Antimatter – Fact and Fiction

To clear things up to start with, antimatter is real.  Many people I talk to (including numerous teachers) have an assumption that antimatter is a mythical construct from science fiction such as Star Trek.  This confusion seems to lie in the fact that, unless you work in a particle collider, there is usually no interaction with antimatter of any sort in daily life.  Relativity has everyday applications, most famously  in nuclear power (and weaponry, which hopefully we’ll never have to experience).  Quantum mechanics is responsible for the transistor and laser technology.  While most people do not understand the intricacies of relativity or quantum theory, they most certainly understand both are very real.  Antimatter has very few practical applications and thus, is mostly heard about in a fictional context.

The story of antimatter began with the attempt by Paul Dirac in the late 1920’s to produce an equation that would describe the properties of an electron traveling near the speed of light.  This was very groundbreaking work as it would necessitate merging quantum mechanics (which describes the properties of atomic particles) with relativity (which describes the properties of matter as it travels at near light speeds).  The end result was an equation which produced an electron with a negative charge and one with a positive charge.  The actual equation is beyond the scope of this post, but I’ll use an analogy which we encounter in beginning algebra.  Take the following equation:

(x + 1)(x – 1) = 0

Since one of the terms in the parenthesis must equal zero to produce the proper result, we arrive at two different solutions, x = 1 and x = -1.  If we are modeling a situation with a zero-bound, that is, a physical constraint that cannot drop below zero, we would typically disregard the x = -1 solution.  An example of this might be in economics where a price of a consumer good cannot drop below $0.00.  Dirac faced this same dilemma.  Electrons have a negative charge.  The initial temptation would be to disregard the positive charge result.  However, Dirac had a different take on this development.  In 1928, Dirac published The quantum theory of the electron.  In this paper, Dirac postulated the existence of an antielectron.  This particle would have the same properties as an electron except for having a positive, rather than negative, charge.

Paul Dirac. Credit: Wiki Commons

This bold prediction was verified four years later by Carl Anderson when he detected particles with the same mass as an electron but with a positive charge.  How does one measure something like this?  Anderson was observing cosmic rays in a cloud chamber.  A magnetic field will move opposite charged particles in opposite directions.  This movement can be photographed as the particles leave trails in the cloud chamber filled with water vapor.  One year later, Dirac would be awarded the Noble Prize and Anderson followed suit in 1936.  Anderson’s paper dubbed the positively charged electron as a positron.

First recorded positron. An electron would have spiraled in the opposite direction. Credit: Carl Anderson, DOI: http://dx.doi.org/10.1103/PhysRev.43.491

During the 1950’s, the antiproton and antineutron would be discovered.  These particles have significantly more mass then positrons and thus, were more difficult to produce.  Protons have a positive charge and antiprotons have a negative charge.  Neutrons are electrically neutral.  However, subatomic particles have charges as well and antineutrons are made of three antiquarks that have opposite charges than the three quarks that neutrons are made of.  So, now that we have established these things are real, how can we use this in an educational setting?

If the student learned about antimatter from science fiction, the first task should be to discern how antimatter is presented in the reading/movie and how it exists in reality.  The most prominent example in popular culture is the use of matter-antimatter propulsion in Star Trek to power starships on interstellar explorations.  The overall premise is not bad in that when matter and antimatter collide, they annihilate each completely into pure energy.  Unlike fission and fusion reactions, which convert a small fraction of available mass into energy, a matter-antimatter reaction is 100% efficient.  So, what stops us from using this as a potential source of energy?

The primary challenges of antimatter production are the cost, amount, and storage of antimatter.  The cost to produce 10 milligrams of positrons is $250 million.  To put that in perspective, it would require about 1100 kilograms (about 1 ton) of antimatter to produce all the energy consumed by the United States in one year.  That would cost $2.75 x 1017 to produce, or about 17,000 times the GDP of the United States.  Obviously, not an equitable trade-off.  When producing antihydrogen, currently the best we can do is produce it on the scale of a few dozen atoms and store it for 1,000 seconds.  The storage problem comes from the fact that antimatter is annihilated as soon as it comes into contact with matter.  And that last fact would make it seem obvious as to why there is not a lot of antimatter in the universe.  However, there is one little problem.

Models of the Big Bang predict equal amounts of matter and antimatter to have been produced when the universe was formed.  The reason being is the laws of physics state matter and antimatter should form in pairs.  If that had happened, all the matter and antimatter would have mutually destroyed each other and we would be left in an universe with no matter and all energy.  That, of course, is not the case.  Very early in the universe’s existence, an asymmetry between matter and antimatter occurred.  For every billion antimatter particles, an extra matter particle existed in the universe.  And that extra particle per billion parts is responsible for the all the matter, including our bodies, in the universe.  How that extra particle of matter was produced remains a mystery for cosmologists to solve.

