What is an atom?

Atoms are the tiny building blocks of our world. They consist of microscopic particles of matter bound together by the forces of nature. The scale of an atom is so small that if you expand an apple to the size of the earth, then its atoms expand to roughly the size of the original apple!

 “If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms — little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.”

Richard Feynman, 1964

The radical idea that all things are made of tiny atoms was first proposed by the Roman poet  Lucretius (b. 90BC), in his poem On the Nature of Things. The essential truth of Lucretius’ idea has survived thousands of years, and had a profound influence on the development of scientific thought. By studying the way atoms interact with light, and the way they interact with each other, scientists have uncovered the structure of atoms, the particles they are made of, and the strange rules governing the motion of particles in the amazing quantum world at the atomic scale.


Most of the atoms we encounter are balanced in a very specific way: their building blocks are arranged so they are electrically neutral. The light-weight electrons carry negative charge and whiz around the heavier protons (positive) and neutrons (neutral) lurking in the dense core of the atom, the nucleus. In 1909 Ernest Rutherford carried out an experiment on gold atoms to probe the inner structure of the atom, revealing just how dense its nucleus really is: the width of the nucleus is typically 100,000 times smaller than the width of the atom, and contains 99% of its mass.


Did anyone ever tell you it was impossible to actually "see" a single atom? Their explanation may have been that because the size of the atom is much smaller than the wavelength of visible light, it is impossible to see. However, here at the University of Otago we have done just that! The trick is to isolate one atom away from any others, and then “take a picture” of the atom using a special kind of laser light.


The key element in the experimental setup is a high aperture lens placed inside the vacuum chamber in which the experiments take place. The aperture refers to the ability of the lense to collect light. The use of the lens is twofold. First, by directing off-resonant laser light through the lens, we form a microscopic confining region (called an optical dipole trap) that can trap one or more atoms at the focal point of the lens.  Second, we can “see” a trapped atom by exposing it to a beam of quasi-resonant light, causing it to flouresce. Becuase we know quite well where the atom is, a large portion of the flourescence can be collected by the lens to create an image of single atom!

Evidence for atoms: Einstein and Brownian motion

Have you ever seen an atom?

In a ground breaking experiment at Otago, single atoms were trapped and observed for the first time with a high probability (90%). To find out more about single atom physics at Otago, visit the Atom Manipulation and Interferometry research group.


The Centre for Quantum Science is  a University of Otago Research Centre hosted by the Department of Physics.


Department of Physics

University of Otago

730 Cumberland Street

Dunedin 9010



Ashton Bradley                       

ashton.bradley [at] otago.ac.nz    


Niels Kjærgaard            

niels.kjaergaard [at] otago.ac.nz