In this article, I will be talking about antimatter and some of its mind-blowing features and incredible mysteries. After that, we’ll also discuss some interesting questions like:
What happens when matter touches antimatter?
Why is our universe entirely made of matter? And really is it?
Is there any mysterious place where we can find galaxies of antimatter?
Does neutral matter (like photons) have an anti-version?
Is gravitational force reversed for antimatter?
How do scientists hold and study antimatter?
Really mind-blowing questions, aren’t they? So, get ready for a journey to the minute physical world of logic which seem illogical, along with a tadka of my philosophy.
So let’s begin!
Physics and Math are very good friends. Physics depends on Math to express the laws of Physics and Math relies on Physics for much of its subject matter. In many of the cases when our intuition becomes an obstacle in understanding the physical world, it is better to rely on Math to guide us. For example, in understanding the bizarre illogical behaviour of quantum particles. So, we can trust that Math describes reality more accurately than our intuition. But not always. Sometimes math makes some weird predictions that make no physical sense. Say, for example, you are designing a missile launch system. So, you have to calculate the correct projectile so that it hits the right position. For that, you need to solve an equation that looks like this: y = ax2 + bx + c to figure out the shooting velocity and launching angle. As the equation has an x2 in it, there are going to be two solutions for the point where the missile will hit the ground. Here, one solution will be the physical one, showing just the right manner in which the missile will be delivered to the correct point. The other solution, however, will give you a nonsensical answer: It will tell you that the initial velocity should be negative, which means you have to shoot the missile backwards and directly at the ground (not in the air). This is a correct mathematical solution, but not a physical one so we have to discard it.
So, in most of the cases we ignore those mathematical solutions which seem illogical in the physical sense. Physicists have been used to this and discard unphysical solutions as mathematical artifacts that are not real insights into our universe.
But be careful, because some of those artifacts might be real, and Nobel Prizes maybe in the wait.
An English physicist, Paul Dirac, in 1928, was working on equations to describe the quantum behaviour of electrons moving at very high speeds. That was the time in physics had blown our naive perception of the universe, not once but twice. One was quantum mechanics which produced such results which led us to rethink the nature of reality at the lowest levels. And the other one was the famous Special Relativity theory which was complex enough to bring a revolution in the whole scientific study.
Dirac was also very much influenced by these mind-blowing theories (Quantum Mechanics and Special Relativity) and by using them he developed an equation (called Dirac Equation) describing the behaviour of fast-moving electrons (relativistic version of the Schrodinger’s wave equation for electrons) that was really amazing and seemed to work except for one little problem.
He found that his equations worked perfectly for the everyday negatively charged electrons, but they also worked for electrons with the opposite electric charge. That means, his equation suggested that the laws of physics would work just as well for a positively charged electron, which he called antielectron. This antielectron was similar to an electron in many ways: it had the same mass and had the same quantum properties. But it had an opposite electric charge. No such particle had ever been observed before.
Some physicists might have been tempted to disregard this result as a mathematical artifact or a negative solution. But Dirac was intrigued as those were no physical laws to prohibit the existence of antielectrons. Looking at his equations he even went further and propose that all particles have a corresponding antiparticle. Hence, he predicted not only a new single particle but a whole new kind of particles. This was really an incredible idea and hard to accept.
But in 1932, a physicist, Carl D. Anderson, discovered the real antielectrons and named them ‘positrons’. This proved the proposal of Dirac. Dirac received on Nobel Prize in physics in 1933 and Anderson got the prize in 1936. Even a complete periodic table of antimatter was proposed by Charles Janet in 1929. At present we have discovered an anti-version of almost every subatomic particle.
The idea of negative matter or antimatter was not very much new. Such ideas appeared in some of the past theories as well.
William Hicks in the 1880s, proposed the possibility of matter with negative gravity. And in the next decade, Karl Pearson suggested the existence of “squirts” from where ‘aether’ flows into “sinks”. The squirts represented normal matter, while the sinks represented negative matter.
