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Antimatter drives sound like science fiction, but they may not be as far as
you think. There’s a version that could, just maybe, launch within your lifetime.
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Warp speed, hyperdrive, jumpdrives, space folding, subspace, wormholes,
infinite improbability drives… We’re pretty good at coming up with ways to travel faster
than light, at least in fiction. Sadly, Einstein’s relativity theory tells us it’s almost certainly
impossible to travel faster than light, at least for macroscopic objects over meaningful distances.
Probably our future exploration of the galaxy will have to take the slow road. That said,
we can certainly do a lot better than the current generation of chemical rockets,
which are still burning stuff for energy, you know, like a steam train. In terms of energy per kilogram
the antimatter drive is near as efficient as you can get within the bounds of
the laws of physics, if you care about those. As such, its
also a staple of science fiction, and it also feels as far of as the warp engine.
But there have been lots of advances in recent years that warrant this update on the timeline
of antimatter-powered space travel. And there are early versions that you might even see launch.
Let’s start with the requisite antimatter review. We’ve talked about antimatter before about how,
in 1928, Paul Dirac discovered it in his equations as he tried to bring quantum mechanics into
agreement with special relativity and found negative energy electron. And how it in 1932 it was discovered
in reality when Carl Anderson noticed cosmic ray electrons curving the wrong way in a magnetic field they proved to be
Dirac's anti-electrons—positrons.
So what is antimatter really? It’s pretty fair to describe it as the mirror image of regular
matter, and every particle of matter has an antimatter counterpart. It's the Bizarro to Superman,
the Wario to Mario, the Dirac to Feynman. The same but oh so different.
Antimatter is inevitable in a universe with the symmetries of ours. If we think
of particles as swirls and vibrations in the quantum fields, well, those fields
can swirl and vibrate backwards? The laws of physics are almost completely symmetric if we,
say, flip all quantum charges to be their opposites, or reflect to their mirror image,
or if we reverse the flow of time. Antimatter is this—charge-reversed and mirror reflected matter,
which can equivalently be thought of as time-reversed matter.
The matter-antimatter symmetry means it’s possible to produce particles matter out of a pure vacuum
as long as you also produce the corresponding particles of antimatter to balance. And also
as long as you have the energy to account for the new mass, according to Einstein’s equation E=mc^2.
This process is called pair production. But if the laws of physics are
symmetric in time this process works backwards. A matter-antimatter
pair can also be un-created—annihilated—to produce energy. And again, E=mc^2, so take the mass,
multiply it by the speed of light, which is a big number, then multiply it by the speed of
light again, and you get a very, very big number. And that’s why antimatter is the ultimate energy-dense
spaceship fuel. For example annihilate an espresso with an anti-espresso and you'll
get an explosion with the power of the modern h-bomb.
There are of course complications to using anti-matter or we'd be flying around with it
and blowing each other up with it already. It's very difficult to make the stuff in quantity
and even more difficult to store. And it's also complicated
to harness it in rocket propulsion. But before we can talk about solving those we
need to dispel a misconception. There’s a commonly repeated statement that matter and antimatter
annihilation releases ‘pure energy.’ There’s not really any such thing as pure energy. Energy
is a property possessed by systems. It can take many forms—mass energy, kinetic, potential, etc.
The most charitable interpretation of “pure energy” is energy that’s easily accessible and
usable. Mass-energy is the least pure or “free” in that sense, and that's the price of it being
the most compact form of energy. And that mass energy isn’t necessarily all liberated in annihilation.
That process will often produce other massive particles, so some energy remains locked away.
The idea of this process producing “pure energy” is probably from electron-positron annihilation.
In that case, the ingoing particles have such low masses that the energy they produce typically
isn’t enough to produce other particles. Instead they annihilate into photons, and the rare
neutrino. The entire energy content of those photons can be captured pretty efficiently.
But to accelerate our spaceship we really need momentum,
not energy. Basically, we need to throw stuff out the back of our ship as fast as possible
so that momentum conservation accelerates us forwards. Although our annihilation photons
have plenty of energy, they are massless and so carry relatively little momentum. Also,
very high energy photons are hard to direct efficiently. Ideally we want to harness whatever
energy we produce to blast massive particles out behind us. It’s certainly possible to do that with
the energy of these electron/positron annihilation photons in an indirect manner, and I’ll come back to options.
But there’s another issue with this “high
efficiency” type of annihilation and that has to do with storage, which I'll also come back to.
But all of this is nudging us towards using more massive versions of anti-matter to power our spaceship.
Massive anti-particles are much harder to create, but let’s see where we’re at with this.
