Let’s not kid ourselves: everything we think we know now is just an approximation to something we haven’t yet found out.
That is the frustrating, exhilarating lesson history teaches us about fundamental theories of nature. Take Newton’s universal law of gravity. It did sterling service describing falling apples and orbiting planets for over two centuries, but eventually gave way to a “righter” theory – Einstein’s general relativity. Ditto the solidly intuitive outlines of classical mechanics: dig down to the level of subatomic particles, and we find them fogged in a haze of quantum uncertainty.
Advertisement
Quantum theory explains matter’s small-scale workings. General relativity describes the universe’s large-scale evolution. Each theory is very right in its own way, but has omissions and inconsistencies that convince us that they, too, are just placeholders for something better. A unified “theory of everything” would take us to places where quantum theory and relativity break down – beyond a black hole’s event horizon, for example, or the very first instants of the universe.
Heady stuff – and many a great mind has come a cropper in the chase. Einstein’s twilight years were largely spent in an isolated, fruitless quest for ultimate enlightenment.
Things haven’t moved on much. String theory, which seeks unification by replacing fundamental particles with tiny scrunched-up strings, has come – and not yet gone, despite much criticism of its lack of testable predictions. Rival approaches such as loop quantum gravity have sprung up, but also brought no breakthrough.
An ill wind for the next 60 years? Mike Duff of Imperial College London, who has been studying theories of everything since the late 1960s, is surprisingly chipper about how much longer it will take. “I’m sceptical of making predictions, ” he says, “but if pressed, I’d say it will take more than 10 years, but less than 100.”
Carlo Rovelli of Aix-Marseille University in France, a leading light in loop quantum gravity, is also optimistic. “Sixty years in the future we will have a theory of quantum gravity, in my opinion,” he says.
If so, that will be a beginning, not an end. Theories earn credibility only gradually. Some predictions of general relativity, such as black holes and gravitational waves, were fully appreciated and explored only in the 1960s. The existence of gravitational waves was finally confirmed earlier this year.
Complicating matters for a unified theory is that it will kick in at insanely high energies far beyond our wit to reproduce. Experimental proof, if any is possible, is likely to be subtle and indirect, perhaps taking the form of patterns imprinted in the cosmic microwave background – radiation left over from the big bang – for example.
For similar reasons, immediate practical benefits are unlikely – although never say never, says Rovelli. “I don’t see technological applications of the theory right now,” he says. “But nobody imagined GPS back in the 1970s when I started studying general relativity.”
So don’t expect any eureka moment – more a long, slow evolution. But equally, expect the unexpected. When Paul Dirac unified quantum theory and Einstein’s special relativity in the 1920s to provide a full description of the electron, he ignored his equation’s prediction of a second, almost identical particle. The discovery just a few years later of the positron opened our eyes to a whole new world that we have yet to explore fully – that of antimatter.
A theory of everything based on string theory, say, might prove the existence of a “multiverse” of further universes, vastly increasing the scope of what we can hope to know – or not. That’s all part of the fun of the chase, says Duff. “Most scientists are trying to become masters at the game,” he says. “Theoretical physicists are still trying to understand the game.”
Read more: We’ve seen the future, and it will blow your mind
New Scientist is 60 years old this week, so how will the next 60 play out? We look forward to how science and technology might upturn the world as we know it
This article appeared in print under the headline “What if… We found a theory of everything?”
