The search for a single overarching theory of nature that describes all the fundamental physical forces and particles has been the major thrust of modern physics. Are we about to reach this elusive goal, or is it turning into a quixotic quest that needs to be abandoned?
Jalees Rehman, MD
Department of Medicine and Department of Pharmacology
University of Illinois at Chicago
I shall let the little I have learnt come to light in order that someone better than I may guess the truth, and his work may demonstrate my error. At this I shall rejoice, and also because I was nevertheless a cause whereby such a truth came to light.
Albrecht Dürer (1471-1528) – Schriftlicher Nachlaß
How will the 20th century be remembered by the historians in the year 2525 – if man is still alive? Will the annals of history label it as the century of human suffering, in light of the millions of lives that were prematurely claimed by two world wars, countless cases of large-scale atrocities, ethnic cleansing and genocide? Or perhaps as the century in which hygiene, vaccinations and antibiotics helped save millions of lives, subjugated colonies gained independence from their oppressors and in which the majority of countries finally recognized women as full citizens, granting them the right to vote and reproductive control over their own bodies?
Considering that the 20th century ended less than two decades ago, any attempt to ascertain what will be deemed significant in the coming centuries would be pure speculation. Nevertheless, when we reflect on what we believe most important about centuries past, it becomes rather obvious what aspects of history leave a long-lasting imprint on the collective consciousness of subsequent generations: expressions of human ingenuity. In school, we may have memorized the dates of when wars started and ended, but we are enthralled by the beauty and insights emanating from the great works of art, music, literature, philosophy and science of centuries past. Testaments to human ingenuity, tenacity and the pursuit of beauty and justice include the development of vaccines and antibiotics, abandoning colonialism and granting equality to women. But there is another, far less known front-runner for the most important intellectual accomplishment in the 20th century: The Standard Model. It is also a shoo-in for the “Most Understated-Name-Ever Award.” What is the Standard Model, and why is it a pinnacle of human achievement?
The Standard Model is to date the most comprehensive and experimentally validated theory of the fundamental physical forces and elementary particles in nature. It unifies three core physical forces, the electromagnetic force, the strong force and the weak force, and describes fermions – elementary “substance” particles, such as quarks, electrons and neutrinos – as well as bosons – elementary “force” particles, such as photons, gluons and the Higgs boson. As such, the Standard Model allows us to understand the very nature of energy and matter and provides a framework for understanding the universe. The Standard Model was finalized in the 1970s, but it is built on some of the conceptual revolutions in physics at the beginning of the 20th century, such as quantum mechanics and the theory of special relativity which challenged traditional views of determinism, time and space.
While there have been many important scientific developments in the past century, the Standard Model stands out as a prime example of how theoretical science and experimental science work like intercalating cogwheels, propelling each other forward. The theoretical framework was formulated in the 1970s based on prior experimental data, but it made definitive predictions regarding the existence of particles that had never been observed. And each subsequent decade, experiments undertaken to test its validity confirmed its predictions, such as the discovery of the top quark in 1995, the tau neutrino in 2000 and the Higgs boson in 2012/2013. This unprecedented pace of experimental testing and theoretical formulation of the Standard Model could only be achieved by the collaborative efforts of thousands of physicists, transcending boundaries of nationality, gender and ideology and united by their common goal of uncovering the fundamental laws of nature.
The Standard Model is part of a grander quest in physics to develop unified theories with the ultimate goal to have one final Unified Theory, which is sometimes referred to as the “Theory of Everything” – quite a baptismal contrast to the humility exuded by the name “Standard Model.”
George Smoot is Professor of Physics at the University of California, Berkeley, the Founding Director of the Berkeley Center for Cosmological Physics and received the Nobel Prize for Physics in 2006 for his work on cosmic microwave background radiation. Smoot suggests that the unification of forces and concepts has been the major thrust of modern physics. This applies to the conceptual combining of mechanical classical physics and statistics to produce statistical mechanics, which formed the foundations of thermodynamics. The Standard Model also arose from a tradition of stepwise unifications, such as the classic unification of electricity and magnetism into electromagnetism by Maxwell in the 19th century. That was followed by the development of a theory that unified weak and electromagnetic forces in the second half of the 20th century and the Standard Model, which also included the strong force.