The use of the term ‘antimatter’ was first done by Arthur Schuster in his writing for Nature in 1898, where he coined the term. He speculated the existence of antiatoms and also hypothesized the existence of a whole antimatter solar systems. He also discussed the possibility of matter and antimatter annihilating each other. His ideas were not a serious theoretical proposal, but a mere speculation. He also believed that antimatter should possess ‘negative gravity’. 
So, Dirac’s ideas were not very much new. But his ideas had some mathematical proofs, which made them more credible.
An antiparticle is just an opposite twin of the original one, but it has some other differences as well. The antiparticle twin is not just different in electric charge but also in charges of the weak and strong nuclear forces.
In fact, scientists have observed antiparticles many times. At present, nearly every charged particle we know of has been confirmed to have an antiparticle. Antiparticles can be easily formed by particle collisions. At CERN, annually a few picograms of antiparticles are produced. Cosmic rays from space colliding with the atmosphere also sometimes contain antiparticles.
Antiparticles are a really good example of the symmetry that we find in physics even at the lowest level. You may imagine particles and antiparticles as two faces of the same coin. Well, copies of particles also happened to exist in different ways: each of the subatomic particles also has two heavier cousins. For example, the electron has the muon and the tau particles which have nearly identical quantum properties (like charge and spin) as an electron but have more mass. So we see, electron has is two heavier cousins and an antiparticle. And of course, the heavier cousins, tau and muon have their anti-versions as well.
And yes, there is no limit to it. According to a speculative theory called ‘supersymmetry’, every particle has yet another kind of mirror, “a superparticle”, that is very much similar to the original particle (same mass, charge etc.) but has different quantum spin. So we see the particles of matter are full of twins, cousins and even clones.
But these raise even more questions: Why do these twin version of particles exist at all? Why don’t we observe them in our day-to-day life?
What will Happen if Particle and Antiparticle Come Close to Each Other?
In science fictions we see many incredible physics born out of the imagination of the writer but most of the things (like time travel, teleportation, faster than light travel etc.) cease to be exactly true in real life. Like in science fictions, it is told that when a particle touches its antiparticle they explode. That sounds ridiculous, doesn’t it?
But, it’s not that fictional. Actually, it turns out to be true. When the twins (particle and antiparticle) meet each other, they don’t just hug and get cozy, they destroy each other completely (villainous!). The two particles vanish and their masses are completely converted into high energy carrying particles like photon or gluon, without leaving a trace of original particles. This process is called ‘Annihilation’. This happens not just with electron and positron, but also when quarks collide with antiquarks or muons meet anti-muons. 
The energy released is very high as mass stores a lot of energy. Einstein’s famous equation E=mc2 suggests that mass and energy are interrelated and interconvertible. We must note here that the speed of light, ‘c’ which is already a large value (300 million m/s), is squared! So a little bit of mass carries a huge amount of energy. If one gram of antiparticles is combined with a gram of normal particles, it would release more than 40 kilotonnes of explosive force, which is more than twice as powerful as atomic bombs used in World War II.
Well, the particles don’t actually touch each other while annihilation. They are not actually tiny little ball (as we sometimes imagine), but quantum mechanical objects. They don’t have any surfaces. When, the two particles (particle and antiparticle) come close to each other, you can think of their quantum mechanical futures as merging and the two particles disappearing into another form of energy (most likely photons). From this energy are the types may also emerge. This is exactly what happens when we smash everyday particles at the Large Hadron Collider to create new kind of particles.
This actually puts up the point that, all particle collision result in the annihilation of original particles into new particles. But what’s the difference in the interaction of particles and antiparticles is that they are mirror versions of each other, with opposite charge and other forces. So they are very much attracted to each other and hence are more likely to collide. Also, they perfectly complement each other, so they annihilate into something neutral (like photons) and don’t create new particles.
The other thing that should be kept in mind is that when particles interact (collide) certain things are conserved. Like we have observed that electric charge can never be created out of nothing nor it can be destroyed. The total electric charge of a particle before and after collision has to be the same. Why is that necessary? We don’t actually know. We observed this pattern in real-life experiments and incorporated these rules in our theories.
When an electron and positron come close to each other, their opposite charge (+1 and -1) pull them towards each other and after they collide the opposite electric charges perfectly cancel out each other allowing all their traces of existence to disappear completely.
