This is a guest article by Rishabh Sharma, a third-year integrated Ph.D. student from the Indian Institute of Science Education and Research (IISER) Tirupati, pursuing a Ph.D. in Experimental High Energy Physics. 

Ever wondered what the universe would look like in the far future? I am not talking about the timescales of millions of years. I am referring to when the universe would have expanded so much that its effects will be visible even on the quantum scale. How will the universe look like? What would happen to all the matter in the cosmos? And most importantly, what particles will constitute the universe in the distant future? Before I answer these questions, I need to introduce you to some fascinating concepts of particle physics. You don’t have to be an expert! Stay with me.

Annihilation as a source of power

In extensive sci-fi literature, matter and antimatter annihilation are used as a source to power the space shuttles. Although theoretically, it would give a scrupulous amount of ‘clean’ energy given our current technological understanding, it is logistically exorbitant to produce antimatter. With a price tag of $62.5 trillion per gram, it is the most expensive substance on Earth. Perhaps, soon, when we can produce it at a much lower price, we can replace our conventional energy sources with the annihilation process.

What is antimatter?

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Paul Dirac (Image: BETTMANN/CORBIS)

The concept of antimatter was born when Paul Dirac brought together two of the greatest achievements of the 20th Century: Quantum Mechanics and the Special Theory of Relativity. The resulting equation was named after him, and it won him a Nobel prize. The solution to the Dirac equation predicts electrons with both positive and negative energy. We can work with positive energy electrons, but how in the world do we make sense of negative energy electrons?

Dirac interpreted these negative energy particles as some sort of ‘anti’ particles having all other properties exactly the same except for electric charge. The ‘usual’ electrons that we know have a negative electric charge, but these negative energy electrons were postulated to have a positive electric charge. Such matter, which was ‘anti’ of ‘normal’ matter, was called antimatter. 

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As per Dirac’s theory, particles in the yellow region are normal particles, while
those in blue region are the antiparticles.

Speaking of which, the word ‘antimatter’ was first used not by Dirac but by Arthur Schuster, who used this term almost 30 years before the predictions by the Dirac equation. 

Discovery of antimatter

But has anyone experimentally observed anti-electrons (antimatter counterpart of electrons)? If some sources are to be believed, then Dmitri Skobeltsyn, a Soviet Physicist, discovered these anti-electrons 5 years before Dirac wrote his legendary equation. Skobeltsyn was studying the Compton effect using the Wilson cloud chamber. Cloud chambers can be thought of as photographic plates to picture the trajectories of particles as they pass through the medium of the chamber. Skobeltsyn very cleverly added a magnetic field to the cloud chamber to differentiate between the trajectories of positively and negatively charged particles.

So, consider a situation in which an electron is curving towards the right. Its antimatter counterpart will curve towards the left. Further, the radius of curvature of these curves also gives us insight into the mass-to-charge ratio of these particles. However, Skobeltsyn did observe particles that had charges opposite to that of electrons but had a trajectory matching with the mass-to-charge ratio of electrons. He did not really go along with the idea and eventually dropped it. 

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Again in 1929, Chung-Yao Chao, a grad student at Caltech, found a particle that behaved like an electron but positively charged. However, he also did not pursue the idea any further. Then, Frédéric and Irène Joliot-Curie in Paris also saw these particles in their photographic plates but thought they were protons and did not pay much attention to them. 

After this spree of being ignored by many experimentalists, Carl Anderson in 1932 finally discovered antielectrons, the first instance of antimatter being experimentally observed. This discovery won him a Nobel Prize in Physics. In his Nobel lecture, he did acknowledge Skobeltsyn and his ingenious idea of introducing magnetic fields in the cloud chamber. 

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Cloud chamber photo of the first positron ever identified by Anderson.. The deflection and direction of the particle’s ion trail indicate that the particle is a positron.

Anderson reported his discovery in Physical Review and asked the journal editor to come up with any name for these antielectrons. This is how the name ‘positron’ had its inception. 

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When an electron meets a positron

Now, what happens when electrons and positrons meet each other when an electron meets its counterpart? They annihilate. Being oppositely electrically charged, they are attracted to each other, and once they meet, they annihilate to form energy in the form of two gamma-ray photons. However, we ask in this article: Can matter and antimatter form stable bound states?

The answer is no, but it gets interesting. If an electron and a positron approach each other, they could interact in a myriad of ways. They can jump right into each other and annihilate, or could even scatter off of each other like in the case of Bhabha scattering, or they could also spiral around each other for a short while before annihilation. Can we quantify this short while? It is of the order of nanosecond (i.e., 1 billionth of a second). So, these are precarious systems, but Martin Deutsch at MIT in 1951 managed to discover them experimentally even in such short time scales.

Positronium was the name given to this transient bound state and is a very weakly bound fragile system with a lifetime of 0.12 nanoseconds. This means that positronium is not a stable particle and would decay rather effortlessly. But here is the interesting part, as promised, however less interesting a positronium might look right now, it could very well be the last atoms standing in our universe with lifetimes of the order of 10 to the power 141 years. That’s 1 followed by 141 zeros, so you read that correctly!

A Positronium represents a bound quantum state where an electron and positron orbit around their common center of mass.
A Positronium represents a bound quantum state where an electron and positron orbit around their common center of mass. (Image: wikipedia creative commons)

Atom, the size of the universe

So, how exactly does positronium go from being a transient particle to a particle with such a large lifetime? The whole thing hinges around proton decay. Protons are thought to be stable particles. They don’t decay easily, and we have never observed a proton decaying so far. Experimentalists put the lower bound of the lifetime of a proton as 1034 years. 

If we haven’t observed the proton decay, it doesn’t necessarily mean that they might never decay, and there is a possibility they could. Hypothetically if protons were to decay, they would have neutral pions and positrons as their decay daughters. Given a substantial amount of time and assuming that protons would eventually decay, in the far future of the universe, all protons would have decayed, and what would already be left would just be positrons, electrons, neutrinos, and photons. 

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In this substantial amount of time it took protons to decay, the universe would also have expanded too much, much greater volumes than it is today. This would decrease the kinetic energy of the particles drifting in space. This means that electrons and positrons will be able to form fragile bonds with each other since they do not have enough kinetic energy to overcome the attraction between the two and spontaneously decay as they do today. 

Due to this weak bond and having almost nothing to intervene in, they will be able to feel each other’s pull even when they are separated by a distance equal to the size of today’s observable universe. These bound electrons and positrons would still be spiraling towards each other ever so slightly that they would eventually fall into each other. Still, owing to the distance between them and their low kinetic energy, it would take a long, long time, perhaps of the order of 10141 years. 

This is how the star of this article, the positronium, would actually be the last atom standing when everything else would have vanished, which makes sense. Understanding positronium could give us hints as to how this universe came into being and help throw some light on where exactly we are headed.  

Oh, by the way, did you know that even a proton and antiproton form a bound state called protonium? More about it in the coming articles.

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