The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [44]
The mid-1980s witnessed the next pivotal development. That’s when a new approach, superstring theory, captured the attention of the world’s physicists. It ameliorated the hostility between general relativity and quantum mechanics, and so provided hope that gravity could be brought within a unified quantum mechanical fold. The era of superstring unification was born. Research proceeded at an intense pace, and thousands of journal pages were quickly filled with calculations that fleshed out aspects of the approach and laid the groundwork for its systematic formulation. An impressive and intricate mathematical structure emerged, but much about superstring theory (string theory, for short) remained mysterious.3
Then, beginning in the mid-1990s, theorists intent on unraveling those mysteries unexpectedly thrust string theory squarely into the multiverse narrative. Researchers had long known that the mathematical methods being used to analyze string theory invoked a variety of approximations and so were ripe for refinement. When some of those refinements were developed, researchers realized that the math suggested plainly that our universe might belong to a multiverse. In fact, the mathematics of string theory suggested not just one but a number of different kinds of multiverses of which we might be a part.
To fully grasp these compelling and contentious developments, and to assess their role in our ongoing search for the deep laws of the cosmos, we need to take a step back and first evaluate the state of string theory.
Quantum Fields Redux
Let’s begin by taking a closer look at the traditional, highly successful framework of quantum field theory. This will prepare us to string unification as well as highlight pivotal connections between these two approaches for formulating nature’s laws.
Classical physics, as we saw in Chapter 3, describes a field as a kind of mist that permeates a region of space and can carry disturbances in the form of ripples and waves. Were Maxwell to describe the light that’s now illuminating this text, for example, he’d wax enthusiastic about electromagnetic waves, produced by the sun or by a nearby lightbulb, undulating across space on their way to the printed page. He’d describe the waves’ movement mathematically, using numbers to delineate the field’s strength and direction at each point in space. An undulating field corresponds to undulating numbers: the field’s numerical value at any given location cycles down and up again.
When quantum mechanics is brought to bear on the concept of a field, the result is quantum field theory, which is characterized by two essential new features. We’ve already encountered both, but they’re worth a refresher. First, quantum uncertainty causes the value of a field at each point in space to jitter randomly—think of the fluctuating inflaton field from inflationary cosmology. Second, quantum mechanics establishes that, somewhat as water is composed of H2O molecules, a field is composed of infinitesimally small particles known as the field’s quanta. For the electromagnetic field, the quanta are photons, and so a quantum theorist would modify Maxwell’s classical description of your lightbulb by saying that the bulb emits a steady stream comprising 100 billion billion photons each second.
Decades of research have established that these features of quantum mechanics as applied to fields are completely general. Every field is subject to quantum jitters. And every field is associated with a species of particle. Electrons are quanta of the electron field. Quarks are quanta of the quark field. For a (very) rough mental image, physicists sometimes think of particles as knots or dense nuggets of their associated field. This visualization notwithstanding, the mathematics of quantum field theory describes these particles as dots or points that have no spatial extent and no internal structure.4
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