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Introduction to the strange world of Quantum Physics
Terminology Note: Quantum physics, quantum theory, quantum mechanics and wave mechanics all refer to the
same field of study in physics. Quantum physics deals with the world on microscopic level.
The discovery in the 1920s of quantum theory bought about the biggest revolution in physics since the time of Isaac
Newton. To understand the full impact of quantum theory we also need to have an understanding of the important
features of what is known as classical physics.
A health warning: Anyone who can contemplate quantum mechanics without getting dizzy hasnt understood it.
A Brief History of Classical Physics:
Newton published his volume on physics called principia (three editions between 1687-1726) and
established the field of physics called mechanics which enabled scientists to describe the motion of objects
using mathematical techniques.
James Maxwell towards the end of 19th century found it was possible to link electricity and magnetism
together in one set of mathematical equations even though electricity and magnetism appeared different in
nature. Maxwell realised that his mathematical equations for electromagnetism have solutions which were
wave like and the velocity of these waves was the speed of light. It was demonstrated that light was made
up of electromagnetic waves.
The theories of Newton and Maxwell are the two main theories on which all of classical physics (as it is now
known) is built. Their descriptive power was so great that at the end of the 19th century it was thought that
all the major problems in physics had been solved and it was just a matter of using the existing theories to
describe the world with greater and greater accuracy.
Characteristics of Classical Physics:
In Newtons mechanics, the laws of motion are written in terms of particle trajectories.
There are no restrictions on the values of physical properties (e.g. energy, speed, position etc.) that
something can possess. Quantities are allowed to vary continuously.
Physical quantities such as velocity and location can all be known with arbitrary precision you just have to
make better measurements.
There are two basic physical phenomena in classical physics which are mutually exclusive. Objects can either
have the form of an extended field (or wave) or a localised particle but not both. Fields (or waves) and
particles are different and independent phenomena, however they can interact with each other e.g.
electromagnetic wave interacting with a charged particle.
The interaction of fields tails off with distance and therefore classical physics is essentially a locally based
theory which means an object can only be affected by things its the immediate neighbourhood.
Classical physics is a completely deterministic view of nature. If you know the values of all physical quantities
describing a situation then you can accurately say what the values of the physical quantities would be at
some later point in the future AND you could say what they would have been at some point earlier in the
past. There is a casual link between past, present and future.
o e.g. using Newtons second law of motion: Force = mass x acceleration
o IF you know all: particle masses, particle positions and velocities at specific time and the forces
acting on all particles at all times.
o THEN you can predict the motion of the particles accurately for all time.
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o The picture of a clockwork universe: A link to deism. God set the universe in motion and then leaves
it on its own.
The quantum revolution
However cracks began to appear in the classical physics picture of the world:
Physicists studying the behaviour of what is known as a black body using classical physics found a disturbing
problem with their mathematical predictions. A black body in physics is one which is a perfect absorber, it
absorbs all electromagnetic radiation that falls on it and then re-emits it. Classical physics predicted a black
body should emit at infinite intensity at high frequencies of radiation. This did not agree with experiments!
The assumption in classical physics was that the black body was absorbing electromagnetic radiation
continuously. Max Planck provided the solution to the problem by suggesting that radiation was emitted and
absorbed from time to time in packets of energy of a definite size. The energy of these so called quanta
(energy packets) would be proportional to the frequency of radiation. The greater the frequency the greater
the energy of the quanta. This was the first indication that light may not be a continuous wave but small
The idea that light came in small quanta was reinforced when Einstein came up with an explanation for the
photoelectric effect which Hertz had discovered in 1887. Here electrons are emitted from a metal when
exposed to electromagnetic waves.
The problem was that classical physics predicted that the whatever kind of electromagnetic radiation you
exposed the metal to, if it was intense enough, electrons should be emitted
However, experiments showed that the emission of electrons depends on the frequency of the
electromagnetic waves rather than the intensity. There is threshold frequency below which no electrons will
be emitted however intense the electromagnetic waves are. Over this frequency electron emission happens
and even a weak intensity beam can eject some electrons.
Einstein used Plancks theory of quanta to give an explanation for the photoelectric effect therefore showing
that electromagnetic radiation is a beam of individual quanta or discrete packets of energy like a particle.
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Diagram of the maximum kinetic energy of electrons emitted as a function of the frequency of light on zinc
Diagram from: http://en.wikipedia.org/wiki/Photoelectric_effect (Accessed 16/9/2014)
Another nail in the coffin of classical physics came when in 1911 Ernest Rutherford discovered that atoms
had a central core made up of a positive charge. This suggested a solar system model for the atom, with a
positive charge at the centre and electrons orbiting around the outside at any distance from the centre.
However classical physics predicted that the orbiting electrons as they move around the centre should emit
electromagnetic waves and therefore lose energy. As they lost energy the electrons would collapse towards
the centre of the atom and the atom become unstable.
Diagram from: http://en.wikipedia.org/wiki/Bohr_model (Accessed 16th September 2014)
The solution to this problem was suggested when Neils Bohr in 1913 following Plancks idea of discrete
energy packets proposed that electrons could only exist at a certain number of discrete distances from the
centre. An atom had a lowest state of energy and when the electron was in this orbit it could not lose any
more energy, therefore the atom became stable and did not collapse in on itself. If an electron moved
between orbits it did so by emitting or absorbing a discrete amount of energy (quanta). Bohrs ideas turned
out to be right, but he had added an ad hoc idea to what was essentially a picture based on classical physics.