“There has long been a goal to include Gravity and make a more unified theory, which is sometimes referred to the Theory of Everything even though we now know that there must be or likely are some additional forces such as dark energy,” says Smoot.
Despite its successes, the Standard Model also has its limitations, and among its most important limitations is that it does not include gravity – the fourth of the fundamental physical forces – and that it also does not account for dark matter and dark energy, hypothesized forms of energy and matter that are thought to form the bulk of mass and energy in the universe and that explain key cosmological phenomena, such as the expansion of the universe.
Gravity is currently best described by the Theory of General Relativity, developed by Albert Einstein in the first half of the 20th century, which replaced Newton’s theory of gravity. Like the Standard Model, the Theory of General Relativity has been repeatedly tested and confirmed by experimental observations. It is also one of the most beautiful theories in physics. Steven Weinberg, who received the 1979 Nobel Prize in Physics together with Abdus Salaam and Sheldon Lee Glashow for their pioneering work in the development of the Standard Model, has described the beauty of General Relativity in his book Dreams of a Final Theory. Weinberg admits that one cannot precisely define what constitutes “beauty” in physics, but one key characteristic of a beautiful scientific theory is the simplicity of the underlying concepts. Despite the appearance of greater mathematical complexity in Einstein’s theory, Weinberg considers it more beautiful than Newton’s theory because the Einsteinian approach rests on one elegant and simple principle – the equivalence of gravitation and inertia. A second characteristic for beautiful scientific theories is inevitability. Every major aspect of a beautiful theory appears so perfect that it cannot be tweaked or improved on. Any attempt to significantly modify Einstein’s Theory of General Relativity would undermine its fundamental concepts, just like any attempts to move around parts of Raphael’s Holy Family would weaken the whole painting.
According to Weinberg, the final unified theory would not only comprise all fundamental physical forces but it would also embody beauty. This quest for unity and beauty may seem like an anachronism, perhaps even quixotic, considering that even after decades of tireless efforts by the world’s most brilliant physicists, it has not yet been possible to combine General Relativity and the Standard Model into a beautiful Unified Theory. Does this mean that it is time to abandon this quest and just accept the fact that there may be multiple distinct theories, such as General Relativity and the Standard Model that are all independently valid? Not according to many of the world’s leading physicists.
Smoot explains, “There are deep underlying reasons and symmetries that make us think that the unification of the forces is not only a beautiful and powerful construct but also necessary to the completeness of Physics.”
How does the concept of beauty in physics relate to other forms of beauty that we experience around us? Beauty in art, music, or literature often manifests itself as a desire of the audience to keep looking, listening, or reading. This form of beauty evokes a desire or longing in its audience. The philosopher Crispin Sartwell explored words for “beauty” used in six different languages, thus illuminating various aspects of beauty across cultures and languages. In his book Six Names of Beauty, Sartwell offers us the following words used to describe beauty and their approximate translations into English:
None of these words is necessarily the exclusive word used for beauty in any given language but it suggests that certain languages can emphasize distinct facets of beauty. A multi-lingual perspective may also help us discover our own deeper concepts of beauty as we find parallel concepts in other languages that may not be readily apparent. A native English speaker, for example, can easily relate “beauty” to light, harmony, wholeness and nobility but a deeper meditation may reveal that humility and imperfection are also aspects of beauty.
A concept of beauty which encompasses beauty, yapha, sundara, to kalon, wabi-sabi, hozho and the thousands of additional words used in other languages make it easier to understand that beauty in physics and beauty in art may be quite similar. The Unified Theory will represent a form of wholeness or completeness of the fundamental laws of physics, it will describe the harmony and balance between particles and forces, it will be expressed in the simplest possible terms, it is an ideal that scientists are striving towards and they hope that it will illuminate the fundamental mysteries of physics.
