einstein

the electrons are recorded as they hit a photographic plate. Various other elements, such as a shutter to open and close the slit instantaneously,
were posited by Einstein in his ingenious efforts to show that position and momentum could in theory be known with precision.
“Einstein would bring along to breakfast a proposal of this kind,” Heisenberg recalled. He did not worry much about Einstein’s machinations, nor
did Pauli. “It will be all right,” they kept saying, “it will be all right.” But Bohr would often get worked up into a muttering frenzy.
The group would usually make their way to the Congress hall together, working on ways to refute Einstein’s problem. “By dinner-time we could
usually prove that his thought experiments did not contradict uncertainty relations,” Heisenberg recalled, and Einstein would concede defeat. “But
next morning he would bring along to breakfast a new thought experiment, generally more complicated than the previous one.” By dinnertime that
would be disproved as well.
Back and forth they went, each lob from Einstein volleyed back by Bohr, who was able to show how the uncertainty principle, in each instance, did
indeed limit the amount of knowable information about a moving electron. “And so it went for several days,” said Heisenberg. “In the end, we—that
is, Bohr, Pauli, and I—knew that we could now be sure of our ground.” 30
“Einstein, I’m ashamed of you,” Ehrenfest scolded. He was upset that Einstein was displaying the same stubbornness toward quantum
mechanics that conservative physicists had once shown toward relativity. “He now behaves toward Bohr exactly as the champions of absolute
simultaneity had behaved toward him.” 31
Einstein’s own remarks, given on the last day of the conference, show that the uncertainty principle was not the only aspect of quantum
mechanics that concerned him. He was also bothered—and later would become even more so—by the way quantum mechanics seemed to permit
action at a distance. In other words, something that happened to one object could, according to the Copenhagen interpretation, instantly determine
how an object located somewhere else would be observed. Particles separated in space are, according to relativity theory, independent. If an
action involving one can immediately affect another some distance away, Einstein noted, “in my opinion it contradicts the relativity postulate.” No
force, including gravity, can propagate faster than the speed of light, he insisted. 32
Einstein may have lost the debates, but he was still the star of the event. De Broglie had been looking forward to meeting him for the first time,
and he was not disappointed. “I was particularly struck by his mild and thoughtful expression, by his general kindness, by his simplicity and by his
friendliness,” he recalled.
The two hit it off well, because de Broglie was trying, like Einstein, to see if there were ways that the causality and certainty of classical physics
could be saved. He had been working on what he called “the theory of the double solution,” which he hoped would provide a classical basis for
wave mechanics.
“The indeterminist school, whose adherents were mainly young and intransigent, met my theory with cold disapproval,” de Broglie recalled.
Einstein, on the other hand, appreciated de Broglie’s efforts, and he rode the train with him to Paris on his way back to Berlin.
At the Gare du Nord they had a farewell talk on the platform. Einstein told de Broglie that all scientific theories, leaving aside their mathematical
expressions, ought to lend themselves to so simple a description “that even a child could understand them.” And what could be less simple,
Einstein continued, than the purely statistical interpretation of wave mechanics! “Carry on,” he told de Broglie as they parted at the station. “You are
on the right track!”
But he wasn’t. By 1928, a consensus had formed that quantum mechanics was correct, and de Broglie relented and adopted that view. “Einstein,
however, stuck to his guns and continued to insist that the purely statistical interpretation of wave mechanics could not possibly be complete,” de
Broglie recalled, with some reverence, years later. 33
Indeed, Einstein remained the stubborn contrarian. “I admire to the highest degree the achievements of the younger generation of physicists that
goes by the name quantum mechanics, and I believe in the deep level of truth of that theory,” he said in 1929 when accepting the Planck medal from
Planck himself. “But”—and there was always a but in any statement of support Einstein gave to quantum theory—“I believe that the restriction to
statistical laws will be a passing one.” 34
The stage was thus set for an even more dramatic Solvay showdown between Einstein and Bohr, this one at the conference of October 1930.
Theoretical physics has rarely seen such an interesting engagement.
This time, in his effort to stump the Bohr-Heisenberg group and restore certainty to mechanics, Einstein devised a more clever thought
experiment. One aspect of the uncertainty principle, previously mentioned, is that there is a trade-off between measuring precisely the momentum
of a particle and its position. In addition, the principle says that a similar uncertainty is inherent in measuring the energy involved in a process and
the time duration of that process.
Einstein’s thought experiment involved a box with a shutter that could open and shut so rapidly that it would allow only one photon to escape at a
time. The shutter is controlled by a precise clock. The box is weighed exactly. Then, at a certain specified moment, the shutter opens and a photon
escapes. The box is now weighed again. The relationship between energy and mass (remember, E=mc 2 ) permitted a precise determination of the
energy of the particle. And we know, from the clock, its exact time of departing the system. So there!
Of course, physical limitations would make it impossible to actually do such an experiment. But in theory, did it refute the uncertainty principle?
Bohr was shaken by the challenge. “He walked from one person to another, trying to persuade them all that this could not be true, that it would
mean the end of physics if Einstein was right,” a participant recorded. “But he could think of no refutation. I will never forget the sight of the two
opponents leaving the university club. Einstein, a majestic figure, walking calmly with a faint ironic smile, and Bohr trotting along by his side,
extremely upset.” 35 (See picture, page 336.)
It was one of the great ironies of scientific debate that, after a sleepless night, Bohr was able to hoist Einstein by his own petard. The thought
experiment had not taken into account Einstein’s own beautiful discovery, the theory of relativity. According to that theory, clocks in stronger
gravitational fields run more slowly than those in weaker gravity. Einstein forgot this, but Bohr remembered. During the release of the photon, the
mass of the box decreases. Because the box is on a spring scale (in order to be weighed), the box will rise a small amount in the earth’s gravity.
That small amount is precisely the amount needed to restore the energy-time uncertainty relation.
“It was essential to take into account the relationship between the rate of a clock and its position in a gravitational field,” Bohr recalled. He gave
Einstein credit for graciously helping to perform the calculations that, in the end, won the day for the uncertainty principle. But Einstein was never
fully convinced. Even a year later, he was still churning out variations of such thought experiments. 36
Quantum mechanics ended up proving to be a successful theory, and Einstein subsequently edged into what could be called his own version of
uncertainty. He no longer denounced quantum mechanics as incorrect, only as incomplete. In 1931, he nominated Heisenberg and Schrödinger for
the Nobel Prize. (They won in 1932 and 1933, along with Dirac.) “I am convinced that this theory undoubtedly contains a part of the ultimate truth,”
Einstein wrote in his nominating letter.
Part of the ultimate truth. There was still, Einstein felt, more to reality than was accounted for in the Copenhagen interpretation of quantum
mechanics.