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Author Topic: Why does light travel at the speed of light? (in a vacuum).  (Read 16428 times)

Offline Airthumbs

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What makes light travel so fast?  Even we emit light in the infra-red, photons, I know, but why are these little packets of photons leaving us with such rapidity...... how many g's are these packets pulling when they go from 0 to light speed, instantly?


 

Offline MikeS

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Why does light travel at the speed of light? (in a vacuum).
« Reply #1 on: 16/05/2011 16:49:49 »
It only has one speed in a vacuum.  It is born and dies at that speed.  It does not experience any g force as it has no mass.
 

Offline Phractality

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Why does light travel at the speed of light? (in a vacuum).
« Reply #2 on: 16/05/2011 17:46:58 »
According to mainstream science, "It just does." They do have a math formula for the speed of light as a function of permeability and permittivity of free space; but those parameters are defined by the speed of light, so the definitions are circular. They say that light requires no medium.

If you can't stomach the idea of a wave with no meadium, ask again in the New Theories section. We're not permitted to share our non-mainstream ideas in the Physics, Astronomy & Cosmology section.
 

Offline Bored chemist

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Why does light travel at the speed of light? (in a vacuum).
« Reply #3 on: 16/05/2011 19:34:03 »
"but those parameters are defined by the speed of light"
No, they can be measured independently.
 

Offline CZARCAR

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Offline Geezer

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Why does light travel at the speed of light? (in a vacuum).
« Reply #5 on: 16/05/2011 23:13:55 »
http://www.physorg.com/news/2011-05-when-the-speed-of-light.html    whats this mean?

Well, it's in nitrogen rather than a vacuum for a start. It might suggest that electromagnetic radiation (light) is susceptible to magnetic fields, which might not be a total surprise, but I'd be interested to see if the results are reproducible when the nitrogen is removed.
 

Offline JP

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Why does light travel at the speed of light? (in a vacuum).
« Reply #6 on: 17/05/2011 04:12:59 »
http://www.physorg.com/news/2011-05-when-the-speed-of-light.html    whats this mean?

Well, it's in nitrogen rather than a vacuum for a start. It might suggest that electromagnetic radiation (light) is susceptible to magnetic fields, which might not be a total surprise, but I'd be interested to see if the results are reproducible when the nitrogen is removed.

It's not actually all that surprising.  First, f you have a magnetic field in a vacuum, you have energy there, so space-time is warped a bit.  With a very high field, I could imagine that you would have to take into account general relativistic effects, which could delay light in some directions compared to others.  I think in this case this isn't what's happening, though, since the field is uniform...

Second, what I think they're getting at is that at very high energy densities, there are quantum effects even in vacuum where EM fields can interact with each other (something which isn't allowed in classical electromagnetism).  Last I heard, these effects were expected to be so tiny that no one had managed to measure them, so they're purely theoretical.  I suspect they're aiming for this effect, since the last line of the abstract reads:
Quote
We were able to measure a nonreciprocity as small as Δn=(52)10-18, which makes its observation in quantum vacuum a conceivable challenge.
 

Offline Phractality

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Why does light travel at the speed of light? (in a vacuum).
« Reply #7 on: 17/05/2011 04:51:14 »
"but those parameters are defined by the speed of light"
No, they can be measured independently.

Quoting from Wikipedia:
Vacuum permittivity
Vacuum permeability

Fc = 1/(4 π ε₀)(q₁q₂/r)

The value of ε₀ is defined by the formula ε₀ = 1/(₀c₀).

Since μ₀ has the defined value 4π 10^−7 H m‾,[3] and c₀ has the defined value 299792458 ms‾,[4] it follows that ε₀ has a defined value given approximately by
ε0 ≈ 8.854187817620... 10‾ Fm‾ (or A2s4kgm‾ in SI base units, or CN‾m‾ or CV‾m‾ using other SI coherent units).[5][6]


So c₀ is NOT derived by the formula c₀ = 1/√(₀ ε₀). It's the other way around.
 


