Yasunori Nomura's 61-Minute Journey Through the Layers of Physics

For some reason, the word “theory” seems to be tacked onto the names of physics theories all the time: Newtonian mechanics, the special theory of relativity, quantum field theory, the Standard Model, string theory, and grand unified theories. While they all have impressive-sounding names, it’s hard to make sense of how they relate to one another.In this 61-minute interview on TBS CROSS DIG with Bloomberg, theoretical physicist Yasunori Nomura (Professor at the University of California, Berkeley; specializing in particle physics and quantum gravity) breaks down the hierarchy of these theories.Dr. Nomura is a researcher known for his work in computational cosmology and quantum gravity, and he’s exceptionally skilled at explaining complex concepts to a general audience. Although the discussion features a string of difficult theoretical names, not a single mathematical equation appears. Yet, the conversation is structured in such a way that the big picture becomes clear.

What I took away from this conversation wasn’t an introduction to physics or an overview of cutting-edge research, but rather a discussion on how to interpret these names. Which theories explain which parts of the universe, and where are the blank spaces? When you rearrange the names of these theories on that map, you realize that many of the mysteries that seemed so difficult are simply a matter of confusion over terminology.

A 61-Minute Journey Through the Map of Physics

Physical theories are layered upon one another. Higher-level theories encompass lower-level ones and, under the right conditions, reproduce the predictions of those lower-level theories.If we map this out, our current location is roughly defined. The microscopic world of matter and forces is well explained by the Standard Model, and its predictions agree closely with observations. The only “continent” on this map that has been fully mapped so far is the Standard Model. The Standard Model is a framework that unifies the particles that make up matter and the three fundamental forces into a single language.

The rest remains uncharted. Quantum gravity—which seeks to describe gravity in the language of quantum mechanics—has not yet been completed; while string theory is one of the leading candidates, it has not been confirmed by observation. Furthermore, the majority of the universe’s mass is accounted for by dark matter, which does not fit into any of the 17 particle types in the Standard Model.The energy balance of the universe is generally considered to be approximately 5% ordinary matter, 27% dark matter, and 68% dark energy (based on 2018 data from the Planck satellite). That 5% encompasses everything—stars, Earth, and all of us. The mapped portion is, in fact, a minority within the universe.

In other words, I interpret this map as one where the blank spaces are larger than the completed ones. Figuring out the order in which to fill in those blank spaces is now the job of theorists. To read this map, three distinctions are necessary: the distinction between phenomena and theories and the nature of theories; the temporal shift in the meaning of words; and the fact that something labeled a “theory” is not necessarily a theory.

Layers of Pomeranians, Bulldogs, Dogs, and “Shake Hands”

At the beginning, Mr. Nomura uses a dog analogy to explain why physics is difficult to understand.

It’s like asking which you prefer: a Pomeranian, a Bulldog, a dog, or a handshake.

[02:56]

Pomeranians and Bulldogs are dog breeds; “dog” is the name of the species; and “shaking hands” is the name of an action. Breeds, species, and actions—the layers are fundamentally different. If you’re asked to line them up in the same column and choose “which one you like,” there’s no way to answer.Physics terminology follows the same structure: the names of phenomena, theories, and the properties of those theories are mixed together without any distinction. That’s why it’s confusing. Nomura cited this arrangement as a way of crafting questions that have no answer.I believe that what makes physics difficult lies not in the calculations, but in organizing these names. In fact, even though no mathematical formulas appear even once during the 61 minutes of this discussion, you can follow which theory explains what right up until the very end.

The Three Layers: Phenomena, Theories, and Properties

Let me break these down into three layers. A phenomenon is something that actually occurs in nature. An apple falling and a paperclip sticking to a magnet are both phenomena. A theory is a framework that explains these phenomena using mathematical equations, while a property is a characteristic of the theory—not the theory itself.

It’s easiest to illustrate this with friction. If you slide an object across a desk, friction comes into play. This is a phenomenon. Behind it lies an electromagnetic force, and macroscopic friction can be described at the microscopic level as the result of this electromagnetic force.The term “friction” is the name of the phenomenon, while “Newtonian mechanics” is the name of the framework that explains it. If you treat them as equivalent, the same confusion as in the dog analogy will arise again. Even with a single phrase like “the true nature of gravity,” the answer depends on whether you’re talking about the phenomenon or the theory.

The same words shift in meaning over time

What makes things tricky is that the meaning of the same word shifts over time. The example Mr. Nomura gives is “yabai.”

