âOnly atoms and the void are real,â said the ancient Greek philosopher Democritus. âWell, actually, itâs a bit more complicated than that. Atoms can also be waves, the void is made up of fields and everyone is going to need to start using the word âquantumâ all the time.â
OK, he said only the first bit. Democritus pioneered atomism â the idea that everything in the universe can be divided into atoms, which can then be divided no further. But as we mark 100 years since the development of quantum mechanics in 1925, I have been wondering whether, somewhere in a corner of the multiverse, he and other ancient philosophers could have come up with a version of the theory millennia earlier. If so, what would that world look like?
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This article is part of a special series celebrating the 100th anniversary of the birth of quantum theory. Read more here.
âThe story could have started 2400 years ago,â says , chief scientist at quantum computing firm Quantinuum. For him, the point of divergence came when Parmenides, another ancient Greek philosopher, declared that the universe is singular and unchanging. He even believed that motion was impossible â something his contemporary Heraclitus reportedly attempted to disprove by waving his arm in front of his face. Parmenides was having none of it: just because the arm was in one place, then another, it doesnât mean we can say it moved, he argued.
Parmenides inspired Democritus, who â quite reasonably â rejected the wacky view that motion was impossible, but embraced the idea of an unchanging reality. His atomism squared these two principles by allowing the world we observe to change, while the underlying, indivisible atoms that make up reality remained the same.
The idea of reality as a snapshot that evolves over time forms the basis of much of classical physics, but Coecke sees this as a detour because the wrong philosopher won out. He has pioneered a view of quantum mechanics that emphasises processes and the relationships between them, much as Heraclitus saw the physics governing matter. âHeraclitus favoured the process view of reality, [where] weâre in a constant flux,â says Coecke.
This relational view was later espoused by Gottfried Wilhelm Leibniz in the 17th century, but not fully developed as a theory of quantum mechanics until the late 20th century â a missed opportunity, says Coecke. âScience could have developed completely different[ly] if we had actually started with Heraclitus and ignored Democritus.â
Perhaps, but others disagree. âIf you consider the individual ideas of a theory, none of them is strange enough not to have been thought at some point in time,â says at the University of Verona in Italy. âThe point is that what we are talking about is not just a bunch of crazy, isolated ideas. Itâs a full-fledged working theory.â
Certainly, if you break down the necessary components of quantum mechanics, it becomes clear that many would have been inaccessible to the ancient Greeks. âYouâre going to have to have a deep understanding of electromagnetism, which the Greeks [didnât] have any idea about, beyond static electricity,â says at Cornell University in New York state. It wasnât until the 1860s that James Clerk Maxwell showed that electricity and magnetism are two sides of the same coin.
You would also need some knowledge of a few other ideas from around the same time. âYou need an understanding of field theory, which comes with [Michael] Faraday. You need an understanding of statistical mechanics, which comes in the 19th century,â says Seth.
Going down the particle path
OK, so the ancient Greeks are probably out. Their ideas reigned until the scientific revolution that kicked off modern science, so what about someone like Isaac Newton? In the 17th century, Newton proposed that light was made up of particles called corpuscles, an intellectual descendant of Democritusâs atoms. His ideas stood against those of Christiaan Huygens, who believed that light only took the form of waves. At the time, Newtonâs corpuscular idea won out and wouldnât be overturned until the 19th century, when experiments by Thomas Young demonstrated lightâs irrefutable wave-based nature.
Today, we know that both Newton and Huygens were headed in the right direction. Wave-particle duality, the idea that fundamental entities like light can behave like both particles and waves, is now a key component of quantum mechanics. So, could Newton have got there earlier? âWith a different worldview, maybe,â says Coecke, but it wouldnât look much like the theory we know today. âPeople would have come up with a different theory of physics, then they wouldnât have called it quantum mechanics, because for them it wouldnât be about quanta,â he says, referring to the discrete packets in which energy radiates.
