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Key insights from
The Elegant Universe: Superstrings,
Hidden Dimensions, and the Quest for the Ultimate Theory
By
Brian Greene
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What you'll learn
Brian Greene (1963-) is an American theoretical physicist,
specializing in superstring theory (commonly called “string theory”). After
getting his PhD from Oxford University, Greene began teaching at Cornell
University in 1990. He then became a professor at Columbia University in
1996, teaching both physics and mathematics. In addition to his 32 years of
teaching, he has written multiple books on theoretical physics for the
general public. Outside of his career as an author, he is also famous for
launching the World Science Festival, a non-profit that seeks to educate
the general public about cutting-edge issues in the sciences. In his first
book, The Elegant Universe, Greene attempts to explain the model of the
universe given by string theory to those who have no formal education in
theoretical physics.
Read on for key insights from The Elegant Universe.
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1. It is not
moving particles, but vibrating strings which make up our world.
Up until the advent of string theory in the middle of the
20th century, our understanding of the physical universe assumed the
building blocks of everything to be particles. Inside the atom were
protons, neutrons, and electrons. Though this seemed to be the bedrock of
the physical universe, further research has identified particles within
protons and neutrons, called quarks. Moreover, experimental data began
suggesting even more particles, such as neutrinos, muons, and taus.
As the number of particles believed to exist increased
through advancements in experimental research, physicists were also trying
to understand the four fundamental forces. These forces—gravity,
electromagnetism, strong nuclear, and weak nuclear—were found to have their
own particles as well. Numerous questions remained, however, concerning how
all these particles interacted. Moreover, how these particles interacted
with other phenomena, such as general relativity and quantum mechanics,
remained unclear.
A theoretical framework that could make sense of these
questions increasingly became the goal of modern physics. The holy grail
would be a complete explanation of the interactions between the fundamental
entities of the universe and the phenomena they comprise. This theory of
everything, often called T.O.E., seemed to elude the particle model of
physics.
In 1968, an alternative model of the universe was
formulated, which began with one simple assumption. The primary claim of
this model—string theory—is that the fundamental physical entities in the
universe are not particles, but actually one-dimensional strings. This
simple conceptual shift opened a brand new frontier for theorists and experimental
researchers alike. If the fundamental entities in the universe are not
microscopic particles, but even tinier strings, then all the laws of the
universe are the results of vibrations. What seemed to be particles are
really just the emanations of vibrating strings, which have certain
frequencies that correspond to the forces and elements already known. So
under this framework, an electron is actually an individual string
vibrating at a specific frequency for electrons.
By replacing the multitude of particles with the simplicity
of strings, modern physics has opened up a brand new avenue of research. If
string theory is accurate, then all of the structures and laws of the
universe are notes in the music of these fundamental threads.
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2. Relativity
shows that our universe is not fixed, but in flux.
Much of contemporary physics is indebted to the work of
Albert Einstein in the early 20th century. His work on relativity
enshrined, amongst other things, the principle of relativity and laid the
foundation for subsequent research in physics. This principle states that
all constant-velocity observers are subject to identical physical laws, and
therefore every observer is justified in claiming that he or she is at
rest.
While this may seem abstract, this is similar to a
phenomenon that happens during road trips. When you are in a car going 60
miles an hour, sometimes it seems that the trees on the side of the road
are whizzing past you, and not you past them. Two considerations check this
feeling. First, our own sensation of the force acting on us, like being
pressed against your car seat when you speed up and leaning forward when
the car brakes are suddenly used. Second, we consciously recognize that
trees don’t move and are stationary objects.
In this principle, it is important to remember that the
observer is moving at a constant velocity, because this means from the
observer’s perspective they are force-free. When the car is maintaining a
constant speed on a smooth road, it induces the feeling that the observer
is at rest, while everything else moves around them. This principle
encompasses Einstein’s point that motion is relative. Whoever or whatever
is measuring the velocity in question actually matters, because all motion
measurements are relative between the objects in question.
This seemingly obscure experience is a good example of the
principle by which Einstein revolutionized physics in the beginning of the
1900s. From this principle concerning the relativity of motion, Einstein
went on to explore how time and even space are relative to each other.
