# Why does light travel so fast

#### vish_al210

but why is it not instantanous 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 travelling 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.... cant 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..

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#### alexam20

maybe because photons are mass less particles.

#### Ash Small

vish_al210;3052624. It is a ratio of distance and time.. ...[/QUOTE said:
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.

#### vish_al210

I'd argue with this,
still dont 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 dont 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 dont remember discussing that.. ???
Well in that question I hope u are refering 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...

Gold Member
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:

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.

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:
From the frame of reference of A, the light will reach the front and back of the railroad car at the same time.

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!

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#### russ_watters

Mentor
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.

Gold Member
The inverse of meters per cycle is cycles per meter.
Good point! :rofl:

#### Rap

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

#### Buckleymanor

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 travelling across the wind would take a longer period to come back to the observer than light travelling with the wind and back.
A popular analogy was that a swimmer travelling up stream and back would take less time than the same swimmer travelling 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 cancelled out.
There would be no difference if the interferometer was at right angles at 180degrees or any specific angle.
The light travelling at right angles to the horizontal would not gain or loose any speed.
Where as the light travelling 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 travelling 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 travelling at different speeds throughout it's journey.

Gold Member
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 travelling across the wind would take a longer period to come back to the observer than light travelling with the wind and back.
A popular analogy was that a swimmer travelling up stream and back would take less time than the same swimmer travelling 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 cancelled out.
There would be no difference if the interferometer was at right angles at 180degrees or any specific angle.
The light travelling at right angles to the horizontal would not gain or loose any speed.
Where as the light travelling 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 travelling 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 travelling 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

Homework Helper
Gold Member
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.

#### Oldfart

The finite speed of light is not a philosophical speed. ......or maybe there will never be an answer),

Well, if the answer may never be known, it sounds philosophical to me -- though I must admit that the speed of philosophy is not well defined...

Thanks for the reply, happy new year!

#### PhanthomJay

Homework Helper
Gold Member
The finite speed of light is not a philosophical speed. ......or maybe there will never be an answer),

Well, if the answer may never be known, it sounds philosophical to me --
you may be right
though I must admit that the speed of philosophy is not well defined...
I thought it was the same as the speed of time :rofl:
Thanks for the reply, happy new year!
Same to you, thanks. I Read Hawking's last chapter just after Dick Clark's countdown. I'm told the million dollar ball dropped, but i guess i was not watching carefully, because I never saw the damn thing drop. Maybe it was one of those Vacuum Fluctuations....

Anyway, Hawking/Mladinow's book fell short of a masterpiece, but nevertheless, it is a MUST read that reflects the latest thinking on the the Universe on both the Quantum and Cosmological scales that one day will be merged, perhaps. It didn't really answer anything that I haven't already learned from the expert responders on the Cosmology forum ..they are second to none in my estimatiion...especially, but not limited to, Chalnoth, who must come from the Great Beyond somewhere.

I did learn , per my understanding, a few new things new, however, such as

-The nature of the Time Dimension in the very very very early stages of the Universe;
- The Sum of Histories leading to the multiverses (I've got a lot of work to due on this, however);
- the nature of Inelligent Life, and,
here's the biggy:
- The Theist may be Gravity....unless i misunderstood, which i probably did.

I'll be posting more questions to the Cosmologists here, on the Cosmology forum.

Gold Member
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,
Excellent PhanthomJay! Just Excellent!

I’ll order that book NOW! I have http://en.wikipedia.org/wiki/The_Universe_in_a_Nutshell" [Broken] and it’s just wonderful + fantastic graphics!

P.S. I also believe in Gravity.

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Gold Member
P.S.2. It’s this one you’re reading, right?

http://en.wikipedia.org/wiki/The_Grand_Design_(book)" [Broken]

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#### PhanthomJay

Homework Helper
Gold Member
P.S.2. It’s this one you’re reading, right?

http://en.wikipedia.org/wiki/The_Grand_Design_(book)" [Broken]

Yeah, that's the One!

I believe in Gravity, ---World without End...Amen!

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