Gravity: A Very Short Introduction

Very Short Introductions #512

by Timothy Clifton

Gravity is one of the four fundamental interactions that exist in nature. It also has the distinction of being the oldest, weakest, and most difficult force to quantize. Understanding gravity is not only essential for understanding the motion of objects on Earth, but also the motion of all celestial objects, and even the expansion of the Universe itself. It was the study of gravity that led Einstein to his profound realizations about the nature of space and time and all astrophysical bodies within it.

In this Very Short Introduction, Timothy Clifton looks at the development of our understanding of gravity since the early observations of Kepler and Newtonian theory. He discusses Einstein's theory of gravity, which now supplants Newton's, and shows how it allows us to understand why the frequency of light changes as it passes through a gravitational field, why GPS satellites need their clocks corrected as they orbit the Earth, and why the orbits of distant neutron stars speed up. Today, almost 100 years after Einstein published his theory of gravity, we have even detected the waves of gravitational radiation that he predicted. Clifton concludes by considering the testing and application of general relativity in astrophysics and cosmology, and looks at dark energy and efforts such as string theory to combine gravity with quantum mechanics.

ABOUT THE The Very Short Introductions series from Oxford University Press contains hundreds of titles in almost every subject area. These pocket-sized books are the perfect way to get ahead in a new subject quickly. Our expert authors combine facts, analysis, perspective, new ideas, and enthusiasm to make interesting and challenging topics highly readable.

Genres: Science, Physics, Nonfiction

128 pages, Paperback

First published April 1, 2017

Reviews

[Stone · Rating 4 out of 5]

Note to self

Gravity
In reality, the phenomenon we refer to as ‘gravity’ is deeply tied up with the nature of space and time. This means that a modern understanding of gravity doesn’t just tell us how objects in the Universe move, it also allows us to understand the behaviour of the very space and time that make up the fabric of the Universe itself.

Gravity is by far the weakest of the four fundamental forces that exist in nature—the others being the electromagnetic force, and the strong and weak nuclear forces.

Unlike Aristotle, Newton didn’t try and seek an explanation for gravity. Instead he quantified its effects, and in the process deduced physical laws that accurately described the motion of not only objects on Earth, but also the motion of the Earth itself, as well as all other bodies in the Solar System.

Time is not a universal concept, unfolding at the same rate for everyone. Time is a personal thing, and depends on our relative motion, with respect to others. Likewise, space is not the fixed backdrop that we think it is. What we think of as distances, and the lengths of objects, are actually dependent on how we are moving.

The freely falling skydiver is brought to Earth because the space-time through which she falls is curved. It is not an external force that tugs her downwards, but her own natural motion through a curved space.

as a person standing on the ground, the pressure you feel on the soles of your feet is due to the rigidity of the Earth pushing you upwards. Again, there is no external force pulling you to Earth. It is only the electrostatic forces in the rocks below your feet that keep the ground rigid, and that prevents you from taking what would otherwise be your natural motion (which would also be free fall).
So, if we free ourselves from defining our motion with respect to the surface of the Earth we realize that the skydiver is not accelerating, while the person who stands on the surface of the Earth is accelerating. Just the opposite of what we usually think.

It wasn’t really the cannonballs that were accelerating away from Galileo at all, it was Galileo that was accelerating away from the cannonballs!

Curvature
Einstein’s solution to this problem was even more amazing. He hypothesized that gravity, instead of being a force that simply pulled things through space, was the result of the curvature of space-time.

in Einstein’s theory, there is no external force called gravity; the motion of every object is just a result of the curvature of space-time. But all objects are moving in the same space-time, so all objects must follow the same paths. In other words, all objects must fall at the same rate, just as Galileo had observed.

The curvature of space-time is usually more irregular than the surface of a sphere, but the basic idea is the same.

Normally, your intuitive description of your own motion would be that you are stationary. But again this is only because of our slavish regard to the Earth as the arbiter of what is at rest and what is moving. Free yourself from this prison, and you realize that you are, in fact, accelerating. You feel a force on the soles of your feet that pushes you upwards, in the same way that you would if you were in a lift that accelerated upwards very quickly. In Einstein’s picture there is no difference between your experience standing on Earth and your experience in the lift. In both situations you are accelerating upwards.

