Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Research Highlight
  • Published:

In retrospect

Nobel 1943: magnetic moment of the proton

Nature Reviews Physics volume 5, page 554 (2023)Cite this article

Subjects

According to Paul Dirac, the magnetic moment of an elementary particle should have been directly proportional to its charge and inversely proportional to its mass. In 1922, Stern and Gerlach had measured the magnetic moment of the electron, and shown that it was quantized, together with its associated angular momentum. At the time, the proton was assumed to also be an elementary particle, so theorists thought that probing its magnetic moment was a waste of time. But in 1933, Stern, along with Immanuel Estermann and Robert Frisch decided to do it anyway. According to Estermann, Wolfgang Pauli “would condescendingly come to the lab and shake his head”.

In their experimental setup (pictured), molecular rays were passed through a magnetic field gradient. By measuring the deflection of the ray, the magnetic moment of the molecules could be deduced. Their result was two and a half times larger than Dirac’s prediction, with an estimated precision of 10%. Today, the discrepancy is often described as the first evidence that the proton was not an elementary particle after all, but one made of smaller constituents.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

References

Original article

  • Frisch, R. & Stern, O. Über die magnetische Ablenkung von Wasserstoffmolekülen und das magnetische Moment des Protons. I. Z. Physik 85, 4–16 (1933)

    Article  ADS  Google Scholar 

Related articles

  • Estermann, I. & Stern, O. Über die magnetische Ablenkung von Wasserstoffmolekülen und das magnetische Moment des Protons. II. Z. Physik 85, 17–24 (1933)

    Article  ADS  Google Scholar 

  • Segrè, E. Otto Stern (1888–1969), A Biographical Memoir (National Academy of Sciences, 1973)

Download references

Author information

Authors and Affiliations

  1. Senior Reporter, Nature https://www.nature.com

    Davide Castelvecchi

Authors
  1. Davide Castelvecchi
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Davide Castelvecchi.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Castelvecchi, D. Nobel 1943: magnetic moment of the proton. Nat Rev Phys 5, 554 (2023). https://doi.org/10.1038/s42254-023-00654-4

Download citation

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s42254-023-00654-4

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing