October 17, 2021

I, Science

The science magazine of Imperial College

The word “anti” comes from ancient Greek, which literally translates as “against” or “opposite to” and nowadays is very commonly used in many contexts from social-politics (anti-social, anti-capitalism) to medicine (antibiotics, antidepressive) and even religion (Antichrist).

In 1932 “anti” prefix officially entered in the world of physical science. Carl Anderson in Pasadena, California, had started an experiment to study the cosmic gamma rays, electromagnetic waves of extremely high frequency (higher than 10,000,000,000 GHz) and therefore very powerful radiations. The rays passed through a so-called “cloud chamber”, a sealed apparatus containing supersaturated vapour of water or alcohol, whose molecules are ionised under the passage of charge particle, and a lead plate. A magnetic field was applied to this apparatus, which allowed the deflection of the charge particle on a specific counter. As the gamma rays passed through the clouds two distinct spots were detected in the two opposite counters indicating that two different equal particles were deflected in opposite directions by the magnetic field. These particles were generated by the gamma rays and must have equal mass but opposite charge. Investigated the mass-charge-ratio, Anderson found that one of the particles was an electron. So, what was the other one? He called it a positron ( a positive electron), and it was the first protagonist of anti-matter and the first “anti” in the history of physics appearing from the Dirac’s Sea.

“Dirac’s what?”

The whole story of anti-matter started three years earlier, in 1929, when the physicist Paul Adrien Maurice Dirac, at Cambridge University came across the equation which combined the quantum mechanic formulation of particles and special relativity. His intent was to explain new observed experimental phenomena such as the Zeeman effect, which could not been explained at the time. But, when he solved its equation, he found that there were values of energies for the particles, which could be negative, and in order to be coherent with the quantum mechanical model he could not discard these values as unphysical. This outcome puzzled him.

One question in particular arose: it was well known that an electron in an atom could lower its energy by emitting photon of same energy. Hence, in principle, an electron could emit very energetic photons to jump into states of negative energies. However this was never observed. Finding an answer to this issue made Dirac suppose that the electrons could not enter in the infinitely accessible energy levels because they were all completely occupied, and for the Pauli’s exclusive principle two particles like electrons could not occupy the exact energy state. This idea of a sea of anti-electrons occupying negative energy levels became popular under the name of Dirac’s Sea.

Dirac hypothesized that if all but one state of negative energies was not occupied then a “hole” was generated in the Dirac’ sea and this “hole” would respond to a magnetic field as if it was a positively charged particle. Within this picture, an electron could have filled the state of the “hole”, and the product would be the emission of extremely high energy photon. Initially, Dirac thought of the proton as the corresponding “hole” in the sea of negative energies of electrons, but the American physicist Julius Oppenheimer disagreed, observing that in the eventuality of an electron jumping from a positive energy level,  the “hole” of negative energy would correspond to the annihilation of electron and proton, with the result of non-existing atoms.

It was one year later, in 1930, that Dirac re-considered the picture and postulated the existence of a particle called an anti-electron, with same mass of the electron but with positive charge. Nevertheless, only with the birth of Quantum e Field theory, did the overall picture change, with anti-electrons considered as real particles and the idea of infinitely occupied negative energy states abolished. After Anderson gave evidence of the existence of anti-electron (called a positron), Dirac received the Nobel Prize for his relativistic quantum theory of the electron. It was 1933 and three years later Anderson would receive the Nobel Prize for his experiment validating Dirac’s theory.

The positron was the first of a series of anti-particles which were produced in the particles accelerators around the world, such as Tevatron in Illinois (US), CERN in Switzerland and KEK in Japan. Both anti-protons and anti-neutrons have been generated since then, from the collisions of electrons at extremely high energies (120 Giga eV). These collisions so far can guarantee a maximum rate of production of 10,000,000 antiprotons per minute (CERN). Although this might seem a big number, it implies that it would take 200 billion years to produce a 1 gram of antiprotons.

In recent years many attempts have been spent in order to isolate anti-atoms for a certain period of time. This is difficult to achieve, because when in contact with any particle the anti-particle is immediately annihilate as predicted by Dirac, generating extremely energetic radiation. Magnetic traps are necessary in order to isolate the charged anti-particles (more problematic is the trapping of anti-neutrons) and this led to the isolation at CERN in 1995 of 9 anti-hydrogen atoms (an anti-proton and a positron). A main problem in isolating anti-matter always existed in the high energetic state of anti-protons and positrons generated after the particles collisions. This makes the anti-hydrogen atom very unstable, so that observation times have been very short (less than microseconds) for decades.

Many laboratories have developed devices capable of “cooling” (reducing the energy) of the anti-particles and in 2002 the ATHENA project announced that they could create the first “cold” anti-hydrogen. In November 2010 another project (ALPHA) announced that trapping of 38 anti-hydrogens for 1 microsecond, and this year in April 2011 the same project managed to trap 306 anti-hydrogen atoms for about 17 minutes. Trapping anti-matter is extremely important for studying its properties with respect to those of conventional matter.

Illustration_Matter-Antimatter_Annihilation (NASA/CXC/M. Weiss)











The concept of matter-antimatter annihilation works also in reverse, with the creation of a particle and antiparticle pairs from gamma rays undergoing perturbations, such as interactions with magnetic field or charged particles. The latter case is also related to the presence of anti-protons in a zone of the Earth’s atmosphere called Van Allen belt, where charged particles of the air are present interacting with Earth’s magnetic field (New Scientist 2824, August 2011). The matter-antimatter creation from radiation must have occurred also in the first moments after the Big Bang, being part of the process of energy-mass conversion required for the creation of the universe. There is, however, one fundamental question, which astrophysicists cannot yet give an answer to: where did all the antimatter go? So far, astrophysicists have observed far more matter in the universe, in the form of stars, galaxies, quasars, pulsars and other astrophysical objects. What happened to the antimatter? Does antimatter have a particular property which resulted in a higher instability so that the universe evolved in favour of matter, or antimatter is massively present in the universe but we cannot distinguish it from conventional matter?

The second hypothesis implies that antimatter must be confined in regions of the Universe very far away from conventional matter, otherwise the encounter of the two entities would imply a huge annihilation. This last point is intrinsically related to understanding the gravitational interactions between antiparticles, as well as particles and antiparticles. The scientific community is divided about it: some physicists believe there is no reason why antimatter should behave differently from conventional matter and postulate an attractive gravitational force for antimatter. Meanwhile, others do not exclude the possibility of a repulsive gravitation between matter and antimatter (the so called fifth force), which would ultimately explain why so many of the questions about our universe still remain unanswered.




Leaving aside the world of antimatter, another “anti” entity showed up back in 1932, with far less exotic and yet still very interesting properties, but this will have to wait until tomorrow…