In the latter situation it is the lift that is responsible for your acceleration. In the former, it is the fact that the Earth is solid that pushes you upwards through space-time, knocking you off your free-fall trajectory. That the surface of the Earth can accelerate upwards at every point on its surface, and remain a solid object, is because it exists in a curved space-time and not in a flat space.

in 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.

if a group of massive bodies are in relative motion (such as in the Solar System, or in a binary pulsar), 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. The scientific way to describe this situation is to say that, in Einstein’s theory, space-time is a dynamical entity.

Gravitational wave
In fact, what the gravitational wave is really doing in this example is changing the amount of space that exists in the directions that are transverse to its direction of propagation. This means that 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. This is only possible because of the fact that space is not fixed in Einstein’s theory but is itself dynamical.
(Wow. Interesting. The gravitational wave change the space inbetween?)

As the wave passes through, it stretches the circle smoothly until it reaches a maximum deformation, at which point it stops and then reverses the process until the direction in which it initially stretched is at a minimum. This stretching and squashing in different directions is illustrated in Figure 9, and continues until the wave has passed.

the effect of the gravitational waves was enormously exaggerated in order to try and make it easier to visualize their effect. In reality, the fluctuations in the shape of the ring should only be expected to be at the level of about one part in a hundred million trillion.

That is, if we made a ring of particles that was a 1,000km across, we shouldn’t expect its shape to change by more than a trillionth of a centimetre. This is obviously very difficult to detect.

Gravitational time dilation
More recently, atomic clocks have been used to probe the same effect. The idea behind these experiments is that beams of light are themselves, in some sense, like clocks. The colour of light is determined by the wavelength of the photons that make up the light, which is itself related to the frequency at which they oscillate. If we were to take these oscillations to be the basis of a clock, by saying that each oscillation is one unit of time, then we can see that the redshifting of light is equivalent to the appearance of clocks at different positions running at different rates.
All we have to do is put two clocks at two different heights and arrange for them to transmit the time they display via radio signals. The difference in the rate at which they appear to tick, according to the radio signals that are observed, is then entirely equivalent to the rate at which a light signal between them should be redshifted.

We can therefore take two highly accurate atomic clocks and put one on a rocket while leaving the other at our feet. If the clock in the rocket transmits its time via radio signals, then we can compare this signal to the time displayed by the clock we kept beside us. They should, in general, be different: an effect known as gravitational time dilation.

Dark Matter
They showed that the collapse due to gravity expected from Einstein’s theory took place just as expected, and that the amount of radiation in the early Universe was compatible with that required by the primordial nucleosynthesis calculations. They also found, however, that there appeared to be a large amount of matter in the Universe that didn’t interact with radiation in any way, other than through its gravitational field. This isn’t how normal matter behaves.

if all matter interacted with light, then the high levels of radiation in the early Universe should have suppressed the seeds of structure in a predictable way. What we see, however, is the level of structure that one should expect if radiation hadn’t done this. The logical conclusion is that there exists matter in the Universe that does not interact with radiation, and that it was the gravitational field of this matter that served as the seeds of the structures that we see around us today.

the initial ripples have a characteristic length. By comparing the scale of these ripples in the background radiation, to the scales that occur in the large-scale structure around us, we can therefore see in quite a direct way how much the Universe has expanded (as the former is the source of the latter). This leads to yet another surprising result. The Universe seems to have expanded more than it should have done if its expansion were dominated by the gravitational field of the matter within it. In other words, the rulers in the late Universe seem to be too big.

What seems clear, as before, is that there appears to be a large amount of matter that we can’t see directly, but whose gravitational field is required to make galaxies move and cluster in the way they do.
This is obviously a very strange state of affairs. The substance that gravitates in this way but cannot be seen is referred to as dark matter. The amount of dark matter required to explain the observations is not small. There needs to be approximately five times as much dark matter as there is ordinary matter.

Recall that we can think of the expansion of the Universe as objects made of matter (such as galaxies) flying away from each other under their mutual gravitational interaction. If this is true, and if gravity is always attractive, then we should expect the large-scale expansion of the Universe to be always decelerating. That is, the expansion should be getting slower over time. The results of the various astronomical observations we have discussed, however, have shown that the expansion is speeding up. The conclusion is that there must be a type of gravitational field that is repulsive—in other words, there seems to be a type of anti-gravity at work when we look at how the Universe expands.
(Anti-gravity to speed up the expansion)

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. This is truly astonishing.
The source of this repulsive gravity is referred to by scientists as dark energy (not to be confused with dark matter). There needs to be around three times as much dark energy as there is dark matter, in order to make the Universe accelerate in its expansion at the current rate.

