# Speed of light

Es99
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Topic 191819

I know this is where the hardcore physicists hang out so I'm sure one of you can point me in the right direction.

Someone over on seti has asked me 'why' the speed of light is what it is. I don't recall ever coming across an explanation for this and any derivation of anything I've seen just states that what it is.

It's set me wondering.

Is it something along the lines that if it were different we wouldn't be here (like the fine structure constant?).

Physics is for gurls!

Joachim Schmidt
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### Speed of light

The speed of light is exactly 299 792 458 m/s because the meter is defined as "the lenght of the path traveled by light in vacuum during a time interval of
1/299 792 458 of a second".
Furthermore the speed of light (c) is a component of the fine-structure constant alpha which is about 1/137. The fine structure constant is fundamental for the physics in the universe. Would it be a bit different, the universe and its physics would be complete different. So you could argue, if the speed of light had been an other value, we wouldn't have existed that way we do.

greets

Es99
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### RE: The speed of light is

Message 45834 in response to message 45833

Quote:

The speed of light is exactly 299 792 458 m/s because the meter is defined as "the lenght of the path traveled by light in vacuum during a time interval of
1/299 792 458 of a second".
Furthermore the speed of light (c) is a component of the fine-structure constant alpha which is about 1/137. The fine structure constant is fundamental for the physics in the universe. Would it be a bit different, the universe and its physics would be complete different. So you could argue, if the speed of light had been an other value, we wouldn't have existed that way we do.

greets

Thanks Joachim! That's what I was thinking it might be. I feel better now. :-)

Physics is for gurls!

tullio
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### RE: The speed of light is

Message 45835 in response to message 45833

Quote:

The speed of light is exactly 299 792 458 m/s because the meter is defined as "the lenght of the path traveled by light in vacuum during a time interval of
1/299 792 458 of a second".
.

greets

Yes, in a vacuum. In other media it is slower. Recently, some physicists have slowed it unbelievably.Greetings.
Tullio

Es99
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### RE: Yes, in a vacuum. In

Message 45836 in response to message 45835

Quote:
Yes, in a vacuum. In other media it is slower. Recently, some physicists have slowed it unbelievably.Greetings.
Tullio

Tell me more. How did they do that? and how slow did they go?

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kinhull
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### I don't know if this helps,

Message 45837 in response to message 45836

I don't know if this helps, it's way over my head.

Quote:

Slow light is the propagation of an optical pulse or other modulation of an optical carrier at a very low group velocity. The term is usually only applied when the velocity is at least hundreds of times slower than the speed of light in a vacuum. There are many mechanisms which can generate slow light, all of which create narrow spectral regions with high dispersion, i.e. peaks in the dispersion relation. Schemes are generally grouped into two categories: material dispersion and waveguide dispersion. Material dispersion mechanisms such as Electromagnetically Induced Transparency (EIT), Coherent Population Oscillation (CPO), and various Four Wave Mixing (FWM) schemes produce a rapid change in refractive index as a function of optical frequency, i.e. they modify the temporal component of a propagating wave. This is done by using a nonlinear effect to modify the dipole response of a medium to a signal or "probe" field. Waveguide dispersion mechanisms such as photonic crystals, Coupled Resonator Optical Waveguides (CROW), and other micro-resonator structures modify the spatial component (k-vector) of a propagating wave.

A predominant figure of merit of slow light schemes is the Delay-Bandwidth Product (DBP). Most slow light schemes can actually offer an arbitrarily long delay for a given device length (length/delay = signal velocity) at the expense of bandwidth. The product of the two is roughly constant. A related figure of merit is the fractional delay, the time a pulse is delayed divided by the total time of the pulse.

tullio
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### RE: RE: Yes, in a vacuum.

Message 45838 in response to message 45836

Quote:
Quote:
Yes, in a vacuum. In other media it is slower. Recently, some physicists have slowed it unbelievably.Greetings.
Tullio

Tell me more. How did they do that? and how slow did they go?

lightspeed
Tullio

Es99
Joined: 9 Sep 05
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Credit: 394,750
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### RE: I don't know if this

Message 45839 in response to message 45837

Quote:
I don't know if this helps, it's way over my head.
Quote:

Slow light is the propagation of an optical pulse or other modulation of an optical carrier at a very low group velocity. The term is usually only applied when the velocity is at least hundreds of times slower than the speed of light in a vacuum. There are many mechanisms which can generate slow light, all of which create narrow spectral regions with high dispersion, i.e. peaks in the dispersion relation. Schemes are generally grouped into two categories: material dispersion and waveguide dispersion. Material dispersion mechanisms such as Electromagnetically Induced Transparency (EIT), Coherent Population Oscillation (CPO), and various Four Wave Mixing (FWM) schemes produce a rapid change in refractive index as a function of optical frequency, i.e. they modify the temporal component of a propagating wave. This is done by using a nonlinear effect to modify the dipole response of a medium to a signal or "probe" field. Waveguide dispersion mechanisms such as photonic crystals, Coupled Resonator Optical Waveguides (CROW), and other micro-resonator structures modify the spatial component (k-vector) of a propagating wave.

A predominant figure of merit of slow light schemes is the Delay-Bandwidth Product (DBP). Most slow light schemes can actually offer an arbitrarily long delay for a given device length (length/delay = signal velocity) at the expense of bandwidth. The product of the two is roughly constant. A related figure of merit is the fractional delay, the time a pulse is delayed divided by the total time of the pulse.

Thanks Kinhull..I'm not sure that is the same thing that Tullio is talking about... and it's making my brain try to remember things I haven't seen or thought about in years.

Ouch.

Physics is for gurls!

Es99
Joined: 9 Sep 05
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### RE: RE: RE: Yes, in a

Message 45840 in response to message 45838

Quote:
Quote:
Quote:
Yes, in a vacuum. In other media it is slower. Recently, some physicists have slowed it unbelievably.Greetings.
Tullio

Tell me more. How did they do that? and how slow did they go?

lightspeed
Tullio

That's very interesting. Thank you for finding that..and I think I even understood most of it!

Physics is for gurls!

Chipper Q
Joined: 20 Feb 05
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### RE: RE: RE: RE: Yes,

Message 45841 in response to message 45840

Quote:
Quote:
Quote:
Quote:
Yes, in a vacuum. In other media it is slower. Recently, some physicists have slowed it unbelievably.Greetings.
Tullio

Tell me more. How did they do that? and how slow did they go?

lightspeed
Tullio

That's very interesting. Thank you for finding that..and I think I even understood most of it!

Frozen Light - really good SciAm article written by Hau. (Hi Es!)

tullio
Joined: 22 Jan 05
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### RE: RE: RE: RE: Quote

Message 45842 in response to message 45841

Quote:
Quote:
Quote:
Quote:
Quote:
Yes, in a vacuum. In other media it is slower. Recently, some physicists have slowed it unbelievably.Greetings.
Tullio

Tell me more. How did they do that? and how slow did they go?

lightspeed
Tullio

That's very interesting. Thank you for finding that..and I think I even understood most of it!

Frozen Light - really good SciAm article written by Hau. (Hi Es!)

Note that Prof. Hau is a woman!
Tullio