# Uncertainty of the speed of light in vacuum

1. Oct 1, 2008

### fluidistic

Hi all,
According to this site : http://physics.nist.gov/cuu/Constants/, the speed of light in vacuum is $$299792458 \frac{m}{s}$$ with no uncertainty! How is that that there's no uncertainty? I like this website because it gives many physics constants with their uncertainties, but here I have to say that I'm septic.

2. Oct 1, 2008

### Hootenanny

Staff Emeritus
The speed of light is defined to be that figure, hence there is no uncertainty.

3. Oct 1, 2008

### phlegmy

yup thats what c is defined as. full stop,,, or:

i appreciate the notion of uncertainty around c but any uncertainty would actually manifest itself in the length of a meter, to explain:

light travels at 299792458 meters every second
this is what has been decided and will not change.

the meter is defined as how far light travels in 1/299792458ths of a second
to further explain:

lets say the guys measuring the speed of light were all drunk and messed up the experiment horribly!! and in actual fact light travels twice as fast as they had measured, what are the implications?

well, funnily enough, the speed of light would remain at 299792458m/s but the meter would have doubled in length.

you could say that instead of the meter doubling in length that the duration of 1 second is halved, but this wouldnt work as the second is linked to radiation from the caesium 133 atom.

its a good one alright

4. Oct 1, 2008

### fluidistic

I'm not sure I understand well. So it has been defined, but light has a real speed which certainly isn't the one defined. So has it been defined (so without uncertainty) in order to make more precise the length of a meter for example? (and I guess many other physics constants are by this way more precise). If yes, what's the meaning of what phlegmy said : measuring the speed of light? If it is already known (and perfectly known...), what's the point of measuring the speed of light?

5. Oct 1, 2008

### Staff: Mentor

Yes.
It wasn't known until it was measured. But at some point, our ability to measure it exceeded the precision of the definition of a "meter". So it made sense to re-define the meter according to something that was known to be a better constant.

6. Oct 1, 2008

### fluidistic

Ah! Thank you, I understand now, all makes sense.

7. Oct 1, 2008

### Hootenanny

Staff Emeritus
That's the point, because of the way the second and the meter are defined any measurement of the speed of light will result in the defined value (but as phlegmy said the length of the meter will change).

To measure the speed of light one would basically measure the time taken for a pulse of light to travel a given distance. Now, because the second is defined by hyperfine atomic transitions (independent of the speed of light) and the meter is defined as the distance a pulse of light travels in 1/c seconds (dependent on the speed of light) when one calculates that speed of light one will always obtain the defined value.

Does that make sense?

Edit: Too slow.

8. Oct 1, 2008

### Type_R

When people talk in this world.....we often use words we shouldn't use because there's a HIGH probability of it being so.

Like Proven
Certain
etc etc.
OF course we can never be 100% certain about a lot of physical things.
But we're certain beyond resonable doubt.....that the speed of light remains constant. It doesn't matter if the light was shot while something was traveling, or standing still. It will still reach its destination at "C"

9. Oct 1, 2008

### HallsofIvy

Staff Emeritus
You're septic? Have you seen a doctor for that condition?

Last edited: Dec 22, 2008
10. Oct 1, 2008

### HallsofIvy

Staff Emeritus
To clarify- it is NOT the "speed of light" that has been "defined"- as you say that is a constant of nature and we cannot just "define" it to be a specific value. But we can define the length of a meter. Whereas the meter was orginally defined to be 1/1000000 (I think that's the right fraction) of the distance from the north pole to the equator, along the longitude of Paris, France, and later defined to be the length of a particular bar of metal kept in store room in Paris (the one kept in Washington, D.C. as the basis for distance units in the U.S.- yes, even the "foot", "yard" and "mile"- was copied from the one in Paris), more recently, the meter was redefined to be the specific length that makes that particular value for the speed of light correct in m/s. The length of the meter is NOT a "constant of nature", it is our creation and we are free to alter it as we please.

The speed of light as given, in m/s is exactly correct because the meter is defined so as to make it exactly correct.

11. Oct 1, 2008

### fluidistic

Sorry, I don't know why I thought this word was used to say that I was doubtful. Even in my mother tongue (the word is septique) I thought it meant doubtful, but it doesn't. How strange! Thanks for pointing that out, I will remember it.
Thanks for the input, now I realize we had to define the speed of light.

Last edited: Oct 1, 2008
12. Oct 1, 2008

### thebluelagoon

I think you meant "sceptic" - that word means doubtful. Septic means... like, nasty. Ill. Rotten.