A Brief History of Time: Chapter 6 Summary & Analysis

Hawking provides a catch up summary here, focusing more on those concepts directly relevant to the black holes he is preparing the reader to consider. As seen already with other concepts, the idea of black holes were around far before the scientific knowledge to back them up, let alone the technology to detect them. Humanity’s curiosity about black holes had not died out before an approach was devised to understand them.

Laplace’s back tracking on the idea of black holes demonstrates the challenges facing the theory, as one of its own proponents lost heart awaiting proof. Nevertheless, the idea has survived since the late 1700s, reflecting the staying power of logical suggestions, regardless of their difficulties in gaining immediate mass acceptance.

Gradually, more theories were established that could support the reasoning behind and offer further exploration of the idea of black holes. Again, Hawking emphasizes the slow build up behind establishing black holes as a valid theory, one dependent on the many discoveries made previous. Scientific understanding is a process dependent on sustained curiosity and on each interconnecting link in the chain, for now.

As black holes are born from stars, Hawking first gives the reader a thorough, and fascinating grounding in a star’s lifecycle. For the most part, stars spend their life in an existential balancing act. As a massive cloud of burning gas, the star faces the contracting pull of its gravity on the one hand, and the outward pressure of its colliding particles on the other. But this stage cannot last forever, as a star’s fuel is finite, so eventually the star’s own gravity will overpower its pressure, and the contracting phase will begin again.

Familiar with stars’ balancing act lifestyle, Chandrasekhar dug a little deeper and found the boundary at which a star’s mass determines its fate. Below the so-called Chandrasekhar limit, after collapsing for a while, the star will again find stability at a smaller size because of the way particles cannot be in the exact same place, giving it structure.

If a star is above the Chandrasekhar limit, this exclusion principle of particles repelling each other is not as strong as the force of the star’s own gravity, as the bigger its mass, the stronger its gravity. But smaller stars can balance this out. They can become a white dwarf (a small, cold but still glowing and stable star that has used up its nuclear fuel), because of the repulsion between the electrons in the star, according to the exclusion principle. Or, smaller stars can become neutron stars after collapsing, which are instead supported by the repelling force between the star’s protons and neutrons. The fact that Landau made a similar discovery at the same time as Chandrasekhar shows the inevitable progress science will make, as one discovery provides the impetus for the next. Humanity’s innate inquisitiveness spurs it onto each subsequent breakthrough.

Eddington opposed Chandrasekhar’s finding because it did not fit with his understanding of the general theory of relativity, and as he considered himself the only person since Einstein to understand it, this was most likely hard for him to accept. Indeed, Einstein himself disagreed with the idea of stars collapsing to a point. Facing such opposition, Chandrasekhar shelved the theory, but its validity held out, and years later was recognized by the highest award in the scientific community.

Oppenheimer provided the next logical step from the work preceding his own, especially relating to Michell’s seminal suggestion that observers would not be able to see such black holes directly. Oppenheimer’s work only gained recognition when telescopes became strong enough to provide observational support. Valid theories can survive the test of time, but ultimately must agree with observation to be accepted. 

Light from collapsing stars will appear redder and redder as the gravitational force strengthens. This is because light loses more energy escaping the star’s surface as the gravity strengthens, lengthening the light’s wavelength, which appears to the human eye as red light. Signals will also seem to take longer to arrive as the star’s gravity strengthens as the star contracts. Eventually, escape will become impossible, for light, signals, and the astronaut. Given that escape is impossible, scientists can only imagine what black holes would look like on the inside.

If someone were brave and inquisitive enough to want to go see a black hole for themselves, they would not be able to anyway, as they would be torn to shreds before the star hit the critical radius after which it becomes a black hole. Perhaps they are better left imagining.

Penrose’s cosmic censorship theory could be said to follow similar logic to the anthropic principle, in the sense that humans cannot see breakdowns in the laws of physics, or else they would be caught up in that same breakdown and die. Thus it is logical that humans cannot continue to experience time (i.e. live) if witnessing that breakdown. Therefore humans continuing to exist will wonder why they cannot personally witness breakdowns in physics.

General relativity offers a set of equations from which various scientists extrapolate varying solutions. Some of these potential applications include ruptures in space-time that would allow short cuts to other sides of the universe, paths known as wormholes. Also like the anthropic principle, Penrose’s cosmic censorship has a stronger version.

Objects cannot emit energy in the form of gravity infinitely. Emitting energy saps energy from the object, which will eventually run out. Energy always has to be accounted for, as determined by physicists’ ever more specific calculations.

From proposing the idea of black holes, to refining the theory, to determining the alternative outcomes of a collapsing star (here a neutron star), to actually finding them in the night’s sky, humanity’s continued pursuit of knowledge is slowly but surely unlocking the mysteries of the universe.

Hawking presents science as an ongoing line of open questions. In this case, the question is how to describe the unseeable. Yet, physicists have found a way, thanks to the predictable outcomes of the laws they have discovered.

Here Hawking shows how the scientific community comes together to solve problems, by expanding on, challenging, and improving each other’s work. Through this collaborative approach, in which any theory can be thrown out at any instant in favor of a more accurate one, humanity’s knowledge of the inner workings of black holes (which cannot even be seen) has gradually been streamlined and distilled into a form that the layman can grasp.

Hawking emphasizes explicitly a fact that has simmered under the surface of his narration throughout the chapter: the idea that black holes started life as a suggestion based on no direct, observable evidence. Nevertheless, the math held out, and in the 1960s the scientific community gained its long-awaited first signs of reassurance.

Here, Hawking expands on what, for now, might be the best available proof of black holes, given they cannot be directly seen. As with virtual particles, scientists can deduce the laws of physics based even on what cannot directly be seen, but only indirectly detected. Only the most curious creatures would have patience for this.

With greater confidence that such objects as black holes do actually exist, cosmologists can make better judgments about what they observe in the universe. Black holes might not be viewable, but they do have a visible effect on the matter around them due to their mass and gravitational pull. Astonomers can now factor this into their calculations, driving humanity’s scientific progress along further.

Finding “primordial” black holes, if they exist, as well as determining some of their properties, could tell physicists about the state of the early universe. Each discovery, theory, or even suggestion leads to the next big question. No wonder Hawking argues that only when humans know everything will they feel satisfied.