The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [42]
*You might think that negative pressure would pull inward and thus be at odds with repulsive—outward-pushing—gravity. Actually, uniform pressure, regardless of its sign, doesn’t push or pull at all. Your eardrums pop only when there is nonuniform pressure, lower on one side than the other. The repulsive push I’m describing here is the gravitational force generated by the presence of the uniform negative pressure. This is a difficult but essential point. Again, whereas the presence of positive mass or positive pressure generates attractive gravity, the presence of negative pressure generates the less familiar repulsive gravity.
*The rapid expansion of space is called inflation, but following the historical pattern of invoking names that end in “on” (electron, proton, neutron, muon, etc.), when physicists refer to the field driving inflation, they drop the second “i.” Hence, inflaton field.
*Among those who played a leading role in this work were Viatcheslav Mukhanov, Gennady Chibisov, Stephen Hawking, Alexei Starobinsky, Alan Guth, So-Young Pi, James Bardeen, Paul Steinhardt, and Michael Turner.
*I stress fundamental particles, like electrons and quarks, because for composite particles, like protons and neutrons (each made from 3 quarks), much of the mass arises from interactions between the constituents (the energy carried by gluons of the strong nuclear force, which bind the quarks inside protons and neutrons, contributes most of the mass of these composite particles).
CHAPTER 4
Unifying Nature’s Laws
On the Road to String Theory
From the big bang to inflation, modern cosmology traces its roots to a single scientific nexus: Einstein’s general theory of relativity. With his new theory of gravity, Einstein upended the accepted conception of a rigid and immutable space and time; science now had to embrace a dynamic cosmos. Contributions of this magnitude are rare. Yet, Einstein dreamed of scaling even greater heights. With the mathematical arsenal and geometric intuition he’d amassed by the 1920s, he set out to develop a unified field theory.
By this, Einstein meant a framework that would stitch all of nature’s forces into a single, coherent mathematical tapestry. Rather than have one set of laws for these physical phenomena and a different set for those, Einstein wanted to fuse all the laws into a seamless whole. History has judged Einstein’s decades of intense work toward unification as having had little lasting impact—the dream was noble, the timing was early—but others have taken up the mantle and made substantial strides, the most refined proposal being string theory.
My previous books The Elegant Universe and The Fabric of the Cosmos covered the history and essential features of string theory. In the years since they appeared, the theory’s general health and status have faced a spate of public questioning. Which is completely reasonable. For all its progress, string theory has yet to make definitive predictions whose experimental investigation could prove the theory right or wrong. As the next three multiverse varieties we will encounter (in Chapters 5 and 6) emerge from a string theoretic perspective, it’s important to address the current state of the theory as well as the prospects for making contact with experimental and observational data. Such is the charge of this chapter.
A Brief History of Unification
At the time Einstein pursued the goal of unification, the known forces were gravity, described by his own general relativity, and electromagnetism, described by Maxwell’s equations. Einstein envisioned melding the two into a single mathematical sentence that would articulate the workings of all nature’s forces. Einstein had high hopes for this unified theory. He considered Maxwell’s nineteenth-century work on unification an archetypal contribution to human thought—and rightly so. Before Maxwell, the electricity flowing through a wire, the force generated by a child’s magnet, and the light streaming to earth from the sun were viewed as three separate, unrelated phenomena. Maxwell revealed