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Topic: Speed of light (Read 6168 times)
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## Speed of light

##### 2002-01-09 21:33:40
Quote
Originally posted by Garf

For comparison, no-one 'proved' what the speed of light is, but we have some pretty accurate values with small error margins for it, and they're widely accepted as the 'truth'.

Perhaps you consider this purely empirical and not a proof, but I'd still like to see you disprove the value of c.

Really

Well, to be pedantic, if I remember my physics correctly, I believe the value of c does arise mathematically as the speed of propagation of the wave solution to Maxwell's equations as applied to free space.  So one could call that "proof." =]

## Speed of light

##### Reply #1 – 2002-01-10 12:28:11
Quote
Originally posted by Delirium

Well, to be pedantic, if I remember my physics correctly, I believe the value of c does arise mathematically as the speed of propagation of the wave solution to Maxwell's equations as applied to free space.  So one could call that "proof." =]

To be even more pedantic: The value of c can, using Maxwell's equations, be calculated from two fundamental constants of nature (the permittivity and permeability; see here for more information). So you first need proof that you have the correct values of those constants

## Speed of light

##### Reply #2 – 2002-01-10 12:52:51
Quote
Originally posted by Lear

To be even more pedantic...

To be _even_ more pedantic: all equations describing physical phenomena (including Maxwell's) are approximations (although quite accurate ones at least within certain limits). All the (more or less) commonly used equations have been designed to model a certain phenomenon or a set of phenomena with certain similar characteristics. They are all (?) based on assumptions that are in turn based on huge amounts of test data. Therefore any result you get using such equations is an approximation in itself, although usually accurate enough to be useful.

## Speed of light

##### Reply #3 – 2002-01-10 13:41:45
The speed of light, c, is known (or should I say defined) exactly.

This is because the length of 1 metre is defined from the speed of light.

see

http://www.what-is-the-speed-of-light.com/

for more details.

Cheers,
David.

## Speed of light

##### Reply #4 – 2002-01-10 14:07:16
Quote
Originally posted by 2Bdecided
The speed of light, c, is known (or should I say defined) exactly.

This is because the length of 1 metre is defined from the speed of light.

see

http://www.what-is-the-speed-of-light.com/

for more details.

Cheers,
David.

This is getting waaaay offtopic.

But...

We don't exactly know how long a metre is then

Edit: And what about places where they don't use the metric system?

--
GCP

## Speed of light

##### Reply #5 – 2002-01-10 14:24:07
Ack!

[nitpick]Yes, c is defined quite accurately, according to modern measurements which in turn doesn't necessarily mean that it is accurate.

The length of 1 meter was originally defined as 1/10,000,000 of a quarter of Earth's meridian. It has later been redefined a few times.
The latest definition of 1 meter (1983, 17. CGPM) is the distance that light travels in a period of 1/299892458 seconds. The number is solely based on the previously measured speed of light so it cannot be used to reversely prove the value of c in any way.
I could probably explain the definitions of meter and second as they are officially defined but I'll bet that nobody here really cares.
[/nitpick]

Moderators: Some parts of this thread should probably be moved to the General Discussion Forum (or maybe even OT )

## Speed of light

##### Reply #6 – 2002-01-10 14:39:44
Garf,

In short 1 meter is defined as the distance traveled by light (usually HeNe or some other laser) in a vacuum in 1/299,792,458 of a second (the speed of light which is considered to be constant is measured as 299,792.458 m/s).

One second in turn is defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

The numbers were chosen so that the previously defined units would be bound to some universal constants.
IIRC tho original definition of 1 meter was a platinum alloy metal bar that had  2 markings 1 meter apart.

End of rant. This really should be moved to some more appropriate forum.

## Speed of light

##### Reply #7 – 2002-01-10 15:00:01
Heretics!

Every child knows that a metre is defined as 1/70 the lenght of the great Xiph in Gizeh.

## Speed of light

##### Reply #8 – 2002-01-10 15:03:18
Quote
Originally posted by YinYang
Heretics!

Every child knows that a metre is defined as 1/70 the lenght of the great Xiph in Gizeh.

(Hopeless attempt at getting ontopic again)

Everybody knows the great Xiph lives in Ogg!

--
GCP

## Speed of light

##### Reply #9 – 2002-01-10 15:04:32

This is the most bizarre thread i've seen on this board.

Keep up the good work, guys!

## Speed of light

##### Reply #10 – 2002-01-22 19:40:21
I agree - this is NICE  lets push it - NOTHIN'  can be proven! it all depends on your view...
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