Antimatter and the Ozma Problem
Image by Felix Mittermeier (Pixabay)
INTRODUCTION
Antimatter and the Ozma Problem were topics that lingered in the back of my mind ever since I was first introduced to it. I remember sitting in the classroom near the end of the school year when my Year Nine Physics Teacher decided to go beyond the specification in the Section of Astrophysics and led the class into a whole new world of Quantum Physics. Back then, I remember being confused about the entire idea; however, I have come to realise that it is important for us to find out more about Particle Physics and the Quantum world, as the knowledge we can acquire from it can help us understand more about the nature of the Universe that we live in, as well as the beginning of it. In understanding Particle Physics, we can turn towards understanding the Ozma Problem and how it was solved, which I will delve into in this article.
THE ATOM
Whether you have or have yet to enter the world of Quantum Physics, you may have learnt that the Atom is defined as the ‘smallest unit of matter’. In fact, the word ‘atom’ means ‘indivisible’ in Greek, as it was previously known that atoms cannot be divided into anything tinier [1]. However, we now know that we can go smaller than the atoms, to the subatomic particles - protons, neutrons and electrons [2] - which are made up of even smaller particles known as quarks and leptons [3].
Protons and neutrons are made of two types of quarks: ‘up’ quark and the ‘down’ quark. These quarks have electrical charges that are fractions of the proton’s charge (an ‘up’ quark has ⅔ positive charge and a ‘down’ quark has ⅓ negative charge). A proton is made up of two ‘up’ quarks and one ‘down’ quark (⅔ + ⅔ -⅓ = 1) so overall the charge adds up to +1, whereas a neutron is made up of two ‘down’ quarks and one ‘up’ quark (⅔ -⅓ -⅓ = 0) which gives the neutron a neutral charge (See Fig. 1) [4].
Fig 1. The sets of up quarks and down quarks in a proton (left) and in a neutron (right)
Source: Wikimedia Commons
The electron (a lepton), the up quark, and the down quark (quarks), which are all fermions, are probably the most well known out of the elementary particles. It is through decades of scientific research and, in particular, the research at CERN with the Large Electron Positron (LEP) collider allowed scientists to discover and learn more about these elementary particles [4]. These quarks also have anti-versions of themselves.
So lets us look into antimatter. 1932 was when the first anti-particle was discovered. Carl Anderson discovered a track left by a cosmic ray that went through a lead sheet that was in a magnetic field. The curvature and the thickness of the track was analysed and the calculations indicated that the track was left by a particle with the same mass as an electron but with the opposite charge - the positron (see Fig.2).
Fig 2. The discovery of the positron. The magnetic field is going into the page and the positron has moved from the bottom and up through the lead sheet. After passing through the lead sheet, It has curved more significantly to the left hand side as its momentum has decreased (r=p/BQ). If it was an electron, it would have curved towards the right hand side.
Source: Wikimedia Commons
It was from then on that Scientists began experimenting and colliding particles in the hopes of discovering more of these mysterious anti-particles.
In the LEP, positrons and electrons were accelerated around the 27-kilometre circumference of the collider and collided with one another when they reached the same point at the same time. This caused an annihilation to occur (as matter is colliding with antimatter). This annihilation was the key moment for scientists, as the goal was to observe in detail what was released in the aftermath of the collision. To do this, the Collider was equipped with four detectors, built around the four collision points within underground halls. They were capable of registering the particles by their energy, momentum and charge, thus allowing physicists to tell what particle reaction happened and what elementary particles were involved [17]. Many of these emergent elementary particles from the annihilation were unknown before the LEP was built; for instance, the two heavier versions of the electron (the ‘muon’ and the ‘tau’), the two heavier versions of the up quark (the ‘charm’ and ‘top’), as well as the two heavier versions of the down quark (the ‘strange’ and the ‘bottom’) [4]. These newly discovered particles were also revealed to have anti-versions of themselves.
Fig 3. The Standard Model of Elementary Particles.
Source: Wikipedia
The LEP was dismantled in 2001 and was replaced by the Large Hadron Collider (LHC), which is the world’s largest and highest-energy particle collider and the largest machine in the world. The name might sound familiar to you because recently in 2012, it became well known for discovering the Higgs Boson [18].
