Precision

From Daniel Holz's post describing the Laser Interferometer Gravitational Wave Observatory, a tool built to detect gravity waves:

There is lots to say about LIGO, but I’d like to focus on one point: the scale on the y-axis of the plot above (which represents strain, the fractional change in the length of the LIGO arms). LIGO is sensitive, over a wide range of frequency, to a strain of better than 1 part in 10²². In other words, it measures changes in the relative length of its 4km arms to better than a thousandth of the size of a proton. This plot should absolutely blow your mind. If not, perhaps I’m being too abstract? This is the equivalent of monitoring changes in the distance between New York and San Francisco to better than one ten billionth the width of a human hair.

Yeah, I'd say that blows my mind.

Here is a picture of one of LIGO's arms, or legs, as the case may be, from its entry page on Wikipedia, and you can see an aerial shot on the LIGO home page. And there's a similar set-up in Europe, called Virgo.