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Antimatter Gravitation

November 13, 2023

I learned a lot of useful home maintenance techniques while working with my father for a few summers during high school. My father was a carpenter-contractor who built custom houses from the ground up. This included laying cast iron sewer lines at the lowest level, to application of roof shingles at the highest level. This was also my first experience with a shoddy Chinese product, a keg of improperly heat treated 16d nails that were more likely to bend, than drive true, when hammered. This was in the early 1960s; now, sixty years later, shoddy merchandise is still a problem.

Every dollar spent on materials was a dollar out of my father's pocket, and the reason for the purchase of these nails was their low cost, the same reason for purchases of many online items today. However, as they say, "you get what you pay for." My father was also critical of his son's using too many nails to affix such things as floor underlayment. His frequent rant was, "Where is it going to fall - Up?" I was reminded of this by the results of a recent experiment at CERN, the ALPHA-g experiment, that finally proves that antimatter behaves gravitationally like ordinary matter and does not fall up.[1-2]

The existence of antimatter was theorized by Paul Dirac (1902-1984) in the eponymous Dirac equation of 1928. The theory was verified by the discovery of the positron, the antimatter version of the electron, by Carl Anderson (1905-1991) in 1932. Aside from their complementary elementary charge, matter and antimatter appear to have identical physical properties, as experiments have confirmed to high precision. This makes all the more mysterious the fact that anti-matter is nearly absent from our universe, where just ordinary matter remains. The gravitation experiment is the latest in a long line of experiments to determine the cause.

P.A.M. Dirac

Paul Dirac (1902-1984).

Dirac's physics contemporaries, including Einstein, noted his unusual personality. Dirac was taciturn and dreaded any small talk and socialization. While at Cambridge, his colleagues defined a unit they called a dirac, which was one word per hour.

Such traits likely derived from an unhappy childhood centered around his strict and authoritarian father, a French teacher, who required his children to speak to him only in French, so they would learn the language. Dirac was silent most of the time because of his limited knowledge of French.

Interestingly, Dirac's undergraduate degree was in electrical engineering, in which he excelled; but, employment was scarce at the time of his graduation. Because of this, he continued his education, eventually getting a PhD at Cambridge in 1926 by writing a dissertation on quantum mechanics, the first such dissertation to be submitted anywhere.

In notable agreement to the 1960 magazine article, "The Unreasonable Effectiveness of Mathematics in the Natural Sciences" by his brother-in-law, Eugene Wigner (1902-1995), Dirac wrote the following in a May, 1963, article, "Why is nature constructed [mathematically]? One can only answer that our present knowledge seems to show that nature is so constructed. We simply have to accept it. One could perhaps describe the situation by saying that God is a mathematician of a very high order, and He used very advanced mathematics in constructing the universe."[3]

(A 1933 Nobel Foundation photograph, via Wikimedia Commons))

A CERN experiment concurrent with ALPHA-g, the AEgIS experiment, plans to measure the gravitation of antimatter by examining the behavior of an antihydrogen beam. An antihydrogen atom is the antimatter complement of hydrogen in which the electron is replaced by a positron, and the proton is replaced by an antiproton. Extreme precision in AEgIS is obtained through the use of interferometry.[4] The cold antihydrogen atoms travel at a speed of a few hundred meters per second.[4] A Moiré deflectometer is used to measure the vertical displacement arising from gravity, since the deflection over a meter's travel is just a few micrometers.[4]

The AEgIS experiment is conceptually simple, but production and handling of the antimatter is critical to its success, and the success of ALPHA-g. One problem is that antimatter combines with normal matter to produce photons in a mutual annihilation, a process that's an important energy source in the Star Trek fictional universe. The complete annihilation of matter and antimatter by mass-energy equivalence is considerably greater than the mere fusion of hydrogen into helium, as happens in our Sun. In that case, just 0.71% of the original rest mass is converted.

An important part of any antimatter experiment is the creation of a sufficient quantity of antihydrogen that's stable for an extended period. CERN scientists were able to capture 309 antihydrogen atoms for up to 1,000 seconds in June, 2012.[5] Importantly, these atoms were contained long enough for many of them to reach their ground state, a necessary condition for an accurate measurement. Trapping and accumulation of antihydrogen atoms are now routine, and up to several thousand atoms have been simultaneously stored for the ALPHA-g experiment.[1]

Since gravity is such a weak force, it's usually masked by stronger electromagnetic forces. The repulsive electric force between two electrons is nearly 1043 times stronger than their gravitational attraction. Antihydrogen atoms are neutral, so electrostatic forces are not an issue, but gravity-level forces are equivalent to a magnetic field of the order 10-6 gauss. Earth's magnetic field varies with location, but it's of the order of 0.5 gauss. The ALPHA-g experiment was designed to overcome this source of error by containing the atoms in a magnetic trap called a Penning trap.

