There was no place for randomness in 17th century science. Newtonian mechanics suggested that if you accurately measure enough data (trajectory, speed, air pressure, friction, etc.) you could calculate the precise outcome of a dice roll. But such a calculation was too complex, so a dice roll was considered random because of a lack of information. Laplace asserted that with enough information one could forecast everything that is going to happen in the universe – and, working backwards, everything that had happened. Such a universe leaves no room for free-will - everything is pre-determined. Thomas Aquinas was aware of this issue and asserted that the universe must contain randomness for humans to have autonomy.
Then in 1859, Maxwell drew attention to the huge variations in outcome resulting from tiny factors affecting the collisions of molecules. With a sufficiently complex system, even the tiniest lack of precision in measurement, or the slightest rounding error in a calculation, could drastically affect the result. This is why the weather so hard to predict - its future state is highly dependent on the initial measurements – and we can never have perfect initial measurements.
Quantum theory does away with cast-iron certainty entirely. "/It appears to us, via quantum experiments, that nature is fundamentally random,/" says Adrian Kent, a mathematician at the University of Cambridge.
Fire a single photon of light at a half-silvered mirror, and it might pass through or be reflected: quantum rules give us no way to tell beforehand. Give an electron a choice of two slits in a wall to pass through, and it chooses at random. Wait for a single radioactive atom to emit a particle, and you might wait a millisecond or a century. This rather lackadaisical attitude to classical certainties could even account for why we are here in the first place. A quantum vacuum containing nothing can randomly and spontaneously generate something. Such a careless energy fluctuation might best explain how our universe began.
The mathematics behind quantum physics starts with the Schrödinger equation, which describes how a quantum particle's properties evolve over time. An electron's position, for example, is given by an "amplitude" smeared over space, and there is a set of mathematical rules you can apply to find the probability that any particular measurement will pinpoint the electron to any particular position.
That's no guarantee the electron will be in that position at any one time. But by repeatedly doing the same measurement, resetting the system each time, the distribution of results will match the Schrödinger equation's predictions. The repeated, predictable patterns of the classical world are ultimately the result of many unpredictable processes.
Say you want to walk through a wall; quantum theory says it's possible. Each one of your atoms has a position that could – randomly – turn out to be on the other side of the wall when it interacts. That event's probability is exceedingly low, and the probability that all of your atoms will simultaneously locate to the other side of the wall is infinitesimally small. A nasty bruise is the sum of all the other probabilities. Welcome to reality.
Einstein was particularly exercised by this probabilistic approach to real-world events, famously complaining it was akin to God playing dice. He conjectured that there must be some missing information that would tell you the measurement's outcome in advance.
In 1964, the physicist John Bell laid out a way to test for such "hidden variables". His idea has since been implemented time and again, mainly using entangled pairs of photons. Entangled particles are a staple feature of the quantum world. They have interacted at some point in the past and now appear to have shared properties, such that a measurement on particle A will instantaneously affect what you get from a measurement on particle B, and vice versa.
What's behind this? The details of Bell's tests are complex and subtle, but the principle is akin to a sport in which two groups of experimenters play according to different rules. Team Alpha assumes that the quantum correlations are down to some hidden exchange of information, and make measurements accordingly. Team Beta, on the other hand, assumes the correlations materialise at random on measurement.
And Team Beta wins every time. The weird correlations of the quantum world derive from fundamental randomness.
Michael Brooks is a consultant for New Scientist
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