Of Particular Significance

About the News that Antimatter Doesn’t “Fall Up”

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 09/27/2023

The press is full of excitement today at the news that anti-matter — hydrogen anti-atoms, specifically, made from positrons and anti-protons instead of electrons and protons — falls down rather than rising up. This has been shown in the ALPHA experiment at CERN. But no theoretical physicist is surprised. Today I’ll explain one of many reasons that no one in the halls of theoretical physics even blinked.

One basic point is that “anti-particle” is not a category but a relationship. We do not say “electrons are particles” and “positrons are anti-particles”, nor do we say “quarks are particles” and “anti-quarks are anti-particles.” Such statements would be inconsistent. Instead we say “quarks and anti-quarks are each others’ anti-particles.” (It’s like the term “opponent” in a football match.) That’s because some particles are their own anti-particles, including photons. If we tried to divide all the types of particles into two categories, it wouldn’t be clear where photons would go; we couldn’t say either that they are particles or that they are anti-particles.

“Anti-particle” is a relationship, exchanging electrons with positrons and leaving photons unchanged. It is not a category; there are no objects that can be said to be “antiparticles”. (If there were, would photons be particles or would they be anti-particles?)

Because of this, we can’t hope that all the types of particles separate into two groups, particles which fall and anti-particle which rise. At best, we would have to guess what would happen to particles that are their own anti-particles. Fortunately, this is easy. There is such a thing as a positronium atom, an exotic atom made from an electron and positron co-orbiting one other. A positronium atom is its own anti-particle; if we flip every particle for its anti-particle, the positronium atom’s electron and positron simply switch places. If indeed gravity pulls down on electrons and pushes up by the same amount on positrons, then gravity’s pull on a positronium atom would cancel. This atom would neither rise nor fall; it would float, feeling no net gravity.

This example would then lead us to expect that particles that are their own anti-particles will float. The logic would apply to photons; they would feel no gravity.

Electrons and quarks make up atoms and are known to fall. If the positrons and anti-quarks that make up anti-atoms were to rise instead, then positronium atoms would float. This would imply that photons float too. But experiments have shown for a century that photons fall.

This would be a consistent picture. But experimentally, it is false: photons do feel gravity. The Sun bends the path of light, a fact that made Einstein famous in 1919, and an object’s strong gravity can create a gravitational lens that completely distorts the appearances of objects behind it. Photons can even orbit black holes. So experiment would force us to accept the picture shown below, if we want positrons to rise. Unfortunately, it is logically inconsistent.

Since photons fall too, we would have to believe either that positronium floats even though photons fall, or that positrons rise even though positronium falls. Neither makes any sense either conceptually or mathematically.

The only consistent picture, then, is that everything falls. Are there any loopholes in this argument? Sure; perhaps the gravity of the Earth, made of atoms, causes electrons and quarks to fall rapidly and causes positrons and anti-quarks to rise slowly, so that positronium still falls, just more gently than ordinary atoms do. (The reverse would be true around a planet made of anti-atoms.) This gives us many more possibilities to consider, and we have to get into more complex questions of what experiment has and hasn’t excluded.

But we would still face another serious problem, because there are anti-quarks inside of protons. (One line of evidence for this is shown here.) If quarks and anti-quarks, which have the same inertial mass (the type of mass that determines how they change speed when pushed) had different gravitational mass (which determines how gravity affects them), then protons and neutrons, too, would not have equal inertial and gravitational mass. [The many gluons inside the protons and neutrons could make this even worse.] Since electrons do have equal inertial and gravitational mass, protons and neutrons would then fall at a different rate than electrons do. The consequence would be that different atoms would fall at slightly different rates. High precision experiments clearly say otherwise. This poses additional obstructions to the idea that anti-quarks (and the anti-protons and anti-neutrons that contain them) could rise in Earth’s gravity.

At best, when it comes to mass and gravity, existing experiments only allow for minor differences between atoms and anti-atoms. To look for subtle effects of any such differences is one of the real, long-term goals of the ALPHA experiment. What they’ve done so far is a neat experimental coup, but despite the headlines, it does not change our basic knowledge of anti-particles in general or of anti-atoms in particular. For that, we have a few years to wait.