If we assume we are imagining a more conventional route to quantum mechanics, Seth says Newton couldnât have gotten there because he just didnât have the experimental data required to inform the development of quantum theory. âNewton, as we know, was one of the greatest minds that the world has ever produced,â he says. âBut the problems that he was solving are not anything like the kinds of problems that weâre trying to solve later, given, say, atomic spectroscopy.â
Spectroscopy, the study of the spectra of light absorbed and emitted by an object, produced one of the key signs that something was wrong with classical physics in the late 19th century. âThe crisis in classical mechanics was essential for the development of quantum theory,â says Seth.
At the time, physicists were struggling to understand the black-body problem, which concerns the behaviour of a theoretical object that absorbs all radiation that hits it. Newton discussed the concept in his 1704 book Opticks, but physicists didnât begin to examine it fully until the development of devices like spectrometers in the 19th century.
Over time, these observations made it clear that the spectra of radiation emitted by such an object would depend on its temperature, with hotter objects emitting more radiation at lower wavelengths â this is why an iron bar, say, glows red, then yellow, and then white as it is heated. But the best theoretical models at the time predicted that the energy released at lower ultraviolet wavelengths could be infinite, which clearly wasnât physically possible.
This stumbling point, which later became known as the ultraviolet catastrophe, was solved by Max Planck in 1900, who took a radical step. Rather than back the accepted view that radiation is continuously emitted by a black body, he proposed it could be released only in quanta. With this assumption, his new model perfectly matched the black-body observations.
But this was only the start of the story. For the next 25 years, giants of physics would wrestle with what this strange idea truly meant. Initially, the idea of energy quantisation was taken only as a calculation trick to produce the correct answer, rather than a reflection of the nature of reality. Physicists were used to such tricks, thanks to the development of statistical mechanics by Ludwig Boltzmann in the 19th century. With classical physics unable to fully model the behaviour of large numbers of particles, such as the motion of atoms in a gas, Boltzmann turned to tools from probability as a way of approximating their actions â a successful breakthrough that would later inspire Planck to pursue quanta.
But these probabilistic methods were seen merely as a reflection of our inability to gather a complete picture of many-particle systems, says Badino. âMost physicists, before quantum mechanics, didnât take probability seriously,â he says. âThey took it as a tool.â
Enter the matrix
During the period from 1900 to 1925, this view of energy quanta as a simple mathematical correction to classical physics was used to develop what is now sometimes called the âold quantum theoryâ, rather than full-blown quantum mechanics. âOld quantum theory is just classical mechanics with a few quantum conditions slapped onto it,â says at the University of Minnesota.
For example, Albert Einstein in 1905 proposed that electrons emitted by light hitting a material, known as the photoelectric effect, could be explained if light were actually made of individual particles, later known as photons; this work would win him the Nobel prize in physics in 1921. And in 1913, Niels Bohr developed the first quantised model of the atom, in which electrons can only take certain energy levels. He was able to fully explain the atomic spectroscopy data of hydrogen, landing him the physics Nobel in 1922.
But, as more data was gathered, Bohrâs model failed at the next simplest case: the helium atom. Physicists began to acknowledge that something was still missing.
It didnât come until 1925, when Werner Heisenberg set out to âestablish a basis of theoretical quantum mechanics founded exclusively on relationships between quantities that in principle are observableâ. In other words, the spectroscopic data, not imagined ideas of electrons orbiting atomic nuclei, must be at the heart of a true quantum theory.
Along with Max Born and Pascual Jordan, Heisenberg developed a âmatrix mechanicsâ version of quantum theory. This uses grids of energy frequency values, or matrices, to describe atomsâ behaviour, finally succeeding in matching theory and experiments at the cost of making our picture of the atom far vaguer â a bitter pill to swallow. âI am convinced that the physics community would not have gone for quantum mechanics if it hadnât been for the pressure of the spectroscopic data,â says Janssen.
