Few topics in science excite the popular imagination more than antimatter, perhaps because the idea sounds like science fiction that you can really believe in. For the same reason, it should not be surprising that when a story about antimatter surfaces in the news cycle—as regularly happens—the real science sometimes gets parked on the shelf and media-promoted pseudoscience takes over.
To put antimatter pseudoscience in perspective, we first need to recall a few things about the genuine article. Atoms of the chemical elements that form the matter of the world around us are themselves constructed from just three varieties of fundamental particle: protons, which carry a positive electric charge; electrons, which carry an equal amount of negative charge; and neutrons, which have no charge at all, although, like the other two, they have magnetic properties. The hydrogen atom is the simplest of all. It consists of a single electron bound by electrical attraction to a nucleus consisting of a single proton. It is therefore electrically neutral. Atoms of heavier elements just have more electrons bound to their nuclei consisting of an equal number of protons and a variable number of neutrons. They are, then, normally neutral.
Even before all this was established, the idea was entertained that a kind of mirror matter might exist made of particles with reverse “signed” attributes like charge and magnetism. Thus Arthur Schuster (Schuster 1898), observing that electric charge plays an important role in nature, asked the rhetorical question, “If there is negative electricity, why not negative gold?” Some thirty years later, English physicist Paul Dirac showed that if quantum mechanics and special relativity—the two cornerstones of modern physics—are to hold simultaneously, it must be possible for counterparts of the three fundamental particles to exist that have the same mass as, but opposite electric, magnetic, and other properties to, the originals. These he called “antiparticles.” We might then legitimately ask: If there are antiparticles, why not antimatter?1 Foreseeing this possibility, Dirac (Dirac 1933) intimated: “We must regard it as an accident that the earth (and presumably the whole solar system) contains a preponderance of negative electrons and positive protons. It is quite possible that for some of the stars it is the other way about. . . . There would be no way of distinguishing them by present astronomical methods.” Positively charged antielectrons, produced in the atmosphere by cosmic rays, were duly discovered (and soon afterwards found emerging from some radioactive substances). A few decades later negatively charged antiprotons were produced in the laboratory by directing fast-moving protons into metal targets. Antineutrons, displaying opposite magnetic properties to neutrons, were almost immediately added to the list, and nuclei of heavy antihydrogen (which consist of one antineutron and one antiproton) followed ten years or so later. Very recently, a few antihelium nuclei have been seen.
Were this the whole story it would not be such an inviting field for pseudoscientific mumbo jumbo. However, Dirac had also shown that antiparticles and their corresponding particles annihilate each other on contact, thereby releasing, via Einstein’s famous equation E=mc2, the energy equivalent of twice their mass. And if antiparticle annihilation produces enormous quantities of energy, why not antimatter bombs and antimatter power generation? Clearly intrigued by the former possibility, members of the U.S. Air Force began to appear at scientific conferences on antimatter in the early 1990s. So did the late science fiction writer Robert L. Forward,2 who had a background in research and a reputation for including only reasonably hard science in his novels. In 1996 a few very fast-moving antihydrogen atoms—antielectrons electrically bound to antiprotons—were produced at the CERN laboratory in Geneva (CERN 1996). Could this breakthrough lead to limitless power sources or unbelievably destructive bombs? Some sections of the world media evidently thought so and exploded with headlines like “Scientists create the fuel of science fiction,” and “Antiworld flashes into view.” Even Sir Joseph Rotblat, world-renowned physicist, Nobel Peace Prize winner, and one of the architects of the 1963 nuclear test ban treaty, warned of antimatter bombs thousands of times more devastating than the hydrogen bomb (Rotblat 1996).
NASA and Hollywood Fantasies
Things ratcheted up several more notches when in 2004 the San Francisco Chronicle published the article “Air Force Pursuing Antimatter Weapons” (Davidson 2004). This concerned a talk given by a certain Kenneth Edwards, director of the U.S. Air Force’s Revolutionary Munitions team, at a conference organized by NASA as part of its Institute for Advanced Concepts (NIAC) program. His report—sprinkled with Old Testament biblical references and maps showing missiles zigzagging over the Middle East—talked enthusiastically of revolutionary antimatter energy sources, rocket propulsion systems, and hand-held (yes indeed!) antimatter weapons.