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14 Responses

  1. Thanks for pointing out the more problematic points with “antigravity” theories. Another would be that now that no difference has been seen the 99 % of energy in protons and antiprotons behaves (is) the same, so probing antiquarks would mean pushing the experiments to 1 % level. (Which seems to be the next generation experiment aims.)

    Re “no debate in the scientific community” there are always fringe opinions, whether these voices merit the label “debate” or not. Science’s article mentions one:

    “Did anyone seriously consider the possibility that antimatter would experience antigravity? In fact, yes. For example, in 2012 Chardin and a colleague hypothesized that the universe might contain equal amounts of matter and antimatter, with the latter subject to antigravity. That idea might seem like a nonstarter, as astronomers haven’t observed antimatter galaxies or knots of matter and antimatter explosively annihilating each other. But antigravity avoids that problem, Chardin says, and it could also do away with two of the biggest puzzles in cosmology: the mysterious dark matter whose gravity keeps the galaxies intact and the even weirder dark energy that is stretching space and accelerating the expansion of the universe.

    Subject to opposite forces, matter and antimatter would separate. Matter would clump to form galaxies. Antimatter would spread as thinly as possible between the galaxies and act like dark energy. Around the galaxies, moats of empty space would open that would prevent matter-antimatter annihilations and, oddly, mimic dark matter dynamically.

    The new result might seem to torpedo Chardin’s model, as it rules out antigravity equal in strength to gravity. However, Chardin notes that all his theory really requires is antimatter that’s subject to some amount of repulsion, and the result isn’t precise enough to rule that out. “Pure antigravity is excluded,” Chardin says. “Beyond that I would be careful.”

    Kostelecky points out that it’s relatively easy to construct a coherent theory in which the effects of gravity on matter and antimatter differ to any degree you like. So it will be important to boost the precision of experiments like ALPHA’s, he says.”

    Finally, the CERN link makes the not so careful claim that “Einstein’s General Relativity is incompatible with currently accepted quantum field theories” and the comment thread add similar claims that gravity isn’t a force and gravitons are fictional. For all cosmological purposes, outside of rapidly spinning black holes and claims that the universe is necessarily de Sitter, a low energy effective quantum gravity field is perfectly compatible (though underscores some non-local vs local properties of quantum fields that explains the equivalence principle). See e.g. Donohue’s work on “Quantum gravity as a low energy effective field theory” – FWIW the late Weinberg supported it in his historical theory review. The de Sitter claim seems odd in a FLRW universe that admits the observed flat solution, and who knows ‘what lurks in the hearts of black holes’!?

    1. The problem with the model of Chardin and Benoit-Levy is that it completely ignores quantum physics and makes no effort to resolve the puzzle posed in this post: they restore the symmetry between matter and antimatter, impose opposite gravitation for them, and do not explain why radiation attracts. It’s a purely classical model, and as such can disregard consistency conditions imposed by quantum theory. But physics is quantum, and I see no way to make the model consistent — independent of Kostelecky’s remark, which I think has nothing to do with the Benoit-Levy/Chardin model (which they call a Dirac-Milne universe.)

  2. Thanks for this helpful outline. The term “anti-matter” has a somewhat mysterious ring to a non-specialist. On a possibly related point, it is generally asserted that dark matter exists, and further that we know/deduce its existence partly through gravitational effects. Could we say that all matter (including anti-matter), discovered or as yet undiscovered, is likely or certain to be subject to gravity? Is this or should it be part of a standard definition of matter?

    1. All physical things (matter or not) are subject to gravity in Einstein’s view (which experiment supports), and because of this, defining “matter” though gravity would not be a good idea.

      The word “matter” has a whole host of different definitions (see my previous post, which addresses this issue in part). But all the definitions agree that ordinary atoms are examples of matter, and light (and more generally electromagnetic waves of all sorts, including radio waves, gamma rays, and everything in between) is not matter. Both atoms and light are subject to gravity, and so, as far as we can tell, is everything else.

