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A Brief History of Time | Quotes


It may not be possible to express [a complete unified theory of physics] in a single fundamental formulation.


Stephen Hawking begins at once to identify the objective of physicists as finding the set of underlying principles that govern the laws of the universe on all levels. However, he also indicates that the very different situations found in large-scale and small-scale investigations would change how we view those laws, even though it would not change the point that the universe at all levels obeys rational laws within the grasp of human understanding.


An expanding universe does not preclude a creator.

Chapter 1

Stephen Hawking recognizes how the general progression of scientific discoveries has greatly enlarged our perceptions of the universe—especially the realization that it is neither fixed nor static, but moving and changing. Although this factor is one that relativity has in common with quantum mechanics, Hawking here suggests that its expansion also implies a beginning point for the creation of the universe and everything in it. However, Hawking attributes this creator role to physics, not God.


These two [partial] theories [of the universe] ... cannot both be correct.

Chapter 1

The dilemma physicists face in their attempt to unveil the laws governing matter (or here, a "whole" theory that holds true in every instance), both at the miniscule level of subatomic particles (quantum physics) and the vast level of galaxies (relativity), is that while large bodies of matter obey the law of gravitational force, miniscule building blocks of matter do not. He therefore labels these two theories as partial ones that cannot be always correct in every case.


The energy which an object has due to its motion will add to its mass.

Chapter 2

In this quote, Stephen Hawking explains the way mass and energy are "equivalent" in a body's motion (over time through space) as expressed by Einstein's famous equation. This equivalence also mathematically explains why we can accelerate matter to near the speed of light but not equal to or greater than that speed.


The discovery ... the universe is expanding is one of the great intellectual revolutions.

Chapter 3

Edwin Hubble's discovery of other galaxies in the universe other than our own was at first met with considerable skepticism. But his subsequent finding that these galaxies are also rapidly moving away from each other is even more startling. Such movement implies that at one time all matter was compacted densely together, a speculation that led to the big bang theory. Three models of what an expanding universe might mean over time are presented on page 46.


The ... more accurately you try to measure the position of the particle, the less accurately ... its speed.

Chapter 4

Stephen Hawking here encapsulates the difficulty physicists are faced with in determining an absolute or fixed position of a particle by which to measure its speed at the quantum (or very tiny) level. This problem is expressed as the uncertainty principle, established by Werner Heisenberg (1901–76). This principle applies to all types of particles and all methods of determining their velocity or position.


The ... unification of the electromagnetic and weak nuclear forces led to ... a grand unified theory.

Chapter 5

Stephen Hawking proceeds to explain that the cluster of partial theories referred to as grand unified theories (GUTs) are not very grand, nor are they completely unified. While some of them bring together two or three of the four fundamental forces found in nature, not one of them encompasses all four by itself. And it is the so-called weak gravitational force that applies to the large-scale level of stars, planets, and galaxies, but not to the small-scale level of quantum mechanics.


Black holes are ... a theory ... developed ... before there was any evidence.

Chapter 6

As Stephen Hawking points out beginning in Chapter 1, individuals first observed the phenomena in the world and then attempted to devise a way to explain them. These explanations, or models, are (regardless of who forms them) then tested against further observations supported by increasingly improved technology. However, in the case of black holes (and also quarks), no one has yet been in a position to actually observe one. The theory about what happens when a star runs out of its fuel means that stars with a large mass may simply explode to avoid gravitational collapse. However, it was also reasonable to expect that not all of them did that, indicating that a large star's mass would cause it to implode in on itself to become a black hole.


The nondecreasing behavior of a black hole's area was very reminiscent of ... entropy.

Chapter 7

Stephen Hawking's observation on the characteristic of a black hole to increase (because energy and matter fall into it, but can't leave) acts as a stepping stone to observe how, in our perception of the advance of time, an ordered system (such as a teacup) is always subject to an increase—but never a decrease—of disorder.


This idea ... that time and space should be finite "without boundary" is just a proposal.

Chapter 8

Stephen Hawking hedges his statements in a way to be as accurate as possible about what he is, and is not, stating in this idea. While it makes some sense in terms of its logic relative to observations about how objects behave, Hawking reminds us that it is still under investigation. It shows promise to Hawking in that it supports a narrow grouping of conditions that are not only possible but also probable. Such conditions support the anthropic principle, which states that out of all possible histories the universe might have had, only a very few could support a life form like humans that could wonder about the nature of the universe.


Thus time became a more personal concept, relative to the observer who measured it.

Chapter 9

The speed of light is one of the very few fixed quantities in the universe, as Stephen Hawking explained in Chapter 2. He further elaborates that this speed is constant regardless of either the position or the movement of the observer. However, this fixed attribute comes at the price of absolute space and time. A clock distant from the core of Earth, for example, will be slower than one close to it. Or one of a pair of twins who travels through space will age more slowly than the other who remains on Earth. Therefore, there is no such thing as an absolute time—it is relative only.


We can't get [particles] beyond the speed-of-light barrier.

Chapter 10

This barrier to achieving an understanding of particles and their behavior at the quantum level is constantly being pushed by such massive devices as the Large Hadron Collider, which went into operation in 2008. As of 2016, the collider achieved a proton–proton collision rate of close to 1 billion collisions per second. While this is extremely impressive, the speed-of-light barrier remains in place because the energy it would take to do surpass it rises exponentially the closer to the speed of light such particles are accelerated.


Finding a theory that unifies gravity with ... other forces is ... [difficult because] general relativity ... does not incorporate the uncertainty principle.

Chapter 11

Although physicists (from Einstein onward) have regarded a unified theory that applies to both the small-scale and large-scale levels as "the holy grail of physics," it has yet to be defined. But Stephen Hawking is able to outline the steps it would take to find it. He argues that this difficulty could be overcome if the general theory of relativity, which includes the law of gravity (rendering it classical), could be combined with the uncertainty principle, which rules quantum mechanics (and is not classical). At the present time, however, such attempts lead to seemingly absurd infinities that would cancel each other out.


Our goal is a complete understanding of the events around us and of our own existence.

Chapter 11

In this statement, Stephen Hawking refers to the farthest edges of the discoveries physicists make as the means to discover first what is the universe and all existence. But his statements throughout A Brief History of Time make it clear this is only the beginning of understanding existence. If a point of saturation (or the point at which all possible observations no longer indicate others yet to be discovered) is reached by a unified theory, then it becomes the task of other thinkers such as philosophers, theologians, and ordinary people to derive meaning, a feat Hawking describes as the pinnacle of human intelligence.


We shall all ... be able to take part in the discussion of ... why ... we and the universe exist.

Chapter 12

Stephen Hawking's intent in writing A Brief History of Time has been to show how scientific inquiries in the past had been undertaken by thoughtful individuals in a wide range of disciplines, including philosophy, religion, and psychology. However, the recent task of reconciling the laws that govern the behavior of subatomic units of particles (quantum mechanics) with those governing an expanding universe (relativity) is one task only mathematicians and physicists can approach. If a unified theory can be found to bring them together, then once again people representing many perspectives can offer insights into what we experience as existence.

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