Ever since the notion of antimatter came into existence, it has not only fueled scientific discussions and controversies but has also been a protagonist of many supernatural tales. In the book and film named Angels and Demons, Professor Langdon tried to save Vatican City from an antimatter bomb. Moreover, even in the Star Trek’s Starship Enterprise, matter-antimatter annihilation propulsion was used for faster-than-light travel.
So, one thing is clear by now, antimatter is important stuff of science fiction & is quite famous in the reel world. But, is it as significant in our real world too? How often do we encounter it in our daily lives? And if it’s present in our real world, how it got detected for the first time? Let’s find out!
The beginning of the twentieth century was one of the most happening eras in the history of physics. That was the time when the early foundations of Physics were completely shaken by the dramatic entry of two exotic new theories, the theory of relativity and quantum physics. Where the theory of relativity dealt with objects moving with relativistic velocities, ie velocities comparable to the velocity of light, the quantum theory was nothing less than a scripture governing the behavior of the microscopic world.
But, whenever we feel that we have found the answer to all the questions, nature rolls its dice one again, and eventually, a new problem was ready to knock at the door of Physicists. The problem was that quantum theory was not relativistic, which means that the quantum description worked only for particles moving slowly, and not for those at high velocities, close to the speed of light. So now, the task was to arrive at something that would combine both relativity and quantum physics. And, this is when the concept of antimatter made its entry.
The most important equation that described the dynamics of the quantum systems was the Schrodinger Wave Equation. But this equation was only good for particles moving at non-relativistic speeds. So scientists had to find a new equation that would explain the dynamics of relativistic particles – an equation that would connect special relativity with quantum mechanics.
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Dirac and Feynman’s description of Antimatter :
In 1928, Paul Dirac came up with a strange equation, which combined quantum theory and special relativity, to describe the behavior of an electron. Undoubtedly, The Dirac equation won him the 1933 Nobel Prize but also posed another problem – just as the equation x2=1 can have two possible solutions x=1 or x=-1, Dirac’s equation could also have two solutions, one for an electron with positive energy, and one for an electron with negative energy. But as far as we know, the energy of a particle must always be a positive number! So, what to do now?
From this, Dirac interpreted that these particles with negative energy are not ordinary particles, but their antiparticles. He claimed that for every particle that exists, there also exists a corresponding antiparticle, exactly matching the particle but with the opposite charge. This means that there should be an “antielectron” identical in every way to an electron but with a positive electric charge. All the other properties and quantum numbers are also closely related but with the signs of all charges reversed. And, the matter composed by taking these antiparticles as the basic building blocks is what we refer to as antimatter!
Although particles and their antiparticles have opposite charges, this doesn’t mean that electrically neutral particles are identical to their antiparticles. The neutron, for example, is made out of quarks, while the antineutron from antiquarks. The massless particles like photons and neutrinos are their own antiparticles. The world of antiparticles is crazy, isn’t it? But the craziness was not over yet. Another striking revelation was all ready to make its presence felt. In 1949, the celebrated Physicist Richard Feynman came with his own description of antiparticles. He quoted that Antiparticles are just the normal particles traveling backward in time. Interesting, isn’t it?
Experimental evidences of antimatter :
Our world relies on evidence. No theory is considered complete, unless and until an experimental proof is present in its support and so was the case with antimatter. Eventually, as the theories about antimatter started making noise, experimentalists all over the globe set themselves on work in order to make some extraordinary discoveries as follows:
In 1932, Carl Anderson, a young professor was studying showers of cosmic particles in a cloud chamber. He saw a track left by “something positively charged, and with the same mass as an electron”. After a continuous effort of one year, he concluded the tracks were actually of antielectrons, which he later termed as “positrons”, for its positive charge. This discovery was the first experimental milestone in the story of antimatter.
More on particle physics:
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Although the antiprotons were known to be present in cosmic rays as a result of collisions of cosmic ray protons with nuclei in the interstellar medium, their artificial production required a tremendous amount of energy which was not possible without the use of an artificial source. However, before 1954, no accelerator in the world was able to achieve more than half of the minimum energy predicted to be necessary. But, after the Bevatron, a particle accelerator at Lawrence Berkeley National Laboratory began operating, the hunt was finally over.
In 1955, antiprotons were first produced by bombarding a copper target with high-energy protons from the proton synchrotron after achieving energy of 6 billion electron-volt range. This discovery was made by the Berkeley physicists Emilio Segrè and Owen Chamberlain, for which they were awarded the 1959 Nobel Prize in Physics. Shortly after the experimental discovery of the antiproton, the antineutron was discovered in proton-proton collisions at the Bevatron in 1956.
Anti nuclei and Antiatoms:
By 1965, all three particles that make up atoms, ie. electrons, protons, and neutrons were known to each have an antiparticle. Now the question that arose was that if particles bound together to form atoms, so do antiparticles also bound together in antiatoms! Physicists wanted to know how subatomic antiparticles behave when they come together.
Finally, the answer to the antinuclei question was found in 1965 with the observation of the antideuteron, a nucleus of antimatter made out of an antiproton plus an antineutron. The goal was simultaneously achieved by two teams of physicists, one led by Antonino Zichichi using the Proton Synchrotron at CERN, and the other led by Leon Lederman, using the Alternating Gradient Synchrotron (AGS) accelerator at the Brookhaven National Laboratory, New York.
After the discovery of the anti nucleus, the next goal was to produce an antiatom. This goal was finally realized in 1995 when 9 antihydrogen atoms were produced during collisions between antiprotons and xenon atoms over a period of 3 weeks at CERN’s Low Energy Antiproton Ring (LEAR) facility.
Quantitative analysis of artificial antimatter produced till date :
Although the hunt for antimatter began about 9 decades ago, things aren’t very smooth even now. Antimatter is around us, the cosmic rays shower a generous amount of antimatter daily on us. Even the bananas in your fruit basket release one positron about every 75 minutes. Still, we haven’t been able to capture and produce a significant amount of antimatter to date. Do you know, even if all the antimatter ever made by humans to date is annihilated at once, the energy produced wouldn’t even be enough to boil a cup of tea!
The reason behind this is that whenever antimatter meets matter, it annihilates, ie both matter and antimatter disappear leading to an explosion of energy! And, as we know how abundant matter is in this universe, it becomes so easy for antimatter to get annihilated even before getting captured! So, to overcome these issues, antimatter particles are created in ultra high-speed collisions, which cost a lot of money and resources. To be precise, it takes $63.5 trillion dollars to produce a single gram of antimatter! No wonder why we haven’t produced even half a gram yet!
Some unanswered questions:
Many theories claim that during the big bang, an equal amount of matter and antimatter should have been created. But, if this is the case, why haven’t we annihilated yet? Where is our corresponding antimatter? Even in his Nobel Lecture, Dirac speculated on the existence of a completely new universe made out of antimatter. So, if it exists, where is it? Do we really have our anti selves in some other Universe? All these mysteries haven’t unfolded yet! So although, we know a lot about antimatter, yet we don’t know enough! A lot more unexpected is yet to come!
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