The discovery of the Higgs Boson is particularly fascinating because, in the 1970s, scientists found out that electricity, magnetism, light and certain types of radioactivity are all manifestations of a force called the electroweak force. The equations detailing the electroweak force correctly describes its force-carrying particles: the photon, the W bosons and Z bosons. All of these emerge with no mass, which is correct for the photon, but not for the W and Z bosons, whose masses are approximately 100 times that of a proton. It is later when theorists Robert Brout, François Englert and Peter Higgs solved the problem. They proposed that W and Z bosons interact with an invisible field, which is now called the ‘Higgs field’. After the Big Bang, the Higgs field was zero, but as the universe cooled and expanded, the field grew, which meant that any particle interacting with it would acquire mass (The Higgs Effect). The more the particle interacts with this field, the heavier it becomes. Photons, as they don’t have a mass, don’t interact with it. The Higgs boson in turn became the associated particle of the Higgs field [19].
Fig 4. Collision at the LHC that produced a Higgs Boson and a Z Boson. The two grey projections represent the particles that decayed from a bottom and an anti-bottom quark, which was likely to have decayed from the Higgs Boson. The green lines represent electrons and positrons, which likely decayed from a Z boson. [20]
Source: Thomas McCauley ©2018 CERN (https://cds.cern.ch/record/2642472?ln=en)
Therefore, discovering antimatter and using it in theories and experiments are fundamental in discovering how the universe began, and the nature of it at a quantum scale.
NATURE OF THE UNIVERSE
Einstein’s theory of relativity reveals the interconnection between energy and matter, perfectly shown in his famous equation E=mc2. It shows how energy can congeal itself into matter, and in reverse, how matter can turn into energy. The nature of the universe can be further explained when Einstein’s theory is combined with Newton’s Laws of Motion. Take a box for example. To move the box, you have to exert a force onto it, and this resultant force will cause it to accelerate in the direction of the force. This acceleration, as it depends on the force, also depends, in proportion, on the mass of the box. According to Newton’s Laws of motion, if a body is stationary and you apply a force on it for a second, then the speed will increase by some amount. If you apply the same force on the same body again, then the body will increase in speed by the same amount. However, in Einstein’s theory of relativity, this change in speed alters - the next push will increase its speed less than what it did on the first push. If the body is travelling near to the speed of light, pushing it would increase the speed by a negligible amount [4]. Therefore, Newton’s Laws of motion are only accurate for day to day motion, not in the situation where particles are moving in an accelerator at high relativistic speeds.
In contrast to Newton’s Laws of Motion, Einstein’s theory states that the mass of a body increases the faster it travels. Near to the speed of light, its mass becomes infinite. Hence, it is impossible to accelerate the object to the speed of light [4]; light is the only thing that can reach the speed of 299,792,458 metres per second [5].
Take a moving particle. It has energy congealed in its matter, and energy in its motion, which is known as its kinetic energy. The total energy E of the moving particle is not the sum of these two forms of energy, but the square root of the sum of the square of the energy of motion pc(product of momentum and speed of light) and the square of the energy in its mass mc2. This may seem familiar to you, as this applies Pythagoras’ theorem. The length of the hypotenuse is proportional to the energy of the moving particle (See Fig. 5) [4].
Fig 5. Conflation of Einstein’s theory of Relativity with Pythagoras’ theorem. The amount of energy of a body is the square root of the sum of the square of the energy of the body when stationary, mc2, and the square of the energy of its motion, pc. [4]
Source: Wikimedia Commons
This means that a photon travelling at the speed of light, with no mass, has energy due to its motion. As the law of the conservation of energy states, energy cannot be created nor destroyed but can be transferred from one form to another. This shows how the energy of a photon can be transformed into energy trapped in matter. But how is it possible for an electron with a negative electrical charge to come from the energy of photons, which has no electric charge? Under the principle of charge conservation, the only way is that a positron, the anti-version of the electron, is also made. Scientists believe that this was what happened right after the Big Bang, the birth of the universe - light consisting of massive amounts of energy congealed into pieces of matter and antimatter, and when matter and antimatter meet, they annihilate one another, which in turn releases energy. Hence, Einstein’s theory of relativity provided us with the key into the world we live in as well as the one into the antiworld; now, we just need to find the lock [4].
