Remember that story about Galileo dropping stuff off the Leaning Tower of Pisa to figure out if gravity acts on everything the exact same way?

And because of that pesky air resistance on Earth, an astronaut later demonstrated it was true by dropping a feather and a hammer on the surface of the Moon? Well, hundreds of years after Galileo, in one of the most sophisticated science labs on the planet, physicists tried to answer a nother fundamental question about gravity: Does antimatter fall just like regular matter?

And of course, they tested it out by dropping it. [intro] Particle physics is all about studying the fundamental building blocks of the universe. Most of the universe we’re all familiar with is made up of just a few subatomic particles: Electrons, protons, and neutrons. But it turns out, each kind of particle has an antimatter counterpart.

Usually, scientists denote this by slapping the prefix “anti-” onto the name. So the antimatter counterpart of an electron is an antielectron. It’s also known as a positron, which highlights the key difference  between matter and antimatter twins.

They have opposite electric charges. So, electrons are negatively charged, while antielectrons have a positive  charge of the same strength. Other than that, the two are completely identical: the same mass, the same size… Or at least, we think they’re identical.

There’s a lot we don’t know about antimatter, because testing it is really difficult. See, when an antimatter particle hits a matter particle of the same type, the two annihilate each  other in a flash of light.   So even if scientists can use fancy toys like particle accelerators to make a bunch of their own antimatter— and they definitely do— it doesn’t tend to stick around very long. It eventually bumps into stray matter particles.

But thanks to technological progress, we’ve developed ways to make not just larger  quantities of antimatter particles in one go, but to trap and store them for fairly long periods. Usually, this involves putting the antimatter in a vacuum chamber, so there aren’t any stray air molecules for them to react with, and in a magnetic field to  suspend the antiparticles so they don’t bounce against  the solid walls, either. This technique is such a well-oiled machine that antimatter has found  some real-world applications, like being used in medical scanners to identify cancerous tumors.

But there are lots of mysteries  surrounding antimatter. One of the biggest is why there’s very little of it in the universe, at least compared to regular matter. And physicists are particularly worried about this because their best theories say that, if the two are completely  identical except for their charge, they should have been made in equal quantities during the Big Bang.

But fortunately for you and me, there seems to have been  the slightest of imbalances, causing only most of the regular matter to annihilate with all that primordial antimatter. Rather than assume there must have been different amounts of matter and antimatter at the dawn of reality, some physicists think that  some force of nature out there may treat the two differently. For example, it might be possible that gravity tugs on antiparticles differently to regular ones, despite the fact that they’re supposed to have the exact same mass.

However, if that example were actually true, it would fly in the face of Einstein’s famous equivalence principle, which is the bedrock of his  General Relativity theory. Basically, the theory takes it as a starting point that all mass is treated the same by gravity, regardless of source. So experimental physicists don’t expect gravity to act on antimatter any differently.

But that’s no excuse for everyone to hang up their lab coats and go through life assuming! We actually have to test our theories, people. That’s kinda a requisite of the whole “sciencing” thing.

And that brings us to an experiment called ALPHA-g at the particle physics lab CERN. The experiment is incredibly sophisticated. It involves subatomic particle collisions to make the antimatter, clever tricks to cool the batch to near-absolute zero, and powerful magnetic fields to trap and move it… But at its heart, ALPHA-g is also pretty simple.

The researchers literally just put antihydrogen atoms in a box and… checked to see if they fell down. Kind of a souped-up version of Galileo’s apocryphal Leaning Tower of Pisa test. Antihydrogen, by the way, is the antimatter version of regular hydrogen… the lightest and most common atom in the universe.

Regular hydrogen is made of one negatively-charged electron bound to one positively-charged proton. That makes it roughly electrically neutral. And antihydrogen is the same but in reverse, with a positively-charged antielectron bound to a negatively-charged antiproton.

Now, you may be wondering why the scientists went  through all that extra effort to make antimatter atoms instead of relying on a fleet of lone antielectrons or antiprotons. Well, they kind of had to, because the electric force is way, way, way stronger than the gravitational one. Unlike an individual positive  or negative particle, a neutrally-charged atom won’t be affected nearly as much by any stray electric and  magnetic fields in the lab, and the experiment can really hone in on just the influence of gravity.

And when the experimenters dropped about one hundred antihydrogen atoms in their fancy science box, they found that about eighty percent of them fell out the bottom of the box a short while later. That may sound odd at first, but even for regular matter, atoms are so small that the force of them bumping into each other can be stronger than the tug of gravity. So in a cloud of a hundred atoms, regular or antimatter, some will be launched upwards just by collisions, and take way too long to fall down for the experiment to spot them.

That eighty percent falling figure wound up matching their team’s simulations. And after some more analysis, they found that the antihydrogen fell at a rate that was roughly the same as normal hydrogen’s. But these early results, which were published in September 2023, still have a large margin of error.

They technically don’t rule out  the possibility that gravity is around twenty to fifty percent weaker for antihydrogen, which would be absolutely earth-shattering if more sensitive follow-up experiments lead in that direction. But at the very least, ALPHA-g has probably ruled out the idea that antimatter doesn’t react  to the force of gravity at all, and completely nixed the chance that it’s somehow flipped for antihydrogen. That antimatter falls up.

So while we may not see any exciting, unexpected new physics emerging from these results, it’s reassuring to know that things fall down, even when antimatter comes into the picture. Galileo would be proud. This episode of SciShow is  brought to you by…merch!

Specifically, some awesome stickers that are repping basically the opposite of what Galileo and those  CERN scientists were doing. And what’s the opposite of dropping? Well, floating, up, up, and away.

That’s right, these stickers are all about balloons! But not just any balloons. They’re space balloons, which have done everything from help humans send communications signals around the world, to prevent a bunch of rovers from crashing smack dab into the surface of Mars.

You can pick a pack up by heading over to DFTBA.com/SciShow right now. And as always, thanks for watching. [ OUTRO ]