This won’t happen if we smoosh to other particles with same charge. For example, if we smoosh two electrons having charge (-1 and -1) and as a whole a charge of -2 has to be conserved, which won’t allow total annihilation of the electrons and may produce some new charged particles.
Well, electric charge is not the only thing that has to be conserved. You may ask, if we need two particles with equal and opposite charges, can an electron with charge -1 and an anti-muon with charge +1 annihilate? Well, the answer is you cannot. There is yet another law in our universe about smooshing, that says that “electronness” and “muonness” (basically quantum properties) have to be conserved. We can’t destroy an electron with a non-electron. It only works for its antiparticle, positron. Same is the rule for its cousins, muon and tau.
Why does the universe have these weird rules? We don’t know. Maybe one day will find that these rules are a consequence of some simpler theory of particles. Maybe antiparticles can give us some clues about that.
Antimatter and Possibility of Our Anti-versions
We saw, antiparticles are identical to normal particles and can annihilate each other completely turning the whole mass into energy. But it gets even more interesting than that.
It turns out that, just like regular particles, antiparticles can assemble themselves to form anti-version of more complex particles like protons and neutrons. For example, if we take two anti-down quarks and one anti-up quark and combine them we can potentially make an anti-neutron. This antineutron will still be electrically neutral just like a regular neutron, but its insides will be made of antiparticles. Again we can make an anti-proton by combining two anti-up quarks and one anti-down quark, which will be similar to a proton but with the negative charge.
After this we can go one step ahead and create more complex structures. Once we have antineutrons, antiprotons and antielectrons, we can potentially make antiatoms! A positron (antielectron) and a negative proton will be just like the regular versions except with charges reversed. If we bring an antielectron and an antiproton together, the antielectron will orbit around antiproton and we will get an antihydrogen! And in the similar way if we assemble enough antiparticles we can make anti-version of anything. For example, if we combine two antihydrogens with one anti-oxygen, we will probably get anti-H2O or anti-water. This anti-water will have the same feel and physical features like regular water. But if we drink it, we will explode in big flashes of energy which will not be that refreshing.
Again if you go further, we can find an anti-version of anything, and even of protein and DNA. And if that happens you may find an anti-Earth and an anti-you who will look just like yourself except that anti-he/anti-she will be made of antimatter [taken into account the probability of anything in the infinite universe]. That would be incredible, wouldn’t it?
In fact, there is nothing fundamentally “mattery” about our kind of matter and there’s nothing “antimattery” about antimatter. If we were somehow made of antiparticles, then we would have called that ‘matter’ and the regular ones ‘antimatter’. These are just arbitrary names and nothing else.
But, all this discussion about antimatter raises one question: Where is all this antimatter?
But before that we have another question to answer, why matter, not antimatter?
Now we know that antiparticles exist and Dirac’s formula does a fabulous job in describing their behaviour. But that doesn’t mean we fully understand them. De facto, this peculiar phenomenon of the universe generates more questions than answers. For example, why do these antiparticles exist? You may answer that, theory of particles requires them. But does this answer fully explain their existence?
Another question can be, is antimatter exactly opposite of the regular one or are there some minute differences in behaviour, texture, flavour, etc? Does it feel gravity the same way as regular mattery objects feel, or does it feel it the opposite way?
Well, the biggest of this question is a simple one: Why is our world made of matter and not antimatter?
If you are positive enough to handle the negativity of the negative matter, read on to learn more about these mysteries. Don’t worry it’s… free of charge. :p
Why the Universe, not the Anti-Universe?
There is one important difference between matter and antimatter: Matter is everywhere, but antimatter is nowhere to be found, at least not in our observable universe. So it seems that the universe has a lot more matter than antimatter.
As matter and antimatter are opposite versions of each other, then according to physics we would expect that the same number of particles and antiparticles were created during the Big Bang (remember the symmetry in Physics). But, if that had happened then each particle and antiparticle would have annihilated each other when they met and eventually converted all matter in the universe into photons and energy. Since you’re alive and reading this article, and you are pretty sure that you are not made out of light, so we can say that this didn’t happen. Therefore, it turns out that there might have been some preference for matter over antimatter. But the universe doesn’t play dice!