THE ELECTRON SPIN
We know that light is made up of photons (quanta of electromagnetic radiation) and the energy of each photon is the product of Planck’s constant and the frequency, E=hf. In an atom of an element, there are discrete and unique energy levels due to the number of quantum waves that can fit into a ‘loop’. Therefore, when electrons move from a high energy level to a lower energy level, the discrete energy difference between the two levels is released in the form of photons, which give out a specific wavelength, and in turn, a specific colour. The colour emitted is unique for each element. This atomic spectrum can be observed by adding the element into a flame and looking at the light through prisms or diffraction grating.
In 1896, Peter Zeeman, the Dutch spectroscopist, noticed that when powerful magnets were placed near his samples, the yellow lines emitted by the sodium changed slightly. The yellow lines changed from being sharp and defined into broad. It was later discovered that the broadening lines were actually due to a separation of one line into multiple lines. Why did this happen?
This is because the electron has its own magnetism; in other words, the magnetic field interacts with the magnetic dipole moment that is associated with its orbital angular momentum [13]. This in turn affects the energy of the sodium samples (each of the levels split into substates of equal energy [14]), which results in an alteration of the atomic spectra.
It has effectively been shown that an electron can act as a small bar magnet with a north and south pole and that it has an intrinsic rotary motion known as ‘spin’, which can orientate itself clockwise or anticlockwise in a magnetic field. It’s important to note that spin is an ‘intrinsic rotary motion’ because an electron, in reality, is not a ball but an infinitely small point that cannot spin.
Spin is an odd physical phenomenon that is still challenging among physicists to explain. It is like the spin of a planet in that it gives a particle angular momentum and a small magnetic field known as a magnetic moment; however, due to the size of subatomic particles like the electron, its surface would have to be moving faster than the speed of light to produce the measured magnetic moments (which is impossible). Moreover, spin is quantised, so only certain discrete spins are allowed. This can be demonstrated using the Stern-Gerlach experiment [15] (See Fig. 3).
In the Stern-Gerlach experiment, a beam of silver atoms is ejected into an inhomogeneous magnetic field. According to classical physics, it is expected that the magnetic moments of the silver atoms are randomly orientated, so they should be deflected by different amounts depending on their orientations. However, they found that half of the silver atoms were deflected upwards and half of them were deflected downwards by the same amount (two discrete points of accumulation in the machine). These two states are known as ‘spin up’ and ‘spin down’, showing the quantised nature of the spin. If you look at the silver atom, there are in total 47 electrons. In 46 of these electrons, each spin-up is paired with one spin down. The spins neutralise each other, so what is left is one unpaired electron - the 5s electron. This electron can be either spin up, spin down or any superposition of these two states - which means that its spin can point in any direction. As all the silver atoms have spins pointing in different directions, they are effectively unpolarised. The inhomogeneous magnetic field, therefore, acts as a filter and forces the spin of a silver atom to take a random orientation in either the same or the opposite direction of the magnetic field. If the spin state of a silver atom is closer to up, it is very unlikely to change its direction to down [16].
DIRAC
Fig 6. Dirac’s equation
Source: BBC (©StellarioCama)
Dirac’s equation (See Fig. 6) was derived in 1928 and it combined quantum mechanics with Einstein’s theory of relativity (the behaviour of fast-moving bodies) [4]. His equation was revolutionary because it revealed the existence of the antiworld - the equation works not only for an electron but also for a positron (the antiparticle of the electron) [12].
Quantum mechanics deals with the motion of tiny particles; their small size brings about uncertainty in the accuracy of their position in time and space. In 1926, Erwin Schrodinger derived the equation of quantum mechanics for slow-moving particles (‘slow-moving’ relative to the speed of light) known as The Schrodinger's Equation. It explained how electrons behave in atoms, and that an electron in a hydrogen atom is moving with a speed of about two thousand kilometres per second. It also explained why the orbital motion of electrons in atoms caused spectral lines to multiply in magnetic fields (but doesn’t explain electrons’ ‘spin’) [4].
