“There are some theories of extra dimensions that you can’t probe with the large scales that we worked with”
around the world are looking for physics beyond the Standard Model. Another is the search for dark matter, the mysterious shadowy entity that acts like a glue holding large structures in the universe together. “There’s nothing in the Standard Model to explain that,” says Tina Potter from the University of Cambridge, UK.
She is working on the ATLAS experiment at the LHC in the hope of finding evidence of a theory called supersymmetry – the most popular theory of physics beyond the Standard Model. It says that every particle in the Standard Model has an as-yetundiscovered partner. That instantly doubles the number of particles physicists are searching for. “Some of these particles are fantastic candidates for dark matter – so if we find them it would plug one of the big holes in our knowledge about the universe,” Potter says.
It isn’t the theory’s only appeal – some versions of string theory also rely on supersymmetry being true. Those that do are called supersymmetrical string theories – or superstring theories. There are five versions in total. However, in order to get the maths to work, all five superstring theories require there to be ten dimensions. That’s the three of space and one of time we experience, plus six so-called ‘hyperspace’ dimensions that we don’t. Those hyperspace dimensions would be small enough to remain out of sight. An alternative theory known as M-theory suggests that all five superstring theories are variants of an 11-dimensional reality. These superstring theories became fashionable in the 1980s and overtook the original version of string theory in popularity. That’s called bosonic string theory, and it suggests the strings vibrate across a staggering 26 dimensions!
Finding evidence of supersymmetry won’t in itself confirm superstring theory, but a failure to do so would make the whole idea even harder to stomach. In that regard the LHC may be able to help. It has just reached the end of its latest run of particle-smashing experiments, producing a swarm of important data for physicists to pick over.
“We are hoping to find something there,” says Potter. “It’s all hands on deck to analyse it as quickly as possible.” Meanwhile, the LHC will be shut down in December 2018 and upgraded ready to produce even more energetic collisions in around 18 months’ time. “Next time we turn on we’ll double the amount of data we have now,” Potter says. “Any hints of supersymmetry we see in this dataset will be confirmed in the next one.”
So slowly but surely physicists are making headway. Larger additional dimensions appear ruled out by the latest gravitational wave discoveries, but there remains hope that experiments such as the Large Hadron Collider could confirm supersymmetry and bolster the somewhat flimsy circumstantial case for string theory with its smaller hyperspaces. As always in physics, more experiments and better data bring sharper insights. They could one day prove that Nature paints on a canvas of many more dimensions than meet the eye. Curiouser and curiouser indeed.
The ATLAS experiment at the Large Hadron Collider is just one way physicists are looking for evidence of supersymmetry
A neutron star merger known as GW170817 was picked up by the LIGO detector in 2017