With the increasing convergence of particle physics, astrophysics, and cosmology, the European Space Agency (ESA) this week published a road map of space missions and technology development that it will aim for between 2015 and 2025. Besides testing the fundamental laws of physics, the program includes the search for gravitational waves and their sources, quantum mechanical experiments in space, the elucidation of the nature of dark energy and dark matter, and the search of antimatter in space.
Researchers in these fields are increasingly turning to space missions to answer fundamental questions in physics. They are in something of a catch-22 situation at the moment: To better understand the structure of the universe, they need a better knowledge of physics, but to get a better understanding of physics, they have to improve their understanding of the structure of the universe.
The cornerstones of fundamental physics, general relativity, which describes how masses interact through gravity, and quantum mechanics—or more specifically the standard model, a theoretical framework describing fundamental particles and their interactions—have withstood numerous tests. But these two pillars stand apart from each other, coexisting but not interacting. Many researchers hope that, in the clean environment of space, they may be able to narrow the gap between quantum theory and relativity and find common ground.
Missions described in the road map include:
- LISA Pathfinder. An ESA technology demonstrator to be launched in 2012 that will test the technology for the ambitious LISA mission, an ESA/NASA project for the detection of gravitational waves. LISA will consist of three spacecraft forming a triangle in space with 5-million-kilometer-long sides. Acting as a giant interferometer, LISA will detect deformations of space-time caused by passing gravitational waves--a key prediction of general relativity.
- MICROSCOPE. A collaborative project between ESA and the French space agency (CNES) that will test the equality between gravitational mass and inertial mass, known as the equivalence principle, with unprecedented precision. The equivalence principle is the basis of general relativity, and the sensitivity of the experiment could allow the detection of new forces that may help to unify quantum theory with relativity. The launch of MICROSCOPE is planned for late 2012.
- Atomic Clock Ensemble in Space. ACES consists of a new generation of atomic clocks to be placed aboard the international space station in 2013. The microgravity environment will allow precise measurements of the gravitational redshift caused by Earth, as well as the search for variations of fundamental constants over time.
- EUCLID. ESA is studying this mission for a possible launch in 2017 or 2018. It would carry a telescope and map the distribution of galaxies in the universe. The observation of oscillations of ordinary matter and of weak gravitational lensing would help to assess the amount of dark matter and dark energy in the universe.
Astrophysicist Martin Rees of the University of Cambridge, the U.K.'s Astronomer Royal, is somewhat sceptical about certain projects under consideration, both by NASA and ESA: "I genuinely think it is hard to justify very expensive space projects for just one fundamental number. I think [NASA's] Gravity Probe B and AMS-2 [the Alpha Magnetic Spectrometer - 02 shortly to be placed aboard the international space station] are very bad choices." Rees is more supportive of the information about fundamental physics that can be gleaned from more general missions such as Gaia, BepiColombo, and EUCLID that yield a rich flow of data of interest to astronomy and cosmology generally.