue about the wrding,.. but not what I expressed.. ;)
Still... enlighten me... well on a PM though, not on this thread.. I don't want all to laught out loud on my ignorance... just kidding ;o ...
take care...
Merry Christmas... and anyways a wonderful year ending...

Just because I choose to argue with you doesn't mean you're wrong.

And you're not the only one to have said that, just the most recent.

I suppose another way to put it is that wavelength divided by period (I think that's the correct term) is also a constant (the speed of light)

I was just wondering if re-phrasing the question using terms like wavelength, frequency and period would help, instead of using distance and time?

Seasonal greetings to you too

 

[vish_al210]

Thank you Ash...

Hey nothing serious.. And I ain't very touchy touchy... so don't worry.. no hard feelings.. ;)
It was supposed to be humourous.. anyway..

But how can one reference wavelength but not distance -- refer frequency but not time..
I am a little puzzled..
But I hope we both agree that unlike what Rap was considering.. speed of light ( I wld like to restrict to the scenario in any particular medium) is a constant regardless of what measure of units one chooses to express it.. As an independent wave period (wavelength/timeperiod or wavelength*frequency) or an average of many such waves over a distance and period of time (distance travelled/time taken to reach there)...
tc..

 

[Ash Small]

Likewise, Vish.

Well, to answer your question, or try to,...the wavelength is the 'natural' unit, as is the period, ...different wavelengths have different periods, but wavelength/period is always a constant.

There is a wavelength of 299 792 458 m that has a period of 1 s and frequency of 1 s^-1, but there are all the other wavelengths as well. Why just consider selected wavelengths, and not the others?

(just considering distance and time didn't seem to be going anywhere)

Is there anything to be gained by asking 'Why do different wavelengths not have the same frequency?' and 'Why is the relationship a constant?'

 

[vish_al210]

Thanks Ash.. well, I get wht u r saying.. maybe I should also change my presentation of speed (atleast for a wave)..
But was the question in ur last statement directed to me or in general.. as I don't remember discussing that.. ?
Well in that question I hope u are referring to only the constituents of the light wave and not all other waves like sound, AC waveform, etc, as u may be battered by a flurry of q's and A's.. Just a safety clause thought... ;)

 

[vish_al210]

And I guess both of us saying the same thing..is not helping and also maybe we have pushed this thread off topic.. hope not...

 

[DevilsAvocado]

Gentlemen; Corvettes, wavelengths, frequencies and Pi is all interesting, but I think some of you are missing the point, discovered by this most distinguished man:

[PLAIN]http://upload.wikimedia.org/wikipedia/en/a/a7/Einstein_TIME_Person_of_the_Century.jpg

If you leave out Albert Einstein and his special theory of relativity (SR), you will never understand anything about the speed of light. Special relativity incorporates the principle that the speed of light is the same for all inertial observers regardless of the state of motion of the source.

We measure speed as the length 'something' has moved, divided by the time it took for it to move, right?

So how is this achieved!?

Well, if the speed of light is invariant (constant) for all observers in all inertial frame of reference, something else must change, right?

And this is exactly what happens according to SR; length contraction, time dilation and relativity of simultaneity, contradicting the classical notion that the duration of the time interval between two events is equal for all observers.

All of the SR predictions have been experimentally verified; if you are using GPS, it wouldn’t work without correction for the time dilation effect, resulting in a delay about 7 μs/day. Without corrections for SR & GR, errors of roughly 10 km/day would accumulate. This is a fact.

If SR states that that all uniform motion is relative, and that there is no absolute and well-defined state of rest (no privileged reference frames) – from mechanics to all the laws of physics, including the laws of electrodynamics, why bother about "the speed of light"??

We can’t even agree on what the "Universal Time" is, and what "Now" means, for God’s sake!

The 4D spacetime is a "stretchy rubber" and the only constant that’s the same in all inertial frame of reference is the speed of light = 1, period.

[URL]http://upload.wikimedia.org/wikipedia/commons/e/e4/Lorentz_transform_of_world_line.gif[/URL]
Spacetime along the world line of a
rapidly accelerating observer

Am I talking bull?

Well, let’s take this classical example of a speeding railroad car, demonstrating the relativity of simultaneity. Observer A is onboard and observer B is standing on the platform, and a flash of light is given off at the center of the railroad car just as the two observers pass each other:

[URL]http://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/Traincar_Relativity1.svg/250px-Traincar_Relativity1.svg.png[/URL]
From the frame of reference of A, the light will reach the front and back of the railroad car at the same time.

[URL]http://upload.wikimedia.org/wikipedia/commons/thumb/7/72/Traincar_Relativity2.svg/294px-Traincar_Relativity2.svg.png[/URL]
From the frame of reference of B, the light will strike the back of the railroad car before it reaches the front.​

This is the only possible solution, without variant (faster than) speed of light, from the frame of reference of B.

But what happens if we put two 1967 Corvette Sting Ray in the middle of the railroad car, going in opposite directions at the exactly the same speed??

Well, they will of course strike the back and the front at the exactly the same time, for both observers A & B.

That’s the only "real" difference between a Corvette Sting Ray and speeding light.

Happy holidays!