So we had position in 1932. And the next anti-thing discovered
was 2000 times heavier, the anti-proton in 1955.
Followed quickly by the antineutron And then
we started to produce antinuclei with anti-protons and anti-neutrons together. First anti-deuterium
then anti-tritium then antihelium-4. More exotic anti-nuclei followed.
with the current record being anti-hyperhydrogen-4.
All of these heavier particles are identified in the debris of high-energy particle collisions,
which means they’re both rare and difficult to capture. There are two steps to this:
first slow the particle down then trap it. These collision products start
out moving fast—often at a good fraction of light speed--and they’re moving in a random
direction. So they need to be channeled and then slowed. The world leaders in this antimatter trapping are
at CERN’s Antiproton Decelerator. It’s an anti-accelerator for capturing anti-protons.
Once slowed, the antiparticle needs to be trapped. This is arguably the
harder part. If antimatter touches matter it annihilates, which means there’s no such thing
as an antimatter-resistant material container that isn’t itself made of
antimatter. The solution is a non-material container, which means a force field,
and the electromagnetic field offers good options for both charged and non-charged antimatter.
The most famous containment device for charged antimatter is the Penning trap,
in which electrodes at either end of the trap create an electric field that keep
charged particles in the middle along the axis,
while a magnetic field along the axis prevents any from drifting away in the radial direction.
This is the technology used by the BASE collaboration,
who have managed to store a hundred antiprotons for a full year in their larger Penning trap.
Electromagnetic containment like this relies on the anti-particle having a
net charge. But that introduces a new problem. Like charges repel,
and so end up as a big diffuse cloud—which defeats the point of using antimatter
as a compact energy-dense fuel. Now, if you want to keep the particles close together you need a colossal
EM field with a similarly colossal anti-matter containment device again defeating the point.
This why we can't just us positrons or just anti-protons as fuel.
Happily there’s a work around. If you trap both positrons and antiprotons in,
for example, a Penning trap, and then cool them, they’ll combine
into anti-hydrogen atoms. Of course, now you have the problem that the resulting
anti-atom is electrically neutral and so immediately escapes the Penning trap,
which is presumably a bad thing. A more sophisticated trap is needed.
The current trick is to use the fact that the anti-hydrogen atom has a magnetic moment—like
a little bar magnet. It’ll try to align and move in the direction of a magnetic gradient.
So if you have a magnetic field with a minimum value in all 3 dimensions, any anti-hydrogen in that
field will move towards the minimum and get stuck there. This is called a magnetic minimum trap.
Because the new anti-hydrogen will immediately escape its Penning trap after formed, the
magnetic minimum trap has to be superimposed directly over the Penning trap. Now,
the magnetic minimum trap is far weaker than the Penning trap. And so anti-hydrogen
needs to be really cold, colder than around 1 Kelvin to have a chance of remaining trapped.
Various novel methods like laser cooling are used to further chill this antihydrogen.
But despite best efforts, this whole process is still very inefficient, with only a tiny
fraction of produced anti-protons and positrons being converted to captured anti-hydrogen.
It’s also difficult to keep the much of this stuff trapped for long. The current
record is by the ALPHA collaboration. The team trapped 112 antiatoms for times
ranging from one-fifth of a second to up to 1,000 seconds.Not exactly long enough
for an interstellar journey. But now that we have actual anti-hydrogen, a new possibility opens up.
Anti-hydrogen is expected to behave essentially identically to regular
hydrogen. And that means that if we have enough of it it can undergo phase transitions into
more useful forms. Hydrogen freezes at 14 Kelvin at atmospheric pressure,
and so the hope is that enough antihydrogen would solidify at that point. That’s the
dream actually— antihydrogen ice fuel pellets suspended in magnetic fields.
OK, so we have our antimatter fuel. Let’s look at how we can use it.
Bring a hydrogen and antihydrogen together and first the electron and positron annihilate
producing high-energy gamma rays. The following proton-antiproton annihilation
is way more complicated. That’s because we’re really annihilating three quarks with three
antiquarks. When the hydrogen and antihydrogen come together, the individual quarks don’t all
annihilate. That only happens to the first quark and antiquark to make contact. They’re likely to
produce particles like a gluon or W boson, which in turn creates more quarks, fragmenting the two
baryons into a shrapnel of particles like pions. Sure enough, this is exactly how antiprotons were
discovered in collision experiments by Segre and Chamberlain in 1955.
