Wednesday, July 27, 2011

Antimatter

We humans have always been fond of symmetry. Our society has created numerous methods of classification based on the symmetric properties of our world: good and evil, beauty and ugliness, large and small, rich and poor etc. In general, we have classified things into two opposing categories. One may think that this is purely artificial, in the sense that the world isn't just black and white and some things do not have an opposite, for example many of these opposing concepts such as beauty and ugliness exist only because of our subjective way of appreciating beauty (a human concept). I don't think that anyone prior to the 20th century could have imagined that matter itself has an opposite. Antimatter is no artificial concept or mathematical tool that scientists created in order to satisfy certain equations, it is a real existing thing.

What exactly is antimatter?

Antimatter is the exact opposite of matter, which it annihilates when the two come into contact. Does annihilation mean that both matter and antimatter vanish? No. Annihilation means the contact between the two results in the production of energy. Einstein's famous equation E = mc^2 shows that mass and energy are equivalent, one can change into the other and viceversa, and one way in which we can transform mass into energy is by colliding matter and antimatter. I'll get back to this later on.
To address the issue of the nature of antimatter let us first consider what matter is made out of. Our bodies, the computer, the floor, all surrounding objects, the Earth, the Moon, the Sun and all other planets, stars, galaxies etc are made up of the same thing: atoms. It's pretty incredible when you think about it, that since atoms were formed, in the first stages of the Big Bang and the formation of the Universe, they've been recycled in various bodies such as gas nebulas which can eventually form stars and planets that make up star systems, all of which form galaxies. Our atoms, and the atoms of everything we see around us came from stars and nebulas, we are literally star dust.
Going even deeper now, what are atoms made of? The building blocks of atoms are: protons, neutrons and electrons. Protons and neutrons are heavy particles which bind together and form the nucleus of the atom. The protons have a positive electrical charge, while the neutrons are electrically neutral. Overall, the nucleus is positively charged because of the protons, and surrounding the nucleus are the negatively charged electrons.


Things go deeper still. The electron doesn't seem to be made up of anything else and in the Standard Model of Particle Physics it is considered an elementary particle, a particle with no further substructure. However, this is not the case with the proton and the neutron. The proton and the neutron are quite similar, they have almost the same mass. The keyword here is 'almost'. The proton is lighter than the neutron by a very small amount. Why is the proton slightly lighter?  The answer comes from the internal structure of these particles. Yes, experiments have shown that both protons and neutrons are actually each made up of three new particles called quarks. Quarks come in six flavors: up, down, top, bottom, strange, charm. The proton is made up of two up quarks and one down quark, while the neutron is made up of two down quarks and one up quark.

Neutron
Proton
                       

The up quark and the down quark have different masses, thus explaining the difference in mass between the proton and the neutron. Quarks also have charge. The up quark has a positive charge equal to +2/3 times the elementary charge, while the down quark has a negative charge equal to -1/3 times the elementary charge. We can now understand the charges of the proton and the neutron, the proton will have charge 2/3 + 2/3 - 1/3 = +1 times the elementary charge (positive), while the neutron has 2/3 - 1/3 - 1/3 = 0 times the elementary charge, so neutral charge.
It is currently unknown if quarks have an internal structure of their own, but apart from them, there are many many more particles in the standard model. Our world is quite diverse when it comes to particles, yet most of us are only familiar with the more common ones.
This is all very interesting but does it have to do with antimatter? Well, it turns out that each particle has a corresponding anitparticle. Antiparticles have opposite charge with respect to their matter counterparts, for example the antiproton is negatively charged. The antineutron is neutral, but is composed of antiquarks whose charges are the opposite of the quarks that make up the neutron (the same applies for the proton). The antiparticles have exactly the same mass as their counterparts, so an antiproton weighs exactly as much as a proton.
Basically, antimatter is the mirror reflection of matter. Antiatoms would consist of a negatively charged nucleus made up of antiprotons and antineutrons, surrounded by a cloud of positively charged antielectrons (called positrons). For all intents and purposes, these antiatoms would behave exactly like normal atoms and you could build an Earth entirely out of antimatter and it would, in theory, look exactly like our own (there is no color difference between matter and antimatter, for example an antileaf would still be green). Unfortunately we haven't experimented much with antimatter, very few antiatoms have been produced in particle accelerators and no source of antimatter has been found anywhere within our solar system. In fact, no meaningful quantities of antimatter have been observed anywhere.
One might ask: if matter and antimatter are symmetric how is that there seems to be more matter in the Universe? Why did the Universe prefer matter over antimatter? These questions have not been answered thus far. This represents one of the major unsolved problems in physics, something which scientists are trying to answer. It is believed that matter and antimatter are not actually perfectly symmetrical and some difference exists, which led to the production of far more matter than antimatter during the Big Bang. The nature of this symmetry breaking is unknown.