In some ether-based models, light propagates like an acoustic shear wave in a solid according to the formula cs = √(G/ρ) where K and G are the bulk modulus and shear modulus of the elastic materials, respectively. The density and shear modulus of the ether are unknown, but this formula gives us their ratio and provides a rational explanation for WHY c is what it is. But this in not mainstream science, so don't give this answer on your final exam in college. Go the the New Theories section to learn more about ether theories.
 

Offline Bored chemist

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Why does light travel at the speed of light? (in a vacuum).
« Reply #8 on: 17/05/2011 07:19:20 »
Phractality, in that case, the speed of light is what it is because they defined it that way.
 

Offline GrapperJ

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Why does light travel at the speed of light? (in a vacuum).
« Reply #9 on: 17/05/2011 20:25:16 »
newbielink:http://zidbits.com/2011/04/why-cant-anything-go-faster-than-the-speed-of-light/ [nonactive] has a decent layman writeup on light speed.

In short, don't confuse the speed of photons with "light speed". Light speed is a maximum limit and a function of reality and space/time itself. Asking why light speed is what it is is like asking why is doesn't our reality have 6 spatial dimensions. It just does. If light speed was different, our reality and universe would also be very different.
« Last Edit: 17/05/2011 20:29:44 by GrapperJ »
 

Offline Phractality

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Why does light travel at the speed of light? (in a vacuum).
« Reply #10 on: 17/05/2011 21:18:59 »
Phractality, in that case, the speed of light is what it is because they defined it that way.

Originally, the meter and second were defined in terms of Earth's size and rotation. Earth's polar circumference was 4 x 10^7 meters, and the mean solar day was 86,400 seconds. The meter was then standardized to the length of a certain platinum rod (which turned out to be slightly less than 1/(4 x 10^7) of Earth's polar circumference). We took precise measures of the speed of light using those meters and seconds. As our methods of measurement became more precise, we discovered that Earth's size and rotation are not perfectly constant; meters and seconds change very slightly in proportion to the speed of light. So we changed the definitions of meter and second, making the speed of light 299792458 m/s by definition. So that is how we ended up with that number for the speed of light.

What gave the speed of light that ratio to Earth's size and rotation, in the first place, is still a mystery.
 

Offline yor_on

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Why does light travel at the speed of light? (in a vacuum).
« Reply #11 on: 18/05/2011 00:07:30 »
Yeah, It's our foremost constant. But it was a very interesting experiment you found Czarcar

"for the first time, that light does not travel at the same speed in opposing directions in a gas where an electromagnetic field reigns. The measured difference in velocity is around one billionth m/s (i.e. 10-9 m/s, which is equivalent to 10-18 times the speed of light). This infinitesimal difference, predicted by theory, is caused by the magnetic and electric fields."

What that makes me think of is the photons 'momentum' and symmetry, seems like a similar effect to me as the idea of a photons 'recoil'? Then again, we can down convert it if we like to a question of what governs our lives, and there we have the idea of everything carried by 'virtual particles' as the 'force carriers'. Using that definition it is no miracle that a magnetic field can influence the photon, as it is the origin of 'electromagnetism'. And as JP points out, if we go to a quantum plane there is no place without those virtual particles, aka photons in this case.
 

Offline Airthumbs

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Why does light travel at the speed of light? (in a vacuum).
« Reply #12 on: 27/06/2011 00:41:36 »
I appreciate all the responses but I am afraid you have all left me, well not entirely satisfied.  :P  The best answer I found to my question went along the lines of....."it just is, so don't ask"  and that sounds more like a religious response then a scientific one to be honest.

When people talk about light they say it has a source, that the light is "emitted from that source".  So a massless photon is emitted from a chemical reaction.  Just before the reaction takes place the photon does not exist, it is therefore not moving at any speed at all. 

What I am struggling to get at is, how can it be that at the exact point of existance the photon can only exist at the speed of light.  Does anyone understand my confusion here?  You see for that photon to have a source and if that source is stationary then how can it be instantly travelling at the speed of light, it's just not possible, unless the photon was in existance before that and was always travelling at the speed of light....  :-\

Maybe the acceleration of this photon to light speed is happening so fast it is not detectable?  Anyway I cannot accept that photons are "born" at the speed of light.  They only reach the speed of light in our universe because they can and that is the limit.

