Antimatter Made Simply
What is antimatter?
What happens if matter and antimatter interact?
How was antimatter discovered?
Why don't we usually come across antimatter in our daily lives?
All these questions and many more come to one's mind when thinking about antimatter.
But, first things first!
Let us first define and understand what antimatter is.
Antimatter is opposite to matter, literally.
For every sub-atomic particle, such an electron, a proton, a neutron, etc. there exists an
antiparticle such as an anti-electron, anti-proton, and anti-neutron.
The antiparticle will have the same mass as the particle, but it will differ in the sign
of its charge and other quantum numbers.
Some of these quantum numbers are the lepton number – one for the electron and each of
the other five members of lepton family, and the baryon number – one third for the each
of the six quarks that make up the baryon family.
Antiparticles are expected to interact with other antiparticles in exactly the same way
ordinary particles interact with each other.
The laws of physics are (almost) symmetric when it comes to matter and antimatter.
In fact, a Universe made up of antimatter would be indistinguishable from ours!
Going into a little bit more detail, the anti-electron is called a positron (for positive electron)
and it is positively charged with a lepton number of minus 1.
The positron has exactly the same mass as the proton.
The proton is a positively charged sub-atomic particle made up of three quarks giving it
a baryon number of one.
Its antiparticle, the antiproton, has the same mass as the proton but it is negatively
charged with a baryon number of minus one (minus one third contributed from each of
the three antiquarks that make up the antiproton).
Neutral sub-atomic particles, such neutrons, are interesting.
The antineutron has the same mass and zero charge as the neutron, but it will have a
baryon number of minus one (again, minus one third coming from each of the three antiquarks
that make up the antineutron).
So what happens when matter and antimatter interact?
The answer is fireworks!
When a positron interacts with an electron they both annihilate to produce two X-ray
energy photons!
So, we are in luck that antimatter is so scarce in the Universe nowadays.
Otherwise, we would have been burned by X-rays and gamma-rays every time matter and antimatter
interacted.
As a matter fact, if you were to meet your anti-self then both of you would annihilate
and release the equivalent energy of roughly 2500 megatons, almost one third the total
energy of the world's arsenal of nuclear weapons!
Antimatter could sure make for mighty powerful spaceship engines, IF you can find a way to
produce macroscopic amounts of antimatter AND a way to store it in isolation!
Some modern particle accelerators regularly produce antiprotons for use in high energy
physics experiments.
Now that we know what antimatter is, let us see how physicists discovered it.
Back in the year 1928, Paul Dirac, one of the founding fathers of the new science of
quantum mechanics, was trying to solve an equation that included the effects of the
theory of special relativity to describe the behavior of electrons in the microscopic world.
To his surprise, the equation admitted solutions that corresponded to electrons with negative
energy going back in time!
Any insecure student of physics would have blushed with embarrassment and redid the math.
However, Dirac was sure of his math.
Instead, he reinterpreted his problematic solution to denote an “anti-electron”
with positive energy going forward in time.
Four years later, an experimental physicist by the name of Carl Anderson proved Dirac
right by actually observing the anti-electron, the positron.
The discovery of the positron earned Anderson the Nobel Prize in 1936.
Dirac had already won the Nobel in 1933 for his contributions to atomic physics.
Are you wondering where Anderson's positrons came from?
The positrons that Anderson discovered originated in atmospheric showers of sub-atomic particles
that result when very high energy cosmic rays (mostly protons) interact with atoms in Earth's
atmosphere.
The story of the prediction and subsequent discovery of the positron is instructive and
shows how big discoveries in physics are often made.
The device used by Anderson is called a cloud chamber and was a standard instrument used
in nuclear physics labs at the time.
A particle going through a cloud chamber would leave sort of a trail of bubbles in its wake.
Applying a known magnetic field would curve the particle according to its charge.
The positron in Anderson's cloud chambers curved in exactly the same way as electrons,
but in the opposite direction.
As a matter of fact, these oppositely-curving tracks were observed several years BEFORE
Anderson, at around the time of Dirac's prediction, but none of the physicists who observed them
pursued the issue further.
It was Anderson who persisted and tested and retested his results and thus confirmed the
reality of the positron.
This is in a way similar to Dirac when he came across his negative energy electrons.
He did not simply brush the solutions aside as a mathematical curiosity but instead reinterpreted
them in a way that made us discover a completely new side to the Universe!
You can discover other sides of our Universe on our Youtube channel.
But, let us continue with the story of antimatter.
The reason that antimatter is rare in today's Universe is that antimatter is created by
very rare nuclear reactions that produce very small amounts of it.
The antimatter that we get from this is mostly at the sub-atomic level: positrons, antiprotons,
antineutrons, and antimesons.
Mesons are the proton's and neutron's lighter cousins that are a made up of an quark and
antiquark.
Yes, physicists have created and studied antihydrogen.
But that was under very difficult laboratory conditions that required complete isolation
of the antihydrogen atoms so that they don't interact with the walls of their container
and annihilate in jet of gamma-rays.
Right now you are probably wondering if antimatter has any useful purpose at all, apart from
allowing physicists to learn more about the Universe.
The answer is yes!
