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Wikipedia says:

Robert Oppenheimer argued strongly against the proton being the negative-energy electron solution to Dirac's equation. He asserted that if it were, the hydrogen atom would rapidly self-destruct.[5] Persuaded by Oppenheimer's argument, Dirac published a paper in 1931 that predicted the existence of an as-yet-unobserved particle that he called an "anti-electron" that would have the same mass and the opposite charge as an electron and that would mutually annihilate upon contact with an electron.[6]

On the other hand, this paper by Kragh says:

In his paper of 1931 Dirac also predicted the antielectron as a new particle soon to be known as the positron. The previous year, at a time when he still believed that the antielectron was a proton, he proposed proton-electron annihilation (𝑝+ 𝑒− → 2𝛾) with the sole argument, “There appears to be no reason why such processes should not actually occur somewhere in the world. They would be consistent with all the general laws of Nature” (Dirac 1930).

On the surface, these appear to be contradictory. Can anyone explain what's going on? Did Dirac originally imagine that the rate of annihilation would just be very low for some reason?

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    $\begingroup$ Why do you think there's any sort of contradiction? Dirac proposed something, Oppenheimer made a convincing objection so then Dirac modified his hypothesis. $\endgroup$ – Spencer Jan 13 at 23:26
  • $\begingroup$ The "time when he still believed" was "previous year" from 1931, i.e. 1930. So their accounts are complementary. Oppenheimer gave him a "reason why such processes should not actually occur somewhere in the world", observed stability of hydrogen. And, according to Kragh, he gave a "sole argument" so we have no idea why he did not think of that earlier. $\endgroup$ – Conifold Jan 14 at 1:17

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