Please explain how we can measure the speed of light

[Lemur866]

@Crafter_Man

Since 1983, the speed of light in a vacuum has been defined as 299792458 m/s. Because it is defined, it cannot be measured.

But that isn’t defining the speed at which light travels in a vacuum so much as defining how long a meter is. The length of a second is defined by the speed of some vibrating particles. Once you know how long a second is, you measure how long it takes light to travel in a second, and define a meter as 1/299792458th of that distance.

[Alex_Dubinsky]

@Musicat

What if someone measures it and obtains a different, but accurate number? Which shall we change – the length of a meter or the second?

Good question. We would change the meter. (The second being defined by oscillations of cesium.)

Edit: whoops. I should refresh my pages…

[Chronos]

Unless you are saying they looked up the wrong number for the speed of light or measured the wavelength wrong, I’d say you can trust it.

Yes, but if you’re going to go down that route, then you might as well just “measure the speed of light” by getting a physics textbook and looking it up in the table of constants on the back cover. That’s not really a measurement, and neither is just looking up the frequency listed on the back of your microwave oven.

[Musicat]

@Crafter_Man

It is now a defined number. Defined means defined. When they defined it, they chose a number that matched closely (thought not exactly) with previously measured speeds.

You have three numbers here: the speed of light, wavelength and frequency. Fix any two, the third is defined as a function.

So you are saying that either the length of the meter and/or the second comes first? If so, I can accept that the speed of light is fixed (=defined) with regards to those values.

Or, the speed of light is fixed and both of the other parameters are dependent upon that, and upon each other.

I don’t care which came first, the chick…er, speed of light, I just want to be clear.

[Crafter_Man]

@Musicat

You have three numbers here: the speed of light, wavelength and frequency. Fix any two, the third is defined as a function.

So you are saying that either the length of the meter and/or the second comes first? If so, I can accept that the speed of light is fixed (=defined) with regards to those values.

Or, the speed of light is fixed and both of the other parameters are dependent upon that, and upon each other.

I don’t care which came first, the chick…er, speed of light, I just want to be clear.

The starting point is the definition of the second. A second is defined as “the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.” The second is independent of everything else. It is simply, “the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.”

Now let’s talk about distance. The meter is defined as the distance travelled by light in a vacuum in (299792458)[sup]-1[/sup] of a second. Regardless of the speed of light, the meter is defined as the distance travelled by light in a vacuum in (299792458)[sup]-1[/sup] of a second. When you see a light wave coming, “simply” mark off the distance it travels during 30.6633189884984 periods of cesium 133 radiation. (And yes, I do see a bit of a problem here. Not sure how to resolve or measure the fractional period. Perhaps this is a source of error.)

Now let’s talk about the speed of light. According to the definition of the meter, light travels 1 meter in (299792458)[sup]-1[/sup] seconds. Therefore, light travels 299792458 meters in 1 second.