Conventional ways of measuring time and space by clocks and rulers
approximate and ignore the way that time, space, and motion are interwoven
concepts. In contradiction to previous scientific conceptions of space and
time as absolute and unchangeable, Einstein showed there is no way our
experimental observations can be objective. Space and time do not exist as
objective concepts in physics, because observation is always
observer-dependent. Thus, to gain a proper explanation of the physical
universe, the relativity of motion, time, and space must be considered.
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3. In quantum
mechanics, physicists find themselves unable to observe phenomena in a
controlled environment.
While relativity is an important feature of physics on the
macroscopic level, quantum mechanics revolutionized theoretical physics on
the microscopic level. At first glance, quantum mechanics does not seem to
be very good science because it seems to neglect the rigor and certainty
which physics seeks to ascertain in its explanations. In fact, quantum
mechanics explicitly makes use of concepts such as uncertainty,
probability, and wave-particle duality.
This latter notion is one of the main features of quantum
mechanics. In the 1920s, scientists theorized and then showed
experimentally that individual electrons act like waves. By firing one
electron at a time toward a screen that could record its impact, the data
suggested that an electron, when repeatedly fired at the screen, will mark
out wave-like patterns. This data confirmed a bizarre yet inevitable
theory: Particles of matter have a wave-like character.
This contradicts our basic intuition that matter is solid
and sturdy. Here was experimental proof, however, that on the microscopic
level, particles have wave-like behavior that shakes up our understanding
of the sub-atomic structure of the universe. While this does not change our
day to day life, it does provide a major obstacle to sub-atomic research in
physics.
Quantum mechanics is unable to predict as much as we
normally expect. On the microscopic level, physicists have come up against
barriers to their very own research because of the nature of particles and
their wave-like interactions. Namely, physicists are unable to observe the
subatomic layer of our universe as we observe other layers because doing so
bombards the particles in question with particles of light, called photons,
thus disrupting the experiment to begin with. It has been shown that we can
know some information about a particle in these experiments, but our
observation itself invalidates the experiment.
Werner Heisenberg’s famous Uncertainty Principle
encapsulates this dilemma facing quantum mechanics. Merely observing
various particles changes their position and velocity because of their
collision with the photons that enable researchers to examine them. Put
more simply, because humans need to shine light upon these subatomic
phenomena in order to see them, they are also introducing more particles
into their experiments, obscuring their intended research.
Despite both its overt challenge to standard research
techniques as well as its bizarre features like wave-particle duality,
quantum mechanics is a lively field of inquiry. While the particle view of
physics has a hard time handling these challenges, string theory provides
an alternative explanation. A so-called particle’s wave-like
characteristics are much more at home in a theory which fundamentally
regards the “particles” as vibrating strings.
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4. To understand
the fundamental nature of the universe according to string theory, we must
reject our intuitions about it.
Relativity, quantum mechanics, and string theory all show
that for our understanding of the universe to continue developing, we must
be willing to alter or reject our most basic intuitions about it. On
macroscopic and microscopic scales, distant from human experience and life,
the universe holds deep secrets which baffle our best non-technical
guesses.
One of the most significant ideas of the 20th century, which
has found its place in string theory, is that there are more than three
spatial dimensions. We experience and learn explicitly at a young age that
things can be measured by their length, width, and depth. This is a bedrock
assumption that frames our physical experience of the world. We can even
map out our bodies in three dimensions. Yet in string theory, it has become
commonplace to speak of up to seven hidden spatial dimensions. These seven
new dimensions have been mathematically sketched out and have helped make
sense of some experimental data within string theory research.
How can this be when it is clear that space is three
dimensional? Again, we must question our observational capacities when it
comes to theories that challenge our own certainties. While it is true that
only three spatial dimensions are extended enough to be known by creatures
such as ourselves, string theorists remark that there are more spatial
dimensions that are hidden, beyond our grasp. What we see is not all there
is. Rather, beyond our most refined experimental technology lie seven
curled-up spatial dimensions that are presently inaccessible to us.
