This episode of SciShow Space is sponsored by Awesome Socks Club, a sock subscription for charity!

Click the link in the description and sign up between now and December 11th to get a new pair of fun socks each month in 2021. {♫Intro♫}. Gravity dominates our lives, but of the four fundamental forces, it’s the hardest to study.

And that’s because it’s unimaginably weak compared to the other fundamental forces. So to measure its effects with ordinary instruments, we need to study massive objects with a whole lot of gravity. Like… our Sun.

Its effect is strongest on Mercury, which orbits closest to it. And that’s why, for years, this little planet has been a VIP in revealing the secrets of this force. It all started back in 1859, when the French astronomer Urbain Le Verrier noticed that something seemed off about Mercury’s orbit around the Sun.

Like the other planets in the solar system, Mercury travels along an elliptical orbit, and over time, the orientation of that ellipse changes. This is known as the precession of perihelion. It happens to all planets to some extent, as they’re tugged around by distant giant planets in the solar system.

But Mercury’s perihelion seemed to be processing a little more than it should. At the time, to calculate each planet’s precession, scientists just factored in its mass, plus the masses and distances of other objects in the solar system. That’s all you need for Newton’s equations of gravity.

And that worked pretty well overall. But Mercury’s perihelion precessed just a fraction of a degree per century too much. And at the time, scientists couldn’t explain it.

Some suggested that there was an undiscovered planet between Mercury and the Sun, which they called Vulcan. But as the years passed and Vulcan never turned up, it became clear that they needed a different answer. And in 1915, they got one.

That year, Albert Einstein published his general theory of relativity, which completely changed our understanding of gravity. He described the gravity as the curvature of spacetime, which you can think of as a fabric that gets distorted by anything with mass. That distortion then affects the motion of other objects traveling through those distortions.

And Mercury is so close to the Sun that the Sun’s dent in spacetime has an especially noticeable effect on Mercury’s orbit. According to the theory, it should make Mercury’s orbit precess slightly faster. And sure enough, when Einstein recalculated the precession of Mercury’s perihelion using the equations of general relativity, his new prediction almost exactly matched observations.

This was the first major success for general relativity, and a sign that Einstein was on the right track. But general relativity didn’t just predict how gravity would affect matter. The theory also predicted that light would bend as it traveled through curved spacetime.

And in 1964, the American astrophysicist Irwin Shapiro suggested a way to test that. He proposed bouncing radio waves off a planet as it passed behind the Sun. The idea was that the signal would have to pass through the Sun’s gravity well to hit the planet, so its path would be curved.

And that curved path would be longer than a straight line between Earth and the planet. As a result, the reflected signal should come back later than if it had traveled in a straight line. For this experiment, Mercury was the perfect candidate.

Since the diameter of its orbit is so small compared to the other planets, a light beam traveling from Earth and back would spend a significant fraction of its travel time in that gravity well. That way, it would be easier to tell how much extra time it took compared to the time it should take light to travel the same distance in a straight line. So in a 1971 study, a team at Arecibo observatory bounced radar signals off the planet’s surface, just as it was about to disappear behind the Sun.

As predicted, the returning echoes were noticeably delayed. And for light, that’s kind of a big deal. So, it gave scientists pretty good reasons to trust general relativity.

But they still had one more detail to check out. And that’s what’s called the equivalence principle. This is another aspect of Einstein’s theory: It says that the effects of gravity are impossible to distinguish from the effects of acceleration, and so the two are equivalent.

This appears to be true, as far as we can tell. Like, if you were in an elevator accelerating toward the ground in freefall, you’d feel like you were floating. Technically, you wouldn’t be able to tell whether the elevator broke or gravity stopped working.

And that’s because you can’t tell the effects of gravity and acceleration apart. At least, we can’t. But there’s still no actual proof that gravity and acceleration are exactly the same.

So to test the limits of this theory, scientists turned to Mercury again. The authors of a 2018 study analyzed seven years of precise positional data from the. Mercury orbiter MESSENGER.

And they used it to accurately reconstruct the probe’s path through space—which in turn let them determine Mercury’s path through space. Then, they compared that to Earth’s path around the Sun. That was key, because if gravity and acceleration are equivalent, then any two objects in the same gravitational field should accelerate equally.

It’s like how if you drop a lemon and a watermelon from a rooftop, they’ll hit the ground at the same time, even though they have totally different masses. But if gravity and acceleration are not equivalent, objects with different masses should accelerate at different rates. In that case, Mercury and Earth would experience a different pull toward the Sun.

If that happened, you’d be able to tell based on variations in the distance between the two planets over a couple years. But the measurements from this experiment upheld the equivalence principle, with 10 times more certainty than ever before. Today, our knowledge of gravity is still evolving, and we still rely on Mercury for new insights.

Because even after hundreds of years of study, this planet nestled close to the Sun is one of the best places to probe this unusual force. Thanks for watching this episode of SciShow Space! While you’re here, I want to tell you about a new charity project called the Awesome Socks Club!

It was started by John and Hank Green, and if you sign up, you can get a fancy new pair of socks each month in 2021. The socks are indeed awesome, but the best part of this is that 100% of after-tax profits will go toward efforts to decrease maternal and child mortality in Sierra Leone, which is one of the most dangerous places to be pregnant in the world. If you want to join the Awesome Socks Club, make sure you subscribe anytime between now and December 11th so we know how many socks to make!

You can sign up at the link in the description. {♫Outro♫}