einstein

Wolfgang Pauli wrote Heisenberg a long and angry letter.“Einstein has once again expressed himself publicly on quantum mechanics (together
with Podolsky and Rosen—no good company, by the way),” he fumed. “As is well known, every time that happens it is a catastrophe.” 9
When the EPR paper reached Niels Bohr in Copenhagen, he realized that he had once again been cast in the role, which he played so well at
the Solvay Conferences, of defending quantum mechanics from yet another Einstein assault. “This onslaught came down on us as a bolt from the
blue,” a colleague of Bohr’s reported. “Its effect on Bohr was remarkable.” He had often reacted to such situations by wandering around and
muttering, “Einstein . . . Einstein . . . Einstein!” This time he added some collaborative doggerel as well: “Podolsky, Opodolsky, Iopodolsky,
Siopodolsky . . .” 10
“Everything else was abandoned,” Bohr’s colleague recalled. “We had to clear up such a misunderstanding at once.”Even with such intensity, it
took Bohr more than six weeks of fretting, writing, revising, dictating, and talking aloud before he finally sent off his response to EPR.
It was longer than the original paper. In it Bohr backed away somewhat from what had been an aspect of the uncertainty principle: that the
mechanical disturbance caused by the act of observation was a cause of the uncertainty. He admitted that in Einstein’s thought experiment, “there
is no question of a mechanical disturbance of the system under investigation.” 11
This was an important admission. Until then, the disturbance caused by a measurement had been part of Bohr’s physical explanation of quantum
uncertainty. At the Solvay Conferences, he had rebutted Einstein’s ingenious thought experiments by showing that the simultaneous knowledge of,
say, position and momentum was impossible at least in part because determining one attribute caused a disturbance that made it impossible to
then measure the other attribute precisely.
However, using his concept of complementarity, Bohr added a significant caveat. He pointed out that the two particles were part of one whole
phenomenon. Because they have interacted, the two particles are therefore “entangled.” They are part of one whole phenomenon or one whole
system that has one quantum function.
In addition, the EPR paper did not, as Bohr noted, truly dispel the uncertainty principle, which says that it is not possible to know both the precise
position and momentum of a particle at the same moment. Einstein is correct, that if we measure the position of particle A, we can indeed know
the position of its distant twin B. Likewise, if we measure the momentum of A, we can know the momentum of B. However, even if we can
imagine measuring the position and then the momentum of particle A, and thus ascribe a “reality” to those attributes in particle B, we cannot in fact
measure both these attributes precisely at any one time for particle A, and thus we cannot know them both precisely for particle B. Brian Greene,
discussing Bohr’s response, has put it simply: “If you don’t have both of these attributes of the right-moving particle in hand, you don’t have them for
the left-moving particle either. Thus there is no conflict with the uncertainty principle.” 12
Einstein continued to insist, however, that he had pinpointed an important example of the incompleteness of quantum mechanics by showing how
it violated the principle of separability, which holds that two systems that are spatially separated have an independent existence. It likewise violated
the related principle of locality, which says that an action on one of these systems cannot immediately affect the other. As an adherent of field
theory, which defines reality using a spacetime continuum, Einstein believed that separability was a fundamental feature of nature. And as a
defender of his own theory of relativity, which rid Newton’s cosmos of spooky action at a distance and decreed instead that such actions obey the
speed limit of light, he believed in locality as well. 13
Schrödinger’s Cat
Despite his success as a quantum pioneer, Erwin Schrödinger was among those rooting for Einstein to succeed in deflating the Copenhagen
consensus. Their alliance had been forged at the Solvay Conferences, where Einstein played God’s advocate and Schrödinger looked on with a
mix of curiosity and sympathy. It was a lonely struggle, Einstein lamented in a letter to Schrödinger in 1928: “The Heisenberg-Bohr tranquilizing
philosophy—or religion?—is so delicately contrived that, for the time being, it provides a gentle pillow for the true believer from which he cannot
very easily be aroused.” 14
So it was not surprising that Schrödinger sent Einstein a congratulatory note as soon as he read the EPR paper. “You have publicly caught
dogmatic quantum mechanics by its throat,” he wrote. A few weeks later, he added happily, “Like a pike in a goldfish pond it has stirred everyone
up.” 15
Schrödinger had just visited Princeton, and Einstein was still hoping, in vain, that Flexner might be convinced to hire him for the Institute. In his
subsequent flurry of exchanges with Schrödinger, Einstein began conspiring with him on ways to poke holes in quantum mechanics.
“I do not believe in it,” Einstein declared flatly. He ridiculed as “spiritualistic” the notion that there could be “spooky action at a distance,” and he
attacked the idea that there was no reality beyond our ability to observe things. “This epistemology-soaked orgy ought to burn itself out,” he said.
“No doubt, however, you smile at me and think that, after all, many a young whore turns into an old praying sister, and many a young revolutionary
becomes an old reactionary.” 16 Schrödinger did smile, he told Einstein in his reply, because he had likewise edged from revolutionary to old
reactionary.
On one issue Einstein and Schrödinger diverged. Schrödinger did not feel that the concept of locality was sacred. He even coined the term that
we now use, entanglement, to describe the correlations that exist between two particles that have interacted but are now distant from each other.
The quantum states of two particles that have interacted must subsequently be described together, with any changes to one particle instantly being
reflected in the other, no matter how far apart they now are. “Entanglement of predictions arises from the fact that the two bodies at some earlier
time formed in a true sense one system, that is were interacting, and have left behind traces on each other,” Schrödinger wrote. “If two separated
bodies enter a situation in which they influence each other, and separate again, then there occurs what I have just called entanglement of our
knowledge of the two bodies.” 17
Einstein and Schrödinger together began exploring another way—one that did not hinge on issues of locality or separation—to raise questions
about quantum mechanics. Their new approach was to look at what would occur when an event in the quantum realm, which includes subatomic
particles, interacted with objects in the macro world, which includes those things we normally see in our daily lives.
In the quantum realm, there is no definite location of a particle, such as an electron, at any given moment. Instead, a mathematical function, known
as a wave function, describes the probability of finding the particle in some particular place. These wave functions also describe quantum states,
such as the probability that an atom will, when observed, be decayed or not. In 1925, Schrödinger had come up with his famous equation that
described these waves, which spread and smear throughout space. His equation defined the probability that a particle, when observed, will be
found in a particular place or state. 18
According to the Copenhagen interpretation developed by Niels Bohr and his fellow pioneers of quantum mechanics, until such an observation is