Frank Wilczek is the Herman Feshbach Professor of Physics at the Massachusetts Institute of Technology and received the 2004 Nobel Prize in Physics together with David Gross and David Politzer for their seminal work on the nature of the strong force. In addition to his work as one of the world’s leading theoretical physicists, Wilczek is actively engaged in communicating the core concepts of physics to a non-specialist audience via his website and several books. His most recent book, A Beautiful Question, is a profound exploration of the physics, aesthetics and philosophy surrounding the search for the Unified Theory. He also replaces the title Standard Model with the more appropriate term Core Theory. He avoids the hyperbolic and misleading expression “Theory of Everything” and instead points out that the Core Theory is a perpetual work in progress, continuously striving for greater beauty and unity. He is optimistic that in the near-future, the Core Theory will be able to accommodate the electromagnetic, weak, strong and gravitational forces as well as describe dark energy and dark matter.
One of the routes that Wilczek maps out for the evolution of the Core Theory is supersymmetry, the idea that when “force” particles (bosons) move into quantum dimensions, they become “substance” particles (fermions) and vice versa. This expanded (“super”) symmetry between bosons and fermions posits the existence of novel superpartner particles. Quarks and leptons, for example, would have squarks and sleptons as their hypothetical boson superpartners. Gauginos, on the other hand, would be hypothetical fermion superpartners of gauge bosons. The elegance and beauty of this model is that it would offer the possibility of including gravity in a possible unified Core Theory that includes hypothetical gravitons and their equally hypothetical superpartner gravitinos. One of the challenges of uniting all four forces is that gravity is by far the weakest of all the physical forces. But supersymmetry and the presence of the hypothesized superpartners, suggests that there exist certain high energy levels at which the different forces begin to approximate each other. Wilczek states that this would still not represent the complete unification of the forces, but it could be a major first step. However, Wilczek also clearly explains that the experimental evidence for supersymmetry and the hypothesized superpartner particles is lacking. The Large Hadron Collider (LHC) at CERN in Geneva that experimentally validated the Higgs boson in 2012/2013 may need to significantly ramp up its energy to generate these particles.
Another path toward a Unified Theory that is being actively pursued by theoretical physicists is string theory, also referred to as superstring theory and M-theory in its later iterations. A brief introduction to the concepts of string theory can be found in The Grand Design by Stephen Hawking and Leonard Mlodinow, and a far more comprehensive one is in The Fabric of the Cosmos by Brian Greene. Through string theory, these physicists are also able to achieve some degree of a unification of the fundamental forces on a theoretical level, but they invoke the presence of nine spatial dimensions (string theory) or even ten spatial dimensions (M-theory) in addition to one time dimension. Proponents of string theory view it as a major conceptual revolution of our concept of time and space, likening it to the paradigm shifts in our concepts of space, time and causality that were triggered by the theories of relativity and quantum mechanics one century ago.
However, there is one key difference that sets apart string theory from the theories of relativity and quantum mechanics. Even though the latter changed our view of the world, they did not fundamentally alter how we pursue science. Successful science always combines theories with empirically-testable hypotheses and predictions. Quantum mechanics, the special theory of relativity, the general theory of relativity and the Standard Model have all been validated by experiments. Supersymmetry has not yet been experimentally validated, but it makes claims that could be eventually be tested and confirmed if the proposed superpartner particles are found at the LHC. However, it is difficult to even conceive of experiments that could prove the existence of nine or ten spatial dimensions. A string of critical books has recently been published that attack string theory on its lack of clearly defined hypotheses and predictions that would allow for its experimental validations. Farewell to Reality by Jim Baggott, Not Even Wrong by Peter Woit and The Trouble with Physics by Lee Smolin question whether a theory without experimental validation can even be considered scientific or whether it has begun taking on the feature of a religious dogma.
To a certain extent, similar criticisms can also be brought forward against supersymmetry since there is no definitive experimental proof to back it up, just circumstantial evidence as described by Wilczek in A Beautiful Question. One response to critics of supersymmetry is that the proposed superpartner particles are so rare and require such high energy to generate that we may not yet have the necessary technology to even test the theory. The Higgs particle, for example, was predicted by the Standard Model several decades before the LHC was able to confirm its existence. It is therefore quite possible that the LHC will find evidence for supersymmetry in the next years as the energy levels are ramped up. But at what point does one abandon a scientific idea or theory as not validated by experiments, and how long does one continue to test it? And at what point does clinging to ideas that have not or cannot be experimentally validated become non-scientific? The search for the Unified Theory has raised profound questions not only about the nature of matter and energy but also the definition of science and the scientific methodology.