It took about another ten years before a fully quantum mechanical picture of the atom would be formulated.
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Louis de Broglie made the suggestion that if light had particle-like properties as well as wave-like properties,
then we can expect particles to have wave-like properties. This is known as wave-particle duality
sometimes a microscopic object will behave like a wave and sometimes it will behave like a particle.
Quantum mechanics was fully born when in 1925 Erwin Schrodinger discovered his so called wave equation
which describes microscopic phenomena of particles:
( ) ( ) ( )
Quantum mechanics has been a very successfully theory and the reason we have mobile phones and laptops
we can carry around today are all because of the principles of quantum mechanics. Our understanding of
mathematics and consequences of the quantum mechanical picture had developed radically since the early
years of the theory.
The consequences of the quantum revolution
The quantum revolution bought about a radical change in our view of the world. Classical physics still works well at
the macroscopic (large scale) level we experience in everyday life but quantum mechanical effects become more
important at the microscopic level (small scale). What are some of the characteristics of quantum theory?
Physical properties of particles can vary continuously or be limited to discrete values.
An object could be anywhere in space, therefore its position can vary continuously.
An object (e.g. in the case of an atom which we mentioned earlier) can only take on discrete energy values.
Whereas classical physics spoke about certainties, quantum physics talks about probabilities. Classical physics would
say a particle is actually at a specific place at a specific time, but standard quantum physics speaks about the
probabilities of particles being found (or measured) at a specific place at a specific time.
The probability of a particle being found (or measured) at position, x, at time, t, = | ( )| (known as the Born rule)
Standard quantum physics says that there is some kind of probability built into the way nature works.
The Heisenberg Uncertainty Principle and complementarity
It turns out that the wave-like nature of particles means that there is a deep connection between the physical
property of momentum (in overly simplistic terms we can consider this a measure of particle speed) and position.
The more accurately we measure the position of a particle, the less accurate its momentum is. The more accurately
we measure the momentum the less accurate its speed is. This is known as the uncertainty principle which was
discovered by Heisenberg in 1927. It says that we cannot simultaneously determine both position and momentum of
particle at the microscopic level with arbitrary precision. The inability to determine both position and momentum at
the same time is not because of the inaccuracy of our measuring device but is inherent in the wave properties of
matter. This different to classical physics where a particles position and momentum can be measured to arbitrary
The Heisenberg uncertainty principle is an example of what is known as complementarity. In quantum physics some
physical properties of matter form complementary pairs. Any attempt to measure one property of a pair will lead to
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uncertainty in the complementary property. Another example of a complementary pair of properties in quantum
physics is time and energy.
Wave-particle duality and complementarity
Another kind of complementarity spoken about in Quantum physics is linked to the complementary properties of
matter or wave-particle duality. Niels Bohr emphasised that we should take the wave-like and particle-like properties
of matter together and not assume they implied a contradiction. Each picture complemented the other rather than
conflicting. The emphasis was they each picture corresponded to different and mutually exclusive sets of
experiments. You could set up experiments to explore the wave-like properties of matter and you could set up
experiments to explore the particle like properties of matter but you could not do both at the same time. We think
of an electron as having both wave-like aspects and the particle-like aspects. These are different forms of the same
material object which we call an electron. It is how we interact with that electron which determines which aspect
(particle-like or wave-like) we see exhibited.
Superposition of states
In classical physics a particle could only be in one state, but in quantum physics a particle can be in a mixture of
many states at the same time. The story is told that Paul Dirac who is important in the early development of
quantum physics described it like this:
He took a piece of chalk and broke it in two. Placing one fragment on one side of his lectern and the other on the
other side, Dirac said that classically there is a state where the piece of chalk is here and one where the chalk is
there, and these are the only two possibilities. Replace the chalk, however, by an electron and in the quantum
world there are not only states of here and there but there are also of whole host of others states that are
mixtures of these possibilities a bit of here and a bit of there added together. (Polkinghorne, Quantum Theory:
A very short introduction, p.21)
This is counterintuitive but this it is what distinguishes the quantum world from the classical world. This is known as
the superposition of states and is responsible for quantum phenomena we can see. This can be demonstrated by
what is known as the double slit experiment:
Fire a beam of electrons at a double slit and detect them on a screen the other side.
16th September 2014)
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If only one electron is arriving at the screen at a time, which slit did the electron go through?
From a classical physics perspective, because an electron is a particle it can only go through one slit at a time. If it
went through the top slit then we could ignore the bottom slit and temporarily close it up and this would have no
effect on the experiment outcome.
However from a quantum physics point of view if we close a slit, the probability of an electron being detected at
specific point on the screen is different from if both slits were open. The presence of the other slit is significant and
affects the results of the experiment. In quantum physics we have a superposition (mixture) of the states for the
electron both going through the top slit and the bottom slit which makes the electron behave like a wave and
produce the characteristic pattern of light and dark shades on the screen.
In some way we have to say the electron went through both slits. From a classical physics perspective this is
nonsense, from a quantum physics perspective it makes perfect sense. The consequences of a superposition of
states can be huge!
Determinism and Indeterminism
In classical physics if you know the all the quantities which describe the state of a system. You can describe how the
system evolves in time. The previous state of the system determines the later states.
In quantum physics where you work on the microscopic level it is different. The superposition principle me...