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"surface tension" physical type is registered as J m^2 instead of J / m^2 #20681

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"surface tension" physical type is registered as J m^2 instead of J / m^2#20681
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@rcgiri-physics
rcgiri-physics
opened on Oct 23, 2026

Hi, thanks for maintaining astropy. In astropy/units/physical.py:

(si.W * si.m**2 * si.Hz**-1, "surface tension"),

W m^2 Hz^-1 is J m^2, but surface tension is J / m^2 = N / m. With astropy 8.0.2:

>>> (u.J / u.m**2).physical_type
PhysicalType('spectral flux density')
>>> (u.N / u.m).physical_type
PhysicalType('spectral flux density')
>>> (u.J * u.m**2).physical_type
PhysicalType('surface tension')

J/m^2 has the same dimension as the jansky (kg s^-2). I think the intended fix is to drop this entry and add "surface tension" to the names for the astrophys.Jy dimension, the same way si.J carries {"energy", "work", "torque"}.

I found this while testing physverify, a small verification tool I'm working on, against public codes. The tests and this write-up were done with help from an AI assistant; the numbers come from actually running the code.

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  1. Mrun25 commented on Oct 23, 2026

    @Mrun25
    Mrun25
    on Oct 23, 2026 · Hidden as spam

  2. pmocz commented on Oct 23, 2026

    @pmocz
    pmocz
    on Oct 23, 2026 · Hidden as spam
  3. pllim
    added
    units
    on Oct 23, 2026

  4. pllim commented on Oct 23, 2026

    @pllim
    pllim
    on Oct 23, 2026
    Member

    Thanks for the report. Can you please check the software behavior also in

    1. https://pypi.org/project/astropy/7.2.3/
    2. dev version via https://docs.astropy.org/en/latest/install.html#install-pre-built-development-version

    and then report back? Thanks!

  5. rcgiri-physics commented on Oct 23, 2026

    @rcgiri-physics
    rcgiri-physics
    on Oct 23, 2026
    Author

    Thanks @pllim. I checked both, and the behaviour is the same as in 8.0.2:

    7.2.3
J / m2 -> PhysicalType('spectral flux density')
N / m -> PhysicalType('spectral flux density')
m2 J -> PhysicalType('surface tension')

8.1.0.dev835+gce87eb782
J / m2 -> PhysicalType('spectral flux density')
N / m -> PhysicalType('spectral flux density')
m2 J -> PhysicalType('surface tension')

  6. pllim commented on Oct 28, 2026

    @pllim
    pllim
    on Oct 28, 2026
    Member

    Thanks, @rcgiri-physics . I agree with your assessment. Looks like it was added by @namurphy in #11204 , probably ported the definition from PlasmaPy community. Perhaps he could clarify?

  7. pllim
    added
    Bug
    on Oct 28, 2026

  8. namurphy commented on Oct 28, 2026

    @namurphy
    namurphy
    on Oct 28, 2026
    Contributor

    Great catch! A PR was opened a few days ago by @pmocz at #20683 and then automatically closed as per the policy for new contributors. Could this PR be re-opened?

  9. pllim commented on Oct 28, 2026

    @pllim
    pllim
    on Oct 28, 2026
    Member

    There were actually 2 PRs but unfortunately both appeared to be AI generated, thus similar looking. If we want to adhere to our own AI policy here, at least the author from #20682 made an attempt to introduce human on Slack. @namurphy , do you want me to re-open #20682 for consideration instead? Thanks!

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