I recently asked Frank Wilczek whether he felt that supersymmetry is still a prime candidate for a Unified Core Theory:
I’m still optimistic about ‘low-energy,’ i.e. accessible, supersymmetry, since the circumstantial case for it is very strong. The Large Hadron Collider (LHC) will have a good shot at it in the next few years. So far they haven’t seen any direct positive evidence, however, which has been disappointing for many people. For a long time, I’ve been worried that the LHC might not have quite enough energy for the job.
Wilczek believes that the current energy levels at the LHC may be able to generate gauginos, the superpartners of gauge bosons, but that the generation of squarks and sleptons may require higher levels of energy. Wilczek agrees that the testability and experimental validation of supersymmetry is essential for it to become part of a Unified Core Theory. For Wilczek, experimental testability is an essential part of a theory’s beauty. A Unified Theory also needs to address other important issues that are not yet resolved by General Relativity and the Standard Model, such as the question of dark matter and dark energy.
Unified Theories’ that do not address these questions in a satisfying way don’t deserve the name. String theory, in particular, falls short. (Frank Wilzek)
Does one need string theory or M-theory to develop a Unified Theory?
Wilczek is convinced we can make important progress at present without string theory or M-theory but cannot rule out that they may prove crucial in the long run. “They have a lot of potential, but they haven’t yet proved themselves to be essential parts of the description of Nature. It’s not clear, at least to me, what problems they’ll solve, or even what they are exactly.”
Wilczek also points out that there are several other exciting questions in physics that will attract the next generation of physicists, some of which he recently outlined in an article “Physics in 100 years.” The search for a Unified Theory, the validation of supersymmetry and understanding dark matter or dark energy will be complemented by great discoveries in gravitational wave astronomy, quantum computation and many other areas of research.
This also relates to an important point made by the Nobel Laureate Robert Laughlin in his book A Different Universe. An excessive focus on identifying the elementary forces and particles brings about a culture of extreme reductionism, assuming that the whole universe can be explained by knowing the building blocks. Taking apart a Lego sculpture and then laying out all the colored Lego bricks in front of us does not really tell us much about the beauty and splendor of the sculpture. Laughlin hopes and predicts that the Age of Reductionism will soon be replaced by an Age of Emergence in which the focus gradually shifts toward understanding how complex structures arise from the fundamental particles and atoms identified during the Age of Reductionism. Just like the simplicity of reductionism, the complexity of emergence also embodies beauty. Instead of heralding the end of physics (as is implied in expressions such as “Theory of Everything”), Unified Theories are just a starting point for the more complex questions in physics.
The evolving Core Theories of physics exemplify the human intellect and spirit and have challenged our perceptions of reality. We often marvel at the splendor of palaces and monuments, but the enduring legacy of any civilization can be found in its ideas. While all the seven wonders of the ancient world have crumbled except for the pyramids of Giza, many of the ideas of ancient scholars have survived, and their magnificence continues to inspire us even after millennia. Even though the great discoveries in physics were often spearheaded by few scientists, they ultimately represent a collaborative effort of several thousands of scientists who worked together to experimentally validate theories and, to a larger extent, also a conjoined effort of all humankind who directly or indirectly contribute to these efforts, perhaps even by allowing public funding to support the heroic struggles.
If we want to establish a legacy of our civilization that truly matters to our future descendants, it will not be achieved by erecting huge monuments but by expanding efforts to find simple and complex beauty in our universe. The past century of basic research in physics has been among the most fruitful periods in the history of any science, but it has also reminded us that we need to invest resources and provide continuing support for it to thrive. The universality of natural laws and the spirit of scientific collaboration across cultural and political boundaries may ultimately serve as a model for a more peaceful human co-existence. In a world that is too often polarized by political and religious ideologies, the search for a unified theory of the fundamental physical forces encapsulates the greater human quest for beauty. We may not always agree on what constitutes beauty but humans in all cultures are united in their longing for beauty. The arduous pursuit of beauty itself embodies beauty and is an opportunity for all humankind to share in the joy of this journey.