Forces of the World Unite!

by Nicholas Mee on June 3, 2013

The aim of physicists is to provide a concise, but accurate description of the universe around us. At first glance, our world seems to be composed of a wealth of different materials controlled by a multitude of different forces. But one of the great triumphs of modern physics is the realization that these forces are actually separate faces of a few fundamental forces. This unification of the forces, as it is known, has become one of the main goals of physics.

The Newtonian Revolution

The first great advance in this direction was Newton’s demonstration that the force that holds us to the ground is the same force that holds the planets in orbit around the sun. We are so familiar with this idea that it is hard to imagine a time when it was not common knowledge. But prior to the Newtonian synthesis, less than 350 years ago, the connection between the fall of an apple and the celestial dance of the planets was completely unknown.

Saturn’s beautiful rings held within the planet’s orbital embrace. (copyright ESA)

Power to the People!

Another unification occurred in the Victorian era when the experiments of Michael Faraday were encapsulated in the mathematics of James Clerk-Maxwell’s theory of electromagnetism – a unified theory of the electric and magnetic forces. Faraday and Maxwell showed how these two forces were intimately entwined. They are really just two sides of the same coin.

This breakthrough was of the utmost significance both theoretically and practically. It led directly to the development of the electrical industry, as well as radio, television, the telephone, radar and much else.

Jimi Hendrix, one of the 20th century’s most creative users of electricity.

Four Forces

Remarkably, by the middle of the 20th century all physical phenomena could be understood in terms of just four forces:

Gravity – which keeps our feet on the ground,

Electromagnetism – which holds atoms together and gives us the whole science of chemistry,

the Strong Force – which operates within the nucleus of an atom and binds the nucleus together,

and, last but not least, another nuclear force – the Weak Force.

We Are Stardust!

The weak force is critical for the synthesis of the elements in the stars. It is called the weak force because it is much weaker than electromagnetism.

As you probably know, the vast energy output of the Sun is produced by the gradual conversion of hydrogen into helium. In very massive stars helium will eventually be converted into carbon and oxygen and then even heavier atoms such as neon, sulphur, silicon and iron.

Cosmic Firework Displays!

Extremely massive stars end their lives in a supernova explosion which releases huge amounts of energy and in the process much heavier atoms are created. All these atoms are dispersed throughout the galaxy by the supernova explosion to form gas clouds that eventually coalesce into the next generation of stars. Ourselves and everything else around us is formed from atoms that were cooked up within stars in this way.

None of this could happen without the weak force. The weakness of the weak force is very fortunate for us because if it were as strong as electromagnetism, then the Sun and stars would have burnt out long before life could have evolved.

Photon Ping Pong

In the late 1940s physicists Richard Feynman, Julian Schwinger, Sin-Itiro Tomonaga and Freeman Dyson pieced together an incredibly accurate quantum theory of electromagnetism known as QED (Quantum Electrodynamics). This incredibly precise theory describes the electromagnetic force as due to the exchange of particles known as photons. (Photons are the fundamental particles from which light is formed.) According to QED photons are continually being passed back and forth between charged particles such as electrons and protons.

Sin-Itiro Tomonaga who independently invented QED amidst the war torn ruins of Tokyo following World War 2. (Physicists seem to make great subjects for stamps.)

The Electroweak Force

A decade later Julian Schwinger and Sheldon Glashow suggested that the weak force might be described in a similar way, with the force mediated by a triplet of particles known as the W-plus, the W-minus and the Z-nought.

But there was a major problem with this idea! The theory would only work if the exchange particles were massless (like the photon) and it was clear that the W and Z particles must be very heavy, otherwise they would already have been seen in particle physics experiments.

The Fabric of the Universe

The solution, provided by Peter Higgs and two other research teams, killed two birds with one stone. It gave theorists a way for the W and Z particles to be massive without destroying their delicate theory, but also uniting the electromagnetic and weak forces into a single force. Despite their obvious differences these two forces could now be seen as two aspects of a single unified electroweak force.

The Higgs mechanism, as it is called, introduces a new quantum field that is built into the fabric of the universe. This field affects some fundamental particles as they move through space.

The electroweak force is mediated by a quartet of exchange particles. One of these is unaffected by the Higgs field and carries on regardless. This is the photon. Its exchange produces the electromagnetic force, which is a long-range powerful force holding atoms together and powering our laptops and televisions.

Can You Feel The Force?

But the other three particles that mediate electroweak interactions do feel the Higgs force. The result is that these particles move through space as though they are wading through treacle. They are bogged down and become very heavy. The force that they produce is therefore very weak – it is the weak force.

These particles, the Ws and Z were finally tracked down at CERN in the early 1980s. Their mass is almost 100 times the mass of a proton, which makes them heavier than an atom of krypton. Because they are so heavy the weak force is weak, stars shine for billions of years, and we are here to look up at them. Without the Higgs mechanism and its effect on these particles, we would not be here.

Exciting the Higgs Field

With the discovery of the Ws and the Z it was pretty clear that electroweak unification was a great success, and so the eventual discovery of the Higgs particle was much anticipated. The Higgs particle is the excitation or particle associated with the Higgs field.

Even though Higgs particles would have been produced in abundance in the very early universe, they would rapidly have decayed into other lighter particles. So they all disappeared long ago. This is why it is necessary to construct a huge proton collider – the Large Hadron Collider (LHC) – to recreate the conditions in which they are produced.

The innards of the CMS detector at the Large Hadron Collider.
(copyright CERN, Geneva)

The Big Announcement

On 4th July 2012 CERN announced that the LHC had found a new particle with the properties expected of the Higgs boson. The initial announcement was rather tentative as they were careful to err on the side of caution. Much more data has now been analysed and it all stacks up. A new particle has definitely been found and it is almost certainly the Higgs particle.

The discovery of the Higgs completes the unification of the electromagnetic and weak forces – the greatest unification of the forces since Faraday and Maxwell 150 years ago.

Further Information

For more information about the unity of physics and the adventures of Peter Higgs and his colleagues see my book Higgs Force: Cosmic Symmetry Shattered.

This article was originally written as a guest post for the TES Science Blog. You can see the original article here: TES Science – Blog


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