We haven’t heard much of Edwards in recent years. We have, however, heard a lot about the 2009 movie version of Dan Brown’s Angels and Demons. In Brown’s book a lump of antimatter produced by the CERN Large Hadron Collider (LHC) is stolen and used to create mayhem in the Vatican as an act of revenge against the church for its persecution of Galileo. CERN had had an embarrassing mishap the previous year when the LHC had to be switched off for serious repairs shortly after it was inaugurated. Evidently thinking that all publicity is good publicity, it allowed the Hollywood promotional machine to take over its premises and use its good name as a platform to sell the movie, the lead actors, and their preferred brands of fashionable consumer goods. It also, apparently, chose not to point out on that occasion that the LHC has exactly the wrong characteristics for making antimatter anyway. Rather than having too little energy to function as efficient producers of antiparticles, its colliding protons have far too much. Even the 1996 antihydrogen atoms, made from antiprotons emerging from less violent collisions in a much lower energy accelerator, were moving so fast that they zipped through the laboratory in nanoseconds and annihilated on encountering the first obstacle they met.
Collecting Antiparticles for Research, Fuel, and Bombs
Here is the big problem: Whether you just want to study antiparticles closely or use them in fuel cells or bombs, you must hold large numbers of them still rather than have them self-destruct in nanoseconds. For this you need some kind of bottle. An antiparticle striking the walls of any ordinary bottle will, of course, instantly annihilate. It is not too difficult, however, to construct “bottles” with “walls” made of electric and magnetic fields, which will send any approaching electrically charged antiparticle backward if it is moving slowly enough. Since antiparticles will also annihilate if they hit any gas molecules remaining in the bottle, its air must be pumped out to a level such that few will be lost in this way in the course of a given experiment. Even at one billionth of the pressure of the atmosphere, each cubic centimeter of air contains many billions of molecules, so this is not easy.
Particles and antiparticles of either charge can nevertheless now be bottled without much trouble. In late 2010, again at CERN and again without recourse to the LHC, simultaneously bottled antiprotons and antielectrons were once more induced to bind together into a small number of antihydrogen atoms (Hangst 2011, Yamazaki 2011). This time they were moving very slowly—only a few hundred meters per second. At such low speeds, a configuration of electric and magnetic fields could be found that was able to bottle even these electrically neutral entities, albeit for only a fraction of a second. This has triggered yet another wave of pseudoscientific speculation about antimatter fuel and weaponry.
Running the Numbers
How are we to deal with these fevered imaginings? In his book Superstition, Robert Park (Park 2008) demolished Gerald O’Neil’s 1974 fantasy of solving the world’s overpopulation problem by accommodating the surplus in space colonies. This he did by what he calls “running the numbers.” Let’s try to highlight the absurdity of these antimatter fantasies by doing the same.
First we can discard the idea of collecting antiatoms (antihydrogen, for example) for such purposes instead of their component antiparticles. Getting the latter to bind together presents enormous technical headaches, which is why it has taken so many years to synthesize a mere handful of antihydrogen atoms. Moreover, there is no concomitant gain in the resulting annihilation energy yield, since it is the unbound components that annihilate anyway. We can also reject antielectrons and antineutrons as fuel or bomb material since the former have only about 1/2000 the mass of antiprotons (and therefore produce that much less energy per annihilation)3 and the latter because after about fifteen minutes, they decay into antiprotons anyway. We are thus left with antiprotons as our fuel or high explosive.
Suppose then that we had bottled every antiproton ever produced at CERN since 1956, the year they were first observed. How much energy would be released if we opened the bottle and allowed them all to annihilate? A rough guess at CERN’s aggregated antiproton yield since 1956 is about one hundred trillion (1014). Current technology limits the largest number that can be bottled as described above to about 10 million (107). Even with the best vacuum presently achievable, these will only survive against annihilation by air molecules for a few weeks. Let us nevertheless put our faith in technological advances, dismiss such objections as the product of insufficiently imaginative minds, and do our energy calculation as if we could indeed have bottled all 1014 antiprotons produced at CERN over the fifty-five years since 1956.
The energy equivalent of the proton and antiproton masses amounts to a few ten-billionths (3×10-10) of a joule.4 One joule will power a one-watt flashlamp bulb for one second. Our 1014 bottled antiprotons would therefore produce 30,000 joules, just about enough energy to light a sixty-watt bulb for eight or nine minutes. Trying to solve the world’s energy problems by antimatter annihilation evidently brings an entirely new dimension to the idea of doing things the hard way.
Now let’s calculate how long it would take to accumulate enough antiprotons to get the explosive power of a large hydrogen bomb, say ten megatons, with an energy equivalent of around forty thousand trillion (4.18×1016) joules. Presently we can bottle about two million antiprotons per minute, equivalent to six ten-thousandths (6×10-4) of a joule. The accumulation time needed for our 10 Mt bomb is therefore seventy million trillion (7×1019) minutes.
