Gravity

“The purpose of this book is to give the reader a very brief introduction to various different aspects of gravity. We start by looking at the way in which the theory of gravity developed historically, before moving on to an outline of how it is understood by scientists today. We will then consider the consequences of gravitational physics on the Earth, in the Solar System, and in the Universe as a whole. The final chapter describes some of the frontiers of current research in theoretical gravitational physics.”

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I was not super impressed by this book, mainly because the level of coverage was not quite as high as has been the level of coverage of some of the other physics books in the OUP – A Brief Introduction series. But it’s definitely an okay book about this topic, I was much closer to a three star rating on goodreads than a one star rating, and I did learn some new things from it. I might still change my mind about my two-star rating of the book.

I’ll cover the book the same way I’ve covered some of the other books in the series; I’ll post some quotes with some observations of interest, and then I’ll add some supplementary links towards the end of the post. ‘As usual’ (see e.g. also the introductory remarks to this post) I’ll add links to topics even if I have previously, perhaps on multiple occasions, added the same links when covering other books – the idea behind the links is to remind me – and indicate to you – which kinds of topics are covered in the book.

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“[O]ver large distances it is gravity that dominates. This is because gravity is only ever attractive and because it can never be screened. So while most large objects are electrically neutral, they can never be gravitationally neutral. The gravitational force between objects with mass always acts to pull those objects together, and always increases as they become more massive.”

“The challenges involved in testing Newton’s law of gravity in the laboratory arise principally due to the weakness of the gravitational force compared to the other forces of nature. This weakness means that even the smallest residual electric charges on a piece of experimental equipment can totally overwhelm the gravitational force, making it impossible to measure. All experimental equipment therefore needs to be prepared with the greatest of care, and the inevitable electric charges that sneak through have to be screened by introducing metal shields that reduce their influence. This makes the construction of laboratory experiments to test gravity extremely difficult, and explains why we have so far only probed gravity down to scales a little below 1mm (this can be compared to around a billionth of a billionth of a millimetre for the electric force).”

“There are a large number of effects that result from Einstein’s theory. […] [T]he anomalous orbit of the planet Mercury; the bending of starlight around the Sun; the time delay of radio signals as they pass by the Sun; and the behaviour of gyroscopes in orbit around the Earth […] are four of the most prominent relativistic gravitational effects that can be observed in the Solar System.” [As an aside, I only yesterday watched the first ~20 minutes of the first of Nima Arkani-Hamed’s lectures on the topic of ‘Robustness of GR. Attempts to Modify Gravity’, which was recently uploaded on the IAS youtube channel, before I concluded that I was probably not going to be able to follow the lecture – I would have been able to tell Hamed, on account of having read this book, that the name of the ‘American’ astronomer whose name eluded him early on in the lecture (5 minutes in or so) was John Couch Adams (who was in fact British, not American)].

“[T]he overall picture we are left with is very encouraging for Einstein’s theory of gravity. The foundational assumptions of this theory, such as the constancy of mass and the Universality of Free Fall, have been tested to extremely high accuracy. The inverse square law that formed the basis of Newton’s theory, and which is a good first approximation to Einstein’s theory, has been tested from the sub-millimetre scale all the way up to astrophysical scales. […] We […] have very good evidence that Newton’s inverse square law is a good approximation to gravity over a wide range of distance scales. These scales range from a fraction of a millimetre, to hundreds of millions of metres. […] We are also now in possession of a number of accurate experimental results that probe the tiny, subtle effects that result from Einstein’s theory specifically. This data allows us direct experimental insight into the relationship between matter and the curvature of space-time, and all of it is so far in good agreement with Einstein’s predictions.”

“[A]ll of the objects in the Solar System are, relatively speaking, rather slow moving and not very dense. […] If we set our sights a little further though, we can find objects that are much more extreme than anything we have available nearby. […] observations of them have allowed us to explore gravity in ways that are simply impossible in our own Solar System. The extreme nature of these objects amplifies the effects of Einstein’s theory […] Just as the orbit of Mercury precesses around the Sun so too the neutron stars in the Hulse–Taylor binary system precess around each other. To compare with similar effects in our Solar System, the orbit of the Hulse–Taylor pulsar precesses as much in a day as Mercury does in a century.”

“[I]n Einstein’s theory, gravity is due to the curvature of space-time. Massive objects like stars and planets deform the shape of the space-time in which they exist, so that other bodies that move through it appear to have their trajectories bent. It is the mistaken interpretation of the motion of these bodies as occurring in a flat space that leads us to infer that there is a force called gravity. In fact, it is just the curvature of space-time that is at work. […] The relevance of this for gravitational waves is that if a group of massive bodies are in relative motion […], then the curvature of the space-time in which they exist is not usually fixed in time. The curvature of the space-time is set by the massive bodies, so if the bodies are in motion, the curvature of space-time should be expected to be constantly changing. […] in Einstein’s theory, space-time is a dynamical entity. As an example of this, consider the supernovae […] Before their cores collapse, leading to catastrophic explosion, they are relatively stable objects […] After they explode they settle down to a neutron star or a black hole, and once again return to a relatively stable state, with a gravitational field that doesn’t change much with time. During the explosion, however, they eject huge amounts of mass and energy. Their gravitational field changes rapidly throughout this process, and therefore so does the curvature of the space-time around them.

