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<   No. 3300   2013-10-06   >

Comic #3300

1 {photo of a spiral fountain}
1 Caption: Pro and Anti

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Erwin Schrödinger
Erwin Schrödinger. Public domain image from Wikimedia Commons.
The Ancient Greeks began with four elements: earth, fire, water, and air. The tail end of alchemy and the birth of chemistry saw these refined into a more accurate picture. There were dozens of elemental substances which could be combined in various ways, but not transmuted one into another. These elements included iron, copper, gold, carbon, sulphur, oxygen, and so on. Dmitri Mendeleev laid them out in a table according to periodicities in their properties.

It was when we discovered that the atoms of the elements were in fact not indivisible that we unlocked the secret of the periodic table. This also explained the mystery of electricity. Atoms were composed of what we describe as positively and negatively charged particles, respectively dubbed protons and electrons. Experiments showed the positive part was large and central, and the negative electrons were relatively tiny and distributed around the atom. Careful measurements determined that there were electrically neutral particles too, called neutrons. These were arranged with the protons in a small nucleus at the core of each atom, the bulk of an atom being empty space buzzing with tiny electrons, like moths around a flame.

This structure also explained the strange phenomenon of radioactivity. Atoms could change into atoms of another element by emitting either an alpha particle (comprised of two neutrons and two protons) or a beta particle (an electron) from their nucleus. In the case of an electron being emitted, the electron was formed by the process of a neutron "breaking apart" into a proton and an electron. In either case, the number of protons in the nucleus changes, and this number is what determines what element the atom is. So elements can be transformed from one into another. We figured out how to force this to happen too. By either splitting atoms or combining atoms we can create new atoms, and potentially release vast amounts of energy. And it turned out that fusing atoms was the power source of the stars, explaining a mystery some people thought could never be solved.

But this is not the end of the story of the atom. The fact that a neutron can fall apart into a proton and an electron hints at something deeper still. The next stage in unravelling the mysteries of the atom occurred rapidly, with a spectacular mixture of theoretical insights and experimental observations.

Paul Dirac
Paul Dirac. Public domain image from Wikimedia Commons.
In 1926, Erwin Schrödinger published his now famous wave equation, which described the possible positions of particles in regions of different energy potentials. This sounds a bit esoteric, but it can be used, for example, to calculate the distribution of electrons in an electrical field, such as around an atomic nucleus. It was an elaboration of the basic idea that electrons "orbit" around the nucleus, but taking into account then-recent findings on the wave properties of particles by Max Planck and Louis de Broglie. There is a lot more that can be said about Schrödinger's equation, but that is a side track for another day.

Just two years later, in 1928, Paul Dirac modified Schrödinger's equation to take into account Einstein's special relativity. He pointed out that the solutions to his new equation allowed the existence of a particle similar to an electron, but with a positive electric charge, opposite to that of the electron. Dirac treated this as a curiosity, and hoped that some future refinement of his theory would resolve the seeming problem. The "problem" was however resolved in a completely different way, when in 1932 Carl David Anderson noticed some strange particle tracks in cloud chamber experiments with cosmic rays. Electrons would produce spiral tracks, influenced by a magnetic field. These new particles also produced spirals, but turning in the opposite direction. This implied they had the same mass as an electron but a positive charge. Anderson had found Dirac's positively charged "anti" electrons. He called them positrons.

Positrons were the first example of antimatter shown to exist - particles that are weird mirror images of ordinary particles. They have the same mass as their normal matter counterparts, but other fundamental properties such as electric charge are the opposite. Another thing about positrons is that if one of them encounters an electron, both the positron and the electron vanish, generating a burst of energy in the form of a pair of gamma rays. All of their mass is converted into energy, leaving behind no particles at all. This reinforces the idea that the positron is a sort of backwards/negative/opposite version of an electron.

In 1930, between Dirac's prediction and Anderson's discovery of the positron, Wolfgang Pauli proposed another theoretical idea to explain a puzzling property of radioactive beta decay. Balancing the energy, momentum, and angular momentum of the particles before and after a beta decay can only be done if there is a third particle involved in the reaction, electrically neutral and extremely light, perhaps even massless. It was later called a neutrino. Untangling this took some time because the existence of the neutron itself was only confirmed by James Chadwick in 1932.

Irène Joliot-Curie
Irène Joliot-Curie. Public domain image from Wikimedia Commons.
In beta decay, a neutron decays into a proton, an electron, and a neutrino. Protons and neutrons are relatively heavy particles, with almost identical mass. These heavy particles were collectively termed baryons (from the Greek barys, meaning "heavy"). Possible reactions seemed to conserve the number of baryons. If you assign a property called "baryon number" such that protons and neutrons have baryon number 1, while electrons and neutrinos (and any other non-baryons) have baryon number zero, then the total baryon number in any reaction is conserved. This became a useful way of looking at particle reactions.

In a similar way, the light particles, electrons and neutrinos, were later (in 1948) termed leptons (from the Greek leptos, meaning small). Electrons were defined to have lepton number 1. To balance lepton number in the same way as baryon number, the neutrino formed in a beta decay had to have lepton number -1. In other words, it was an anti-neutrino. By building up experimental observations, in a manner very similar to the early chemical experiments of Antoine Lavoisier and his contemporaries, particle physicists realised there were certain rules governing how these particles transformed one into another. Electric charge before and after a reaction was conserved. Baryon number was conserved. Lepton number was conserved.

