The ultimate aim for theorists is to write down a "theory of everything", which would describe all the known forces of nature using the same set of equations. But one of the major hurdles facing them is gravity, which is the only force not included in the Standard Model of particle physics. This is partly because gravity, as described in theories by Newton and Einstein, is many orders of magnitude smaller than the other forces – a curious discrepancy known as the hierarchy problem.

Some cosmologists have suggested that the hierarchy problem would disappear if we were to assume that our universe's four familiar dimensions – three for movement in space, and one for time – form a single "brane" in a higher-dimensional bulk. In this braneworld, the three forces described by the Standard Model would act along our brane in the normal way. Gravity, however, would be able to spread throughout the bulk, leaving us to observe just a fraction of its attractive force.

In the most popular braneworld models, the extra dimensions are distorted so that they occupy volumes up to a millimetre in size. Therefore to catch a glimpse of them one must look for deviations in Newton's gravitational inverse-square law at equally small scales. In January this year Dan Kapner and colleagues of the University of Washington in the US used a torsion-balance experiment to prove the law holds down to 55 µm – small enough to make the idea of extra dimensions less plausible (see related story: "Pendulum swings away from dark energy"). However, Dimitrios Psaltis from the University of Arizona now claims to have used the age of a black hole to put a new limit on the theoretical size of the extra dimensions, which might explain why they haven't been found yet.

Black holes are often expected to have an extremely long life because their rate of evaporation is fairly slow. But if extra dimensions do exist then some of this evaporation could take place in the bulk of the braneworld, and so would proportionally reduce the black hole's lifespan. Psaltis took recent measurements of the 3D velocity and position of the black hole XTEJ118+480 to reconstruct its trajectory. This in turn allowed him to calculate when it last passed the galactic plane, and thus the earliest time the black hole could have been formed.

Psaltis found that XTEJ118+480 must be older than 11 Myr, which in braneworld terms means the extra dimensions could extend over volumes no larger 80 µm. Although this is not quite as small as the experimental limits set by the Washington team, it does shrink the target area for future tests of the inverse-square law.

However, Ruth Gregory of Durham University, who has also researched the cosmological effects of a braneworld, told *Physics Web* that the equation Psaltis used to calculate the lifetime of a black hole is not based on hard theoretical ground. "I certainly won’t be taking it to the bank," she said. "The main problem is that we don’t know what a black hole looks like on a brane. But it is good that experimentalists are taking observations to put the theory to the test."