One of the most popular downloadable screensavers on the Web is SETI@home, which uses idle time on private PCs to analyse radio-astronomy signals in search of extra-terrestrial intelligence. The program has been downloaded by more than 5 million users, resulting in what is by many measures the most powerful distributed computer on Earth. Many similar projects have been launched in the past few years, and in February, as part of the Year of Physics activities, Einstein@home started operation.

This project is analysing gravitational wave data from the Laser Interferometer Gravitational Wave Observatory (LIGO) in the US and from the German-British GEO 600 gravitational wave observatory. The project is looking in particular at data coming from extremely dense, rapidly rotating stars, which are believed to be either quark stars or neutron stars. Some of these compact stars may not be perfectly spherical, and if this is the case they should emit characteristic gravitational waves that LIGO and GEO 600 should detect.

CERN has also been investigating this approach to distributed computing with a program called LHC@home. This was launched as part of the public outreach events for CERN's 50th anniversary last year, and was relaunched in February as part of the Year of Physics activities. LHC@home runs the program SixTrack, which simulates particles travelling around the Large Hadron Collider (LHC) to test the long-term stability of particle orbits. Since the particles in the LHC are relativistic, and indeed the whole purpose of the LHC is to use the famous equation E = mc2 to generate new particles, LHC@home provides a good starting point to engage the public in a discussion of Einstein's special theory of relativity. This is one of his three major papers published in 1905 with a centenary being celebrated in this Year of Physics.

Both Einstein@home and LHC@home are based on the Berkeley Open Infrastructure for Network Computing (BOINC), an open-source platform launched by the team behind SETI@home. BOINC benefits from the experience of SETI@home, in particular concerning issues such as computer security and ensuring that users are properly credited for their contributions (see CERN Courier September 2004 p62). This latter point is important because users are fiercely competitive about how much computer power they contribute, and this competition is part of what makes public resource computing so popular. The users also take considerable interest in the underlying science - the outreach part of the activity - and there is usually a lively discussion in the LHC Café, one of the project's user notice boards, which is moderated occasionally by people at CERN.

When LHC@home was made public last September the first 1000 users joined within less than 24 hours, and the project had to set a limit of 5000 users to ensure that the large flow of data being sent back could be handled adequately. The results so far are promising. Being able to increase computing power by one or two orders of magnitude, compared with what can be achieved with dedicated resources, will allow physicists at CERN to gain qualitatively new insight into the performance of the LHC, uncovering for example narrow regions of beam instability that would have been overlooked in coarser parameter searches. BOINC allows users to decide how much of their PC's idle time they want to apportion to each project. You can join Einstein@home and LHC@home, and indeed several other projects that run on BOINC, by going to

Compiled by Hannelore Hämmerle and Nicole Crémel