BUG: Missing Constant in Documentation

Describe your issue.

The table in the documentation that lists constants is missing the characteristic impedance of a vacuum. This is because the constant is not listed directly in the CODATA 2018 table (this occured in #10920); however, it has been maintained in scipy since January 2020 (see #11343). Because the code that handles the documentation of constants defines obsolete values as those not in the CODATA 2018 table, the constant isn’t included in the documentation. I’m a little out of my depth of expertise here, but I’m not sure that the characteristic impedance of a vacuum should be counted as an obsolete constant. I’m not really sure why it’s not included in the CODATA 2018 table, despite the fact that it is still listed on NIST’s website as a fundamental constant. Is it fair to say that this should not be included as an obsolete constant? I’d love to hear from anybody who knows more about this subject and its nuances, thanks!

Reproducing Code Example

>>> from scipy import constants
>>> constants.physical_constants['characteristic impedance of vacuum']
(376.73031366686166, 'ohm', 5.61366546036269e-08)
>>> 'characteristic impedance of vacuum' in constants._obsolete_constants
True

Error message

N/A

SciPy/NumPy/Python version information

1.7.3 1.21.5 sys.version_info(major=3, minor=7, micro=11, releaselevel=‘final’, serial=0)

[jakobjakobson13]

Given that the redefinition using the exact_values approach was added as a stopgap, should that now be removed as well?

You are right.

[jakobjakobson13]

In the two articles (https://physics.nist.gov/cuu/pdf/JPCRD2018CODATA.pdf, https://physics.nist.gov/cuu/pdf/RevModPhys.93.025010.pdf) listed by NIST on the 2018 adjustment the characteristic impedance of a vacuum is listed as well.