Antimatter is mysterious, dangerous, and rare. In fiction, it's at the core of Isaac Asimov's positronic brains, the engines on the Enterprise, and the bomb in Dan Brown's Angels and Demons. But in the real world, antimatter is fairly mundane stuff. If the entire universe turned into antimatter, we'd barely notice. Or would we?

Top image: NASA/Hubble.

Our own Charlie Jane Anders wanted to know:


Last week, researchers announced they had found a method of measuring the gravitational mass of antihydrogen. Does this mean we can WEIGH ANTIMATTER? Or, if not, what does it actually mean?

Before getting into the nitty-gritty, let me assuage your curiosity with a) Yes, but at the moment we can only guess its weight with the accuracy of the world's worst carnival barker, and b) If it turns out that antimatter weighs more or less (or god forbid, the negative) of ordinary matter, it means that we've got to seriously rethink what we know about gravity.

But before getting into any of that, let me say a word or two about what antimatter actually is.


A quick antimatter primer

Every type of particle in the universe has an antiparticle – a sort of evil twin version of itself, with the opposite electrical and nuclear charges. An electron, for instance, has its counterpart, the positron, which has a positive electric charge, rather than a negative one. A proton has the boringly named "antiproton" with a negative charge. In fact, antimatter is so similar to ordinary matter that a few particles, notably the photon and the Higgs Boson, seem to be their own antiparticles.

In almost every way that counts, matter and antimatter are just two sides of the same coin – one of the central symmetries of the laws of the universe. Every experiment we've ever done, for instance, nets the same number of antimatter as matter particles.

Likewise, if you happen to encounter your antimatter twin, whatever you do, do not touch him or her. When matter and antimatter come into contact with one another, they annihilate entirely, unleashing a maelstrom of energy equivalent to Einstein's famous E=mc^2.

Of course, this being io9, you probably already knew that.

It's worth noting that matter and antimatter must be at least a little bit different. You are, after all, made of one and not the other, so the very early universe presumably had to distinguish between the two. But those differences are primarily related to subtleties in the weak nuclear force that we don't need to sweat for now.

Every experiment that we've ever done also suggests that it takes the same amount of energy to make a particle and its antiparticle. In other words, an electron and a positron have the same mass. This clearly has to be true for the Higgs which, as I noted, is its own antiparticle.

But just because it's just as difficult to push around an antiproton as a proton doesn't mean that gravity affects them the same way. Could it be that antimatter has antigravity?

The Eötvös Experiment

In the late 17th century, Isaac Newton realized that "mass" means two very different things, depending on the context.

On the one side, your bathroom scale resists the gravitational force between you and the earth and reads a larger number the more massive you are. This is gravitational mass.

Inertial Mass, on the other hand, has nothing at all to do with gravity. It's a measure of how hard it is to accelerate something — or to decelerate it, once it’s in motion.

Despite the fact that the two are so intimately related (and that either can be used seamlessly in "Yo mama" jokes), there is no obvious reason that your gravitational mass (which creates the force between you and the earth) and your inertial mass (how hard you are to move) should have anything to do with one another.

And yet, they do. Galileo is so famous because, among much else, he showed that gravity accelerates objects independent of their mass, by comparing the rate that wheels of different sizes and density rolled down a hill. In other words, the ratio of gravitational to inertial mass seems to be a fixed constant of nature for some reason.

In 1885, a Hungarian Physicist named Lorand Eötvös devised a series of experiments that determined the ratio of gravitational to inertial mass to insane precision. He essentially pitted the "real" gravitational force of the earth against the centrifugal force caused by the rotation of the earth. If you've ridden a gravitron, you'll know that a spinning platform creates a sensation kind of like gravity, but without the actual gravitational field. The spinning of the earth should only care about the inertial mass, while the gravitational field cares about the gravitational mass. Eötvös and his successors showed that the two are the same to better than a part in 10 million.

Of course, this doesn't explain why the two are the same.

