Albert Einstein is known for his haircut, theories of relativity and belief that “the fact that [the physical world] is comprehensible is a miracle.”
What he meant was that via science, math and our own neurons, humans can deduce physical laws that the universe seems to obey. Those laws explain the phenomena we see around us—bulbs lighting up, hammers coming down or atoms sticking together and splitting apart—and let us predict future events such as the merging of galaxies, explosion of stars or creation of extreme conditions in particle accelerators.
But even with these laws and a lot of expertise, scientists don’t truly comprehend the universe yet—they’re not even close. What is dark matter, the invisible substance that serves as gravitational scaffolding for galaxies, or dark energy, the enigmatic force that powers the accelerating expansion of the universe? Both terms share their common gloom because physicists (and everybody else) are in the dark about whatever’s behind them. But such mysteries only add urgency to the incremental quest for a fuller understanding of what makes the cosmos tick.
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Some physicists believe this fuller understanding might involve a “theory of everything” (TOE): a single underlying theoretical framework that governs the universe. Other physicists, meanwhile, don’t believe the universe is quite as comprehensible as Einstein implied, and, in their opinion, this makes the search for a TOE a waste of time.
Both sides agree that humans won’t ever find a theory of everything everything. No matter how successful a TOE might be at explaining the universe from first principles, it is unlikely to ever account for why you prefer extra pickles on your cheeseburgers or have an irrational fear of clowns. When physicists wax poetic (or shake their fists) about a TOE, they mean something very specific. “What they’re talking about is unifying all the forces of nature into a single one,” says physicist Katherine Freese, a professor at the University of Texas at Austin.
To date, scientists have uncovered just four such fundamental forces. “There’s electromagnetism,” Freese continues. “So electricity and magnetism—everybody knows about those.”
Everybody also knows about the force that makes you fall and embarrass yourself: gravity.
The remaining two are more obscure: the strong force binds protons and neutrons together within atomic nuclei, whereas the weak force helps atoms and subatomic particles to fall apart via a form of radioactive decay.
Developing a single theoretical framework that brings those forces together—by describing them as manifestations of one larger force—is a physicist’s narrow version of the “everything” in a TOE.
Still, “the unification of the four fundamental forces, if verified experimentally one day, will be admirable and a great feat—but it will be far from the TOE, the truth of the universe,” says Demetris Nicolaides, a theoretical physicist at Raritan Valley Community College and author of the book In Search of a Theory of Everything: The Philosophy behind Physics. But, hey, a human’s got to try.
Scientists have good reason to think they can form a theory to at least describe their limited “everything.” After all, some unification has already occurred: physicist James Clerk Maxwell brought light, electricity and magnetism together more than 100 years ago by defining them as individual features of the larger force of electromagnetism.
The weak force was the next to join the force family, after scientists developed high-energy particle accelerators. Inside these devices, particles can collide at nearly the speed of light. “It’s effectively probing the universe at higher energies, which corresponds to going to earlier in the universe,” Freese says. The greater the energy of a collision, the closer it may come to replicating the almost incomprehensibly hot and dense conditions thought to have prevailed in the early moments after the big bang. When scientists access such “young cosmos” states with particle accelerators, they see electromagnetism and the weak force acting as one single force—the electroweak force—suggesting that in the early universe, these two forces were one.
Freese suspects the strong force would join them if particle accelerators could reach energies high enough to simulate the even hotter, even younger universe in which the particles mediating the strong force would appear. But the technology almost certainly won’t improve enough in our lifetime to accomplish this, she says.
Wrangling the final (and, surprisingly enough, weakest) force, gravity, is a much harder task: Electromagnetism, as well as the strong and weak forces, can be shown to fundamentally follow the strange-but-calculable quantum rules. Yet gravity is, at present, best described by Einstein’s general theory of relativity, which concerns the universe at larger scales. These two frameworks do not play nice with each other; quantum mechanics and relativity effectively dictate separate and contradictory rules for the cosmos. Quantum theory typically deals with the universe in tiny chunks, or quanta, while general relativity takes the cosmos to be continuous even at the smallest scales.
“The paramount challenge in finding a TOE is finding a successful quantum version of gravity, that is, to combine the rules of quantum theory with the rules of Einstein’s theory of general relativity—or to find new rules completely,” Nicolaides says. Until scientists have a theory of quantum gravity, they’re likely to meet with little success in uniting gravity with the other three forces.
As always, theorists have some speculative ideas. One is called loop quantum gravity, which posits that space is made up of tiny, indivisible pieces. Under this theory, spacetime itself would become quantized, which would allow scientists to understand the behavior of large-scale spacetime through a quantum lens. There’s also string theory, which describes the universe as made of almost unimaginably small vibrating strings and, in current versions, postulates the existence of at least 10 dimensions. In this theory, vibrating strings would create gravitons, tiny particles that act under quantum mechanical laws but carry gravitational force. “String theory raised hopes in the 1980s,” says Carlo Rovelli, a prominent proponent of loop quantum gravity who holds a visiting research chair at the Perimeter Institute for Theoretical Physics in Ontario. But it’s not a decent TOE candidate, in his view, because it doesn’t have the best track record. “It has not delivered after half a century,” Rovelli notes. (To be fair, loop quantum gravity hasn’t exactly brought home abundant bacon, either.)
Although Rovelli works on quantum gravity, he thinks searching for TOEs is futile. “There are plenty of open questions that we do not know how to answer, and I think it is more realistic to try to solve them one at a time rather than trying a single theory of everything,” he says. “Also, ‘everything’ is far too much. The world is complex and is better approached with a multiplicity of theoretical tools.”
There’s also the rather bleak view, espoused by Nicolaides and others, that a TOE—one that is even broader than physicists’ definition of such a theory—must exist somewhere out there, but humans might not ever find it. And even if we do, “everything” would still not be truly everything. “We could, at least in principle, know the cause of every phenomenon but one,” he says. “We could not know or explain the most interesting of the phenomena: why there is something instead of nothing, why there is a ‘nature’ in the first place or ‘Why this nature with these laws? Why not some other type?’ Science can’t answer that.”
But scientists will undoubtedly keep trying to tiptoe toward unification anyway. “The approach physicists have taken to the universe is ‘simplify, simplify, simplify,’” Freese says. “If you can look out there, and you see ‘the wind does this’ and ‘the chair does that,’ and you can describe them all with a single equation, then you’ve gotten somewhere. And you can make predictions for what everything else is gonna do.” That, to make an understatement, has led to lots of major advances throughout history.
If physicists ever do suss out a TOE, the advances to emerge from it could perhaps profoundly alter the course of human history. Or perhaps instead a TOE would spark no major advances at all and would only offer breakthrough insights for realms and regimes so far removed from human experience as to be immaterial to everyone’s everyday lives. Freese, for one, remains optimistic: “It would change things the way that major fundamental advances always do,” she says. “You don’t know what they’re going to be until you get there”—which, of course, is something that physics can’t predict.