Gravity is caused by acceleration, so by sitting at my desk, in the gravitational field of the Earth, I am being exposed to a small number of particles that wouldn’t be there if I were falling freely. The number is too small to measure, but if I moved my desk so that it was near a black hole (where gravity is much stronger) then it would be an entirely different story. I would be bombarded with high-energy particles and radiation.

We noted earlier that the acceleration of cosmic expansion is said to be caused by dark energy, but we didn’t go into any detail about what dark energy might be. The truth is that we don’t yet know what dark energy is, but we do have a favourite candidate: the cosmological constant.
(so a number?)
A cosmological constant can be thought of as a universal gravitational force, pulling or pushing all particles in the Universe together at the same rate. This is exactly the sort of thing that’s required to make the Universe accelerate. All we have to do is make sure the cosmological constant is set up to push things apart, give it the right magnitude, and it will make the expansion of the Universe accelerate. In fact, it’s by far the simplest way to make the Universe accelerate.

[Duke Jeopardy · Rating 4 out of 5]

This is the book to read if (for some reason) you're looking for a succinct summary of the current state of understanding of gravity. Most all of the book flows from Einstein's theory of gravity positing that mass distorts the fabric of space-time. The book covers experiments that validate Einstein's theory (some 100 years after his theory) as well as the ramifications of his theory including gravity waves and many aspects of cosmology. The book is definitely accessible for non-scientists and can be finished in a week. I wouldn't be surprised if an ongoing understanding of gravity becomes one of the single biggest advances in the next 50 years.

[Jason Turner · Rating 5 out of 5]

From Cavendish’s first lab test of the inverse square law to the debate grounds of the cosmological constant’s potential outcomes (or existence). I’m very satisfied with myself for completing this book, because now I have a more grounded understanding of the differences between classical physics, the emergence of quantum and particle physics, the complications between each theory, and most importantly the tests that give weight to them all. Though the final words on gravity have yet to be written, all signs point to the remarkable, and unlikely, nature of our perfect universe.

[Edward Canade · Rating 5 out of 5]

I remember one afternoon stopping by a brook tumbling down a small waterfall and thinking how in the end, gravity always wins.

I'd highly recommend this little book to any lay person who enjoyed science in school and is looking for a way to replenish that nerdy part of your brain that seeks a little extra stimulation every now and then. I can't say I understood everything presented here. For example, how the idea of a curved space time universe, as first advanced by Albert Einstein, fits into the currently accepted view that the universe is flat, (rather than saddle shaped or a three dimensional spherical) still causes me problems. Another thing that leaves me shaking my head is that if all the universe is expanding how do galaxies combine? Shouldn't they be be forever getting further apart? And there is this, "In Einstein's theory , gravity is the result of the curvature of space-time. There is no external force that pulls things together. The apparent way in which large bodies move toward each other is simply a result of the curvature of space-time. The Earth, for example, is not pulled towards the Sun, it is simply in free fall, following the shortest path available to it in a curved space time." Okay...

And as for detecting gravitational waves passing through the Earth, generated by the merging of two huge black holes, that's really bizzaro. First detected on 14 September 2015, the author explains, "The signal lasted on 0.2 of a second, and appeared to make the 4km-long arms of the interferometer change their length by about 1/1,000 of the width of a proton."

So from Newton mechanics, to Einstein's theory of relativity, to the study of cosmology and galaxy supper cluster, with a splash into dark matter, dark energy, (repulsive gravity?), and modern quantum mechanics, and a flash mention of multiverses, and where do we go from here, give your brain a little exercise and enjoy.

[tiago · Rating 5 out of 5]

5/5
https://app.thestorygraph.com/reviews...

[Semih · Rating 3 out of 5]

Her VSI kitabı gibi hızla biten ve yoğun detaylar içeren bir kitaptı. Enteresan bir iki detay yakaladım bu okumada. Mesela bir astronotun yerçekimsizlik hissetmesi hep Dünya'ya yaptığı serbest düşüşle örneklenir ama Galileo'nun bıraktığı topların düşüşünün Galiele'nin onlardan uzaklaşmasıyla açıklanabileceği belirtilmez.

