What If...

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What If...
Each choice we make necessarily precludes others we might have made.  Or so we thought.  But what if Everett were right?  The year 2033 - on the front pages of the world’s papers, the news that a quantum computer has been processing more bits, or qubits to be exact, of information than there are atoms in the visible universe.  The debate about the ‘many worlds’ interpretation continues with renewed vigour.
I look down into the chasm where the water below swirls and eddies.  Can it be true that endless permutations of my identity are being played out across the many parallel streams of reality?  I am gripped by sense of vertigo.  Somewhere out there, another self, braver than I now feel.  One who will not hesitate to cross the rope bridge that spans this chasm.  And I am ashamed of myself as the others of my party, already on the bridge, urge me on.  In another universe, the branch of the tree my other self had been climbing when she was seven had not snapped.  In this universe, it snapped and I fell, fracturing several bones.  The child, I once was, is here too, in my thoughts.  ‘The little monkey’, my mother called me as I once scaled the tallest trees without fear.  Both urging me on as I hesitate.  Both mocking me for my timidity.  Both strangers to me. 
Is the child ever a stranger to the adult?  Even after the accident, I believe the essence of me had not changed.  I still painted, was still reprimanded for my dreaminess and untidy ways.  I had just lost my nerve, that was all.  And my other self/selves, how had that random event of the tree branch’s not snapping altered the course of her life?  Enough to make us now strangers to one another?  We parted ways with that single event and have been on different trajectories ever since. 
“Don’t look down,” a member of my party calls back to me.  They are safe on the other cliff-edge now.  I grasp the guide ropes tightly with both hands as I feel the single rope sway slightly beneath my feet.  Only a metre from the cliff and I cannot make the next step, cannot slide my hands along the guide rope.  And I look down.  Fixated on the patterns made by those swirling eddies below.  Each one a topologically stable form in the constant flux of the flowing stream.  Each one’s form subtly changing in small incremental steps through time.  Yet if enough time had passed would it be right to say that the eddy directly below me shares its identity with its later incarnation some distance downstream?  Were I to watch it continuously, however, I could not have pinpointed the moment when the one becomes the other.  
Was it the failure of the branch to snap in those other universes where I continue to be brave, the moment in which our characters diverged?  Possibly not.  But rather they diverged as a result of small variations of no moment leading out from that event, or non-event, which are amplified as they traverse the many branches of the multiverse.  I must be the brave one now as those others on the opposite cliff-edge urge me forward.  As I release my grip on the guide rope and slide my hand along sufficiently to take the next step, I hesitate.  Just long enough.  Before an enormous crack.  The branch of a eucalypt crashes down on the bridge not two metres from where I am standing.  I feel the reverberation of its fall along the length of the rope.
An echo of terror from another self.  Has my choice condemned her?  If I had first heeded that summons to courage from the child I once was, or a self for whom the branch had not snapped when she was seven, I would now be with my companions on the opposite cliff-edge.  At least I am safe though separated from my party.  It could have been worse.  And that other self, one who like me had not followed with the others, but waited fearfully on the this side, who did not hesitate as I did at the crucial moment, what has been her fate?  All possible choices I could have made in that moment and all those leading up to it have been played out across the multiverse.  Remorse, it makes no sense to feel remorse.  For there are yet others who have also been saved by my cowardice - or her last minute act of courage.  And others for whom the choice of neither of us on this fateful day has determined their destiny.  All destinies of all my selves following all possible paths.  On this cliff-edge, in this universe, I am consoled with the knowledge that I have been saved by only one choice.  A choice that has precluded a potentially fatal one.

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Quantum Theories

U is for ... Uncertainty Principle

One of the most famous ideas in science, this declares that it is impossible to know all the physical attributes of a quantum particle or system simultaneously.

P is for ... Probability

Quantum mechanics is a probabilistic theory: it does not give definite answers, but only the probability that an experiment will come up with a particular answer. This was the source of Einstein’s objection that God “does not play dice” with the universe.

L is for ... Large Hadron Collider (LHC)

At CERN in Geneva, Switzerland, this machine is smashing apart particles in order to discover their constituent parts and the quantum laws that govern their behaviour.

A is for ... Act of observation

Some people believe this changes everything in the quantum world, even bringing things into existence.

A is for ... Alice and Bob

In quantum experiments, these are the names traditionally given to the people transmitting and receiving information. In quantum cryptography, an eavesdropper called Eve tries to intercept the information.

Q is for ... Qubit

One quantum bit of information is known as a qubit (pronounced Q-bit). The ability of quantum particles to exist in many different states at once means a single quantum object can represent multiple qubits at once, opening up the possibility of extremely fast information processing.

X is for ... X-ray

In 1923 Arthur Compton shone X-rays onto a block of graphite and found that they bounced off with their energy reduced exactly as would be expected if they were composed of particles colliding with electrons in the graphite. This was the first indication of radiation’s particle-like nature.

M is for ... Multiverse

Our most successful theories of cosmology suggest that our universe is one of many universes that bubble off from one another. It’s not clear whether it will ever be possible to detect these other universes.

B is for ... Bose-Einstein Condensate (BEC)

At extremely low temperatures, quantum rules mean that atoms can come together and behave as if they are one giant super-atom.

Y is for ... Young's Double Slit Experiment

In 1801, Thomas Young proved light was a wave, and overthrew Newton’s idea that light was a “corpuscle”.

O is for ... Objective reality

Niels Bohr, one of the founding fathers of quantum physics, said there is no such thing as objective reality. All we can talk about, he said, is the results of measurements we make.

K is for ... Kaon

These are particles that carry a quantum property called strangeness. Some fundamental particles have the property known as charm!

