Will we drown in a sea of jelly?

September 7, 2001

Theories of Earth's demise from the BA Festival of Science in Glasgow, September 3-7

Some physicists say all matter is poised to disintegrate. Ben Allanach asks if we should be losing sleep over the prospect: Cosmic blasts, heatwaves, floods and symmetrical jelly?

Imagine a universe with no light. Stars are extinguished, the Sun goes out and plants can no longer grow. Imagine that electricity no longer works. There is no television or telecommunications. Even worse, matter itself simply disintegrates. The Earth is destroyed, the human race with it.

Just a bad dream? Physicists have been forced to face up to such a catastrophe because their theories say it could happen at any moment. It is all based on a model of particle physics called supersymmetry.

The smallest bits of matter or fundamental particles are well described by the Standard Model, a simple theory that describes the relationship between particles. So far the theory has stood up against experimental tests.

But most physicists believe that the Standard Model will be superseded by a more complete theory because it leaves so many questions unanswered. For example, basic calculations under the Standard Model indicate that fundamental particles should be some billion billion billion times heavier than what has been observed.

Particle theorists construct speculative models that answer questions such as these. In the past, this approach has paved the way to discovery. Einstein developed his theory of general relativity in just this way - first a theory is proposed to explain some ambiguity; then experimental tests are performed to prove or disprove the theory.

Many attempts to solve the lightness of matter problem have failed. Some fell because a mathematical inconsistency was eventually found in the model, or because they made predictions that contradicted available data. One theory that explains the lightness of matter and that has made it over the hurdles is supersymmetry.

Supersymmetry predicts that each kind of subatomic particle has a super-partner, a heavier, ghostly partner that has almost identical properties. If they exist, these super-partners of ordinary matter could be produced and measured in high-energy colliders such as those used at Cern in Switzerland or Fermilab in Chicago, which accelerate particles close to the speed of light, smash them together and measure any new form of matter produced. The aim is to reach higher energies than have been reached before, allowing heavier forms of matter to be produced. Producing and measuring super-partners would be solid evidence for the supersymmetry theory, so the race to find them is on.

But strangely, super-partners could induce the end of the world as we know it by changing the underlying vacuum of the universe over which all matter exists. This vacuum is not completely empty - rather, small random fluctuations occur, where pairs of particles pop out and annihilate each other. Every object feels the effect of these transient particles and in this way the vacuum determines some of the properties of matter.

The Edinburgh-based physicist Peter Higgs used these ideas about the vacuum to explain why some particles have mass. In the very early universe, all of the known particles were massless. Within the first second after the big bang, the universe cooled and the vacuum changed, causing matter to acquire a mass.

Higgs's theory is based on a mathematical structure too complex to explain here, but an analogy will serve us adequately. It is as if the vacuum contains a jelly of so-called Higgs particles that slows down any matter moving through it. Objects that resist movement have mass, by definition. In the very early universe, the jelly was too runny to provide any resistance and therefore everything was massless.

In the supersymmetry theory, the vacuum in our universe is unstable. The soft jelly of Higgs particles could suddenly change into a much thicker jelly made of super-partner particles. This would be bad news because particles of light, or photons, which are currently massless, would become immensely heavy. Nature would no longer have enough energy to produce them and all would become dark. Because photons are responsible for the electric and magnetic forces, these forces would no longer exist.

This would be the death knell for the poor atom, because it is the electric force that binds electrons to the nucleus. Atoms would lose all their electrons; protons and neutrons would decay; and the force that binds them together would change strength. All matter would disintegrate to leave a strange, new disorder.

The universe's vacuum prefers to have the lowest possible energy, and because the new region of jellified super-partners would have lower energy than the rest of the universe, it would infect the space around it. The region of new vacuum would then expand at the speed of light, wreaking dark havoc and disintegration.

Italian physicist Alessandro Strumia has investigated the probability of the super-partner jellifying. "The chance vacuum fluctuations that trigger the change have a well-defined probability of occurring, depending on the properties of the super-partners," he says.

However, he suggests the catastrophe is not imminent. His calculations indicate that the probability of this event happening is minuscule, even in a time as long as the age of our universe: "It is certainly not big enough to keep me awake at night," Strumia says.

Ben Allanach is a research associate at Cern in Switzerland.

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