Abandoning the idea of the atom as a neat package of particles and instead embracing its true reality as a fuzzy blob of probability was the conceptual leap that allowed modern quantum mechanics to be born. Such a leap would have been impossible for the ancient Greeks, who saw randomness and chance as merely the will of the gods. Newton would also have found it difficult to fathom, given his laws of motion dealt in certainty and absolutes. Even Einstein, who lived through the leap, struggled with it, famously stating that âGod does not play diceâ with the universe. And really, we are still struggling with the interpretation of quantum mechanics today, says Janssen. âA hundred years later, thereâs still absolutely no agreement on what it is all supposed to mean.â
So what was it that allowed Heisenberg to rethink reality in such a radical way? There is an argument to be made that the specific sociological conditions in which he and his contemporaries were operating were key to the development of quantum mechanics.
âThe category of theoretical physicist emerges in the last part of the 19th century in Germany, and itâs worth noting that thatâs where all the work gets done,â says Seth. Before that period, all physicists were experimentalists at heart, he says: âMax Planck is essentially the first person who gets a full professorship in something called theoretical physics.â
While experimentalists used reality to inform theory, the new-breed theoretical physicists were willing to let mathematics prescribe how the universe really operates, says Badino. âTheoretical physics is a completely different kind of game.â
The key ingredient
For Seth and Badino, the emergence of theoretical physics as a discipline in its own right places a hard limit on how early quantum mechanics could have been developed; at most, it could have arrived only a few decades before it did. Janssen thinks there is even less wiggle room, pointing to key developments by Einstein and Bohr from 1916 to 1918 that would go on to inspire Heisenberg. âYou canât push it much further back than 1918,â he says.
Of course, Germany â indeed the whole of Europe â was rather preoccupied during that specific period. âWe cannot neglect the fact that there was a war,â says Badino. But did the first world war accelerate or delay the birth of quantum mechanics? Probably the latter, says Seth. âCommunicating scientific ideas slows down dramatically,â he says, and âthere is a whole generation of young, brilliant physicists who dieâ.
We canât know exactly what contributions these people would have made, but it is hard to argue that the war effort drove physics forward â a marked contrast to the second world war, during which physics was inextricably linked to the development of the atomic bomb and mathematics to the creation of the computer.
That said, some people claim that Germanyâs cultural circumstances after the first world war did play a role in quantum mechanics. In 1971, historian of science Paul Forman argued that the freewheeling, permissive culture of the Weimar Republic (the name given to the German state between 1918 and 1933), along with an antagonism towards traditional âexactâ science, was a breeding ground for a theory that would dispense with the tidy idea of electrons orbiting atoms in favour of something far messier.
âThe reason that you have quantum mechanics developing in that period is that the physicists are eager to fall in line with that zeitgeist,â says Janssen, speaking of Formanâs thesis, but this remains a contentious idea. Janssen personally disagrees with Forman, saying quantum mechanics took hold simply because it worked. âIt solved a number of problems,â he says. âI donât see any big influence of the zeitgeist here.â
With all that in mind, it seems my multiverse thought experiment has brought us back to where we started: The quantum revolution began 100 years ago, and there wasnât much opportunity for it to start earlier. But hang on â what about starting later?
âWhat are the most important ingredients of quantum mechanics?â says Coecke. âPeople will have different views on that. But all these features came at different points in time, not all in the 1920s.â
For example, one interpretation of quantum mechanics returns to the concept that the probabilistic elements of the theory really are just mathematical tools, reflecting our incomplete knowledge of the universe rather than holding any deeper meaning. This âhidden variableâ theory supposes there actually is a concrete reality; we just canât measure it.
Many physicists, including Einstein, liked that idea, but John Stewart Bell slapped them down in the 1960s. He demonstrated that any hidden variable theory would fail to fully reproduce the weirdness and wonder of quantum mechanics. His âBell testsâ, as they became known, allowed experimentalists to prove that the world is truly quantum, by closing possible loopholes that would permit hidden variables.
Coecke argues it was only after these experiments that quantum mechanics was fully accepted â and points out that the final loophole was closed only in 2015. âVery, very recent,â he says. In other words, forget the centenary â perhaps we should be celebrating merely the 10th anniversary of quantum mechanics.