Like any system that is pushed out of equilibrium and made to change rapidly, this causes disturbances in the form of waves. A more down-to-earth example of a wave is what happens when you throw a stone into a previously still pond. The water in the pond was initially in a steady state, but the stone causes a rapid change in the amount of water at one point. The water in the pond tries to return to its tranquil initial state, which results in the propagation of the disturbance, in the form of ripples that move away from the point where the stone landed. Likewise, a loud noise in a previously quiet room originates from a change in air pressure at a point (e.g. a stereo speaker). The disturbance in the air pressure propagates outwards as a pressure wave as the air tries to return to a stable state, and we perceive these pressure waves as sound. So it is with gravity. If the curvature of space-time is pushed out of equilibrium, by the motion of mass or energy, then this disturbance travels outwards as waves. This is exactly what occurs when a star collapses and its outer envelope is ejected by the subsequent explosion. […] The speed with which waves propagate usually depends on the medium through which they travel. […] The medium for gravitational waves is space-time itself, and according to Einstein’s theory, they propagate at exactly the same speed as light. […] [If a gravitational wave passes through a cloud of gas,] the gravitational wave is not a wave in the gas, but rather a propagating disturbance in the space-time in which the gas exists. […] although the atoms in the gas might be closer together (or further apart) than they were before the wave passed through them, it is not because the atoms have moved, but because the amount of space between them has been decreased (or increased) by the wave. The gravitational wave changes the distance between objects by altering how much space there is in between them, not by moving them within a fixed space.”

“If we look at the right galaxies, or collect enough data, […] we can use it to determine the gravitational fields that exist in space. […] we find that there is more gravity than we expected there to be, from the astrophysical bodies that we can see directly. There appears to be a lot of mass, which bends light via its gravitational field, but that does not interact with the light in any other way. […] Moving to even smaller scales, we can look at how individual galaxies behave. It has been known since the 1970s that the rate at which galaxies rotate is too high. What I mean is that if the only source of gravity in a galaxy was the visible matter within it (mostly stars and gas), then any galaxy that rotated as fast as those we see around us would tear itself apart. […] That they do not fly apart, despite their rapid rotation, strongly suggests that the gravitational fields within them are larger than we initially suspected. Again, the logical conclusion is that there appears to be matter in galaxies that we cannot see but which contributes to the gravitational field. […] Many of the different physical processes that occur in the Universe lead to the same surprising conclusion: the gravitational fields we infer, by looking at the Universe around us, require there to be more matter than we can see with our telescopes. Beyond this, in order for the largest structures in the Universe to have evolved into their current state, and in order for the seeds of these structures to look the way they do in the CMB, this new matter cannot be allowed to interact with light at all (or, at most, interact only very weakly). This means that not only do we not see this matter, but that it cannot be seen at all using light, because light is required to pass straight through it. […] The substance that gravitates in this way but cannot be seen is referred to as dark matter. […] There needs to be approximately five times as much dark matter as there is ordinary matter. […] the evidence for the existence of dark matter comes from so many different sources that it is hard to argue with it.”

“[T]here seems to be a type of anti-gravity at work when we look at how the Universe expands. This anti-gravity is required in order to force matter apart, rather than pull it together, so that the expansion of the Universe can accelerate. […] The source of this repulsive gravity is referred to by scientists as dark energy […] our current overall picture of the Universe is as follows: only around 5 per cent of the energy in the Universe is in the form of normal matter; about 25 per cent is thought to be in the form of the gravitationally attractive dark matter; and the remaining 70 per cent is thought to be in the form of the gravitationally repulsive dark energy. These proportions, give or take a few percentage points here and there, seem sufficient to explain all astronomical observations that have been made to date. The total of all three of these types of energy, added together, also seems to be just the right amount to make space flat […] The flat Universe, filled with mostly dark energy and dark matter, is usually referred to as the Concordance Model of the Universe. Among astronomers, it is now the consensus view that this is the model of the Universe that best fits their data.”

 

The universality of free fall.
Galileo’s Leaning Tower of Pisa experiment.
Isaac Newton/Philosophiæ Naturalis Principia Mathematica/Newton’s law of universal gravitation.
Kepler’s laws of planetary motion.
Luminiferous aether.
Special relativity.
Spacetime.
General relativity.
Spacetime curvature.
Pound–Rebka experiment.
Gravitational time dilation.
Gravitational redshift space-probe experiment (Essot & Levine).
Michelson–Morley experiment.
Hughes–Drever experiment.
Tests of special relativity.
Eötvös experiment.
Torsion balance.
Cavendish experiment.
LAGEOS.
Interferometry.
Geodetic precession.
Frame-dragging.
Gravity Probe B.
White dwarf/neutron star/supernova/gravitational collapse/black hole.
Hulse–Taylor binary.
Arecibo Observatory.
PSR J1738+0333.
Gravitational wave.
Square Kilometre Array.
PSR J0337+1715.
LIGO.
Weber bar.
MiniGrail.
Laser Interferometer Space Antenna.
Edwin Hubble/Hubble’s Law.
Physical cosmology.
Alexander Friedmann/Friedmann equations.
Cosmological constant.
Georges Lemaître.
Ralph Asher Alpher/Robert Hermann/CMB/Arno Penzias/Robert Wilson.
Cosmic Background Explorer.
The BOOMERanG experiment.
Millimeter Anisotropy eXperiment IMaging Array.
Wilkinson Microwave Anisotropy Probe.
High-Z Supernova Search Team.
CfA Redshift Survey/CfA2 Great Wall/2dF Galaxy Redshift Survey/Sloan Digital Sky Survey/Sloan Great Wall.
Gravitational lensing.
Inflation (cosmology).
Lambda-CDM model.
BICEP2.
Large Synoptic Survey Telescope.
Grand Unified Theory.
Renormalization (quantum theory).
String theory.
Loop quantum gravity.
Unruh effect.
Hawking radiation.
Anthropic principle.