However, there can be two possibilities to explain this inequality:
Possibility – 1
As Albert Einstein said:
“For every one billion particles of antimatter there were one billion and one particles of matter. And when the mutual annihilation was complete, one billionth remained – and that’s our present universe.”
It suggests that during the Big Bang, slightly more matter was created than antimatter. Again the vast majority of matter and antimatter annihilated themselves into oblivion and the tiny bits of matter that were left resulted in the creation of all the galaxies, stars, planets and dark matter that exist today.
This possibility does explain the mattery universe we see today, but it has some problems. It does answer the question, “Why is the universe made of matter and not antimatter?”, but again it raises an equivalent question, “Why did the universe begin with more matter than antimatter?”. Sadly enough, we don’t have an answer to this question. And again our present theories about the beginning of the universe are inconsistent with anything asymmetrical during the initial production of matter and antimatter.
Possibility – 2
It is possible that equal amounts of matter and antimatter were created during the Big Bang. But there was something in the particles that caused to be more matter than antimatter.
This is possible if there are some physical reactions that destroy antimatter faster or create more matter than antimatter. As particles and antiparticles are created all the time, even a small difference in how they are destroyed or created can add up to a large imbalance.
So, what do you think, Possibility-2 is quite possible, isn’t it? But, again how likely the universe has inherent preference for making and preserving matter over antimatter. As I said, most of the Physics is fully symmetric. And as far as we know, what particles can do, antiparticles can reversely do. For example, when a neutron undergoes a nuclear beta decay, it releases a proton, an electron and an antineutrino, and in exact opposite way when an antineutron undergoes a nuclear beta decay, it releases an antiproton, an antielectron and a neutrino.
So there must be something else that can explain the preference for matter over antimatter. But at present we don’t know what it is.
Maybe Antimatter is Somewhere Hidden
What if we got it all wrong? Maybe, there are equal amounts of matter and antimatter in the universe, but they are all separated into different regions due to cosmic inflation in the primordial time of the universe. We know that the Earth and its neighborhood are definitely made of matter, but what if other things beyond them are made of antimatter?
If you’re interested in the origin of the universe and its properties you may check out our this post: How big is the Universe? What is its Shape?
Matter and antimatter are so similar that we can’t tell if a distant star or galaxy is made of matter or antimatter. An antimatter star would have the same nuclear reactions and generate photons the same way with the same energies and an antimatter galaxy will also have the same chemistry and absorption and emission spectra as a normal matter galaxy. So it becomes near to impossible for physicists to determine if a distant celestial body is made of antimatter just by looking at its physical features. But we are forgetting one thing: when matter comes in contact of antimatter they annihilate with big flashes of light and other electromagnetic waves.
So, let’s take a look on Earth and its surroundings i.e. the Moon and neighboring planets. Earth is made of matter and if it had any antimatter on it, it would have reacted explosively. Now let’s look at the moon, if moon was made of antimatter, then every time it was hit by a matter meteor there would be a explosion with a giant flash of light. And we know that even an antimatter meteor of size of a raisin could cause an explosion as powerful as an atomic bomb. The Earth and the Moon are constantly bombarded with matter meteors, small and big. So, we can be sure that at least the Earth and the Moon aren’t made of antimatter.
Same is the case for Mars and other planets of our solar system. If Mars was made of antimatter, we would see the Mars light up with exploding photons. In fact, if there was any significant concentration of antimatter near a region with matter, you would see constant annihilations and release of photons at the border between the matter and the antimatter region. Do we see such phenomenon in our neighborhood? No. So, we can be pretty sure that our Solar System is not made of antimatter.
Astronomers are now searching for entire solar systems made of antimatter in our galaxy, but haven’t found any hint of that. They have even considered the possibility of entire galaxies made of antimatter. NASA is also trying to find if such galaxies exist by looking for X-Ray and gamma ray signatures of annihilation events in colliding superclusters. Unfortunately, they have found no such signals, and are pretty confident that our entire cluster of galaxies is all made of matter.
