First put forward by Sir Isaac Newton, the theory of gravity states that all particles have an attractive gravitational force that draws them together. This operates over long distances, and although it is weak, can build up if enough particles have joined together. This is what sets the earth in orbit around the sun and causes large stars to collapse in on themselves. It is considered one of the four main types of energy.
Gravity Quotes in A Brief History of Time
The A Brief History of Time quotes below are all either spoken by Gravity or refer to Gravity. For each quote, you can also see the other terms and themes related to it (each theme is indicated by its own dot and icon, like this one:
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Chapter 1
Quotes
As far as Kepler was concerned, elliptical orbits were merely an ad hoc hypothesis, and a rather repugnant one at that, because ellipses were clearly less perfect than circles. […] he could not reconcile them with his idea that the planets were made to orbit the sun by magnetic forces.
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It is an interesting reflection on the general climate of thought before the twentieth century that no one had suggested that the universe was expanding or contracting. [...] this may have been due to people’s tendency to believe in eternal truths, as well as the comfort they found in the thought that even though they may grow old and die, the universe is eternal and unchanging.
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Chapter 2
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The Aristotelian tradition also held that one could work out all the laws that govern the universe by pure thought: it was not necessary to check by observation. So no one until Galileo bothered to see whether bodies of different weight did in fact fall at different speeds.
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Newton was very worried by this lack of absolute position, or absolute space, as it was called, because it did not accord with his idea of an absolute God. In fact, he refused to accept lack of absolute space, even though it was implied by his laws.
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Chapter 6
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The hostility of other scientists, particularly Eddington, his former teacher and the leading authority on the structure of stars, persuaded Chandrasekhar to abandon this line of work […] However, when he was awarded the Nobel Prize in 1983, it was […] for his early work on the limiting mass of cold stars.
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Chapter 8
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We don’t yet have a complete and consistent theory that combines quantum mechanics and gravity. However, we are fairly certain of some features that such a unified theory should have.
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Gravity Term Timeline in A Brief History of Time
The timeline below shows where the term Gravity appears in A Brief History of Time. The colored dots and icons indicate which themes are associated with that appearance.
Chapter 1
...up his ideas about how things move in space and time. His law of universal gravitation suggested everything is attracted to everything else, with the force being stronger when those things...
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...all, but very far away and hard to measure. In fact, given his idea of gravity, these stars should all be moving around each other, and at some point should fall...
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...that it might be expanding. Instead they toyed with the idea that at great distances gravity could be repulsive, rather than attractive. It allowed the stars to remain in equilibrium. But...
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...fire, and water is simple, but cannot make any predictions. By contrast, Newton’s theory of gravity, which is determined by mass and distance, is even simpler, but can accurately predict the...
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...theories. For example, Mercury’s movement diverged slightly from predictions made by applying Newton’s law of gravity. Albert Einstein’s slightly different prediction, via his general theory of relativity, matched with what was...
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...the whole picture. But, that is how progress has been made so far. For example, gravity depends only on mass, not the content of an object, so we do not need...
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...mechanics—both great achievements of the first half of the 20th century. The first relates to gravity and large to really large-scale structures of the universe. The second relates to miniscule matter...
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Chapter 2
Newton also discovered the law of gravity, which is the idea that every object attracts every other object proportionally to its mass;...
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This law of gravity also states that the force is proportionally smaller the further away the objects are from...
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Ignoring gravitational effects, like Einstein and Poincaré did back in 1905, the resulting theory is called the...
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But this theory is inconsistent with Newton's laws on gravity, in which distance is a factor, meaning moving an object would affect the force applied...
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The sun's mass and resultant gravitational force curves space-time so that although the earth travels straight in four-dimensional space-time, it looks...
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...seems to not take straight paths through three-dimensional space. Light should also be bent by gravity, according to general relativity. Light cones near the sun ought to bend slightly inward, because...
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...the higher its frequency (or waves per second). Light loses energy to escape the earth's gravitational field, making its the frequency slow and in turn making it look to an observer...
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Chapter 3
...should have guessed it, as otherwise the universe would have contracted under the influence of gravity. But as the universe is expanding, this cancels out that gravitational pull. If the universe...
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People could have realized the universe was expanding from Newton's theory of gravity, but everyone at the time seemed set on believing in a static universe. Even Einstein...
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Three models obey Friedmann's assumptions, though he only suggested one himself. In the first, gravity can slow and eventually stop the expansion. Finally, the universe will contract again. In the...
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...the first model, or theory, the universe is finite, yet does not have a boundary. Gravity bends space around itself, so space is curved, like the surface of the earth. When...
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...we need to know the universe's rate of expansion and average density, to determine if gravity will slow or stop the expansion. We know from the Doppler effect that galaxies are...
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Chapter 4
...chips. The only areas of science that not yet been integrated into this theory are gravity and the larger structure of the universe.
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...because it does not include quantum mechanics. This does not lead to inconsistency, though, as gravitational forces are so weak compared to other forces. But gravity would be much stronger in...
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Chapter 5
Aristotle thought the universe was made up of earth and water, which tended to sink (gravity), and wind and fire, which tended to rise (levity). He thought matter was continuous, meaning...
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...force particles have. They’re observable in the form of “classical” waves, such as light or gravity. Scientists have created four classes of forces for these types of particles, which they hope...
