In 1916, Albert Einstein published his famous general theory of relativity.

And in it he claimed that gravity isn't a force, it's just the result of matter curving spacetime. The mere presence of stuff can change the possible paths that other stuff can take.

Naturally, physicists wanted proof for this bonkers theory. But Einstein thought some of that proof was entirely out of reach. In 1922, he wrote that some of general relativity effects were, quote, “so small that confirmation of them by laboratory experiments is not to be thought of." Challenge accepted.

Albert. It took a research project spanning 40 years and four of the most perfectly spherical balls ever made, but scientists have finally proven this key aspect of general relativity. [ intro music ] For this episode, we're focusing on two specific effects of general relativity. The first is the geodetic effect.

Basically, Earth's mere presence distorts spacetime around it. Say you wanted to measure the length of an orbit around the Earth, which is directly on top of the Earth's equator. Instead of it being exactly two times pi times earth’s radius, you know the equation for the circumference of a circle.

The geodetic effect would make your measurement a couple of centimeters shorter. Yes, I did just say a couple centimeters for a circumference that is roughly 4 billion centimeters in length. So you can understand why Einstein thought it'd be impossible to observe.

The second effect is the frame dragging effect, where Earth's rotating motion drags spacetime around it. Kind of like how if you spin a ball in honey, the friction of the ball's surface drags some honey along in a twisting motion. Over the years, physicists have run different experiments to try to capture these fingerprints of general relativity.

Like in the 1970s, Gravity Probe A confirmed the time part of space time distortion where time passes slower closer to a mass. For this experiment, scientists shot a super precise clock called a maser, straight up to an altitude of about 10,000km and let it free fall back down. The research team then compared its time to an identical clock, which spent the whole time in a lab, and they confirmed the Earth clock lagged behind, as predicted.

Another space based experiment from the 70s sought to verify the frame dragging effect. Here, scientists use two 60-centimeter wide disco ball looking satellites, called the Laser Geodynamics Satellite, or LAGEOS, and its sibling, LAGEOS II. Scientists tracked the satellite's orbital paths by shooting lasers at them to see if they would shift or rotate.

And their first findings, published in 1997 and 1998, gave a tepid, controversial thumbs up. Both of the satellites orbit did change, but the results had a large 20 to 30% error, with some scientists arguing that the actual error was even higher. Luckily, by the 90s, there was already an experiment in the works that was hoping to save the day.

Gravity probe B was a step up to its predecessor in both precision and scope, aiming to test both the geodetic and frame dragging effects. Two physicists independently came up with the idea. George Pughin 1959 and Leonard Schiff in 1960.

All it required was a guide star, a telescope, and a spinning, freely rotatable object inside that telescope called a gyroscope. Out of 1400 contending stars, the Gravity Probe team picked IM Pegasi due to its brightness, position, and trackability. It's bright enough to be seen with the naked eye, and as the name suggests, you can find it right next to the constellation Pegasus.

The next part of the plan was to send the telescope into a polar orbit, meaning it crosses directly over both the north and south poles. Then the telescope gets pointed permanently at IM Pegasi to serve as a reference axis, and the gyroscope is set spinning so its spin axis is also pointing toward IM Pegasi. Over time, scientists would measure how much the gyroscopes axis shifts.

If general relativity isn't a thing, it should remain pointed at the guide star. But if it is a thing, the geodetic effect should tilt the gyroscope in the north south direction at a rate of about 0.0018 degrees per year, and the frame dragging effect should tilt it in the east west direction at an even smaller rate of 0.000011 degrees per year. Now, working with tiny decimals like that is a pain.

So instead, astronomers use a unit called milliarcseconds, where one milliarcsecond is one-3,600,000th of a degree. for comparison, that's the width of a human hair seen from like 16 kilometers away. So in physicists speak, general relativity predicts 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. It's okay.

You don't need to memorize these numbers. There's no test at the end. According to the father of relativity, this was impossible to measure, and for decades he was right.

Our equipment simply wasn't precise enough. But Leonard Schiff was undeterred. He pulled together a team of physicists, and in 1964, he got initial funding from NASA that kicked off a more than 40 years long project.

Part of the reason why the project lasted so long was because scientists had to literally invent more than a dozen new technologies for the experiment to work. A huge amount of effort went into the metaphorical star of the show, the gyroscope, a.k.a. the spinning ball. For robust results, the team decided to make four of them.

That way, 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 4200 rpm.

So fast, so furious. For precise data, the scientists needed the spinning boss to wobble as little as possible, which meant they had to be super symmetrical and smooth. So engineers made the balls out of fuzed quartz for consistent density throughout.

They also invented a new machine to polish the balls individually by grinding an abrasive slurry against them from four different sides. To ensure the ball stayed perfectly spherical, engineers painstakingly matched the pressure on each side by compensating for asymmetrical effects, such as the ball's weight and the polishers spinning forces. The result was literally the smoothest balls ever made, with only a few atoms thick of roughness.

