The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos

by Brian Greene

Cosmic Destiny

Distance and Brightness

A Type Ia supernova occurs when a white dwarf star pulls material from the surface of a companion, typically a nearby red giant that it’s orbiting. Well-developed physics of stellar structure establishes that if the white dwarf pulls away enough material (so that its total mass increases to about 1.4 times that of the sun), it can no longer support its own weight. The bloated dwarf star collapses, setting off an explosion so violent that the light generated rivals the combined output of the other 100 billion or so stars residing in the galaxy it inhabits.

Whose Distance Is It, Anyway?

The cosmological conversion factor is called the universe’s scale factor; in an expanding universe, the scale factor increases with time.

The Colors of Cosmology

Cosmic Acceleration

For the last 7 billion years, contrary to long-held expectations, the expansion of space has not been slowing down. It’s been speeding up.

A hundred billion years from now, any galaxies not now resident in our neighborhood (a gravitationally bound cluster of about a dozen galaxies called our “local group”) will exit our cosmic horizon and enter a realm permanently beyond our capacity to see.

The Cosmological Constant

The data also allowed the researchers to fix the numerical value of the cosmological constant—the amount of dark energy suffusing space. Expressing the result in terms of an equivalent amount of mass, as is conventional among physicists (using E = mc2 in the less familiar form, m = E/c2), the researchers showed that the supernova data required a cosmological constant of just under 10–29 grams in every cubic centimeter.8 The outward push of such a small cosmological constant would have been trumped for the first 7 billion years by the inward pull of ordinary matter and energy, in keeping with the observational data. But the expansion of space would have diluted ordinary matter and energy, ultimately allowing the cosmological constant to gain the upper hand. Remember, the cosmological constant does not dilute; the repulsive gravity supplied by a cosmological constant is an intrinsic feature of space—every cubic meter of space contributes the same outward push, dictated by the cosmological constant’s value. And so the more space there is between any two objects, arising from cosmic expansion, the stronger the force driving them apart. By about the 7-billion-year mark, the cosmological constant’s repulsive gravity would have carried the day; the universe’s expansion has been speeding up ever since, just as the data in Figure 6.2 attest.

Instead, for reasons that will shortly become apparent, the natural choice is to express the cosmological constant’s value as a multiple of the so-called Planck mass (about 10–5 grams) per cubic Planck length (a cube that measures about 10–33 centimeters on each side and so has a volume of 10–99 cubic centimeters). In these units, the cosmological constant’s measured value is about 10–123, the tiny number that opened this chapter.

Explaining Zero

Everyone knew that you can’t trust quantum field theory on super-small distance scales. Jitters with wavelengths as small as the Planck scale, 10–33 centimeters, and smaller, have energy (and from m = E/c2, mass equivalent) so large that the gravitational force matters. To describe them properly requires a framework that melds quantum mechanics and general relativity.

But the immediate and more pragmatic response among researchers was simply to declare that the calculations should disregard jitters on scales smaller than the Planck length. Failure to implement this exclusion would extend a quantum field theory calculation into a realm clearly beyond its range of validity. The expectation was that we will one day understand string theory or quantum gravity well enough to deal with the super-small jitters quantitatively, but the interim stopgap was to mathematically quarantine the most pernicious fluctuations.

But even so, researchers found that the resulting answer for the energy jitters, while finite, was still gargantuan, about 1094 grams per cubic centimeter.

While a large finite number for the energy that suffuses space is better than an infinite one, physicists realized the dire need for dramatically reducing the result from their calculations.

One physicist who challenged the orthodoxy was the Nobel laureate Steven Weinberg.* In a paper published in 1987, more than a decade before the revolutionary supernova measurements, Weinberg suggested an alternative theoretical scheme that yielded a decidedly different outcome: a cosmological constant that is small but not zero. Weinberg’s calculations were based on one of the most polarizing concepts to have gripped the physics community in decades—a principle some revere and others vilify, a principle some call profound and others call silly. Its official, if misleading, name is the anthropic principle.

Cosmological Anthropics

Clearly, then, the degree to which you are swayed by the anthropic approach depends on the degree to which you are convinced of its three essential assumptions: (1) our universe is part of a multiverse; (2) from universe to universe in the multiverse, the constants take on a broad range of possible values; and (3) for most variations of the constants away from the values we measure, life as we know it would fail to take hold.

Life, Galaxies, and Nature’s Numbers

From Vice to Virtue

The Final Step, in Brief

The String Landscape

Quantum Tunneling in the Landscape

The Rest of Physics?

Is This Science?

CHAPTER 7

Science and the Multiverse

On Inference, Explanation, and Prediction

The Soul of Science

Accessible Multiverses

Science and the Inaccessible I: Can it be scientifically justifiable to invoke unobservable universes?

So for confidence in a theory to grow we don’t require that all of its features be verifiable; a robust and varied assortment of confirmed predictions is enough. Scientific work going back well over a century has accepted that a theory may invoke hidden, inaccessible elements—provided it also makes interesting, novel, and testable predictions about an abundance of observable phenomena.

It may give one confidence about the theory's claims regarding unobservables, but it can't declare that those unobservables are any more real than anything imaginable that doesn't contradict observed reality.

Science and the Inaccessible II: So much for principle; where do we stand in practice?

The Brane, Cyclic, and Landscape Multiverses are based on string theory, so they suffer multiple uncertainties. Remarkable as string theory may be, rich as its mathematical structure may have become, the dearth of testable predictions, and the concomitant absence of contact with observations or experiments, relegates it to the realm of scientific speculation.

Or religion.

Predictions in a Multiverse I: If the universes constituting a multiverse are inaccessible, can they nevertheless meaningfully contribute to making predictions?

The essential feature of such “predictive multiverses” is that they’re not composed from a grab-bag of constituent universes. Instead, the capacity to make predictions emerges from the multiverse evincing an underlying mathematical pattern: physical properties are distributed across the constituent universes in a sharply skewed or highly correlated manner.

But, would it play in Vegas?

Predictions in a Multiverse II: So much for principle; where do we stand in practice?

Predictions in a Multiverse III: Anthropic reasoning

Prediction in a Multiverse IV: What will it take?

Dividing Up Infinity

A Further Contrarian Concern

Mysteries and Multiverses: Can a multiverse provide explanatory power of which we’d otherwise be deprived?

But I and many others have come to realize that although some fundamental features of the universe are suited for such precise mathematical predictions, others are not—or, at the very least, it’s logically possible that there may be features that stand beyond precise prediction.

A bias born of your investment in String Theory.

But to turn away from a multiverse because it could lead us down a blind alley is equally dangerous. In doing so, we might well be turning a blind eye to reality.

And yet, you ignore how blind you are to the lack of progress made by string theorists.

CHAPTER 8

The Many Worlds of Quantum Measurement

The Quantum Multiverse

Quantum Reality