Science in Plain English · Particle Physics · 2023
A plain-English review of the landmark 2023 ALPHA experiment that finally answered one of the oldest questions in physics — and why one man who died in 1727 deserves a share of the credit.
Reviewing: Anderson et al., Nature 621, 716–722 (2023) · CERN ALPHA Collaboration
Newton watched an apple fall. That simple observation gave us the theory of gravity. Three hundred years later, a team of roughly fifty scientists from across the world gathered at one of the most complex machines ever built — a particle accelerator buried beneath the Swiss-French border — to ask a question that Newton never could have imagined: what if you made an apple out of antimatter? Would it fall the same way?
Nobody knew. And that not-knowing had gnawed at physicists for nearly a century. In September 2023, the answer finally arrived, published in the journal Nature under the modest title: “Observation of the effect of gravity on the motion of antimatter.”
It is, quietly, one of the most important experimental results in modern physics. This article will walk you through what they did, who did it, what they found, and why it matters — without a single equation.
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First Things First: What Is Antimatter?
Everything in the universe is made of matter. But physics tells us there is a mirror-image version called antimatter, where particles are identical in every way except their electric charge is flipped. When a particle of matter meets its antimatter twin, they instantly destroy each other in a burst of pure energy.
Antimatter isn’t science fiction — we can make it. Scientists have created antihydrogen atoms at CERN, and tiny amounts of antimatter are produced naturally when cosmic rays from space collide with Earth’s atmosphere. It’s just incredibly short-lived, because it annihilates the moment it touches regular matter.
The great unsolved mystery is this: the Big Bang should have created equal amounts of matter and antimatter, which would have wiped each other out completely — leaving nothing but energy and an empty universe. Yet here we are. Something caused a tiny imbalance — roughly one extra matter particle for every billion pairs — and that leftover matter became every star, planet, and galaxy we see today. Nobody knows why that imbalance existed, and it remains one of the biggest unsolved questions in all of science.
And What Is Antihydrogen, Exactly?
Antihydrogen is the simplest antimatter atom — the mirror image of ordinary hydrogen. Where a normal hydrogen atom has a positively charged proton at its core with a negatively charged electron orbiting it, antihydrogen flips that completely: it has a negatively charged antiproton at its core with a positively charged positron orbiting it.
Hydrogen is the most studied atom in science — we know its behaviour with extraordinary precision. That makes antihydrogen the perfect measuring tool. If you create antihydrogen and find even the tiniest difference in how it behaves compared to hydrogen, that’s a massive deal. It could point to why matter won over antimatter in the early universe — solving, in part, the deepest mystery in physics: why there’s something rather than nothing.
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The Experiment: They Just Dropped It
For decades, one deceptively simple question had never been answered: does antimatter fall down? It sounds almost embarrassingly basic. But it genuinely wasn’t known. Some theoretical models suggested antimatter might actually fall upward — that gravity might repel it rather than attract it. If true, that would have rewritten physics and potentially explained the accelerating expansion of the universe.
The problem was practical. You can’t just drop antimatter — the moment it touches anything, it explodes into energy. You first need to create it, which is extraordinarily difficult. Then you need to hold it in place using powerful magnetic fields, without it ever touching the container walls. And only then, with extraordinary delicacy, can you turn off the trap and watch which way the atoms drift.
That is, in essence, what the ALPHA team did. They built a vertical apparatus — called ALPHA-g, the “g” standing for the gravitational constant — specifically designed for this purpose. It took over thirty years of accumulated research to get to this point.
“In physics, you don’t really know something until you observe it. This is the first direct experiment to actually observe a gravitational effect on the motion of antimatter. It’s a milestone in the study of antimatter, which still mystifies us due to its apparent absence in the Universe.”
— Prof. Jeffrey Hangst, ALPHA Spokesperson, CERN, 2023
They created antihydrogen by combining antiprotons — slowed down to near-stillness in CERN’s Antiproton Decelerator — with positrons captured from a radioactive sodium source. They then trapped the resulting neutral antihydrogen atoms in a magnetic minimum trap, preventing annihilation. Finally, they slowly turned off the magnetic field and recorded where the atoms went — up or down — as they escaped.
To be sure they weren’t being fooled by magnetic effects mimicking gravity, they repeated the experiment several times with different magnetic bias settings. The physics of the trap is complicated; the question being asked is not. Which way did it go?
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What They Found — in Plain English
In Plain English
Antimatter falls down. Just like everything else.
The team found that antihydrogen atoms, when released from their magnetic trap, fell toward Earth at a rate consistent with ordinary gravity — roughly the same rate as a regular hydrogen atom would fall. The odds of getting these results if gravity didn’t act on antimatter at all were about 1 in 3,000. And the odds of antimatter actually falling upward? So astronomically small that the scientists essentially said the number was meaningless — it simply isn’t happening.
