Death by Black Hole: And Other Cosmic Quandaries - Neil deGrasse Tyson (2014)



Particle physics gets my vote as the subject with the most comical jargon in the physical sciences. Where else could a neutral vector boson be exchanged between a negative muon and a muon neutrino? Or how about the gluon that gets exchanged between a strange quark and a charmed quark? Alongside these seemingly countless particles with peculiar names is a parallel universe of antiparticles that are collectively known as antimatter. In spite of its continued appearance in science fiction stories, antimatter is decidedly nonfiction. And yes, it does tend to annihilate on contact with ordinary matter.

The universe reveals a peculiar romance between antiparticles and particles. They can be born together out of pure energy, and they can die together (annihilate) as their combined mass gets reconverted back to energy. In 1932, the American physicist Carl David Anderson discovered the antielectron, the positively charged antimatter counterpart to the negatively charged electron. Since then, antiparticles of all varieties have been routinely made in the world’s particle accelerators, but only recently have antiparticles been assembled into whole atoms. An international group led by Walter Oelert of the Institute for Nuclear Physics Research in Jülich, Germany, has created atoms where an antielectron was happily bound to an antiproton. Meet antihydrogen. These first anti-atoms were created in the particle accelerator of the European Organization for Nuclear Research (better known by its French acronym CERN) in Geneva, Switzerland, where many modern contributions to particle physics have occurred.

The method is simple: create a bunch of antielectrons and a bunch of antiprotons, bring them together at a suitable temperature and density, and hope that they combine to make atoms. In the first round of experiments, Oelert’s team produced nine atoms of antihydrogen. But in a world dominated by ordinary matter, life as an antimatter atom can be precarious. The antihydrogen survived for less than 40 nanoseconds (40 billionths of a second) before annihilating with ordinary atoms.

The discovery of the antielectron was one of the great triumphs of theoretical physics, for its existence had been predicted just a few years earlier by the British-born physicist Paul A. M. Dirac. In his equation for the energy of an electron, Dirac noticed two sets of solutions: one positive and one negative. The positive solution accounted for the observed properties of the ordinary electron, but the negative solution initially defied interpretation—it had no obvious correspondence to the real world.

Equations with double solutions are not unusual. One of the simplest examples is the answer to the question, “What number times itself equals nine?” Is it 3 or—3? Of course, the answer is both, because 3 × 3 = 9 and—3 ×—3 = 9. Equations carry no guarantee that their solutions correspond to events in the real world, but if a mathematical model of a physical phenomenon is correct, then manipulating its equations can be as useful as (and much easier than) manipulating the entire universe. As in the case of Dirac and antimatter, such steps often lead to verifiable predictions, and if the predictions cannot be verified, then the theory must be discarded. Regardless of the physical outcome, a mathematical model ensures that the conclusions you might draw are logical and internally consistent.

QUANTUM THEORY, also known as quantum physics, was developed in the 1920s and is the subfield of physics that describes matter on the scale of atomic and subatomic particles. Using the newly established quantum rules, Dirac postulated that occasionally a phantom electron from the “other side” might pop into this world as an ordinary electron, thus leaving behind a hole in the sea of negative energies. The hole, Dirac suggested, would experimentally reveal itself as a positively charged antielectron, or what has come to be known as a positron.

Subatomic particles have many measurable features. If a particular property can have an opposite value, then the antiparticle version will have the opposite value but will otherwise be identical. The most obvious example is electric charge: the positron resembles the electron except that the positron has a positive charge while the electron has a negative one. Similarly, the antiproton is the oppositely charged, antiparticle of the proton.

Believe it or not, the chargeless neutron also has an antiparticle. It’s called—you guessed it—the antineutron. The antineutron is endowed with an opposite zero charge to the ordinary neutron. This arithmetic magic derives from the particular triplet of fractionally charged particles (quarks) that compose neutrons. The quarks that compose the neutron have charges–1/3,–1/3, +2/3, while those in the antineutron have 1/3, 1/3,–2/3. Each set of three add to zero net charge yet, as you can see, the corresponding components have opposite charges.

