Serendipity (Part 2) - The Antimatter Particle_ Positron - A Great Example of Serendipitous Discovery

Serendipity is the faculty of making fortunate and unexpected discoveries by accident. Part 1 may be reached here.

How the existence of antimatter was theoretically predicted and experimentally observed makes a great story.  Serendipity played a role at all stages of the discovery.  

Prediction of Antimatter: In 1927 Paul Dirac was trying to include relativistic effects in the formulation of the non-relativistic quantum theory as proposed by Schrodinger and Heisenberg.  Their theory needed to introduce some ad hoc properties like particle spin and magnetic moments which were difficult to justify (for an electron to have spin one-half, its surface would need to rotate at speeds greater than the speed of light!!).  

Dirac found that his theory was eminently successful in explaining the origin of particle spin and magnetic moments of the electron, except that there was a problem.  

The Problem & Dirac's Solution: Dirac's theory required symmetry in solutions such that a particle (for example an electron) with positive energy must have a twin with a negative energy - there was no escaping the situation. Real electrons have positive kinetic energy and that is fine, but the concept of negative energy states was a problem. Positive energy electrons will fall to negative energy states and make the system unphysical.

This was unexpected and unsought-for result of Dirac's theory. Dirac solved this problem by postulating 'Dirac-sea' that was to say that all the negative energy states are already full and positive energy electrons have no where to fall. Any holes in the negative energy sea may diffuse around and behave like a positive electron (or a positron).  If an electron falls into the hole then the hole is filled with the energy difference released as a packet of energy (the electron and a positron no longer exist -  annihilation of matter).  Later developments in theories in the form of Quantum Electrodynamics (QED) have done away with the need to introduce the negative energy sea.  

Dirac showed sagacity in dealing with the unexpected result and predicted the existence of antimatter (positron and other  antiparticles).  Dirac was awarded Nobel Prize in 1933 for predicting the existence of the positron.  In many surveys, Paul Dirac is ranked as the fourth most important scientist behind Albert Einstein, Isaac Newton and James Clerk Maxwell. I have had the pleasure of giving talks on the three luminaries and they are available to read here by clicking on their names.  In the Appendix, I give a brief introduction to Paul Dirac  - I am sure you have not met a person of such unusual character before. 

Experimental Observation of the Positron: Carl Anderson is credited with the observation of the positron in 1932 - Anderson was awarded Nobel Prize in 1936 for his discovery of the positron.  The story is really quite fascinating in that other scientists, even before Dirac predicted it, had observed positrons but had failed to see the significance of their measurements.  They either ignored it or tried to give bogus explanations. As Pascal had said - 'Chance only favours the prepared mind'.

Before I discuss the observation of the positron, let me digress and describe briefly how particles are experimentally detected; the detector is called a cloud chamber invented by CTR Wilson (Nobel Prize 1927), a device that makes the path of charged particles passing through the chamber visible; the path may be photographed for analysis.  When placed in a magnetic field, the path bends in an arc which allows one to calculate the velocity, charge  and mass of the particle.  

Positrons are created when high energy cosmic ray particles collide with nuclei in a medium.

Missed Opportunities: Several researchers missed the chance of detecting positrons:

1. In 1928, Dmitri Skobeltsyn had observed tracks in a cloud chamber that looked like electron tracks but were curved in the opposite direction in the magnetic field.  He chose to ignore them.

2.  In 1930, Chung-Yao Chao observed positron track but did not attribute them to a positive charged particle.  Chao was a fellow student of Carl Anderson, and Anderson later acknowledged that his work was inspired by Chao.

3. In April 1931, a few months before Anderson's discovery, Frederic and Irene Joliot-Curie missed the opportunity to discover the positron in their experiment using a Wilson cloud chamber.  The Curies did not use cosmic rays but were bombarding aluminium and boron with alpha particles - they observed electron tracks that curved in the wrong direction indicating a positively charged electron (positron).   However, they interpreted the tracks as being due to electrons that have been scattered back into the equipment! 

The story of Irene and Frederic Joliot-Curie is a fascinating one and may be read here. The Curies had the world's most powerful alpha source and were very well placed for making novel discoveries; they also appear to have a habit of missing out on important observations  - they could have won three extra Nobel Prizes.  

Besides the positron, the Curies misinterpreted their results and missed the discovery of the neutron for which James Chadwick was awarded Nobel Prize in 1935. They also ignored the presence of lanthanum after  bombarding uranium with alpha particles and thus missed out on the observation of nuclear fission - Otto Hahn and Fritz Strassmann repeated their experiment in 1938 - Otto Hahn was awarded 1944 Nobel Prize for this.

The Curies did win the 1935 Nobel Prize for discovering induced radioactivity showing that radioactive elements may be produced in the laboratory (basis for positron emission tomography (PET) scanners in medical diagnosis).

4. In late 1932, Blackett and Occhialini confirmed the existence of the positron using a cloud chamber equipped with Geiger counters - a much more efficient set up.  

Their experiment also observed the production of an electron-positron pair from energetic cosmic gamma rays thus confirming conversion of energy into matter as predicted by Einstein's famous equation E = mc2

Blackett and Occhialini published their work a few months after Carl Anderson and missed out on being the first to report the discovery of the positron.  Blackett did win the 1948 Nobel Prize for his work on cloud chambers and cosmic radiation.