Recent experiments have succeeded in actually stopping light, Hau's team stopped light for one-thousandth of a second!  This process involves the slowing down of light.  Are you all saying that after this experiment the light just shot off instantaneously at the speed of light?  I'm not having it!  ;D

 

Offline yor_on

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Why does light travel at the speed of light? (in a vacuum).
« Reply #13 on: 27/06/2011 04:26:49 »
Well, as I see it there has to be presumptions with any experiment deciding a 'photons path' Airthumbs :) Photons only exist in interactions. You can think of photons/waves three ways, as something 'propagating', as something being a 'constant', as something being the best 'clock' we have.

If you ignore 'propagating' you will still have a constant, and a clock. I can't see any experiment showing us a single photons propagation. The only way to define it practically is by 'weak measurements', and then you fall back on measurements over a large amount of 'photons' defining it as they are not only 'similar', but 'indivisible', which they in fact are not. If they were 'there could only be one' as they say in Highlander. Otherwise it will have to be done theoretically.

As for light only having one 'speed'. It's massless, inertia is defined by mass, so is accelerations. So in a way you can see it as a direct result of light having no mass. without it light has no reason to accelerate as I see it. If we accept the recoil as a definition of 'something' reducing the energy of whatever it is experiencing that recoil, then we can use that as a definition of something 'leaving' or 'disappearing'. And we know that photons annihilate in their final interaction, meaning that all light you see is photons dying. But in between those two states we can't test on a single photon. Any path defined must be a statistical approach, and as confirmed by lights duality we can't even state if it is a photon or a wave in between, as I see it.

So, assuming a propagation, a photon is massless. Assuming no 'propagation' you have two interactions (recoil-annihilation) defined by a constant, that also is a clock.
 

Offline Heikki Rinnemaa

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Why does light travel at the speed of light? (in a vacuum).
« Reply #14 on: 27/06/2011 06:27:51 »
 :)

Hi.

My opinion is that light does not have constant speed.

I draw image where is two possibilityes,,,A and B,,,and i dont no any natural reasons why light goes like B. But natural function is that A.

Light start to goes speed 0 and accelerate some maximum speed,,,can be more than measured 300000km/s near sun-earth,, and then decelerate,,and enough faraway from light-source light speed is  again 0km/s.

Speed is not constant,,even light is particle or matter-vibration,,it is matter-particle with mass,,but if you think that it is wave-vibration without matter,,i dont agree that because i dont have any needs to say what light is,,it is still and support our life.


 :)

 

Offline lightarrow

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Why does light travel at the speed of light? (in a vacuum).
« Reply #15 on: 27/06/2011 14:09:16 »
Light start to goes speed 0 and accelerate some maximum speed,
Ok, be Δt the time interval needed to a photon to accelerate from 0 to c, in a specific frame of reference. Now let's make the measurement in another frame, which is moving at velocity v with respect to the first. The time interval is not the same anylonger, it becomes Δt*sqrt[1-(v/c)2]. The final speed, however, is still the same, because light' speed is invariant (= frame independent) so the (average) acceleration is different. Does it make any sense? What if v --> c? The acceleration becomes infinite?
 

Offline Mr. Data

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Why does light travel at the speed of light? (in a vacuum).
« Reply #16 on: 27/06/2011 19:47:04 »
As has been mentioned, permitivitty and permeability explain exactly why light travels at the speed it does. However, this speed can be changed depending on the medium, so light for instance would travel slower in a universe where it's vacuum energy density is higher (just as an example) and not that I believe parallel universes exist.
 

Offline lightarrow

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Why does light travel at the speed of light? (in a vacuum).
« Reply #17 on: 28/06/2011 07:02:16 »
As has been mentioned, permitivitty and permeability explain exactly why light travels at the speed it does.
And how do you explain the numerical values of those parameters without knowing light' speed? Any measure of them can be traced to a light' speed measure, actually.
 