So what happened to all the antimatter that was created during the Big Bang?  It was destroyed as it collided with matter and can be observed as the cosmic microwave background radiation (CMB).  Originally released as high energy gamma rays, the CMB has been redshifted all the way to the radio end of the spectrum by the time it reaches Earth.  If our eyes could detect radio waves, instead of seeing stars in a dark sky at night, we would see a glow everywhere we looked.  That is because we are embedded within the remnants of the Big Bang.

CMB as imaged by the Planck mission. Cutting a sphere from pole to pole produces an oval when laid flat. This is a map of the entire celestial sphere and demonstrates the CMB is ubiquitous. Credit: ESA and the Planck Collaboration.

The discovery of the CMB in the 1960’s was a crucial piece of evidence in favor of the Big Bang theory over its then competitor, the Steady State theory.  The CMB produces what is known as a black body spectrum that is emitted by masses in a hot, dense, state.  The early universe with massive matter/antimatter annihilation would be in such a state.  And this leads us back to the original question, does antimatter exist today and can it have any practical purposes?

In its natural state, antimatter exists mostly in the form of cosmic rays.  The term ray is a bit of a misnomer.  Cosmic rays consist of atomic nuclei, mostly hydrogen protons, traveling near the speed of light.  Some cosmic rays are produced by the Sun but most originate outside the Solar System.  Their exact source is a mystery to astronomers although supernovae and jets emanating from black holes are suspected to be the primary culprits.  Trace amounts of positrons have been detected in cosmic rays in space before they reach the Earth’s atmosphere.  When cosmic rays collide into atmospheric molecules, the protons are shattered into various sub-atomic particles.  When a cosmic ray collided into Carl Anderson’s cloud chamber, one of the particles produced was that positron which became the first observed antimatter particle in 1932.

Today, supercolliders such as CERN in Switzerland produce antimatter in the name manner.  Protons are accelerated to very high speeds and sent colliding into a metal barrier.  The impact breaks apart the proton into various sub-atomic particles.  Positrons can also be created via radioactive decay and this process provides one of the few practical applications of antimatter.  Positron Emission Tomography (PET) scans operate on this principle.  Radioactive material is produced by a cyclotron on the hospital site and injected into the patient.  As positrons are released, they collide with matter in the body which mutually annihilate and eject gamma rays in opposite directions.  The scanner then detects the gamma rays to produce images of areas of the body that x-rays cannot.

What most people really want to know about antimatter is, can it be used to produce warp drive propulsion?  If the production and storage aspects of antimatter are solved, it must be understood that gamma rays produced by matter-antimatter collisions are random in direction.  That is, it acts more like a bomb than a rocket.  However, warp drive would not act like a conventional rocket.  Even a conventional antimatter rocket could not propel a spacecraft past the speed of light.  As a rocket accelerates closer to the speed of light, according to relativity theory, its mass would approach infinity.  No matter how much antimatter you could annihilate, you could not produce enough energy to break the light barrier.

Original 11 foot model of U.S.S. Enterprise. Model was used to represent a starship 947 feet long and 190,000 tons. The fictional Enterprise is almost as long as the Eiffel Tower is high and three times heavier than an aircraft carrier. Credit: Mark Avino, National Air and Space Museum, Smithsonian Institution

The concept of warp drive operates in a different principle.  Warp drive does not push a rocket through space, but rather compresses space in front of the spacecraft to, in effect, shorten the distance between two points.  The space is then expanded behind the spacecraft.  Is this possible?  Right now, no.  The space-time fabric is not very pliable.  The estimates on the amount of mass/energy required to propel a starship the size of Star Trek’s U.S.S. Enterprise range from all the mass in the universe to the mass the size of Jupiter.  Radical breakthroughs in both physics and engineering are required to make warp drive possible.

A unification of quantum mechanics and relativity might provide a pathway to warp drive.  The operative word here is might.  We simply do not know what such an advancement would bring.  When Max Planck discovered energy was transmitted in discrete packets rather than a continuous stream in 1901, he had no idea that development would lead to such applications as lasers, digital cameras, LED lights, cell phones, and modern computers.  And so it is today, we can only speculate what such theoretical advancements might bring for future applications to harness the great potential energy of antimatter.  Perhaps, we’ll have the good fortune to see our current students working to solve those problems.

*Image on top of post are bubble chamber tracks from first antihydrogen atoms produced at CERN in 1995.  Credit:  CERN

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