Further confirmation came from Frédéric and Irène Joliot-Curie (the daughter of Marie and Pierre Curie) in 1934, when they observed a new type of radioactive decay. In their experiments they realised the alchemist's dream of transmuting one element into another, and at the same time discovered something unexpected. They took a sample of aluminium and bombarded it with alpha particles. Some of the particles reacted with aluminium atoms to produce atoms of phosphorus. Aluminium normally has 13 protons and 14 neutrons. Naturally occurring phosphorus atoms all have 15 protons and 16 neutrons (the isotope phosphorus-31). So adding 2 protons and 2 neutrons from the alpha particle should produce exactly what is needed. However, to balance the energy of the reaction, one neutron is ejected, leaving behind 15 protons and just 15 neutrons in the phosphorus atom - a different isotope, phosphorus-30, which is not found in nature. The reason it is never found in nature is that this new isotope is radioactively unstable, and decays within a few minutes. How it decays is interesting.

A nucleus of phosphorus-30 has not enough neutrons to be stable. Looked at another way, it has too many protons. In the resulting decay, a proton turns into a neutron, ejecting a positron and a neutrino. This is sort of the opposite reaction to beta decay, in which a neutron turns into a proton, ejecting an electron and an antineutrino. This new form of decay was called beta-plus decay, or positron emission, and its discovery won the Joliot-Curie's the Nobel Prize for Chemistry in 1935. Positron emission is less likely to occur than beta decay because the total mass of the products can be greater than the mass of the initial particles (depending on the available binding energy of the nucleus), so energy may need to be added in to account for the mass difference. This energy can be available in nuclei that have been transmuted from another nucleus, with residual energy from the first reaction left over and stored as an "excited state" in the nucleus. Finally, for this new form of decay, the neutrino produced is a normal matter neutrino, since its lepton number of 1 is needed to balance the production of a positron with lepton number -1.

Emilio Segrè
Emilio Segrè. Public domain image from Wikimedia Commons.
But Dirac's antimatter predictions didn't stop there. His equations also allowed antiparticle counterparts of the proton and neutron. These took longer to be confirmed, but in 1955 Emilio Segrè and Owen Chamberlain managed to detect antiprotons in particle collision experiments using the Bevatron particle accelerator at Lawrence Berkeley National Laboratory. (Chalk up another Nobel Prize for our story - this discovery earned Segrè and Chamberlain the 1959 Physics Prize.) As you might expect, the antiproton is the same mass as a proton, but has negative electric charge and a baryon number of -1. Just a year later, in 1956, Bruce Cork used the same Bevatron machine to confirm the existence of the antineutron. This was a bit trickier, since neutrons have no electric charge, but the negative baryon number can be confirmed by observing what reactions involving antineutrons are possible.

The most characteristic reaction of antimatter particles is annihilation when they meet their normal matter counterparts. We've already mentioned this with electrons and positrons. The same thing happens when an antiproton and a proton meet - they vanish in a burst of energy, released as gamma rays. And as protons are nearly 2000 times as massive as an electron, the amount of energy released is correspondingly higher. And this is where we get our modern science fiction concepts of antimatter engines from. A tiny amount of antimatter can be made to produce an enormous quantity of energy, simply by mixing it with normal matter. Because of the enormous multiplier of the speed of light squared in Einstein's mass-energy equivalence, the amounts of antimatter needed are truly tiny - a few grams can release energy equivalent to all but the largest nuclear bombs ever built. Antimatter annihilation is roughly a thousand times more powerful than any nuclear reaction, because all of the mass is consumed and turned into energy, rather than leaving behind reaction products.

The difficulties in using antimatter as an energy source are manifold. First, you need to create the antimatter. With our current technology, we can create only incredibly small quantities of antimatter, and doing so takes more energy than we can retrieve by using it as a fuel. Secondly, there is the problem of storing it. You can't put antimatter into a fuel tank, because it will react with the fuel tank and destroy it in an enormous explosion. You can't put it in anything physical. This includes air - it will react catastrophically with air just as easily. One potential way to contain antimatter is inside a vacuum chamber with a magnetic field - the theory is understood but building such a device is another thing (not that it's worth doing so, since we don't have enough antimatter to put into it). The third problem is that if anything goes wrong, there's no hope of a harmless fuel leak, or getting a Scottish engineer onto the job and fixing things. Any failure whatsoever means an instant, enormous explosion.

PET scanner
Medical PET scanner. Public domain image by Jens Langner from Wikimedia Commons.
However, there are things we can and do use antimatter for. The most prominent is medicine. It's relatively easy to create artificial isotopes like the phosphorous-30 that the Joliot-Curies generated. Typical isotopes used for medicine include carbon-11, nitrogen-13, and oxygen-15. These are easily absorbed by the human body when delivered and carried through the bloodstream to various organs. They decay over a few minutes by positron emission. This is useful because when a positron is emitted inside the human body, it almost immediately encounters an electron and the two annihilate, releasing a pair of gamma rays that can be detected outside the body and triangulated back to the position of the decay. (This could not be done with a standard beta decay isotope, as that produces an electron, which is simply absorbed by tissue within the body.) The result is a scan of the tissues within the body that absorb the introduced isotopes, which can be useful for diagnosing various medical conditions such as cancers. This form of medical scanning is known as positron emission tomography, or PET. The amount of gamma radiation produced is small, but needs to be taken into account when deciding whether the scan is justified for the patient. In many cases, it can be life-saving.

Antimatter began as a theoretical curiosity, and was later confirmed as a reality. It has been applied to our flights of fancy about the future, as well as to our practical suite of tools for modern medicine. Like many things in science, it was first worked on by cutting edge research proposing odd things that could have no foreseeable practical implications, and has become a valuable part of our modern technology that improves people's health and lifestyles. It's nice to imagine what bizarre things science might possibly do in the future, and then realise that some of those things are a reality right now.

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