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By Tom Lancaster and Stephen J. Blundell

Sometimes it’s the fly in the ointment, the thing that spoils the purity of the whole picture, which leads to the big advances in science. That’s exactly what happened at a conference in Shelter Island, New York in 1947 when a group of physicists gathered to discuss the latest breakthroughs in their field which seemed at first sight to make everything more complicated.

Isidor Rabi reported experimental results from Columbia University that showed that the g-factor for the electron, a property reflecting its magnetic moment, was not precisely two, as Paul Dirac’s beautiful theory of the electron had predicted, but came out to be a messy 2.00244 (though the modern value is very slightly lower than this). And Willis Lamb, also at Columbia, explained how two energy levels in the hydrogen atom which were supposed (again according to Dirac) to be coincident were very slightly displaced from each other (an effect now known as the Lamb shift).

These were apparently messy, annoying and disruptive results that ruined a pure, dignified and elegant theory. But physicists like a challenge, and the conference attendees included Hans Bethe, Julian Schwinger, and Richard Feynman, all three of whom would attack the problem. The key insight was to realize that there are a multitude of quantum processes that can occur, and which had been forgotten. An electron is not just an electron, but is surrounded by a cloud of virtual particles: photons, electrons, and antielectrons, popping in and out of existence. These higher order processes are most pictorially described by Feynman diagrams, simple cartoons containing dots, arrows and wiggly lines, each one a shorthand for a mathematical term in a complex calculation but summarizing a physical interaction in an elegant form.

These diagrams can be used to show how the basic interaction between electrons and light is altered by quantum processes, an effect which tweaks its magnetic moment. This slightly shifts the “g-factor” and gives a prediction which has been verified experimentally to many decimal places. It also affects the way in which the spin and orbital angular momentum behave and this can be used to explain the Lamb shift. These tiny effects signal a vacuum that is not empty but teeming with quantum life, myriad interactions shimmering around every particle

Feynman diagrams first appeared in print sixty-five years ago this year, so they have now reached statutory retirement age. But rather than being put out to grass, Feynman’s cartoons are still used to make calculations and describe physical processes. They are at the foundation of modern quantum field theory, and if we ever figure out how to make a theory of quantum gravity, it is pretty likely Feynman diagrams will be in the description. It’s a reminder of why detailed measurements are needed in physics. Those little discrepancies can lead to revolutions in understanding.

Tom Lancaster was a Research Fellow in Physics at the University of Oxford, before becoming a Lecturer at the University of Durham in 2012. Stephen J. Blundell is a Professor of Physics at the University of Oxford and a Fellow of Mansfield College, Oxford. They are co-authors of Quantum Field Theory for the Gifted Amateur.

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Quick! What’s behind you right now? Did you peek over to see desks, the wallpaper, students, books, or toys? Were those objects there even before you looked at them? Are they there now, even though you’re reading this instead of seeing them? As strange as it sounds, some scientists believe that nothing exists definitely until someone measures it, such as you did with your eyes and ears. These scientists work in a field of science called Quantum Mechanics.

In the early 1900s, smarty-pants scientists like Albert Einstein, Niels Bohr, and Werner Heisenberg studied, experimented and argued over the question of what light was made of. Light was very mysterious to scientists at the time, because in some experiments it acted like a wave, similar to the invisible radio and magnetic waves all around us. In other experiments though, light acted like a particle, a solid object like a Pop Tart, a textbook, a penny, a skyscraper… Anything that’s in one place and that weighs something is a particle. It didn’t seem to make sense for something to be an invisible wave and a solid particle at the same time, but in test after test, light was both! You might think it was time for these scientists to turn in their labcoats and get new jobs… this was too hard to figure out! Instead of giving up though, the scientists continued experimenting and studying the subject until they found a solution: light is a wave until it gets observed, then it becomes a solid particle!

This was huge news for scientists. If light acts like this, then other solid objects may not be so solid after all too. The scientists studying Quantum Mechanics presented this thought-provoking possibility: that that the world is actually a wave of possibilities until we observe it, then it becomes the solid place we can feel, touch, taste and smell. It’s a bit like hiding trash under your bed: if you can’t see it, it’s not there!

On 4 July 2012, scientists at CERN’s Large Hadron Collider (LHC) facility in Geneva announced the discovery of a new elementary particle they believe is consistent with the long-sought Higgs boson, or ‘god particle’. Our understanding of the fundamental nature of matter — everything in our visible universe and everything we are — is about to take a giant leap forward. So, what is the Higgs boson and why is it so important? What role does it play in the structure of material substance? We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the next week to explain some of the mysteries of the Higgs.

By Jim Baggott

We know that the physical universe is constructed from elementary matter particles (such as electrons and quarks) and the particles that transmit forces between them (such as photons). Matter particles have physical characteristics that we classify as fermions. Force particles are bosons.

In quantum field theory, these particles are represented in terms of invisible energy ‘fields’ that extend through space. Think of your childhood experiences playing with magnets. As you push the north poles of two bar magnets together, you feel the resistance between them grow in strength. This is the result of the interaction of two invisible, but nevertheless very real, magnetic fields. The force of resistance you experience as you push the magnets together is carried by invisible (or ‘virtual’) photons passing between them.

