Particles of matter and anti-matter respond equally to the same force in a gravitational field, at least to within the capacity of an experiment at CERN to distinguish. The new finding accords with the Standard Model of Particle Physics. However, it provides no assistance in solving one of the biggest problems in physics: explaining why is there more matter in the universe than antimatter.
The universe has a lot of matter and very little anti-matter, unless we make it ourselves. That's fortunate for us – a universe with large amounts of antimatter just waiting to annihilate any matter it encountered would be a much more dangerous place. It's also confusing since we would expect the Big Bang to have created equal amounts of each. The shortage of anti-matter means we also know very little about how it behaves.
To explain the preponderance of matter, there must be some way that anti-matter is not perfectly symmetrical with matter in its properties. Finding that asymmetry has become one of science's great quests – one that has so far failed to find its goal, but taught us plenty on the way. The latest example, published in Nature, reveals the asymmetry does not lie in responses to gravity, or if it does the difference is so small it is within the study's tight margins of error.
A team led by CERN's Dr Stefan Ulmer confined anti-protons (the anti-matter equivalent of protons) and negatively charged hydrogen ions in a Penning trap. The trap forces particles to move in a near-circle with a frequency dependent on the magnetic field strength of the trap and the charge-to-mass ratio of the particle.
Testing how gravity affects anti-matter was not the primary purpose of the experiment. It was actually an attempt to find out if there are differences in the charge to mass ratio of protons and anti-protons, another possible explanation for the much-sought asymmetry.
The team compared the frequencies of the anti-protons and ions in different fields. "By doing this, we were able to obtain a result that they are essentially equivalent, to a degree four times more precise than previous measures,” Ulmer said in a statement. In another release Ulmer noted “The charge-to-mass ratio is now the most precisely measured property of the antiproton.”
Based on 24,000 comparisons, the authors concluded the mass-to-charge ratio is the same to a precision of 16-parts-per-trillion.
The observations were taken over 18 months, allowing comparison of behavior when the Earth was at both its furthest and closest from the Sun, to see if changes in solar gravity would have any effect. The precision here was much less – three parts in 100 – but Ulmer said; “This limit is comparable to the initial precision goals of experiments that aim to drop antihydrogen in the Earth’s gravitational field.”
The two particles chosen were not exactly equivalent. The hydrogen ions consist of a proton and two electrons, whereas the anti-protons stand alone. The experiment was conducted this way so both sides of the comparison would have a negative charge, rather than one being positive and the other negative. However, the study allows for the extra mass of the two electrons, and again finds no differences between matter and anti-matter on this measure.
The work is a vindication of the Standard Model, which treats some properties of matter and anti-matter as identical, while others are thought to be the exact opposite of each other. It is accepted we will need to go beyond the Standard Model to explain the universe's asymmetry, and the findings tighten the limits on models that attempt to do this.