If the balls were the size of the Earth, the tallest mountain would only be 1.5 meters high. To verify this, the engineers had to create another machine that could precisely measure smoothness, and that machines measurement tool also had to be insanely smooth. A never ending cycle of smoothness.

Now, just to have these perfect balls wasn't enough to properly detect the geodetic and frame dragging effects. Nothing could touch the balls and alter their spin, including whatever tool needed to measure their drift. So as a first step, scientists got each ball to float inside its own case or housing.

To do this, they coated the balls in a 1.2 micrometer thick layer of niobium metal and surrounded each ball's housing with six charged electrodes. Since the ball now had a conductive metal coating, the electrodes would attract opposite charges on the ball surface toward them. Overall, this caused the ball to be evenly pulled in all six directions and thus float.

Just barely. The gap between any parts of the ball and its housing was just 32,000,000th of a meter wide. Next, to measure the floating balls without touching them, scientists leveraged niobium magnetic properties.

They turned the niobium coatings into superconductors by cooling everything with liquid helium down to 1.8 Kelvin, just 1.8 degrees above the coldest possible temperature in the universe. As a superconductor, the niobium now had its electrons unmoored from their atoms, and as the ball spun, the electrons on the surface would lag behind and meander across the surface, creating a magnetic dipole precisely align with the ball's spin axis. Scientists call this phenomenon the London moment, which was named after a scientist, not the city by adding a coil on each balls housing and detectors named squids.

Engineers could magnetically measure the London moment and therefore the direction of the gyroscopes axes touch free. So much engineering to just make one measurement. Well, one measurement times four.

Now, the balls were mechanically isolated, but they could still be disturbed by other forces and fields and engineers went to great lengths to minimize these too. For example, to cancel out any external rotational forces on the gyroscopes, engineers designed the entire spacecraft to rotate once every 78 seconds. They also blocked Earth's magnetic field from messing with the Squids readings by putting them in carefully designed nested lead bags, and they reduce stray magnetic fields from the niobium superconductors by developing a special heating and cooling procedure.

On top of all this, gravity probe itself could also be nudged off track by space events like solar radiation, so engineers built a micro thruster system that could steer the craft back into place using puffs of gas, about 1/100 the strength of a human exhale. And in a case of recycling brilliance, the gas they use for those pathetically weak breaths came from the helium they had been using to cool everything down. There's even more mind blowing engineering behind this mission.

You could spend hours going down internet rabbit holes. Our writer sure did, but for now, we're going to stop here and shift into what happened after the experiment was built, which brought even more hurdles. Or we will right after this ad break.

Because just like all the scientists behind Gravity Probe B, SciShow needs funding too This video is supported by Uncommon Goods, the place to find creative gifts for everyone you know. Uncommon goods looks for products that are high quality, unique and often handmade or made in the US. Many are crafted by independent artists and small businesses, making them truly one of a kind.

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As they say, Uncommon Goods. We're all out of the ordinary. launching the spacecraft required precise timing to ensure alignment with IM Pegasi. In fact, across the three days it could launch.

The launch window was only one second long. They missed the first day due to wind. But on the second day, April 20th, 2004, Gravity Probe B was able to launch.

Once in space, the mission was limited to 16 months. That's how long the helium coolant will last. During that time, researchers saw three unexpected behaviors from the gyroscopes that threatened the data accuracy.

Diagnosing these behaviors took five years to figure out as scientists searched through more than one terabyte of data. Eventually, they found a clue in some housekeeping data meant for monitoring stray magnetic fields. Apparently, while the balls were very nearly perfect spheres, geometrically, they were not electrically, and neither were their housings.

The electrical interactions between these asymmetries explain all three unexpected behaviors. Knowing this, scientists corrected their analysis and came up with the following results for the geodetic effect The gyroscopes axes tilted in a north south direction at an average rate of 6,601.8 milliarcseconds per year, Nailing the predicted value to within 0.07%. For the frame dragging effect.

The average tilt rate in the east west direction was 37.2 milliarcseconds per year. This matched the predicted value to within 5.1%, but the measurement had a less than ideal 19% uncertainty. Due to the corrections the researchers had to incorporate into their analysis.

Still, the results were reliable enough to confirm that these two general relativity effects exist, and the Gravity Probe b team finally published their landmark paper in 2011. Scientists always seek to do better, though. So 15 years later, in 2026, physicists took another stab at observing the frame dragging effect and succeeded.

They improve on the method from the late goals experiment, tracking how the orbital paths of two satellites change across a span of almost three years. The measure changes match the predicted values for the frame dragging effect to within 0.1%, with only 0.3% of uncertainty. So thanks, Einstein, for challenging us to prove you wrong and in so doing, prove you right. [ outro music ]