A Brief History of Time

Author: Stephen Hawking

Finished: April 01, 2020

This book blew my mind away. The physical book that I read was published in 1988 and actually used by my dad when he was in college (so, it's really old). The thought that comes to mind is: How timeless is time. Or rather, how timeless is humanity's musing about time?

I found profound the ending (conclusion) of the book, in which Stephen Hawking talks about how humans have always wondered about the creation of the universe, and how even if we were able to figure out the unified theory of physics, it would only explain the "how" the universe was formed; the "why" we as humans would work together to keep pondering.

"Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe? The usual approach of science of constructing a mathematical model cannot answer the questions of why there should be a universe for the model to describe. Why does the universe go to all the bother of existing? Is the unified theory so compelling that it brings about its own existence? Or does it need a creator, and, if so, does he have any other effect on the universe? And who created him" (174)

In the conclusion, Hawking also describes how scientists until now have been too occupied with figuring out WHAT the universe is and not WHY. Philosophers have traditionally asked the WHY questions but now the scientific theories have gotten too technical.

"If we do discover a complete theory, it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason - for then we would know the mind of God." (175)

Maybe half of the book went over my head, but I still enjoyed it. It blew my mind more than half of the time. Even having two physics parents didn't help; some of these concepts are just really confusing. However, seeing as how my purpose wasn't to understand all physics concepts in their entirety, but to get the big picture, I really enjoyed this book. Stephen Hawking was able to explain many of the major physics and astronomy related breakthroughs in the past couple hundred years in concise and narrative terms that made reading this book enjoyable.


  • Aristotle vs. Newton and Galileo

    • Aristotle - preferred state of rest (17) which any body would take up if it were not driven by some force or impulse
    • Newton - there is no absolute space; there is no unique standard of rest
    • Einstein - there is no absolute time (theory of relativity); each observer must have his own measure of time
    • Space and time are now both dynamic quantities (33)
  • Friedmann Model

    • at some point in the past the distance between neighboring galaxies must have been zero
    • big bang - "density of the universe and the curvature of space-time would have been infinite" (46)
    • can't deal with infinite numbers in math, so general theory of relativity breaks down at the singularity
    • curvature of space time is infinite in the big bang singularity
  • General relativity is an incomplete theory
  • Uncertainty Principle

    • Marquis de Laplace (nineteenth century) argued that the universe is completely deterministic (scientific determinism)
    • Werner Heisenberg (1926) - Uncertainty Principle

      • "In order to predict the future position and velocity of a particle, one has to be able to measure its present position and velocity accurately" (54) ... the more accurately you try to measure the position of the particle, the less accurately you can measure its speed, and vice versa
      • Uncertainty principle puts an end to Laplace's dream of deterministic universe model
      • "One certainly cannot predict future events exactly if one cannot even measure the present state of the universe precisely!" (55)
    • Schrodinger, Heisenberg, Paul Dirac → Quantum mechanics (based on uncertainty principle)
    • Einstein - never accepted universe was governed by change ("God does not play dice")
  • Elementary Particles

    • Aristotle - matter is continuous
    • Democritus - matter is grainy
    • Atom is smaller than wavelength of light, so we cannot "look" at the parts of an atom ... We need to use something with much smaller wavelength! (65)
  • Black holes

    • "Stars with masses about the Chandrasekhar limit ... have a big problem when they come to the end of their fuel. In some cases they may explode or manage to throw off enough matter to reduce their mass below the limit and so avoid catastrophic gravitational collapse, but it was difficult to believe that this always happened, no matter how big the star. How would it know that it had to lose weight?" (84) (haha)
    • 1965 - Penrose and Hawking - according to general relativity, there must be a singularity of infinite density and space-time curvature within a black hole ... laws of science would break down (88)

      • "God abhors a naked singularity"
      • weak cosmic censorship - protects observers from outside black hole from the consequences of breakdown of predictability that occurs at the singularity
      • for the person falling into the black hole, "the singularity would always lie in his future and never in his past" (89) whaaat
      • event horizon = boundary of the region of space-time from which it is not possible to escape - one-way membrane
    • Brandon Carter (1970), David Robinson (1973)

      • "a black hole has no hair"
      • i.e. after gravitational collapse of black hole, its size and shape depends ONLY on its mass and rate of rotation, not on the nature of the body that had collapsed to form it (92)

        • literally reduced to just the mass!
  • Origin and Fate of the Universe

    • Theory of relativity → space-time begins with big bang singularity, will come either to a big crunch singularity, or at a singularity inside a black hole
    • Theory of quantum → conservation of mass, black hole and its singularity evaporates away and disappears
    • "DOES THE UNIVERSE IN FACT HAVE A BEGINNING OR AN END? And if so, what are they like?" (115)
    • if universe has the beginning at Big Bang, who wrote the initial configurations?
    • anthropic principle - "we see the universe the way it is because we exist" (124)
    • using imaginary time (using imaginary numbers) and then the distinction between space and time disappears completely (134)
    • Feynman - sum over histories
    • No-boundary theory (from quantum theory of gravity) - there would be no boundary to space-time and so there would be no need to specify the behavior at the boundary" (136)

      • Universe is completely self-contained and not be affected by anything outside itself
      • "It would neither be created nor destroyed. It would just BE" (136)

    "So long as the universe had a beginning, we could suppose it had a creator. But if the universe is really self-contained, having no boundary or edge, it would have neither beginning nor end; it would simply be. What place, then, for a creator? (141)

  • The arrows of time

    • thermodynamic arrow of time (direction of time in which disorder or entropy increases)
    • psychological arrow of time (subjective sense of direction of time; how we feel time passes) - affected by thermodynamic time
    • cosmological arrow of time (direction of time in which universe is expanding)
  • Unification of physics

    • what would it mean if we really did discover the ultimate theory of the universe?
    • "in newton's time it was possible for an educated person to have a grasp of the whole of human knowledge, at least in outline" (167)
    • but development of science so fast!
    • theories are always being changed to account for new observations ... "you have to be a specialist"
    • "rate of progress is so rapid that what one learns at school is always a bit out of date"

    "A complete, consistent, unified theory is only the first step: our goal is a complete understanding of the events around us, and of our own existence" (169)

meta thoughts

  • What if the animals on earth (i.e. bees, hives) follow galaxy-level patterns based on faint gravitational (or other energy we don't know about) pulses? Like how queen bee acts (Connection with Richard Dawkins' Selfish Gene?)
  • Are we just unconsciously listening to pulses of the universe's vibrating breath - it directs the movement of our blood cells, the spin of our electrons

    • i.e. entropy stems from universe's set of laws, applied to our solar system microcosm
    • universe as a living system
  • Entropy in humans - in young life we want to be chaotic and move around and travel and stuff, but as we get older we care more about settling down. Do we reflect patterns of the universe?
  • Beginning of universe (hot) ... and how chaos bleeds to stability, like human life tends to ... What poets and systems engineers share ... finding and connecting patterns and analogies!!!
  • The first primitive forms of life consumed different materials from what we consume now, and released oxygen, and this changed the atmosphere to the composition that it has today (121)

    • What if carbon emission and climate change are just allowing for the next stage of evolution of Earth to allow for new evolutionary behavior and for development of other forms of life?