illustration of a black hole, swirling light around it

A Brief History of Time: Stephen Hawking’s 1988 classic theoretical physics book

A single “theory of everything” exists. We just haven’t found it yet.

That’s one of the main arguments from theoretical physicist Stephen Hawking (1942-2018), as articulated in his 1988 bestselling book A Brief History of Time. The book helped make him one of his generation’s best known intellectuals, and he used an array of impressive technologies to help him continue to shape public thought during his long battle with ALS. It helped popularize many obscure and complex ideas.

Though he didn’t win a Nobel Prize in his lifetime and he occupied a kind of celebrity status, he did contribute meaningfully to his field. In 1974, in his early 30s, Hawking argued that black holes would emit heat energy, so-called Hawking radiation, which would mean that, unless they otherwise added mass, a black hole could eventually vanish. He helped us discover that black holes might not even be, you know, black. That work gave him needed pedigree to write this book, which is a relatively breezy read while also citing much of the most exciting ideas in theoretical physics and even cosmology.

As a hobbyist consumer of pop science, I’ve long wanted to read this text. Much of what he wrote about has been covered by an array of science Youtubers and writers I follow. Yet I still got much from the book. Do read it. Below I share my notes from the book for myself.

Here are my notes:

  • Newton law of universal gravitation from 1687 (the first great unification) was superseded by Einstein’s theory of relativity (the second) in 1915 but it is still used for practical purposes because it’s simple and accurate for most of our practical purposes, excepting for extremely massive and dense objects, or at very small distances.
  • Universal Theory of Everything is the goal of today’s theoretical physics
  • General theory of relativity and quantum mechanics are our two partial theories for the universe today.
  • All the light we see in the universe is from the past (the sun’s light is from 8 minutes ago). Therefore we are always only now reacting to the past
  • God does not play dice”: despite his contributions Einstein doubted the unpredictability that quantum mechanics introduces
  • We understand “4 fundamental forces” of the universe, listed from strongest to weakest:
    • the strong nuclear force
    • the electromagnetic force
    • the weak nuclear force
    • gravity
  • “Grand unifying theory” has a more specific usage
  • Quarks (what we consider elementary particles today) outnumber antiquarks allowing matter to accumulate and life to form
  • I don’t yet fully grasp CPT symmetry
  • Quantum mechanics predicts that light is both wave and particle
  • In 1783, John Mitchell predicted stars so dense no light could emit; since 1969, we’ve called them black holes
  • Chandrasekhar’s Limit describes the stability and instability of stars relative to their mass
  • The cosmic censorship hypothesis, described in 1960 by Roger Penrose, says we can’t see singularities past an event horizon
  • “A black hole has no hair” is a popular way of describing that a black hole has no known features outside of a few elemental features: mass, electric charge, and angular momentum
  • Temperature is average energy of its particles
  • “The whole history of science has been the gradual realization that events do not happen in an arbitrary manner, but that they reflect a certain underlying order, which may or may not be divinely inspired” -Stephen Hawking
  • Anthropic principle: “we see the universe the way it is because we exist” and the strong and weak forms of this says this is why the universe is this age and other features
  • In 1979, Alan Guth put forward the cosmic inflation theory we know as the Big Bang
  • The total energy of the universe is zero ”because matter clusters and anti particles exist elsewhere. It’s just a clumpy universe.”
  • Hawking introduced that space Time is finite but has no boundary (like the curved earth but in 4d)
  • “A theory should make predictions that can be tested with observation.”
  • If there is no absolute time and imaginary time goes backward and forward then “why do we remember the past but not the future?” The laws of science do not distinguish between the past and future.
  • At least three kinds of arrows of time
    • Thermodynamic: second law in which more disorder is created in a closed system
    • Psychological: in which we remember the past but not the future
    • Cosmological: in which the universe is expanding rather than contracting
  • Hawking argues the psychological arrow exists because it follows second law of disorder increasing (the energy to make something orderly uses orderly food energy and releases heat that adds more disorder to the universe). Psychological and thermodynamic are essentially the same arrow, he argues. the cosmological arrow will someday flip when the universe begins to contract down to the “big crunch.”
  • Universe began smooth and orderly and has gotten lumpy and disorderly over time
  • Universe may begin to contract in ten thousand million years: will entropy reverse?
  • Hawking once thought entropy would reverse but he now believes that entropy will always continue. He admits his previous thinking as a mistake and cites Einstein as someone else who made a notable mistake in the science (his defense of the cosmological constant) and admitted it
  • Intelligent life as we know it will only exist in the expanding stage of the universe, he argues
  • Kurt Gödel (1906-1978) put among Aristotle and Gottlob Frege (1848-1925) as influential logicians
  • Einstein-Rosen Bridge wormholes allow travel without being faster than speed of light by bending dimensions
  • Negative energy balance
  • Casimir effect for time travel by using the gravitational pull of black holes
  • Perhaps the past is fixed so you can only travel into the future, which is why we’ve never been visited by anyone from the future
  • If traveling to past, this may either be “consistent histories” (no free will) or “alternative histories” like Back to the Future
  • Chronology protection conjecture: perhaps we can’t travel into the past with the knowledge we have in the future
  • 1928 Max Born said “ Physics as we know it will be over in six months.” He was wrong.
  • World sheets in string theory
  • There is the anthropic argument for why we have 3D space and 1d time even if string theory predicts 10 or 26 dimensions. The rest of those dimensions are infinitely tiny and turned in on themselves. The anthropic argument is the universe was built for us, and 3D allows gravity to work enough and we can exist and move in space; any more and planets gravity wouldn’t work the same
  • Author believe there is a unified theory of physics because there are only three options:
    • There is one and we can find it if we are smart enough
    • There isn’t one, just an endless maze of ever more complicated puzzles that dig deeper
    • There isn’t one and there can’t be because there is randomness (god)
  • St Augustine (354-430) argued it is a misunderstanding to think God exists in time at all; Gdod created time so lives outside of that known universe. He wrote: “For that very time You made, nor could times pass by before You made times.”
  • Arguing against the second option listed above about the united theory, Hawking notes that there is the Planck limit (length) to “boxes on boxes” of ever smaller elementary particular; Hawking believes we do eventually reach that final level, even if we aren’t there now. So, Hawking argues there is somewhere order between general relativity and quantum.
  • In the 19th century science became too technical for philosophers to keep up and ask “why does the universe exist,” rather than “what are the laws of the universe” as physicists ask.
  • Wittgenstein: “the sole remaining task of philosophy is the analysis of language “
  • We may end with the Big Crunch