# [Python-ideas] Why not picoseconds?

Stephan Houben stephanh42 at gmail.com
Mon Oct 16 03:46:00 EDT 2017

```Hi all,

I realize this is a bit of a pet peeve of me, since
in my day job I sometimes get people complaining that
numerical data is "off" in the sixteenth significant digit
(i.e. it was stored as a double).

I have a bunch of favorite comparisons to make this clear
how accurate a "double" really is: you can measure the
distance to Neptune down to a millimeter with it, or
the time from the extinction of the dinosaurs until
now down to half-second resolution.

Unfortunately for my argument, measuring time is one
of the most accurate physical measurements we can make,
and the best of the best exceed double-precision accuracy.

For example, the best atomic clock
achieves a measurement uncertainty of 3e-18, which is about two order of
magnitudes more accurate than what double-precision gives you;
the latter runs out of steam at 2.2e-16.

So I realize there is obvious a strong need for (the illusion of)
such precise time keeping in the Python API <wink>.

Interestingly, that 2.2e-16 pretty much aligns with the accuracy of the
cesium atomic clocks which are currently used to *define* the second.
So we move to this new API, we should provide our own definition
of the second, since those rough SI seconds are just too imprecise for that.

Stephan

2017-10-16 9:03 GMT+02:00 Stefan Krah <stefan at bytereef.org>:

> On Mon, Oct 16, 2017 at 08:40:33AM +0200, Stephan Houben wrote:
> > "The problem is that Python returns time as a floatting point number
> > > which is usually a 64-bit binary floatting number (in the IEEE 754
> > > format). This type starts to loose nanoseconds after 104 days."
> > >
> > >
> > Do we realize that at this level of accuracy, relativistic time
> dilatation
> > due
> > to continental drift starts to matter?
>
> tai64na has supported attoseconds for quite some time:
>
>     https://cr.yp.to/libtai/tai64.html
>
> The relativity issue is declared to be out of scope for the document. :-)
>
>
> Stefan Krah
>
>
>
>
>
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