Oscar Klein tried to generalise Schrodinger’s theory by using E2 and Einstein's hypotenuse relation. The square root of 25 can be either +5 or -5 (positive or negative). Since you can’t have negative length, the negative answer was rejected (taken as false); however, it left people feeling unsure. Dirac wanted to write an expression for the energy of an electron using E instead of E2 (using a way other than square rooting the whole Einstein Hypotenuse equation). He aimed to find an equation showing how a sum of some amount of mc2 and pc would give E (in this case, the energy of an electron) [4].
Imagine a right-angled triangle with the side lengths of 3, 4 and 5 (5 being the length of the hypotenuse), and as I have mentioned above, we are taking some amount of ‘3’ and ‘4’ and they should add up to 5. This can be written as
4a + 3b = 5
Then, square the equation to get
16a2 + 12ab + 12ba + 9b2 = 25
This equation should be the same as the hypotenuse form, where 16 + 9 = 25, which means that a2 = 1, b2 = 1, and ab + ab = 0. However, there are no numbers when squared would give 1 but whose product ab would be zero [4]. This is not only for 3,4 and 5; but for any combination. Effectively, we are trying to match the two equations
E2 = b2 (mc2)2 + a2(pc)2 + a × b[(pc) × (mc2)] + b × a[(mc2) × (pc)]
and
E2 = (mc2)2 + (pc)2
This cannot be solved using numbers but it can with matrices.
The two matrices that can solve Dirac’s problem are
a2 and b2 each equal to 1 and if you multiply ab and ba, you get
So ab + ba = 0,
which solves Dirac’s problem [4].
ANTIMATTER & CP SYMMETRY
Antimatter, as the name suggests, is the opposite of matter. For instance, in the anti-world, positrons (anti-version of the electron) and antiprotons (anti-version of the proton) would exist. However, if these particles and their respective antiparticles are the same but different at the same time, what does that mean? It means that the anti-version of a particle (made of matter) would have the opposite charge and would be mirrored, in other words, it would show parity. This is known as CP Symmetry - C for Charge and P for Parity [4]. The thing that stays the same between the particle and its anti-version is its mass.
To help with the understanding of CP Symmetry, I have included a diagram (See Fig. 7) that illustrates this.
Fig 7. ‘Day and Night’ - A painting made by Maurits Cornelis Escher in 1938. The painting is mirrored (parity) and its charge is swapped over (represented by the colour change between black and white). As you can see, after changing its ‘charge’ and parity, the painting afterwards (on the right bottom corner) is identical to the original one (on the left top corner).
Source: Talk by Tara Shears - Antimatter: Why the anti-world matters (The Royal Institution) [3]
MIRROR SYMMETRY & PARITY
A right-handed coordinate system is useful in finding out what a mirror image shows. The coordinate system shows the x-axis (represented by the index finger), the y-axis (represented by the middle finger) and the z-axis (represented by the thumb) (See Fig. 8) [7].
Fig 8. The Right Hand Coordinate System and the Left Hand Coordinate System (which are mirror images of each other). For the right-hand coordinate system, the direction at which the thumb is pointing at is along the z-axis, the index finger is pointing in the direction along the x-axis and the middle finger is pointing the direction along the y-axis (for the left-hand coordinate system, it is along the -y-axis).
Source: Scratchapixel
If you place your right hand next to the mirror, then the mirror image should show the x-axis and the z-axis going in the same direction as that of your right hand; however, the y-axis becomes -y. Hence, the mirror image is a left-handed coordinate system (See Fig. 7) [7]. This demonstrates how mirror reflections show a change in handedness. Imagine a screw. If you turn it clockwise next to a mirror, the mirror image should show the screw turning anticlockwise. In mechanics terminology, the screw we are turning clockwise is a right-handed screw and the mirror image shows a left-handed screw. In Mirror Symmetry (also known as parity conservation), there should be no change in handedness; therefore, the laws of nature should show no preference for right-handedness or left-handedness [7].
Parity can be described as a transformation - a mirror reflection and a rotation of 180° about the new y-axis [7].
(x, y, z) (x, -y, -z)
(x, -y, -z) (-x, -y, -z)
This transformation can be seen with electromagnetism. If you place a solenoid next to a mirror, then using the right hand grip rule, your thumb should show the direction of the magnetic field when you fingers are curled up in the direction of conventional current. If you apply the same rule for the mirror image of the solenoid, you will find that the direction of the magnetic field will be opposite to that of the solenoid itself [7].
