alpha particles/alpha decay Alpha particles are helium nuclei consisting of two protons and two neutrons. With beta particles and gamma radiation, they are one of three types of radiation produced in radioactive decay, when the nucleus of an atom loses some of its mass with a release of energy.
Bohm diffusion A mathematical relationship for the rate at which a plasma (a collection of charged ions) diffuses under the influence of a magnetic field. This process is much more complex than the diffusion of a gas, but is described by a simple formula involving only temperature, magnetic field strength and a constant.
Manhattan Project The Allied Second World War project to make an atomic bomb, set up in response to intelligence that Germany was attempting to build one. Led by and located in the United States, but with significant contributions from the UK and Canada. Named after the temporary headquarters of the Army faction, based on Broadway, the project pulled together many sites, though the eventual focus was on Los Alamos in New Mexico. The first bomb test, codenamed Trinity, took place at what is now the White Sands Proving Ground on 16 July 1945, less than four weeks before the bomb’s deployment in Japan.
photon A quantum particle of light and the carrier of the electromagnetic force. Until the 20th century light was thought to be a wave, but both theory and experiment showed that it can also be treated as a massless particle.
superposition Superposition is a fundamental behaviour of quantum theory that has no equivalent in the macro world of objects we see around us. It says that where a quantum particle has a state that, say, has two possible values – such as spin, which can be ‘up’ or ‘down’ – it will not have an actual value but simply have a probability of being in one state or the other, until it is measured when it collapses to a single, actual value. A tossed coin is a real world item with two states. Before we look at the coin it could be heads or tails with 50 per cent probability – but we know that it actually has one of these values. One side is face up. A quantum particle, though, has no value, just the probabilities while it is in superposition.
thought experiment An experiment that is not actually carried out, but that can be used to demonstrate a concept or an idea. Schrödinger’s Cat (see here) is probably the best-known thought experiment in physics, but Einstein, for example, was always using them, particularly in his attempts to discredit quantum theory. His best-known, the EPR thought experiment (see here), led to the development of actual experiments demonstrating quantum entanglement.
wavefunction In quantum physics, the wavefunction is a mathematical formula that describes the behaviour of a quantum state of the particle, which evolves over time according to the Schrödinger wave equation. The wave in question, which spreads out over time, does not describe the particle itself, but rather the probability of a quantum state having a particular value – so, for instance, it can describe the probability of finding a particle in different locations. The probability is given by the square of the wavefunction.
zero time tunnelling Because a quantum particle does not have an exact location before it is observed, it can pass through an obstacle that it should not be able to get past. This process is known as quantum tunnelling. In experiments in which a particle is measured passing along a route that includes a barrier, the particles tunnelling through the barrier appear to spend no time inside the barrier, hence ‘zero time tunnelling’.
Everyone has experienced a sophisticated quantum device called a beam splitter, because a glass window is a great example. Stand in a room at night with the light on and look out of the window. You can see yourself reflected clearly. But go outside and you can also see into the room. While some light from the room reflects back in – maybe 5 per cent – most passes through. (It occurs all the time, but is only obvious at night, as the reflection is barely visible in daylight.) This presents an interesting problem. Newton thought light was made of particles, but couldn’t explain why a particular particle reflected or passed through glass. He thought it might be caused by imperfections in the surface, although this isn’t supported experimentally. We now know it is due to the quantum nature of photons. We can’t tell if a photon will reflect, merely the probability. The effect is even more remarkable because the percentage reflected from the inner surface depends on the thickness of the glass. The incoming photons are influenced by this because as quantum particles they are sufficiently spread out to interact with the whole sheet, not just the surface.
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A window is a quantum device called a beam splitter that allows a percentage of photons through. This baffled Newton, unaware of the probabilistic nature of quantum particles.
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Beam splitters can entangle particles, even collections of particles. The process starts with a photon sent unobserved through a beam splitter. It is in a superposition of states – we only have probabilities of whether it reflected off or through. Each path interacts with a cloud of atoms ‘say’ then passes through a polarizing beam splitter, choosing direction on polarization. Finally, the photon is detected, triggering the entanglement of the atom clouds. The detail involves messy maths – but the process works.
RELATED THEORIES
3-SECOND BIOGRAPHIES
ISAAC NEWTON
1642–1727
British physicist most famous for gravitation and laws of motion, but also very active in optics
MICHAEL HORNE
1943–
Leading American physicist in quantum theory and entanglement, another beam splitter expert
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Brian Clegg
Most light passes through window glass, but a proportion is reflected, a process Newton struggled to explain.
