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deSitter
Still in Old School

Reged: 12/09/04

Re: C [Re: sirchz]
#5987889 - 07/24/13 02:12 PM

True, but C, like Planck's constant, is not on the same footing as dynamical constants such as the electron mass and the gravitational constant. All the parameters in the Standard Model are dynamical constants.

-drl

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Postmaster

Reged: 04/28/03

Loc: Atlanta, GA
Re: C [Re: deSitter]
#5987911 - 07/24/13 02:28 PM

So basically you are saying that in General Relativity, the other constants that contain units of distance or time are defined relative to C. So the question isn't why is C ~3x10^9 m/s, but why is the electron so small, gravity so weak, etc.

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deSitter
Still in Old School

Reged: 12/09/04

#5987987 - 07/24/13 03:22 PM

Yes that's a legitimate question - see Dirac's "Large Numbers Hypothesis".

The explanation of C is quite rigorous because both Euclidean and Minkowskian geometry are examples of "affine geometries" in the sense of projective geometry. Every metric geometry can be embedded in projective geometry by positing within it the ideal domain. If the metric geometry has an indefinite signature and is degenerate, the characteristic constant is real. That is the case with relativity.

The explanation of Planck's constant is not yet at hand in so categorical a way, but many (raises hand) expect a similar geometric and invariant-theoretic derivation some day. In practice, both C and h are set to 1 to simplify calculations. They can always be restored at the end.

-drl

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PhilCo126
Post Laureate

Reged: 01/14/05

Loc: coastline of Belgium
Re: C [Re: deSitter]
#5988082 - 07/24/13 04:31 PM

C = Celeritas ( Latin for "speed" )

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: C [Re: PhilCo126]
#5988707 - 07/24/13 11:21 PM

Truly, I appreciate all the discussion that is going into this. But, is it possible for one of you to do a "Jarad" or a "Dave" and put these thoughts into even simpler words for me to begin to understand them. Why is c~3x10^9 m/s in our universe now and seemingly, for most of its current history? Why can't it be larger or smaller? Thanks guys (and gals?) Otto

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llanitedave
Humble Megalomaniac

Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: C [Re: Otto Piechowski]
#5988794 - 07/25/13 12:22 AM

Danny's reference to the value of c being '1' is spot on. It's the fundamental constant of the universe, not an arbitrary value. That's why he called it the "natural" value. If the number seems arbitrary to us, it's because we are simply using the wrong perspective. It's humanity, and our own length and time scales that are arbitrary, not the geometry of spacetime.

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Mister T
Pooh-Bah

Reged: 02/01/08

Loc: Upstate NY
Re: C [Re: Otto Piechowski]
#5989055 - 07/25/13 06:56 AM

Light travels at the speed light travels

299,792,458 m/s or 186,282 mi/s are values we have calculated relative to the units we have chosen as our standards.

On Greezledorp III, c = 42 fremulangs/quiszzenferfulmops

(roughly translated Gleezledorpese is a VERY complex and nuanced language)

If you want to know the physical parameters that enable/confine c to it measured value, you will have to go to a planet where they have a much more advanced understanding of Wave-icle Physics than we do.

probably a planet where they kids have Large Hadron Colliders sitting in their closets collecting dust because they are bored with them....

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: C [Re: Mister T]
#5989062 - 07/25/13 07:07 AM

Everyone,

Thank you.

(If there is more to say, by all means say it.)

Otto

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sirchz
super member

Reged: 09/21/09

Re: C [Re: Otto Piechowski]
#5989634 - 07/25/13 02:03 PM

Quote:

Why is c~3x10^9 m/s in our universe now and seemingly, for most of its current history? Why can't it be larger or smaller?

Questions of why are dangerous territory. Rather than discussing "c", the speed of light, most discussions I've seen revolve around the fine structure constant. Alpha is a measure the electromagnetic interaction, which is quite important. I'm not an expert in this, so I'll stop and simply refer you to the links below for more information, especially the Economist article.

Economist Article on the Fine Structure Constant
Wikipedia - Fine Structure Constant

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petrus45
sage

Reged: 01/31/11

Loc: SW Ohio / N Ky.
Re: C [Re: AstroGabe]
#5993863 - 07/27/13 11:11 PM

Quote:

Particles with mass travel slower than the speed of light since the space-time momentum four-vector cannot be a null. That's a way of saying that since space and time are interconnected, and that the energy and momentum are interconnected, then the fact that a rest mass exists, the particle travels through space and time at different rates. Essentially, the world line of the particle is constrained to be time-like.

