- Smolin, Lee.
Einstein's Unfinished Revolution.
New York: Penguin Press, 2019.
ISBN 978-1-59420-619-1.
-
In the closing years of the nineteenth century, one of those
nagging little discrepancies vexing physicists was the behaviour
of the
photoelectric
effect. Originally discovered in 1887, the phenomenon causes
certain metals, when illuminated by light, to absorb the light
and emit electrons. The perplexing point was that there was a
minimum wavelength (colour of light) necessary for electron
emission, and for longer wavelengths, no electrons would be
emitted at all, regardless of the intensity of the beam of light.
For example, a certain metal might emit electrons
when illuminated by green, blue, violet, and ultraviolet light, with
the intensity of electron emission proportional to the light
intensity, but red or yellow light, regardless of how intense,
would not result in a single electron being emitted.
This didn't make any sense. According to
Maxwell's
wave theory of light, which was almost universally
accepted and had passed stringent experimental tests, the
energy of light depended upon the amplitude of the wave
(its intensity), not the wavelength (or, reciprocally,
its frequency). And yet the photoelectric effect didn't
behave that way—it appeared that whatever was causing the
electrons to be emitted depended on the wavelength
of the light, and what's more, there was a sharp cut-off
below which no electrons would be emitted at all.
In 1905, in one of his
“miracle
year” papers,
“On a Heuristic Viewpoint Concerning the Production and
Transformation of Light”, Albert Einstein suggested a
solution to the puzzle. He argued that light did not propagate
as a wave at all, but rather in discrete particles, or “quanta”,
later named “photons”, whose energy was proportional
to the wavelength of the light. This neatly explained the
behaviour of the photoelectric effect. Light with a wavelength
longer than the cut-off point was transmitted by photons whose
energy was too low to knock electrons out of metal they
illuminated, while those above the threshold could liberate
electrons. The intensity of the light was a measure of the
number of photons in the beam, unrelated to the
energy of the individual photons.
This paper became one of the cornerstones of the revolutionary
theory of
quantum
mechanics, the complete working out of which occupied much
of the twentieth century. Quantum mechanics underlies the
standard model
of particle physics, which is arguably the most thoroughly tested
theory in the history of physics, with no experiment showing
results which contradict its predictions since it was formulated
in the 1970s. Quantum mechanics is necessary to explain the
operation of the electronic and optoelectronic devices upon
which our modern computing and communication infrastructure
is built, and describes every aspect of physical
chemistry.
But quantum mechanics is weird. Consider: if light
consists of little particles, like bullets, then why
when you shine a beam of light on a
barrier
with two slits do you get an interference pattern with bright
and dark bands precisely as you get with, say, water waves?
And if you send a single photon at a time and try to measure
which slit it went through, you find it always went through one
or the other, but then the interference pattern goes away.
It seems like whether the photon behaves as a wave or a particle
depends upon how you look at it. If you have an hour, here is
grand master explainer
Richard Feynman
(who won his own Nobel Prize in 1965 for reconciling the
quantum mechanical theory of light and the electron with
Einstein's
special
relativity) exploring how profoundly weird the
double slit
experiment is.
Fundamentally, quantum mechanics seems to violate the principle
of realism, which the author defines as follows.
The belief that there is an objective physical world whose
properties are independent of what human beings know or
which experiments we choose to do. Realists also believe
that there is no obstacle in principle to our obtaining
complete knowledge of this world.
This has been part of the scientific worldview since antiquity
and yet quantum mechanics, confirmed by innumerable experiments,
appears to indicate we must abandon it. Quantum mechanics says
that what you observe depends on what you choose to measure; that
there is an absolute limit upon the precision with which you
can measure pairs of properties (for example position and momentum)
set by the
uncertainty
principle;
that it isn't possible to predict the outcome of experiments
but only the probability among a variety of outcomes;
and that particles which are widely separated
in space and time but which have interacted in the past are
entangled
and display correlations which no classical mechanistic theory
can explain—Einstein called the latter “spooky
action at a distance”. Once again, all of these effects
have been confirmed by precision experiments and are not
fairy castles erected by theorists.