But what’s beyond our observable universe we can’t say. Also, the voids between clusters of galaxies are large enough that if there was a boundary between matter and antimatter out there, it would be too faint to see. Perhaps, the space beyond the observable universe is full of antimatter, who knows!
However, it is more likely that the rest of the universe is also made of matter. This is because, a universe organized into cluster of matter galaxies and antimatter galaxies would have required the matter and antimatter to be widely separated in the early universe, which will again raise a whole new set of questions. (But no one is stopping you to imagine the possibilities and think about the consequences.)
Do Neutral Particles have Anti-versions?
Does every particle have an anti-version? So far we saw that every particle, that has some type of charge, had an antiparticle. But we cannot say the same for neutral particles. For example, we have never observed the anti-version of a photon (a neutral particle with no charge) i.e. anti-photon. Some say that the photon is its own antiparticle. Let’s see what Wikipedia says:
“A truly neutral particle is a subatomic particle with all his charges equal to zero. This not only requires particles to be electrically neutral, but also requires that all the other charges (like the color charge) are neutral. Such a particle will be its own antiparticle… Known examples of such elementary particles include photons, Z bosons and Higgs bosons…”
However, this argument seems more like avoiding the question than answering it. Ha ha! 😀
(i.e. does being your own best friend mean you have no friends?)
So, Z-bosons and gluons are also devoid of antiparticle twins. You may observe that all these particles carry force, and so do the charged particles, but they do have antiparticles. Why some particles have antiparticles and others don’t? (Think about it) We can’t say.
Scientists believe that the neutrino, which is also a neutral particle with zero electric charge, probably has an antiparticle with opposite values of the charges associated with the weak nuclear force (called “hypercharge”). But neutrinos are themselves mysterious little particles that are difficult to study, so it is possible that they are also their own anti-versions.
How can we Study Antimatter?
How fantastic it would be if we could build antiobjects from antiparticles. It would not only be fantastic but also educational: we could learn from them how antimatter is different from regular matter and it could also explain why antimatter exists.
However, making experiments with antiobjects is really hard, and at present near to impossible. The research on antiparticles is still at the infancy phase. Only recently, scientists have succeeded in creating antihydrogen using antiprotons and antielectrons. In 2010 scientists at Switzerland-based European Organization for Nuclear Research (CERN)succeeded in creating a few hundred atoms of it and trapping them for about twenty minutes. Looks quite impressive, doesn’t it? But it is not enough to answer all the big questions about antimatter. Imagine how little we could have learnt about the universe if we were allowed only to see a small number of hydrogen atoms for a few minutes.
Although we are making good progress in antimatter research, we won’t learn much unless we develop better techniques to create larger number of antiparticles and also develop methods for storing them safely. The scientists at CERN have, however, developed a method to store antiparticle safely at least for a few minutes.
When the particle scientists at CERN generate antihydrogen atoms, which are electrically neutral (unlike positron and antielectron which can be controlled by electric field) they take advantage of the magnetic properties of antihydrogen. They used superconducting magnets to generate a strong magnetic field to control and trap antihydrogen atoms. If the antihydrogen atoms have low enough energy, they can stay in this magnetic “bathtub” for a longer time. 
At present, only a few picograms of antimatter are created annually at CERN, which means that it would take millions of years to make antimatter equivalent to half a raisin!
Little More About Antimatter and Anti-gravity
So far we know that antimatter exist and have an opposite charge compared to its regular counterpart and that when it comes in contact with matter, they annihilate and turn into light.
However, there are a lot many questions about antimatter that are still unanswered. On the first hand we don’t know why antimatter exist at all? And again we know that the universe definitely have some preference for matter over antimatter. But why is it so? We don’t know. These questions may give you a headache and may make you wary of antimatter. But think about the awesome features of antimatter. This argument may give your mind some refreshment:
“The Feynman – Stueckelberg interpretation states that antimatter and antiparticles are regular particles travelling backwards in time.”(Wikipedia)
Wait, hold on to your excitement and read the sentence again. It is just an interpretation of the behavior of antiparticles, not that they are actually travelling backwards in time.
[Well, there are yet some exciting mysteries of time which you can check out here: What is Time? Is Time-Travel Possible?]