Consider, for example, a piece of paper. We normally regard
such an object as two dimensional, having both length and width, but no
depth. Yet if we were to examine the edge of our paper under a microscope,
we would surprisingly discover a thickness that entails heretofore unseen
depth. While we may continue to regard paper as two dimensional for our
purposes, it nevertheless remains true that on a small scale, under a good
magnifying glass, paper is three dimensional.
Likewise, string theorists conclude that there are multiple
spatial dimensions that are too small for human interaction, but are open
to the fundamental strings that are even smaller than the quarks we
currently see. Strings vibrate through all 10 of the spatial dimensions
that exist, and this is extremely significant for our understanding of the
universe. If strings operate in and are constrained by the spatial
dimensions they inhabit, then the shape of these dimensions affects how a
string vibrates. If a string’s vibrations are dependent on the warp and
shape of the spaces it occupies, then that means all of the physical laws
and elements in our universe are affected as well. Therefore, the 10 spatial
dimensions actually have a major impact on the way our universe is
constructed, even though we only perceive three of them.
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5. String theory
informs us that black holes may one day be as small as particles.
Amongst the exciting and unnerving natural phenomena of the
universe, black holes are likely the most well known. Though we usually
imagine them to be humongous gaps in space, sucking in everything up to and
including light, modern physics shows that black holes need not be big. It
is not the size of the clump of matter that makes a black hole, but rather
the pressure it experiences because of gravity. To form properly, lighter
and therefore smaller black holes must be crushed much more than larger
masses would.
Scientists also readily accept that black holes do not last
forever. Eventually, though this has yet to be observed, even black holes
will diminish in mass until they are no more. Though we have been raised to
think that nothing escapes a black hole, we have to recognize that they are
ringed by energy, a specific radiation that they give off as they consume
more matter. Black holes are always bleeding off energy even as they take
in matter, meaning that their overall mass is diminishing, if only
infinitesimally.
Within string theory, the question of dying black holes has
the makings of an answer. Over an incredibly long time, longer than
recorded human history, a black hole will shrink as it loses its mass in
the form of energy. Eventually, it will lose all of its mass and become
indistinguishable from a massless particle, because of its reduced size.
Just as massless particles such as photons can be understood as vibrations
of strings, it seems that black holes can be similarly understood.
Though some time must pass before observation of black holes
disappearing can become a reality, the theoretical underpinnings of string
theory can account for the decline of these massive entities in the cosmos.
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6. String theory
challenges and modifies our cosmological assumptions.
Cosmology, the science of the origin and development of the
universe, is an especially significant area of theoretical physics. When
people mention the big bang, they are referring to the standard
cosmological model of modern physics. From nothing—a zero-point—all of the
energy and matter that has ever existed exploded into empty space. Over
millions of years, stars swirled into existence, nebulae and asteroids and
eventually planets as well.
With the advent of string theory, crucial modifications of
the standard big bang model have emerged. First, the supposed
zero-point—the nothing from which colossal energy and matter exploded into
existence—simply cannot be real. If the fundamental stuff of the universe
is string, not particle, then there has to be a minimum size of the
universe—at least the length of its basic strings. The universe did not
erupt from nothing, but from its minimum starting point.
String theory also provides a possible answer for why our
existence is three dimensional when there may actually be 10 spatial
dimensions. While the universe was at its smallest, the ten dimensions of
space were all curled up, rather than three of them being extended as we
commonly experience. When the expansion of the big bang occurred, colossal
temperature fluctuations disrupted the string vibrations in such a way that
three of the dimensions uncurled. Why this occurred—rather than four, five,
or even all 10 of the dimensions unfurling—is still somewhat unknown,
though speculations abound.
Though numerous questions persist, string theory has
provided a brand new paradigm for physicists to engage the universe.
Moreover, this new paradigm has challenged our intuitions about the world
around us while also connecting and confirming experimental data that
otherwise has had no warrant. It remains to be seen what the 21st century
has in store, and if the next big breakthrough will further enhance string
theory, or replace it.
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Endnotes
These insights are
just an introduction. If you're ready to dive deeper, pick up a copy of The
Elegant Universe here. And since we get a commission on
every sale, your purchase will help keep this newsletter free.
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