Why simplicity works

Why is simplicity so unreasonably effective at scientific explanation? |  Aeon Essays
Detail of Measurement of the Earth (Eratosthenes) (1966), by Crockett Johnson. Courtesy Ruth Krauss in memory of Crockett Johnson/NMAH

Does the existence of a multiverse hold the key for why nature’s laws seem so simple?

Johnjoe McFadden is professor of molecular genetics at the University of Surrey in the UK. His books include Quantum Evolution (2011); Human Nature: Fact and Fiction (2006); Life on the Edge: The Coming of Age of Quantum Biology (2014), co-authored with Jim Al-Khalili; and Life Is Simple (2021).

Edited by Sally Davies

It’s May 1964 and, on a low hillside in New Jersey, the physicists Robert Woodrow Wilson and Arno Allan Penzias are listening in on the Universe. They are standing beneath what looks like a gargantuan ear trumpet attached to a garden shed: the Holmdel Horn Antenna, built by Bell Laboratories to investigate microwaves as an alternative to radio waves for telecommunication. When interest in microwave communication waned, Bell lent out the Holmdel horn to interested scientists.

The Holmdel Horn Antenna in New Jersey, United States. Courtesy Wikipedia

Penzias and Wilson were interested. Both aged around 30, they planned to map the sky with microwaves. But they were baffled: when they pointed the horn at a dark region beyond the galaxy and only sparsely populated with stars, instead of the silence they expected, they detected a kind of background hiss – a hiss that filled the entire sky.

Meanwhile, the physicist Robert H Dicke was working on a related puzzle. Two decades earlier, Dicke had invented the microwave detector. Now he and his lab were trying to develop sensitive instruments to test the cosmological predictions that emerged from Albert Einstein’s general theory of relativity, particularly how it related to Edwin Hubble’s astonishing discovery that the Universe is expanding. The reigning, steady-state theory claimed that the Universe had always been expanding, balanced by a continuous creation of new matter. The rival theorists, including Dicke, took expansion at its face value, running it backwards in time to propose that, about 14 billion years ago, the Universe burst into existence in a cataclysmic explosion from a very tiny point.

An exploding universe should have left a uniform faint cloud of microwave radiation, which Dicke’s team was determined to find. News of the group’s efforts reached Penzias and Wilson, prompting Penzias to give Dicke a call. Over a brownbag lunch, Dicke’s colleagues recall him picking up the receiver, repeating phrases such as ‘horn antenna’ and nodding. After hanging up, he turned to his group and said: ‘Well boys, we’ve been scooped.’ Dicke realised that Penzias and Wilson had discovered the Big Bang.

The uniformity of the cosmic microwave background (CMB) tells us that, at its birth, ‘the Universe has turned out to be stunningly simple,’ as Neil Turok, director emeritus of the Perimeter Institute for Theoretical Physics in Ontario, Canada, put it at a public lecture in 2015. ‘[W]e don’t understand how nature got away with it,’ he added. A few decades after Penzias and Wilson’s discovery, NASA’s Cosmic Background Explorer satellite measured faint ripples in the CMB, with variations in radiation intensity of less than one part in 100,000. That’s a lot less than the variation in whiteness you’d see in the cleanest, whitest sheet of paper you’ve ever seen.

Wind forward 13.8 billion years, and, with its trillions of galaxies and zillions of stars and planets, the Universe is far from simple. On at least one planet, it has even managed to generate a multitude of life forms capable of comprehending both the complexity of our Universe and the puzzle of its simple origins. Yet, despite being so rich in complexity, some of these life forms, particularly those we now call scientists, retain a fondness for that defining characteristic of our primitive Universe: simplicity.

The Franciscan friar William of Occam (1285-1347) wasn’t the first to express a preference for simplicity, though he’s most associated with its implications for reason. The principle known as Occam’s Razor insists that, given several accounts of a problem, we should choose the simplest. The razor ‘shaves off’ unnecessary explanations, and is often expressed in the form ‘entities should not be multiplied beyond necessity’. So, if you pass a house and hear barking and purring, then you should think a dog and a cat are the family pets, rather than a dog, a cat and a rabbit. Of course, a bunny might also be enjoying the family’s hospitality, but the existing data provides no support for the more complex model. Occam’s Razor says that we should keep models, theories or explanations simple until proven otherwise – in this case, perhaps until sighting a fluffy tail through the window…



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