# Thread: Why is c so important?

1. ## Why is c so important?

It seems that in Einstein's universe, everything that is everything; time, space, matter energy, motion, all must bow before the speed of light. But why? Why is the speed of light so fundamental? It seems odd; after all, light could not exist without a universe to exist in, and yet the universe seems to be less important than light.

2. Originally Posted by parallaxicality
It seems that in Einstein's universe, everything that is everything; time, space, matter energy, motion, all must bow before the speed of light. But why? Why is the speed of light so fundamental? It seems odd; after all, light could not exist without a universe to exist in, and yet the universe seems to be less important than light.
It might help to not think of it as "the speed of light". c is something more general, the maximum relative velocity of two objects, the maximum velocity at which energy, information, or the effect of any cause can propagate. It's not that c is the speed of light, light just travels (in a vacuum) at c.

3. Well said. Yeah, it is a property of the universe that light does conform to.

4. Originally Posted by parallaxicality
It seems that in Einstein's universe, everything that is everything; time, space, matter energy, motion, all must bow before the speed of light. But why? Why is the speed of light so fundamental? It seems odd; after all, light could not exist without a universe to exist in, and yet the universe seems to be less important than light.
If you look closely at the foundations of special relativity, you will find that it is based on just two postulates. 1) The speed of light is a fixed constant in all inertial reference frames. 2) The laws of physics are the same in all inertial reference frames.

It turns out that in working carefully through the derivation of the Lorentz transformation, which is the heart of special relativity, one need only assume that there is some phenomena that propagates with speed "x" in all inertial reference frames. One can then derive all of special relativity and obtain the Lorentz transformation with "x" playing the role of "c". This means that "x" would be a limiting speed for the speed of massive particles and for the transmission of information. The derivation also shows that there can be only one such speed that is independent of the reference frame. It happens that, on the basis of experimental evidence, the speed of light, c, is independent of the inertial reference frame.

It turns out that general relativity predicts the existence of gravitational waves and that they propagate at the speed of light. So, you could replace the speed of light with the speed of gravity, the only wrinkle being that special relativity specifically neglects gravitational effects. But by passing to general relativity you would have the same importance for the speed of gravity that you have for the speed of light, and they are the same.

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Originally Posted by cjameshuff
It might help to not think of it as "the speed of light". c is something more general, the maximum relative velocity of two objects, the maximum velocity at which energy, information, or the effect of any cause can propagate. It's not that c is the speed of light, light just travels (in a vacuum) at c.
Wow, that's very enlightening, cjameshuff! After all these years, I finally get it! Thanks a lot.

Hope you're right.

I think DrRocket said you're right.

6. Originally Posted by cjameshuff
It might help to not think of it as "the speed of light". c is something more general, the maximum relative velocity of two objects, the maximum velocity at which energy, information, or the effect of any cause can propagate. It's not that c is the speed of light, light just travels (in a vacuum) at c.
That would have been my answer. But being the humble brainiac that I am, I decided to let someone else provide the answer. Sweet of me, no?

Joking

Interesting.

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Originally Posted by cjameshuff
It might help to not think of it as "the speed of light". c is something more general, the maximum relative velocity of two objects, the maximum velocity at which energy, information, or the effect of any cause can propagate. It's not that c is the speed of light, light just travels (in a vacuum) at c.
This is one of those things I love in science (or anything else - it happens to me all the time) - where something is impossible to get your head round, then someone explains it to you really simply and you think 'Oh wow, that's so obvious! How did I not figure that one out?'

By the way, why is c described as the speed of light specifically? I always thought that electromagnetic waves all travelled at the same speed. Is it just because light is the easiest to observe?

8. Originally Posted by azimuth
[Snip!] By the way, why is c described as the speed of light specifically? I always thought that electromagnetic waves all travelled at the same speed. Is it just because light is the easiest to observe?
A lot of us prefer "light" as a convenient short-hand for "electromagnetic radiation" or even "EM radiation" regardless of the wavelength. If we really want to be specific about the wavelength or frequency band we say "infrared" or "ultraviolet" or "X-rays", etc. When we mean the kind of light that we can see with our eyes we say "visible light".

Also, visible light was the first form of light that we attempted to measure the velocity of.

9. Originally Posted by parallaxicality
It seems that in Einstein's universe, everything that is everything; time, space, matter energy, motion, all must bow before the speed of light. But why? Why is the speed of light so fundamental? It seems odd; after all, light could not exist without a universe to exist in, and yet the universe seems to be less important than light.
I guess you say that because of E=mc2 and the maximum speed being c? I'm not sure why that would make c more important than the universe but in the right units, c=1 and E=m without the involvement of c.

10. Besides, without C you get scurvy.

11. Originally Posted by mike alexander
Besides, without C you get scurvy.
Orange you clever!

12. Originally Posted by Swift
Orange you clever!
Sublime

13. I would never allow mere sub-limes on my ship!

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Originally Posted by Argos
Well said. Yeah, it is a property of the universe that light does conform to.
And nothing else conforms to c. Nothing else can reach it; nothing else can surpass it.

However, c affects everything else, so much so that there's little which isn't affected in one way or another by this constant.

Therefore, I would say that the universe conforms to c, rather than the other way around.

15. dare I ask why our universe seems tied to c?

16. Knot if you value your sanity--it's all tangled up, and there are a lot of loose ends which tend to leave you adrift. At c

Wow, that brings us back to scurvy, doesn't it? Maybe there is something to that idea of spacetime scurvature.

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Think, for a minute, how lightening fast men's heads turn towards scurvatious women...

18. surely you mean "scurvaceous"

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Originally Posted by parallaxicality
surely you mean "scurvaceous"
Shirley, that's what I must have meant!

20. And stop calling parallaxicality "Shirley".

21. Because without C, what would come after B?

22. Originally Posted by KaiYeves
Because without C, what would come after B?
Are you saying, in order to C we have to B? G, that makes sense.

23. Yc? Icc. Ucc2. Im2b<c&ur2. Oicur<c&im22u.

24. Originally Posted by mugaliens
And nothing else conforms to c. Nothing else can reach it;
How about ripples in spacetime [G waves]?

Therefore, I would say that the universe conforms to c, rather than the other way around.
The inflation, the expansion of the universe, are not confined to c.

25. So...is there a reason that light moves at that particular speed? Or is it just fundemental?

26. If you write Maxwell's equations in a form that is invariant over the Lorentz transformations you can derive the speed of light from DrRocket's x to get that EM radiation has to propagate at the same velocity as x.

27. Originally Posted by DrRocket
The derivation also shows that there can be only one such speed that is independent of the reference frame. It happens that, on the basis of experimental evidence, the speed of light, c, is independent of the inertial reference frame.
Originally Posted by HenrikOlsen
If you write Maxwell's equations in a form that is invariant over the Lorentz transformations you can derive the speed of light from DrRocket's x to get that EM radiation has to propagate at the same velocity as x.
But there are still constants that have to be experimentally determined, and if you don't assume light is EM you still have to verify that light does have that speed.

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