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Comment by japanuspus | Hacker News Reader

As an atomic physicist, it is great to see clever atomic physics experiments opening up new fields in high-energy physics. The team has been working incredibly hard for this: Jeff Hangst who leads the experiment was already working on this when he taught my undergrad electrodynamics course back in 1998!

An important piece of background info for non-atomic-physicists is that the Hydrogen 1S-2S transition frequency is probably the most accurately measured physical property in the world, thanks to the pioneering work by T. Hansch. Back in 2011 they reported an uncertainty of 4e-12, https://arxiv.org/abs/1107.3101.

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Awesome. I share your admiration for such long quests and it reminds me of a similar quest to reach Bose Einstein Condensation of atomic hydrogen, the basis for the mentioned hires spectroscopy. See: https://cds.cern.ch/record/375046/files/9812038.pdf

I thought the charge on an electron had been measured out to 10^-19? One of my professors posted the journal article from the NIST team that did so.

Here's a reference: https://physics.nist.gov/cgi-bin/cuu/Value?e

Your reference gives an uncertainty of 6.1 x 10-9

The abstract says 4e-15, not 4-12?

So does the paper. The paper also says this, right before explaining that it's a component of the ~4e-15 number, though:

>A conservative estimate of the height difference between clock and experiment contributes a gravitational red shift uncertainty of 7.7 × 10−17.

So, it appears the 4e-15 is not an actual measured value, but a synthetic value based on estimation and measurement.

Yeah that's right, the red shift is just one contribution to the total uncertainty.

They're all listed in table I and the 4.2 * 10^-15 just comes from adding them in quadrature.

When physicists say "estimate" in this context, they mean "take into account this fourth-order tiny effect by using an approximate theory of said effect with accuracy of a few percent, and then use 1.2 meters when it's really 1.0 so you overestimate by 10%".

It's like when you take a caliper and measure the width of an object, you typically neglect the error due to thermal expansion of the caliper since it's hotter/colder than when it was marked with gradations. If you're really anal about uncertainty (like these guys), you'd estimate the effect of this expansion and say "a conservative estimate says this contributes an uncertainty of 0.0003 mm".

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