If a ball rolls up a hill but lacks the energy to get to the top, it will never reach the other side. This seems obvious, but in quantum physics it isn’t true. A quantum object such as an electron or a photon can pass through a barrier even if in classical terms it doesn’t have enough energy. This so-called quantum tunnelling is a consequence of quantum particles not having a well-defined location, just the wavefunction that describes the probability of finding the object at different points in space. The presence of a barrier attenuates the wavefunction but doesn’t shrink it to nothing even on the far side: there is a finite, if small, chance that the object might be found there. Tunnelling plays an important role in several natural phenomena. It is what enables alpha particles to escape from the strong binding forces of an atomic nucleus in radioactive decay; it can speed up the rate of some chemical processes and allows some chemical reactions to proceed in cold interstellar space. The effect is used technologically, for example in certain kinds of diode in which electrons tunnel across the junction between two types of semiconductor.
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Quantum tunnelling is the penetration of quantum particles through a barrier even though in classical terms the particles lack enough energy to surmount it.
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Quantum tunnelling can be useful in semiconductor microelectronics, but it is also a nuisance. As transistors on silicon chips get smaller, the insulating layers separating components become thinner, just a few atoms across. This makes them leaky barriers, since electrons may tunnel through them. Then it becomes impossible to turn the devices off. So far the problem has been solved by replacing the silicon dioxide insulator with a better one made of hafnium dioxide.
RELATED THEORIES
3-SECOND BIOGRAPHIES
FRIEDRICH HUND
1896–1997
German physicist, a pioneer of quantum chemistry who first recognized the importance of tunnelling in the light-emission spectra of molecules
GEORGE GAMOW
1904–68
Russian-born American theoretical physicist who recognized the importance of quantum tunnelling in alpha decay
1947– & 1933–2013
German and Swiss inventors of the scanning tunnelling microscope in the 1980s, for which they received the 1986 Nobel Prize in Physics
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Philip Ball
A tunnelling particle gets around a barrier by moving from A to B without going through the space in between.
A remarkable outcome of quantum physics is that photons can travel faster than the speed of light. Such ‘superluminal’ experiments send photons towards a barrier through which they should not pass. Quantum theory tells us that a photon’s location is not fixed and there is a small probability that it will already be at the other side of the barrier. So a few photons instantly pass through it and carry on. If the barrier is 1 unit of distance wide, and the photon travels the same distance either side of the barrier, it traverses 3 units of distance in the time light usually takes to cover 2 units, moving at 1.5 times the speed of light. Early experimenter Raymond Chiao insists that it is impossible to send a signal this way, because photons that get through a barrier are random. However in 1995, Günter Nimtz modulated the tunnelling beam, demonstrating this by playing a recording of Mozart’s Symphony No. 40 transmitted at over four times the speed of light. There remains dispute over whether the signal truly travels faster than light or is simply distorted by the process, rather like a runner leaning forward to break the tape first.
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Because quantum particles that tunnel through a barrier do so in zero time, photons undergoing tunnelling appear to travel faster than light.
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Most early experiments used highly technical barriers called undersized waveguides or photonic lattices. Nimtz often uses an example of tunnelling discovered by Newton – frustrated total internal reflection. When a beam of light enters a prism at a right angle it bounces off the back of the glass. Newton discovered that a second prism, placed close but not touching, enables part of the beam to flow through instead of reflecting. This happens because photons tunnel through the barrier formed by the gap.
RELATED THEORIES
3-SECOND BIOGRAPHIES
ISAAC NEWTON
1642–1727
British physicist famous for gravitation and laws of motion, but also very active in optics
GÜNTER NIMTZ
1936–
German physicist who has worked on the impact of electromagnetic radiation on humans and superluminal effects of quantum tunnelling
RAYMOND CHIAO
1940–
American physicist who specializes in quantum optics
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Brian Clegg
Nimtz used tunnelling between two prisms to send a Mozart symphony faster than light.
As quantum theory took shape in the 1920s, it looked ever stranger. The Schrödinger equation implied that particles could act like waves. Quantum particles could exist in superpositions. Heisenberg formulated his uncertainty principle. What did it all mean? Niels Bohr, working in Copenhagen and assisted by Heisenberg and others, provided a disconcerting analysis, now known as the Copenhagen Interpretation, which broke with the longstanding belief that science can always find things out. It accepts that in quantum theory there are unanswerable questions, and that one experiment might not be consistent with another. All we can know about the world is what we can measure. Take the classic experiment of a beam of photons being fired through two parallel slits. If you don’t ask which slit the photons pass through, they act like waves forming an interference pattern of bright and dark on the far side. If you set up the apparatus to detect which slit they go through, there’s no interference pattern. But which slit did they ‘really’ go through to cause interference? According to the Copenhagen Interpretation you can’t ask that. There is no essential reality beyond the quantum description, nothing more fundamental than probabilities and measurements.