Every time a patient undergoes a so-called PET scan to detect tumors in the body, antimatter
is involved.
PET is acronym for POSITRON Emission Tomography.
The patient is injected with radioactive sugars that accumulate at tumor sites.
These radioactive sugars then gradually decay emitting positrons in the process.
A specialized and highly sophisticated instrument then maps out the locations from which the
positrons are emanating, thereby giving a highly detailed view inside the human body.
Patients who have had PET scans remain radioactive for a few days and are usually advised to
stay away from children and pregnant women, given the risks posed by radiation to children
and fetuses.
OK, so we now know what antimatter is; we know how it interacts with matter, we know
how to produce it here on Earth and how it is produced in Nature – basically under
very special conditions and in very small amounts, and that it is exists for very short
periods of time because it interacts quickly with ordinary matter.
But what we still don't yet is why all stuff in the Universe – planets, stars, galaxies
– is predominantly NOT antimatter.
This issue is so important in physics that it even has its own name: “the matter-antimatter
asymmetry problem.”
Addressing this issue requires that we delve into cosmology and high energy physics, also
known as particle physics.
Full disclosure: The solution to the above problem is still being worked out by physicists
as we speak.
Nevertheless, we will try to get a very simplified version of the known parts of the answer.
The theoretical framework within which to understand the matter-antimatter asymmetry
is three-pronged and was formulated over fifty years ago by physicist Andrei Sakharov.
Our quest begins right at the beginning: the Big Bang.
The Big Bang is the instant when our Universe was born.
The Big Bang created EQUAL amounts of matter and antimatter.
For every electron there was a positron, for every quark there was an antiquark.
Mind you, protons and neutrons formed much later.
They formed a few minutes after the Big Bang, at which time almost all antimatter had disappeared!
But, we are getting ahead of ourselves here.
Let us go back to the Big Bang.
During the first fraction of a fraction of a second after the Big Bang the Universe was
an extremely hot soup of particles and antiparticles and intense radiation.
Particles and antiparticles annihilated each other to produce highly energetic photons,
and the energetic photons could interact with each other to produce again particle-antiparticle
pairs, all happening under the watchful eyes of the laws of particle physics.
The Universe would have remained in this fiery thermal equilibrium for eons without anything
interesting happening had it not been for an astounding cosmological fact, the first
prong.
The Universe is expanding!
The expansion of the Universe lead to cooling.
The energy of radiation decreased to a point where photon interactions were no longer energetic
enough to produce particle-antiparticle pairs.
The result is that the refueling of the abundance of the pairs stopped.
If no other processes had interfered, the particles and antiparticles would have annihilated
each other with time and we would have been left with a universe filled only with photons
and NO matter or antimatter at all!
We wouldn't be here to discuss it.
The effect of the expansion of the Universe is to stop the particle-antiparticle pairs
from being replenished.
Now, other high energy physics processes must come into play to produce the matter-antimatter
asymmetry, which bring us to the second prong.
If you recall, antiparticles have the opposite baryon number to particles.
High energy physics reactions have to conserve energy, momentum, charge, lepton number, and
baryon number.
A quantity is said to be conserved if its value before the reaction is the same as after
the reaction.
All the laws of high energy physics require that the above-mentioned quantities be strictly
conserved, or so it seems.
The very absence of antimatter in the Universe requires that the conservation of baryon number
must be violated to some extent under certain conditions in order to produce the matter-antimatter
asymmetry observed in today's Universe.
To what extent?
To about one part in one billion!
How do we know that?
Astrophysicists know that there are roughly a billion photons for every particle in the
Universe.
This means that for every billion and ONE particles there were only a billion antiparticles
out there.
The billion particles will annihilate with the billion antiparticles leading roughly
to a billion photons, with the upshot being that when all is said and done we will have
ONE particle and a billion photons, the particle to photon ratio observed by astrophysicists.
However, there is no evidence yet in particle physics labs of the violation of baryon number
conservation.
Moreover, there is no theoretical process within the accepted paradigm of particle physics,
the Standard Model, in which the baryon number is not conserved.
Evidently, gaining a full understanding the matter-antimatter asymmetry requires new physics.
The third prong needed to understand the matter-antimatter asymmetry is the so-called CP violation.
The laws of physics, including conservation laws, should be the same if you were to change
the signs of the charges, C, of all the particles involved in a reaction, and look at the reaction
through a mirror (P: Parity).
CP violation occurs when the outcomes of a reaction change upon inverting the sign of
the charges and taking the mirror image of the reaction.
CP violation is needed to produce more particles than antiparticles in the very early Universe.
Glimpses of CP violation have been observed in particle physics labs since the 1980s in
reactions involving K-mesons (referred to as kaons) and other sub-atomic particles.
However, the amount of detected violation is not enough to explain the full magnitude
of the matter-antimatter asymmetry.
Naturally, more experiments are still needed!
Antimatter has been a source of profound insights into physics for almost a century.
We know almost everything about antimatter, except its rarity.
Understanding why this is case will definitely teach us new physics.
This story is currently being written in major particle physics labs in the world such as
CERN in Geneva, Switzerland.
With so much effort being put in, it is only a question of time before the matter-antimatter
asymmetry is answered.
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Stay tuned!