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Kindle Notes & Highlights
by
Brian Greene
Read between
April 20 - September 25, 2020
String Theory and Mathematics
The State of String Theory: An Evaluation
The theory will remain speculative until a convincing link to experiment or observation is forged.
GOAL: Unite gravity and quantum mechanics
IS GOAL REQUIRED?: Yes.
STATUS: Excellent.
GOAL: Unify all forces
IS GOAL REQUIRED?: No.
STATUS: Excellent.
GOAL: Incorporate key breakthroughs from past research
IS GOAL REQUIRED?: No.
STATUS: Excellent.
GOAL: Explain particle properties
IS GOAL REQUIRED?: No.
STATUS: Indeterminate; no predictions.
GOAL: Experimental confirmation
IS GOAL REQUIRED?:
Yes.
This highlight has been truncated due to consecutive passage length restrictions.
STATUS: Indeterminate; no predictions.
GOAL: Cure singularities
IS GOAL REQUIRED?: Yes.
STATUS: Excellent.
GOAL: Black hole entropy
IS GOAL REQUIRED?: Yes.
STATUS: Excellent.
GOAL: Mathematical contributions
IS GOAL REQUIRED?: No.
STATUS: Excellent.
CHAPTER 5
Hovering Universes in Nearby Dimensions
The Brane and Cyclic Multiverses
The fact is, the vast majority of phenomena, from the motion of planets to the interactions of particles, are just too complex to be described mathematically with complete precision. Instead, the task of the theoretical physicist is to figure out which complications in a given context can be discarded, yielding a manageable mathematical formulation that still captures essential details.
Beyond Approximations
Duality
Branes
Branes and Parallel Worlds
It is here that parallel universes make their stringy entrance.
Sticky Branes and Gravity’s Tentacles
Time, Cycles, and the Multiverse
The Past and Future of Cyclic Universes
Zanstra showed that there couldn’t have been an infinite number of cycles preceding our own. The wrench in the cosmological works was the Second Law of Thermodynamics.
As Tolman himself had realized, the equations of general relativity link the entropy content of the universe with the duration of a given cycle. More entropy means more disordered particles squeezed together when the universe shrinks; that generates a more powerful rebound, space expands further, and so the cycle lasts longer. Looking back from today, the Second Law then implies that ever-earlier cycles would have had ever-less entropy (because the Second Law says that entropy increases toward the future, it must decrease toward the past),* and would thus have had ever-shorter durations.
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Steinhardt and company claim that their new version of cyclical cosmology avoids this pitfall. In their approach, the cycles arise not from a universe expanding, contracting, and expanding again but rather from the separation between braneworlds expanding, contracting, and expanding again. The branes themselves continually expand—and they do so throughout each and every cycle. Entropy builds from one cycle to the next, just as the Second Law requires, but because the branes expand the entropy is spread over ever-larger spatial volumes. The total entropy goes up, but the entropy density goes
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The Cyclic Multiverse is widely known within the physics community but is viewed, almost as widely, with much skepticism. Observations have the capacity to change this. If evidence for braneworlds emerges from the Large Hadron Collider, and if signs of gravitational waves from the early universe remain elusive, the Cyclic Multiverse will likely garner increased support.
In Flux
CHAPTER 6
New Thinking About an Old Constant
The Landscape Multiverse
The Return of the Cosmological Constant
In any case, one thing stands. The general theory of relativity allows the inclusion of the cosmological constant in the field equations. One day, our actual knowledge of the composition of the fixed star sky, the apparent motions of fixed stars, and the position of spectral lines as a function of distance, will probably have come far enough for us to be able to decide empirically the question of whether or not the cosmological constant vanishes. Conviction is a good motive, but a bad judge.
