The Big Questions: Physics (30 page)

The quantum version of information theory says an entangled pair can carry only two bits of information. If those two bits encode something like ‘the spins are the same when measured in the X dimension’, and ‘the spins are opposite when measured in the Y dimension’, that gives a description of the spin states of both particles – but leaves no room for information about the spin of an individual particle.

That’s why the first measurement appears to give a random result, yet the result of the second measurement can be predicted with perfect accuracy. Though it gives the illusion of a ‘spooky’ transfer of information between the particles, it’s actually just that the first measurement gives us more information. Given the first result, and the nature of the link between the spins, the second particle’s spin can be deduced with simple logic.

Quantum researchers have only just begun to appreciate that information might be the key to understanding their discipline, and don’t yet have many solid explanations for how this might work. But if information does lie at the root of quantum theory, that seems somehow appropriate. We are living in what has been dubbed the ‘information age’, where optical fibres and satellite transmissions fire information around the world at astonishing speeds and intensities. All these technologies work because of our understanding of the quantum world – the laser and the microchip are both spin-offs from quantum theory. It seems only right that the last question in physics should tie information theory and quantum theory together.

So where does this leave us in the search for the ultimate nature of reality? If you can describe anything as an ‘it’, as a real entity, it ultimately appears to come from a bit of information – or a large collection of them. We get, as the physicist John Archibald Wheeler put it, ‘it from bit’. In 1990, Wheeler declared that,
‘Tomorrow we will have learned to understand and express all of physics in the language of information.’ That ‘tomorrow’ has not yet come, but perhaps it is appearing on the horizon at last.

However, we simply cannot know how far we are on the path to discovering the ultimate nature of reality. During this century our investigations of reality have taken us from the realm of the atomic to the subatomic, right down to the idea of energetic fluctuations in the fabric of space and time. It seems that the fundamental nature of reality goes deeper than this, into abstract notions of mathematics and information. But is that the end?

‘Tomorrow we will have learned to understand and express all of physics in the language of information.’

JOHN ARCHIBALD WHEELER

Physicists are painfully aware that any and all their conjectures could be a million miles from the truth. They work within the current limits of knowledge and the limits of the human imagination. Both seem to recede as we discover more about the world, but never disappear. If the end of physics is on the horizon today, it is worth remembering that it has always seemed to be there. It would be hubris to think we are taking the final steps towards understanding the very core of reality; there is undoubtedly plenty of distance left for physicists to cover. But when the journey is so deeply fascinating, that can only be cause for celebration.

absolute zero

–273 celsius: the temperature at which no substance contains any heat energy.

alpha radiation

A non-penetrating but potentially damaging form of radiation. Alpha particles are identical to the helium nucleus, containing two protons and two neutrons.

anthropic principle

The notion that asking why the universe is as we see it is essentially pointless because we could not exist in any universe that is substantially different.

antimatter

The ‘nemesis’ of matter: every matter particle has an antimatter counterpart. When a particle and its antiparticle collide, they annihilate one another.

beta radiation

Radiation composed of high energy electrons or their antiparticles (positrons) that is readily stopped by a thin sheet of metal. It is caused by nuclear processes that involve the weak nuclear force.

Big Bang

The moment, nearly 14 billion years ago, when the universe came into existence.

black hole

A patch of space–time where gravity is so strong that nothing (not even light) can escape. Black holes are often formed when a giant star collapses under the pull of its own gravity.

boson

A particle with a quantum ‘spin’ number that is an integer. Bosons are the particles through which forces act.

closed timelike curve

A region of space–time that, if followed, brings one back to the same moment in time.

compactification

The process by which the ‘extra’ dimensions suggested by many modern theories of physics remain undetected.

cosmic microwave background radiation

The radiation released around 300,000 years after the Big Bang. The CMB radiation contains many clues to the nature of the early universe.

dark energy

A mysterious form of energy that many physicists believe to be responsible for the accelerating expansion of the universe.

dark matter

Hypothetical matter that, according to most astronomers, makes up nearly a quarter of the mass/energy in the universe. Dark matter is scattered throughout the universe, but concentrated in haloes around galaxies and galaxy clusters.

double slit experiment

An experiment originally created to demonstrate the wave nature of light. It has also been used to show that quantum particles have wave-like characteristics.

electromagnetic force

A force that causes particles with an electric charge to repel or attract one another, depending on the signs of their charge.

electron

A subatomic particle, believed to carry the fundamental unit of electric charge.

electroweak interaction

A force, thought to exist in the hot conditions of the early universe, which gave rise to the electromagnetic and weak forces.

entanglement

A phenomenon that occurs when two quantum particles interact. They become linked, with information about both particles residing in each one.