Well, there is one more amazing question about antimatter: Do antiparticles feel gravity the same way as matter particles do?
Although we know many things about antimatter and our current theory predicts that it feels gravity just like normal matter**, yet we cannot actually observe it because we don’t have enough quantity of antimatter to experimentally test it. Gravity is such a weak force (and you know it depends on mass) that we need a very large number of particles to measure it. And again antimatter is so rare and unstable that accurate gravitational experiments are nearly impossible.
Nevertheless, in 2013 CERN’s anti-atom factory, ALPHA carried out the gravitational experiments on the few antihydrogen atoms which they trapped using magnetic fields. They tried to observe the behavior of the anti-atoms when they were released from the magnetic trap. In that condition, the antihydrogen atoms were experiencing no other force except gravity. And there were detectors to detect the movement of antihydrogen atoms, to check whether they rise up or fall down under the effect of gravity.
The experiment also tried to determine whether the effect of gravity on the antimatter was equivalent to that of matter. It tried to determine the effect using the ratio of the antihydrogen’s gravitational mass Mg to its inertial mass Mi (its resistance to change of motion under a force). And the weak equivalent principle states that inertial mass should be equal to gravitational mass and it has been tested to high precision for matter. If the weak equivalence principle holds (i.e. gravitational effects is similar to that of matter) then the ratio F=Mg/Mi must be equal to 1. And if antimatter experiences the opposite i.e. anti-gravity, then F should be equal to -1.
But the results of the ALPHA experiment didn’t even come close to that. The experiment concluded that the ratio was probably no more than 110or less than -75 (i.e. -75 < F < 110). So we cannot conclusively say that antimatter experiences anti-gravity or not. 
However, CERN is also working on a new experiment named GBAR, which stands for ‘Gravitational Behavior of Antihydrogen at Rest’. The setup of components of this project is still not complete, which is expected to be completed by September 2018. This experiment can provide some more accurate data on the gravitational property of antimatter.
But what if antimatter feels gravity differently from the regular matter? Remember the defining feature of antiparticles, that their electromagnetic, weak and strong force charges are reversed. Is it possible that antiparticles also have their “gravity charge” reversed? And if that happens then antiparticles would feel gravity the opposite way. Imagine if we somehow figure out how to create and harness the antigravity property of these antimaterials, then those flying cars and antigravity boots we fantasized about in our childhood days might actually become a reality!
** Remember Einstein’s theory which suggests that the fabric of time and space bends due to the effect of mass. It will require negative mass for antiparticles to have antigravity. Which is not possible (Or is it?).
So we see how incredible is the physics of our universe. Of course, as everything has been created out of nothing, and to conserve that nothingness it needed antimatter. But, can we have something like anti-energy which can eat up regular energy? Well, nothing has been found similar to that. Energy is considered neutral, and it is also speculated that it leaks out in higher dimensions (we’ll discuss about these in detail in some other episodes). We haven’t observed anti-photons, and anti-version of light, though it sounds interesting.
Now coming back to anti-matter. Energy stored in matter and antimatter is very high. We can generate a humongous amount of energy from antiparticle annihilation. This leads our engineers to speculate that this energy can be used to power spacecraft. Antimatter powered spacecraft can be an efficient way to explore this vast and mysterious universe. However, NASA cautions there is a huge catch with this idea: It costs around $100 billion for a milligram of antimatter. Again it requires a huge amount of energy to create antimatter, more than what we can get back from an antimatter reaction. So, at present its not economically and technically viable implement this idea. 
But this hasn’t stopped NASA and other agencies from working to improve the technology to make antimatter spacecraft possible. It is, however, speculated that it will be possible to use antimatter in about 40-60 years in the future.
Image Source :
 – https://en.wikipedia.org/wiki/Antimatter
 – https://en.wikipedia.org/wiki/Annihilation
 – https://home.cern/about/engineering/storing-antimatter
 – https://arstechnica.com/science/2013/04/does-antimatter-fall-up-experiment-could-provide-the-answer/
 – https://www.livescience.com/32387-what-is-antimatter.html
[•] – Jorge Cham and Daniel Whiteson (2017) : We Have No Idea
[•] – Other various references are inside the post