The Final Frontier: Antimatter Propulsion
Common in science fiction, an antimatter powered rocket is not new. In fact, the propulsion system was first proposed by Eugen Sänger in 1953. When seen powering the Enterprise in “Star Trek” to speeds faster than light, using antimatter as an energy source seems completely in the realm of fiction. However, this futuristic technology may not be as far off as you think.
Let’s take a step back. What is antimatter?
There’s no trick, it’s exactly what it sounds like. The prefix ‘anti’ has Greek origin and means, “against, opposed to, opposite of, instead.” Antiparticles make up antimatter just as regular particles make up regular matter. Antiparticles and their particle counterparts share all the same properties aside from one thing: they hold opposite electric charges. Other than that, antiparticles are exactly the same as regular particles.
Before we get into antimatter propulsion systems, let’s review the history of antimatter. Modern antimatter was first proposed by the British physicist Paul Dirac in 1928. While studying special relativity and quantum mechanics, Dirac was solving a relativist quantum mechanics equation to describe the behavior of an electron moving at a relativistic speed. He found two viable solutions to the equation: one for an electron with a positive charge and one with a negative charge.
Take the square root of 4. Just like Dirac’s, this equation has two possible solutions, one with a negative charge, and one with a positive charge.(x = 2 or x = −2).
Dirac suggested we should revise Einstein’s famous equation E=mc² because of an entire new category of particles. He said that Einstein left out that the “m” in the equation, the mass, could have negative properties as well as positive.
However, there was a caveat. Classical physics didn’t include both negative and positive properties for each particle. By revising Einstein’s equation to
E = + or — mc², it would assume an entire new kind of particle. And so, the existence of antimatter was possible.
So, Dirac proposed that the electron had a corresponding antiparticle, the antielectron or positron, with all the same properties, just an opposite charge. The equation won Dirac the Nobel Prize in 1933.
In 1932, four years after Dirac suggested the existence of the positron, it was discovered by Carl Anderson, proving Dirac right. Anderson had not set out to hunt antimatter — he found it accidentally. He had built his own cloud chamber to determine the composition of cosmic rays, high energy particles that enter Earth’s atmosphere from space. By surrounding the chamber with an electromagnet, the ionizing particles passing through the chamber would curve and bend, which allowed Anderson to determine whether particles passing through were positively or negatively charged.
Anderson took hundreds of photographs of tracks taken by cosmic ray particles. The most famous is the one at left. This photo shows a curve in the trajectory of a particle identical to an electron. However, the curve suggested the particle was positively charged, whereas an electron is negatively charged.
Anderson’s discovery won him a Nobel Prize in Physics in 1936. At the age of 31, he was the youngest person at that time ever to be awarded. Paul Dirac’s prediction had finally been proven.
In 1955, physicists Emilio Segrè and Owen Chamberlain at the University of California, Berkeley produced an antiproton for the first time. It was produced at the Bevatron Particle Accelerator by slamming protons into a fixed target. Unlike an electron, a proton is not a fundamental particle. This means it’s composed of even smaller particles called quarks. A proton is made of two up quarks and one down quark, while an antiproton is made up of two up antiquarks and one down antiquark. Segrè and Chamberlain were awarded the 1959 Nobel Prize in Physics for the discovery.
The antineutron was produced only a year after, in 1956, also at the Bevatron Particle Accelerator at the University of California, Berkeley, by colliding protons and antiprotons.
More recently, in 1995, physicists at CERN (the acronym is French, standing for “Conseil Européen pour la Recherche Nucléaire”, or European Council for Nuclear Research) announced that they had successfully produced the first atoms of antihydrogen at the Low Energy Antiproton Ring (LEAR).
Hydrogen is the simplest atom, which has one negatively charged electron orbiting one positively charged proton. The antimatter counterpart, antihydrogen, consists of one positively charged positron orbiting one negatively charged antiproton. Each of the nine anti-hydrogen atoms that were created at LEAR only lasted 40 nanoseconds (40 billionths of a second).
Fortunately and unfortunately there is no known remaining macroscopic antimatter in the universe today. It’s unfortunate because it means to study antimatter, we have to create our own. And we can create antimatter through the use of high-energy particle colliders, like the ones mentioned above.
However, it’s fortunate that antimatter doesn’t exist macroscopically, because when matter and antimatter meet, they annihilate one another. If there were macroscopic amounts of antimatter on the Earth and in the universe today, we might see things randomly annihilating into pure energy. Thankfully that’s not the case and we don’t have to worry about that.
A good way to think about matter-antimatter annihilation is using positive and negative numbers, such as -1 + 1. If you add them together, they would cancel out to give you a sum of 0. What’s misleading here is that when antimatter and matter annihilate, they do produce something: pure energy.
So how do we create our own antimatter? I mentioned that the first antiprotons and neutrons were made in particle colliders, but what are they?
Particle colliders are a type of particle accelerator. Accelerators are lined with powerful supermagnets that generate electromagnetic fields to accelerate and steer particles. When a particle is sent through an accelerator, it slams into a target. This creates more particles, some of which are antiparticles.