entropy

A measure of the disorder of a physical system. The entropy of a closed system always increases.

ether

A fluid that was thought to fill the universe and carry electromagnetic waves such as light. Its existence was disproved in 1887.

fermion

A matter particle whose quantum spin is half of a whole number.

fractal

A geometry that looks the same whatever the scale on which it is viewed.

fundamental forces

The four fundamental forces are: strong; electromagnetic; weak; gravity. See the table on
page 204
.

gamma radiation

A highly penetrating radiation that results from the emission of high-energy (gamma ray) photons in nuclear reactions.

gas

A fluid composed of particles that are only very weakly attracted to one another.

general relativity

Albert Einstein’s description of the warping of space and time by the presence of mass and energy.

geodynamo

The turbulent ball of churning molten iron that occupies the inner core of the Earth and creates a self-sustaining magnetic field.

gluon

The particle that mediates the strong nuclear force.

grand unified theories (GUTs)

Theories that attempt to define how three of the forces of nature (strong, weak and electromagnetic) were once one in the conditions of the early universe.

gravity

The attractive force that acts between particles with mass or energy.

Heisenberg uncertainty principle

A rule that imposes a limit on the accuracy with which certain combinations of properties can be known for any quantum particle or system.

hidden extra dimensions

Certain theories suggest that there are more spatial dimensions than the ones (up–down; back–forth; across) we experience.

Higgs boson

A hypothetical particle that is thought to mediate the Higgs field, the source of certain types of mass.

inflation

A theory that suggests the universe went through a period of ultra-fast expansion just after the Big Bang.

interference

A wave phenomenon where two waves interact, producing a wave whose properties depend on the size and relative phase of the original waves.

inverse square law

When a force between two spatially separated objects diminishes, by an amount proportional to the square of the distance between them, as they are moved further apart.

kinetic energy

Energy associated with motion.

liquid

A state of matter in which particles have some attractive forces acting between them, and can only be further separated by an input of energy.

magnetosphere

The magnetic field surrounding the Earth.

many worlds theory

The idea, originated by Hugh Everett III, that every quantum event creates a new and separate universe. Only quantum particles such as electrons are able to sense the existence of these other worlds.

mass

The attribute of an object that gives resistance to acceleration (inertial mass) or responds to and creates the gravitational force (gravitational mass). Physicists believe the two types of mass are equivalent but this is not proven.

Maxwell’s equations

The equations, laid out by James Clerk Maxwell, that define how electric and magnetic fields behave and interact.

momentum

The product of the mass and velocity of a particle.

multiverse

A universe composed of myriad smaller universes, usually without any connection that allows passage between them.

neutron

A particle composed of three quarks, but with no net electric charge.

nucleus

The central core of an atom where almost all of its mass is concentrated.

particle accelerator

A machine used to smash subatomic particles into one another. Analysis of the resulting debris can give clues to the fundamental constituents of matter and the nature of the universe.

perpetual motion machine

A hypothetical machine that would do useful work with no energy being supplied.

photon

A particle of light or other electromagnetic energy.

potential energy

The energy an object contains because of its position within a field (usually gravitational or electric).

proton

A positively charged particle composed of three quarks.

quantum

A fundamental unit, originally of energy but now applied to anything indivisible, such as the electron charge, in subatomic physics.

quantum chromodynamics (QCD)

A theory that describes the interactions of quarks and gluons in nuclear physics.

quantum electrodynamics

A theory that describes the interaction of the electromagnetic force with matter.

quantum gravity

A theory that will bring together quantum theory and relativity to create an overarching description of how things behave over very small and very large scales. String theory is one attempt to build a quantum gravity theory.

quark

A subatomic particle associated with atomic nuclei. Protons and neutrons are each composed of three quarks.

Schrödinger equation

The equation used to define how matter behaves at subatomic scales.

solar wind

A stream of charged particles that fly off from the surface of the sun. The solar wind is responsible for the
aurora borealis
(the northern and southern lights).

solid

The state of matter where particles are strongly bound together in a rigid structure.

space–time

The four dimensional fabric of the universe. Any one point in space–time is known as an ‘event’: a specific time and location.

special relativity

Einstein’s 1905 theory that laid out a way for the laws of physics to be the same for all observers, regardless of how they were moving through space and time. He later generalized the theory to include the effects of gravitational fields.

standard model

The theory of physics that describes the properties of all known particles, and their interactions.

string theory

A description of the subatomic world where particles and forces are described by the vibrations of strings and loops of energy.

strong force

The short-ranged force that binds quarks into protons and neutrons, and overcomes the repulsion of the positively charged protons to hold atomic nuclei together.

superconductor

A material that offers no resistance to the flow of electrical current.

superfluid