When someone says, “This is a crazy book,” no one thinks it’s a bad thing.

[03:57]

Nowadays, “yabai” can also be used as an interjection. This happens with scientific terms as well. Names were given based on the meaning at the time they were coined, but they sound different in light of later common knowledge.The interpretation that shifts in the meaning of scientific terms occur through the same mechanism as changes in the meaning of everyday language is merely my own analogy; I won’t make any definitive claims. Still, I have a strong sense that if we ignore the history of words, we’ll lose the ability to read the map.

Holography and the Uncertainty Principle are names for “properties”

The third source of confusion is treating a property as a theory in and of itself. The uncertainty principle is a property of quantum mechanics, not a theory in itself.The holographic principle is also a property attributed to quantum gravity. For example, when we say “quantum mechanics is probabilistic,” “probabilistic” refers to a property of the theory of quantum mechanics—it is not the name of a separate theory.

Once an important property is identified, a theory named after that property is proposed. The AdS/CFT correspondence demonstrated by Maldacena in 1997 is an example that concretely realizes the holographic principle and has become one of the main streams of research in quantum gravity.The sequence—where a property comes first and a theory is discovered later—serves as a clue when interpreting names. Since searching for theories based on properties is a common practice in physics, even when encountering the name “holographic theory,” one can distinguish that it refers to a different line of research rather than a proliferation of theories.The name “superstring theory” is similar; it derives from the fact that it is a theory that posits strings as fundamental units. However, the name alone does not reveal which layer the theory pertains to.

An increase in names is distinct from an increase in descriptions of the world.

Until the framework breaks down and a higher-level theory emerges

Physics is the discipline of expressing the rules of nature through mathematical equations; theories are not created once and left as is, but are broken and rebuilt. Mr. Nomura first distinguishes between the framework and the phenomena using language.

“Newtonian mechanics” and “friction” are not concepts on the same level.

[05:22]

What Mr. Nomura is distinguishing here is the framework for describing the rules and the phenomena explained within that framework. I took this single statement as an overview of the entire article.

The difference in layers between Newtonian mechanics and friction

Newtonian mechanics is the framework for defining rules, while friction is a phenomenon explained within that framework.Friction can be described as a secondary manifestation of electromagnetic forces. Keeping the framework and the phenomena separate is fundamental when analyzing theoretical relationships. No matter how precise the equation for friction may be, it does not call into question the validity of the framework of Newtonian mechanics. The questions being asked about the framework and the phenomena are fundamentally different.

Two Breakdowns: The World of Light Speed and the Microscopic World

Newtonian mechanics broke down in two directions: the world of speeds approaching the speed of light and the world of extremely small scales.

In motion at speeds close to the speed of light, predictions diverged from observations, and Einstein’s special theory of relativity took over there.In the microscopic world, the behavior of atoms and light could not be explained, leading to the development of quantum mechanics. It was clear exactly where these two theories diverged. Therefore, the old theory was not discarded; rather, it survived within the new theory as an approximation under certain conditions. What broke down was its scope of application, not its correctness.

New theories do not erase old ones. At speeds sufficiently slower than the speed of light, the predictions of the special theory of relativity reduce to those of Newtonian mechanics.On the scale of everyday life, the results of quantum mechanics approach those of classical mechanics. Theories do not replace one another; rather, the higher-level theory encompasses the lower-level one. To me, this seems to be the backbone of this discussion.

From Special to General

The “special” in the Special Theory of Relativity means that it is limited to flat spacetime that does not include gravity. The General Theory of Relativity was an extension that incorporated gravity; it was completed in 1915 and gained widespread acceptance after the deflection of light was confirmed during the observation of a total solar eclipse in 1919.

The core of the theory is the assertion that acceleration and gravity cannot be distinguished in principle. To a person inside, standing in an accelerating elevator is indistinguishable from being pressed against the floor of a spaceship.Although this runs counter to everyday intuition, it is essential to the theory’s internal consistency. The sequence so far—Newtonian mechanics, the Special Theory of Relativity, and the General Theory of Relativity—shows that the scope of application has expanded step by step. One framework is broken, a higher-level framework emerges, and then that, too, is broken. Modern physics exists at the end of this cycle of repetition.Amid this accumulation, gravity alone remained the last to fail to reconcile with quantum mechanics.

