Among Art Hobson’s and Gregory Derry’s letters and Sean Carroll’s July Quick Study (page 62), only Derry’s letter addresses the point I was trying to make in my June Quick Study (page 62): Viewing probabilities as personal judgments eliminates the quantum measurement problem and enables one to make better sense of quantum mechanics.

Hobson’s letter expounds his own realistic view of quantum states and their collapse. It belongs with the three examples I mention that eliminate the physicist from the story.

Carroll takes what I write about the consequences of a personalist interpretation of probability to be an example of an epistemic interpretation of quantum mechanics. That misses my point.

In 1926 Max Born noted that the content of quantum states was the probabilities that they enabled one to calculate. It is strange that after thus elevating probability to a new and foundational role, no physicists then or for the next three-quarters of a century thought it important to reexamine the meaning of probability. For most physicists, probabilities are user-independent frequencies, but for most statisticians, they are guides to action by the person who made the probability assignment. If physicists in 1926 had held a personalist view of probability, it would have required them from the very beginning to hold a personalist (“epistemic”) view of quantum states. There would have been no need for an “interpretation.”

I have comments on several issues raised by Derry. Max Jammer and many others have indeed written for over half a century that quantum states are nothing more than formal devices for encapsulating probabilities of observation. But nobody before Carlton Caves, Christopher Fuchs, and Rüdiger Schack ever added that if probabilities are viewed as personal judgments of the person who assigns them, then that same view must be taken of quantum states.

Derry quotes Niels Bohr’s statement that he does not “appeal to the observing subject.” Later in that paragraph, Bohr adds that “all subjectivity is avoided by proper attention to the circumstances required for the well-defined use of elementary physical concepts.”1 That does contradict my reading of the two Bohr quotations that appear in my Quick Study. By “experience,” Bohr must have meant collective rather than individual experience. I doubt that Bohr took a personalist view of probability. That Bohr, however, was a personalist is argued interestingly by Ulrich Mohrhoff.2 

I quote Bruno de Finetti on “Fairies and Witches” only to give a poetic statement of the unfamiliar view of probability that I am inviting physicists to examine. My point is that if de Finetti is correct, then it would profoundly affect our understanding of quantum mechanics. For me, the illumination it sheds on the interpretation of quantum mechanics is all by itself a compelling reason for adopting a personalist view of probability.

The expansion of my argument that Derry looks forward to reading can be found in the article of mine3 cited in my Quick Study along with the rather more technical article4 by Fuchs and Schack that inspired mine.

1.
N.
Bohr
,
Essays 1958–1962 on Atomic Physics and Human Knowledge
,
Ox Bow Press
(
1987
), p.
7
.
3.
N. D.
Mermin
,
Rep. Prog. Phys.
82
,
012002
(
2019
).
4.
C. A.
Fuchs
,
R.
Schack
,
Rev. Mod. Phys.
85
,
1693
(
2013
).
5.
A.
Hobson
,
Physics Today
75
(
11
),
13
(
2022
).
6.
G. N.
Derry
,
Physics Today
75
(
11
),
12
(
2022
).
7.
S. M.
Carroll
,
Physics Today
75
(
7
),
62
(
2022
).
8.
N. D.
Mermin
,
Physics Today
75
(
6
),
62
(
2022
).