Gravity and antimater
(appeared in Feb 2016)

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Looking for something you can see only when it’s gone can take a long time, says S.Ananthanarayanan.

Star Trek and Dan Browne’s Angels and Demons have brought the word, antimatter, into everyday vocabulary. Stated simply, it is a kind of matter that annihilates, or disappears into a flash of energy, as soon as it contacts ordinary matter. How then can we, who live in an ordinary world, ever learn anything about this kind of stuff? Is it for real?

While all matter is made up, ultimately, of combinations of just three elementary particles, the electron, the proton and the neutron, a class of antiparticles, the positron, the antiproton and the antineutron and also antiparticles of other fundamental particles found in nuclear reactions, has been both predicted in theory and detected in the lab, and in cosmic rays. The antiparticle of a particle is exactly like the mother particle, except that it has the opposite charge, and also the opposite magnetic moment, because of the reversal of its components. But because of the fact that antiparticles arise essentially to balance the counter particle, with the help of the energy equivalent to the mass of both the particles, a particle-antiparticle pair cannot exist at the same place – they destroy each other, giving off just gamma rays, sometimes with a rearrangement of their components, if there are any.

But antiparticles behave normally with other antiparticles, and can form anti-atoms which have antiprotons in negatively charged nuclei and positrons in orbit, and even anti-molecules. There may exist somewhere, in fact, a whole anti-universe which is just like the one we are familiar with. But nearer home, we have real experience with a simple atom, the antihydrogen, which forms when positrons and antiprotons, which arise in radioactivity and collisions in accelerators, are brought together. A group of researchers at Liverpool, Berkeley, Lausanne, Manchester, Warrington (UK), London, Geneva Toronto, Vancouver, Aarhus, Stockholm, Calgary and Yavne, Israel report in the journal, Nature a further confirmation that atoms of antihydrogen are electrically neutral , just like ordinary hydrogen.

Significance

Now, why is this significant and why is it important? It is significant because work of just any kind with antimatter presents problems that do not exist with ordinary matter, the main problem being that antimatter is short-lived and rarely stays around in quantity. For all this, positrons and antiprotons, which are charged antiparticles, can be confined with the help of electric and magnetic fields and studied. Their masses and charges have thus been estimated with very high accuracy, as being equal to those of electrons and protons. And as the charge on the electron or proton are equal, to a very high degree of accuracy, the hydrogen atom is also expected to be electrically neutral, which has been confirmed to a very reliable extent in experiment.

This conclusion, however, has not been confirmed equally well for the antihydrogen atom. In the case of ordinary hydrogen, it is reasonably simple to generate a stream of atoms which can be subjected to strong electric fields before they are detected, to see if the stream is even slightly deflected. But with antihydrogen, there is first the problem of creating the atoms, by combination of positrons and antiprotons and then the difficulty of containing or creating a stream of the atoms. Positrons and antiprotons are charged particles and can be controlled by fields and kept away from contact with ordinary matter, but the antihydrogen atom is neutral (or very nearly so) and it strays, to be annihilated. Getting antihydrogen atoms and then definitely testing their electrical neutrality has thus been a challenge.

As the hydrogen atom has a magnetic moment, however, using magnetic fields becomes a way to trap the electrically neutral antihydrogen atom. Atoms which are not moving too fast can thus be confined within a ring of stronger magnetic field, like a golf ball in a depression. But the trap is not a strong one and it is only a single anti-atom that is generally trapped. And even this atom can be detected only when it gets out of the trap and annihilates. The best test of neutrality so far was of 2014, where antihydrogen atoms were created and confined within the trap in the ALPHA facility at CERN, near Geneva and subjected to electric fields as they escaped, to see if they deflected. The results, which were published in Nature Communications, were that the atoms were neutral to the eighth decimal place, which was a great advance over the previous limit of 1997. The current advance, whose result is a 20-fold improvement, uses a different method, not of deflecting emerging atoms, but testing if electric fields are able to knock atoms out the weak magnetic trap. Comparatively slow-moving antihydrogen atoms were trapped in a shallow magnetic depression and then subjected to pulses of electric fields. In case the atoms had any electric charge, it was worked out they would gain energy and escape the trap. The experiment was then to apply the field for some time and then switch off the trap, to see if any anti-atoms were left!

Importance

The fact that antihydrogen is found to be neutral does not come as any surprise, as all the theory, and this is exceedingly successful theory, says that this is the way it should be. In fact, finding anything else would be not just astonishing but would also upset our entire understanding of physics and the nature of things. The reason there is interest in being sure of the charge neutrality of antihydrogen is to eliminate uncertainty in experiments in another area of interest - to test the gravitational properties of antimatter.

For all the success of quantum physics and the General Theory of Relativity, the fact is that there is no bridge between the two areas of study and there are still gaping holes in our understanding of the universe. On the one hand, it is not clear why the universe is all ordinary matter and no antimatter. In the field of cosmology there is no proven mechanism that leads to the expansion of the universe and also the way galaxies are seen to rotate. Unlike the solar system, where the outer planets go round slower than the inner planets, in the case of a galaxy, the outermost regions go round much faster than expected. The only way to explain this seems to be by proposing some invisible matter as pervading the galaxy, to show itself only through gravity.

Another difficulty is with the Big Bang Theory of the origin of the universe – that theory says there should be a difference in the level of left-over radiation as seen in different parts of the universe. But what is observed is that the radiation is uniform over distances so large that there could have been no communication from opposite ends, within the known age of the universe. How this is so can be answered only by proposing very fast expansion during the first split seconds, so that places which were once in contact have now been flung so far apart.

Repulsive gravity

Finding devices to fix these deficiencies in the current theory calls for setting the universe free to expand, because of the presence of matter whose gravitational interaction would be repulsive, in place of being attractive. If antimatter were this kind of entity, the fact would pave the way to much progress in developing a theory of the nature and origin of the universe. The General Theory of Relativity itself would need refinement but finding candidate samples of dark matter or energy would be an advance indeed.

The force of gravity, however, is so weak that it is only the presence of a 6 million billion billion kg earth below us that has made it possible for us to be intuitively aware of gravity. While measurement of gravitational forces between positrons or antiprotons, whose weight is in billionths of a billionth of the billionth of a gram, is hence hard enough, the fact these objects are affected by electric fields makes detection of how gravity affects them simply out of question. Hence the interest in trying our luck with the antihydrogen atom, which may at least be electrically neutral .

But even with the antihydrogen atom, the force of gravity would be so weak that the slightest whisper of electrical effects would wreck the measurements. Experiments to show that the atom is electrically neutral cannot hence be accurate enough, to push as far back as possible the presence of electric effects that could lead us astray in case we were able to measure gravitational properties of antimatter.

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