What Is Real?: The Unfinished Quest for the Meaning of Quantum Physics

by Adam Becker

made Wheeler a natural choice to be Everett’s advisor.

interested in the measurement problem.

Everett became friends with Petersen, and, through him, he gained access to Bohr.

Wheeler “was preaching this idea that you ought to just look at the equations and obey the fundamentals of physics while you follow their conclusions and give them a serious hearing,” according to Misner.

What he found was far more astonishing than anything in his beloved science-fiction stories.

We met the measurement problem back in Chapter 1. The problem, in a nutshell, is this: Quantum wave functions move along nice and smoothly, always obeying one simple and deterministic law, the Schrödinger equation—except when they don’t.

what constitutes a “measurement” anyway—

central puzzle of quantum physics.

von Neumann’s approach doesn’t even tell you what measurements are.

Everett argued that this line of reasoning leads inevitably to solipsism—

possible way out of this dilemma—but at the cost of a unified picture of the world free of contradiction, a price Everett was (justifiably) unwilling to pay.

“The time has come… to treat [quantum physics] in its own right as a fundamental theory without any dependence on classical physics, and to derive classical physics from it.”

Rejecting both von Neumann and Bohr, Everett came up with his own solution to the measurement problem.

he insisted that a single universal wave function was all there was: a massive mathematical object describing the quantum states of all objects in the entire universe.

never collapsing, but splitting instead.

“many-worlds” interpretation of quantum physics.

let’s take a second look at a simple quantum experiment, even simpler than the double slit: Schrödinger’s cat.

means they get entangled.

wave functions keep getting entangled:

measurement causes the wave function to collapse.

you get entangled with the shared wave function of the box and everything in it.

So when you perform this experiment, Everett said, both outcomes occur—and you split in two.

Schrödinger equation dictates that each branch will carry on independently of the others, with hardly any interaction between branches.

Eventually, we have a single complicated and messy wave function for the entire universe—the universal wave function.

each person in each branch of the universal wave function, their world appears to be the only world,

This is a hallmark of the many-worlds interpretation: the appearance of a single world, despite the true existence of many.

finished a draft of his thesis in January 1956, Wheeler was the first to see it.

In discussions of this paradox with graduate students and staff members here at Princeton, and with Niels Bohr, Everett brought to light new features of the problem that make it in and of itself an appropriate subject for an outstanding thesis when further developed.… [Everett] really is an original man.” But Wheeler was caught between several competing interests.

quantum cosmology;

Wheeler idolized Bohr,

Wheeler was a political animal;

in exactly the way that Einstein could not.

Wheeler was also unwilling to give up Everett’s theory of a universal wave function—

title itself… like so many of the ideas in it, need further analysis and rephrasing.”

Everett after his Copenhagen visit, Wheeler at first sounded hopeful and clear-eyed about the work yet to be done.

real issue is the words that are to be attached to the quantities of the formalism.”

Bohr] would welcome very much a several weeks’ visit from you to thrash this out.… Unless and until you have fought out the issues of interpretation one by one with Bohr, I won’t feel happy about the conclusions to be drawn from a piece of work as far reaching as yours. Please go (and see me too each way if you can!). So

Bohr, Petersen, and others in Copenhagen were less enthusiastic about Everett’s ideas than Wheeler thought.

lack of an adequate understanding of the measuring process.

Ultimately, he dismissed Everett’s ideas as either “a matter of theology” or “metaphysics,” since the extra worlds postulated by Everett could never be seen or perceived directly in any way.

imprimatur,

Wheeler

vigorously defending the universal wave function—

have vigorously supported and expect to support in the future the current and inescapable approach to the measurement problem.

Moreover, I think I may say that this very fine and able and independently thinking young man has gradually come to accept the present approach to the measurement problem as correct and self consistent,

Wheeler followed up with another letter to Everett,

“Your thesis must receive heavy revision of words and discussion, very little of mathematics, before I can rightfully take the responsibility to recommend it for acceptance.

humanly impossible to come to agreement on all issues unless you and I are in the same place for several weeks,

Copenhagen immediately, Everett didn’t go.

“I believe that a number of misunderstandings will evaporate when it has been read more carefully