The Universe’s extra bits

source: CERN Bulletin

Parallel universes, unknown forms of matter, extra dimensions….This is not cheap science fiction but very concrete physics theories that the scientists are trying to confirm with the LHC and other ongoing experiments. Although it's enough to make us dream about going to a parallel world for the weekend, let’s keep our feet firmly on the ground and try to work out what all these things really are…

Given the astonishing fact that 96% of the Universe is actually unknown, we can think of filling it with all sorts of weird and exotic things. Extra dimensions and parallel universes may indeed be real, that is, their existence is accepted by a large community of scientists who have worked out mathematical models and physical constraints. “The idea of a fifth dimension was first introduced by Kaluza and Klein at the beginning of the last century in an attempt to unify gravity and electromagnetism”, confirms Ignatios Antoniadis from CERN’s Theory Group. “I personally got involved in the study of extra dimensions in the 1990s, when I was working at the Ecole Polytechnique in Paris”. 

Today, we know that extra dimensions could hide new forms of matter and energy as well as long-expected – yet undiscovered – particles, such as the graviton or the constituents of dark matter. Unfortunately, based on our current knowledge of Nature, scientists are not able to predict the exact number of possible extra dimensions. Some theoretical models allow for just one or two dimensions beside the four (three for space + time, according to Einstein’s relativity theory) that we experience every day, while the so-called string theories go as far as predicting the existence of 6 or more additional dimensions. “All the current theories that model the extra dimensions are equally valid and need to be tested with experiments”, says Antoniadis.

Scientists believe that there could be two different types of extra dimensions: one in which light can propagate (we will call it ‘electromagnetic’) and another in which light cannot propagate and with which we can only interact gravitationally (we will call it ‘gravitational’). Strictly speaking, the ‘electromagnetic’ type would still be part of our Universe as the type of interactions and the behaviour of light would be exactly the same. Therefore, parallel universes could only exist in ‘gravitational’ extra dimensions, those in which the photon cannot propagate.

The common feature of the two types of extra dimensions is that they must be finite and small, otherwise we would have already observed them. “The smaller the extra dimension, the higher the energy we need to probe it”, explains Antoniadis. “‘Gravitational’ extra dimensions could be much larger (up to almost a millimetre), because in this case the persisting lack of information we have about them could be due to the fact that our experimental apparatuses can only interact with these dimensions through gravitation, a very weak force at the particle scale in our dimensions”. 

The LHC is producing particle collisions at 7 TeV, a very high energy, which will even increase to 14 TeV after the long machine shutdown planned for 2012. Providing such a high energy to particles could enable them to enter the extra dimensions, interact and then return to our dimensions to enter the detector and leave a track that would carry the information of the travel. “Both types of extra dimensions can potentially be probed by experiments at the LHC”, confirms Antoniadis. “The high-energy proton-proton collisions could produce gravitons that would travel not only in our dimensions but also in possible extra ones. In the gravitational type of extra dimensions, the strength of gravitation would be much higher than in ours and therefore the graviton would most likely disappear from our dimensions and stay in the extra ones. This would result in some missing energy– carried by the escaping graviton – in the detectors. This missing energy would be associated with other specific features and characteristic bits of information that would uniquely identify it as an escaped graviton. As for the ‘electromagnetic’ types of extra dimensions, they could be probed even more directly because particles can enter them very shortly, leaving a clear signal in our detectors that would directly confirm their existence.”

For the first time, the LHC experiments are collecting data at energies that can potentially enable scientists to probe extra dimensions. However, before booking our trip to the closest parallel universe, let’s allow a couple of more years for their data analysis to provide us with conclusive results!

by CERN Bulletin