Kütleçekim alanına dair şöyle bir anlatı yapıyor mesela:
"Let’s start at the beginning of time. If the Universe was smaller and hotter in the past, then if we consider earlier and earlier times, then the density of matter should be expected to become larger and larger. Now, it turns out that not all types of matter increase in density at the same rate, as we go back in time in this way. The density of light (or radiation, as it’s often referred to by physicists) increases at a faster rate than the density of most other types of matter. This means that at very early times the density of radiation can be even higher than the density of the electrons, neutrons, and protons that make up normal matter. In this case the gravitational field of radiation becomes the dominant influence on the expansion of the Universe." Bu da beni field etkileşimlerini öğrenmem için tetikliyor.

Ya da fiziksel mekanizma olarak basit görünen ama sonuçları açısından çığır açıcı bazı düşünceleri dile getiriyor. Bu kitaplarda beğendiğim birinci kısım bunun gibi detaylar:
"... background radiation is no exception to this phenomenon, and as it passes by massive objects, its trajectory is bent by their gravitational fields. This distorts the pattern of ripples in a calculable way. It changes what the ripples look like, and is an effect that was observed by Planck. Another effect that can be seen in the background radiation is due to the evolution of gravitational fields as the Universe expands. If this happens, then a photon that enters a gravitational field with a given amplitude could find itself leaving a field with a different amplitude. The difference in these amplitudes gives (or takes away) energy from the photon. Comparing observations of this effect to the theoretical predictions gives further evidence that the expansion of the Universe is speeding up."

İkinci beğendiğim şey ise son kısımdaki Further Readings kaynakları 8)

[Arno Mosikyan · Rating 5 out of 5]

Time is not a universal concept, unfolding at the same rate for everyone. Time is a personal thing, and depends on our relative motion, with respect to others. Likewise, space is not the fixed backdrop that we think it is. What we think of as distances, and the lengths of objects, are actually dependent on how we are moving.

Einstein’s solution to this problem was even more amazing. He hypothesized that gravity, instead of being a force that simply pulled things through space, was the result of the curvature of space-time.

The beauty of this idea is that we now no longer need to include gravity as an extra force that exists in the Universe. In this new picture, the only thing responsible for the attraction of massive bodies is space-time itself (which has to be there anyway). This is the fundamental idea behind the General Theory of Relativity.

So, if we free ourselves from defining our motion with respect to the surface of the Earth we realize that the skydiver is not accelerating, while the person who stands on the surface of the Earth is accelerating.

It wasn’t really the cannonballs that were accelerating away from Galileo at all, it was Galileo that was accelerating away from the cannonballs!

A change in energy of a photon results in a change in its colour (its wavelength), and so a beam of light traveling through a gravitational field should be expected to have a different colour depending on how far it is above the source of that field.

The gravitational interaction is fundamental to the study of cosmology, as gravity dominates over all other forces on large distance scales.

The effects of the war between gravity and radiation are imprinted as tiny ripples in the CMB.

So 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.

[miffythereader · Rating 5 out of 5]

An exemplary delivery of the current collective understanding of gravity, both suitable for non-physicists and certainly aspiring ones! I found that the non-literary yet professional language used to describe the concepts was very thoughtful, easy to follow, and considerate. The author didn’t display an unnecessary show of his intelligence by reversing the aforementioned, which in totality, consolidated the intended use of the book as an introduction to gravity. For me, it hovered over the things I was uneasy about in an elucidative way and expounded the commonsensical ideas in a new light of further understanding, so the tiny book proved its intention without dumbing things down and integrating sesquipedalian words— with little to jargon. I love it!


“To understand the full profundity of what Einstein achieved, and the beauty of the theory that resulted, there is no alternative but to delve deeper into the maths and physics. This should be a rewarding experience, as Einstein’s theory of gravity lets us understand space and time as they really are. It lets us imagine universes that never were, as well comprehend the one in which we live. It lets us calculate what happens in environments so alien and exotic that our everyday understanding of reality is turned entirely on its head. Yet it is almost certainly incomplete. The final words on gravity have yet to be written.”