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The first class is gravity. Every particle feels gravity, according to its mass and energy. It is also the weakest...
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...This affects only electrically-charged particles like electrons, but not gravitons. It is much stronger than gravity, and comes in the form of positive or negative charge. If two charges match, such...
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...the last three forces together, though the name doesn’t quite fit as they don’t include gravity. The idea is that at some high energy, the strong nuclear force would be weakened,...
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Grand unified theories of these forces have not yet incorporated gravity. But gravity is a weak force and doesn’t factor much on the atomic scale. Yet,...
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Chapter 6
...In Newton’s time, people argued whether light was a particle or a wave, and how gravity would therefore affect it. Under quantum mechanics, scientists know it is both. Gravity will affect...
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...said that any star that was big and dense enough would have such a strong gravitational pull that light could not escape it. We cannot see them, but we should be...
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But one cannot compare light to other matter that gravity drags in, like a cannonball falling back to earth. Light travels at a fixed speed...
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...when a large volume of gas begins to collapse in on itself under its own gravity—usually it is mostly hydrogen. The increasing number of atomic collisions taking place as the gas...
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...worked out how big a star needed to be to support itself against its own gravity after using up all its fuel. According to the Pauli exclusion principle, matter cannot be...
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...faster than everything else. After a certain point, if the star is dense enough, its gravity will outweigh its expanding force. The Chandrasekhar limit states a star one and a half...
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...theories couldn’t be proven with technology in his day. Essentially, his work states that stars’ gravitational pull changes light’s path through space-time. Finally, the light is pulled so strongly, it cannot...
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...take an infinite time to arrive because it would not be able to escape the gravitational pull. In fact, each previous signal would take longer and longer to arrive, and the...
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Because gravity is strongest at the star’s surface, the difference in gravity between the astronaut’s head and...
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General relativity states that moving objects give off gravitational waves, which bend space-time. These waves carry energy away from the object producing them. Slowly,...
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...they will be orbiting each other so quickly our current technology would pick up the gravitational waves.
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...there were no black holes in reality. But Penrose and Wheeler said a non-rotating star’s gravitational waves during its collapse would make it spherical. In 1963, Roy Kerr extended this to...
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...cannot see seems impossible. But Michell suggested in 1783 we can measure a black hole’s gravitational effects on the material around it. There are examples of systems where stars orbit some...
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Chapter 7
This new idea about black hole radiation suggests gravitational collapse is not so final after all. Mass or energy lost into a black hole...
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Chapter 8
...particles together to form more atoms. In denser than average regions of the universe, the gravitational force of this clumping matter would have slowed expansion. Some regions would stop expanding altogether...
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...passed, hydrogen and helium would form into smaller clouds and collapse, due to their own gravity. Contraction would force collisions between atoms, raising the particles’ temperature, starting nuclear fusion reactions. This...
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...heat and light. Bigger stars will use up their fuel much quicker to balance their gravitational force, creating carbon and oxygen as they contract again. The central portions of the star...
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...give the universe more energy than if the symmetry had broken. This extra energy has anti-gravitational effects due to strong repulsion, acting like Einstein’s cosmological constant. These areas would increasingly expand,...
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...All matter has positive energy, and thus repels other matter; at the same time the gravitational force attracts all matter, and so could be said to be a negatively charged force...
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...values in some regions of the early universe.” The energy in those fields would have anti-gravity effects, like a cosmological constant, increasing the rate of expansion. The energy would slowly decrease...
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...which involve the break down of scientific law. Really, what singularity theories show is that gravity becomes so strong that we need to return to the quantum level, and use a...
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There is no consistent theory that combines quantum theory and gravity. If there were, it should involve Feynmann’s sum over histories proposal, which states particles move...
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Another feature of the unified theory of quantum mechanics and general relativity is that gravity is represented in a curved space-time. Applying the sum over histories to Einstein’s ideas on...
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...might be no singularities in imaginary time, undoing Hawking’s earlier work. But, singularity theories showed gravity to be so powerful at these points that it had to be considered on the...
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Chapter 9
To unify gravity with quantum mechanics, one must apply imaginary time. Imaginary time is no different from the...
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...cosmological arrow, but that is not what we observe. One requires a quantum theory of gravity to know how it all began, rather than guess.
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Chapter 11
Previous chapters covered general relativity, incomplete gravity theories, and the three forces that can be combined in grand unified theories, although these...
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...When incorporating the uncertainty principle and general relativity one can either adjust the strength of gravity or the cosmological constant. But this still will not remove all the infinities from the...
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In the 1970s, a possible solution was offered, called supergravity. It combined the graviton, the gravity wave-carrying particle, with other particles with different spin. These...
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...or emission of energy and particles is represented by the merging or division of strings. Gravity passing from the sun to the earth would previously be seen as a graviton passing...
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In the 1970s Joel Scherk and John Schwarz said string theory could describe gravity, but only if the tension were significantly higher. This would leave most of general relativity’s...
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There are also problems with more dimensions. Gravitational forces are increasingly weaker at the same distance with more dimensions at play. This would...
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...to explore, and they may well find a new layer of particles beyond quarks. But gravity might limit this otherwise infinite series of discoveries as we achieve higher energy production rates....
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Chapter 12
...to predict events. But, the question comes back round to how these laws were chosen. Gravity has taken prominence in this book because it forms the large-scale structure of the universe,...
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