How a 40-Year Gyroscope Experiment Helped Confirm Two Effects Einstein Thought Unmeasurable

In 1916, Albert Einstein published his general theory of relativity. In it, he argued that gravity is not a force but the result of matter curving spacetime. The mere presence of matter, he held, can change the paths that other matter can take.

Physicists naturally wanted proof for this theory, but Einstein himself thought some of that proof was out of reach. In 1922 he wrote that some effects of general relativity were, in his words, "so small that confirmation of them by laboratory experiments is not to be thought of."

The video describes the answer to Einstein's doubt as a research project spanning 40 years, which used four of the most perfectly spherical balls ever made. Scientists have now proven this key aspect of general relativity, the narrator says.

The video says it will focus on two specific effects of general relativity.

The first effect is the geodetic effect: Earth's presence distorts the spacetime around it. The video gives an example of measuring the length of an orbit directly above the equator. Instead of exactly 2πr, the geodetic effect would make the measurement a couple of centimeters shorter, for a circumference of roughly 4 billion centimeters. That is why Einstein thought it would be impossible to observe.

The second effect is frame dragging, in which Earth's rotation drags spacetime around with it. The video compares this to spinning a ball in honey, where friction drags some honey along in a twisting motion.

Over the years, physicists ran several experiments to capture these effects. In the 1970s, Gravity Probe A confirmed the time part of spacetime distortion, in which time passes more slowly closer to a mass. The team shot a super-precise clock called a maser up to an altitude of about 10,000 kilometers and let it fall back down. Compared with an identical clock that stayed in a lab, the Earth clock lagged behind, as predicted.

Another space experiment from the 1970s aimed to verify frame dragging. It used two 60-centimeter satellites called LAGEOS and LAGEOS II, tracking their orbits by shooting lasers at them to see whether they shifted or rotated. Its first findings, published in 1997 and 1998, were described as a tepid, controversial thumbs up: both satellites' orbits did change, but the results carried a 20 to 30% error, and some scientists argued the actual error was even higher.

By the 1990s, an experiment called Gravity Probe B was in the works, aiming to improve on its predecessor in both precision and scope and to test both the geodetic and frame-dragging effects. Physicists George Pugh in 1959 and Leonard Schiff in 1960 had independently come up with the idea. It required a guide star, a telescope, and a freely rotating gyroscope inside the telescope.

From 1,400 contending stars, the team picked IM Pegasi for its brightness, position and trackability. It is bright enough to be seen with the naked eye and lies next to the constellation Pegasus. The telescope was to be sent into a polar orbit, crossing over both poles, and pointed permanently at IM Pegasi as a reference axis, with the gyroscope set spinning so its axis also pointed at the star.

If general relativity were wrong, the gyroscope should stay pointed at the guide star. But if it were right, the geodetic effect should tilt it north-south at about 0.0018 degrees per year, and frame dragging should tilt it east-west at about 0.000011 degrees per year. Astronomers express such tiny values in milliarcseconds, where one milliarcsecond is one 3,600,000th of a degree, about the width of a human hair seen from roughly 16 kilometers away.

In the units physicists use, general relativity predicts that the geodetic and frame-dragging effects should tilt Gravity Probe B's gyroscope at rates of 6,606.1 and 39.2 milliarcseconds per year, respectively. The video notes viewers do not need to memorize these numbers.

According to Einstein, these effects were impossible to measure, and for decades he was right, because the equipment was simply not precise enough.

Leonard Schiff was undeterred. He pulled together a team of physicists, and in 1964 he received initial funding from NASA that started a project lasting more than 40 years. Part of the reason it lasted so long, the video says, is that scientists had to invent more than a dozen new technologies for the experiment to work.

The team decided to make four gyroscopes so they could average the data and reduce the impact of measurement error. Each ball was 38 millimeters in diameter, roughly the size of a ping-pong ball. Two were set to spin clockwise and two counterclockwise, all at 4,200 rpm.