“The Solar System, including the Earth, is our most immediate laboratory for observing the consequences of gravity.”

These particular paragraphs rendered me speechless and left me in absolute awe. Gotta love physics. 😍

[Deidre · Rating 5 out of 5]

This is the second of these Very Short Introductions that I have read. I picked up the first to see if it would be a good classroom instructional resource. I have a Ph.D. in that subject content, and I was very impressed with how well it was written. In attempts to remove that bias, and experience this from more of a novice/student perspective, I picked up this book. I have an undergraduate minor in Physics from years ago so essentially minimal education in this area. My opinion remains unchanged. Brilliant! Clifton does an excellent job of explaining this complicated topic in an easily digestible way. Through his explanations I was able to ponder and make new connections between classical mechanics (an undergrad course I took) and special relativity (a topic I have no formal education in what so ever). A favorite aspect is that the reference list is diversified, and items are denoted as to whether they require education in the subject area or if they are appropriate for a novice.

[Santosh · Rating 5 out of 5]

There are lots of unknowns to be discovered...like "quantum theory of gravity" & "dark matter/energy".
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.

While astonishing discoveries have already been made by observing gravitational systems outside of the Solar System, it is highly likely that we have even more to look forward to in the future. The reason for this optimism is partly due to the construction of a new generation of telescopes, the largest of which is known as the SKA (Square Kilometer Array). The SKA will be a telescope designed to receive radio waves from distant sources, and it will be built on a scale never before seen on Earth.

[Chris · Rating 5 out of 5]

Very short indeed. The 100 page synopsis was useful to read. I read this in preparation for a job application (lets see how that goes!), in general the gravity elements are facinating. The forward work in the field was most applicable. This book does a good job at indicating how predicitve the various developments in gravity model are with respect to their confirmation levels and the challenges related to experimental confirmation.
If you are looking for a short path for understanding the current field and major topics this is a good read. Caveat this does not go into detail or any of the math (good or bad you decide).

[Ryo · Rating 4 out of 5]

重力，作為最古老且難以量化的力量，同時也是自然界中四大基本力(重力、電磁力、強核力、弱合力）之一，其實際運作的方式並不明顯，對於觀察來說有點困難，重力不只告訴大眾天體在宇宙如何運行，也讓我們明白整個宇宙結構中的時間與空間是如何表現。

上一本的”黑洞”，已說明黑洞是宇宙中”重力極強”的區域，而重力又是怎麼讓黑洞這樣的空間產生？ 本書主要在介紹重力的多重面向，先從理論(牛頓與愛因斯坦)再到現今科學界如何理解重力，雖然有些問題仍是無解，但至少我們能藉由探討重力的現象來從另一個角度看待我們生存的地球、太陽系及宇宙全貌。

讀完本書你會發現世界跟你想的可能不一樣（當然如果你是物理相關科系應該會覺得這本只是小兒科😆)，還有很多有待發掘的東西，人類的好奇心不只侷限在藍色星球，我們一直想往外太空看，不過宇宙的尺度對於有限的科技與有限的壽命來說，真的大的太多了，若要理解前人的理論，除了深入鑽研數學及物理，沒有別的方法，對於有興趣的人，這會是一個很棒的體驗。

💡重力總是具有相同的強度嗎？
💡光有自己的重力場嗎？

[Bharath Beeravelly · Rating 4 out of 5]

I am onto this journey of learning and knowing more about Astrophysics, especially the spacetime continuum. And this book seems like a very good start to have a quick overview of Gravity in general. I would recommend this book to people who are enthusiastic about learning gravity quickly. Although, it got kinda heavy in between, I really like the concept of "A Very Short Introduction"

[Maria · Rating 5 out of 5]

This series always delivers. The book covered key history and concepts in the study of gravity, and presented a few more cutting edge topics (quantum loop theory, multiverse, etc.) I especially like the transitions between the topics and the overaching framework within which they are discussed.

[Daniel Wright · Rating 3 out of 5]

Chapter 1: From Newton to Einstein
Chapter 2: Gravity in the Solar System
Chapter 3: Extrasolar tests of gravity
Chapter 4: Gravitational waves
Chapter 5: Cosmology
Chapter 6: Frontiers of gravitational physics

[Karl Hallbjörnsson · Rating 4 out of 5]

Felt like reading some lay-physics and this really hit the spot.