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Posted 19 August 2007

Astronomy needs Quantum Mechanics

by Frikkie de Bruyn

Astronomy, the scientific study of heavenly bodies has a strange relationship with quantum mechanics, the study of the behaviour of atoms, sub-atomic particles and radiation. The strangeness of this relationship may be ascribed to the weirdness of quantum mechanics, such as the uncertainty principle. Astronomy is not alone in this uneasy relationship. In fact, the weirdness of quantum mechanics raises some serious questions about the reality that quantum mechanics describes. The uncertainty principle specifies that we cannot know simultaneously the exact location and motion of a particle or even the time in which a particle acquires a certain amount of energy. This is why physicists refer to electrons in an atom as if they were a negatively charged cloud surrounding the nucleus. We cannot really describe an electron as a small particle following an orbit around the nucleus. It is much more interesting to describe electrons as a negatively charged cloud and it gives us a better and more sophisticated model of the atom. This view is apparently in contrast to the clear description of the electron as a particle in Einstein’s photoelectric experiment. We are forced to accept this weirdness of quantum mechanics in which a particle such as the electron is described as a wave and a particle known as the wave-particle duality. The same applies to the photon. The astronomer uses particles/waves and the interaction of light (photons) with matter in observation of planets, stars, galaxies and the interstellar medium such as the gas and dust clouds of the Orion nebula. Let’s take a closer look at how the astronomer is completely dependant on the quantum interactions and behaviour.

If light did not interact with matter you would not be able to read these words and most important, the basis of astronomy, observation, would not be possible. In fact you and I would not exist because photosynthesis would not be possible and life would not exist. Let us look at the hydrogen atom to understand the importance of light ad matter better. The hydrogen atom is both simple and common. Roughly 90 percent of all atoms in the Universe are hydrogen. The electron orbits the nucleus at strictly amounts of binding energy. In physics these levels are called energy levels. We can say that the lowest or closest orbit to the nucleus is the smallest and most tightly bound orbit in its permitted energy level. The electron can move from one energy level to another by supplying enough energy to make up the difference between the two energy levels. It is like moving an object from a low shelf to a high shelf, the greater the distance to move the object the more energy we need to raise he object. The amount of energy needed to move the electron is the energy difference between the two energy levels.

If the electron moves from a lower to a higher level we say the atom is excited. That is, energy has been added to the atom to enable the electron to move to a higher level. If the electron falls back to a lower level the energy is released. Atoms can be excited by collisions. If two electrons collide both may have electrons move to a higher level. This happens very often in hot gas, such as the star forming areas in interstellar gas clouds where atoms move rapidly and collide often. An electron can also gain energy by absorbing a photon provided the photon has exactly the right amount of energy to enable the electron to move to a higher level. Thinking of a photon as a wave, the wavelength, which determines the amount of energy, must exactly of the right length for the electron to absorb the energy.

Atoms, like human beings, cannot exist in an excited state forever. The excited atom is unstable and usually gives up the energy within 10-6 to 10-9 seconds and returns the electron to its lowest energy level. And now, after all the quantum mechanics above, we arrive at the significance of all this for the astronomer. The atom has a surplus energy and it emits this surplus energy in the form of a photon. It is this photon that travels from the star or being reflected by a planet that enables the astronomer to observe the object. But equally important is the exact energy levels in an atom referred to above. No two atoms from different elements have the same energy levels. Each type of atom or ion (atom which loses electrons or gain extra electrons) has its unique sets of energy levels; each type absorbs and emits photons with a unique set of wavelengths. The astronomer can therefore identify the elements in a star or gas cloud by studying the characteristic wavelengths of light absorbed or emitted. Stars are classified according to spectral types, based on the unique wavelength per element, from O to M. The story is more complicated than this with the temperature of the star playing a very important role. These problems were eventually solved by an English astronomer, Cecilia Payne, showing that over 90 percent of the atoms in a star consist of hydrogen.

The wavelength of the photons emitted by atoms in a star can also tell the astronomer if the star is moving away from Earth or approaching us. This is known as the Doppler shift (shorter and longer sound waves) and we experience it everyday with a car approaching and eventually receding from us. A star approaching us will emit photons with a shorter wavelength (blue shift) and photons with a longer wavelength (red shift) if the star is receding from us. Stars and galaxies very far away emit photons shifted very far to the red. There is a relationship between the lengths of the red shifted photons reaching us and the speed at which the star is receding. This tells astronomers that the Universe is expanding and by analysing the photons from supernovae at various distances from Earth, astronomers determined in 1998 that the expansion of the Universe is accelerating. This is by no means the only ways in which astronomy uses quantum effects. But I think it is sufficient to make us realize that we constantly use quantum effects in astronomy. Next time we use a telescope, spare a thought for the wonder of the quantum world enabling us to live and study the heavenly bodies.

Frikkie de Bruyn

Barrow, John D. The Constants of Nature. Random House, London, U.K. 2002
Ford, K.W. Quantum Physics The Quantum World. Harvard University Press, Massachusetts London, U.K. 2004
Smolin, Lee. Three Roads to Quantum Gravity. Orion Books, London, U.K. 2000 
Hawking, S.W. The Universe in a Nutshell. Bantam Press. U.K. 2001

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