*This question was originally answered on Quora by Jack Fraser, An Under Grad Physicist at Oxford University.*

How do scientists measure the speed of light? We don’t. No, seriously, we don’t measure the speed of light (which *always* refers to the speed *in a vacuum*). We know *exactly *what the speed of light is.

It is: c=299792458*ms*^{−1}

And that is absolutely 100% accurate, with no measurement errors. But Jack, I hear you say, *what are you talking about? *The reason we know that that’s *exactly* the speed of light is that ** we defined it to be that number. **We then take our definition of a second (the length of time for a certain number of periods of the radiation emitted in hyperfine transitions in caesium-133), and from that,

**we define a meter.**So the thing we would be measuring

*is what a meter is.*

We use the speed of light as a fixed velocity, from which all observers can define their own length scale. To *measure* the speed of light would require an external definition of what a meter is, and since about the 1970s, we don’t have one.

If you *did* want to measure the speed of light using this external distance reference, it’s easy to test; you just release a light pulse at t=0 towards a mirror and then time how long it takes to get back to you. This is the exact principle that radar/sonar work on (although again, they measure the *distance* knowing the *speed—*but it works either way around).

**Some background:**

The meter was originally defined after the French Revolution, in about 1799. It was defined as 1⁄10,000,000 the distance between the equator and the pole.

The “meter” was formally defined from 1889 as the length of a platinum rod, held in a vault in Paris.

From this definition of a meter (and an old definition of a second), we measured (using the mirror-timing method, or based on astronomical observations) the speed of light to be *about* 299792458, plus a non-integer bit, and error bars from the measurement errors.

Eventually, we realized that having a meter defined by *something there was only one of *was a bit annoying. So, we attempted to define it in a way that *anyone* could replicate without having to refer to a “standard object.” Therefore, we *redefined the meter—*using the speed of light.

The official definition of a meter today is:

1⁄299792458 of the distance travelled by light in a vacuum, in 1 second.

Using the caesium definition of a second.

Therefore, this was* exactly* equivalent to defining the speed of light to be the number given above. We chose that number (and not a more convenient number like 300,000,000), because that number changed the definition of a meter by only a fraction of a fraction of a percent, but made everything all nice and integer-y.

A consequence of using this definition is that **any attempt to measure the speed of light is cyclical; **you *must* use a “meter” to measure it at some point, which relies on the speed of light.

Therefore what you *actually* do now, when you “measure” the speed of light (in a vacuum), is actually “measure how accurate your measuring instruments are.”

*This question was originally answered on Quora by Jack Fraser, An Under Grad Physicist at Oxford University.*