The ALPHA-g experiment at CERN

The ALPHA-g experiment at CERN.

The ALPHA Collaboration consists of scientists from twenty-six institutions.[1,6]

(CERN photograph. Click for larger image.)

The antiprotons for the ALPHA-g experiment were produced by particle collisions in CERN's accelerators, and they were slowed through deceleration in a ring.[2] The antiprotons and positrons were then mixed to form atoms of antihydrogen that are contained in a vertically oriented antihydrogen trap.[2] In the experiment, trap doors at the top and bottom were opened to detect the up/down motion of the atoms in Earth's gravitational field.[1] There is some thermal motion; so, the gravity is inferred from the difference in the numbers moving upwards and downwards.[1] While it's been determined that antimatter falls down, and not up, there's the further question of whether the falling rate is the same as for matter.[2]


  1. E. K. Anderson, W. Bertsche, N. M. Bhatt, G. Bonomi, A. Capra, I. Carli, C. L. Cesar, M. Charlton, A. Christensen, R. Collister, A. Cridland Mathad, D. Duque Quiceno, S. Eriksson, A. Evans, N. Evetts, S. Fabbri, J. Fajans, A. Ferwerda, T. Friesen, M. C. Fujiwara, D. R. Gill, L. M. Golino, M. B. Gomes Gonçalves, P. Grandemange, P. Granum, J. S. Hangst, M. E. Hayden, D. Hodgkinson, E. D. Hunter, C. A. Isaac, A. J. U. Jimenez, M. A. Johnson, J. M. Jones, S. A. Jones, S. Jonsell, A. Khramov, N. Madsen, L. Martin, N. Massacret, D. Maxwell, J. T. K. McKenna, S. Menary, T. Momose, M. Mostamand, P. S. Mullan, J. Nauta, K. Olchanski, A. N. Oliveira, J. Peszka, A. Powell, C. Ø. Rasmussen, F. Robicheaux, R. L. Sacramento, M. Sameed, E. Sarid, J. Schoonwater, D. M. Silveira, J. Singh, G. Smith, C. So, S. Stracka, G. Stutter, T. D. Tharp, K. A. Thompson, R. I. Thompson, E. Thorpe-Woods, C. Torkzaban, M. Urioni, P. Woosaree, and J. S. Wurtele, "Observation of the effect of gravity on the motion of antimatter," Nature, vol. 621, no. 7980 (September 28, 2023), pp. 716-722, https://doi.org/10.1038/s41586-023-06527-1. This is an open access article with a PDF file available at the same URL.
  2. Pallab Ghosh, "Scientists get closer to solving mystery of antimatter, BBC News, September 27, 2023.
  3. Paul Dirac, "The Evolution of the Physicist's Picture of Nature," Scientific American, May, 1963 (Republished, June 25, 2010).
  4. G. Testera, A.S. Belov, G. Bonomi, I. Boscolo, N. Brambilla, R. S. Brusa, V.M. Byakov, L. Cabaret, C. Canali, C. Carraro, F. Castelli, S. Cialdi, M. de Combarieu, D. Comparat, G. Consolati, N. Djourelov, M. Doser, G. Drobychev, A. Dupasquier, D. Fabris, R. Ferragut, G. Ferrari, A. Fischer, A. Fontana, P. Forget, L. Formaro, M. Lunardon, A. Gervasini, M.G. Giammarchi, S.N. Gninenko, G. Gribakin, R. Heyne, S.D. Hogan, A. Kellerbauer, D. Krasnicky, V. Lagomarsino, G. Manuzio, S. Mariazzi, V.A. Matveev, F. Merkt, S. Moretto, C. Morhard, G. Nebbia, P. Nedelec, M.K. Oberthaler, P. Pari, V. Petracek, M. Prevedelli, I. Y. Al-Qaradawi, F. Quasso, O. Rohne, S. Pesente, A. Rotondi, S. Stapnes, D. Sillou, S.V. Stepanov, H. H. Stroke, G. Tino, A. Vairo, G. Viesti, H. Walters, U. Warring, S. Zavatarelli, et al., "Formation Of A Cold Antihydrogen Beam in AEGIS For Gravity Measurements," arXiv, May 30, 2008.
  5. The ALPHA Collaboration, "Confinement of antihydrogen for 1,000 seconds," Nature Physics, vol. 7, no. 7 (June 5, 2011), pp. 558-564.
  6. ALPHA Collaboration Website at CERN.

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