A Theory of Everything
When I was at school, it all seemed so simple. The physical world, at the smallest level, was made up from atoms. Atoms were like the solar system, with electrons whizzing around a nucleus, and this was a system just like the world we knew at the macro level, where planets whizzed around the sun. Now, we knew there were some complications. Electrons had their orbits, and they could jump from one orbit to another, and it appeared that those jumps were carefully scripted, so they could only go from one defined level to another, as if you were going from one level of a building up to the next. And, yes, there was another complication, as it turned out that in the nucleus there were two things: neutrons and protons. The protons had a positive electromagnetic force, while the electrons travelling around them had a negative force: the only reason the electrons didn’t hurtle down towards the protons and annihilate each other is because they were travelling in their orbits at speed: just like an aeroplane not falling to earth because it travelled fast enough to ensure it created lift? Well, not quite like that, but it would do. Those other items in the nucleus were neutrons, and they were called that because they didn’t have an electric charge.
There was more. In a way I found exciting at the time, this model of the nature of the physical universe also made sense of lots of chemistry, and from there on to many other things. Atoms could be linked together to form molecules. Some molecules were ‘stable’, like oxygen, which in our daily lives compromises two oxygen atoms linked together to form the O2 molecule (and later I learnt these was another stable form, O3, ozone, which was the reason you could smell something funny by the tracks of electric trains in the London Underground). Then we went on to compounds, like hydrochloric acid, which was a combination of hydrogen and chlorine, and this was interesting because it was really in two parts. Part of the molecule could break away and link up with another substance, and that would lead to other combinations like sodium chloride (the sodium element combined with the chlorine bit from hydrochloride acid. It was like the parts in a Meccano set!
Of course, it all got complicated, and eventually scientists were taking substances apart and discovering they were made up of very complex combinations of atoms, often several, sometimes even scores and even hundreds of atoms. However, it all made sense. However, I think all that was falling apart long before I was at school, although I didn’t know it at the time. Although it was somewhat beyond my schoolboy science classes, at least until I reached the final years of secondary education. Somewhat later I was to confront the science of what’s truly fundamental, and the amazing world of theoretical physics. At that point all my schoolboy knowledge was cast aside, and I learnt that our physical reality is shaped by a bewildering and complex world of particles, fields, together with many laws and rules that nature played by.
Where was all this leading? I was off on a different path by the time I was well into my university studies (I’d abandoned science for social anthropology), but even back then I was aware and know much more clearly today, our understanding of ‘reality’ remains limited and incomplete. Despite this the animating hope of many scientists today and throughout history) is that we will be able to formulate a ‘Theory of Everything’, (with that marvellous acronym TOE) where one set of universal equations and one framework will describe literally every aspect of our physical reality.
When most of us think about science, we don’t often think about something very fundamental to the enterprise: what the goal of it all might be. Clearly reality is a complicated place, and the only tools we have to guide us in understanding the nature of our world rely on what we can observe, measure, and test through experiments. When we take account of that huge body of observational and experimental knowledge, we have a record of all the phenomena that we know exist. The enterprise of science, then, seeks to make sense of the huge body of empirical data, and then seeks to explain it in as simple and conclusive way as possible, to maximize our predictive power concerning natural phenomena, doing so with as few assumptions which seem absolutely necessary.
As many writers today remind us, we have seen incredible advances in our understanding of the physical world when compared to what we understood when I was at school. Now it appears we can analyse just about everything we can directly detect and measure, and do so precisely, even exquisitely. The ‘Standard Model’ of elementary particles lists four key influences that underpin our world, the electromagnetic, strong nuclear, and weak nuclear forces along with general relativity and gravity. Then there is the inflationary Big Bang which describes our cosmic origins, when those four key forces first appeared, only to evolve and become independent. It makes for a compelling story. Unfortunately, current mysteries like dark matter, dark energy, and the baryogenesis puzzle to do with asymmetry together hint that there’s more to the Universe than we currently understand.
The elusive goal that motivates many scientists is the belief tall of these key forces can be brought together into a ‘Theory of Everything’. However, despite its fascination, some argue that there is not a Theory of Everything out there to be found at all, that the goal is an illusion.
