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“Finally his screen lit up and clicking resumed, slowly this time. "With [a] top-down [approach] we put humankind back in the center [of cosmological theory] Interestingly, this is what gives us control." "In a quantum universe we switch on the light", I added. ”

“"I wrote in Brief History." I completed the thought for him: "that we are just a chemical scum on a medium-sized planet orbiting an average star in an ordinary galaxy." His eyebrows went up in assent. That was the old bottom-up Hawking appeared on the screen. From a God's-eye view we are but an irrelevant speck.”

“Look around you; identify as many law-like patterns in your data as you can; use these to construct histories of the universe ending up like the one you observe; add them together to create your past. So, instead of a background of absolutes, top-down cosmology prioritizes the historical nature of everything.”

“Far-fetched in his time, Wheeler's vision of a participatory universe would wind up as a centerpiece of our top-down cosmology forty years later. Hawking took Wheeler's observer's participance seriously—very seriously and applied it not just to retroactively determine the paths of quantum particles but of the universe as a whole.”

“The delayed-choice experiment illustrates vividly and strikingly that the process of observation in quantum mechanics introduces a subtle form of teleology into physics, a backward-in-time component.”

“the unobserved past in quantum mechanics exists as a spectrum of possibilities only—a wave function. Much like electrons or radioactive decay particles, fuzzy photon wave functions morph into a definite reality only when the future to which they give rise has been fully settled, i.e., observed.”

“We are, of course, all too familiar with backward-in-time reasoning from thinking about other layers of evolution, from biological evolution to human history. I briefly described in chapter 1 how history at all levels is shaped by the chance outcomes of countless branching events. These frozen accidents add a retrospective component to the study of history…”

“history at the very deepest level emerges backward in time. It is as if a constant flux of quantum acts of observation retroactively carves out the outcome of the big bang, from the number of dimensions that grow large to the types of forces and particles that arise. This renders the past contingent on the present, a further reduction of causality…”

“observership in quantum cosmology isn't a mere afterthought or an anthropic post-selection principle acting in a giant preexisting multiverse, but an agency operating at a deeper level, an indispensable part of the continual process through which physical reality—and physical theory, we argue—come about.”

“Whereas Bohr argued that all but one outcome survives, Everett claimed this is only the view from within a given branch of history.”

“The universe we see around us, this branch of reality, is the collective result of innumerable such environmental acts of observation. Having registered and built upon countless chance outcomes, over a period of billions of years, each contributing a few bits of information to our branch of history, this is how the world around us has acquired its specificity.”

“AT THE LEVEL of experience...every act of observing amounts to some sort of pruning of the branching tree of possible futures. A measurement situation in quantum theory is like a fork in the road, where history divides into two or more separate branches. In the experience of any given ob-server, at such branching points, only one of the branches survives. More precisely, on each branch, only that branch survives.”

“de Finetti in 1974 wrote: "My thesis… is...probability does not exist… only subjective probabilities exist, the degree of belief in the occurrence of an event attributed by a given person at a given instant and with a given set of information."…Throughout our lives, most of us gain confidence in subjective probabilities because we find that results we deem likely happen often, and those that we don't happen rarely.”

“Everett's framework, in which probabilities wiggle their way into quantum theory in a more subtle…way, much like the way probability enters our thinking in day-to-day life. Whether we ponder the weather, the lottery, or the shape of the next gravitational wave passing through planet Earth, we all use subjective probabilities all the time to quantify our uncertainty in situations where we have incomplete knowledge.”

“Everett himself said he sought somehow to bridge the positions of Einstein and Bohr. He claimed their differences were a matter of perspective and described his scheme as "objectively deterministic, with probability appearing at the subjective level." This is an interesting point. In the early Copenhagen formulation of quantum mechanics, probabilities were axiomatic and fundamental.”

“Everett pulled down Bohr's wall separating the quantum microworld from the classical macroworld. His key idea was to take the math behind quantum mechanics seriously and to apply it to everything. Suppose there is no collapse, he suggested, but only a single universal wave function that includes observers and everything else, evolving gently and smoothly...”

“Niels Bohr, on the other hand, who had a background in philosophy as well as mathematics, had a profound intuition that quantum mechanics was consistent. Bohr took seriously the central tenet of quantum mechanics that observership—the very questions we ask of nature—affects how nature manifests itself. "No phenomenon is a real phenomenon until it is an observed phenomenon," he held.”

“To Einstein, the probabilistic nature of quantum mechanics signaled that the theory was incomplete, that there had to be a deeper-lying framework that permitted an objectively real description of physical reality, regardless of any acts of observation. "The [quantum] theory produces a good deal but hardly brings us closer to the secret of the Old One," he wrote to Bohr. "I am convinced...He does not play dice"”

“[Stephen:] The history of the universe depends on the question you ask. Good night.”

“Stephen put together one more line: "I think that a proper quantum outlook (onto the universe] will lead to a different philosophy of cosmology in which we work from the top down, backward in time, starting from the surface of our observations." I was startled—Stephen's new top-down philosophy would seem to upend the relation between cause and effect in cosmological theory.”

“The Cartesian answer to the scientific revolution was to move the Archimedean point inward, to man himself, and to choose the human mind as the ultimate point of reference. The dawn of the modern age threw men back upon themselves. From Dubito ergo sum, "I doubt, therefore I am," came Cogito ergo sum, "I think, therefore I am."”

“Hannah Arendt, one of the twentieth century's most celebrated thinkers, sharply articulated this uncomfortable, straddled position in The Human Condition: "The great strides of Galileo proved that both the worst fear of human speculation-that our senses might betray us—and its most presumptuous hope–the Archimedean wish for a point outside from which to unlock universal knowledge–could only come true together."”

“Now while the ontological status of the physical laws hardly matters in the controlled environment of laboratories, it explodes in our faces when we ponder their deeper origin—let alone when we inquire about their biophilic character.”

“Anticipating Stephen, I responded emphatically: A God's-eye view is obviously fallacious in cosmology. We are within the universe, not somehow outside it. Stephen assented and concentrated intensely on composing his next phrase. The failure to recognize this, he clicked, has led us into a blind alley. We need a new philosophy [of physics] for cosmology. Ah, I laughed, time for philosophy at last!”

“We had this old idea, that there was a universe out there, and here is man, the observer, safely protected from the universe by a six-inch slab of plate glass. Now we learn from the quantum world that even to ob-serve so minuscule an object as an electron we have to shatter the plate glass; we have to reach in there...” -John Archibald Wheeler

“The sheer vastness of the possibilities in biological evolution means that any kind of causal deterministic explanation of why we have this particular tree of life is doomed to fail. This is why biologists work ex post facto, describing how a given outcome leads back to a specific sequence of branchings.”

“anthropic multiverse cosmology says, we have intrinsically biofriendly laws of physics merely because we could hardly have evolved in a universe where the physical conditions preclude our existence. In a sense the anthropic principle says that we find the physics of the observable universe the way it is because we are here.”

“quantum jitters…of spacetime create a self-reinforcing cycle of ever more frantic jittering that destroys its own basic structure. And unlike other fields, which undulate in a fixed background of space and time, gravity is spacetime. This is the crux of the difficulty when trying to reconcile gravity with quantum theory.”

“life built its stupendous complexity on an inconceivably large number of frozen accidents… From the functionalities of individual organisms over the characteristics of species to the taxonomy of the tree of life, law-like patterns in biology encode the outcomes of countless chance events, which, over a period of billions of years, in a co-evolving environment, have enabled layer upon layer of complexity to emerge”