If successful scientific theories can be thought of as cures for stubborn problems, quantum physics was the wonder drug of the 20th century. It successfully explained phenomena such as radioactivity and antimatter, and no other theory can match its description of how light and particles behave on small scales.
But it can also be mind-bending. Quantum objects can exist in multiple states and places at the same time, requiring a mastery of statistics to describe them. Rife with uncertainty and riddled with paradoxes, the theory has been criticised for casting doubt on the notion of an objective reality - a concept many physicists, including Albert Einstein, have found hard to swallow.
Today, scientists are grappling with these philosophical conundrums, trying to harness quantum's bizarre properties to advance technology, and struggling to weave quantum physics and general relativity into a seamless theory of quantum gravity.
The birth of an idea
Quantum theory began to take shape in the early 20th century, when classical ideas failed to explain some observations. Previous theories allowed atoms to vibrate at any frequency, leading to incorrect predictions that they could radiate infinite amounts of energy - a problem known as the ultraviolet catastrophe.
In 1900, Max Planck solved this problem by assuming atoms can vibrate only at specific, or quantised, frequencies. Then, in 1905, Einstein cracked the mystery of the photoelectric effect, whereby light falling on metal releases electrons of specific energies. The existing theory of light as waves failed to explain the effect, but Einstein provided a neat solution by suggesting light came in discrete packages of energy called photons - a brain wave that won him the Nobel Prize for Physics in 1921.
In fact, light's chameleon-like ability to behave as either a particle or a wave, depending on the experimental setup, has long stymied scientists. Danish physicist Niels Bohr explained this wave-particle duality by doing away with the concept of a reality separate from one's observations. In his "Copenhagen interpretation", Bohr argued that the very act of measurement affects what we observe.
One controversial experiment recently challenged this either/or scenario of light by apparently detecting evidence of both wave- and particle-like behaviour simultaneously. The work suggests there may be no such thing as photons - light appears quantised only because of the way it interacts with matter.
Other interpretations of quantum theory - of which there are at least half a dozen - deal with the measurement problem by suggesting even more far-fetched concepts than a universe dependent on measurement. The popular many worlds interpretation suggests quantum objects display several behaviours because they inhabit an infinite number of parallel universes.
For about 70 years, this wave-particle duality was explained by another unsettling tenet of quantum theory - the Heisenberg uncertainty principle. Formulated by Werner Heisenberg in 1927 and recently made more precise, the theory puts an upper limit on knowledge. It says one can never know both the position and momentum of a quantum object - measuring one invariably changes the other.
Bohr defeated Einstein in a series of thought experiments in the 1920s and 1930s using this principle, but more recent work suggests the underlying cause of the duality seen in experiments is a phenomenon called entanglement.
Entanglement is the idea that in the quantum world, objects are not independent if they have interacted with each other or come into being through the same process. They become linked, or entangled, such that changing one invariably affects the other, no matter how far apart they are - something Einstein called "spooky action at a distance".
This may be involved in superconductivity and may even explain why objects have mass. It also holds promise for "teleporting" particles across vast distances - assuming everyone agrees on a reference frame. The first teleportation of a quantum state occurred in 1998, and scientists have been gradually entangling more and more particles, different kinds of particles, and large particles.
Entanglement may also provide a nearly uncrackable method of communication. Quantum cryptographers can send "keys" to decode encrypted information using quantum particles. Any attempt to intercept the particles will disturb their quantum state - an interference that could then be detected.
In April 2004, Austrian financial institutions performed the first money transfer encrypted by quantum keys, and in June, the first encrypted computer network with more than two nodes was set up across 10 kilometres in Cambridge, Massachusetts, US.
But keeping quantum particles entangled is a tricky business. Researchers are working on how to maximise the particles' signal and distance travelled. Using a sensitive photon detector, researchers in the UK recently sent encrypted photons down the length of a 100-kilometre fibre optic cable. Researchers in the US devised a scheme to entangle successive clouds of atoms in the hopes of one day making a quantum link between the US cities of Washington, DC, and New York.