From the formulation of the modern quantum theory in the 1920s,
often called the
Copenhagen
interpretation after the location of the institute where
one of its architects,
Neils Bohr,
worked, a number of eminent
physicists including Einstein and
Louis de Broglie
were deeply
disturbed by its apparent jettisoning of the principle of realism
in favour of what they considered a quasi-mystical view in which
the act of “measurement” (whatever that means) caused
a physical change
(wave
function collapse) in the state of a system. This seemed to
imply that the photon, or electron, or anything else, did not have
a physical position until it interacted with something else: until
then it was just an immaterial wave function which filled all of
space and (when squared) gave the probability of finding it at
that location.
In 1927, de Broglie proposed a
pilot wave theory
as a realist alternative to the Copenhagen interpretation. In
the pilot wave theory there is a real particle, which has a
definite position and momentum at all times. It is guided in
its motion by a pilot wave which fills all of space and is
defined by the medium through which it propagates. We cannot predict
the exact outcome of measuring the particle because we cannot have
infinitely precise knowledge of its initial position and
momentum, but in principle these quantities exist and are
real. There is no “measurement problem” because
we always detect the particle, not the pilot wave which guides it.
In its original formulation, the pilot wave theory exactly reproduced
the predictions of the Copenhagen formulation, and hence was not a
competing theory but rather an alternative
interpretation
of the equations of quantum mechanics. Many physicists who preferred
to “shut up and calculate” considered interpretations a
pointless exercise in phil-oss-o-phy, but de Broglie and
Einstein placed great value on retaining the principle of realism
as a cornerstone of theoretical physics. Lee Smolin sketches an
alternative reality in which “all the bright, ambitious students
flocked to Paris in the 1930s to follow de Broglie, and wrote textbooks
on pilot wave theory, while Bohr became a footnote, disparaged for
the obscurity of his unnecessary philosophy”. But that wasn't
what happened: among those few physicists who pondered what the
equations meant about how the world really works, the Copenhagen
view remained dominant.
In the 1950s, independently,
David Bohm
invented a pilot wave theory which he developed into a complete
theory of nonrelativistic quantum mechanics. To this day, a
small community of “Bohmians” continue to explore
the implications of his theory, working on extending it to be
compatible with special relativity. From a philosophical
standpoint the de Broglie-Bohm theory is unsatisfying in that it
involves a pilot wave which guides a particle, but upon which
the particle does not act. This is an
“unmoved
mover”, which all of our experience of physics argues
does not exist. For example, Newton's third law of motion holds
that every action has an equal and opposite reaction, and in
Einstein's general relativity, spacetime tells mass-energy how to
move while mass-energy tells spacetime how to curve. It seems
odd that the pilot wave could be immune from influence of the
particle it guides. A few physicists, such as Jack Sarfatti, have
proposed “post-quantum” extensions to Bohm's theory
in which there is back-reaction from the particle on the
pilot wave, and argue that this phenomenon might be accessible to
experimental tests which would distinguish post-quantum phenomena
from the predictions of orthodox quantum mechanics. A few
non-physicist crackpots have suggested these phenomena might even
explain
flying saucers.
Moving on from pilot wave theory, the author explores other attempts
to create a realist interpretation of quantum mechanics:
objective
collapse of the wave function, as in the
Penrose
interpretation; the
many
worlds interpretation (which Smolin calls “magical
realism”); and
decoherence
of the wavefunction due to interaction with the environment. He
rejects all of them as unsatisfying, because they fail to address
glaring lacunæ in quantum theory which are apparent from its
very equations.
The twentieth century gave us two pillars of theoretical physics:
quantum mechanics and
general
relativity—Einstein's geometric theory of gravitation.
Both have been tested to great precision, but they are fundamentally
incompatible with one another. Quantum mechanics describes the very
small: elementary particles, atoms, and molecules. General relativity
describes the very large: stars, planets, galaxies, black holes, and
the universe as a whole. In the middle, where we live our lives,
neither much affects the things we observe, which is why their
predictions seem counter-intuitive to us. But when you try to put
the two theories together, to create a theory of
quantum gravity,
the pieces don't fit. Quantum mechanics assumes there is a
universal clock which ticks at the same rate everywhere in the
universe. But general relativity tells us this isn't so: a simple
experiment shows that a clock runs slower when it's in a
gravitational field. Quantum mechanics says that it isn't possible
to determine the position of a particle without its interacting with
another particle, but general relativity requires the knowledge of
precise positions of particles to determine how spacetime curves and
governs the trajectories of other particles. There are a multitude
of more gnarly and technical problems in what Stephen
Hawking called “consummating the fiery marriage between
quantum mechanics and general relativity”. In particular,
the equations of quantum mechanics are
linear,
which means you can add together two valid solutions and get
another valid solution, while general relativity is
nonlinear,
where trying to disentangle the relationships of parts of the
systems quickly goes pear-shaped and many of the mathematical
tools physicists use to understand systems (in particular,
perturbation
theory) blow up in their faces.