Antimatter can seem to pop into existence out of thin air. If a pair of gamma rays have sufficiently high energy, they can interact and spontaneously transform themselves into an electron-positron pair, thus converting a lot of energy into a little bit of matter as described by the famous 1905 equation of Albert Einstein:

E = mc2

which, in plain English reads

Energy = (mass) × (speed of light)2

which, in even plainer English reads

Energy = (mass) × (a very big number)

In the language of Dirac’s original interpretation, the gamma ray kicked an electron out of the domain of negative energies to create an ordinary electron and an electron hole. The reverse is also possible. If a particle and an antiparticle collide, they will annihilate by refilling the hole and emitting gamma rays. Gamma rays are the sort of radiation you should avoid. Want proof? Just remember how the comic strip character “The Hulk” became big, green, and ugly.

If you somehow managed to manufacture a blob of antiparticles at home, you would immediately have a storage problem, because your antiparticles would annihilate with any conventional sack or grocery bag (either paper or plastic) in which you chose to carry them. A cleverer solution traps the charged antiparticles within the confines of a strong magnetic field, where they are repelled by the magnetic walls. With the magnetic field embedded in a vacuum, the antiparticles are also rendered safe from annihilation with ordinary matter. This magnetic equivalent of a bottle is also the bag of choice when handling other container-hostile materials such as the 100-million-degree glowing gases of (controlled) nuclear fusion experiments. The real storage problem arises after you have created whole (and therefore electrically neutral) anti-atoms, because they do not normally rebound from a magnetic wall. It would be best to keep your positrons and antiprotons separate until absolutely necessary.

IT TAKES AT least as much energy to generate antimatter as you recover when it annihilates to become energy again. Unless you had a full tank of fuel in advance, self-generating antimatter engines would slowly suck energy from your starship. I don’t know whether they knew about this on the original Star Trek television and film series but I seem to remember that Captain Kirk was always asking for “more power” from the matter-antimatter drives and Scotty was always saying that “the engines can’t take it.”

While there is no reason to expect a difference, the properties of antihydrogen have not yet been shown to be identical to the corresponding properties of ordinary hydrogen. Two obvious things to check are the detailed behavior of the positron in the bound company of an antiproton—does it obey all the laws of quantum theory? And the strength of an anti-atom’s force of gravity—does it exhibit antigravity instead of ordinary gravity? On the atomic scales, the force of gravity between particles is unmeasurably small. Actions are instead dominated by atomic and nuclear forces, both of which are much, much stronger than gravity. What you need are enough anti-atoms to make ordinary-sized objects so that their bulk properties can be measured and compared with ordinary matter. If a set of billiard balls (and, of course, the billiard table and the cue sticks) were made of antimatter, would a game of antipool be indistinguishable from a game of pool? Would an anti-eightball fall to Earth at exactly the same rate as an ordinary eightball? Would antiplanets orbit an antistar in exactly the same way that ordinary planets orbit ordinary stars?

I am philosophically convinced that the bulk properties of antimatter will prove to be identical to those of ordinary matter—normal gravity, normal collisions, normal light, normal pool sharking, etc. Unfortunately, this means that an antigalaxy on a collision course with the Milky Way would be indistinguishable from an ordinary galaxy until it was too late to do anything about it. But this fearsome fate cannot be common in the universe because, for example, if a single antistar annihilated with a single ordinary star, then the conversion of matter to gamma-ray energy would be swift and total. Two stars with masses similar to that of the Sun (each with about 1057 particles) would become so luminous that the colliding system would temporarily outproduce all the energy of all the stars of a hundred million galaxies. There is no compelling evidence that such an event has ever occurred. So, as best as we can judge, the universe is dominated by ordinary matter. In other words, being annihilated need not be one of your safety concerns on that next intergalactic voyage.

Still, the universe remains disturbingly imbalanced: when created, every antiparticle is always accompanied by its particle counterpart, yet ordinary particles seem to be perfectly happy without their antiparticles. Are there hidden pockets of antimatter in the universe that account for the imbalance? Was a law of physics violated (or an unknown law of physics at work) during the early universe that forever tipped the balance in favor of matter over antimatter? We may never know the answers to these questions, but in the meantime, if an alien lands on your front lawn and extends an appendage as a gesture of greeting, before you get friendly, toss it an eightball. If the appendage explodes, then the alien was probably made of antimatter. If not, then you can proceed to take it to your leader.