Frank Close has a question. “If you step off the top of a cliff, how does the Earth down there ‘know’ you are up there for it to attract you?” It’s a question that has taxed many illustrious minds before him. Newton’s law of gravitation first allowed such apparently instantaneous “action at a distance”, but he himself was not a fan, describing it in a letter as “so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it”.
Today we ascribe such absurdities to fields. “The idea of some physical mediation – a field of influence – is more satisfying,” says Close, a physicist at the University of Oxford. Earth’s gravitational field, for example, extends out into space in all directions, tugging at smaller objects like the moon and us on top of a cliff; the Earth itself is under the spell of the sun’s gravitational field.
Advertisement
But hang on: what exactly is a field?
On one level, it is just a map. “Ultimately, a field is something that depends on position,” says , a theoretical physicist at the Massachusetts Institute of Technology. A gravitational field tells us the strength of gravity at different points in space. Temperatures or isobars on a weather chart are a field. A field is a mathematical abstraction – numbers spread over space.
But there is more to it than that. Witness what physicist Michael Faraday saw in the 19th century, and many a schoolkid has since: iron filings neatly ordering themselves along the lines of a magnetic field, reaching out into space from the magnet itself and influencing nearby objects (though at the speed of light, not instantaneously). “It made a huge impression on Faraday, that this strange thing had a physical reality,” says Wilczek.
Arguably the modern world is built on the principle of electromagnetic induction that Faraday developed out of his new understanding of fields: magnetic fields and electric fields power the motors of our civilisation. A mere abstraction?
The modern era has shed some further light on fields, but also added confusion. Quantum fields – ultimately, the electromagnetic field is one – have tangible products in the form of particles, which pop up as disturbances within them. For the electromagnetic field, this entity is the photon. The Higgs field, long postulated to pervade empty space and to give elementary particles their mass, was discovered in 2012 by squeezing out its particles in high-energy collisions.
But quantum fields are complicated beasts, formed of “superpositions” of many classical fields. That’s far away from anything we can envisage as a map, or delineate as neat lines. “At that point I have to rely on equations,” says Wilczek, who for his work on the quantum fields of the strong nuclear force.
One thing’s for sure: fields are everywhere. Quantum theory teaches us that even seemingly empty space is a roiling broth of fields and their associated particles. “The idea that nothing’s there is extremely naive,” says Wilczek. Aside from anything else, fields are the proof that nature does indeed abhor a vacuum.
Read more: “Get your head around the 13 boldest ideas in science”
This article appeared in print under the headline “How to think about fields”
