EXCERPTS:
In the entire universe, as far as we can tell, matter and not antimatter is the norm. It seems that the destruction of antimatter was one of the first acts after the Big Bang.
If a lump of matter is your fuel, then antimatter is the spark that will release its energy in ways that, theoretically at least, cannot be bettered in nature. This is why some have cited the Tunguska event to have been the result of antimatter hitting the atmosphere.
The positron did not pre-exist within that atom any more than a bark exists inside a dog; it was the energy release that created it.
The positron flies away from the atom and lives only so long as it avoids meeting an electron. As our world is made of atoms, which all contain electrons, the positron soon bumps into one, these counterbalanced opposites disappearing in a flash of gamma rays, which is light far beyond the part of the spectrum that our eyes can see. Special instruments however can detect these rays, which are exploited in medicine in the PET scanner— positron emission tomography. Antimatter destroys, but in controlled circumstances this can paradoxically be a life saver.
On a larger scale, nature produces positrons in the heart of the sun. The sunlight that shines on us today is in part a result of positrons that were created in the centre of the sun some 100,000 years ago, only to be annihilated almost immediately. The sun is mostly hydrogen, the simplest element. In its centre where the temperature exceeds 10 million degrees, the hydrogen atoms are disrupted into their component pieces, electrons and protons swarming independently and at random. The protons occasionally bump into one another and through a sequence of processes link together, eventually forming the seed of helium, which is the next simplest element. Helium is the ash from this fusion reaction and has less mass than the protons that were used to make it. This loss in mass has turned into energy, E = mc2 at work, which is ultimately the energy that emerges as sunlight. So what do positrons have to do with this? A helium nucleus contains two protons and two neutrons. Under suitable circumstances a proton can change into a neutron and emit energy some of which materializes as a positron, similar to what happens in the positron emitters of earthly medicine. The positron finds itself in the heart of the sun, where there are lots of electrons, and is instantly destroyed, turned into gamma rays. These try to rush away at the speed of light but are interrupted by the crowd of electrically charged particles, electrons, and protons that form the seething star. Buffeted this way and that, repeatedly absorbed by electrons and then emitted with less energy than before, it will take a hundred thousand years before gamma rays manage to reach the surface, hundreds of thousands of kilometers above. In doing so the rays lose lots of energy, their character changing from X-rays to ultra-violet and at last into the rainbow of colours that are visible to our eyes. So daylight is the result of antimatter being produced in the heart of the sun and, in part, of its annihilation.
It would require more than a billion atoms in a chemical explosive to produce as much energy as could be liberated by the annihilation of a single electron. Annihilate a single gram of antimatter, (about 1/25th of an ounce), and you would obtain as much energy as you could get from the fuel tanks of two dozen conventional space shuttles. Positron energy conversion would be a revolutionary energy source which would interest those who wage war as just half a gram explosively equates to 20 kilotons, the size of the bomb at Hiroshima.4 It is no surprise then that if antimatter can be produced and stored until needed, it has the potential for power that would interest the space industry, or for weapons that would excite the military. I have no doubt that these possibilities are being actively investigated. This book will tell the story of antimatter, what it is, how it was discovered, how we can make it, and what opportunities and threats it could pose. It will also assess the reality of antimatter as fuel for space odysseys and for weapons.
So, what is antimatter? Saying that it is the opposite of matter is easy on the ear, but what actually is ‘opposite’ about it?
Nearly all of the atoms of oxygen that you breathe, and of the carbon in your skin or the ink on this page, were made in stars about five billion years ago when the earth was first forming. So we are all stardust or, if you are less romantic, nuclear waste, for stars are nuclear furnaces with hydrogen as their primary fuel, starlight their energy output and assorted elements their ‘ash’ or waste products.
So even at the basic level of atoms, matter and antimatter look the same: the source of their contrast is buried deeper still.
The force of gravity rules the galaxies, planets, and falling apples, and keeps our feet on the ground. However it is electric and magnetic forces that give us shape and structure. The electromagnetic force is much more powerful than gravity, but in bulk matter the attractions and repulsions of the positive and negative charges tend to cancel out, leaving the all-attractive force of gravity as dominant.
Such a swapping of charges would turn what we know as matter into what we call antimatter. An anti-atom of antihydrogen would consist of a negative ‘antiproton’ encircled by a positively charged ‘positron’. Paul Dirac, who first predicted that such a mirror image of matter should exist, summarized this enigma on receiving his Nobel Prize in 1933:
We must regard it rather 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, these stars being built up mainly of [positively charged electrons] and negative protons.
When large numbers of atoms are involved, Newton’s laws of certainty emerge from the underlying quantum rules.
It is these quantum laws, when combined with Einstein’s theory of relativity, that reveal that just one form of matter is not enough: the act of creation in the Big Bang must have made two counterbalanced varieties.
Said another way: in a millionth of a second an electron makes more circuits of the central proton than the earth has made around the sun in its entire history.
