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.
Quantum weirdness
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.
Uncertainty rules
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.
Secure networks
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.
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| In this computer artwork, the funnels represent several different universes being created at the same time. Each parallel universe may have different physical laws to our universe (central funnel), with differing levels of stability and expansion. Some universes may even be devoid of any matter (empty funnel, right) (Image: Mark Garlick / SPL) |
Lightning-fast computers
Quantum computers are another long-term goal. Because quantum particles can exist in multiple states at the same time, they could be used to carry out many calculations at once, factoring a 300-digit number in just seconds compared to the years required by conventional computers.
But to maintain their multi-state nature, particles must remain isolated long enough to carry out the calculations - a very challenging condition. Nonetheless, some progress has been made in this area. A trio of electrons, the building blocks of classical computers, were entangled in a semiconductor in 2003, and the first quantum calculation was made with a single calcium ion in 2002. In October 2004, the first quantum memory component was built from a string of caesium atoms.
But particles of matter interact so easily with others that their quantum states are preserved for very short times - just billionths of a second. Photons, on the other hand, maintain their states about a million times longer because they are less prone to interact with each other. But they are also hard to store, as they travel, literally, at the speed of light.
In 2001, scientists managed to stop light in its tracks, overcoming one practical hurdle. And the first quantum logic gate - the brains behind quantum computers - was created with light in 2003.
Quantum gravity
While three of the four fundamental forces of nature - those operating on very small scales - are well accounted for by quantum theory, gravity is its Achilles heel. This force works on a much larger scale and quantum theory has been powerless so far to explain it.
A number of bizarre theories have been proposed to bridge this gap, many of which suggest that the very fabric of space-time bubbles up with random quantum fluctuations - a foam of wormholes and infinitesimal black holes.
Such a foam is thought to have filled the universe during the big bang, dimpling space-time so that structures such as stars and galaxies could later take shape.
The most popular quantum gravity theory says that particles and forces arise from the vibrations of tiny loops - or strings - just 10-35 metres long. Another says that space and time are discrete at the smallest scales, emerging from abstractions called "spin networks".
One recent theory, called "doubly special relativity", tweaks Einstein's idea of one cosmic invariant - the speed of light - and adds another at a very small scale. The controversial theory accounts for gravity, inflation, and dark energy. Physicists are now devising observations and experiments that could test the competing theories.
Economies of scale
Quantum physics is usually thought to act on light and particles smaller than molecules. Some researchers believe there must be some cut-off point where classical physics takes over, such as the point where the weak pull of gravity overwhelms other forces (in fact, gravity's effect on neutrons was recently measured). But macroscopic objects can obey quantum rules if they don't get entangled.
Certainly, harnessing troops of atoms or photons that follow quantum laws holds great technological promise. Recent work cooling atoms to near absolute zero have produced new forms of matter called Bose-Einstein and fermionic condensates. These have been used to create laser beams made of atoms that etch precise patterns on surfaces, and might one day lead to superconductors that work at room temperature.
All of these hopes suggest that, as queasy as quantum can be, it remains likely to be the most powerful scientific cure-all for years to come.
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