Many theoretical physicists are, at heart, fundamentally egalitarian. They believe that all of the universe’s inhabitants, from the invisibly tiny to the inconceivably large, should be bound by the same rules. It is a mindset that has proven not only fair-minded but productive: many of the most storied breakthroughs in fundamental physics have come about by bringing new phenomena under the jurisdiction of old laws.
In the 19th century, the Scottish physicist James Clerk Maxwell revealed that electricity, magnetism and the behaviour of light were distinct aspects of the same phenomenon, and could all be described with the same elegant framework. A century later, physicists like Steven Weinberg extended this unifying zeal yet further to include two more fundamental forces of nature into what is now known as the Standard Model of particle physics.
But the mission remains incomplete. Most confoundingly, the universe’s smallest scales are governed by quantum theory while its largest inhabitants obey the rules of general relativity. The two systems remain irreconcilable. Combining the two into a so-called theory of quantum gravity would be one of the richest prizes in the field.
In this week’s edition, we explore some of the approaches that may lead to such a breakthrough. Most require us to rethink the way the universe behaves. String theory, for example, whose mathematical elegance has been marred for decades by a lack of testable predictions, says that all the universe’s particles are born from the vibrations of tiny string-like entities. Entropic gravity, meanwhile, suggests that the fabric of reality emerges from a web of connections between the universe’s quantum particles. Others, with fantastical names such as loop quantum gravity and causal dynamical triangulation, make even greater demands on our imaginations.
To the majority of physicists whose work is of a more practical bent, such theoretical speculation is at best entertaining and at worst misguided. There is, after all, no reason to believe that a single “theory of everything” which could circumscribe the universe with one equation will ever be found. Even if one were to emerge, the “everything” it purports to describe would be sadly limited. Just as a knowledge of the American constitution, say, is not enough to exactly reconstruct the behaviours of 330m Americans, access to the ultimate laws of nature will leave plenty of room for surprises.
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