The emergence of substance from pure energy, of which the purest form is light, is almost biblical in scope. With antimatter, the negative image of matter, we make contact with the gods of creation.
There’s matter, like the electron; antimatter, like the positron; and then there are things that are neither matter nor antimatter. The most familiar example of something that is beyond substance is electromagnetic radiation. All electromagnetic radiation, from gamma rays through X-rays and ultra-violet to visible light, infrared, and radio waves, consists of photons of different energies.
Matter and antimatter can cancel one another out, their annihilation leaving non-substance in the form of photons; if the conditions are right this sequence can happen in reverse where photons turn into pieces of matter and antimatter. The photon of light is just one example of over a hundred known examples of particles that are non-substance. Such entities are known as ‘bosons’, after the Indian physicist Satyendranath Bose. By contrast, substantial particles that are basic pieces of matter or of antimatter are known as ‘fermions’, after the Italian Enrico Fermi. The behaviour of fermions is described by Dirac’s equation; bosons follow different rules.
One of the results of Dirac’s work was the discovery that the electromagnetic field itself is ruled by quantum theory. Photons are particle-like bundles of electromagnetic radiation, and they transmit the electromagnetic force as they flit between one charged particle and another. In modern ‘quantum field theory’, not just the electromagnetic force but all the forces are transmitted by bosons. What the photon does for the electromagnetic force, so the ‘graviton’ is believed to do for gravity. No one has yet detected a graviton, but few doubt that it exists and that some day it will be found.
The battle between matter and antimatter in the universe was fought fourteen billion years ago, and matter won.
Fermions give rise to structure, they have stability and lead to life. We are formed from atoms that have existed for billions of years, it is only now that they are configured in combinations that think they are us. We breathe in oxygen, exhale carbon dioxide, grow and die, but our atoms will go on. Their basic pieces will recombine with infinite variety into the distant future, so long as they do not meet antimatter. Dehmelt had proved what the theorists suspected: we live in a world of matter, but the vacuum is full of both ‘virtual’ antimatter and ‘virtual’ matter, virtual in the sense that it does not materialize (maybe that should read ‘anti-materialize’) but whose presence can be inferred by its effects on passing particles of matter.
One of the implications of Stueckelberg’s diagrams was that an antiparticle could be regarded as a particle that was travelling backwards in time.
This creates mental images of antimatter being truly exotic; that in watching positrons we are sensing electrons arriving from the future. If in the first moment matter and antimatter emerged equally from the Big Bang, an instant later they should have annihilated one another. This gives another perspective on the question. The mystery is less about why antimatter has disappeared, and more a question of why has matter survived? Perhaps the answer is that there is some difference between them, that they are not perfect mirrors of one another.
The complex interactions that make amino-acids, DNA, and life will equally allow anti-elements to make everything in anti-DNA, even anti-life.
The chemistry of antimatter is the same as matter: antiplanets and antimatter in all its forms are as realizable as the more familiar matter which dominates the known universe. Are antigalaxies of antistars surrounded by antiplanets of antimatter awaiting unsuspecting astronauts in the far reaches of the universe? How sure are we that there is no antimatter at large, out there somewhere?
How can we know the make-up of a distant star, seen only as a faint candle across the vastness of space? All we see from earth is the starlight and as we have no reason to suppose that the spectra of the anti-elements are any different from those of the elements, we cannot tell stars from antistars simply by looking out into the night sky.
All of the evidence suggests that, with the exception of transient antiparticles produced like the above, everything within several hundred million light years of us is made of matter. This is a huge volume, to be sure, but only a fraction of the visible universe. There is still a lot of unexplored space where antimatter could dominate. Could matter and antimatter have become separated into large independent domains?
On the average every five cubic metres of outer space contains one proton, no antiprotons and ten billion quanta of radiation. Everything that we know about the early universe, from theory, observations and the results of experiments at LEP, suggests that in the hot aftermath of the Big Bang those numbers would have been ten billion quanta of radiation, ten billion antiprotons, and ten billion and one protons. The inference is that one of the first acts after creation was a Great Annihilation such that the matter-dominated universe today is made from the surviving one out of ten billion protons. Everything out there today is the remnant of an even grander creation.
So whereas chemical reactions convert just one part in a billion of matter’s trapped energy, nuclear reactions can liberate up to about 1 per cent. If we could transform larger fractions of matter into energy, our ambitions would expand in parallel. In principle we could liberate the full mc2 latent within matter into energy. That is the promise of antimatter.
All cultures have wondered about their origins, and the paradox of how something came from nothing. Why the Big Bang occurred no one yet knows, but out of its energy everything that we know was born. And it is beams of antimatter, first antiprotons and then positrons, that have enabled us to simulate the early universe in experiments, and begin to understand what it was like when less than a billionth of a second old. This is an astounding achievement of the human intellect: of groups of atoms collected together and able to think, to look out in wonder at the universe that made us, and build machines that can revisit our origins in the Big Bang. And the tool that made all this possible is antimatter. With such inspirations in fact, who needs fiction?