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The Copenhagen Interpretation states that it is not meaningful to think of anything more fundamental to quantum systems than what we are able to measure.
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A central component of the Copenhagen Interpretation is Bohr’s notion of complementarity. This means you can answer some questions with one experiment, and others with another but they won’t necessarily give consistent results. In one you see a particle; in another a wave. Neither is more true: both are needed. Although many physicists accept this, they are less enthusiastic now about the readiness of Bohr and his followers to extend complementarity to biology, ethics, religion, politics and psychology.
RELATED THEORIES
HEISENBERG’S UNCERTAINTY PRINCIPLE
3-SECOND BIOGRAPHY
1935–
American physicist who famously paraphrased the Copenhagen interpretation as ‘Shut up and calculate!’
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Philip Ball
Complementarity says that light can act as waves (top) or as particles (bottom) but not both simultaneously.
There is an alternative to the Copenhagen Interpretation of what happens when an attempt is made to measure a quantum system, causing the wave function to collapse. It states that there is no such measurement problem in quantum physics since particles are only ever in one place at any time even when no one is around to look at them. For example, in the double-slit experiment, particles do not pass through both slits simultaneously, instead each particle only passes through one slit. In this model, the wave function serves to determine the distribution of particles at the end of the experiment and the apparent localized collapse of the wave function is just the result of making a particular measurement at a particular time on discrete particles that were already following clearly determined paths. Such a causal and deterministic approach to quantum mechanics is in stark contrast to the more widely accepted probabilistic treatment. This radically different view is known as the Bohm Interpretation after David Bohm, the American-born theoretical physicist who developed it. Similar ideas were proposed in the early days of quantum mechanics by Louis de Broglie and the explanation is sometimes called the de Broglie-Bohm theory.
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Bohm attempted to remove the element of chance from quantum mechanics, challenging the widely accepted theory.
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If reality is ultimately based on some deterministic framework, such that every aspect of the world that we inhabit, including the workings of our brains, is determined by the physical laws governing the whole of the universe, then does free will really exist?
RELATED THEORIES
3-SECOND BIOGRAPHY
1892–1987
French physicist who originally pioneered a causal approach to quantum mechanics
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Leon Clifford
Bohm’s interpretation would restore a clockwork universe where everything is predetermined.
A quest for deeper order in the world defined the life of theoretical physicist David Bohm who cast doubt on the Copenhagen Interpretation. This search inspired his work in quantum mechanics, caused him to flirt with Eastern mysticism in his later years and led him during the 1930s towards communism; an affiliation that ultimately required him to leave the United States, the country of his birth, and live in effective exile.
In 1949 Bohm refused to testify to Congress against his former PhD supervisor and suspected communist sympathiser, Robert Oppenheimer. Bohm was arrested and charged with contempt of Congress. He was later tried and acquitted, but the scandal led to him being fired from his job at Princeton. After this, Bohm worked abroad, moving first to Brazil in 1951, then to Israel in 1955. He finally settled in the UK in 1957 and became Professor of Theoretical Physics in 1961 at Birkbeck College, University of London, where he developed the detail of his interpretation of quantum theory.
Two great intellectual friendships influenced Bohm: the physicist Albert Einstein at Princeton and the philosopher Jiddu Krishnamurti in London; both men in their different ways aided Bohm as he sought out order in science and society. Einstein’s nagging doubts about quantum mechanics and his view that ‘God does not play dice’ undoubtedly struck a chord with the young Bohm, while Krishnamurti’s spiritual viewpoint helped Bohm put his idea of the oneness of the universe into a philosophical context.
Bohm came to believe that there is some deeper reality to the universe and that the world we see around us is akin to a ghost, a projection of this hidden truth. For Bohm, true reality could only be glimpsed by a mind that was free from the self-deceptions created by the very process of thinking. His interpretation was that the universe we see – the universe of space and time and particles and quantum mechanics – unfolds naturally out of this deeper underlying reality, what he called the implicate order.
Bohm’s belief resulted in a new interpretation of quantum mechanics, one that invokes a wave function for the whole universe, that can evolve according to the Schrödinger equation and that is deterministic, guiding the path of every particle in existence. This causal and deterministic treatment contrasts with the probabilistic explanation of the Copenhagen Interpretation. Bohm’s unorthodox view of quantum mechanics has never been widely accepted by physicists, but remains a coherent alternative interpretation.
