Galileo’s most famous experiment has taken a trip to outer space. The result shows Einstein was right — again. The testing confirms a major part Einstein’s theory of gravity. What’s more, it was about 10 times more precise than earlier such tests.
According to science lore, Galileo dropped two balls from the Leaning Tower of Pisa. In fact, it seems unlikely that Galileo actually carried out this experiment. But he supposedly was trying to show that those balls would fall with the same acceleration. Their change in speed due to gravity would be the same, no matter what they were made from or how much they weighed.
Scientists have now performed a similar experiment in a satellite. They dropped two hollow cylinders and measured them for more than 120 orbits. The cylinders were in free fall for about eight days’ worth of time.
This experiment was designed to test what’s known as the equivalence principle. It’s a foundation of Einstein’s gravity theory, the general theory of relativity. The equivalence principle states that two different ways of defining an object’s mass are actually the same.
Now stick with us here, because this can get a bit complicated. Mass is a way of quantifying how much “stuff” an object is made up of. One way to define mass is by how easily an object moves when pushed. That’s the kind of mass called “inertial mass.” An object with more inertial mass requires a more forceful push to increase its speed by a certain amount. A second way to define mass is by how strong a gravitational pull an object feels. That’s known as “gravitational mass.”
Einstein’s equivalence principle holds that this inertial mass is equal to the gravitational mass. And if that’s true, then two different objects should fall at the same rate (at least when they’re in a vacuum, where air resistance is eliminated). In a perfect vacuum, for instance, a feather will accelerate just as much as will a ball of iron.
In the satellite experiment, known as MICROSCOPE, the two cylinders’ accelerations did match — at least to within two-trillionths of one percent! The satellite, by the way, is still collecting additional data.
The result is “fantastic,” says physicist Stephan Schlamminger. He works at OTH Regensburg in Germany and was not involved with the research. “It’s just great to have a more precise measurement of the equivalence principle,” he says. Why? “Because it’s one of the most fundamental tenets of gravity.”
Researchers behind the new experiment described their findings December 4 in Physical Review Letters.
What the experiment did
The tests used one hollow cylinder made from a platinum alloy. It was centered inside a larger hollow cylinder. This one was made from a titanium alloy. General relativity predicts that despite their different masses and their being made of different materials, both cylinders should fall at the same rate.
Any violation of the equivalence principle, however, would allow one to fall slightly faster than the other.
Because the satellite they were on was orbiting Earth, the two objects were allowed to go into “free fall.” Equipment on the satellite then used electrical forces to keep the two cylinders aligned. If the equivalence principle did not hold up, those electrical forces would have to change over time to keep the cylinders in line. But they didn’t.
“If we see any difference in the acceleration [of the two objects] it would be a signature of violation” of the equivalence principle, says Manuel Rodrigues. In fact, he reports, they saw no hint of such a variation. Rodrigues is a MICROSCOPE researcher who works at the French aerospace lab ONERA. It’s in Palaiseau.
With about 10 times the precision of previous tests, the result is “very impressive,” says Jens Gundlach. He’s a physicist of the University of Washington in Seattle. But, he adds, “The results are still not as precise as what I think they can get [on] a satellite.”
Performing the tests in space got around certain pitfalls of land-based experiments. For instance, even the flow of groundwater below the soil surface can slightly alter the mass of the surrounding terrain (and thereby change the gravitational force it exerts on other objects). Yet satellite tests are not perfect, either. Slight temperature changes in the spacecraft can cause parts of the test equipment to expand or contract. And that limits how well the scientists can test the equivalence principle.
MICROSCOPE’s ultimate goal is to beat other measurements by not just 10-fold, but by a factor of 100. With more data still to be analyzed, the scientists think there’s a chance they may reach that mark.
Confirming the equivalence principle doesn’t prove that the current view of how gravity works is fully settled. For instance, scientists still don’t know how to combine general relativity with quantum mechanics, the physics of the very small.
“The two theories seem to be very different,” Rodrigues says. Yet he notes that “people would like to merge these two.” Some attempts to do so predict that the equivalence principle might not hold up. But any violation of that equivalence might be too small for even today's best experiments to pick up. That’s why scientists think the equivalence principle is worth testing ever more precisely — even if it means shipping their experiments off into space.
acceleration A change in the speed or direction of some object.
aerospace A research field devoted to the study of Earth’s atmosphere and the space beyond or to aircraft that travel in the atmosphere and space.
alloy A blend of two or more metals in which the individual elements are thoroughly mixed at a microscopic level.
factor Something that plays a role in a particular condition or event; a contributor.
field An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory. (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
frequency The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.
fundamental Something that is basic or serves as the foundation for another thing or idea.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
groundwater Water that is held underground in the soil or in pores and crevices in rock.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
matter Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.
platinum A naturally occurring silver-white metallic element that remains stable (does not corrode) in air. It is used in jewelry, electronics, chemical processing and some dental crowns.
quantum mechanics A branch of physics dealing with the behavior of matter on the scale of atoms or subatomic particles.
relativity (in physics) A theory developed by physicist Albert Einstein showing that neither space nor time are constant, but instead affected by one’s velocity and the mass of things in your vicinity.
resistance (in physics) Something that keeps a physical material (such as a block of wood, flow of water or air) from moving freely, usually because it provides friction to impede its motion.
tenet An underlying idea, principle or belief — as in something generally believed to be true.
Journal: P. Touboul et al. The MICROSCOPE mission: first results of a space test of the Equivalence Principle. Physical Review Letters. Vol. 119, December 4, 2017, p. 231101. doi: 10.1103/PhysRevLett.119.231101.