The Standard Model of particle physics: The absolutely amazing theory of almost everything

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How does our world work on a subatomic level?
Varsha Y S, CC BY-SA

By Glenn Starkman, Case Western Reserve University

The Standard Model. What dull name for the most accurate scientific theory known to human beings.

More than a quarter of the Nobel Prizes in physics of the last century are direct inputs to or direct results of the Standard Model. Yet its name suggests that if you can afford a few extra dollars a month you should buy the upgrade. As a theoretical physicist, I’d prefer The Absolutely Amazing Theory of Almost Everything. That’s what the Standard Model really is.

Many recall the excitement among scientists and media over the 2012 discovery of the Higgs boson. But that much-ballyhooed event didn’t come out of the blue – it capped a five-decade undefeated streak for the Standard Model. Every fundamental force but gravity is included in it. Every attempt to overturn it to demonstrate in the laboratory that it must be substantially reworked – and there have been many over the past 50 years – has failed.

In short, the Standard Model answers this question: What is everything made of, and how does it hold together?

The smallest building blocks

But these elements can be broken down further.
Rubén Vera Koster, CC BY-SA

You know, of course, that the world around us is made of molecules, and molecules are made of atoms. Chemist Dmitri Mendeleev figured that out in the 1860s and organized all atoms – that is, the elements – into the periodic table that you probably studied in middle school. But there are 118 different chemical elements. There’s antimony, arsenic, aluminum, selenium … and 114 more.

Physicists like things simple. We want to boil things down to their essence, a few basic building blocks. Over a hundred chemical elements is not simple. The ancients believed that everything is made of just five elements – earth, water, fire, air and aether. Five is much simpler than 118. It’s also wrong.

By 1932, scientists knew that all those atoms are made of just three particles – neutrons, protons and electrons. The neutrons and protons are bound together tightly into the nucleus. The electrons, thousands of times lighter, whirl around the nucleus at speeds approaching that of light. Physicists Planck, Bohr, Schroedinger, Heisenberg and friends had invented a new science – quantum mechanics – to explain this motion.

That would have been a satisfying place to stop. Just three particles. Three is even simpler than five. But held together how? The negatively charged electrons and positively charged protons are bound together by electromagnetism. But the protons are all huddled together in the nucleus and their positive charges should be pushing them powerfully apart. The neutral neutrons can’t help.

What binds these protons and neutrons together? “Divine intervention” a man on a Toronto street corner told me; he had a pamphlet, I could read all about it. But this scenario seemed like a lot of trouble even for a divine being – keeping tabs on every single one of the universe’s 10⁸⁰ protons and neutrons and bending them to its will.

Expanding the zoo of particles

Meanwhile, nature cruelly declined to keep its zoo of particles to just three. Really four, because we should count the photon, the particle of light that Einstein described. Four grew to five when Anderson measured electrons with positive charge – positrons – striking the Earth from outer space. At least Dirac had predicted these first anti-matter particles. Five became six when the pion, which Yukawa predicted would hold the nucleus together, was found.

Then came the muon – 200 times heavier than the electron, but otherwise a twin. “Who ordered that?” I.I. Rabi quipped. That sums it up. Number seven. Not only not simple, redundant.

By the 1960s there were hundreds of “fundamental” particles. In place of the well-organized periodic table, there were just long lists of baryons (heavy particles like protons and neutrons), mesons (like Yukawa’s pions) and leptons (light particles like the electron, and the elusive neutrinos) – with no organization and no guiding principles.

Into this breach sidled the Standard Model. It was not an overnight flash of brilliance. No Archimedes leapt out of a bathtub shouting “eureka.” Instead, there was a series of crucial insights by a few key individuals in the mid-1960s that transformed this quagmire into a simple theory, and then five decades of experimental verification and theoretical elaboration.

Quarks. They come in six varieties we call flavors. Like ice cream, except not as tasty. Instead of vanilla, chocolate and so on, we have up, down, strange, charm, bottom and top. In 1964, Gell-Mann and Zweig taught us the recipes: Mix and match any three quarks to get a baryon. Protons are two ups and a down quark bound together; neutrons are two downs and an up. Choose one quark and one antiquark to get a meson. A pion is an up or a down quark bound to an anti-up or an anti-down. All the material of our daily lives is made of just up and down quarks and anti-quarks and electrons.