Gabe: I'm not particularly well-versed in the shop talk on these issues, but it seems like you're saying one of the qualities of things that have "mass" is that they travel at some speed slower than "c." Is this "slowing" below "c" what actually confers mass? I've heard this "mass-conferring" issue has something to do with the Higgs Field and the Higgs-Boson. I'm picturing a big net slowing light particules down at which point they convert into mass. I'm sure this is ridiculously crude for those who understand the math, but is this image in any way useful in visualizing these concepts?

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jhayes_tucson
professor emeritus

Reged: 08/26/12

Loc: Bend, OR
Re: C [Re: Otto Piechowski]
#5993968 - 07/28/13 12:41 AM

Quote:

Truly, I appreciate all the discussion that is going into this. But, is it possible for one of you to do a "Jarad" or a "Dave" and put these thoughts into even simpler words for me to begin to understand them. Why is c~3x10^9 m/s in our universe now and seemingly, for most of its current history? Why can't it be larger or smaller? Thanks guys (and gals?) Otto

The universe is full of constants and the value of 'c' (in a vacuum) is one of them. It is something that we measure; like the value of Planck's constant or the value of 'G' (the gravitational constant.) It is what it is. Physics is a search for fundamentals and there may be a way to compute some of these constants from more fundamental models one of these days but as far as I know, 'c' is still considered a constant value.
John

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AstroGabe
sage

Reged: 01/10/10

Loc: SE Wisconsin
Re: C [Re: petrus45]
#6017540 - 08/10/13 12:31 PM

Hi Peter,
Yes, if a particle has mass it must necessarily travel with a velocity less than the speed of light. Your analogy isn't far off.
I wrote a quick article for our club last year that has a similar analogy and a bit more explanation of its importance. Starting on pg 4:

Gabe

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: C [Re: AstroGabe]
#6017545 - 08/10/13 12:34 PM

What a nice elegant explanation of what causes things to have mass! Thank you, Gabe. Otto

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PeterR280
Pooh-Bah

Reged: 05/27/13

Re: C [Re: sirchz]
#6018270 - 08/10/13 09:29 PM

Maxwell's equations give solutions to electromagnetic wave equations. they also imply that the speed of light is a constant but it wasn't until Einstein that c was recognized as the speed limit of the universe. The number c is just that. There is nothing in physics now that predicts that speed from other constants. Who knows in the future.

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llanitedave
Humble Megalomaniac

Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: C [Re: PeterR280]
#6018284 - 08/10/13 09:48 PM

No discussion on c is complete without referencing the definitive book on the subject:

K & R

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: C [Re: llanitedave]
#6018309 - 08/10/13 10:25 PM

Not nice Dave.

But funny.

Otto

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scopethis
Postmaster

Reged: 05/30/08

Loc: Kingman, Ks
Re: C [Re: Otto Piechowski]
#6018752 - 08/11/13 10:35 AM

doesn't the Universe have to expand faster than c?

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GregLee1
professor emeritus

Reged: 07/21/13

Loc: Waimanalo, HI
Re: C [Re: scopethis]
#6019031 - 08/11/13 01:15 PM

Quote:

doesn't the Universe have to expand faster than c?

In my very partial understanding, it's the space part of the universe that expands faster than c, not any material. This is from a tutorial article at The Physics Forums :

Quote:

The material that emitted the light which we are currently receiving as CMB is now at a distance from us of 45 billon LY. The light managed to get here in slightly less than the age of the expansion which is 13.7 billion years. The material was much closer to us when the light started its journey. It didn't break any speed laws. Yet it seems to have come all the way across the balloon surface to us from a point which is now 45 billion LY away. This might strike you as paradoxical but it isn't really. The balloon analogy shows you how light can cover enormous distances in less than the expansion age.

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GregLee1
professor emeritus

Reged: 07/21/13

Loc: Waimanalo, HI
Re: C [Re: Otto Piechowski]
#6019193 - 08/11/13 02:52 PM

Quote:

(1) why does "c" remain a constant in any given universe;

Does it? As it happens, just yesterday I caught part of an interview (on Morgan Freeman's Science Channel series) with a physicist telling about his alternative to Inflation. Rather than the uniformity of the universe being due to exchange between its parts when it was small (before Inflation), the early exchanges were possible because c was small (and there was no inflation). Sorry, I didn't get the name.

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PeterR280
Pooh-Bah

Reged: 05/27/13

Re: C [Re: GregLee1]
#6019987 - 08/11/13 11:59 PM

Maybe the universe stays the same size and the "constants" change over time.

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