The modern idea of a Theory of Everything goes back more than 100 years, to the early days of general relativity. Einstein was able, starting in 1915, to successfully describe the observed phenomenon of gravitation. The presence, distribution, and motion of matter and energy through spacetime determined the curvature and evolution of that spacetime fabric, and then the curvature of that spacetime fabric determined the future trajectories and fates of every particle that exists within that spacetime. Put simply, general relativity took the idea of special relativity and unified it with the idea of gravitation, creating the powerful framework that many would argue was the most important of Einstein’s astonishing accomplishments.
When I was learning about science at school we were being taught about science prior to Einstein, with some brief references to what he had concluded. Before his theories there had been a different approach, Maxwell’s classical theory about electromagnetism, with four central principles:
- the speed of light was the ultimate speed limit at which anything could travel,
- particles and interactions could be described in terms of fields and charges,
- electromagnetism vs relativistically invariant, and
- energy and momentum were always conserved.
Maxwell’s (classical) theory put the previously distinct notions of electricity and magnetism together into a unified footing.
Within four years from the publication of Einstein’s theory of general relativity scientists were working to unify this theory with Maxwell’s principles. However, it turned out that despite some similarities the two theories also exhibited several fundamental differences. Despite this, it was the first 20th Century attempt at a Theory of Everything. Einstein’s general relativity was already a four dimensional theory (adding the dimension of time to our familiar three dimensional view of matter in the world), but Maxwell’s electromagnetism required four separate degrees of freedom in addition, meaning that the same four dimensions used in Einstein’s theory would be insufficient to hold general relativity and electromagnetism together in a single, unified framework.
Theoretical physicists weren’t discouraged, and attempted to solve the mismatch by taking a dramatic leap into a fifth dimension, allowing general relativity and electromagnetism to be unified. Alas, in a way that has become familiar with integrating approaches since then, there were some new inconvenient problems. The postulated fifth dimension couldn’t impact anything in our four-dimensional spacetime; it must somehow ‘disappear’ from all the equations that impacted the observable physical world. Moreover, scientists knew the universe didn’t merely conform to Maxwell’s classical electromagnetism, but required more, especially it required a quantum description for electromagnetism (at least), and other limiting postulates.
However, this was merely the beginning of formulating what would turn out to be many proposals that drew on extra dimensions. In one sense this was unproblematic, as in theory there could be more than three spatial dimensions to our Universe so long as those ‘extra’ dimensions were below a certain critical size that experiments had already explored. However, as soon as scientists began to talk about the notion of a Theory of Everything, their suppositions almost always required the addition of new entities — particles, fields, interactions, etc. — whose existence was already either ruled out or highly constrained by observations, measurements, and experiments by known results. If there is a fifth dimension, it had to be so tiny and its effects so weak that it would not affect the body of data scientists had already collected and which revealed no evidence for its existence.
The quest for a Theory of Everything was to lead to enormous advances in physics during the 20th century, in nuclear physics, quantum physics, and particle physics. The combination of novel experimental results and new theoretical developments has helped us understand what appear to be the full suite of particles that exist in the Universe, what rules they followed in interacting and binding together, and how the forces that governed them behaved. The result today is the Standard Model of elementary particles, simultaneously simple and contradictorily, full of complexities.
As a schoolboy I learnt about atoms and their building blocks, the trio of protons, neutrons, and electrons. Rather, now the electron is just the lightest of three generations of charged leptons: along with the muon and tau lepton. Then there are their antiparticles, plus a species of neutrino (and antineutrino) that is the corresponding ‘uncharged lepton’ to each of the charged leptons. Confused? What’s more, protons and neutrons are no longer considered fundamental particles, but are composite particles composed of quarks and gluons. Guess what: there are three generations of quarks, with the up-and-down quarks (making up the first generation) having charm-and-strange and then top-and-bottom quarks as their heavier-generation counterparts. Getting even more confused? Hang on …Meanwhile, there are eight massless gluons (mediating the strong nuclear force), one massless photon (mediating the electromagnetic force), and three very massive W-and-Z bosons (mediating the weak nuclear force), plus the Higgs boson to complete the Standard Model. Yes, it does seem confusing, but despite this veritable zoo of particles, every particle-based experiment performed, and every detector set up to observe particles ever concocted has only found evidence of these particles and these particles alone, with the properties given to them by the Standard Model framework.