      The word “anti-matter” has its own ambiguities, but again, everyone agrees that anti-atoms are anti-matter (even though, using other definitions, they are also matter!) The word’s mysterious ring comes from science fiction, not science. Within science, there are some mysteries, such as why the universe has more quarks than anti-quarks and more electrons than positrons — but there is no mysteriousness. Positrons were discovered almost a century ago, and positrons are used in medical technology (PET scans). Anti-quarks are found in protons and neutrons. The equations for these types of particles are fully known and have been studied in great detail.

      1. Dr.Strassler:
        In Eisenstein’s description of gravity, isn’t anything that has energy subject to gravity? The greater the energy density, the greater the gravitational effect? Photons are subjected to gravity, because although they have no “mass”, they do have energy. In Newton’s version of gravity, “mass” is the dominant contributor, only because it has the highest energy density thing that is obvious (observable). Although, if I remember correctly, Newton himself had no concept of energy.

        1. Yes, that’s correct. Any attempt to measure whether anti-matter behaves differently from matter in the Earth’s gravitational field is testing whether Einstein’s description is correct. A surprise would indicate either a failure of Einstein’s viewpoint or the existence of a new force that is non-gravitational and affects atoms and anti-atoms differently.

      2. If we’d instead followed the Jack Williamson books from the early 1950s, we’d call it “seetee” (CT, contra-terrene), which I always thought was cute. But even those books from back then also used the term anti-matter.

        It surprises me sometimes how much of my earliest science education came from the SF books of that era (1950s and 1960s). Not likely to happen with modern writing, but back then there was a lot of SCIENCE fiction.

  3. This is, to me, checking an important box. Of course, no one I think expected that antimatter would experience anti-gravity – certain no physicist I know thought that way. But, you don’t really know until you measure it, and so this seems like a worthy measurement to make.

  4. “Fall”, “Rise” ?? I have heard thing “fall” into a gravity well but this is the first time I read the word “rise” wrt to gravity.

    I was under the impression, in accordance with Einstein’s theory that particles, mass neither falls nor rises but rather moves in the curved space created by the larger mass. In other words each mass creates their own curvature in space and the mass just flow in their own resultant vectors.

    So, what’s this “floating” all about? For something to “float” it would require a magnetic field, yes/no?

    1. It’s a matter of language and perspective (and to some extent, theory.) Which is a nice relation to Professor Strassler’s last post.

      If you take the einsteinian view, the describing gravity as a force is incorrect, it’s a pseudoforce in the manner of centripetal force. On the other hand particle physics tends to treat it as one, and perhaps this is why it vexes them so, they are treating as real things (like gravitons) that just aren’t.

      But trying to be ‘accurate’ with language is a minefield. Unless you wish to fill your words full of footnotes and asides (As Professor Strassler does in regards to matter in this article) then you’re bound to confuse more than clarify. There must be a balance struck, especially when dealing with laymen, where you can be understood but also concise.

      1. In my view, your statement about particle physicists isn’t correct. First, particle physicists know that when gravity is weak, it is perfectly ok to treat it just like any force; you can switch language to that of Einstein’s anytime you want. (In Kaluza-Klein theory, electromagnetism is equally fictitious.) Second, gravitational waves are real — they carry energy and make LIGO vibrate, and they can rip your body apart if they’re strong enough. Gravitons are just quanta of gravitational waves. These things are real, and not the reason gravity vexes particle physicists (or anyone else.) Only when you get to the non-linear quantum effects of gravity and try to make sense of them at ultra-short distances are there any problems.

  5. Dr.Strassler:
    Maybe I’m missing something here, but I didn’t think there was any debate or issue with the way Anti-matter would fall in a gravitational field. Antimatter is still “stuff” in that it has positive mass, just the opposite charge. Why would it fall up? The positron has the same mass as the electron, just a positive charge. What debate am I missing here?

    1. The news media, in its reporting today, described it as though it was somewhat of a debate. The point of this blog post is that to explain that indeed there was no debate in the scientific community, and to explain why there was no debate. So you are not missing a scientific debate, only a news media blitz.

    2. It’s a lot like the possibility of faster-than-light particles; it’s not really considered a serious possibility in the scientific community, at least not in any extreme sense. But it’s been postulated, some passionately advocate for it, and the public is much less aware of WHY it isn’t expected. As such it’s easy to cultivate the idea that it’s a puzzling controversy that could change physics at any moment. A look at any youtube comments section is illustrative there.

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