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“Even though much remains to be learned before we will be able to recount a black hole's history by reading its ashes—the Hawking radiation-many theorists now agree that there is no longer a real paradox. Moreover, I'd argue that this development is something entirely different. Moving from a single spacetime to emergent spacetimes has truly foundational implications.”

“Einstein showed that gravity is a manifestation of warped spacetime. Holography goes further and postulates that warped spacetime is woven from quantum entanglement.”

“entanglement between the inside and the outside of an evaporating black hole can be thought of as generating a wormhole stretching across the horizon. It is as if the particles of Hawking radiation together with their antiparticle partners behind the horizon collectively sew their own space-time bridge, transforming an old black hole from a hermit kingdom into sort of drive-through.”

“a quantum mechanical link... Physicists say they remain "entangled." The entanglement means that if you measure the emitted radiation on its own, it looks like random thermal radiation. But if one were able to consider the members of the pairs jointly, one would find that they do contain information encoded in fine correlations that interlink their individual properties.”

“Hawking radiation originates in quantum jitters of fields near the horizon of black holes. These jitters give rise to particle-antiparticle pairs. Whenever the antiparticle falls into the black hole, its partner particle can escape into the distant universe, where it shows up as Hawking radiation emitted by the hole...despite their distance the pairs of particles and antiparticles maintain a quantum mechanical link”

“IT WOULD TAKE much more work in Maldacena's holographic laboratory, and the exploration of many more blind alleys, before theorists finally began to discern the escape route out of an old black hole. As a matter of fact, in the years since Stephen's passing, a new generation of black hole physicists, steeped in holography, have realized that perhaps wormholes are involved.”

“"The moment we say that a black hole is described by Schwarzschild's geometry we have an information loss problem," Stephen told his audience in Dublin. "However, information about the exact state is preserved in a different geometry. The confusion and paradox arose because we thought classically, in terms of a single objective spacetime. But Feynman's sum over geometries allows it to be both geometries at once."”

“if determinism, the probabilistic predictability of the universe on the basis of scientific laws, were to break down in the presence of black holes, how could we be sure it doesn't break down in other situations? How could we be sure of our own history, and our memories? "The past tells us who we are" Stephen noted pointedly. "Without it, we lose our identity." ”

“As such the black-hole information paradox became a beacon in the search for a unified theory. Physicists felt that if they could disentangle Hawking's knot and understand what happens to the hidden information when black holes cease to be, they'd be well on their way to marrying the principles of relativity and quantum theory into a single coherent framework.”

“The paradox of the fate of information locked inside evaporating black holes became the most vexing puzzle in theoretical physics of the late twentieth century, bedeviling not one but two generations of physicists. It is in some ways the contemporary analogue of the Mercury anomaly in the nineteenth century, the wobble in Mercury's orbit that defied Newton's theory.”

“This was the first glimmer of holography in modern physics: The storage capacity of black holes isn't determined by their interior volume but by the area of their horizon surface. It is as if black holes do not have an interior but are holograms.”

“Bekenstein and Hawking's entropy formula predicts that black holes have a truly gigantic information storage capacity. Black holes are likely the most space-efficient storage devices in the universe indeed. According to their formula, Sagittarius A, the huge black hole of 4M sun masses lurking in the center of the Milky Way—the shadow of which was first imaged in the spring of 2022—can store no less than 10^80 GB.”

“Hawking's formula exhibits the interplay of different areas. By combining principles from quantum theory and general relativity, Hawking had taken a mathematical risk, but he was rewarded with an insight that neither relativity nor quantum theory alone could ever have provided: Black holes radiate.”

“To his surprise he found that quantum processes near the horizon surface, the point of no return in relativity, gave rise to a tiny flux of thermal radiation streaming away from the black hole in all directions. And he went on to calculate the temperature, T, of a black hole…”

“Today, whenever Planck's spectrum shows up in nature, physicists take this as a telltale sign of an underlying quantum process. This was precisely the sort of process Hawking considered. Stephen looked at black holes from a semiclassical angle, studying the quantum behavior of matter moving about in the classical, warped geometry of a black hole.”

“The 2.7 Kelvin cosmic microwave background radiation, for example, is black-body radiation. It tells us that even the entire observable universe behaves like an ordinary radiating body. In the year 1900, Planck's theoretical derivation of the spectrum of black-body radiation marked the dawn of the quantum revolution.”

“In A Brief History of Time, the early (bottom-up) Hawking famously wrote, "Even if we do find a theory of everything, it is just a set of rules and equations. What is it that breathes fire into the equations?" The answer of the later (top-down) Hawking was: observership. We create the universe as much as the universe creates us.”

“We started out searching for a deeper explanation of the universe's fitness for life in the physical condition at the origin of time. But the quantum cosmology we developed to this suggests we were looking in the wrong direction. Top-down cosmology recognizes that, much like biology's tree of life, physics tree of laws outcome of a Darwinian-like evolution that can only be understood backward in time.”

“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.”