How to Destroy the Universe (14 page)

Once gathered, antimatter is stored in a device known as a Penning trap. This is effectively a magnetic bottle, which holds the electrically charged particles in strong electric and magnetic fields to stop them from annihilating with the container walls. For the same reason all the air is evacuated from the space inside. The antimatter in the trap is also cooled using a laser beam to prevent thermal energy freeing it from the grip of the magnets. Laser cooling works by stimulating the antimatter particles to radiate energy.

Antimatter could offer a major boost to space flight itself. The big problem in deep space exploration is the mass of fuel that spacecraft must carry. Antimatter offers a dense, portable source of energy that could conceivably propel a craft around the Solar System or to distant stars on feasible timescales. NASA has calculated that a 100-ton spacecraft powered by antimatter could reach a speed of 100,000 km/s (62,000 m/s)—fast enough to get to the nearest stars in about 12 years. The spacecraft would use a Penning trap to
store its fuel, from where it would be siphoned into a combustion chamber and mixed with particles of ordinary matter. The two groups of particles annihilate spewing fragments out of the back of the rocket engine at high speed, propelling the spacecraft forward. An antimatter engine would be over 2,000 times more efficient than a space shuttle main engine. The amount of antimatter required would still be large, however, compared to what we have available using current techniques. One option a team at NASA's Marshall Space Flight Center has looked at is to use antimatter to boost a fusion-powered nuclear engine (see
How to build an atomic bomb
). They calculated that just 10 micrograms of antimatter would be sufficient to power a spacecraft on a tour of the Solar System. This propulsion system would enable a journey to the outer limits of Solar System in around a year.

Even 10 micrograms of fuel won't come cheap. But as Lawrence Krauss says in
The Physics of Star Trek
: “It is the ultimate rocket-propulsion technology, and will surely be used if ever we carry rockets to their logical extremes.”

• Small world

• Electronic eyes

• Quantum tunneling

• Atomic force microscopes

• Moving atoms

Atoms are some of the tiniest objects in nature, as small as 32 billionths of a millimeter in size. The smallest thing the human eye can see from a distance of 30 cm (12 in) is about 0.1 mm (0.003 in) across. Atoms are more than three million times smaller again. Even the most powerful desktop microscope falls short by a factor of 1,500. However, during the 20th century, physicists have built a range of new high-tech microscopes that are capable of peering all the way down into the atomic realm.

The word atom was first used nearly 2,500 years ago by the Greek philosopher Democritus to describe the smallest indivisible particles of matter possible. Keep chopping a chunk of matter in half and there comes a
point where what you have cannot be divided any further. In 1803, British physicist John Dalton was one of the first scientists to take seriously the idea that matter was made of atoms. It was a great idea, and explained phenomena such as Brownian motion—how dust grains viewed under a microscope seem to be getting knocked this way and that by an invisible force. In Dalton's theory the invisible force came from collisions with atoms. Atoms also laid the foundations for the emerging science of chemistry. The fact that chemical elements are made of indivisible chunks explained why the products of chemical reactions always come in small whole-number proportions—because it's impossible to make a mass of a chemical that weighs less than the mass of one atom. But despite their importance, no one had much clue what these tiny entities looked like. That would start to change in the 19th century as physicists began to piece together a theoretical picture of how each atom is built.

In 1896, British physicist J.J. Thomson was studying cathode rays—beams of particles given off by a negatively charged electric terminal that has been heated up. When he measured the mass of these particles, he found that it was tiny. Thomson had discovered the electron, a minuscule subatomic particle that orbits in the outer reaches of every atom. Later, two other particles would be discovered—the positively charged proton and the electrically neutral neutron, which
inhabit the dense core of an atom called the nucleus. At first, it was thought that electrons orbited the nucleus like the planets orbit the Sun. This view changed in the 1920s with the development of quantum mechanics. In this view, electrons are complicated three-dimensional waves that oscillate around the nucleus in accordance with the laws of quantum theory.

In the early 1930s, the German physicist Ernst Ruska and electrical engineer Mark Knoll took advantage of the wave nature of electrons to design a new kind of microscope capable of magnifications of one million times, a factor of 500 better than an optical microscope. A few years earlier, the French physicist Louis de Broglie had figured out an equation to calculate the equivalent wavelength of a particle from its momentum—a measure of the impetus of a moving body given by its speed multiplied by its mass. When Ruska and Knoll did this for electrons they found that the de Broglie wavelength of a typical electron is hundreds of thousands of times shorter than the typical wavelength of light. The shorter the wavelength, the greater the level of detail it can resolve, so a microscope that uses an electron beam offers far greater resolution.

The two researchers now had to figure out how to channel and focus beams of electrons in the same way
that the lenses and mirrors of an ordinary microscope channel light. The design they finally came up with generates a beam of electrons using a device called an electron gun, a negative electrical terminal that's heated to give the electrons enough thermal energy to escape from the metal. These electrons are then attracted and accelerated by a positively charged grid and focused into a tight beam using a magnet.

The magnet directs the electron beam onto the object to be studied. As the electrons pass through the object, some of them are deflected by its internal structure. When they emerge on the other side, the beam and the image imprinted on it by this structure is magnified by another set of magnetic lenses and then focused onto a fluorescent screen where it can be viewed. Because the electrons are transmitted through the specimen being observed, this technique is known as transmission electron microscopy (TEM). The first TEM was built in 1931. A few years later, Knoll and colleagues built a variant that was able to capture wider-field images. It did this by deflecting the path of the initial electron beam using a set of electrical coils. By varying the current in the coils the beam could be made to scan back and forth across the specimen object—rather like an old tube-based television set that scans an electron beam rapidly back and forth across a phosphor-coated screen to form a picture. This design of electron microscope is normally known as a scanning electron microscope, or SEM for
short. The beam in an SEM bounces off the specimen rather than passing through it.

