A new satellite is about to take on quite the challenge after it reached space on Monday, with plans to test one of the fundamental principles of physics at the very core of Einstein’s theory of general relativity.
The craft, called MicroSCOPE, is a joint French-European mission that aims to test the equivalence principle, also known as the "universality of free fall." The principle states that two objects fall with the exact same acceleration in the same gravity field. We already know this to be correct with an accuracy of 3 parts per 100,000 billion.
MicroSCOPE will push the limit to one part in 10 million billion. Once in orbit, the satellite will release two different pieces of metal – one titanium and the other a platinum-rhodium alloy. The two objects will be in perfect and permanent free fall, shielded from the perturbations one may encounter on Earth.
The most famous test of the principle in popular culture is probably when Galileo dropped weights from the Leaning Tower of Pisa (although it might have only been a thought experiment). Nonetheless, many have tried to look for a difference without success.
In Einstein's reformulation of the theory of gravity, he observed that the gravitational force experienced while standing on a massive body is equivalent to the pseudo-force generated in an accelerated frame of reference.
Everything on Earth experiences an acceleration towards the ground of about 9.81 m/s2. If you were suddenly put in a spaceship with no windows in deep space and moving at that acceleration, you wouldn’t be able to work out that you’re not on Earth anymore. Even if you had the most sophisticated technology, without external knowledge, gravity and dynamic acceleration would appear identical.
If MicroSCOPE finds a difference between them, it would point to a severe limitation of general relativity. But even if the principle holds, we are still looking for an explanation of where it comes from. Only when relativity and quantum mechanics are put together in the grand unified theory will the existence of this principle become clear.