The Standard Model of elementary particles provides an ingredients list for everything around us.
Fermi National Accelerator Laboratory, CC BY

Simple. Well, simple-ish, because keeping those quarks bound is a feat. They are tied to one another so tightly that you never ever find a quark or anti-quark on its own. The theory of that binding, and the particles called gluons (chuckle) that are responsible, is called quantum chromodynamics. It’s a vital piece of the Standard Model, but mathematically difficult, even posing an unsolved problem of basic mathematics. We physicists do our best to calculate with it, but we’re still learning how.

The other aspect of the Standard Model is “A Model of Leptons.” That’s the name of the landmark 1967 paper by Steven Weinberg that pulled together quantum mechanics with the vital pieces of knowledge of how particles interact and organized the two into a single theory. It incorporated the familiar electromagnetism, joined it with what physicists called “the weak force” that causes certain radioactive decays, and explained that they were different aspects of the same force. It incorporated the Higgs mechanism for giving mass to fundamental particles.

Since then, the Standard Model has predicted the results of experiment after experiment, including the discovery of several varieties of quarks and of the W and Z bosons – heavy particles that are for weak interactions what the photon is for electromagnetism. The possibility that neutrinos aren’t massless was overlooked in the 1960s, but slipped easily into the Standard Model in the 1990s, a few decades late to the party.

3D view of an event recorded at the CERN particle accelerator showing characteristics expected from the decay of the SM Higgs boson to a pair of photons (dashed yellow lines and green towers).
McCauley, Thomas; Taylor, Lucas; for the CMS Collaboration CERN, CC BY-SA

Discovering the Higgs boson in 2012, long predicted by the Standard Model and long sought after, was a thrill but not a surprise. It was yet another crucial victory for the Standard Model over the dark forces that particle physicists have repeatedly warned loomed over the horizon. Concerned that the Standard Model didn’t adequately embody their expectations of simplicity, worried about its mathematical self-consistency, or looking ahead to the eventual necessity to bring the force of gravity into the fold, physicists have made numerous proposals for theories beyond the Standard Model. These bear exciting names like Grand Unified Theories, Supersymmetry, Technicolor, and String Theory.

Sadly, at least for their proponents, beyond-the-Standard-Model theories have not yet successfully predicted any new experimental phenomenon or any experimental discrepancy with the Standard Model.

The ConversationAfter five decades, far from requiring an upgrade, the Standard Model is worthy of celebration as the Absolutely Amazing Theory of Almost Everything.

Glenn Starkman, Distinguished University Professor of Physics, Case Western Reserve University

This article was originally published on The Conversation. Read the original article.

Experiment shows Einstein’s quantum ‘spooky action’ approaches the human scale

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An illustration of the two 20-micrometre-wide vibrating drumheads, each composed of trillions of atoms, in an entangled quantum state of motion.
Petja Hyttinen and Olli Hanhirova, ARKH Architects Ltd., Author provided

By Matt Woolley, UNSW

Quantum physics is often defined as the physics of the very small – think atoms, electrons and photons.

But we have managed to demonstrate one of the quirky features of quantum physics at a much larger scale. In a paper published today in Nature, we describe how we were able to create quantum entanglement of the motion of objects composed of many billions of atoms.

Entanglement is where two objects that may be separated by an arbitrary distance are somehow connected: a measurement on one object leads to a change in the results of measurements made on the other – what Albert Einstein called “spooky action at a distance”.




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Entanglement has been demonstrated for microscopic-scale systems, such as those involving photons, ions and electron spins. But a number of challenges remained before we could demonstrate entanglement on a larger scale.

Before I look at how we solved some of those challenges we need to understand a bit more about quantum physics.

The size of things

Does size really matter in quantum physics? Well, kind of. There is, in fact, nothing in the formulation of quantum mechanics that mandates that it should apply only to the very small.

