einstein

during the war about the cosmological implications of general relativity. “Inertia is simply an interaction between masses, not an effect in which
‘space’ of itself is involved, separate from the observed mass,” Einstein had declared. 23 But Schwarzschild disagreed with that assessment.
And now, four years later, Einstein had changed his mind. In his Leiden speech, unlike in his 1916 interpretation of general relativity, Einstein
accepted that his gravitational field theory implied that empty space had physical qualities. The mechanical behavior of an object hovering in empty
space, like Newton’s bucket, “depends not only on relative velocities but also on its state of rotation.” And that meant “space is endowed with
physical qualities.”
As he admitted outright, this meant that he was now abandoning Mach’s principle. Among other things, Mach’s idea that inertia is caused by the
presence of all of the distant bodies in the universe implied that these bodies could instantly have an effect on an object, even though they were far
apart. Einstein’s theory of relativity did not accept instant actions at a distance. Even gravity did not exert its force instantly, but only through
changes in the gravitational field that obeyed the speed limit of light. “Inertial resistance to acceleration in relation to distant masses supposes
action at a distance,” Einstein lectured. “Be-cause the modern physicist does not accept such a thing as action at a distance, he comes back to the
ether, which has to serve as medium for the effects of inertia.” 24
It is an issue that still causes dispute, but Einstein seemed to believe, at least when he gave his Leiden lecture, that according to general relativity
as he now saw it, the water in Newton’s bucket would be pushed up the walls even if it were spinning in a universe devoid of any other objects. “In
contradiction to what Mach would have predicted,” Brian Greene writes, “even in an otherwise empty universe, you will feel pressed against the
inner wall of the spinning bucket . . . In general relativity, empty spacetime provides a benchmark for accelerated motion.” 25
The inertia pushing the water up the wall was caused by its rotation with respect to the metric field, which Einstein now reincarnated as an ether.
As a result, he had to face the possibility that general relativity did not necessarily eliminate the concept of absolute motion, at least with respect to
the metric of spacetime. 26
It was not exactly a retreat, nor was it a return to the nineteenth-century concept of the ether. But it was a more conservative way of looking at the
universe, and it represented a break from the radicalism of Mach that Einstein had once embraced.
This clearly made Einstein uncomfortable. The best way to eliminate the need for an ether that existed separately from matter, he concluded,
would be to find his elusive unified field theory. What a glory that would be! “The contrast between ether and matter would fade away,” he said, “and,
through the general theory of relativity, the whole of physics would become a complete system of thought.” 27
Niels Bohr, Lasers, and “Chance”
By far the most important manifestation of Einstein’s midlife transition from a revolutionary to a conservative was his hardening attitude toward
quantum theory, which in the mid-1920s produced a radical new system of mechanics. His qualms about this new quantum mechanics, and his
search for a unifying theory that would reconcile it with relativity and restore certainty to nature, would dominate—and to some extent diminish—the
second half of his scientific career.
He had once been a fearless quantum pioneer. Together with Max Planck, he launched the revolution at the beginning of the century; unlike
Planck, he had been one of the few scientists who truly believed in the physical reality of quanta—that light actually came in packets of energy.
These quanta behaved at times like particles. They were indivisible units, not part of a continuum.
In his 1909 Salzburg address, he had predicted that physics would have to reconcile itself to a duality in which light could be regarded as both
wave and particle. And at the first Solvay Conference in 1911, he had declared that “these discontinuities, which we find so distasteful in Planck’s
theory, seem really to exist in nature.” 28
This caused Planck, who resisted the notion that his quanta actually had a physical reality, to say of Einstein, in his recommendation that he be
elected to the Prussian Academy, “His hypothesis of light quanta may have gone overboard.” Other scientists likewise resisted Einstein’s quantum
hypothesis. Walther Nernst called it “probably the strangest thing ever thought up,” and Robert Millikan called it “wholly untenable,” even after
confirming its predictive power in his lab. 29
A new phase of the quantum revolution was launched in 1913, when Niels Bohr came up with a revised model for the structure of the atom. Six
years younger than Einstein, brilliant yet rather shy and inarticulate, Bohr was Danish and thus able to draw from the work on quantum theory being
done by Germans such as Planck and Einstein and also from the work on the structure of the atom being done by the Englishmen J. J. Thomson
and Ernest Rutherford. “At the time, quantum theory was a German invention which had scarcely penetrated to England at all,” recalled Arthur
Eddington. 30
Bohr had gone to study with Thomson in Cambridge. But the mumbling Dane and brusque Brit had trouble communicating. So Bohr migrated up
to Manchester to work with the more gregarious Rutherford, who had devised a model of the atom that featured a positively charged nucleus
around which tiny negatively charged electrons orbited. 31
Bohr made a refinement based on the fact that these electrons did not collapse into the nucleus and emit a continuous spectrum of radiation, as
classical physics would suggest. In Bohr’s new model, which was based on studying the hydrogen atom, an electron circled a nucleus at certain
permitted orbits in states with discrete energies. The atom could absorb energy from radiation (such as light) only in increments that would kick the
electron up a notch to another permitted orbit. Likewise, the atom could emit radiation only in increments that would drop the electron down to
another permitted orbit.
When an electron moved from one orbit to the next, it was a quantum leap. In other words, it was a disconnected and discontinuous shift from one
level to another, with no meandering in between. Bohr went on to show how this model accounted for the lines in the spectrum of light emitted by the
hydrogen atom.
Einstein was both impressed and a little jealous when he heard of Bohr’s theory. As one scientist reported to Rutherford, “He told me that he had
once similar ideas but he did not dare to publish them.” Einstein later declared of Bohr’s discovery, “This is the highest form of musicality in the
sphere of thought.” 32
Einstein used Bohr’s model as the foundation for a series of papers in 1916, the most important of which, “On the Quantum Theory of Radiation,”
was also formally published in a journal in 1917. 33
Einstein began with a thought experiment in which a chamber is filled with a cloud of atoms. They are being bathed by light (or any form of
electromagnetic radiation). Einstein then combined Bohr’s model of the atom with Max Planck’s theory of the quanta. If each change in an electron
orbit corresponded to the absorption or emission of one light quantum, then—presto!—it resulted in a new and better way to derive Planck’s
formula for explaining blackbody radiation. As Einstein boasted to Michele Besso, “A brilliant idea dawned on me about radiation absorption and
emission. It will interest you. An astonishingly simple derivation, I should say the derivation of Planck’s formula. A thoroughly quantized affair.” 34