Image resolutions with these techniques can reach scales of 1 nanometre (nm)—a millionth of a millimeter—allowing crisp images of blood cells, micro-organisms, viruses and crystal structures in materials. It still isn't quite good enough to see atoms, but it was a massive step forward.

A machine to see atoms would finally become reality in 1981 when German physicists Gerd Binnig and Heinrich Rohrer invented the scanning tunneling microscope, or STM. Rather than illuminating the target specimen with electrons—or any other form of particles or radiation—the STM makes use of a quantum phenomenon known as tunneling. In classical (non-quantum) physics, two electrical charges of the same sign repel each other. The electric fields set up by the two charges interact with one another to push them apart and this force of repulsion can only be overcome by applying a greater force, i.e. shoving them together very hard. In quantum physics, however, this isn't quite true. Electric charges of the same sign still repel each other but down in the quantum world particles behave like waves—their position in space isn't tied down to a single point but is instead smeared out
over a fuzzy patch. This uncertainty in position can bring two charged particles together without the need to apply as much external force to them.

Tunneling plays a major role in understanding the nuclear fusion reactions at the heart of the Sun. Fusion works by joining together atomic nuclei, resulting in the release of energy. However, nuclei are all positively charged so must be forced together in order to overcome the mutual repulsion they feel. This is achieved by heating the nuclei up, which makes them jiggle around violently and smash together. To collide them with enough force to do this would ordinarily require temperatures well in excess of that found in the Sun's core. Tunneling solves the problem, reducing the force, and hence the temperature, needed for fusion to take place. STMs take advantage of quantum tunneling by dragging a tiny probe across the surface of the specimen object being studied. The probe doesn't actually make contact with the surface but remains around two atomic widths away. The tip of the probe itself is extremely sharp—usually just a single atom thick. If a tiny voltage is applied between the tip of the probe and the specimen, a stream of negatively charged electron particles are able to quantum tunnel between the two, setting up an electrical current. The strength of the current varies with the size of the gap between the probe and the surface. A computer alters the probe height to keep the current constant. The changes in
height as the probe is dragged across the surface reveal the surface's texture. By scanning the needle back and forth across the surface, a picture of its lumps and bumps can be built up.

This technique can achieve resolutions of up to 0.01 of a nanometre—1/100,000,000 of a millimeter—good enough to pick out individual atoms, showing them up as peaks that each look rather like a wizard's hat. Technically, these aren't actual images of the atoms but representations of their position based on interactions between their outer cloud of electrons and the STM probe.

The one problem with STM is that the voltage needed to generate the tunneling current means that it can only be used to map out the surfaces of electrically conducting materials. That all changed in 1986 with the invention of the atomic force microscope, or AFM. This device is similar to an STM but rather than using a tunneling current, the tip of the probe itself drags over the surface of the sample object. The probe is mounted on a cantilever arm, which moves up and down in response to the surface contours of the sample. This movement is detected by shining a laser beam on the cantilever—as the cantilever moves, it shifts the angle by which the beam is reflected, which in turn can
be measured by a light sensor. The readings are fed to a computer which uses them to build a relief map of the surface and displays it on a screen. AFMs can also be used to measure the mechanical properties of materials, such as their elasticity and roughness. Because there is no requirement for the sample to conduct electricity, AFM is used to study all kinds of materials—from plastics to biological samples.

In 1989, researchers Donald Eigler and Erhard Schweizer were able to spell out the letters “IBM” in xenon atoms by nudging the atoms around with an STM probe. The technique works by getting the tip of the probe so close to an atom that the atom actually sticks to it and can be dragged across the surface and deposited where it's needed. This means that an STM probe doubles as an atomic toolkit that can be used to engineer structures on the tiniest scales. Most recently it has been used to alter the bond structure between atoms inside molecules, changing the molecules' chemical properties. In the space of just over 200 years, our understanding of the atomic world has gone from one of complete ignorance to being able to print the fundamental atomic structure of matter on a T-shirt.

• Ancient alchemy

• Radioactivity

• Structure of the atom

• Nuclear transmutation

• The final piece

• Nuclear waste

• Making gold

The alchemists of old searched in vain for a way to turn humble base metals into gold. Although largely based on pseudoscience and superstition, alchemy laid the foundations for what became the modern science of chemistry. But even that wasn't up to making gold. The solution finally came in the 1930s with the development of nuclear physics: a way to tinker with the very heart of an atom, and ultimately transform one chemical element into another.

Alchemists searched for a legendary substance known as the philosopher's stone, which was believed to have
the power to turn ordinary base metals, such as lead, into precious metals, such as gold. But the work of these early alchemists had little to do with any real scientific understanding of the structure of matter. Unfortunately for them, they did find small amounts of gold in the residues of their experiments, which just led them further from the truth. These traces were already in their original samples of ore, and had been separated out by the primitive chemical processes they were using. It turns out that it
is
possible to turn lead into gold, but the secret lay a million miles from the pseudoscientific ramblings of the alchemists. It had to wait for the development of modern physics in the early 20th century.

In the 1800s, physicists had realized that matter was made of tiny, indivisible chunks known as atoms (see
How to see an atom
). It was discovered in 1896 that some atoms sporadically spit out particles in a process known as radioactivity. This seemed to take one of three different forms—called alpha, beta and gamma. Alpha radiation consisted of positively charged heavy particles; beta radiation consisted of high-speed negatively charged electrons; gamma radiation was essentially electromagnetic radiation—similar to light but with much higher energy.