The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [30]
In the second half of the twentieth century, physicists united the field concept with their burgeoning understanding of the microworld encapsulated by quantum mechanics. The result, quantum field theory, provides a mathematical framework for our most refined theories of matter and nature’s forces. Using it, physicists have established that in addition to electric and magnetic fields, there exists a whole panoply of others with names like strong and weak nuclear fields and electron, quark, and neutrino fields. One field that to date remains wholly hypothetical, the inflaton field, provides a theoretical basis for inflationary cosmology.*
Quantum Fields and Inflation
Fields carry energy. Qualitatively, we know this because fields accomplish tasks that require energy, such as causing objects (like paper clips) to move. Quantitatively, the equations of quantum field theory show us how, given the numerical value of a field at a particular location, to calculate the amount of energy it contains. Typically, the larger the value, the larger the energy. A field’s value can vary from place to place, but should it be constant, taking the same value everywhere, it would fill space with the same energy at every point. Guth’s critical insight was that such uniform field configurations fill space not only with uniform energy but also with uniform negative pressure. And with that, he found a physical mechanism to generate repulsive gravity.
To see why a uniform field yields negative pressure, think first about a more ordinary situation that involves positive pressure: the opening of a bottle of Dom Pérignon. As you slowly remove the cork, you can feel the positive pressure of the champagne’s carbon dioxide pushing outward, driving the cork from the bottle and into your hand. A fact you can directly verify is that this outward exertion drains a little energy from the champagne. You know those vapor tendrils you see near the bottle’s neck when the cork is out? They form because the energy expended by the champagne in pushing against the cork results in a drop in temperature, which, much as with your breath on a wintry day, causes surrounding water vapor to condense.
Now imagine replacing the champagne with something less festive but more pedagogical—a field whose value is uniform throughout the bottle. When you remove the cork this time, your experience will be very different. As you slide the cork outward, you make a little extra volume inside the bottle available for the field to permeate. Since a uniform field contributes the same energy at every location, the larger the volume the field fills, the greater the total energy the bottle contains. Which means that, unlike with champagne, the act of removing the cork adds energy to the bottle.
How could that be? Where would the energy come from? Well, think about what happens if the bottle’s contents, rather than pushing the cork outward, pull the cork inward. This would require you to pull on the cork to remove it, an exertion of effort that in turn would transfer energy from your muscles to the contents of the bottle. To explain the increase in the bottle’s energy we thus conclude that, unlike champagne, which pushes outward, a uniform field sucks inward. That’s what we mean by a uniform field’s resulting in a negative—not positive—pressure.
Although there’s no sommelier uncorking the cosmos, the same conclusion holds: if there’s a field—the hypothetical inflaton field—that