Quantum Field Theory as the “OS” and the 17 Types of Particles

Physics had two major theories: quantum mechanics and the theory of relativity. However, there was no theory that satisfied both simultaneously.Quantum mechanics cannot handle particles moving at speeds close to the speed of light, and the theory of relativity does not include quantum effects. Quantum field theory is the framework designed to incorporate both. It did not become practical immediately after its formulation. It was only after the renormalization method—which addresses the infinities that appear in calculations—was later developed that it finally became a useful tool.

Mr. Nomura compares this framework to an operating system.

Since it’s a “box”—or rather, an OS—you’re free to choose which apps to install on it

[18:39]

Quantum field theory is a general-purpose framework that can accommodate any particle or force. However, it does not tell us which specific particles and forces actually exist in the universe. Even if you boot up the OS, a list of pre-installed apps does not appear.Experiments determine what’s inside, and that work constitutes the core of particle physics. The power of this metaphor lies in the fact that it drives home the point that half the work of particle physics involves investigating what’s inside.

The OS for Elementary Particles

The work of physics is twofold: the task of building the “OS”—quantum field theory—and the task of using experiments to find the “apps” to run on it. The Standard Model is the result of the latter. The framework and what is placed on top of it have been treated as separate problems from the very beginning.

The Standard Model

The Standard Model is the result of this work. It is a theory built by placing experimentally discovered particles onto the foundation of quantum field theory. It consists of a total of 17 particles—six types of quarks, six types of leptons, four types of gauge particles that transmit forces, and one type of Higgs particle—and explains matter along with the three fundamental forces: the electromagnetic force, the weak force, and the strong force.Depending on how you count them, some say there are 16. The difference between 17 and 16 is simply a matter of counting conventions; it does not change the substance of the theory. Quarks and leptons make up matter, gauge particles mediate forces, and the Higgs particle is involved in mass.

The predictions of this theory agree very well with observations. To me, a system this well-rounded seems exceptional even within the field of physics. That is why it is called the “Standard” Model and holds a standard position. However, the Standard Model answers the question of “what exists,” but it does not answer the question of “why there are exactly 17 types.”“What is included” and “why it is included” are separate questions.

The “Mixing of Forces” Created by the Higgs

Forces can all be described in the same form, known as gauge theory. The electromagnetic force is mediated by the exchange of photons, and the strong force is mediated by the exchange of gluons. However, attempts to unify the electromagnetic and weak forces into a single theory have not been successful. The Higgs particle was introduced to resolve this problem.When the Higgs permeates the vacuum, it creates a “mixing of forces,” endowing the particles that carry the weak force with mass.In July 2012, the Higgs boson was discovered at CERN’s LHC, and the discovery was awarded the Nobel Prize in Physics the following year, in 2013. The final piece of the Standard Model has been filled in by experimental evidence.

The Two Gaps Remaining Before Gravity

A quantum version of gravity that cannot yet be expressed mathematically

Gravity is not included in the Standard Model. While the three fundamental forces—electromagnetism, the weak force, and the strong force—can be described using the language of quantum field theory, gravity alone causes the theory to break down when combined with quantum mechanics. Quantum gravity, the quantum version of gravity, is the whitest area on this map.

One of the leading candidates is string theory, which will be the focus of this article. String theory is a concept that views the smallest units of matter not as particles but as vibrating strings; while it works well mathematically, it has not been fully confirmed by observation. Furthermore, it can only be formulated mathematically under limited conditions.The Standard Model excels primarily in scenarios where gravity is weak and quantum effects are mild. In extreme situations where both forces act simultaneously, we’re at a loss for words.

What’s confusing here is the relationship between quantum gravity and the “theory of everything.” The emphasis shifts depending on whether the discussion centers on quantizing gravity or defining all particles and forces, making them conceptually distinct. While many physicists believe they point to the same ultimate form, to me, this seems to lie only within the realm of speculation. I won’t make any definitive claims.

Invisible Mass: Dark Matter

There is also a major gap outside the Standard Model: the fact that the majority of the universe’s mass does not fit into any of the 17 particle types defined by the Standard Model. Mr. Nomura describes the path to this discovery as follows:

“As long as matter exists, we will inevitably detect gravity… By observing how stars move within a galaxy, we can determine how much mass is present. On the other hand, we can count only what is visible, and that is nowhere near enough.”

[49:08]

When we measure the velocities of stars in galaxies, we find there is mass that cannot be explained by visible stars and gas alone. In the 1970s, observations of rotation curves by Bella Rubin and others provided quantitative clues, and the same discrepancy was repeatedly observed regardless of the type of galaxy. I interpreted this not as a matter concerning a single celestial body, but as a matter concerning the structure of the universe as a whole.We know for certain that dark matter “exists” based on its gravitational effects, but its detailed properties remain unknown.

