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Linné on line arrow Physics and the Cosmos arrow Physics and the Cosmos, Glossary

Physics and the Cosmos, Glossary

Alpha particle An alpha particle is the atomic nucleus of a helium atom.
Amorphous Amorphous materials do not have any ordered structure.
Black body radiation All bodies send out electromagnetic radiation due to the motion of the atoms. The frequency distribution of the radiation depends on the energy distribution and is thereby a measure of the temperature. The mean energy of this radiation covers all possible frequencies, but for us the most prominent ones are the heat radiation and visible light. Based on theoretical arguments the frequency distribution should follow Planck's radiation law, but in most cases it is only a good approximation. The spectrum that is in best agreement with Planck's law is from the cosmic microwave background. A hypothetical body which fulfills Planck's law is called an ideal black body radiator.
Catalyst A substance that facilitates a chemical or physical reaction without being affected itself. This happens through one of the substances binding to the catalyst, which lowers the threshold energy for the reaction and makes it easier for the other substance to react with the first one compared to if it was free.
Cerenkov light When charged particles travel faster than the speed of light in a given medium, the propagation of the electromagnetic field has certain properties. A shock wave consisting of Cerenkov light (photons) is formed. This can be compared with the sonic bang that is formed when a plane travels faster than the speed of sound. However, particles can only travel faster than the speed of light in a medium (such as water or ice) where the speed of light is smaller than in vacuum. One example of Cerenkov light is the bluish light that is emitted from water where one stores spent nuclear fuel. It should be noted that this blue light is not any more dangerous than any other blue light.
Conservation laws Within physics one has found that certain quantities are conserved, which is described by so called laws of conservation. Within classical mechanics we know that for example energy, momentum and angular momentum are conserved quantities. However, they can be converted between different forms and redistributed between different objects.

Another quantity that is conserved is charge. In particle and nuclear physics positive and negative charges can annihilate each other but the total net charge is always the same. Furthermore, the number of atoms of a given type is always conserved in chemical processes, as well as the number of baryons in nuclear physics processes.

Within particle physics one has introduced another type of property called quantum numbers that are associated with different kinds of particles, which one believes are conserved in most reactions. Two examples are lepton number and baryon number. However, under certain cinditions these conservation laws are broken.

Critical mass The mass of the Universe which makes the expansion rate approach a limit when the Universe becomes very old. If the mass of the Universe is lower than the critical mass it will expand for ever. One says that the Universe is open. On the other hand, if the mass is larger, the Universe will start to contract at some point. One says that the Universe is closed.
Crystal Crystalline materials have an ordered structure where the atoms or molecules have well defined positions. In addition, crystalline materials have well defined melting and boiling temperatures.
Electronvolt, eV Measure of energy used on atomic and subatomic levels, one electronvolt = 1.6*10–19Joule.
Hadron Particle consisting of three quarks or a quark anti-quark pair.
Muon A "cousin" of the electron, in other words a particle with very similar properties to the electrons. However, there are two important differences: the muon is 200 times heavier than the electron, and it is unstable. Its lifetime is long compared to many other unstable particles. See also more general information about leptons.
Neutralino A hypothetical particle which is predicted in supersymmetric extensions of the standard model. In most such models the neutralino is the lightest supersymmetric partner of one of the particles in the standard model and therefore it is stable. Today there could be a remnant of such particles from the Big Bang. In that case this would be part of the so called dark matter.
Neutrino A particle (lepton) that does not have any electric charge; its mass is extremely small, and it only interacts through the weak interaction. However, it is necessary in the standard model and its existence has been proved experimentally (Nobel price 1995 to Frederick Reines) in spite of its weak interaction. In fact the interaction is so weak that most neutrinos which hit the earth will go straight through it.
Positron The anti particle of the electron. In other words a particle with the opposite quantum numbers compared to the electron but with all the other properties, such as the mass, being the same.
Quantum chromodynamics Quantum chromodynamics (QCD) is the theory which describes the strong interaction between quarks and gluons. In most cases the theory works well, but at low energies it is more difficult to make precise calculations. Research is ongoing.
The solar neutrino problem For a long time one could not understand why the measured fluxes of neutrinos produced by fusion of light atomic nuclei in the sun did not agree with predictions from solar models describing the solar cycle. The measured fluxes were only about half of what one expected. Today we know that the explanation is that the neutrinos produced in the sun oscillate into other types of neutrinos on the way to the earth; in earlier experiments one did not look for these other types of neutrinos.
The standard model The model that is used today to describe the interaction between fundamental particles. It describes the electromagnetic force, the weak nuclear force and the strong nuclear force. On the other hand it does not include gravitation and it is a big challenge for physicists to include this in the model as well.
Threshold energy Some reactions cannot take place unless the reacting substances are in relative motion, in other words have kinetic energy. The lowest energy that is needed for such a reaction is called the threshold energy.