In short: Einstein’s general theory of relativity, which predicts that gravity pulls everything the same way regardless of what it’s made of, passed yet another test. Even antimatter plays by the same rules.
The precise measurement the team reported was a gravitational acceleration of 0.75 ± 0.13 g (where g is the standard gravitational acceleration at Earth’s surface). That is, within their experimental uncertainty, antihydrogen falls at roughly the same rate as regular matter. The measurement is not yet precise enough to rule out small differences — that is the next chapter of this research — but it rules out the big, dramatic possibility that antimatter falls the wrong way entirely.
But Wait — What About the Skeptics?
Science is a conversation, not a decree. A subsequent paper by independent physicist Marcoen Cabbolet argued that the ALPHA data are actually inconclusive — that the same results could, with 88.7% certainty, be interpreted as evidence that Einstein’s equivalence principle is violated. This is a minority view in the physics community, and the ALPHA team stands by their analysis. But it illustrates something healthy: even landmark results get challenged. The scientific process demands it, and future, more precise measurements will settle the matter definitively.
From the Paper — The Authors’ Own Conclusion
We conclude that the dynamic behaviour of antihydrogen atoms is consistent with the existence of an attractive gravitational force between these atoms and the Earth. We estimate a probability of 2.9 × 10⁻⁴ that a result, at least as extreme as that observed here, could occur under the assumption that gravity does not act on antihydrogen. The probability that our data are consistent with the repulsive gravity simulation is so small as to be quantitatively meaningless (less than 10⁻¹⁵). Consequently, we can rule out the existence of repulsive gravity of magnitude 1g between the Earth and antimatter. The results are thus far in conformity with the predictions of General Relativity.
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Why Does This Matter?
There are several reasons scientists cared about this question — and not all of them are purely academic.
Testing Einstein. General Relativity has passed every test thrown at it since 1915. But it has never been tested with antimatter. Science demands you check everything. Any crack in the theory could point toward a more complete understanding of the universe.
The expansion of the universe. If antimatter fell upward, it would mean matter and antimatter gravitationally repel each other. That could have explained why the universe is expanding at an accelerating rate — enormous, invisible regions of antimatter somewhere pushing everything apart. A tidy solution to another enormous unsolved problem. It didn’t pan out, but it was a genuine scientific hope.
And yes — the dream of propulsion. If antimatter repelled gravity, you’d have a theoretical path toward what science fiction has called an antigravity drive. Combine that with the staggering energy released when matter and antimatter annihilate — gram for gram the most energetic reaction known to physics, millions of times more powerful than nuclear fission — and you have the basic ingredients that Star Trek’s warp drive runs on. The cruel irony is that even if antimatter had fallen upward, a practical drive would still be centuries away. CERN produces antimatter at a rate where, if you collected it for a million years, you’d have enough to boil a cup of coffee.
But ruling things out is not failure. Every eliminated possibility narrows the search. And the fact that antimatter behaves so consistently with regular matter under gravity actually deepens the mystery of why the universe contains so little of it — which means something profound is still waiting to be discovered.
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The People Behind the Paper
The paper was authored by the ALPHA Collaboration — an international team of approximately fifty scientists from seventeen institutions across nine countries. In the world of big physics, such papers are genuinely collective achievements; no single person “did” this experiment. It emerged from decades of shared work, accumulated technique, and inherited knowledge.
The three corresponding authors — the scientists who take final responsibility for the work — are among the most respected figures in antimatter research.
Prof. Jeffrey S. Hangst — The Architect
Hangst is the founder and long-serving spokesperson of the ALPHA collaboration, based at Aarhus University in Denmark. A graduate of MIT and the University of Chicago, he has been stationed at CERN virtually full-time since 2001. He was a founding member of the predecessor ATHENA experiment, which in 2002 first synthesised antihydrogen from trapped plasmas — a result featured on the front page of the New York Times. In 2010 his ALPHA team demonstrated the first-ever trapping of antihydrogen atoms. He has received the European Physical Society’s accelerator award, the John Dawson Award for Excellence in Plasma Physics Research, and the Ångström Medal from Uppsala University. He is a Fellow of the American Physical Society and a member of the Royal Danish Academy of Sciences. Outside the lab, he plays guitar in a cover band called Diracula — a nod to Dirac, the physicist who first theorised antimatter — and counts Roger Waters of Pink Floyd among his CERN visitors. He grew up in Ellwood City, Pennsylvania.
Prof. William Bertsche — University of Manchester
Bertsche is based at the University of Manchester and the Cockcroft Institute, one of the UK’s premier accelerator science centres. He has been a core member of the ALPHA collaboration for many years, specialising in the plasma physics and trapping techniques that make antihydrogen experiments possible. His work on the precise magnetic trapping of neutral antimatter atoms is central to what ALPHA-g achieved.