Offline imatfaal

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Why does light travel at the speed of light? (in a vacuum).
« Reply #18 on: 28/06/2011 11:58:28 »
As has been mentioned, permitivitty and permeability explain exactly why light travels at the speed it does.
And how do you explain the numerical values of those parameters without knowing light' speed? Any measure of them can be traced to a light' speed measure, actually.

L-Arrow - are you sure about that?  Whilst most measurements do use the relationship through maxwell's equations - there is no requirement that this is the case.  Nowdays both permitivity and permeability are defined by their relation with the speed of light BUT the wonder of Maxwell's equations is that they can also be calculated without reference to the speed of light
 

Offline Mr. Data

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Why does light travel at the speed of light? (in a vacuum).
« Reply #19 on: 28/06/2011 13:05:52 »
As has been mentioned, permitivitty and permeability explain exactly why light travels at the speed it does.
And how do you explain the numerical values of those parameters without knowing light' speed? Any measure of them can be traced to a light' speed measure, actually.

By numerical values, think of the speed of light as a consequence of the permitivitty and permeability of spacetime. You have your light speed, then you know the permitivitty and permeability have exact values. They fit - just like every postulation from Maxwells equation.
 

Offline yor_on

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Why does light travel at the speed of light? (in a vacuum).
« Reply #20 on: 28/06/2011 23:43:15 »
You know Imatfaal. Looking at it, it seems as if Maxwell had some twenty equations originally, other sources stating that it was Heaviside that whittled them into their modern configuration? I've tried to look into it but it seems a quagmire to get the history straight there :)

Here's a online version of his book(s) James Clerk Maxwell, A Treatise on Electricity And Magnetism Vol 1 of 2.

But Heaviside interest me a lot. He reminds me of that Russian mathematician that recently refused money and prizes. Both of them seem to have had their own brand of integrity.
« Last Edit: 28/06/2011 23:56:35 by yor_on »
 

Offline imatfaal

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Why does light travel at the speed of light? (in a vacuum).
« Reply #21 on: 29/06/2011 11:35:38 »
Maxwell didn't entirely write any of 'his equations'.  (two are Gauss, one Ampere and one Faraday - or is it two Faraday and one Gauss). It is the use, juxtapostion, and fundamental interelatedness of them in his paper On the Physical Lines of Force that means that we call them Maxwell's equations.
 

Offline Heikki Rinnemaa

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Why does light travel at the speed of light? (in a vacuum).
« Reply #22 on: 03/07/2011 06:42:44 »
 :)


http://en.wikipedia.org/wiki/Electromagnetic_radiation

http://en.wikipedia.org/wiki/Electromagnetic_spectrum


Those all vibration cannot exist pure matterless place,,, and therefore all wave-vibration can--must be matter-vibration,,,

Who says that colors is matter-vibration,,wave-frequence,,,?


Perhaps it is particle speed-motion and particle construction join-movement,,,

- speed
- shape or construction



 :)


 

Offline JP

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Why does light travel at the speed of light? (in a vacuum).
« Reply #23 on: 03/07/2011 15:46:57 »
Those all vibration cannot exist pure matterless place,,,

Sure they can.
 

Offline lightarrow

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Why does light travel at the speed of light? (in a vacuum).
« Reply #24 on: 06/07/2011 20:54:50 »
And how do you explain the numerical values of those parameters without knowing light' speed? Any measure of them can be traced to a light' speed measure, actually.

L-Arrow - are you sure about that?  Whilst most measurements do use the relationship through maxwell's equations - there is no requirement that this is the case.  Nowdays both permitivity and permeability are defined by their relation with the speed of light BUT the wonder of Maxwell's equations is that they can also be calculated without reference to the speed of light
You know tha μ0 is defined from the Ampre (μ0 = 4π*10-7), so it remains ε0. How can you measure it? You shoul for example measure the coulombian force between two charges at a specific distance because of:

F = (1/4πε0) q1*q2/r2

I wonder if it's possible to measure the charges q1 and q2 in Coulomb, without knowing ε0.
 

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Why does light travel at the speed of light? (in a vacuum).
« Reply #24 on: 06/07/2011 20:54:50 »

 

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