How was antimatter discovered?

At the start of the 20th century, two physical theories were born which would completely change the way we view the world: the theory of relativity and quantum mechanics. The theory of relativity, which was largely developed by Albert Einstein, dealt with massive objects, objects travelling near the speed of light, gravity and was mostly used in cosmology and astronomy since it accurately described the macrocosm (bodies on a large scale). Quantum mechanics on the other hand, which was developed by many physicists such as Heisenberg, Schrödinger, Bohr, Pauli, Dirac and others, dealt with the microcosm, the world of particles. Both theories had an astonishing power of prediction when it came to their respective fields. Naturally scientists thought that if you could unite relativity and quantum mechanics, you would essentially have a theory of everything. Unfortunately, this unification proved to be much more difficult than expected. The shocking revelation was that quantum mechanics and relativity contradict each other, they cannot both be true in their current forms. This is yet another unsolved problem in physics. 
While there are problems in unifying relativity and quantum mechanics, there are certain areas in which the two theories merge beautifully. One such case is the theory surrounding antimatter.
One of the milestones of quantum mechanics was the development of Schrödinger's equation. In simple terms, this equation gives the evolution and behavior of a particle (or a wave). You only need to know the particle's surroundings and how they affect it and the Schrödinger equation will tell you how the particle behaves. Schrödinger's equation is used, for example, in the study of atoms. The conditions in which the particle 'resides' are given as a term called the Hamiltonian. The only problem with Schrödinger's equation is that it doesn't take relativity into account and what this means is that it will produce wrong results for situations in which we deal with speeds close to the speed of light, or high energy physics. In these cases the classical Hamiltonian wrongly describes the conditions and one needs to introduce a relativistic Hamiltonian. Even in our usual non-relativistic scenarios, the solutions are just almost correct but the approximation is good enough and therefore there is no need for relativity, but obviously not all scenarios are non-relativistic and a general theory is needed. This means that Schrödinger's equation is a special case of a more general equation which takes relativity into account. This equation is called Dirac's equation. Prior to Dirac there were other attempts to incorporate relativity into Schrödinger's equation but only with limited success (resulting in the Klein-Gordon equation for example). The problem was purely mathematical and Dirac solved it (as the story goes, he was staring at his fireplace when the solution came to him) in a quite non-mathematical fashion which was latter proven to be correct. 
Dirac's equation was truly a marvel, uniting both quantum mechanics and relativity, it's predictive power was unchallenged. One of the predictions it made was the existence of antimatter. Dirac's equation allowed for the existence of negative energy solutions, something which is unphysical. To solve this problem, Dirac considered an infinite sea of negative energy particles in which no other particle could go because of the Pauli exclusion principle (a principle which states that two identical particles cannot occupy the same space and would thus repel each other). While no particle could go into Dirac's sea, it was possible for particles to leave the sea of negative energy, for example if an electron were to leave and thus acquire positive energy it would leave behind a 'hole', regarded as a positive energy electron with positive charge ... the positron. Thus we have arrived at antimatter. Dirac's interpretation was however wrong, there is no sea of negative energy particles as this leads to several contradictions, the good explanation came later on, but the existence of antimatter was indeed correct. 
Positrons were later discovered and produced in various experiments, thus confirming the theory. As mentioned earlier antimatter does not have 'antimass', a positron has exactly the same mass as an electron.

