X-rays, dark matter and galaxies in cluster Abell 2744 (credit: NASA, ESA and NASA, ESA, ESO, CXC, and D. Coe (STScI)/J. Merten (Heidelberg/Bologna))
We still find out what stuff is made of by throwing pieces at each other, and watching where the bits fly. This has been the principle behind experiments from Rutherford’s foil to mankind’s biggest machine, the Large Hadron Collider. Astronomers can watch even larger colliders. Collisions on a vast scale happen throughout the Universe – between galaxies and giant clusters of galaxies. Harnessing these natural particle experiments is helping us investigate the next unknown material.
Three ingredients make up clusters of galaxies: stars, swirling clouds of gas, and dark matter. The gas can be seen in X-rays, and stars shine like the Sun, but dark matter is frustratingly invisible. By a factor of six, it is the most common ingredient, yet science knows embarrassingly little of its properties. What we do know is that dark matter is heavy, so it can be mapped via the way its gravity affects visible things nearby.
During cluster collisions, stars almost always pass straight through each other. Like Rutherford’s atomic nuclei, stars are pinpoints of matter separated by vast empty space. Conversely, the clouds of gas crash into each other, and stop. The behaviour of the dark matter reveals its nature. Dark matter in the “Bullet Cluster” whizzed through with the stars, showing its lack of interaction (which is also why it doesn’t shine).
Observations of a “drive-by shooting” in which another cluster was hit by five small galaxies at once, suggest their dark matter slightly slowed. This may be the first, tentative evidence for the low-level interactions expected from dark matter. Later this year, I will observe that collision with the Hubble Space Telescope to measure how much dark matter was deflected. Like Rutherford’s experiment for atoms, I hope this will show us what dark matter is made of.
The idea of atoms as a fundamental building-block of nature stretches back to antiquity. By the 19th century, the chemical theorist John Dalton used the idea of atoms to explain chemical relationships in A new system of chemical philosophy (1803). However, the discovery of the electron by J J Thomson in 1897 was the first step in a chain of research which would first model the atom and then eventually explode the idea of the atom as being indivisible. It would be one of his young students, the New Zealander Ernest Rutherford, who would take the next step.
Based at McGill University in Canada and then at Manchester University in England, Rutherford and his co-workers used experimental method to investigate the nature of the atom. The ’gold-foil experiment’ devised by Hans Geiger and Ernest Marsden in 1909 produced unexpected deflections in alpha particles bombarding a thin foil: Rutherford deduced that the atom had a small nucleus which was responsible for such anomalies and where most of its mass resided. This idea was incorporated into his ‘planetary’ model of the atom.
By 1917 Rutherford demonstrated that by firing alpha particles into nitrogen, he could make a transmutation resulting in the production of hydrogen nuclei: or protons as he would come to call them. He had successfully ‘split’ the atom.
Dr Richard Massey is a University Research Fellow at University of Durham. His work is on the dark Universe above the Earth's atmosphere.
The 'Rutherford's experiments' section of this article was written by staff at the Royal Society.