Speed of light (from

I’m pretty sure that the meter has been altered recently so that the speed of light is exactly 300,000,000 meters per second. I remember reading this in a text at BU 2-3 years ago. Not to be picky, just FYI.


The staff report to which the OP refers, maybe be found here, What’s the origin of the meter and the metric system? Dated: 14-Mar-2001. Follw this link for additional discussion of this topic here at the Straight Dope.

No it hasn’t. The closest thing to it that is true is that the meter is now defined in terms of the speed of light. But the number chosen was picked to match the existing length.

The speed of light is easy to remember: 299,792,458 m/sec

My ingenious astronomy student remembers an easy light mnemonic.

That’s odd. At one point I also had the idea that the meter had been redefined to make c exactly equal to 3e8 m/sec. But I did a web search and found nothing to confirm this, so I figured my brain had hiccupped. Maybe this was suggested at the time and quickly rejected.

It would have been nice if it had been defined in that way to begin with, but with the current definition established, it’d be pretty much impossible to redefine it that way. If they did, then almost every physical constant tabulated in any textbook or reference anywhere would be wrong in the third or fourth decimal place, which would be a terrible headache. Planck’s Constant is 1.0545726610[sup]-34[/sup] kgm[sup]2[/sup]s[sup]-1[/sup], you say? Is that old meters, or new meters? In new meters, that’d be 1.0531140439210[sup]-34[/sup] kg*m[sup]2[/sup]*s[sup]-1[/sup], which may be a very significant difference in some cases.

Of course, for most practical purposes, it’s safe to say that c = 3*10[sup]8[/sup] m/s , and it’s a lot easier to remember than the exact value.

… to say nothing of the need to adjust the measurements of things like length of soccer field, width of carpeting, length of lumber, etc etc etc.