Anderson's Discovery of the Positron: 

This is a picture of one of the first positron tracks observed by Anderson in 1932. It was taken in a cloud chamber in the presence of a magnetic field of 2.4 Tesla pointing into the paper (so the path of a positively charged particle travelling from the bottom of the picture is curved to the left). The cloud chamber (17x17x3 cm) contained a gas supersaturated with water vapour. In the presence of a charged particle (such as a positron), the water vapour condenses into droplets - these droplets mark out the path of the particle. 

The band across the middle is a lead plate, 6 mm thick, which absorbs some of the energy of the particle and slows it down. The radius of curvature of the track above the plate is smaller than that below. This means that the particle is travelling more slowly (23 MeV) above the plate than below it (63 MeV), and hence it must be travelling upwards. From the direction in which the path curves one can deduce that the particle is positively charged. That it is a positron and not a proton can be deduced from the long range of the upper track - a proton would have come to rest in a much shorter distance (~5 mm) 
Carl Anderson won the 1936 Nobel Prize for Physics for this discovery.  

In 1937, Carl Anderson with his student Seth Neddermeyer observed a totally unexpected track in the cloud chamber - the track was made by the passage of a particle of mass 207 times the electron, which had a negative charge exactly the same as that of an electron. This is yet another example of serendipitous discovery  - existence of a muon was absolutely astonishing - it was neither predicted by any theory and nobody was looking for it! When informed about the muon - I. I. Rabi had quipped - 'Who ordered it?' - Muon had no place in the current physics theories. Despite being an unexpected addition, the muon served as the first clue regarding the existence of a new family of elementary particles helping physicists build the Standard Model.

Discussion: the first half of the 20th century was a golden era for physics (and also chemistry) with major breakthroughs in theoretical insights in the form of theory of relativity and quantum mechanics. As is common when a paradigm shift happens in a field of study, a flurry of new ideas and empirical evidence come in quick succession and many of the outstanding problems find resolution.  It is also evident that many discoveries were accidental and serendipity played a significant role.  

The period 1900 to 1950 was such a time when one could say there was an awakening in physical sciences.  

It may be apparent from the presentation above that important new discoveries were forthcoming rapidly and such was the merit of the work that many Nobel Prizes were awarded with only 2 or 3 years gap between discovery and award.  Additionally, many researchers involved were very young - some had only finished their PhD when they did the Nobel Prize winning work.  This is highly unusual as the average age for such work is 40 years and the Nobel Award comes on average after 20 years (at age 60 years) of the seminal work. I had analysed this situation and refer you to my feature here.

Essentially, physicist who were born in 1880 to 1900 were fortunate to be completing their postgraduate degrees in the 1910 to 1930 when many opportunities of making great discoveries became available.  Historically, this happens when there is a paradigm shift - one recent example is the arrival of portable computers (laptops) in mid 1970s.  I reproduce the data below

Most of these innovators are multi-billionaires and were at the right place at the right time.  Notice that most were born between 1947 and 1965 - a short span of 18 years. 

We find a similar trend in physics around 1925 to 1940 when great discoveries were made by those born in 1880s to 1900. The slide highlights the situation: Red shows the age distribution of the laureates at the time of award for all categories.  Blue shows this distribution (numbers multiplied by 20 for clarity) for Nobel Prizes in physics between 1921 and 1940.

Considering that the age at PhD is about 26 years, the data confirms the points highlighted in the above discussion. For a more detailed discussion - See (1,2).

APPENDIX - Paul Dirac (b. - 1902 Bristol, England

                                         d. - 1984 Tallahasse, Florida)

Personality:  Dirac was regarded by his friends and colleagues as unusual in character.  

Niels Bohr called Dirac 'a complete logical genius' and also the 'strangest man' who had ever visited his institute. 

Dirac was known for his precise and taciturn nature.  His colleagues defined a unit called a 'dirac', which was one word per hour!  According to Dirac, his father who was originally from Switzerland wanted his children to  speak only French at home so that they might learn the language; Dirac found that  difficult and decided to remain silent.  

In 1937, Dirac married Margit Wigner, sister of famous physicist Eugene Wigner.  He would introduce Margit to visitors as "Allow me to present Wigner's sister, who is now my wife."

Dirac met Richard Feynman in a conference - after a period of silence, Dirac asked Feynman, "I have an equation, do you have one too?"

According to his biographer, Dirac suffered agonies if forced into socializing or small talk.

Dirac's views on poetry: 'The aim of science is to make difficult things understandable in a simpler way; the aim of poetry is to state simple things in an incomprehensible way'

Dirac was absolutely blunt in his comments.  Niels Bohr was finding it difficult to finish a sentence in his paper - Dirac told him that 'I was taught at school never to start a sentence without knowing the end of it'.  

Undoubtedly, Dirac was regarded a 'strange' genius by his contemporaries.  

Einstein remarked about one of Dirac's paper, 'I am toiling over Dirac.  This balancing on the dizzying path between genius and madness is awful'.  On another occasion, 'I don't understand the details of Dirac at all'.  

For Dirac's contributions to science - there were many - he was active until his death in 1984,  I refer you to his biography in Wiki for details.

Thanks for reading ...