The Equivalence Principle

The fundamental explanation for why inertial and gravitational mass seemed to be the same had to wait a long time – until Einstein came along.

Of course it did.

Einstein's foundation for General Relativity – our modern theory of gravity – rested upon an idea known as "The Equivalence Principle." While he constantly tinkered with the wording, the basic idea is that if I were to cut the cables on your elevator, in the brief, but exhilarating moments before you went crashing to your death, there would be nothing to distinguish your state of free-fall from a true absence of gravity.

This can be flipped around. Suppose I put you on an accelerating rocketship. You would, of course, be pushed to the back of the ship, and it would feel like "back" was "down." According to the Equivalence Principle, there's no local experiment that you could do that could distinguish between a real gravitational field, and the effects of the rocket. A thrown ball would arc downwards (backwards), for instance, and time would even run slower in the back of the ship than at the front, albeit to the tune of about 1 part in a trillion – again, perfectly in accord with what happens in a real gravitational field.

I cannot overstate how important the Equivalence Principle is. Everything, from the structure of Black Holes, to gravitational waves, to the expanding universe drop out of Einstein's equations, and those equations are ultimately founded on the Equivalence Principle.

But it's a big deal in another way as well. If you're rocketing through space with your antimatter twin, both of you will be pushed to the back with the same acceleration. After all, it's really the back of the ship accelerating forward to meet you. If it really is true that nothing can distinguish an accelerating frame from a real gravitational field, then antiparticles had better fall toward the earth with the same acceleration as ordinary ones.

Or to put it another way, antiparticles should have the same gravitational mass as their regular matter counterparts.

Because if not, physics has a lot of explaining to do.

How much does an antiatom weigh?

We've assumed for about a century now that antimatter and matter should both have the same gravitational and inertial mass — and while General Relativity depends on it, we haven't really had a way of weighing antimatter directly, until recently. After all, it's really tough to make antimatter in any considerable quantities, and for the most part, the particles that we do create are things like positrons. Charged particles are good and all, but since all things being equal, electrical forces are something like 10^36 times larger than gravitational forces, what we really want are electrically neutral antiparticles. In other words, we need antiatoms.

In an exciting new article published last week by the ALPHA collaboration at CERN, the team was able to capture 434 anti-hydrogen atoms (positrons orbiting anti-protons) in a magnetic trap. The anti-atoms are then released from the trap and measured in a nearby detector. If most of the anti-atoms fall downwards, then presumably the anti-atoms have positive gravitational mass. If they fall upwards, then they must have negative gravitational mass – antigravity. This is kind of like the story of Galileo dropping masses out of the Leaning Tower of Pisa, except on the atomic level. And also, it actually happened.

The measurement is a toughy. Since they bounce around a fair amount in the magnetic trap, some of the anti-atoms are going to be ejected flying up and some down. In other words, what we get is a ridiculously noisy signal.

So to answer Charlie Jane's original question, we don't measure the mass of the anti-hydrogen, at least not directly. All we do measure is the ratio of the gravitational to inertial mass – a number, mind you, that we expect to be 1.

What does the ALPHA collaboration actually measure? Well, prepare to be underwhelmed! They find that the ratio is somewhere between -65 and 110. This is roughly equivalent to a scale that says a typical adult weight somewhere between negative five and positive eight tons. It's correct as far as it goes, but not terribly helpful. But in the case of antihydrogen, it's not so much the number itself that's so impressive, but the fact that we can measure the mass at all.

Also, just give it time. The errorbars will tighten considerably with more data. So what should you expect when the dust settles? You don't need to take my word for it. As my pal, the physicist Rich Gott (who called my attention to the experiment in the first place) put it:

Dave Goldberg is a Physics Professor at Drexel University, and the author of "A User's Guide to the Universe," and the upcoming "Universe in the Rearview Mirror" (coming July 11!). You should send him your questions about the universe, or even better become a fan of his awesome facebook page.