Think about space-time as a foam. This is what physicists have done to reconcile two revolutionary theories of modern physics: general relativity, which explains large-scale gravitation, and quantum theory, which governs the behaviour of fundamental microscopic particles.

Essentially, microscopic particle "foam" models describe space-time as a granular geometric structure, as opposed to macroscopic bodies, such as planets, where no space time granularity occurs. From an observational point of view, it is a situation like that of a transparent bucket half-filled with sand. If you look at the bucket from far, you can't understand if it contains a fluid or a granular structure, but by coming closer (thus increasing the resolution with which we observe it), we notice the sand granularity.

For decades, this fascinating hypothesis of the space-time description was not experimentally proven, as the effects of the phenomenon are extremely small and difficult to detect. Now, however, a new study by Giovanni Amelino Camelia from the Sapienza University of Rome Physics Department, in collaboration with PhD student Giacomo D'Amico and Sapienza graduates Niccolò Loret and Giacomo Rosati (now at the University of Zagreb and at the University of Cagliari) produced an initial analysis of experimental data with outcomes that seem to underpin the space-time foam scenario.

The research, published in the latest issue of Nature Astronomy, used the data obtained from the Fermi Space Telescope, funded mainly by Nasa with the cooperation of space agencies in Italy, France, Japan and Sweden, and the IceCube Neutrino Observatory, located at the south pole and funded by the US National Science Foundation. The study’s statistical approach analyses all the data collected to date by the Fermi and IceCube observatories to determine the frequency of particle observations (photons or neutrons) with properties attributable to space-time foam.

According to some space-time patterns, the particles that reach us have a travel time that depends, albeit very weakly, on their energy. Finding evidence of this is made more difficult by the fact that the source properties, which are not yet understood, may in some cases mimic the effects of space-time foam. Only with statistical analyses that combine observed properties for a set of particles can one distinguish between effects due to source properties and effects due to space-time foam.

The Sapienza study shows that the data collected so far has the statistical properties that favour a space-time foam-based interpretation, rather than source properties, but the currently available statistical sample is not sufficient to draw any definitive conclusions. "With further data that over the next 4 or 5 years,” says Amelino Camelia, “we will be able to understand whether the specific space-time foam model we have analysed is true. Even if our hypothesis is wrong, it would still represent a significant step further in the study of space-time foam, allowing us to narrow the class of models on which to concentrate our efforts."

Further Information
Giovanni Amelino Camelia
amelino@roma1.infn.it