What’s so small to you is so large to me (excerpt)

What’s so small to you is so large to me (excerpt)

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Among the many reasons I chose to pursue physics was the desire to do something that would have a permanent impact. If I was going to invest so much time, energy, and commitment, I wanted it to be for something with a claim to longevity and truth. Like most people, I thought of scientific advances as ideas that stand the test of time.
 
My friend Anna Christina Büchmann studied English in college while I majored in physics. Ironically, she studied literature for the same reason that drew me to math and science. She loved the way an insightful story lasts for centuries. When discussing Henry Fielding’s novel Tom Jones with her many years later, I learned that the edition I had read and thoroughly enjoyed was the one she helped annotate when she was in graduate school.
 
Tom Jones was published two hundred and fifty years ago, yet its themes and wit resonate to this day. During my first visit to Japan, I read the far older Tale of Genji and marveled at its characters’ immediacy too, despite the thousand years that have elapsed since Murasaki Shibuku wrote about them. Homer created the Odyssey roughly two thousand years earlier. Yet notwithstanding its very different age and context, we continue to relish the tale of Odysseus’s journey and its timeless descriptions of human nature.
 
Scientists rarely read such old - let alone ancient - scientific texts. We usually leave that to historians and literary critics. We nonetheless apply the knowledge that has been acquired over time, whether from Newton in the seventeenth century or Copernicus more than a hundred years earlier still. We might neglect the books themselves, but we are careful to preserve the important ideas they may contain.
 
Science is certainly not the static statement of universal laws we all hear about in elementary school. Nor is it a set of arbitrary rules. Science is an evolving body of knowledge. Many of the ideas we are currently investigating will prove to be wrong or incomplete. Scientific descriptions certainly change as we cross the boundaries that circumscribe what we know and venture into more remote territory where we can glimpse hints of deeper truth beyond.
 
Neils Bohr in 1912 was faced with a challenging choice - abandon classical physics or abandon his belief in observed reality. Bohr wisely chose the former and assumed classical laws don’t apply at the small distances occupied by electrons in an atom. And it was one of the key insights that led to the development of quantum physics.
 
Once Bohr ceded Newton’s laws, at least in this limited regime, he could postulate that electrons occupied fixed energy levels - according to a quantization condition involving a quantity called orbital angular momentum that he proposed. According to Bohr, his quantization rule applied at these smaller scales. The rules were different from the rules that hold at macroscopic scales, such as the Earth circulating around the Sun.
 
Technically, quantum mechanics still applies to these larger systems as well. But the effects are far too small to ever measure or notice. When you observe the orbit of the Earth or any macroscopic object for that matter, quantum mechanics can be ignored. The effects average out in all such measurements so that any prediction you make agrees with its classical counterpart. For measurements on macroscopic scales, classical predictions generally remain extremely good approximations - so good that you can’t distinguish that quantum mechanics is in fact the deeper underlying theory.
 
Classical predictions are analogous to the words and images on an extremely high-resolution computer screen. Underlying them are the many pixels that are like the quantum mechanical atomic substructure. But the images or words are all we generally need (or want) to see.
 
Quantum mechanics constitutes a change in paradigm that becomes apparent only at the atomic scale. Despite Bohr’s radical assumption, he didn’t have to abandon what was known before. He didn’t assume classical Newtonian physics was wrong. He simply assumed that classical laws cease to apply for electrons in an atom. Macroscopic matter, which consists of so many atoms that quantum effects can’t be isolated, obeys Newton’s laws, at least at the level at which anyone could measure the success of its predictions. Newton’s laws are not wrong. We don’t abandon them in the regime in which they apply. But at the atomic scale, Newton’s laws had to fail. And they failed in an observable and spectacular fashion that led to the development of the new rules of quantum mechanics.

About the Author: 
Lisa Randall is Professor of Theoretical Physics at Harvard University and a judge for the open international category of the Quantum Shorts competition. This edited extract is taken from her book Knocking on Heaven's Door, with kind permission of Ecco, an imprint of Harper Collins Publishers.

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Quantum Theories

A is for ... Act of observation

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

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.

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.

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.

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.

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!

G is for ... Gluon

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

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.

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 ... 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.

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.

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.

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.

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.

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.

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.

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!

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.

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.

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.

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.

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.

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.

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.

U is for ... Universe

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

I is for ... Information

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

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.

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”.

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.

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.

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 ... 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.

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.

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.

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.

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!

R is for ... Randomness

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

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.

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.

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.

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.

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.

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.

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.

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.