It’s not surprising to read that many have sought — and are still seeking — the elusive Grand Unified Theory, a theory of everything, one that includes gravity, string theory and additional symmetries, additional dimensions, additional extra particles, or additional unification frameworks. It seems in confronting these ideas there’s an enormous amount of trouble. All of the new ideas necessitate adding further ingredients to our reality: ingredients which can lead to new interactions or decays of the particles we already know about. However, we already have masses of data on how the known (Standard Model) particles interact and decay (or appear forbidden from interacting or decaying), we have to take extreme care that any attempt toward a Theory of Everything doesn’t conflict with already-existing data, particularly with the data we have from particle physics experiments.
One popular approach is string theory (and positive geometry). Instead of one extra dimension, there are many: at least six and as many as 22 in addition to the four we know about. Instead of relying on such esoteric behaviours as magnetic monopoles, extra Higgs sectors, superheavy bosons admitting proton decay, and left-right symmetric features, they have even more. Instead of space, there’s superspace; there’s supergravity; there’s not just the conventional ‘for every Standard Model particle, there’s a superpartner particle’ version of supersymmetry, leading to suggestions there are four new super symmetries and hundreds of additional new particles. It seems as though, by adding more and more and more and ingredients, ingredients that aren’t reflected in observations we grow and worsen, the puzzles we’re facing when it comes to the Universe today.
From the outside, and looking at this confusing array of developments, there’s one obvious question that haunts the scientists: do our theoretical ideas line up with reality? When we formulate attempts at a Theory of Everything, it is important to remember the goals of science are working “to maximize our predictive power of nature’s phenomena with as few assumptions, parameters, and variables as are absolutely necessary” Our current big scientific mysteries compel us to keep seeking truths about the Universe, given many aspects of reality that we cannot yet, fully explain. But relying on loose, superficial analogies and mathematical ingenuity is more than dissatisfying; it’s an approach that loses a fundamental connection with observable, measurable reality.
Unsurprisingly, there are many critics. Paul Davies, (in Schrödingers’s Cat Flap, The Monthly: December 2026) offers a nice if quixotic comment on this state of affairs:
“In a famous remark, Albert Einstein once asked whether the Moon continues to exist when nobody is looking. This startling comment stemmed from Einstein’s deep distrust of a branch of physics called quantum mechanics, the mind-bending theory that brilliantly describes the atomic microworld. Now celebrating its centenary, quantum mechanics is the most successful scientific theory of all time. It accurately explains the behaviour of matter from subatomic particles to stars, and has given us the laser, the transistor, MRI machines, superconductors, AI and much more. Although quantum mechanics underpins much of modern technology, the foundations of the theory make no sense, shredding our everyday notions of reality and defying intuition. A century on, scientists remain deeply divided over what to make of it.”
What is this powerful theory that brings such practical benefits yet appears perplexing and paradoxical? In the mid 1920s scientist found the quantum microworld is riddled with uncertainty. In itself, that is not so troublesome. We are, after all, familiar with uncertainty in daily life. Suppose you toss a coin and keep it concealed between your hands: will it show heads or tails? It’s fifty-fifty: you can look to find out which. The fact that you didn’t know before looking which side of the coin faced up doesn’t affect the fact that it must have already been either heads or tails. Your observation merely uncovered a pre-existing reality. Quantum uncertainty, however, denies that there is a pre-existing reality. Instead, atoms, molecules and subatomic particles don’t actually possess well-defined basic properties, such as position or orientation or speed, in the absence of an actual observation. You can measure, say, the location of an atom and find it to be somewhere. But that doesn’t mean the atom was already there before you looked. Quantum mechanics says asking where the atom was an instant before inspecting is not only pointless, it’s meaningless.: “there is simply no fact of the matter of where the atom was located – a philosophically startling assertion.
In exploring the world of quantum theory and its applications, Davies ends with more philosophical problems. “Is there a real world out there after all, even among atoms and molecules? Or is the unobserved microworld suspended in a state of existence limbo? There are a dozen or so rival attempts to make sense of quantum weirdness, ranging from invoking consciousness to adding new physical processes that collapse superpositions spontaneously into a single reality. But the most widespread attempt to make sense of the theory is to treat the alternative realities in a quantum superposition as “really real” parallel worlds. … Outlandish though the multiverse idea may seem, many distinguished physicists buy into it. … So, does the Moon exist when nobody is looking? A many-worlds advocate would answer yes, but with a vengeance: not only does the Moon exist, but there are also countless versions of the Moon, each existing in a separate branch universe amid an infinity of parallel realities. It is a conclusion that would have Einstein spinning in his grave.”