S is for ... Schrödinger Equation

This is the central equation of quantum theory, and describes how any quantum system will behave, and how its observable qualities are likely to manifest in an experiment.

C is for ... Cryptography

People have been hiding information in messages for millennia, but the quantum world provides a whole new way to do it.

U is for ... Universe

To many researchers, the universe behaves like a gigantic quantum computer that is busy processing all the information it contains.

R is for ... Radioactivity

The atoms of a radioactive substance break apart, emitting particles. It is impossible to predict when the next particle will be emitted as it happens at random. All we can do is give the probability that any particular atom will have decayed by a given time.

C is for ... Computing

The rules of the quantum world mean that we can process information much faster than is possible using the computers we use now.

I is for ... Information

Many researchers working in quantum theory believe that information is the most fundamental building block of reality.

S is for ... Schrödinger’s Cat

A hypothetical experiment in which a cat kept in a closed box can be alive and dead at the same time – as long as nobody lifts the lid to take a look.

R is for ... Randomness

Unpredictability lies at the heart of quantum mechanics. It bothered Einstein, but it also bothers the Dalai Lama.

G is for ... Gluon

These elementary particles hold together the quarks that lie at the heart of matter.

E is for ... Entanglement

When two quantum objects interact, the information they contain becomes shared. This can result in a kind of link between them, where an action performed on one will affect the outcome of an action performed on the other. This “entanglement” applies even if the two particles are half a universe apart.

T is for ... Teleportation

Quantum tricks allow a particle to be transported from one location to another without passing through the intervening space – or that’s how it appears. The reality is that the process is more like faxing, where the information held by one particle is written onto a distant particle.

S is for ... Superposition

Quantum objects can exist in two or more states at once: an electron in superposition, for example, can simultaneously move clockwise and anticlockwise around a ring-shaped conductor.

J is for ... Josephson Junction

This is a narrow constriction in a ring of superconductor. Current can only move around the ring because of quantum laws; the apparatus provides a neat way to investigate the properties of quantum mechanics.

A is for ... Atom

This is the basic building block of matter that creates the world of chemical elements – although it is made up of more fundamental particles.

H is for ... Hawking Radiation

In 1975, Stephen Hawking showed that the principles of quantum mechanics would mean that a black hole emits a slow stream of particles and would eventually evaporate.

W is for ... Wave-particle duality

It is possible to describe an atom, an electron, or a photon as either a wave or a particle. In reality, they are both: a wave and a particle.

P is for ... Planck's Constant

This is one of the universal constants of nature, and relates the energy of a single quantum of radiation to its frequency. It is central to quantum theory and appears in many important formulae, including the Schrödinger Equation.

Q is for ... Quantum biology

A new and growing field that explores whether many biological processes depend on uniquely quantum processes to work. Under particular scrutiny at the moment are photosynthesis, smell and the navigation of migratory birds.

D is for ... Decoherence

Unless it is carefully isolated, a quantum system will “leak” information into its surroundings. This can destroy delicate states such as superposition and entanglement.

L is for ... Light

We used to believe light was a wave, then we discovered it had the properties of a particle that we call a photon. Now we know it, like all elementary quantum objects, is both a wave and a particle!

Z is for ... Zero-point energy

Even at absolute zero, the lowest temperature possible, nothing has zero energy. In these conditions, particles and fields are in their lowest energy state, with an energy proportional to Planck’s constant.

F is for ... Free Will

Ideas at the heart of quantum theory, to do with randomness and the character of the molecules that make up the physical matter of our brains, lead some researchers to suggest humans can’t have free will.

I is for ... Interferometer

Some of the strangest characteristics of quantum theory can be demonstrated by firing a photon into an interferometer: the device’s output is a pattern that can only be explained by the photon passing simultaneously through two widely-separated slits.

H is for ... Hidden Variables

One school of thought says that the strangeness of quantum theory can be put down to a lack of information; if we could find the “hidden variables” the mysteries would all go away.

N is for ... Nonlocality

When two quantum particles are entangled, it can also be said they are “nonlocal”: their physical proximity does not affect the way their quantum states are linked.

M is for ... Many Worlds Theory

Some researchers think the best way to explain the strange characteristics of the quantum world is to allow that each quantum event creates a new universe.

D is for ... Dice

Albert Einstein decided quantum theory couldn’t be right because its reliance on probability means everything is a result of chance. “God doesn’t play dice with the world,” he said.

T is for ... Tunnelling

This happens when quantum objects “borrow” energy in order to bypass an obstacle such as a gap in an electrical circuit. It is possible thanks to the uncertainty principle, and enables quantum particles to do things other particles can’t.

V is for ... Virtual particles

Quantum theory’s uncertainty principle says that since not even empty space can have zero energy, the universe is fizzing with particle-antiparticle pairs that pop in and out of existence. These “virtual” particles are the source of Hawking radiation.

R is for ... Reality

Since the predictions of quantum theory have been right in every experiment ever done, many researchers think it is the best guide we have to the nature of reality. Unfortunately, that still leaves room for plenty of ideas about what reality really is!

B is for ... Bell's Theorem

In 1964, John Bell came up with a way of testing whether quantum theory was a true reflection of reality. In 1982, the results came in – and the world has never been the same since!

G is for ... Gravity

Our best theory of gravity no longer belongs to Isaac Newton. It’s Einstein’s General Theory of Relativity. There’s just one problem: it is incompatible with quantum theory. The effort to tie the two together provides the greatest challenge to physics in the 21st century.

W is for ... Wavefunction

The mathematics of quantum theory associates each quantum object with a wavefunction that appears in the Schrödinger equation and gives the probability of finding it in any given state.