The Existence of Antimatter

An ongoing curiosity for physics has driven me into the depths of particle physics, once again. This time, I decided to share my findings with the readers of the magazine (perhaps to save you some time, just in case you were, too, wondering why on earth does antimatter need to exist?!). Physics enthusiasts, this one is for you.

This short article, with the goal to shed some light on the often peculiar puzzles offered by particle physic, can be simply viewed in two main parts:

• What is antimatter?
• Why does antimatter need to exist?

I'd like to remind you that I am by no means an expert, and all the information that I use and mention for my explanations comes from credible sources, generated by experts in the field of particle physics. I've also linked some resources for further reading at the end of the article, as well as graphs/visualisations to help you grasp concepts such as the Light Cone easier than through my explanations. 😊

WHAT IS ANTIMATTER?

Antimatter can be thought of as the opposite of normal matter: it is identical to matter, apart from charge, which is opposite. So take an electron, for example - because its charge is negative, its identical counterpart will have a positive charge, and is therefore named positron. Antimatter particles are simply termed antiparticles. The positron was the first one to be discovered, having been found by American physicist Carl Anderson in cosmic ray studies (1). But the prevalence of antiparticles does not stop at subatomic level, showing that 'without exception, nature is symmetric', as the author puts it (1). The ALPHA experiment at CERN has even produced antihydrogen atoms from antiprotons and positrons (2). Although antiparticles are most often observed in particle physics labs, where they are produced to be studied, antimatter exists in nature. A common example is again, positrons - they are produced by lightning (3). Since matter and antimatter have opposite charges, their interaction creates equally imposing effects. When the two interact, they annihilate each other, converting their masses to 'pure energy in the form of photons' (1).

WHY DOES ANTIMATTER NEED TO EXIST?

Considering the instant annihilation of particles and antiparticles on their mutual encounter, we can say we're lucky that there's significantly more matter than antimatter. If this were not the case, the Universe would be far more than inhospitable to life: it would not even hold a stable shape. In addition, it seems that antiparticle behaviour is completely determined by particle behaviour (4) .

While physics has yet to explain the asymmetry between matter and antimatter, it can to some extent explain the importance of antimatter.

ANTIPARTICLES AND... TIME TRAVEL 

In the context of physics, antimatter has the remarkable role of 'wedding' quantum mechanics and relativity together, in the words of the legendary theoretical physicist Richard P. Feynman (4). Since special relativity states that no object can travel faster than the speed of light, a stunning visualisation of past, present and future emerges.

When an object travels through space-time, we can imagine its 'past' behind it and its 'future' in front of it – all in space-time, with space as the x-axis and time as the y-axis. The Light Cone represents the area of space-time which can affect the motion (and reality) of the object (see Image 1). Anything outside the Light Cone is 'elsewhere' and cannot affect the object. This is best explained visually, so for further detail, I've linked a couple of videos in the Further Reading list at the end of the article (5).

Of course, as we know, the faster an object moves through space, the lower its speed through time. This is part of a concept known as Terrell rotation, the limit of which is that an object's motion cannot be rotated to make the object move backwards through time. This is because the speed limit of objects in our Universe (imposed by special relativity) is light speed. Anything faster than that, and an object would be travelling through space but not through time.

Therefore, the Light Cone acts as a boundary of causality, since time cannot flow backwards without reversing cause and effect as we know it – inside the Light Cone. What is this implying? That time CAN flow backwards, as long as this happens outside the Light Cone; this way, whatever happens outside the Cone does not influence reality inside (2).

But what does this have to do with antimatter?

Let's return to our first example of the antiparticles of electrons: positrons are simply electrons with a positive charge, and therefore positive energy; since energy can be defined as motion through time, an electron's antiparticle could be seen as an electron with opposite energy, therefore an electron with opposite motion; in other words, travelling backwards through time.

This somewhat bizarre concept, suggested by the Dirac equation, implies that the electron, or at least its antimatter counterpart, can move faster than the speed of light, something deemed impossible by the theory of special relativity. However, the incompatibility of this explanation with Einstein's theory of relativity can be alleviated by another explanation relating to the Light Cone.

The term 'probability distribution' usually refers to all the possible probabilities for a variable within a given range (6). In particle physics, however, the probability distribution of a particle gives the 'probability of finding an electron in a given volume of space' (7).

In physics and mathematics, every outcome, every probability must be taken into account. This often leads to seemingly unlikely probabilities - it has even been suggested that the probability distribution for the positron can somehow leak through the Light Cone, resulting in the probability amplitudes for the particle/antiparticle pair to cancel each other out exactly (because, as we said previously, they are very nearly identical) (2). This means that reality sometimes split up into pairs of particles and antiparticles, but depending on the speed we're moving, so that antiparticles' probability distribution can leak through the Light Cone, into our reality.

To sum it up, it could be concluded that the reason for antiparticles is causality. The existence of antimatter which can somehow spread its probability distribution wave into what we call reality, only to be annihilated by matter, creates the effect of causality as we can experience cause and effect safely inside our Light Cone, without anything outside the Light Cone interfering.

But this sounds like an incomplete explanation, right? That's because it is. What we know about antimatter so far has been confirmed in scientific experiments, however, there are many more details which are likely but not confirmed, including virtual particles. Perhaps in the coming years, new evidence, particularly from particles colliders such as the Large Hadron Collider or LHC, will enable another physics enthusiast to complete this explanation with a part II to this article.

REFERENCES

1. Gregg Wolfe, Erika Gasper, John Stoke, Julie Kretchman, David Anderson, Nathan Czuba, Sudhi Oberoi, Liza Pujji, Irina Lyublinskaya, Douglas Ingram. 33.4 Particles, Patterns, and Conservation Laws. openstax.org. [Online] 12 August 2015. https://openstax.org/books/college-physics-ap-courses/pages/33-4-particles-patterns-and-conservation-laws.

2. The Field Guide to Particle Physics. The Reason for Antiparticles. 2022.

3. Anne Marie Helmenstine, Ph.D. What Is Antimatter? www.thoughtco.com. [Online] 6 February 2019. https://www.thoughtco.com/overview-of-antimatter-608646.

4. Feynman, Richard P. The reason for antiparticles. s.l. : Cambridge University Press, 2014.

5. Pretty Much Physics. YouTube. [Online] 2018. https://youtu.be/OZv3ycr6Jxg?t=96.

6. M, Shruti. What is Probability Distribution: Definition and its Types. www.simplilearn.com. [Online] 15 February 2023. https://www.simplilearn.com/tutorials/statistics-tutorial/what-is-probability-distribution.

7. Representation of Orbitals. chem.libretexts.org. [Online] 18 August 2022. https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/06%3A_Electronic_Structure_of_Atoms/6.06%3A_3D_Representation_of_Orbitals#:~:text=One%20way%20of%20representing%20electron,plot%20of%20the%20pr.