PLASMA – THE FORGOTTEN STATE OF MATTER
Solid, liquid, and gas. We know these three states, yet the fourth one makes up 99% of the visible universe. Plasma is the fuel of stars, and the red-hot state could fuel aircraft engines and perhaps even bring us infinite energy.
We are less familiar with plasma than with solid, liquid and gaseous states of matter. Yet 99% of the visible universe exists in this elevated state.
Huge brooding grey-black banks of cloud fill the sky. Inside them, electrically-charged water drops and ice crystals are travelling up and down the cloud. Negative charges gradually collect at the bottom of the cloud, while Earth’s surface grows ever more positively charged. Then suddenly the tension is equalised by a spectacular energy discharge: lightning.
Every time we see lightning, we are seeing plasma, the fourth fundamental state of matter – as compared with the three more familiar ones: solid, liquid, and gas. It is the intensive energy discharge of lightning that converts the air into plasma, in which atomic nuclei and electrons have been separated. In a split second, 10,000 amperes of current is discharged, travelling between the Earth’s surface and the cloud at supersonic speed and heating the surrounding air to 25,000°C. The intense heat peels electrons off nitrogen and oxygen atoms in the air, converting them from gas into plasma.
In principle, all substances can be converted into plasma. All that is needed to remove electrons from the atomic nuclei is a sufficiently high temperature. Scientists and engineers have already used plasma’s unique qualities in fluorescent tubes and plasma TVs, but we have only just begun to understand and tame the immense energy of plasma. Plasma could replace jet fuel in airliners, and plasma engines are intended to play a central role in our colonisation of the Solar System. If physicists can manipulate this turbulent state with sufficient control, plasma could even become a practically inexhaustible source of energy.
99% of the universe is plasma
All substances change state as temperatures rise. We know this best from water. When the temperature is below zero, water is solid ice, with the atoms locked in a grid. As the temperature rises above freezing point, water becomes liquid – the grid disintegrates, and the molecules move about each other. When the water is heated to the boiling point of 100°C, it then becomes gaseous, in the form of water vapour: the molecules are moving freely in three dimensions.
Those are the three states of matter we encounter in our everyday lives. But if we continue the heating to above 1000°C, the water molecules will split into oxygen and hydrogen atoms. At a temperature of 10,00012,000°C, the transformation into the fourth
state takes place: the heat shakes electrons loose of the atomic nuclei, converting the gas into plasma, where negative electrons and positive ions – made up of protons and neutrons – are moving freely around each other. Their freedom of movement means that plasma is electrically conductive.
More than 99% of the visible universe – the bright stars and the hydrogen clouds in and between galaxies – consists of plasma. The hydrogen clouds are extremely thin, but plasma can also be highly compact, although it is then so hot that the atoms are ripped apart. This is the case at the centre of stars such as the Sun, which consist entirely of plasma. In the Sun’s 15-million-degree-hot core, the pressure is 250 billion times the pressure at Earth’s surface. The intense pressure compresses the hydrogen plasma so much that the hydrogen nuclei overcome their reciprocal electrical rejection, fusing into helium, which generates the energy that makes the Sun shine. Rocky planets such as Earth, orbiting their star in the inhabitable