Read The Big Questions: Physics Online
Authors: Michael Brooks
The Big Questions: Physics (25 page)
The announcement may not have thrown down a gauntlet to the physics community, exactly – Dirac was largely uninterested in what anyone else thought. Nevertheless, the
prediction was out there. And unknown to anyone, the evidence was out there too. Physicists studying cosmic rays, the charged particles that smash into Earth’s atmosphere, creating a cascade of other particles, had seen – but not understood – the signature of an antielectron five years before Dirac made his pronouncement. When passed through a magnetic field, some of these particles bent the ‘wrong way’. It was noted as anomalous, and discussed at scientific meetings around the same time that Dirac was discussing his theoretical ideas. But it was not until 1932 that anyone put two and two together, when Carl Anderson discovered the antielectron in the debris of cosmic ray collisions. The breakthrough won Anderson a Nobel Prize.
Once it was clear that antimatter could exist, it was only natural to ask how much of it there is in the universe. Is it all around? Does it sit unnoticed in antimatter stars in antimatter galaxies? And if so, is there less antimatter than matter in the universe? Would that explain why there is something rather than nothing? The trouble is, answering these questions involves understanding a lot more about antimatter. But how do you study something that annihilates on impact with everything around you?
We have found a few answers out in space and are now reasonably sure that there are no antimatter stars out there, although there are natural sources of antimatter in the universe. One, seen by the INTEGRAL telescope, is a fountain of positively charged electrons, or positrons, that streams out from somewhere near the centre of the Milky Way. There are clues on Earth too. As Carl Anderson showed, we can study antimatter by looking at the debris created when cosmic rays smash into the Earth’s atmosphere. But it’s not an abundant source: high-energy cosmic rays smashing into gas clouds only produces around 3 or 4 tonnes of antimatter per hour in the whole of our solar system.
In fact, our efforts to create antimatter on Earth are even more feeble. The prime source of antimatter is the European Organisation for Nuclear Research (CERN) in Geneva, but it is an incredibly crude process. The CERN researchers simply smash beams of positively charged protons into a lump of metal – copper or tungsten. The result of this is a huge spray of particles, a few of which are negatively charged antiprotons. A few of these few particles spray out in the right direction to be harvested in a trap.
ANTIMATTER PROPULSION
Understanding why matter triumphed over antimatter might prove crucial to the survival of humanity. Even if our species survives ‘local’ disasters, such as runaway climate change, we will eventually face much bigger challenges. In 5 billion years, for instance, our dying sun will expand and engulf the Earth. Even earlier, in just 2 billion years, our galaxy will collide with Andromeda, throwing us into a galactic maelstrom of colliding stars and planets.
To survive such scenarios may require finding a new place to live. Unfortunately, the best forms of transport we have are not going to get us anywhere close to even the nearest substitute Earth. Antimatter, though, might help. Take the best candidate planet we have found so far. Gliese 581c is twenty light years away. To get there in a human lifetime a craft would need to travel at somewhere around half the speed of light – and our chemical-powered rockets don’t travel anywhere near that kind of speed.
Our only hope is to come up with a new propulsion technology – something like the energy released when matter collides with antimatter. One kilogram of antimatter, annihilated with the same amount of normal matter, would release around 10 billion times as much energy as is released by the explosion of a kilogram of TNT. That makes it 1,000 times more efficient, in terms of energy per kilo, than nuclear fission. NASA calculations suggest that a 100-tonne antimatter-powered craft could reach speeds of 100,000 kilometres per second.
That would be possible by creating an antimatter jet. Annihilation creates high energy charged particles that travel at monstrous speeds. Use a system of magnets to shoot those particles out from the back of the spacecraft, and Newton’s third law – every action has an equal and opposite reaction – means a huge kick forwards for the spacecraft. All we have to do now is work out how to build an antimatter drive.
For every 10 billion joules of energy the CERN researchers put into the process, they get one joule’s equivalent back as antimatter. If you annihilated all of the antimatter ever made at CERN, the released energy would power a single electric lightbulb for no longer than a few minutes.
Not that you can store it up to feed into a power grid. Antimatter can’t be allowed to touch normal matter, so it can only be held by the electromagnetic fields of a ‘Penning trap’. This uses magnetic fields to hold the particles away from the physical walls of a container. Scientists can only store antimatter in a Penning trap for a few minutes at a time, and each trap can only hold so many particles: as soon as their mutual repulsion overcomes the repulsion due to the trap’s magnetic field, the antimatter annihilates on the walls of the container.
What’s more, CERN’s traps can only hold around 1,000 billion particles, which sounds like a lot, but isn’t. It is around a hundredth of the number of atoms contained in a child’s balloon – and that would only come from hundreds of millions of years’ worth of production at CERN. The dream of antimatter-powered spaceships taking us to the stars (see box:
Antimatter Propulsion
) will have to wait for a better source of fuel. But the dearth of antimatter hasn’t stopped CERN giving us clues as to why something survived and nothing didn’t.