To get precise data, the spinning balls needed to wobble as little as possible, so they had to be extremely symmetrical and smooth. Engineers made them from fused quartz for consistent density throughout and invented a new machine to polish each ball individually, grinding an abrasive slurry against it from four sides. They compensated for asymmetrical effects such as the ball's weight and the polisher's spinning forces to keep each sphere perfectly round. The video says the result was the smoothest balls ever made, with roughness only a few atoms thick; if a ball were Earth-sized, its tallest mountain would be only 1.5 meters high.

Verifying this required another machine that could measure smoothness precisely, and that measuring tool also had to be extremely smooth.

Nothing could touch the balls and alter their spin, including the tool used to measure their drift, so scientists first had each ball float inside its own case or housing. They coated each ball in a 1.2-micrometer layer of niobium metal and surrounded each housing with six charged electrodes. The conductive coating made the electrodes pull the opposite charges on the ball surface toward them, which evenly pulled the ball in all six directions and kept it floating.

The video says this floating was just barely: the gap between any part of the ball and its housing was only 32 millionths of a meter wide.

To measure the floating balls without touching them, scientists used niobium's magnetic properties. They cooled the niobium with liquid helium to 1.8 kelvin, just 1.8 degrees above the coldest possible temperature in the universe, turning the coatings into superconductors. As a superconductor, the niobium let its electrons move freely, and as the ball spun, the surface electrons lagged and drifted across it, creating a magnetic dipole aligned with the ball's spin axis.

This phenomenon is called the London moment, named after a scientist rather than a city. Engineers added a coil to each ball's housing and used detectors called SQUIDs to read it. This allowed them to magnetically measure the London moment, and therefore the direction of each gyroscope's axis, without touching the ball. The video jokes that this much engineering was needed for one measurement, or one measurement times four.

Even mechanically isolated, the balls could still be disturbed by other forces and fields, so engineers worked to minimize these too. To cancel external rotational forces on the gyroscopes, they designed the whole spacecraft to rotate once every 78 seconds. They blocked Earth's magnetic field from affecting the SQUID readings by placing them in carefully designed nested lead bags, and reduced stray magnetic fields from the niobium superconductors with a special heating and cooling procedure.

Space events such as solar radiation could also nudge the craft off track, so engineers built a micro-thruster system to steer it back into place with puffs of gas, about 1/100 the strength of a human exhale. The video notes that this breathing gas came from the helium that had been used to cool everything down.

Launch had to be timed to align with IM Pegasi. Across the three days the spacecraft could launch, the window was only one second long. The first day was missed due to wind, but Gravity Probe B launched on the second day, April 20, 2004. Once in space, the mission was limited to 16 months, the lifespan of the helium coolant.

During the 16 months, researchers saw three unexpected behaviors from the gyroscopes that threatened the accuracy of the data. Diagnosing them took five years as scientists searched through more than one terabyte of data. They eventually found a clue in housekeeping data meant for monitoring stray magnetic fields. The balls were very nearly perfect spheres geometrically, but not electrically, and neither were their housings. The video says the electrical interactions between these asymmetries explain all three unexpected behaviors.

After correcting their analysis, scientists found that the geodetic effect tilted the gyroscope axes north-south at an average rate of 6,601.8 milliarcseconds per year, matching the predicted value to within 0.07%. For frame dragging, the average east-west tilt rate was 37.2 milliarcseconds per year, which matched the predicted value to within 5.1%, though the measurement had a less than ideal 19% uncertainty because of the corrections the researchers had to incorporate.

Still, the video says the results were reliable enough to confirm that both effects exist. The Gravity Probe B team published its landmark paper in 2011.

Scientists always seek to do better. Fifteen years later, in 2026, physicists tried again to observe frame dragging and succeeded. They improved on the method used in the Gravity Probe B experiment, tracking how the orbital paths of two satellites changed across almost three years. The measured changes matched the predicted frame-dragging values to within 0.1%, with only 0.3% uncertainty.

The video closes by thanking Einstein for challenging physicists to prove him wrong, and in doing so, proving him right.