Positron-electron interaction


Other antiparticles have also been produced such as the antiproton and the antineutron. This has also led to the formation of antiatoms of hydrogen and helium (antihydrogen and antihelium). So far, however, only very small quantities of antimatter have been produced, as producing it in substantial quantities represents a great technological challenge. It is interesting to note that antimatter can be viewed as matter travelling backwards in time. In the above diagram, called a Feynman diagram, the time axis is vertical and goes upward and we can see that that the electron (on the left) goes forward in time, going upward, but the positron on the right goes downward, backward in time. It is believed that this doesn't have any profound implication, but is more a mathematical observation.

Applications

The only currently employed use of antimatter is in Positron Emission Tomography (PET) scans. The idea is to introduce a radionuclide (unstable element) radiating positrons into the human body and then detect the radiation emitted from the positrons annihilating with electrons from the atoms in the body. By capturing this radiation you can create a three-dimensional picture of the area of interest. 
Aside from PET scans no other applications of antimatter exist (other than scientific study of course) because of the extremely small quantities in which it can be produced. If we had large quantities of antimatter, the applications would be endless.
As mentioned earlier, when matter and antimatter come into contact they produce energy proportional to the masses involved, and the constant of proportionality is the speed of light squared, a very very large number. It is because of this large number that relatively small quantities of antimatter produce large amounts of energy. For example one gram of antimatter and one gram of matter would release energy equivalent to three times the nuclear explosion of Hiroshima.


Think about it, an antimatter paperclip could destroy an entire city. It is clear that if substantial quantities of antimatter could be produced, one application would be in the development of weapons. Conventional nuclear reactions (specifically fusion reactions like the ones in nuclear bombs or inside the Sun) convert only up to 0.7% of the mass used into energy, while matter-antimatter collisions convert all of the mass into energy. This could also be useful in power generation. As mentioned before, a small quantity of antimatter can produce a large amount of energy, enough to power a large city for an extensive amount of time. Power plants using antimatter would be the most efficient form of power generation.
Another useful application would be in space travel. Using antimatter to generate propulsion for spacecraft could accelerate them to very near the speed of light. This concept is used widely in science-fiction, for example the spaceships that brought humans to Pandora in the movie Avatar used antimatter for propulsion.
To give you a feel on how much antimatter has been produced, if we could take all of the antimatter produced on Earth it would only be enough to power a small light-bulb for a short amount of time.

Where can we find antimatter?

For now, on Earth, the only sources of antimatter are: particle accelerators, cosmic rays hitting the atmosphere producing positrons and radioactive elements that also produce positrons. Positrons are easy to produce but being extremely light particles, it is next to impossible to produce enough of them to have a measurable effect. In fact, it's been calculated that at the current rate of production of antimatter it would take hundreds of thousands, if not millions, of years to produce a few grams of antimatter, unless of course some sort of breakthrough leads to an efficient method of producing it. If we had the means to directly convert energy into antimatter, a nuclear reactor could produce about one gram per day, which is completely acceptable. 
Can we find antimatter someplace else? Absolutely, however it seems unlikely that we could find it somewhere within our reach. There seems to be no indication of any substantial quantities of antimatter present in our own solar system, and some cosmological models have indicated that antimatter clouds may have formed somewhere near the core of our galaxy.


Sadly there is no way we could get there in the near future. It would thus seem that we have to rely on science to find an efficient means of producing measurable quantities of antimatter. Having such quantities would not only be useful in power generation, propulsion and weaponry but would also help us understand the nature of the apparent asymmetry between matter and antimatter, the reason for the dominance of matter in our Universe, thus bringing us closer to understanding how it came to be.