Fig 9. A positively charged particle in a magnetic field, and its mirror image.
Image Reproduced from Fig. 3 of Daniel, Michael. “The Ozma Problem .” Physics Review, May 1998, p.21 [7]
If you look at a positively charged particle moving in a magnetic field at a certain direction (as seen in Fig.9), then using Fleming’s Left-Hand rule, we can determine the direction of the force on the charged particle. The mirror image would have the positively charged particle moving in the same direction (parallel to that of the original one) but its magnetic field and force would be going in the opposite directions. You can see the parity transformation of the mirror image from the original. What is more astonishing is that this parity transformation obeys the laws of electromagnetism - like charges repel [7]!
THE OZMA PROBLEM
The Ozma Problem poses the scenario where you are a scientist on Earth who just received a mission to communicate with aliens, and your first task is to tell them which direction left is. How can we do this?
This is a challenging task because everything that is controlled by gravity and the strong nuclear force shows mirror symmetry. Which means that up and down, left-hand side and right-hand side, are all relative to one another [6]. For example, if you set up two cameras in a laboratory and record a video of an experiment taking place, then take this original video of the experiment and mirror it, and subsequently send both videos (non-mirrored and mirrored) to another laboratory, the scientists at the other laboratory wouldn’t be able to distinguish between the two - whether one of the videos was real or the mirrored version. This reinforces the fact that the laws of nature are ‘mirror symmetric’ [7].
If there are no shared reference objects between us and the aliens that can help us solve this problem, then how are we supposed to tell them what left is? Most would think that up to this point, this task is virtually impossible. However, In 1956, the Chinese-American physicist Chien Shiung Wu conducted a nuclear physics experiment, and the aim was to see whether or not P-conservation (conservation of parity) also applies to weak nuclear force, just as it does with electromagnetism and strong nuclear force [8].
The results of the experiment have established that the conservation of parity was violated by the weak nuclear force, which meant that it would be possible to distinguish between a mirrored version of the world and the mirror image of the current world, as they would behave differently [8].
Wu’s experiment monitored the β decay of Cobalt-60 atoms that were aligned by a uniform magnetic field (the weak interactions are responsible for β decay) [8]. The alignment of the atoms is important as it minimises the random fluctuations that occur at higher temperatures. When the temperature gets closer to absolute zero, the cobalt nuclei can behave like tiny bar magnets, each of them having a north and south pole. This gives them their ‘spin’ which allows them to line up with the direction of the magnetic field lines [7]. Cobalt-60 is an unstable isotope so it will undergo β decay to the stable isotope of Nickel-60. Electrons and gamma rays are emitted in this process as well [8].
Fig 10. The Beta Decay of Cobalt-60.
Source: Wikipedia [8]
The emission of gamma rays is essential in determining whether the weak nuclear force obeys or violates the conservation of parity. Gamma rays are photons and they go through an electromagnetic (EM) process when released from the Cobalt-60 nuclei. Electromagnetic (EM) radiation obeys the conservation of parity, hence they would be emitted almost symmetrically in all directions (isotropically). Thus, the distribution of the gamma rays acts as a control for the distribution of the emitted electrons. The experiment used this principle, counting the rate of emission for gamma rays and electrons in two distinct directions and making comparisons. If the counting rates for the electrons were similar to those of the gamma rays, then parity would be conserved by the weak interaction. However, if the counting rates were very different, then the weak interaction violates the conservation of parity [8]. Asymmetry is shown as the electrons appear to be emitted predominantly in one direction than the other, which is in the direction opposite to the direction of the magnetic field [7], which in turn is opposite to the nuclear spin (See Fig. 11) [9] [10].
Fig 11. The β decay of Cobalt-60 and its mirror image - notice how the original arrangement shows the rotational axes as left-handed and the mirror-reversed arrangement shows it as right-handed. For your information, the direction of the magnetic field in the original arrangement is going downwards whereas in the mirror-reversed arrangement, the direction of the magnetic field is going upwards (See MIRROR SYMMETRY AND PARITY).
