Uncertainty principle
Heisenberg first outlines his uncertainty relation to Wolfgang Pauli
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Original prompt & settings (JSON) ↗The uncertainty principle first reached the world as a 14-page private letter to Wolfgang Pauli, not as the famous March paper.
23 February 1927: fourteen pages to Pauli
While working in Copenhagen in February 1927, Werner Heisenberg sent Wolfgang Pauli a 14-page letter outlining the new uncertainty relation and his gamma-ray-microscope reasoning. The letter is dated 23 February.
Heisenberg argued that making an electron's position increasingly definite necessarily made its momentum increasingly indefinite, and vice versa.
The imaginary microscope
The gamma-ray-microscope thought experiment connected the attainable spatial resolution with an unavoidable uncertainty in the electron's momentum — gamma rays precisely because their short wavelength promised sharp vision.
Heisenberg had developed the argument while Niels Bohr was away; Bohr returned and challenged part of its optical reasoning before publication.
Which formulation is which
The February letter became the basis of Heisenberg's paper “Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik,” received by the journal on 23 March 1927 — so accounts that treat March as the event are describing a different milestone.
Heisenberg's original relation was approximate. The familiar exact standard-deviation bound of ℏ/2 is Earle Hesse Kennard's later 1927 formulation, and modern treatments distinguish intrinsic statistical spread in a quantum state from measurement error and disturbance, two ideas the popular statement mixes together.
Sources
Researched 23 Aug 2026 6 sources 3 audit passes Date corrected
How this was checked
Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →
Date corrected. The research places this at 1927-02-23.
What the sources leave uncertain
- February is the formulation milestone, independently supported by Heisenberg's dated letter to Pauli. The formal journal article was received on 23 March 1927, so accounts that call March the publication event are describing a different milestone.
- The precise modern inequality using standard deviations and the bound ℏ/2 is Kennard's later 1927 formulation, not exactly the approximate relation as Heisenberg first wrote it.