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ESA to look for the missing link in gravity

September 11, 2002

Although you can never be certain of predicting future developments in science, there is a good chance of a fundamental breakthrough in physics soon. With a series of unique experiments and missions designed to test our understanding of gravity, the European Space Agency (ESA) hopes to get to the very bottom of it.

Scientists will study space phenomena that do not seem to conform to our perceived understanding of gravity. In this way, they hope to develop a greater comprehension of the Universe.

Gravity is one of the four fundamental forces of nature. It shapes the Universe around us, allowing planets, stars and galaxies to form. However, the more scientists study gravity and its effects on celestial objects, the more mysteries they seem to uncover. One example is the so-called `Pioneer anomaly`, named after the NASA space probes Pioneer 10 and 11, on which the effect was first noticed. The anomaly was revealed when a number of spacecraft were seen to be affected by an unknown force that slowed them down. The same behaviour has now been detected on NASA`s Galileo and the joint ESA-NASA Ulysses spacecraft.

Scientists have known for a long time that there appears to be `too much` gravity in the Universe. They can observe the effects of gravitational forces at work, but the origin of these forces cannot be identified. This `excess` of gravity is usually referred to as `the missing mass problem`, since scientists assume that only matter can create gravity. It is therefore supposed that the Universe is filled with large quantities of `dark matter` that has yet to be detected. What if that assumption is wrong?

Some theories suggest that gravity might pull a little harder at extreme distances than had previously been considered, so the concept of dark matter may not even be necessary. Alternatively, the anomalies may be the result of a fifth force of nature: one that is very weak and only shows up in the remotest regions of space. Space is an ideal testing ground to examine the existing theories. In the apparent weightlessness of space, scientists can detect the most delicate of forces and can measure them with extreme accuracy.

Developing an ambitious series of space experiments and missions, ESA is focusing its efforts on testing Albert Einstein`s Theory of General Relativity, the most advanced description of gravity ever formulated. One of the first objectives is the detection of gravitational waves. General Relativity has predicted their existence but, so far, they remain undetected. These waves should travel through space like ripples on a pond. LISA, a joint ESA-NASA mission, will be the first space mission to attempt to detect such gravitational waves. Finding them would be the ultimate test of General Relativity.

A second objective, to be tested by the ESA Gaia and BepiColombo missions, will be to measure precisely how matter distorts space, searching for any deviation in the amount predicted by General Relativity. Microscope, an ESA mission carried out in coordination with the French National Space Agency (CNES), is designed to test a concept from General Relativity called The Principle of Equivalence. According to this, objects are accelerated by gravity in the same way, independent of their mass and chemical composition. If Microscope detects a violation of this principle, it could be the clearest sign yet of a new dimension to gravity, known as quantum gravity.

Quantum gravity is a much-sought-after theory. Its purpose is to reconcile Einstein`s General Relativity with quantum physics, the most advanced theory describing the fundamental forces in Nature, with the exception of gravity. Quantum gravity supposes that space is granular on the smallest of scales. In a similar way, for example, a beach appears smooth from a distance but is actually composed of individual pieces of sand. Hyper, a mission currently under study at ESA would attempt to detect the quantum granularity of space, as one of its investigations into gravity. Looking further into the future, ESA has taken the first steps in defining a mission which would examine directly the Pioneer anomaly.

With this series of missions, ESA will carry out a unique investigation into the very nature of gravity. This may well provide the next fundamental breakthrough in our understanding of the Universe.

European Space Agency (ESA)