Post by Andrei Tchentchik on Aug 25, 2020 15:36:19 GMT 2
(.#497).- What is antimatter?
What is antimatter?
Richard Taillet
Teacher, Physical Researcher.
Published 07/01/2005 - Modified 10/28/2015.
Archives
The word "antimatter" is surrounded by a certain aura of mystery, and raises many questions. We are going to define what antimatter is, present some of its important properties, and then look at the processes that give rise to it. We will insist on the fact that the existence of antimatter is no longer to be proven, we detect it, we manufacture it, we store it, we even use it, for example in medical imaging.
Then, we will present the various observations by which we detect antimatter in our galaxy, indicating the processes that are responsible for its creation.
Then we will see that antimatter plays an important role in discussions of cosmology. In particular, the question of matter-antimatter asymmetry (why does the Universe contain mainly matter and so little antimatter?) Is linked to important questions of fundamental physics.
The sky in gamma rays (seen by EGRET). This map could contain information on the presence of antimatter in our galaxy ... "© Nasa
Finally, we will present some more speculative points on potential unconventional sources of antimatter in the Universe, evaporation of mini black holes and annihilation of dark matter, as well as the perspectives linked to their experimental demonstration.
Richard Taillet
Teacher, Physical Researcher.
Modified 10/28/2015.
Archives
The structure of matter
The matter around us is made up of atoms and molecules, which are made up of electrons and atomic nuclei. These atomic nuclei are themselves made up of protons and neutrons (they are called nucleons). We could take this little decomposition game a step further, because we now know that nucleons are made up of quarks. This is not crucial in what follows, and we will describe matter in terms of nucleons and electrons, without worrying about the internal structure of nucleons. These are charged particles, and a negative charge is conventionally assigned to the electron. We also know other particles, such as neutrinos, muons, taus, etc ... which are not present in the matter around us.
These particles can interact with each other. What does this term mean? We are familiar with the fact that electric charges repel each other if they are of the same sign whereas they attract each other if they are of opposite signs. These are two ways that particles interact, attract or repel, but they are not the only ones. We now know that particles can also transform into one another, for example a muon can transform into an electron by emitting a neutrino and an antineutrino.
At the theoretical level, the interactions were first described by classical electromagnetism, then by more elaborate theories which incorporate elements of quantum physics and special relativity. They are called quantum field theories. Historically, the establishment and understanding of these theories has been long and painful. For example, early versions of quantum field theories revealed enormous theoretical problems and serious inconsistencies.
Antimatter: nothing mysterious
- A theoretical invention: In 1928, Paul Dirac shows that some of these problems are solved if we describe the electron as a more complex particle than it seems, by associating it with another particle, similar in all respects to l electron but with opposite electrical charge. Dirac begins by assuming that this other particle could be the proton, which would have been pretty nice, since the two charged particles that make up matter would actually have been two facets of the same object.
Traces left by the passage of particles in a detector. The two traces of opposite curvatures from the same point indicate the appearance of an electron-positron pair. "
- An experimental discovery: In 1932, a new particle having the same mass as the electron but an opposite charge was discovered by Anderson among the particles found in the atmosphere (they are called cosmic rays, see below) , it is the positron, the particle predicted by Dirac's theory. The first particle of antimatter had just been discovered.
- A general property: For all types of particles, there is a corresponding type of antiparticle. There are antiprotons, antielectrons (positrons), antineutrons (even if they are neutral, they have antiparticles, made of antiquarks).
Some particles are their own antiparticles, like photons (this property of photons has important consequences, we will see later that it makes it difficult to detect hypothetical objects made of antimatter).
- An amazing property: The antiparticles do not have much extraordinary, they look a lot like the usual particles but they have opposite charges. The property which makes them sometimes called mysterious, and which gives them their prefix "anti", is the following: when a particle meets the antiparticle which corresponds to it, a reaction can take place, which leads to annihilation of the two, that is to say their disappearance, with the appearance of other particles, often high energy photons, more precisely γ rays (pronounce gamma).
For this reason, antimatter is very unstable in our environment made of matter, and there is no question of bottling it (an anti-bottle would be suitable, but then no question of placing it on a table ..).
When an electron-positron annihilation occurs, it can create 3 photons, or a pair of photons. The latter case is most likely, and the photons produced then have a remarkable property: they have a well-determined energy of 511 keV (the keV and a multiple of the electron volt eV, 1 keV = 1000 eV, a unit used very commonly in high energy physics. The correspondence with usual units is given by 1 eV = 10-19s Joule), and are emitted in exactly opposite directions (views of the center of mass of the electron-positron pair).
- Nothing mysterious: Since the 1930s, the experimental situation has evolved, of course, and antimatter is now something quite ordinary, we manage to produce and store it, to manipulate positrons and antiprotons, assemble into hydrogen antiatoms (for example, see the ATHENA and ATRAP experiments at CERN), we observe them in the atmosphere, we capture them in detectors, we detect them indirectly in the Galaxy.
We even managed to find uses for antimatter, the most important in practice undoubtedly being the use of the line of annihilation in medical imaging, in scanners with tomography by positron emission (PETscann), but we are thinking also for therapeutic applications, for example using antiprotons to treat cancerous tumors.
An image of the brain obtained by MRI and positron emission tomography
Uses for storing energy or creating new weapons have also been considered, but it seems that these prospects are still distant. A probably fairly realistic view of this type of use can be found in Peter W. Hamilton's science fiction novel "Dawn of the Night". That said, in general, science fiction authors often use antimatter as a miracle resource, in a rather moderated way ...