What then really matters in determining whether an object will behave according to the strange rules of quantum physics, or according to the familiar rules of classical physics?

There are perhaps two ingredients required for the observation of quantum behaviour in an object.

The first is isolation. The world outside is full of sound and fury, such as other matter and radiation. If the object can find a way to isolate itself from this fury, it can evolve according to the simple rules of quantum mechanics.

An object that cannot isolate itself from the fury will find that the richness of quantum dynamics cannot be accessed. So its motion will be well described by the familiar rules of classical physics.

A thrown ball will follow a well-defined trajectory; it will not spread out as one might expect from quantum physics. A rolling stone will go up a hill until the supply of the energy of motion it had at the bottom of the hill is exhausted; it cannot possibly emerge on the other side of the hill as it might according to the rules of quantum tunnelling.

The second ingredient is frequency, the rate at which a confined object vibrates. The emergence of quantum behaviour typically requires that the energy associated with the object (which is related to its frequency of vibration) exceeds the energy associated with the object’s environment (which is related to its temperature).

Even if an object is well isolated from its environment, it will not be perfectly isolated, so the properties of the object’s environment still matter.

Consider light. Photons of light interact only weakly with other photons, so that if we consider light propagating in near vacuum we have a well-isolated system. That’s the first ingredient.

What about frequency? Well, the electric and magnetic fields associated with visible light go up and down around 6×1014 times per second.

In this case, the energy associated with a photon of light vastly exceeds the energy scale of the likely thermal environment. One can tell a similar story for the electronic levels of isolated atoms. Thus very small objects are more likely to possess the ingredients required for the observation of quantum phenomena.

Scale things up a bit

Let’s go bigger and more tangible. Instead of thinking about the electromagnetic fields of light or the electronic levels of an atom, how about the motion of a macroscopic, massive object? Can we make and observe such an object behave according to the rules of quantum physics?

The answer, as we report today, is yes.

In an experiment, performed recently in the laboratory of Professor Mika Sillanpää at Aalto University in Finland, we set up two microfabricated vibrating circular membranes, like drumheads.

Each was about the width of a human hair and we were able to measure them in a state that exhibited the quantum property of entanglement.

The two drumheads were brought into an entangled state through careful driving of a superconducting electrical circuit to which both were coupled.

While these drumheads may seem small on the human scale, they are huge on the atomic scale – each drumhead is composed of trillions of atoms.

These drumheads are the largest objects to be prepared in an entangled state, and this experiment is perhaps the closest approach to a literal implementation of the famous thought experiment of Einstein, Podolsky and Rosen that first studied the phenomenon that became known as entanglement back in 1935.

Why we did it

So why should we take the trouble to demonstrate quantum physics with massive, macroscopic objects? There are two answers: one fundamental and one applied.

On the fundamental side, this demonstration gives us greater confidence that the laws of quantum physics do indeed apply to large objects.

But will this continue to hold true as the size and mass of the objects in such experiments is increased? We don’t know.




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Tabletop experiments with massive objects bring forth the possibility that such a question might one day be answered.

On the applied side, one may ask: what could mechanical quantum systems offer in this electronic age? But mechanical systems are more common than many people realise.

The humble quartz oscillator remains a crucial technology for clocks. Surfaces are imaged using the atomic force microscope, essentially a suspended cantilever that deflects light. Gravitational waves are observed by monitoring the motion of suspended mirrors using laser light.

While quantum control of mechanical systems conceivably offers an advantage in each of these scenarios, mechanical systems offer another advantage: they move, and so they couple to both microwaves and light.

While the processing power of a future quantum computer might rely on microwaves in a low-temperature laboratory environment, quantum communications systems require light propagating through optical fibres or free space.

Mechanical systems can act as intermediaries between these worlds and thereby contribute to the realisation of a quantum internet.

The ConversationWhile it is hard to say exactly where these experiments might ultimately lead, it is clear that the era of massive quantum machines has arrived, and is here to stay.

Matt Woolley, Senior Lecturer in Electrical Engineering, UNSW

This article was originally published on The Conversation. Read the original article.