Physics is concerned with understanding the world around us at the most fundamental level.
Its focus ranges from the origin and fate of the universe to matter on the subatomic scale – and everything in between.
A conventional computer is able to do one thing at a time – that is, read a piece of information (a 'bit') and perform a few basic logic operations on it.
These very simple processes form the basis of everything that current computers can do. They do these simple operations sequentially on a piece of information amazingly fast, so it is possible to achieve quite complex outcomes (such as three-dimensional graphics and interactive computer games) in almost real time via an unbelievably large number of these simple steps.
While the rapid increases in computing power mean these simple steps can be carried out faster and faster, there are fundamental limits to how fast conventional electronic components can operate, and current technology is rapidly approaching these limits. Hence the need for quantum computers.
Quantum computers are different from conventional computers, in that they use electrons and atoms to store information ('qubit') rather than chips. As a result, they can handle billions of pieces of information simultaneously, and so avoid the inevitable bottleneck in conventional computers caused by their limitation to sequentially process one piece of information.
Despite the simplicity of the idea, it is actually extremely difficult to build a quantum computer. This is because electrons and atoms can be easily disturbed by their environment, causing the breakdown of information stored in the quantum computer and thus errors in computation.
So, it may be a while before you have a quantum computer on your desk. But physicists around the world are racing to get there, and have already built a 7-qubit-quantum computer, which demonstrates that the physics indeed works.
The image you see in a mirror is not real – quite obviously so, since it will have a watch on its right arm if you have a watch on your left arm! It is what is called a virtual image. Your brain interprets the light reaching your eyes as having travelled in a straight line from its source, when in fact it has travelled along a 'crooked' line – it has been reflected from the surface of the mirror. So your brain 'sees' an image the same distance 'inside' (or behind) the mirror as you are standing from the mirror.
The fact that the reflected light is interpreted as having travelled in a straight line from 'inside' the mirror is what changes the handedness of the image your brain 'sees'.
Before the mid-1950s, scientists believed that nature was 'mirror symmetric' – that the laws of physics should not have an intrinsic handedness. In other words, they believed that if you were viewing an elementary particle interaction in a darkened room, you could not tell whether you were viewing the real process, or the image of the real process in a mirror. However, evidence emerged to suggest that one of the four fundamental interactions between elementary particles, namely the weak interaction, could in fact distinguish left from right.
An experiment confirming this suspicion was carried out in 1958 by a female physicist, Chien-Shiung Wu. Prior to the experiment, the Nobel Prize-winning Italian physicist Wolfgang Pauli had stated, "I do not believe that the Lord is a weak left-hander, and I am ready to bet a very high sum that the experiments will give symmetric results." He lost his money!
The more dense a star or planet is, the harder it is to escape from its 'gravitational pull'. If an object is sufficiently dense, then nothing, not even light, can escape its gravitational field. In this case, the object is a black hole – black because no radiation can escape.
To get some idea of the incredible densities required, the whole mass of the Earth would have to be compressed into a sphere with a radius of less than one centimetre before it would be dense enough to form a black hole.
Nevertheless, it is believed that the required densities can occur at the end of the life cycles of certain stars, and recently, strong evidence has emerged suggesting that supermassive black holes lie at the centre of many galaxies.
This question has mystified astronomers since they first realised over seventy years ago that the Universe is not static but expanding. Over the last few years and after decades of controversy, resolution of this puzzle is within sight.
The latest studies of supernova explosions vast distances from Earth, and of the pattern of irregularities in the cosmic microwave background, are telling us that not only is the Universe expanding, but that the rate of expansion is accelerating. Many astronomers are now convinced that the Universe will indeed expand forever.
Einstein's Special Theory of Relativity states that it is impossible for any object to travel faster than the speed of light, which is 300 million metres per second.
The reasoning goes like this. We are all probably comfortable with the fact that the heavier an object is, the more work we have to do to achieve a given change in speed – which is why truck engines are bigger than car engines! In fact, as Einstein recognised, it is not the just the mass of an object which determines its resistance to change in speed, but its energy.
This energy consists of the energy the object has when it is at rest (its 'rest energy', proportional to its mass via the famous formula E = mc2), plus the kinetic energy it possesses due to its motion. The faster an object moves, the greater its energy, and hence the greater its resistance to further increases in its speed – meaning we have to do more work to achieve a given additional increase in speed.
This cycle gets out of control as the speed of an object approaches the speed of light, in that it would take infinite amounts of energy to accelerate an object to a speed greater than that of light.
All matter in the Universe is built up from a relatively small number of elementary particles, which include quarks (the constituents of protons and neutrons) and electrons (which, together with protons and neutrons, make up atoms). Associated with each elementary particle is an 'antiparticle' which also occurs in nature. The antiparticle has the same mass as the particle, but has the opposite charge.
A particle and an antiparticle can combine (or "annihilate") to produce a photon, or a particle of light. Conversely, a particle-antiparticle pair can be produced from a photon. So there is a type of symmetry between particles and antiparticles in these processes. However, for some reason not yet entirely explainable, processes that occurred shortly after the creation of the Universe in the Big Bang resulted in a greater number of particles than antiparticles. This is why we see most matter in the Universe made up of protons, neutrons and electrons rather than antiprotons, antineutrons and positrons (the name given to antielectrons).
There is no evidence to suggest that some parts of the Universe contain a preponderance of particles while other parts contain a preponderance of antiparticles – which is probably lucky, as if our Earth happened to venture into a region of the Universe with a preponderance of antiparticles, we may all end up as photons (light)!
One of the most puzzling phenomena observed in the first part of the 20th century was the behaviour of helium at very low temperatures – with 'very low' meaning just two degrees above ABSOLUTE zero (-271ºC). At these temperatures, the density of helium remains constant independent of temperature, but does not solidify. Normal liquids do not do this.
But the most significant and bizarre observation is that at these really cold temperatures, helium will conduct heat better than ANY metal, and shows NO viscosity. The complete absence of viscosity means that the helium can flow without friction – a hallmark of superfluidity.
For liquid helium this means that some truly odd things happen: when cooled in a hanging dish, a thin film will form and creep up and over the sides of the dish as if it were soup in a ladle determined to get back to the pot! These strange properties were later understood to be very natural examples of purely quantum phenomena.
Helium is just one example of matter that is able to exist in its lowest possible energy state as a collective whole, where all particles have exactly the same energy and even the same identities. Being indistinguishable while simultaneously occupying the same lowest possible energy state is a characteristic of what are called "bosons".
Sometimes bosons display this curious kind of condensation into a 'super' phase. Examples are superfluid helium, electrons in superconducting metals and high temperature superconducting ceramics, and most recently, Bose-Einstein condensates of heavy atoms caught in optical laser traps.