There are two shapes of particle accelerators: a circular accelerator where a beam of particles travels repeatedly around a loop, or a linear accelerator where the particle beam travels in a straight trajectory, from one end to the other.
Colliders are a type of accelerator that smash particles together. This technique generates more energy because the energy of the two particles is added together when they collide. The Large Hadron Collider (LHC) located at CERN is the largest and most powerful collider in the world.
All particle accelerators only produce about one or two picograms of antiprotons each year. A picogram is a trillionth of a gram. To put this in some perspective, all of the antiprotons produced at CERN in one year would be enough to light a 100-watt electric light bulb for three seconds. Obviously, we have quite a bit to go.
Now that we know the basic history of antimatter, the different types, and how it’s artificially produced, let’s get back to antimatter propulsion. How did we go from Dirac revising Einstein’s equation in 1928, to Sänger proposing a new kind of rocket in 1953? Two words: specific impulse. Specific impulse is the measure of how efficiently a rocket uses propellant. Specific impulse reaches extremely high amounts when the exhaust velocity produced is equal or near to the speed of light. A higher specific impulse generates more thrust for the same amount of propellant. It’s speculated Sänger believed in the potential of antimatter because of its high levels of specific impulse.
Sänger was drawn to this idea, which he thought possible through matter-antimatter annihilation. Although they were predicted to exist, antiprotons and antineutrons hadn’t been confirmed. At the time, the only antimatter proven to exist was the positron. So all Sänger had to work with was the positron. Don’t get so disappointed — when positrons and electrons are annihilated, gamma rays are produced, the most energetic wave in the electromagnetic spectrum. And gamma rays move at the speed of light.
Antimatter propulsion solved! Not quite.
Sänger coined the name “photon rocket” for his idea. He proposed that through electron-positron annihilation, a beam of gamma rays could be channeled into an exhaust. The problem is directing the gamma rays produced by the annihilation because they emerge in random directions. They are also very dangerous for humans (no they will not turn you into the Hulk) and depending on the exposure, you could damage tissue, your DNA, or even die instantly. And so, gamma rays produced in all directions would be a lethal problem for the crew onboard the photon rocket. Sänger never solved this problem or finished the proposal.
Sänger’s ideas sparked interest in antimatter’s potential long after his initial proposal. The close to 100% conversion to energy during annihilation is the reason for that. The nuclear reactions that power atomic bombs come in distant second place, with only about three percent of their mass converted to energy. Compared to antimatter’s 100%, 3% seems like nothing.
So where can we apply this today?
One of the most exciting applications for antimatter, (besides the small detail that the research gives us clues to the history of our existence) is in the modern space tech industry. Because antimatter is such a potent fuel, it is much better than today’s chemical rockets. For example, tons of chemical fuel are needed to propel a human mission to Mars and nearly 70 percent of the payload would be fuel just to get there, which is very inefficient. In contrast, just tens of milligrams of antimatter will do (for some perspective: a milligram is about one-thousandth the weight of a piece of the original M&M candy).
1g of antimatter = 80 KTON of TNT = 10,000,000 liters of LNG
And we’re back to the beginning: antimatter rockets. So, what are recent proposals for how an antimatter rocket would work?
Although there are many different proposals, we’re going to be going over two types: one that is powered by electrons and positrons, like Sanger’s photon rocket, and one that is powered by protons and anti-protons.
Quick detour: proton-anti-proton annihilation is a bit more complicated than electron and positron annihilation because a proton is not a fundamental particle — it’s made up of quarks. When a proton and its antiproton pair meet, they annihilate into energy and produce mesons through hadronization. Hadronization is the formation of hadrons out of quarks and gluons. Part of the reason why this happens is because really high energies can be converted to mass, as per E = mc2.
When a proton and antiproton meet, one of the quarks from the proton will annihilate with an antiquark from the antiproton. This annihilation will produce a gluon. Gluons are gauge bosons, otherwise known as exchange particles. They carry the strong force and bind, or “glue” quarks together.
The newly produced gluon and remaining two quarks, two antiquarks and other gluons undergo hadronization into a number of mesons. Mesons are in the hadron particle family (particles composed of two are more quarks) and contain one quark and one antiquark.
These newly formed mesons speed away from the spot of annihilation at velocities a significant fraction of the speed of light. The produced mesons are very unstable and will decay into a number of photons, electrons, positrons, and neutrinos. The decay process could be taken advantage of and used to power a spacecraft.
Aaaand we’re back to antimatter propulsion! Although electron-positron annihilation and proton-antiproton annihilation have many differences, they all seem to agree on the basics. Which are as follows:
- We need to be able to produce the antimatter in significant quantities
- We need to be able to contain the antimatter safely
To explain: because antimatter annihilates with normal matter, you can’t just stuff some antimatter in a bottle. Instead, it would have to be separated from normal matter so it doesn’t annihilate everything it comes in contact with.
3. We need to be able to direct the energy from annihilation to propel the ship
The next article in the series (coming out later this month) will take a closer look at recent proposals for antimatter rockets that will boldly take us where no one has gone before.