The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [29]
At this point, Newton would surely shoot you another disapproving look. Ever the skeptic, he would find another problem with your explanation. After catching up on the more intricate details of general relativity by racing through one of the standard textbooks, he would accept the strange fact that gravity can—in principle—be repulsive. But, he’d ask, what’s all this talk of negative pressure permeating space? It’s one thing to use the inward pull of a stretched rubber band as an example of negative pressure. It’s another to argue that billions of years ago, just around the time of the big bang, space was momentarily permeated by an enormous and uniform negative pressure. What thing, or process, or entity has the capacity to supply such a fleeting but pervasive negative pressure?
The genius of inflation’s pioneers was to provide an answer. They showed that the negative pressure required for an antigravity burst naturally emerges from a novel mechanism involving ingredients known as quantum fields. For our story, the details are crucial because the manner in which inflationary expansion comes about is central to the version of parallel universes it yields.
Quantum Fields
In Newton’s day, physics concerned itself with the motion of objects you can see—stones, cannonballs, planets—and the equations he developed closely reflected this focus. Newton’s laws of motion are a mathematical embodiment of how such tangible bodies move when they’re pushed, pulled, or shot through the air. For more than a century, this was a wonderfully fruitful approach. But in the early 1800s, the English scientist Michael Faraday initiated a transformation in thinking with the elusive but demonstrably powerful concept of the field.
Take a strong refrigerator magnet and place it an inch above a paper clip. You know what happens. The clip jumps up and sticks to the magnet’s surface. This demonstration is so commonplace, so thoroughly familiar, that it’s easy to overlook how bizarre it is. Without touching the paper clip, the magnet can make it move. How is this possible? How can an influence be exerted in the absence of any contact with the clip itself? These and a multitude of related considerations led Faraday to postulate that though the magnet proper does not touch the paper clip, the magnet produces something that does. That something is what Faraday called a magnetic field.
We can’t see the fields produced by magnets; we can’t hear them; none of our senses are attuned to them. But that reflects physiological limitations, nothing more. As a flame generates heat, so a magnet generates a magnetic field. Lying beyond the physical boundary of the solid magnet, the magnet’s field is a “mist” or “essence” that fills space and does the magnet’s bidding.
Magnetic fields are but one kind of field. Charged particles give rise to another: electric fields, such as those responsible for the shock you sometimes receive when you reach for a metal doorknob in a room with wall-to-wall wool carpeting. Unexpectedly, Faraday’s experiments showed that electric and magnetic fields are intimately related: he found that a changing electric field generates a magnetic field, and vice versa. In the late 1800s, James Clerk Maxwell put mathematical might behind these insights, describing electric and magnetic fields in terms of numbers assigned to each point in space; the numbers’ values reflect the field’s ability, at that location, to exert influence. Places in space where the magnetic field’s numerical values are large, for instance an MRI’s cavity, are places where metal objects will feel a strong push or pull. Places in space where the electric field’s numerical values are large, for instance the inside of a thundercloud, are places where powerful electrical discharges such as lightning may occur.
Maxwell discovered equations, which now bear his name, that govern how the