By Brian Greene
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.
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.
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.
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.
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.
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.
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.
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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