On Monday, April 25, French scientists launched a satellite that will directly test an important aspect of Albert Einstein’s General Theory of Relativity, modern physics' current model of how gravity works.
Formulated in by Einstein in 1907, the Equivalence Principle states that there exists no observable distinction between inertial mass and gravitational mass. Put another way, if you're standing in a sealed box and experiencing a "downward" force as though you were on Earth, there is no way to tell, from the force alone, if the box is actually on Earth or if it is inside a rocket, far from any planet, smoothly accelerating at 1g. Similarly, if you're floating weightless inside the box, experiencing no acceleration, there is no way to tell if the box is in deep space or plummeting toward Earth.
Einstein's Equivalence Principle helped him generalize his Special Theory of Relativity. Developed by Einstein while working as a patent clerk in Switzerland in 1905, special relativity describes how, because the speed of light remains constant regardless of how fast the light source is moving, observations of space and time can vary according to an object's velocity relative to the observer. Special relativity carries with it several implications that have become fundamental to modern physics, including Einstein's famous formulation of the equivalence of the properties of mass and energy: E = mc2 .
Special relativity works only when you ignore the effects of a gravitational field. But Einstein realized that the Equivalence Principle could allow him to treat free fall and inertial motion – that is, the tendency of a body in motion to remain in motion and continue along a straight line – as the same thing. This insight led to his General Theory of Relativity, which he published in 1915.
But is the Equivalence Principle true? That's what the 660-lb. French satellite launched Monday from French Guiana aboard a Soyuz rocket aims to find out.
"In space," notes a press release from France's space agency, the Centre National d'Études Spatiales, "it is possible to study the relative motion of two bodies in almost perfect and permanent free fall on an orbiting satellite, shielded from perturbations encountered on Earth (notably seismic perturbations), over the course of several months."
Developed by CNES, the Microscope satellite contains two cylindrical masses – one composed of titanium and the other a platinum-rhodium alloy – that are minutely controlled and monitored over time to see if they behave in a manner that confirms Einstein's famous theory. If Einstein was right, the two masses should move identically regardless of their composition.
But "if different accelerations have to be applied," notes the press release, "the principle will be violated: an event that would shake the foundations of physics."
In the century since it was formulated, general relativity has withstood every test that scientists have thrown at it; earlier this year, scientists detected ripples in spacetime caused by the collision of black holes, just as Einstein predicted.
Nevertheless, scientists have yet to reconcile Einstein's theory of gravitation with the Standard Model of particle physics, which describes the interactions between the other known fundamental forces of the cosmos. The Standard Model predicts that general relativity breaks down at very small scales, but nobody has ever observed it doing so.
The Microscope experiment observes the two masses at a precision three orders of magnitude greater than any tests so far performed on Earth.
Any violation of Einstein's Equivalence Principle would open up a new kind of physics.
"It would be the first sign of new physical phenomena, the signatures of new interactions or new forces, which are not explained by our standard physics model," says a CNES press release "It would thus bring into question our knowledge at the interface between the field quantum theory and relativity theories of gravitation as well as the application of these theories to astrophysics and cosmology."