Matter and force particles are then interpreted as fundamental disturbances of these different kinds of fields. We say that these disturbances are the ‘quanta’ of the fields. The electron is the quantum of the electron field. The photon is the quantum of the electromagnetic field, and so on.

In the mid-1960s, quantum field theories were relatively unpopular among theorists. These theories seemed to suggest that force carriers should all be massless particles. This made little sense. Such a conclusion is fine for the photon, which carries the force of electromagnetism and is indeed massless. But it was believed that the carriers of the weak nuclear force, responsible for certain kinds of radioactivity, had to be large, massive particles. Where then did the mass of these particles come from?

In 1964, four research papers appeared proposing a solution. What if, these papers suggested, the universe is pervaded by a different kind of energy field, one that points (it imposes a direction in space) but doesn’t push or pull? Certain kinds of force particle might then interact with this field, thereby gaining mass. Photons would zip through the field, unaffected.

One of these papers, by English theorist Peter Higgs, included a footnote suggesting that such a field could also be expected to have a fundamental disturbance — a quantum of the field. In 1967 Steven Weinberg (and subsequently Abdus Salam) used this mechanism to devise a theory which combined the electromagnetic and weak nuclear forces. Weinberg was able to predict the masses of the carriers of the weak nuclear force: the W and Z bosons. These particles were found at CERN about 16 years later, with masses very close to Weinberg’s original predictions.

By about 1972, the new field was being referred to by most physicists as the Higgs field, and its field quantum was called the Higgs boson. The ‘Higgs mechanism’ became a key ingredient in what was to become known as the standard model of particle physics.

Jim Baggott is author of Higgs: The Invention and Discovery of the ‘God Particle’ and a freelance science writer. He was a lecturer in chemistry at the University of Reading but left to pursue a business career, where he first worked with Shell International Petroleum Company and then as an independent business consultant and trainer. His many books include Atomic: The First War of Physics (Icon, 2009), Beyond Measure: Modern Physics, Philosophy and the Meaning of Quantum Theory (OUP, 2003), A Beginner’s Guide to Reality (Penguin, 2005), and A Quantum Story: A History in 40 Moments (OUP, 2010). Read his previous blog post “Putting the Higgs particle in perspective.”

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By Jim Baggott

If a tree falls in the forest, and there’s nobody around to hear, does it make a sound?

For centuries philosophers have been teasing our intellects with such questions. Of course, the answer depends on how we choose to interpret the use of the word ‘sound’. If by sound we mean compressions and rarefactions in the air which result from the physical disturbances caused by the falling tree and which propagate through the air with audio frequencies, then we might not hesitate to answer in the affirmative.

Here the word ‘sound’ is used to describe a physical phenomenon – the wave disturbance. But sound is also a human experience, the result of physical signals delivered by human sense organs which are synthesized in the mind as a form of perception.

Now, to a large extent, we can interpret the actions of human sense organs in much the same way we interpret mechanical measuring devices. The human auditory apparatus simply translates one set of physical phenomena into another, leading eventually to stimulation of those parts of the brain cortex responsible for the perception of sound. It is here that the distinction comes. Everything to this point is explicable in terms of physics and chemistry, but the process by which we turn electrical signals in the brain into human perception and experience in the mind remains, at present, unfathomable.

Philosophers have long argued that sound, colour, taste, smell and touch are all secondary qualities which exist only in our minds. We have no basis for our common-sense assumption that these secondary qualities reflect or represent reality as it really is. So, if we interpret the word ‘sound’ to mean a human experience rather than a physical phenomenon, then when there is nobody around there is a sense in which the falling tree makes no sound at all.

This business about the distinction between ‘things-in-themselves’ and ‘things-as-they-appear’ has troubled philosophers for as long as the subject has existed, but what does it have to do with modern physics, specifically the story of quantum theory? In fact, such questions have dogged the theory almost from the moment of its inception in the 1920s. Ever since it was discovered that atomic and sub-atomic particles exhibit both localised, particle-like properties and delocalised, wave-like properties physicists have become ravelled in a debate about what we can and can’t know about the ‘true’ nature of physical reality.

Albert Einstein once famously declared that God does not play dice. In essence, a quantum particle such as an electron may be described in terms of a delocalized ‘wavefunction’, with probabilities for appearing ‘here’ or ‘there’. When we look to see where the electron actually is, the wavefunction is said to ‘collapse’ instantaneously, and appears ‘here’ with a frequency consistent with the probability predicted by quantum theory. But there is no predicting precisely where an individual electron will be found. Chance is inherent in the collapse of the wavefunction, and it was this feature of quantum theory that got Einstein so upset. To make matters worse, if the collapse is instantaneous then this implies what Einstein called a ‘spooky action-at-a-distance’ which, he argued, appeared to violate a key postulate of his own special theory of relativity.

So what evidence do we have for this mysterious collapse of the wavefunction? Well, none actually. We postulate the collapse in an attempt to explain how a quantum system with many different possible outcomes before measurement transforms into a system with one and only one result after measurement. To Irish physicist John Bell this seemed to be at best a confidence-trick, at worst a fraud. ‘A theory founded in this way on arguments of manifestly approximate character,’ he wrote some years later, ‘howe