Prof. Joel Fajans — University of California, Berkeley
Fajans is a plasma physicist at UC Berkeley whose expertise in the behaviour of charged particle plasmas has been essential to ALPHA since its early days. Manipulating the positron and antiproton plasmas that combine to form antihydrogen requires deep knowledge of plasma confinement — his speciality. Berkeley has been a key institutional partner in the collaboration throughout its history.
The Wider Team
Beyond the three corresponding authors, the paper lists physicists drawn from institutions spanning Denmark, the UK, Canada, Brazil, Sweden, Israel, the United States, Italy, Switzerland and beyond. Among the institutions represented: Swansea University, TRIUMF (Canada’s national particle accelerator laboratory), the Universidade Federal do Rio de Janeiro, Simon Fraser University, York University (Canada), Ben-Gurion University of the Negev, and the University of British Columbia. This is not one country’s achievement. It is humanity’s.
- S. Hangst -Aarhus University, Denmark & CERN
- Bertsche – University of Manchester & Cockcroft Institute, UK
- Fajans – UC Berkeley, USA
- Charlton -Swansea University, Wales, UK
- C. Fujiwara – TRIUMF, Vancouver, Canada
- L. Cesar – Universidade Federal do Rio de Janeiro, Brazil
- Robicheaux – Purdue University, USA
- Eriksson – Swansea University, Wales, UK
- Madsen – Swansea University, Wales, UK
- Momose – University of British Columbia, Canada
- Jonsell – Stockholm University, Sweden
- K. Anderson – Aarhus University, Denmark
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Where Does This Leave Us?
The ALPHA team is clear that this is a beginning, not a conclusion. Their measurement of gravitational acceleration — roughly 0.75 g — carries an uncertainty large enough that small differences between matter and antimatter gravity cannot yet be ruled out. The next phase involves laser-cooling the antihydrogen atoms to make them slower and more controllable, which will dramatically improve precision. Two other CERN experiments — AEgIS and GBAR — are pursuing the same question using different methods, and their results will either confirm or complicate what ALPHA has found.
Things become truly interesting, physicists say, when the measurement reaches a precision of about one part in ten million. At that level, any deviation from Einstein’s predictions would be nearly impossible to explain within the current framework of physics — and would open a door to genuinely new science.
“If you compare the situation with the sensitivity of the first prototypes of gravitational-wave detectors 50 years ago, which had to be improved by six or seven orders of magnitude before a detection could be made, anything is possible.”
— Diego Blas, Institut de Física d’Altes Energies, Barcelona
That is the spirit in which this result should be read. Not as a closed door — antimatter falls down, nothing to see here — but as the first footstep on a very long road. Newton watched an apple fall and saw a universe of rules. These scientists watched an anti-apple fall for the first time in history, and confirmed that those rules hold. What they haven’t yet done is look close enough to find where, if anywhere, those rules might bend.
That is the next experiment. And after thirty years of work just to get here, they are already running it.
Two Giants, One Truth
There is a pleasing irony buried in this result that tends to get lost in the technical language of the paper. We live in an age that loves to pit scientific giants against each other — Newton versus Einstein, classical physics versus relativity, the old order versus the new. And it is true that Einstein’s general theory of relativity replaced Newton’s law of universal gravitation as our best description of how gravity works. Einstein showed that gravity isn’t really a force at all, but a curvature of spacetime caused by mass — a concept Newton never imagined and likely couldn’t have.
Yet what the ALPHA experiment quietly confirms is that at the most fundamental level, both men were pointing at the same truth.
Newton said: everything falls. It doesn’t matter what it’s made of, where it came from, or how it got here. Drop it, and it falls. Einstein said essentially the same thing with far greater mathematical precision — his Weak Equivalence Principle holds that all masses, regardless of their composition or internal structure, respond identically to gravity. No exceptions.
The ALPHA result is the most extreme test of that shared intuition ever attempted. Not an apple. Not a planet. Not light bending around the sun. But an atom built entirely from antimatter — matter’s own opposite, which didn’t even exist as a concept until a century after Newton died and thirteen years after Einstein published relativity — and it fell. Straight down. Just as both men, separated by two and a half centuries, would have quietly expected.
Newton watched an apple and trusted what he saw. Einstein built the mathematics to explain why. And in a laboratory beneath the Swiss countryside in 2023, fifty scientists confirmed that the universe is still playing by those same rules — even when the apple is made of the strangest stuff that exists.
Perhaps that is the deepest message of this paper. Not that antimatter falls, but that after all this time, after all the revolutions in our understanding, the universe remains stubbornly, elegantly consistent. Newton and Einstein weren’t rivals separated by centuries. They were collaborators separated by time, both chasing the same thing — and both, it turns out, were right.
Source Paper: E. K. Anderson, C. J. Baker, W. Bertsche et al. (ALPHA Collaboration), “Observation of the effect of gravity on the motion of antimatter,” Nature 621, 716–722 (2023).
Available via OSTI: https://www.osti.gov/biblio/2281639
Also published in Nature: https://www.nature.com/articles/s41586-023-06527-1
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