### The Shortcomings of Intuition

The more physicists discover about the inner workings of the universe, the more difficult it is for people to intuitively understand their discoveries. Most discoveries of the twentieth century have run exactly counter to what people believe in their hearts about the world around them. The examples are too numerous to mention, but one of the best-known intuition-foiling insight of the twentieth century is Einstein’s special theory of relativity. Einstein showed, through a series of complicated mathematical proofs, that only the speed of light is constant, and that all other motion is relative. What one person perceives to be happening in the world around him might differ dramatically from what another person perceives—and it’s likely that both people are perfectly correct. Special relativity, Greene warns, is “not in our bones.” The idea that different observers experience space and time differently is not one that we experience in our daily lives. We cannot *feel* special relativity. We must just study the equations, read the explanations, and take scientists’ word for it.

Einstein’s revision of Newton’s particle model of light is another phenomenon that seems, at an intuitive level, too strange to apply to the real world. Einstein and others proved that light has both wavelike *and* particle-like properties, a duality that ran counter to scientists’ long-standing beliefs that light must have one property or the other. Louis de Broglie later showed that all matter shares this dual nature. With macroscopic objects, the wave nature is almost impossible to detect, and the duality only becomes important on small-distance scales. Here we encounter another counterintuitive set of laws. From a perspective of day-to-day observation, nothing makes less sense than particles’ behavior on an ultramicroscopic level.

Quantum physics is even harder to grasp viscerally. Refining a suggestion made by Erwin Schrödinger, Max Born was the first to argue that an electron wave can only be described from a standpoint of probability. We can never know precisely what will come next in the cosmos, just what is likeliest to happen. Schrödinger worked from de Broglie’s conception of dual-nature matter to develop an equation that guides the shape of probability waves (or *wave functions*). Einstein himself—the same man who trampled centuries of human intuition about the speed of light, motion, and gravity—was appalled by this seemingly random and senseless hypothesis. He said, famously, “God does not play dice with the universe.”

Interestingly, after detailing instance after instance of intuitive lapses, Greene offers an intuition-based explanation of string theory’s development and necessity. He says that string theory caught on because physicists have an intuitive objection to the idea that the universe operates according to two contradictory sets of laws. They embrace string theory because it offers the possibility of one coherent Theory of Everything—and this appeals to their intuitive desire for one system.

### Theory of Everything

It is possible, physicists believe, to uncover a single theory that explains every single phenomenon in the universe seamlessly, with no contradictions or “cosmological constants” or other evidence of scientists’ incomplete understanding. Einstein spent the last thirty years of his life seeking a unified field theory that could explain all four forces—the strong force, the weak force, electromagnetism, and gravity—and all of matter within a single theoretical framework. String theorists hope to realize Einstein’s disappointed ambition. They hope to uncover a single quantum mechanical theory that explains the entire universe, and even multiple universes, should they exist.

It is necessary to find a single theory to establish the inevitability of the universe’s composition. For an understanding to be thorough, it can contain no gaps or glitches or internal inconsistencies (like the divide between general relativity and quantum mechanics). String theorists were troubled when they discovered five equally valid mathematical explanations of string theory. This multiplicity of explanations directly undercut the dream of unity. But when Witten showed that these five versions might be five variations on the same formula, string theorists realized once again that arriving at a unified theory will require drastic revisions of what they consider reality.

Perhaps a unified theory will emerge after scientists find, for example, that the universe is composed of a multitude of dimensions invisible to the eye. Or perhaps they will find that the universe humans inhabit is just one of a multitude. Cosmology—the study of the universe’s origins—has generated some eyebrow-raising speculations about the universe’s essential symmetry and coherence. Should any of these speculations be proved, scientists will be that much closer to unveiling the Theory of Everything—the ambition that Greene describes as the “Holy Grail” of modern physics.

### Elegance

String theorists seek to explain a wide range of phenomena with a very small number of versatile, far-reaching ideas—a single set of universal laws. String theory is the most elegant theory yet conceived because it is predicated on the unity of all particles and nuclear forces. The idea of tiny strings in constant vibration is an extremely simple notion, but it may well prove the basis of every event in the universe, and that, to Greene, is the definition of elegance. Throughout the book, Greene uses words like *elegant*, *aesthetic*, and *beautiful* when describing string theory—especially in contrast to the chaotic principles of quantum mechanics. The more physicists learn about the universe, the more elegant and concise their theories will likely be.

### Theory versus Experiment

Ultimately, the efficacy of a physical theory resides in its ability to decipher and predict physical phenomena. Theories have little value until they can be tested, proved, and applied to the real world beyond the blackboard and lab. In the 1980s, Sheldon Glashow reminded string theorists of this crucial aspect of the scientific method. Glashow and other detractors were eager to point out that string theory, in its current formulation, contains far too many gaps and question marks to be experimentally accessible.

Greene regrets this problem, which he believes reflects the sweeping ambition of string theory. Most of the time, theory tends to follow observation and experiment. In string theory, the order of operations is reversed. The elementary particles of matter are simply too small—or, as has recently been suggested, too large—to be detected outside a nuanced theoretical framework. String theorists must know exactly what they are looking for before they can hope to find it. For this reason, they are working feverishly to discover which Calabi-Yau shape is correct. Only after resolving this matter can physicists begin to verify string theory experimentally. Even with such specific information, the challenge of verifying string theory experimentally remains formidable. Locating matter at Planck length is, at the moment, beyond the capabilities of the most advanced technological equipment. But though it may take years or even generations, Greene believes that one day an accelerator will manage to probe that tiny scale. In the meantime, physicists must continue working out the mathematical underpinnings of the theory.

### The Uncertainty Principle

The uncertainty principle, which is the key feature of quantum mechanics, proclaims that some features of the universe—namely the position and velocity of a particle—cannot be known with absolute precision, or at least not at the same time. This uncertainty becomes more exaggerated as distance and time shrink, indicating the chaos and frenzy that are prevalent in the microscopic realm. In the absence of concrete predictions about what occurs at these small-distance scales, physicists must settle for probability equations.

Heisenberg’s uncertainty principle applies specifically to the laws of quantum mechanics. But, in other realms of physics, the element of uncertainty is both the chief motivator and the major stumbling block. For every statement of fact in *The Elegant Universe*, Greene offers an unresolved speculation. Though our knowledge of the universe has advanced incalculably over the last hundred years, we are still surrounded on all sides by mysteries beyond our grasp. But not yet knowing the answers should not prevent us from continuing to ask the questions. Cosmological certainty is a most elusive goal, but Green believes it is a goal worth pursuing.