Source: Wikimedia Commons
Since the direction of the magnetic field in the mirror-reversed arrangement is upwards, it should be in the same direction as the emission of electrons (predicted direction of beta emission if parity were conserved). This is contrary to observation because the experiment has shown that the emission of electrons (beta emission) is going downwards. The mirror-reversed arrangement is not realised in nature so mirror symmetry is violated. In turn, the conservation of parity is violated in the weak nuclear force [7].
So we can tell the aliens that they should do the same experiment as Wu did, and the end that emits the most electrons is the end that we call ‘south’. Then, label the ends of the magnetic axis of the field used for lining up the nuclei, and this in turn can be used for labelling the ends of a magnetic needle. Take a long piece of wire and arrange it to carry electric current away from you and place the magnetic needle above the wire. The north pole of the needle will point in the direction we call ‘left’ [11].
However, we encounter another obstacle. What if the aliens are made out of antimatter and they all live in an anti-world? Could there be a way of determining this? And how would we be able to explain what ‘left’ is to them now? [6]
In order to determine whether or not the aliens are made out of antimatter and live in an anti-world, we can look into the electrically neutral variety of K mesons. When they decay, a pion that is either positively or negatively charged is produced, which is accompanied by an electron or a positron respectively. Asymmetry can be seen in these decays because if matter and antimatter were perfectly opposite to one another, the chance of each decay occurring would be the same. However, in reality, they are a bit different [4].
The Neutral K and the anti-K each have 2 versions: short and long-lived. The long-lived versions (of both the original one and the anti-version one) show a bigger effect in the difference - hence it is used. Whether the K is mirrored or not, the decay of the long-lived K into a positron (along with a negatively charged pion) is always more likely to happen than the decay of the long-lived K into an electron (along with a positively charged pion). Out of 2000 decays, approximately 1003 of them will result in a positron (with negatively charged pion) and 997 of them will result in an electron (with a positively charged pion) [4].
Notify the aliens so that they can identify K, and since we can’t use the name since aliens would call it something else, tell them that it is the thing that weighs slightly more than half the mass of a proton or antiproton. Since they would also call the proton something else, tell them what we mean by the proton - the massive particle in the ‘nucleus’ at the centre of the alien’s simplest atom. Once the aliens have identified K, tell them that we are talking about the electrically neutral one (since there is K+ and K-). We need to tell them that the property that holds the atom together is what we call ‘charge’ and that we are talking about the K with no charge. The alien now knows that we are talking about the long-lived K0 [4].
Then, ask the alien: ‘Is the lightweight particle that is produced most often in the decays of the long-lived K0 (or anti-K0) the same as you find in your atoms, or is it the opposite?’. If the alien answers that it is the same, then there are positrons orbiting the atoms in the alien world, which means that the alien is made of antimatter. If the alien answers that it is the opposite, then the alien is made of matter because electrons are orbiting their atoms, just like what we find in our world. If the aliens are made of antimatter, we can tell them what ‘left’ is by saying that the thing that they decay into less frequently (the electron) is made of matter and is moving in a ‘left-handed’ way [6]. If the aliens are made of matter, then they can conduct the method I have mentioned above that applies the Wu Experiment [4].
SCIENCE FICTION OR REALITY
For several decades, people have been fascinated by antimatter and its annihilation when it meets with matter where both matter and antimatter are destroyed (a self-destruction). This meeting is almost instantaneous and releases a large amount of energy. An annihilation of just one kilogram of antimatter will release about ten billion times the amount of energy given out when a kilogram of TNT explodes [4]. Hence, there is no question as to why people ponder about whether antimatter weapons should be the ‘next big thing’; however, in reality, it is not feasible to make these weapons.
This is because even before making a kilogram of antimatter, making a single gram of it (which would equate with the Hiroshima bomb with a yield of 20 kilotons of TNT) would take a long time. For example, in order to make a gram of antiprotons, you will need 6 × 1023 (Avogadro’s constant) of them. The quickest source is at the Fermilab, USA. In the month of June 2007, they produced 1014 antiprotons. If they were able to do this for a year, they could get approximately 1015, which is equivalent to 1.5 billionths of a gram. Annihilating this amount of antimatter releases 270 Joules only, which is the same amount of energy required to illuminate a single electric light bulb for five seconds [4].
This shows how inefficient the process of making these antiparticles is. We can only make a few of them over a long period of time, and due to the law of the conservation of energy (some energy is wasted in the process of making the antiparticles), the energy released from the annihilation would be less than the total energy we would have to put into the whole project!