Leon Clifford
20 December 1917
Born in Wilkes-Barre, Pennsylvania, United States
1939
Awarded BSc from Pennsylvania State College
1940
Joins Robert Oppenheimer at University of California, Berkeley, as postgraduate research student
1943
Awarded PhD on the basis of research into nuclear scattering. Works at Berkeley Radiation Laboratory, contributing calculations for the Manhattan Project
1947
Moves to become Assistant Professor of Physics at Princeton, where he works alongside Albert Einstein and conducts research into plasmas, metals and quantum mechanics
1949
Discovers a law governing how plasmas diffuse in magnetic fields – now called Bohm diffusion– and publishes a paper describing the phenomenon
1951
Moves to Brazil and publishes his first book, Quantum Theory, taking a conventional view
1957
Moves to the UK and publishes Causality and Chance in Modern Physics, which expounds his deterministic view of quantum mechanics
1959
With Yakir Aharonov, discovers the Aharonov-Bohm Effect showing that electromagnetic potentials are real, not simply mathematical concepts
1980
Publishes Wholeness and the Implicate Order, outlining his belief that there is a deeper underlying basis for reality
1990
Elected a Fellow of the Royal Society
27 October 1992
Dies in London
1993
The Undivided Universe: An Ontological Interpretation of Quantum Theory, a key text explaining the Bohm Interpretation, co-written with Basil Hiley, published posthumously
Quantum wavefunctions collapse when attempts are made to observe and measure quantum systems. When this occurs, all possible states of the quantum system coalesce into the one observed state, a phenomenon that has given rise to the Copenhagen, Many Worlds and Bohm interpretations of quantum mechanics. But the questions about what causes the wavefunction to collapse and at what point in the measurement process the actual collapse occurs remain subjects for debate. One suggestion (no longer widely held) was that the wavefunction only collapses when a conscious observer is involved in the measurement. Conscious observers can only see the world in one way, therefore they must be in one state or another and cannot be simultaneously in many states; it is this requirement of consciousness to be in a single state that forces the wavefunction to collapse. To explain the idea, physicist Eugene Wigner proposed a version of the famous Schrödinger’s Cat thought experiment in which a friend was placed in the box with the cat. Wigner suggested that the presence of the conscious mind of the friend would cause the wavefunction to collapse inside the box, crystallizing the state of the cat as either alive or dead.
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It seems that you can affect the quantum world just by looking at it – but does this require consciousness?
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The possibility that our minds somehow interact with the quantum world raises the question of whether our consciousness may itself be a quantum phenomenon. After all, our brains are made from atoms and use electrical signals, which are all subject to the laws of physics. Could quantum mechanics one day provide an explanation for the mystery of human consciousness?
RELATED THEORIES
3-SECOND BIOGRAPHIES
EUGENE WIGNER
1902–95
Hungarian-born physicist who first proposed that collapse of the wavefunction occurred as a result of interacting with our consciousness
1903–57
Hungarian-born mathematician, who saw consciousness as a part of the chain involved in the collapsing quantum wavefunction
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Leon Clifford
Some physicists have suggested that it takes a conscious observer, like a human being, to cause a wavefunction to collapse.
Many physicists accept the Copenhagen Interpretation of quantum physics that quantum particles genuinely can be in more than one state simultaneously, and that the probability wave that predicts their position enables them to act as if they were in more than one place. For some, though, this is a step too far. Hugh Everett was determined to find a way of rationalizing the strange behaviour of quantum particles. In his controversial PhD thesis he presented the theory that would dominate his working life, the Many Worlds Interpretation. It dispenses with the idea of waveforms collapsing to provide a specific value on being observed. Instead, according to Many Worlds, each time a quantum particle can have more than one state, the world branches. The particle exists in one state in one version of the universe and in the other in the second. The reality we experience is just a single path through each of these worlds. This means we no longer need worry how a photon or electron somehow interferes with itself when passing through a double-slit experiment – in one universe it goes through the first slit, in another it goes through the second. Although we only experience a single universe directly, we can see the result of the different universes interacting in the interference pattern of light and dark fringes produced by the double slits.
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Originally called relative state formulation, the 1957 theory now known as Many Worlds proposes the existence of an infinite set of parallel universes.
3-MINUTE THOUGHT
If Many Worlds is true then that most famous paradox of quantum theory, Schrödinger’s Cat, is no longer a problem. In one world the cat is alive, in another it is dead. It can not be both simultaneously in the same world. All possible outcomes of each quantum decision exist in their own branches of reality. However, many physicists believe the extra complexity of Many Worlds, requiring a new world each time every particle in the universe undergoes a change, is too high a price to pay to get around the strangeness of the Copenhagen Interpretation.
RELATED THEORIES
3-SECOND BIOGRAPHIES
HUGH EVERETT III
1930–1982
American physicist who first proposed the interpretation
BRYCE DEWITT
1923–2004
American physicist who popularized and named the Many Worlds Interpretation
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Brian Clegg
In Many Worlds, whenever there is more than one possible outcome at the quantum level, there is a different world to accommodate it.
