It is one of the most frequently asked questions in school science lessons: has anyone ever seen an atom? The answer is yes. Using electron microscopes capable of magnifications of a million times and more, atoms are now routinely observed up close and personal in labs around the world. And last month saw the passing of the British-born American physicist who made that possible: Professor Albert Crewe.
During the 1970s, Prof Crewe made headlines by unveiling images and movies of individual atoms. While the fundamental building blocks of matter had been glimpsed by Erwin Muller at Penn State University in 1955, it was Prof Crewe and his colleagues at the University of Chicago who developed electron microscopy to the point where atoms could be studied in detail as a matter of routine. As such, it was they who delivered the final knockout blow in the bitter debate about the very existence of atoms - which, incredibly enough, was still being dismissed as metaphysical nonsense by leading scientists as recently as the turn of the 20th century.
Ever since, the controversy has stood as a warning to the baleful effects of letting philosophy influence the progress of science. Ironically, the first proponents of atoms were themselves philosophers. During the 5th century BC, the Greek scholar Leucippus and his student Democritus argued that in a constantly changing world there must be entities that remain constant, and fundamental, and called these entities "atoms" - from the Greek for "indivisible". The somewhat flaky nature of these arguments and the lack of any hard evidence for the reality of atoms inevitably prompted criticism from other philosophers - notably Aristotle, whose rejection of the "atomic hypothesis" choked off any further developments for more than 2,000 years.
Once the Aristotelian world view was abandoned during the Renaissance, the atomic hypothesis started to show its power. By the mid-1700s, the Swiss mathematician Daniel Bernoulli had pointed out that the notion of atoms could explain the behaviour of gases. Around 50 years later the English chemist John Dalton showed that atoms could be the basis of all chemistry. The most far-reaching implications of the atomic hypothesis were uncovered by the 19th-century Austrian physicist Ludwig Boltzmann. In 1872, the 28-year-old wunderkind made the startling claim that atoms could explain all the fundamental properties of heat. While no one had ever seen an atom, the general view was that they zoomed around and collided with one another at random. Boltzmann pictured a vast number of atoms trapped in a box, and derived an equation describing their behaviour. As it seemed reasonable to assume that vast numbers of atoms were involved, Boltzmann turned to statistics to capture their overall behaviour. The result, now called Boltzmann's Equation, showed that regardless of how the atoms move, their behaviour follows a specific mathematical formula. Known as Maxwell's Distribution, this formula was already known to explain many features of gases and liquids. What Boltzmann had done was to show precisely why the formula worked.
Or at least, he had if one believed in atoms. Unfortunately for Boltzmann and for science, not everyone did. And in 1894, Boltzmann found himself sharing his department at the University of Vienna with Ernst Mach, a renowned philosopher and physicist and bitter critic of the concept of atoms. Mach was a leading member of the Empiricist school of philosophy, which insisted that only concepts capable of empirical verification by human senses had any place in science. And since no one had ever seen or felt the existence of atoms directly, Mach deemed them to be no more than convenient fictions.
It is hard to credit so bizarre an argument, which blithely ignores the possibility that technology might one day allow atoms to be seen directly - as indeed it did. No less astonishing is the fact that Mach was not alone in his beliefs. Among his fellow anti-atomists was the distinguished chemist Friedrich Ostwald of Leipzig University, who teamed up with Mach to launch a sustained attack on Boltzmann's work.
Both sides arguing their case so vociferously that even the protagonists wondered if things were getting out of hand. Their concerns were not misplaced. Exhausted by the constant attacks on his work, Boltzmann became increasingly depressed, and during a family holiday in September 1906, he hanged himself. Tragically, Boltzmann did not know just how close he had come to seeing his atomic theory vindicated. The evidence centred on a baffling phenomenon observed 80 years earlier by a Scottish naturalist. While observing grains of pollen under a microscope, Robert Brown noticed tiny specks dancing around within fluid-filled voids in the pollen. He suspected they were some form of life until he observed the same motion among microscopic particles of patently lifeless stone.
The cause of this "Brownian motion" went unexplained until the publication of a paper in 1905 by a young Swiss patent clerk called Albert Einstein, who showed it was consistent with otherwise invisible atoms striking the particles and making them dance around. Further experiments confirmed Einstein's arguments, and by 1908 the evidence for atoms was so strong that even Ostwald conceded that his erstwhile adversary had been right. While cold comfort for Boltzmann, the debacle stands as a monument to the dangers of letting philosophical arguments trump quantitative experiments.
Today, most scientists remain deeply sceptical of the role of philosophy in their work. Yet it is starting to creep once again into debates over ideas at the cutting edge of fundamental physics. There is, for example, mounting interest in the idea that the visible universe is just a tiny part of a much grander "multiverse". According to some theorists, its existence would resolve a number of conceptual problems in fundamental science. However, critics argue that as we can never observe anything beyond our visible universe, all talk of the multiverse is untestable speculation - and thus violates the definition of science put forward by the late philosopher of science, Sir Karl Popper.
Certainly, much of the work on the multiverse sounds like thinly disguised science fiction. Yet to reject its reality on purely philosophical grounds smacks of the defeatism of Mach and his followers. For who is to say that scientists will never repeat the achievement of the late Prof Crewe, and give us the means to routinely observe the unobservable? Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England