Like divorced parents during the festive season, physics is made of two beloved authorities who just can’t get on – general relativity and quantum mechanics.
For decades, many researchers have pinned their hopes of unification on something called string theory. On the up side it points to a curious connection between gravity and the behaviour of subatomic particles. On the down side … it’s string theory.
Like many physicists from all over the world, theorists from Princeton University have chased the dream of bringing together the estranged parents of physics for decades into what is known as a theory of everything.
As Princeton communications manager Catherine Zandonella explained recently, they have achieved some success, at least when it comes to a mathematical common ground of sorts.
“The key insight is that gravity, the force that brings baseballs back to Earth and governs the growth of black holes, is mathematically relatable to the peculiar antics of the subatomic particles that make up all the matter around us,” says Zandonella.
“This relatability between gravity and subatomic particles provides a sort of Rosetta stone for physics.”
This common ground isn’t quite the same thing as a marriage of bickering theories, but a means of translating relative languages could at least get them talking.
Unfortunately such insight comes courtesy of the string theory framework that continues to divide researchers – a mathematical model most of us have heard of, but few of us really grasp.
The starting place for all this mess has to do with infinities. If you plug numbers into a formula and fail to get a meaningful answer, your formula is effectively useless at making predictions.
Quantum theory gives us solid answers for most interactions between particles. Not only are the answers meaningful, they correspond neatly with experimental figures.
Unfortunately gravity is an odd duck. Right now we explain the weak attraction between distant masses by describing the shape of spacetime, according to Einstein’s theory of general relativity.
Combining the probability-based mathematics of discrete force-carrying particles with the spacetime background of general relativity just gives you nonsense in the form of an infinite answer. Like two people shouting past each other.
Having gravity on board with the two nuclear forces and electromagnetism would give us a simpler, all-encompassing theory that describes how our Universe emerges from a single operating system of physics.
One potential solution is to see all particles in an entirely different way, one that preserves their discrete, probabilistic nature while weaving in the 4D background of spacetime.
String theory does this by portraying particles as a single dimension that changes shape within multiple dimensions. Or, to put it another way, a line that ‘hums’ at a range of frequencies, which accounts for a particle’s unique characteristics.
This actually works for a range of things in physics. In the 1990s, three Princeton researchers applied string theory to the strong force particles that glue quarks together into protons and neutrons – particles humorously called gluons.
They found the same approach could also help them understand how black holes radiated energy.
A few years later, another Princeton researcher named Juan Maldacena developed a more general explanation of big things like spacetime-distorting masses and the field theory that gives rise to subatomic particles.
Twenty years on, string theory continues to inspire this shared language between arguing parents of physics, in the form of anti-de Sitter space/conformal field theory, or AdS/CFT correspondence.
“We hope to understand the singularity inside the black hole,” says Maldacena. “Understanding this would probably lead to interesting lessons for the Big Bang.”
Only recently, Stanford University physicist Leonard Susskind published his explanation of the expanding inner volume of black holes based on AdS/CFT correspondence.
As elegant as this model is, a nice idea needs to be exclusively observed in action for it to become a convincing theory. Especially when quite literally the bedrock of everything is at stake.
Even the founding father of string theory has his doubts on whether the mathematics represents something real, or is a shallow reflection of a deeper truth.
“My guess is, the theory of the real world may have things to do with string theory, but it’s not string theory in its formal, rigorous, mathematical sense,” Susskind recently said in an interview.
“The exact thing – which I call string theory, which is this mathematical structure – is not going to be able to, by itself, describe particles.”
String theory might get the parents talking, and might even show us the way to observations that connect general relativity with quantum mechanics.
But we’ve clearly got a long way to go to get the physics family together. It’s probably not going to happen in time for Christmas. Or any Christmas soon, for that matter.