My Thoughts
Giving 2.5/5 because I found it underwhelming and just rekindling some of my interests on the subject of simulation. There were some interesting new elements, but most of it felt like a revision.
Notes
- If you wanted to store even a single bit of information for each molecule, you’d need to increase the current storage capacity of the world’s computing centres by a factor of 10^21. So a weather forecast cannot be constructed on the basis of atoms and molecules, and a simulation that tries to tell us about it…
- So let’s start with something more down to Earth, the weather forecast. In uncertain times, isn’t it reassuring to hear an expert predict what will happen tomorrow or next week? Like scientific soothsayers, meteorologists guide us through life, advising what our day will be like, all with the aid of computer simulation.
- …thematically to weather prediction. While the word fluid [evokes] a liquid like oil, petrol, or water, to a physicist nearly everything is a fluid: air, glassy ice, plasma in the sun, and the gas in galaxies are all examples. Consequently, the three Navier-Stokes equations come from the behavior of material…
- The second Navier-Stokes equation expresses how different parts of material push upon each other. In other words, it’s all about forces. Weather forecasters often talk about pressure, which is another name for jostling at a microscopic level. On the scale of entire weather systems, high pressure pushes material outwards.
- …universes started to seem even stranger. During the 1980s, telescopes around the world continued to [refine] our understanding of the cosmic web. Davis had been busy focusing on the simulations, but another of the original mapping team’s leaders, Margaret Geller, suspected there was more to discover. From childhood, Geller had been fascinated by three-dimensional patterns.
- …been enlarged in some gigantic photocopier. If a simulated universe were about 80% dark energy (the currently agreed figure is slightly [under] 70%), with the remainder mostly dark matter, the computational and real universes came back into agreement. This implied that if astronomers could measure the expansion of the universe directly, [they’d] find it speeding up…
- …through an interlocking network of observations, theory and simulations, building on all these [1970s] and 1980s insights. [I’ll] present some more of the evidence in the coming chapters. That is not to say…
- …small fraction of a second after the Big Bang. Stars are profoundly different in this respect. They are relative latecomers, first appearing at least 100 million years after the universe began. Stars, unlike dark matter particles, take time to be fashioned from clouds of hydrogen and helium gas. These gases are subject to forces other than gravity.
- …and Andromeda is [our nearest] large neighbour, but they are far from the only galaxies. The 1997 film Contact starts with a flight over Earth, after which the camera pulls back and our planet starts receding. We pass the moon and Mars, fly through the asteroid belt and [zip] past Jupiter and Saturn until the sun and the solar system…
- …[how our] solar system and Earth came to be born in the Milky Way. The existence of galaxies, their histories, their different sizes and shapes, studying all this using computer simulations appealed to me as I started my PhD in 2005. There was something attractive about capturing and studying the building…
- …simplified into a set of best-guess rules. In particular, the birth, life, and death of stars, the nuclear furnaces that make galaxies visible, are by necessity described loosely rather than in a rigorously principled way. This is the same idea as the weather sub-grid. Raindrops and leaves are too small and numerous to include in simulation.
- …be taken seriously is a skill in itself. Modern galaxy simulations trace how materials assemble into galaxies over time, beginning shortly after the birth of our universe. But they cannot be [expected] to reproduce every aspect of this long process. Such an end is unattainable. Instead, they provide a sketch of cosmological history. The sketch isn’t…
- …fundamental question which still plagues contemporary simulations. How quickly do stars form from clouds of gas? Without the answer to this question, we don’t know exactly how bright each part of a simulated universe should be or how the brightness should vary over time. As Tinsley’s criticism of Sandage’s program illustrated, making an erroneous assumption about the brightness of galaxies…
- …distorted by the gravitational influence of a dark halo. This is a long way from seeing the dark halo directly, but measurements of the lensing effect are at least consistent with the presence of an extended halo [of] material. The first serious computer simulations of dark matter focused on the cosmic web, which is a much larger scale structure than individual galaxies.
- …and adjusts the pressure it exerts on surrounding particles accordingly. Taken together, the rules give rise to the same kind of complex swirling motions that I explained earlier for the weather, but the motions are now expressed in terms of moving particles rather than [a] grid. One of the pioneers of the new approach, Joe Monaghan, named it “Smoothed Particle Hydrodynamics”. Monaghan…
- …so flat that when seen from the side, they appear as a razor-thin line of light. [We went to] lunch, [where] they happened to [serve] American-style pizza, and I asked Governato, a proud Italian, why his simulations looked a bit too much like thick-crust monstrosities rather than the thin-crust cuties they ought to be. [With a] trace of irritation, he explains that other simulations had a…
- …results from simulations are what makes them mean something. Nobody should take at face value everything that emerges from a simulation, because we have undeniably cheated to make galaxies fit inside computers. But if we can make predictions of something which turns out to agree with the real universe, we can build confidence in the tale the simulation reveals. This is the second…
- …detected passing through Earth. Consequently, in the last decade, black holes have been demonstrated to exist beyond any reasonable doubt. The Nobel Prizes in Physics in 2017 and 2020 were conferred on a total of six pioneers who’ve contributed to the stack of evidence. Two of the six, Andrea Ghez and Reinhard Genzel, found a spectacular black hole, millions of…
- …drawn for the task at hand. Even so, this approach requires cunning tricks to turn Einstein’s famously mind-bending theory of gravity, general relativity, into something a computer can handle. The theory is tried and tested, but it has strange consequences…
- …different from that other weapon. The layers thrown into space glow extraordinarily brightly at first, then fade over time. The Crab Nebula is a beautiful example of a nearby supernova remnant. A thousand years after its initial explosion was recorded, as visible in the daytime by Chinese and Japanese astronomers, it continues to glow, cool and expand.
- …collapsing star simulations were relatively straightforward and went without a hitch. Yet it took until 2005 to complete the next step, which was investigating a collision between two black holes to study the production of gravitational waves. Relativity is an enormous topic, taking years to learn and decades to master, and when simulating more…
- …from the perspective of an unfortunate astronaut falling in along with the collapsing star itself. Seen by a more safety-conscious astronomer at a distance, the simulated scenario looks dramatically different. As the collapsing star reaches the Schwarzschild radius, a few kilometers across, the flow of time itself is disrupted. The collapse appears to slow until it’s…
- …brightly shining quasar, tinged blue in colour due to the extreme heat. After a central shroud of dust that currently obscures the galactic centre burns away, an eerie new light would illuminate the night sky, shining a thousand times brighter than Venus. Such a source would be visible in the sky even during the day, despite being more than a billion times further away than the sun.
- …vast amounts of energy which heat the surroundings. The galaxies appear to be on fire, glowing smoke [filling] the camera. In reality, this is superheated gas from the vicinity of each galaxy’s supermassive black hole. The smoldering galactic embers start to caramelise; they’re pulled together by gravity and coalesce in the middle…