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Richard Ruiz | Univ. of Pittsburgh | U.S.A.

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Weighing Antimatter

How much does antimatter weigh?

It is a great question and to be honest physicists don’t know. In fact, it is a great question precisely because we don’t know. To clarify: I am talking about “weight,” not “mass.” I wrote a few words at the bottom of this post about the difference between the two. For now I will just say that mass is what makes pushing or pulling something in a new direction harder; weight is that pull, by a planet’s gravity, on things that have mass. In the Universe, there are some kinds of matter that do not have mass, like photons (packets of light), and thus are also weightless. Other kinds of matter, like protons & electrons, do have mass and consequentially weigh something.

Figure 1: CERN’s Atomic Spectroscopy And Collisions Using Slow Antiprotons (ASACUSA) Experiment. (Photo: CERN)

Okay, so here is where things get interesting. Back in the 1920′s a guy named Paul Dirac discovered the theory of antimatter.  The theory not only predicted that each piece of matter has an “antimatter partner” but also that the two partners have the same mass. This morning, the ASACUSA Experiment (Fig. 1) at CERN announced that the anti-proton has the same mass as its partner, the proton. Well, at least up to experiment’s capabilities of resolving the two. Anyone keeping track of CERN’s anti-matter physics program, or has watched the first 15 minutes of “Demons & Angles,” might know that the lab has been making significant progress trapping and collecting anti-hydrogen. While the amount being produced at CERN may not be enough to make a small city-state disappear, it is close to the amount needed to determine the weight of anti-hydrogen. This is good news for physicists at Fermilab who are working on the Antimatter Gravity Experiment (AGE), the goal of which is to measure anti-hydrogen’s weight. Interesting, no?

Figure 2: A hydrogen atom consists of an electron and a proton orbiting around one another, and are kept together because of their mutual electric attraction. Similarly, an anti-hydrogen atom consists of a positron (anti-electron) and an anti-proton. (Image: Wikipedia)

Now for the exciting part. Our theories, e.g. the time-tested Standard Model of Physics, only say that matter-antimatter partners should have the same mass. There is NO reason whatsoever, other than helping one sleep at night, that the partners should have the same weight. Since weight is innately related to gravity, any measurement of an individual anti-particle’s weight is inherently a measurement of gravity at the quantum scale. Additionally, any description of the behavior of antimatter acting under gravity is at the very least a stepping stone to Quantum Gravity. Quantum Gravity, by the way, is the theory of gravity at the microscopic scale; it does not really exist, yet; and is preventing physicists from constructing a full description (theory) of our universe. Determining that the proton and anti-proton have the same mass makes it easier to spot any differences in their weight. On top of that, if there is a difference in the weight of hydrogen & anti-hydrogen, then it might also explain why there is so much more matter in the universe than antimatter.

If you are not excited by now, I give up. :) Note: A big thanks to @symmetrymag for bringing this news to my attention.

A Few Words on Mass vs. Weight


Physically, “inertia” is the natural resistance to a change in movement; a measurement of inertia is called “mass.” One way to think about mass is if you & I were running down a football pitch, side-by-side, and you tried pushing me over. Mass is that bit of resistance you feel when you try pushing me over. If I were twice as tall, it would be harder to push me over. If I were half as tall, it would be easier to push me over. Next time you are playing football, like right after you read this Quantum Diaries post, try it out. “Weight” is that specific, attractive pull (force) a planet has on an object. The big difference is that mass is universal property of an object whereas weight can vary. A single electron will always have the same mass but a human will weigh less and less the further away he/she is from the Earth. Since this rock I like to call home is approximately a sphere, the gravitational pull it has at its surface is approximately constant. Consequentially, the difference between 1 lb (a unit of force) and 1 kg (a unit of mass) is a numerical constant. I hope this helped.


Happy Colliding.

- richard (@bravelittlemuon)


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3 Responses to “Weighing Antimatter”

  1. [...] As usual, Richard has written a really good explanation of his topic. Read the full article here. [...]

  2. Uncle Al says:

    “matter-antimatter partners should have the same mass. There is NO reason whatsoever, other than helping one sleep at night, that the partners should have the same weight.” There are excellent reasons why the Equivalence Principle (EP) is true for all measurable observables – and a simple experiment in existing apparatus to violate the EP with an unmeasurable observable, without contradiction.

    Any differential system with (virtual) antimatter-contributing components versus none qualifies. Eötvös experiments detect no composition or field EP violations to 5·10^(-14) difference/average. Lunar laser ranging observes zero Nordtvedt effect. 1.74 solar-mass 465.1 Hz pulsar PSR J1903+0327 plus a 1.05 solar-mass star are a 95.17-day orbit binary system: 15.3% (AP4 model) vs. 0.0001% gravitational binding energy, 1.8·10^11 vs. 30 surface gees, 2·10^8 gauss vs. 5 gauss magnetic field; compressed superfluid neutrons and superconductive protons vs. proton-electron plasma, extreme isospin and lepton number divergence; and pulsar 11% (AP4) of lightspeed equatorial spin velocity are differentially EP-inert for orbit, periastron precession, and gravitation radiation orbital decay. 1.97 solar-mass 317.5 Hz PSR J1614-2230 and a 0.5 solar-mass He-C-O white dwarf contrast pulsars with Fermi-degenerate matter, 20% versus 0.01% gravitational binding energy, to no EP anomaly.

    Teleparallel gravitation (“Fernparallelismus,” arxiv:0812.0034, physics/0503046, 0405142) in Weitzenböck spacetime with spacetime curvature (analogous to Lorentz force) is chiral. (metaphoric) Left and right shoes will vacuum free fall non-identically given a vacuum left foot. Socks (geometrically achiral atomic mass distributions) and unmassed bosons (e.g., photons, arxiv:0912.5057, 0905.1929, 0706.2031, 1106.1068) will be differentially inert. The quantitative difference between opposite shoes cannot be measured, physically or electronically,


    Test mass reduction to practice is outside physics. Crystallography builds mathematically perfect opposite shoes, emergent at atomic scale and self-similar to kilogram size, as 11 pairs of enantiomorphic space groups in 230 space groups total. Three of those pairs have no opposite sense or racemic screw axes within a single space group. Chemically and macroscopically identical, opposite geometric parity atomic mass distributions can falsify the EP without cotradiction of any prior observation at any scale in any venue,

    Two geometric parity Eotvos experiments.
    P3(1) | P3(2) gamma-glycine 127.1 atoms/nm^3
    P3(1)21 | P3(2)21 alpha-quartz 79.64 atoms/nm^3

    Somebody should look. The worst it can do is succeed.

    But what of Noether’s theorems, vacuum isotropy, and conservation of angular momentum! Noether is valid for continuous or approximate discontinuous symemtries. Geometric parity is the singular absolutely discontinuous external symmetry. Conservation of angular momentum would have a subtle trace violation, sourcing Milgrom acceleration for massed fermions in MOND. What fun!

    Physics fundamentally postulates vacuum mirror symmetry then manually inserts flurries of symmetry breakings when theory fails vs. obseravation. Quantum gravitation, SUSY, and dark matter are disasters. Mathematics can model anything. Rigorous derivation from defective postulates does not make deeply parameterized complex models real world true,


  3. [...] Weighing Antimatter (quantumdiaries.org) [...]

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