This is roughly 10,000 times the age of the universe—rather a tall order, you might think. Technophiles will nevertheless again argue that future improvements should be taken into account. Let’s be generous and allow them a factor of 20 billion or so to cover this, bringing the accumulation time down to a mere 7,000 years. So to have made such a bomb available now, with these quite impossible improvements, we would “only” have had to start accumulating around the dawn of recorded history. No time out for equipment maintenance or breakdowns; no holiday, meals, or comfort breaks for the operators of course; just continuous accumulation, day and night, week after week, month after month, year in, year out, century after century.
Still More Problems
There is one overriding consideration I haven’t yet mentioned: any antiparticles we make in the laboratory, we create out of energy itself (E=mc2 is here working backward, so to speak). So when they annihilate we only get back energy we originally used up. Not all of it by any means. Nature is not so kind as to give us back even the energy we expended, having decreed that antiparticle creation is a hopelessly inefficient way of storing energy.
What this suggests is that we would be better off going and getting a few bucketfuls of the stuff from one of those antimatter stars. Here I refer you once again to Robert Park’s book (Park 2008) in which he shows that travel to even a nearby star within a human lifetime would consume many thousands of times the entire annual energy production of Earth. And as if this wasn’t enough, astrophysicists, far and wide in the cosmos though they have looked, have never seen even a hint of any such stars.
Back to Real Science
Why this should be so is one of the great mysteries of modern physics. It is as if nature provided herself with two Lego kits for assembling different worlds but then left one of them in the box. To the best of our knowledge the instruction manuals—the laws of nature—are identical for the two kits. Have we perhaps misunderstood these laws?
When all else fails, read the manual is worthwhile advice for anyone faced with computer hardware behaving in similarly mysterious ways. Likewise, a guiding principle of real science is that our understanding of even well-established natural laws should always remain open to question. Careful re-reading of what might be called the fine print of these laws may yet reveal some minute asymmetry between matter and antimatter that has so far escaped our attention but that on a cosmic scale results in the world we see rather than the one we expected to see. Such is indeed the aim of current space-based (AMS 2011) and laboratory (Hangst 2011; Yamazaki 2011) experiments.5 But that, of course, is another matter.
1. The media normally make no distinction between antiparticles—the fundamental building blocks of antimatter—and antimatter itself (antiatoms, antimolecules, antistars etc.). In this article, I use antimatter in the latter, more correct sense.
2. Bob Forward was both likeable and knowledgeable, and I got on well with him on the few occasions we met. His pet project was neither power generation nor weaponry but an antimatter space propulsion system, in which annihilating antielectrons (which are usually and illogically known as positrons) heat a working rocket propellant instead of being used as a primary fuel.
3. NIAC, buried by NASA in 2007, was resurrected in 2011. Apparently following up Forward’s antielectron idea (www.nasa.gov/exploration/home/antimatter_spaceship.html), it nevertheless estimated in 2006 that “only” ten trillion trillion antielectrons (ten milligrams) would be needed for a manned trip to Mars along these lines. This is outside the scope of the present article, but their subsequent silence implies that even this relatively modest scheme got nowhere.
4. 2mc2, with m=1.67×10-27 kg and c=3×108 m/s.
5. In antimatter research things are currently moving somewhat faster than a speeding bullet. Since March 2011 when I began to write this article, the Alpha Magnetic Spectrometer (AMS above) has been launched to the International Space Station, antihydrogen atoms have been bottled on the order of fifteen minutes, the antiproton has been “weighed” with precision equivalent to weighing the Eiffel tower on a machine sensitive enough to detect a sparrow landing on it, and a kind of Van Allen belt of trapped antiprotons has been detected near Earth. Not surprisingly, some reports of these items of science news have indulged in the usual hand-waving fantasies about weapons, power, and space propulsion. Discussion of these developments must, however, await a future article.
AMS 2011. http://ams.cern.ch/.
CERN. 1996. Atoms of antimatter. CERN Courier 36 (2): 1–3.
Davidson, K. 2004. Air force pursuing antimatter weapons, Program was touted publicly, then became official gag order. San Francisco Chronicle (Oct 4).
Dirac, P.A.M. 1933. Nobel Prize lecture. Available at http://nobelprize.org/nobel_prizes/physics/laureates/1933/dirac-lecture.html.
Hangst, J. 2011. ALPHA collaboration gets antihydrogen in the trap. CERN Courier 51(2): 13–15. Available at http://cerncourier.com/cws/article/cern/45129.
Park, Robert. 2008. Superstition: Belief in the Age of Science, Princeton University Press.
Rotblat, Sir J. 1996. Private view. Financial Times (Jan 13/14).
Schuster, A. 1898. Potential matter—A holiday dream. Nature 58: 367.
Yamazaki, Y. 2011. At the cusp in ASACUSA. CERN Courier 51(2): 17–19. Available at http://cerncourier.com/cws/article/cern/45130.