Another reason why making weapons of antimatter is unrealistic is the way in which we have to store them. This means that we have to ensure that the antimatter doesn’t get in contact with matter. You would need a high vacuum and a container with strong electric and magnetic fields. This is possible as scientists have successfully stored antiparticles in Penning traps for many weeks, but there is a limit to how many you can keep in one bottle. When lots of charged particles are in a small volume, they will repel one another. Hence, it becomes more difficult to keep them inside the magnetic bottle. Approximately a million antiprotons is the largest number successfully stored however that is actually many billions of times smaller than the number of antiprotons required for a gram [4].
One suggestion is storing antiprotons and positrons together. As antiprotons are negatively charged and positrons are positively charged, this takes away the problem of electric repulsion. However, this poses another problem due to the fact that if the antiprotons and positrons paired up, the overall net charge would be 0 (neutral). Which means that the electric and magnetic fields cannot keep the antiparticles inside anymore as they can only affect charged particles, so they will move out and annihilate. There are also other potential ways in storing antiparticles like antihydrogen atoms; however, these ways also place a limit to how many can be stored per bottle [4].
Regardless of these limitations, research into fuelling spacecrafts with antimatter continues. In the Cassini-Huygens probe to Saturn, more than half of its weight was in its fuel and oxidiser tanks, and the launch vehicle weighed more than 180 times the probe itself. If antimatter fuel could be used, then a mass equivalent to a grain of rice could power a spaceship to Mars instead of using three tonnes of chemical propellant. Antimatter fuel for space travel at the moment is still not possible because in order to store even one millionth of the amount needed for the Mars trip, a lot of electric force is needed to push on the walls of the fuel tank (due to the electrical repulsion between the antiparticles of the fuel). This means that the equipment in producing this strong electric and magnetic field in the antimatter fuel tank would weigh a lot, which counteracts the primary advantage of antimatter fuel [4].
Fig 12. An artist’s rendition of an antimatter propulsion system
Source: Wikimedia Commons
In the media, Science Fiction has made false statements about antimatter weapons. In Dan Brown’s Angels and Demons, it mentions how antimatter annihilation results in ‘No byproducts. No radiation. No pollution.’, which is false since a large amount of energy is released in the form of gamma rays [4].
CONCLUSION
If you have read up to this point, I hope that this article has offered you a good insight into concepts ranging from the existence of antimatter to the realms of the universe. Indeed, the world of quantum physics is a complicated one, and there is so much more about it that I didn’t include in this article. During my research, I was utterly astounded by how physicists could derive equations that link space, energy and momentum together in such an impeccable way. I am confident that more discoveries in quantum physics will soon come to fruition, and they will help us understand things that we have yet to find out. There is just so much out there that we still do not know.
‘Those who are not shocked when they first come across quantum theory cannot possibly have understood it.’
- Neil Bohr
BIBLIOGRAPHY
[1] Sharp, Tim. “What Is an Atom?” LiveScience, Purch, 11 Sept. 2019, www.livescience.com/37206-atom-definition.html.
[2] “Origins: CERN: Ideas: The Building Blocks Of Matter.” Exploratorium, www.exploratorium.edu/origins/cern/ideas/standard.html#:~:text=Scientists%20once%20thought%20the%20most,particles%20called%20protons%20and%20neutrons.
[3] “Tara Shears - Antimatter: Why the Anti-World Matters.” Performance by Tara Shears, YouTube, The Royal Institution, 18 Oct. 2013, www.youtube.com/watch?v=0Fy6oiIRwJc
[4] Close, Frank E. Antimatter. Oxford University Press, 2018.
[5] The Editors of Encyclopaedia Britannica. “Speed of Light.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 17 May 2019, www.britannica.com/science/speed-of-light.
[6] Reich, Henry, director. How to Tell Matter From Antimatter | CP Violation & The Ozma Problem. YouTube, Minutephysics, 26 Feb. 2020, www.youtube.com/watch?v=Elt0Gt9Cb6Q
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[8] “Wu Experiment.” Wikipedia, Wikimedia Foundation, 29 June 2020, en.wikipedia.org/wiki/Wu_experiment.
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