However, this does not mean that the framework of quantum field theory itself has been broken. It is an area where various candidates for expanding the Standard Model to include unknown particles and interactions are being considered. The view that the true nature of dark matter is related to extra dimensions or hidden sectors in string theory remains, based on current observations, in the realm of conjecture, and I regard it as just one hypothesis among many.

Grand Unification and the 2012 Higgs Are Two Different Things

Another gap lies in the question of why three forces that are similar in form exist as three separate entities. All three forces share the same form—that of gauge theories. If so, wouldn’t they merge back into a single force at high energies? This line of thinking is encapsulated in Grand Unified Theories, the prototype of which is the SU(5) model proposed by George and Glashow in 1974.It predicts proton decay, but this has never been observed. As long as proton decay remains unobserved, Grand Unified Theory will remain a hypothesis. Nevertheless, the appeal of this line of thought lies in the fact that the three forces share the same underlying structure.

What’s confusing here is the name. The particle predicted by the Grand Unified Theory is sometimes called the “Higgs particle,” but it is distinct from the one discovered at the LHC in 2012.The observed Higgs is a piece of the Standard Model, while the Higgs of the Grand Unified Theory is a separate, yet-to-be-discovered particle. The same name refers to entities on different levels. This is precisely where the dog analogy from the beginning comes into play again.

“Big Bang” and “Inflation” Are Names for “Phenomena”

Cosmological terminology also falls into the same trap. Both the Big Bang theory and the theory of inflation are names given to phenomena that occurred based on theories of elementary particles; they do not refer to different layers of theory. The question “What came before the Big Bang?” ultimately boils down to a matter of definition: where exactly do we draw the line for what constitutes the Big Bang?

Inflation is the idea that there was a rapid expansion immediately after the beginning of the universe; it was proposed independently in 1981 by Katsuhiko Sato and Alan Guth.The fluctuations observed in the cosmic microwave background radiation are consistent with this picture. Both inflation and the Big Bang are names given to explanations demanded by observations; they are not stories invented out of thin air.

The term “Big Bang” itself originated when Fred Hoyle used it sarcastically during a BBC radio broadcast in 1949, and it subsequently gained widespread use. Later, in 1964, the cosmic microwave background radiation was discovered, and it became widely accepted that the universe had a hot beginning.Penzias and Wilson were awarded the Nobel Prize in Physics in 1978 for this discovery. Once we recognize that words are merely labels attached to phenomena—and that questions of definition come first—most of the controversy surrounding this topic disappears. Even the terms “before” and “after” change in meaning depending on where the reference point is set.

The View That Gravity Is Not a “Force”

Another area where the “layers of naming” come into play is the way we count “forces.” Generally, the forces of nature are counted as four: gravity, the electromagnetic force, the weak force, and the strong force. However, in the theory of general relativity, gravity is not treated as a force.The other three forces can be described in terms of the exchange of gauge particles, whereas gravity is described as a distortion of spacetime itself. This is why some argue that “gravity is not a force.” Counting them as four is a way of speaking that aligns with everyday intuition; in terms of theoretical structure, they are divided into three forces and one non-force.

How the World Looks When You Have a Map of Words

Mr. Nomura concludes by saying:

If we do not define words, we risk becoming confused by things that are completely meaningless

[59:38]

These 61 minutes were not an introduction to physics or a look at cutting-edge research, but rather a discussion of the methodology for organizing the names of theories. After listening to this conversation, my perspective on physical theories changed slightly.Once you can distinguish between the different layers, the fragments you see in the news—such as “quantization of gravity,” “grand unified theory,” and “the true nature of dark matter”—appear as specific locations on a map.

If we reframe this as a conclusion about “how the world works,” the most important lesson from this map is that the uncharted portions are far more extensive than the mapped ones. The Standard Model is well-constructed. However, within the universe, the parts that have been explained are actually in the minority. The vast majority of the remainder consists of areas that have been given names but whose details remain undefined.

I believe we should accept these blank spaces not as flaws, but as the current reality. The habit of distinguishing between layers of meaning is not limited to physics. It happens everywhere that the same words shift in meaning over time, and that labels differ from the phenomena they describe. If we develop the ability to distinguish between the names of theories and the names of properties, the big picture of physics will come together.When we look at these “white” areas, we see not confusion but the next question. That is what this dialogue has left us with.