F I N .
What is antimatter?
Richard Taillet
Teacher, Physical Researcher.
Published 07/01/2005 - Modified 10/28/2015.
Archives
The word "antimatter" is surrounded by a certain aura of mystery, and raises many questions. We are going to define what antimatter is, present some of its important properties, and then look at the processes that give rise to it. We will insist on the fact that the existence of antimatter is no longer to be proven, we detect it, we manufacture it, we store it, we even use it, for example in medical imaging.
Then, we will present the various observations by which we detect antimatter in our galaxy, indicating the processes that are responsible for its creation.
Then we will see that antimatter plays an important role in discussions of cosmology. In particular, the question of matter-antimatter asymmetry (why does the Universe contain mainly matter and so little antimatter?) Is linked to important questions of fundamental physics.
The sky in gamma rays (seen by EGRET). This map could contain information on the presence of antimatter in our galaxy ... "© Nasa
Finally, we will present some more speculative points on potential unconventional sources of antimatter in the Universe, evaporation of mini black holes and annihilation of dark matter, as well as the perspectives linked to their experimental demonstration.
Richard Taillet
Teacher, Physical Researcher.
Modified 10/28/2015.
Archives
The structure of matter
The matter around us is made up of atoms and molecules, which are made up of electrons and atomic nuclei. These atomic nuclei are themselves made up of protons and neutrons (they are called nucleons). We could take this little decomposition game a step further, because we now know that nucleons are made up of quarks. This is not crucial in what follows, and we will describe matter in terms of nucleons and electrons, without worrying about the internal structure of nucleons. These are charged particles, and a negative charge is conventionally assigned to the electron. We also know other particles, such as neutrinos, muons, taus, etc ... which are not present in the matter around us.
These particles can interact with each other. What does this term mean? We are familiar with the fact that electric charges repel each other if they are of the same sign whereas they attract each other if they are of opposite signs. These are two ways that particles interact, attract or repel, but they are not the only ones. We now know that particles can also transform into one another, for example a muon can transform into an electron by emitting a neutrino and an antineutrino.
At the theoretical level, the interactions were first described by classical electromagnetism, then by more elaborate theories which incorporate elements of quantum physics and special relativity. They are called quantum field theories. Historically, the establishment and understanding of these theories has been long and painful. For example, early versions of quantum field theories revealed enormous theoretical problems and serious inconsistencies.
Antimatter: nothing mysterious
- A theoretical invention: In 1928, Paul Dirac shows that some of these problems are solved if we describe the electron as a more complex particle than it seems, by associating it with another particle, similar in all respects to l electron but with opposite electrical charge. Dirac begins by assuming that this other particle could be the proton, which would have been pretty nice, since the two charged particles that make up matter would actually have been two facets of the same object.
Traces left by the passage of particles in a detector. The two traces of opposite curvatures from the same point indicate the appearance of an electron-positron pair. "
- An experimental discovery: In 1932, a new particle having the same mass as the electron but an opposite charge was discovered by Anderson among the particles found in the atmosphere (they are called cosmic rays, see below) , it is the positron, the particle predicted by Dirac's theory. The first particle of antimatter had just been discovered.
- A general property: For all types of particles, there is a corresponding type of antiparticle. There are antiprotons, antielectrons (positrons), antineutrons (even if they are neutral, they have antiparticles, made of antiquarks).
Some particles are their own antiparticles, like photons (this property of photons has important consequences, we will see later that it makes it difficult to detect hypothetical objects made of antimatter).
- An amazing property: The antiparticles do not have much extraordinary, they look a lot like the usual particles but they have opposite charges. The property which makes them sometimes called mysterious, and which gives them their prefix "anti", is the following: when a particle meets the antiparticle which corresponds to it, a reaction can take place, which leads to annihilation of the two, that is to say their disappearance, with the appearance of other particles, often high energy photons, more precisely γ rays (pronounce gamma).
For this reason, antimatter is very unstable in our environment made of matter, and there is no question of bottling it (an anti-bottle would be suitable, but then no question of placing it on a table ..).
When an electron-positron annihilation occurs, it can create 3 photons, or a pair of photons. The latter case is most likely, and the photons produced then have a remarkable property: they have a well-determined energy of 511 keV (the keV and a multiple of the electron volt eV, 1 keV = 1000 eV, a unit used very commonly in high energy physics. The correspondence with usual units is given by 1 eV = 10-19s Joule), and are emitted in exactly opposite directions (views of the center of mass of the electron-positron pair).
- Nothing mysterious: Since the 1930s, the experimental situation has evolved, of course, and antimatter is now something quite ordinary, we manage to produce and store it, to manipulate positrons and antiprotons, assemble into hydrogen antiatoms (for example, see the ATHENA and ATRAP experiments at CERN), we observe them in the atmosphere, we capture them in detectors, we detect them indirectly in the Galaxy.
We even managed to find uses for antimatter, the most important in practice undoubtedly being the use of the line of annihilation in medical imaging, in scanners with tomography by positron emission (PETscann), but we are thinking also for therapeutic applications, for example using antiprotons to treat cancerous tumors.
An image of the brain obtained by MRI and positron emission tomography
Uses for storing energy or creating new weapons have also been considered, but it seems that these prospects are still distant. A probably fairly realistic view of this type of use can be found in Peter W. Hamilton's science fiction novel "Dawn of the Night". That said, in general, science fiction authors often use antimatter as a miracle resource, in a rather moderated way ...
F I N .
