Thursday, 6 October 2016

From a clockwork universe to the uncertainty of quantum mechanics

Before the turn of the 20th century, most people (physicists included) thought the universe ran like a clockwork machine. If you could measure the properties of the universe accurately enough, you could predict how it was to behave in the future. It was, in other words, "deterministic". Much of this was down to the ideas of Newton.

Newtonian mechanics

It's probably fair to say that no single individual has had a greater influence on the scientific view of the world than Sir Isaac Newton. He was a genius (a god in the physics world!), but like all genii he lived in his particular era of history. In 1543, a century before Newton's birth, Nicolaus Copernicus launched a scientific revolution by rejecting the prevailing Earth-centred view of the Universe in favour of a heliocentric view in which the Earth moved round the Sun. Galileo was summoned to appear before the Inquisition in 1633 charged with heresy for supporting Copernicus' ideas. As a result Galileo was "shown the instruments of torture".

Newton's great achievement was to provide a synthesis of scientific knowledge to explain why the planets went round the sun (among other things). He discovered a convincing quantitative framework that seemed to underlie everything else – he proposed his law of gravity. By combining this law with his general laws of motion, Newton was able to demonstrate mathematically that a single planet would move around the Sun in an elliptical orbit. For the first time, scientists felt they understood the fundamentals, and it seemed that future advances would merely fill in the details of Newton's grand vision.

An orrery – a mechanical clockwork model of the motions of the planets in the solar system

Newton's discoveries became the basis for much more study, and the upshot of this was a mechanical world-view that regarded the Universe as something that unfolded like clockwork – predictable and mechanistic. People thought that once this mechanism had been set in motion, its future development was, in principle, entirely predictable. Hence the Universe was thought to be "deterministic", and physicists felt very safe with this idea.

This mechanistic view still prevailed two centuries later (up to the end of the 19th century) as scientists continued to stand on Newton's large shoulders and think they just had to "fill in the details". For example, a stormy sea may look random and unpredictable, but this is just a consequence of its complexity and the huge number of water molecules involved. In the mechanistic view, if you had a big enough computer and accurate knowledge of the starting conditions, such a system would be entirely predictable.

Cracks in the clockwork

However, some cracks in Newtons clockwork mechanism were starting to appear. In the late 19th century, a number of discoveries happened that just couldn't be explained by the old model – including the discovery of the photoelectric effect by Heinrich Hertz (1887) and of the electron by J. J. Thomson (1897) and the fact that electric charge occurs in indivisible units called quanta (Millikan, 1909).

Along came Einstein in the early 20th century. He put forward new theories of gravity and energy (he won the Nobel Prize in 1921 for his explanation of Hertz's photoelectric effect). In 1913, Nils Bohr explained the discrete spectral emission lines of the hydrogen atom, again by using the idea of quantization and what later came to be known as "photons" (1926). The "quantum revolution" had begun!

Over the last 75 years or so, quantum mechanics has brought a profound change in human thinking, particularly around the notion of "indeterminism".

The quantum revolution

Quantum physics is concerned primarily with things at the microscopic scale such as atoms and molecules, and how they move and interact. In the quantum world we find a very serious kind of unpredictability that cannot be blamed on our ignorance of the details or our lack of computation clout. Instead it turns out to be a fundamental feature of nature. In the realm of atoms, all we can do is calculate probabilities for different outcomes – and we can never, even in principle, do any better.

A 3D quantum view of an atom formed of protons and neutrons in the nucleus, surrounded by electrons. Electrons aren't really in orbits, but more in fuzzy "probability zones" that look like shells and lobes.

One example is the radioactive decay of an atomic nucleus. Unstable nuclei (e.g. uranium-238) will "spontaneously" decay into a more stable form by emitting a particle. Quantum mechanics allows us to predict with high accuracy the time after which half of a collection of unstable nuclei will have decayed (the half-life), but not when one particular nucleus will have decayed.

Strange behaviour

The problem is things at the quantum level just don't behave like things on a macroscopic level. One of the inherent differences is that single particles (like an electron) sometimes behave as if they're solid "particles" and sometimes behave as if they are waves. This paradoxical behaviour has been known since Thomas Young's double-slit experiment way back in 1805. The fact is they're both, and neither. The concept of a solid particle (like a snooker ball) is inadequate, and the idea of a wave (like a water wave) is also inadequate. What we call particles (like an electron) are really more like packets of wave-like energy and they just appear to behave differently in different circumstances.

We're taught at school that an atom is composed of a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus (similar in structure to the solar system). This model was introduced by Niels Bohr and Ernest Rutherford in 1913. It's a helpful way of thinking of things, but not entirely correct. The electrons are not balls whizzing around in an orbit. All we can say is that there's a certain region within which we're most likely to find the electron wave-packet when we look.

in 1927, Werner Heisenberg discovered what he called the "uncertainty principle". It says that one can never know at the same time the precise location and velocity of a "particle". The better you know one, the less certain you can be about the other. It's a consequence of their very nature.


After hundreds of years of thinking of the Universe mechanistically, this old mindset has filtered down into society as a whole. We all, to some degree, view the world as acting like clockwork. If I do this, that happens; cause produces effect – it's safe, secure, predictable and dependable.

But reality isn't like that. Uncertainty and probability are built in.

Uncertainty, fuzziness, indeterminacy are wonderful things! In his essay on the "Seven Radical Principles of Wise Decision Making", Martin Boronson comments that it's because we deeply despise uncertainty that we value decisiveness so much. However it's in holding that uncertainty that and being ok with it that creativity can happen. "New ideas only emerge if we can sustain the tension and anxiety [of the uncertainty] and wait."

I am a member of the Zenways sangha led by Zen master Daizan Skinner Roshi, and I teach meditation, mindfulness and yoga at the ZenYoga studio in Camberwell, London. See my website for further details.

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  1. Very nice, thanks Mark! I needed a clean up of my physics knowledge and just got it. Plural of genius is genii by the way.

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