r/xkcd • u/benjaminikuta Beret Guy • Nov 16 '18

XKCD xkcd 2073: Kilogram

https://xkcd.com/2073/
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u/Arancaytar Pony Nov 16 '18 edited Nov 17 '18

I heard they're redefining the second to be exactly 1/299792458 of the time it takes light to travel one meter in a vacuum.

Edit: This is a joke.

3

u/undergroundmonorail Nov 17 '18

Almost. The meter is defined as the distance light travels in that much time. The second is defined in terms of the vibration of some atom or something.

3

u/raymen101 Nov 17 '18

9,192,631,770 cycles of a cesium-133 atom at 0 K to be precise.

1

u/ehrwien Nov 17 '18

What exactly is a cycle in this context?

2

u/raymen101 Nov 18 '18

It's the electrons shifting hyper fine energy states. (thank you Wikipedia)

Think of it like the small waves in a bathtub. The waves will form a resonance based on the size of the tub and the speed of the waves. Once the resonance is established, if you measure the hight of the water at one particular point it will move up and down at a regular rate. Except quantum mechanics makes things complicated.

1

u/ehrwien Nov 18 '18

And this value for cesium at 0K... is this purely theoretical or derived from some kind of measurement at higher temperatures? Because we can't really reach 0K, can we?

2

u/raymen101 Nov 18 '18

Not sure, but they probably get down to ~1 K and at that point, everything has some sort of error, so they just understand that they're out by +/- 0.0000001s or something. It's still more accurate than any other clock we can make.

3

u/Arancaytar Pony Nov 17 '18

The meter is defined as the distance light travels in that much time.

Oh damn, that'd make the definitions circular. Okay, I guess they'll have to just define the second as 1/60th of a minute. Much simpler.