Ultimately, Smolin argues, giving up realism means abandoning what
science is all about: figuring out what is really going on.
The incompatibility of quantum mechanics and general relativity
provides clues that there may be a deeper theory to which
both are approximations that work in certain domains (just as
Newtonian mechanics is an approximation of special relativity
which works when velocities are much less than the speed of
light). Many people have tried and failed to “quantise
general relativity”. Smolin suggests the problem is that
quantum theory itself is incomplete: there is a deeper
theory, a realistic one, to which our existing theory is
only an approximation which works in the present universe where
spacetime is nearly flat. He suggests that candidate theories
must contain a number of fundamental principles. They must be
background
independent, like general relativity, and discard such concepts
as fixed space and a universal clock, making both dynamic and
defined based upon the components of a system. Everything must
be relational: there is no absolute space or time; everything is defined
in relation to something else. Everything must have a cause, and
there must be a chain of causation for every event which traces
back to its causes; these causes flow only in one direction. There is
reciprocity: any object which acts upon another object is acted upon
by that object. Finally, there is the “identity of indescernibles”:
two objects which have exactly the same properties are the same
object (this is a little tricky, but the idea is that if you
cannot in some way distinguish two objects [for example, by their
having different causes in their history], then they are the same
object).
This argues that what we perceive, at the human scale and even in
our particle physics experiments, as space and time are actually
emergent properties of something deeper which was manifest in
the early universe and in extreme conditions such as gravitational
collapse to black holes, but hidden in the bland conditions which
permit us to exist. Further, what we believe to be “laws”
and “constants” may simply be precedents established by
the universe as it tries to figure out how to handle novel
circumstances. Just as complex systems like markets and evolution
in ecosystems have rules that change based upon events within
them, maybe the universe is “making it up as it goes along”,
and in the early universe, far from today's near-equilibrium, wild
and crazy things happened which may explain some of the puzzling
properties of the universe we observe today.
This needn't forever remain in the realm of speculation. It is
easy, for example, to synthesise a protein which has never existed
before in the universe (it's an example of a
combinatorial
explosion). You might try, for example, to crystallise this novel
protein and see how difficult it is, then try again later and see if
the universe has learned how to do it. To be extra careful, do it first
on the International Space Station and then in a lab on the Earth.
I suggested this almost twenty years ago as a test of
Rupert
Sheldrake's theory of morphic resonance, but (although
doubtless Smolin would shun me for associating his theory
with that one), it might produce interesting results.
The book concludes with a very personal look at the challenges
facing a working scientist who has concluded the paradigm
accepted by the overwhelming majority of his or her peers is
incomplete and cannot be remedied by incremental changes based
upon the existing foundation. He notes:
There is no more reasonable bet than that our current
knowledge is incomplete. In every era of the past our
knowledge was incomplete; why should our period be any
different? Certainly the puzzles we face are at least
as formidable as any in the past. But almost nobody bets
this way. This puzzles me.
Well, it doesn't puzzle me. Ever since I learned classical
economics, I've always learned to look at the incentives in
a system. When you regard academia today, there is huge risk
and little reward to get out a new notebook, look at the
first blank page, and strike out in an entirely new direction.
Maybe if you were a twenty-something patent examiner in a small city
in Switzerland in 1905 with no academic career or reputation at
risk you might go back to first principles and overturn space, time,
and the wave theory of light all in one year, but today's
institutional structure makes it almost impossible for a
young researcher (and revolutionary ideas usually come from
the young) to strike out in a new direction. It is a blessing
that we have deep thinkers such as Lee Smolin setting aside the
easy path to retirement to ask these deep questions today.
Here is a
lecture by
the author at the Perimeter Institute about the topics
discussed in the book. He concentrates mostly on the problems
with quantum theory and not the speculative solutions discussed
in the latter part of the book.
May 2019 