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#26 llanitedave

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Posted 24 July 2013 - 11:22 PM

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.

#27 Mister T

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Posted 25 July 2013 - 05:56 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.... :p

#28 Otto Piechowski

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Posted 25 July 2013 - 06:07 AM

Everyone,

Thank you.

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

Otto

#29 sirchz

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Posted 25 July 2013 - 01:03 PM

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

#30 petrus45

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Posted 27 July 2013 - 10:11 PM

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?

#31 jhayes_tucson

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Posted 27 July 2013 - 11:41 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


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

#32 AstroGabe

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Posted 10 August 2013 - 11:31 AM

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:

http://www.rasastro....r Fall 2012.pdf

Gabe

#33 Otto Piechowski

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Posted 10 August 2013 - 11:34 AM

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

#34 PeterR280

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Posted 10 August 2013 - 08:29 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.

#35 llanitedave

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Posted 10 August 2013 - 08:48 PM

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

K & R

:whistle:

#36 Otto Piechowski

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Posted 10 August 2013 - 09:25 PM

Not nice Dave.

But funny.

Otto

#37 scopethis

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Posted 11 August 2013 - 09:35 AM

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

#38 GregLee1

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Posted 11 August 2013 - 12:15 PM

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 :

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.



#39 GregLee1

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Posted 11 August 2013 - 01:52 PM

(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.

#40 PeterR280

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Posted 11 August 2013 - 10:59 PM

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

#41 EJN

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Posted 11 August 2013 - 11:34 PM

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

K & R

:whistle:


Does this mean that if faster-than-light travel were possible,
it would be C++ ?

:rimshot:

#42 llanitedave

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Posted 12 August 2013 - 01:12 AM

Unless, of course, the universe is interpreted.

#43 Otto Piechowski

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Posted 12 August 2013 - 11:12 AM

It might have been drl, but refresh my memory if this is wrong.

Didn't he say what Greg and Peter have been saying, that inflation may not have happened, and maybe its the constants which are changing? Something like that?

Otto

#44 scopethis

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Posted 12 August 2013 - 11:38 AM

that would seem to truly uphold the Relativity Theory, that everything is relative to the viewer...

#45 Qwickdraw

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Posted 12 August 2013 - 04:09 PM

Good question.

Danny can probably give a much better answer than I can. But for at least a first level answer, C is determined from 2 other physical properties of the vacuum - permeability (the magnetic constant) and permittivity (the electric constant). Wiki link

As to why those two physical constants are what they are, I can't give you an answer beyond they are built into the nature of our space-time. Maybe someone else can add a deeper insight.

Jarad


Of course ad nauseum, you can now ask yourself, self, why than is the magnetic constant the electric constant at the values they are? I suppose you can kick this down the road as long as you can maintain your sanity.

#46 llanitedave

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Posted 12 August 2013 - 10:15 PM

If the constants are changing, they'd all have to change in sync with one another, which in a practical sense means they aren't changing.

Otherwise, the values of the weak force and the strong force would also be changing. Atoms might be possible in some eras but not others, stellar characteristics and reactions would change over time, and there would be no continuity to the cosmos.

On the contrary, we have firm evidence that the constants of nature which affect matter as we know it have been constant for at least 1.7 billion years on Earth, from among other things studying the products from the Oklo fission events. The fact that type 1A supernovae appear identical throughout the history of the observable universe is also pretty strong evidence in favor of constant constants.






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