Experiments in CERN’s Large Electron and Positron Collider (LEP) tell us that the Big Bang would have created a universe in a situation where five cubic metres of space held 10 billion antiprotons, and 10 billion and one protons. Today, that same space contains one proton and no antiprotons. At some point in our history, matter and antimatter met and annihilated one another, leaving just one proton per five cubic metres. These protons eventually came together and formed the universe we know today. So what created that initial imbalance of particles?
In the late 1960s, the Russian physicist and dissident Andrei Sakharov set his mind to solving this puzzle. The detective story is not yet finished, but we do have a clue about what created the initial imbalance between matter and antimatter. It seems to be an odd little particle called a neutrino. Sakharov’s biggest clue came in 1964, when physicists found something odd about the weak force, which governs radioactive decay and various other processes that take place in the nucleus of an atom. The weak force, unlike every other force, does not act quite the same way on matter as on antimatter.
Inside every proton are three particles called quarks. Quarks have their own antimatter particles, the antiquarks. The weak force, it turns out, treats quarks and antiquarks differently. This disparity in the way the weak force deals with matter and antimatter means that the laws of physics must somehow be subtly different for the two. That, in turn, means that the conservation laws that cover things such as energy and momentum cannot be applied to matter and antimatter. There must exist natural processes that allow the balance between the two to change.
So what are those processes? One clue is that they must have occurred in a period of cosmic upheaval, where reactions were taking place between particles, antiparticles and radiation. If the reactions took place at one rate for particles, and another rate for antiparticles, you would get a net difference in the amounts that survived. The early universe, which was a long way out of thermal equilibrium and thus buzzing with transformations of energy into particles, and particles into different particles, was perfect for creating such an imbalance.
ANNIHILATION IS NOT THE ONLY FRUIT
Investigations of strange phenomena in space might tell us more about why there is something rather than nothing. When matter and antimatter meet, the result is a gamma ray. So when, in 1997, a NASA satellite saw a stream of gamma rays emerging from somewhere near the centre of the Milky Way, the obvious conclusion was that clouds of matter were annihilating with clouds of antimatter, producing what has come to be known as an ‘antimatter fountain’.
There are other possible explanations. It could be that a black hole is producing a jet of particles, or that fragments of a supernova are undergoing radioactive decay. The most interesting idea, though, is that a whole new type of chemistry – antimatter chemistry – is at work.
Paul Dirac’s prediction that matter and antimatter will annihilate on contact has been borne out in countless experiments, but there is certainly more to it than just an explosion. For a start, antimatter has been known to bounce off matter in some circumstances. CERN researchers have put antiprotons and antielectrons (positrons) into the same antimatter traps, and combined them to make antihydrogen atoms (hydrogen has one electron and one proton; antihydrogen has a positron and an antiproton). Antihydrogen wasn’t the only thing they found. For a few microseconds, antiprotons were joining together with the protons from hydrogen molecules without exploding away into pure energy. Annihilation isn’t inevitable, it seems – at least not immediately.
That’s as far as Sakharov got, but we have now gone much further. We know that the hot, dense conditions just after the Big Bang were perfect for creating a host of particles that would never be seen in our cold, empty universe. And one of them, known as the ‘majoron’, is the reason why there is something rather than nothing.
As the majoron ages, theory suggests that it does not respect the symmetry laws that create equal numbers of particles and antiparticles from one decaying particle. The majoron decays to form particles called neutrinos, which are tiny uncharged particles that zip around the universe at almost the speed of light. Majorons also produce the neutrino’s antiparticle, the antineutrino. But there is no compulsion for the majoron to form equal numbers of neutrinos and antineutrinos.
During their lifetime, the neutrinos and antineutrinos will collide with electrons and positrons to form quarks and antiquarks. If there is an excess of neutrinos over antineutrinos, that means more quarks than antiquarks will form. So when annihilation happens between quarks and antiquarks, there is matter left over.
It is a pleasing solution to the problem at hand, but there is a fly in the ointment. This is an unproven theory: we have yet to see direct evidence of the majoron. Some indirect evidence may come into view at CERN’s Large Hadron Collider, but our experiments are not yet powerful enough to recreate the conditions at the time before annihilation took place and observe the majoron at work. When it comes to working out why there is something rather than nothing, time travel back to the first moments of creation might provide our only dependable answer.
Human nature, the laws of physics, and the march of technological progress
In 1998, almost no one you’d meet on the street would have given this question a moment’s thought. By the end of 1999, the possibility had been discussed by millions of people around the globe. Why? Because they had seen
The Matrix.
The central premise of the film is that the human population of Earth is lying in vats of nutrient, their energy being harvested by a race of machines.