A Revolution in Physics
157
an explicit, well-formed version of external events. Instead, the observer has
the burden of giving a fixed shape to the information he or she receives. As
explained earlier, without an observer nothing actual or specific is happening.
That is the underlying theme of this book.
In fundamental physics this idea has been present, in a latent form, for
over a century. The great physicist Niels Bohr wrote in 1929 that the purpose
of science is not to uncover “the real essence of phenomena” but to disclose
“relations between the many aspects of our experience”. There is no such thing
as the “real essence” of physical events, because events can only be described
or visualized by living observers. That is, events assume a definite form only in
living experience.
An elementary particle such as an electron does not have a given position,
mass or speed if it is not observed. Instead it exists in the form of what is called
a wave function which reveals the probability of obtaining certain values when
it is measured. Yet if it is directly observed, the wave function collapses and the
particle is in a fixed state, with a specific position and velocity at each instant.
This simple fact makes it evident that at the subatomic level, the observer
participates in the phenomenon under observation. It is not clear how this
can be, and quantum theory has been stumped for a hundred years by this
quandary.
It is only in the last decade that a reasonable answer has been proposed: The
underlying idea is that the wave function’s probabilities do not stand for the
likelihood that a certain material fact is true in the physical world. Instead, it
represents the experimenter’s degree of belief that the fact is true. This approach
is called quantum Bayesianism, which is usually shortened to QBism. In order
to explain this position, I must say a few words about the Bayesian concept of
probability.
Let the letter H stand for some possible fact in the real world. Then P(H )
stands for the probability that the proposed fact H is true. If D is some other
fact, then P(H |D) is the probability of H being true, assuming that D is
true. Likewise, P(D) is the probability that D is true, and P(D|H ) is the
probability that D is true assuming H is true. Mathematically, Bayes’ Rule is
given by the formula
P(H |D) = P(D|H )P(H )/P(D)
The formula is irrelevant here: What needs to be retained is that these four
quantities are related, so that any one of them can be obtained if we know the
other three.
In applications of this formula, H stands for our hypothesis: A hypothesis is
a fact that we believe to be true at the start. You may also think of a hypothesis
157
an explicit, well-formed version of external events. Instead, the observer has
the burden of giving a fixed shape to the information he or she receives. As
explained earlier, without an observer nothing actual or specific is happening.
That is the underlying theme of this book.
In fundamental physics this idea has been present, in a latent form, for
over a century. The great physicist Niels Bohr wrote in 1929 that the purpose
of science is not to uncover “the real essence of phenomena” but to disclose
“relations between the many aspects of our experience”. There is no such thing
as the “real essence” of physical events, because events can only be described
or visualized by living observers. That is, events assume a definite form only in
living experience.
An elementary particle such as an electron does not have a given position,
mass or speed if it is not observed. Instead it exists in the form of what is called
a wave function which reveals the probability of obtaining certain values when
it is measured. Yet if it is directly observed, the wave function collapses and the
particle is in a fixed state, with a specific position and velocity at each instant.
This simple fact makes it evident that at the subatomic level, the observer
participates in the phenomenon under observation. It is not clear how this
can be, and quantum theory has been stumped for a hundred years by this
quandary.
It is only in the last decade that a reasonable answer has been proposed: The
underlying idea is that the wave function’s probabilities do not stand for the
likelihood that a certain material fact is true in the physical world. Instead, it
represents the experimenter’s degree of belief that the fact is true. This approach
is called quantum Bayesianism, which is usually shortened to QBism. In order
to explain this position, I must say a few words about the Bayesian concept of
probability.
Let the letter H stand for some possible fact in the real world. Then P(H )
stands for the probability that the proposed fact H is true. If D is some other
fact, then P(H |D) is the probability of H being true, assuming that D is
true. Likewise, P(D) is the probability that D is true, and P(D|H ) is the
probability that D is true assuming H is true. Mathematically, Bayes’ Rule is
given by the formula
P(H |D) = P(D|H )P(H )/P(D)
The formula is irrelevant here: What needs to be retained is that these four
quantities are related, so that any one of them can be obtained if we know the
other three.
In applications of this formula, H stands for our hypothesis: A hypothesis is
a fact that we believe to be true at the start. You may also think of a hypothesis