These pions give us another approach at antimatter propulsion. They can be used
directly as our “working mass”, which means the stuff you throw out the back for momentum
exchange, your thrust. This design is called a ‘pion rocket’. It’s possible because the
charged pions can be channeled by a magnetic field. Unfortunately, the energy released in
hydrogen-antihydrogen annihilation also ends up as neutral pions,
gamma rays, and neutrinos that ignore our magnetic field. If we
really don’t want to waste the energy from these, there other ways to capture it.
One approach is to capture the photons and kinetic energy of the particles to generate
electricity. That electricity can then be used to accelerate other particles to provide thrust,
as in an ion drive, which we already employ for very steady,
if slow acceleration in space craft. That can be done whether or not the annihilation products are also
used for propulsion. And that makes it also an option for electron-positron annihilation.
OK, so how long before we get our first antimatter craft? Well, quite a while
for our first crewed interstellar craft, but maybe not so long for our first unmanned probe,
especially for our own solar system. That might be possible if we use antimatter in
a sort of hybrid mode with nuclear fission or fusion.
Now nuclear power is proposed in a couple of space travel scenarios. The more prosaic
it to use a fission or fusion reactor generating electricity to power an
ion drive. A more radical, but actually perfectly within our technological grasp
is nuclear pulse propulsion like the Orion project. In these, a series of
fission or fusion bombs are detonated behind the ship or behind a sail to propel it.
Both direct nuclear reactor and nuclear pulse propulsion suffer from the same challenge—the size
of the device needed. Fusion reactors need to be huge to sustain a reaction with net-positive output,
and this is why we don’t yet have a commercial fusion reactor. Nukes need to be
big in order to initiate their chain reaction. But if a small amount of antimatter can be
employed to kick off the reaction, then these devices can be made much smaller.
Let’s take the H-bomb example. A key component of the hydrogen bomb is some
sort of fission core—typically a ball of plutonium. This is surrounded by a
layer of heavy hydrogen. The plutonium is detonated as a fission bomb,
providing the heat and neutrons needed to ignite fusion in the hydrogen.
Because of the need for a critical mass of plutonium, there’s a minimum size for
a hydrogen bomb, which means a minimum explosive output. That in turn means that any Orion-type
spaceship needs to be pretty huge to capture and withstand that energy. But what if we replace some or all of the
plutonium core with a tiny grain of antimatter? Then we can build a much smaller device of the same style,
with the annihilation of the antimatter providing the energy and particle bombardment
needed to ignite a smaller fission core, or even directly ignite fusion. Either way,
this enables a more manageable thermonuclear explosion, which would work on a smaller craft.
This approach is called antimatter-catalyzed nuclear pulse propulsion, and more generally,
antimatter-catalyzed fusion or fission may be useful in powering spacecraft in various ways.
Really, the main advantage of doing this that we need far less antimatter because most of the
energy is from the “classical” nuclear fuel. The amount of antimatter needed is as low as
micrograms by some estimates. It’ll take us a few decades to produce that much antimatter
at current rates, but then we can build an antimatter-catalyzed fission craft that can
reach the Oort cloud—further than anything we've ever set out there— and that would be in a mere 10 years travel time.
Or so at least one proposal has calculated. Other proposals for non-crewed craft are also within distant,
but not sci-fi-future level reach. It’s remotely possible that the first antimatter-enabled launch will be
in our lifetimes! For some of us, anyway. And not of all of this is just theoretical. Early experiments have shown that
antiprotons cause extreme amplification of fission in non-critical heavy metal samples.
So we started out wanting a not-too-disappointing alternative to the impossible FTL
drives. What’s it really going to take to build a proper sci-fi antimatter drive? The main holdup
is the rate at which we can produce and store antimatter. Once we can make enough to solidify,
it gets a bit easier with the storage. At current rates this is going to take centuries to millennia.
So we can build more and bigger colliders that are devoted just to
antimatter harvesting. But there’s also the possibility of harvesting it in space.
Space is full of radiation and high energy cosmic rays, with high energy collisions
making antiprotons all the time. The PAMELA satellite discovered that Earth’s magnetic field confines
antiprotons and positrons produced by those collisions. So maybe we can
harvest this antimatter in the Van Allen belts to fuel up for an interstellar
journey. A side advantage to harvesting in space is safety. I’m not sure I’d want to
blasting off from Earth’s surface on top of a stash of delicately suspended antimatter.
So, that’s where we stand with antimatter drives. They’re not quite around the corner,
but also there are plausible paths to the first versions. And at least for those versions the
technical challenges seem pretty solvable. Our first crewed, interstellar antimatter-powered
spaceship is not in our lifetime, but if we want it to happen it will. One day we can cross the
galaxy by annihilating the reflections in the quantum symmetries of spacetime.
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