 

[russ_watters]

I'd argue with this, I've tried to answer this question before and got into trouble with a moderator, but the answer I'd give this time is that it is not, as several people have said, a ratio of distance and time (or similar).

The answer is wavelength times frequency. (wavelength in metres, maybe, and frequency per second, maybe.)

Whenever you multiply wavelength by frequency you get the speed of light, in whatever units you choose to use, much the same as circumferance divided by diameter gives Pi.

It is the relationship between wavelength and frequency that we should be discussing.

(Maybe I'll get into trouble again?)

$$\lambda = \fract{v}{f}$$

That's a pretty basic relationship that offers nothing profound to say about the speed of light.

...different wavelengths have different periods, but wavelength/period is always a constant.

Um, well, yes - they are inverses of each other. That's also nothing profound. It's basically just a definition. The inverse of meters per cycle is cycles per meter.

 

[DevilsAvocado]

The inverse of meters per cycle is cycles per meter.

Good point! :rofl:

 

[Rap]

Very interested, please osp details.

Sorry, I cannot give the reference either. I read the same thing on Wikipedia, on some relativity-related article, and it sounded valid.

 

[Buckleymanor]

Very interested, please osp details.

If I remember the main premis of the M.M. experiment it was that if there was any difference in the speed of light in one direction to another direction it would show up in a decoherent interferance pattern when the two beams of light from the interferometer were recombined.
There wavelengths would be out of sink.
The two beams of light were configuered at right angles to each other and the whole apparatus could be rotated.
It was thought at the time that if there was an aether wind light traveling across the wind would take a longer period to come back to the observer than light traveling with the wind and back.
A popular analogy was that a swimmer traveling up stream and back would take less time than the same swimmer traveling the same distance across the stream and back.
Or was it the other way round.
Anyway it was decided that there would be a difference.
The problem with this is that if there is an increase in speed in one direction after the light hits the mirror it will loose any gain in speed in the return journey to the observer.
The amount of speed lost will be exactly the same amount as any gained it will be exactly canceled out.
There would be no difference if the interferometer was at right angles at 180degrees or any specific angle.
The light traveling at right angles to the horizontal would not gain or loose any speed.
Where as the light traveling along the horizontal would gain or loose speed in one direction then gain or loose speed in the other.
The net result being that when the light from the horizontal and that at right angles was recombined both beams would be traveling at exactly the same speed and moveing along the same plane.
So result would show no increase in speed as far as the observer was concerned allthough the light had been traveling at different speeds throughout it's journey.

 

[DevilsAvocado]

And you did take the distance between the Sun and the Earth into your calculations?

 

[jhmar]

If I remember the main premis of the M.M. experiment it was that if there was any difference in the speed of light in one direction to another direction it would show up in a decoherent interferance pattern when the two beams of light from the interferometer were recombined.
There wavelengths would be out of sink.
The two beams of light were configuered at right angles to each other and the whole apparatus could be rotated.
It was thought at the time that if there was an aether wind light traveling across the wind would take a longer period to come back to the observer than light traveling with the wind and back.
A popular analogy was that a swimmer traveling up stream and back would take less time than the same swimmer traveling the same distance across the stream and back.
Or was it the other way round.
Anyway it was decided that there would be a difference.
The problem with this is that if there is an increase in speed in one direction after the light hits the mirror it will loose any gain in speed in the return journey to the observer.
The amount of speed lost will be exactly the same amount as any gained it will be exactly canceled out.
There would be no difference if the interferometer was at right angles at 180degrees or any specific angle.
The light traveling at right angles to the horizontal would not gain or loose any speed.
Where as the light traveling along the horizontal would gain or loose speed in one direction then gain or loose speed in the other.
The net result being that when the light from the horizontal and that at right angles was recombined both beams would be traveling at exactly the same speed and moveing along the same plane.
So result would show no increase in speed as far as the observer was concerned allthough the light had been traveling at different speeds throughout it's journey.

Perhaps the following is what you are referring to:

http://en.wikipedia.org/wiki/Michelson–Morley_experiment

It is very interesting, but it does not proove Einstein's theory right or wrong, it simply negates the experiment; my sugestion is a none mathematical hypothesis of the action that causes Einstein theory to work.

 

[PhanthomJay]

I got the feeling from some of the posts here that the speed of light is just what it is, sort of a philosophical speed, and was wondering if it might be possible for humans, after a million years of math/phiysics, to change that outlook, to prove that c must be what it is, rather than just accept is as a sort of "god number." ... is such a feat fundamentally impossible?

The finite speed of light is not a philosophical speed. In our Universe, it is a function of space permeability, permissivity, Planck's constant, and all that other good stuff, which, in turn , are functions of the Fine Structure Constant of OUR universe, which happens to be , as was pointed out earlier, approximately 1/137. So it is best to ask why this constant is what it is, rather than ask why the speed of light is what it is, because now you avoid having to include the the standard definitions of meters and seconds, which puts you in an endless cyclic loop of defining one in terms of the other.

So why is the Fine Structure Constant of our Universe 1/137? I think Hawking/Mlodinow in their latest book hint at this answer (well, its not yet an answer and won't be until further advances in theories of quantum gravity, M-Theory, TOE, etc., or maybe there will never be an answer), but until I finish the final chapter, I'll provide comment later on my understanding, for what it may be worth.