
rational
consistent with or based on or using reason
This would be a revolution in our view of the unification of the laws of science but it would not change the most important point: that the universe is governed by a set of
rational laws that we can discover and understand.

impose
compel to behave in a certain way
In an unchanging universe a beginning in time is something that has to be
imposed by some being outside the universe; there is no physical necessity for a beginning.

physical
relating to the sciences dealing with matter and energy
One can imagine that God created the universe at literally any time in the past. On the other hand, if the universe is expanding, there may be
physical reasons why there had to be a beginning.

theory
a wellsubstantiated explanation of some aspect of the world
I shall take the simpleminded view that a
theory is just a model of the universe, or a restricted part of it, and a set of rules that relate quantities in the model to observations that we make. It exists only in our minds and does not have any other reality (whatever that might mean).

observation
facts learned by watching attentively
A theory is a good theory if it satisfies two requirements. It must accurately describe a large class of
observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future
observations.

provisional
under terms not final or fully worked out or agreed upon
Any physical theory is always
provisional, in the sense that it is only a hypothesis: you can never prove it.

modify
cause to change; make different
Each time new experiments are observed to agree with the predictions the theory survives, and our confidence in it is increased; but if ever a new observation is found to disagree, we have to abandon or
modify the theory.

partial
being or affecting only a segment
Today scientists describe the universe in terms of two basic
partial theories—the general theory of relativity and quantum mechanics.

relativity
the theory that space and time are not absolute concepts
The general theory of
relativity describes the force of gravity and the largescale structure of the universe, that is, the structure on scales from only a few miles to as large as a million million million million (1 with twentyfour zeros after it) miles, the size of the observable universe.

phenomenon
any state or process known through the senses
Quantum mechanics, on the other hand, deals with
phenomena on extremely small scales, such as a millionth of a millionth of an inch.

endeavor
earnest and conscientious activity intended to do something
One of the major
endeavors in physics today, and the major theme of this book, is the search for a new theory that will incorporate them both—a quantum theory of gravity.

indicate
give evidence of
Galileo’s measurements
indicated that each body increased its speed at the same rate, no matter what its weight.

basis
the fundamental assumptions from which something is begun
Galileo’s measurements were used by Newton as the
basis of his laws of motion.

explicit
precisely and clearly expressed or readily observable
It also meant that whenever a body is not acted on by any force, it will keep on moving in a straight line at the same speed. This idea was first stated explicitly in Newton’s Principia Mathematica, published in 1687, and is known as Newton’s first law.

proportional
having a constant ratio
This states that the body will accelerate, or change its speed, at a rate that is
proportional to the force. (For example, the acceleration is twice as great if the force is twice as great.)

gravity
the force of attraction between all masses in the universe
In addition to his laws of motion, Newton discovered a law to describe the force of
gravity, which states that every body attracts every other body with a force that is proportional to the mass of each body.

acceleration
(physics) a rate of increase of velocity
One can now see why all bodies fall at the same rate: a body of twice the weight will have twice the force of gravity pulling it down, but it will also have twice the mass. According to Newton’s second law, these two effects will exactly cancel each other, so the
acceleration will be the same in all cases.

accuracy
the quality of being near to the true value
Newton’s law of gravity says that the gravitational attraction of a star is exactly one quarter that of a similar star at half the distance. This law predicts the orbits of the earth, the moon, and the planets with great
accuracy.

standard
a basis for comparison
The lack of an absolute
standard of rest meant that one could not determine whether two events that took place at different times occurred in the same position in space.

absolute
expressing finality with no implication of possible change
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.

finite
bounded in magnitude or spatial or temporal extent
The fact that light travels at a
finite, but very high, speed was first discovered in 1676 by the Danish astronomer Ole Christensen Roemer.

propagation
the movement of a wave through a medium
A proper theory of the
propagation of light didn’t come until 1865, when the British physicist James Clerk Maxwell succeeded in unifying the partial theories that up to then had been used to describe the forces of electricity and magnetism.

predict
indicate by signs
Maxwell’s theory
predicted that radio or light waves should travel at a certain fixed speed.

postulate
a proposition accepted as true to provide a logical basis
The fundamental
postulate of the theory of relativity, as it was called, was that the laws of science should be the same for all freely moving observers, no matter what their speed.

equivalence
a state of being essentially comparable or equally balanced
Perhaps the best known are the
equivalence of mass and energy, summed up in Einstein’s famous equation E=mc2 (where E is energy, m is mass, and c is the speed of light), and the law that nothing may travel faster than the speed of light. Because of the
equivalence of energy and mass, the energy which an object has due to its motion will add to its mass.

mass
the property of a body that causes it to have weight
As an object approaches the speed of light, its
mass rises ever more quickly, so it takes more and more energy to speed it up further. It can in fact never reach the speed of light, because by then its
mass would have become infinite, and by the equivalence of
mass and energy, it would have taken an infinite amount of energy to get it there.

intrinsic
belonging to a thing by its very nature
Only light, or other waves that have no
intrinsic mass, can move at the speed of light.

event
something that happens at a given place and time
(An
event, in this sense, is something that takes place at a single point in space, at a specified point in time.)

measure
express as a number or quantity
In the theory of relativity, we now define distance in terms of time and the speed of light, so it follows automatically that every observer will
measure light to have the same speed (by definition, 1 meter per 0.000000003335640952 second).

combine
gather in a mass, sum, or whole
We must accept that time is not completely separate from and independent of space, but is
combined with it to form an object called spacetime.

coordinate
a number that identifies a position relative to an axis
It is a matter of common experience that one can describe the position of a point in space by three numbers, or
coordinates.

distinction
a discrimination between things as different
In relativity, there is no real
distinction between the space and time coordinates, just as there is no real difference between any two space coordinates.

specify
define clearly
It is often helpful to think of the four coordinates of an event as
specifying its position in a fourdimensional space called spacetime.

consequence
a phenomenon that is caused by some previous phenomenon
Einstein made the revolutionary suggestion that gravity is not a force like other forces, but is a
consequence of the fact that spacetime is not flat, as had been previously assumed: it is curved, or “warped,” by the distribution of mass and energy in it.

orbit
the path of a celestial body in its revolution about another
The mass of the sun curves spacetime in such a way that although the earth follows a straight path in fourdimensional spacetime, it appears to us to move along a circular
orbit in threedimensional space. In fact, the
orbits of the planets predicted by general relativity are almost exactly the same as those predicted by the Newtonian theory of gravity.

ellipse
a closed plane curve with an oval shape
However, in the case of Mercury, which, being the nearest planet to the sun, feels the strongest gravitational effects, and has a rather elongated orbit, general relativity predicts that the long axis of the
ellipse should rotate about the sun at a rate of about one degree in ten thousand years.

deflect
turn from a straight course or fixed direction
This means that light from a distant star that happened to pass near the sun would be
deflected through a small angle, causing the star to appear in a different position to an observer on the earth.

frequency
the number of observations in a given statistical category
Another prediction of general relativity is that time should appear to run slower near a massive body like the earth. This is because there is a relation between the energy of light and its
frequency (that is, the number of waves of light per second): the greater the energy, the higher the
frequency.

dynamic
relating to the forces that cause motions of bodies
Space and time are now
dynamic quantities: when a body moves, or a force acts, it affects the curvature of space and time—and in turn the structure of spacetime affects the way in which bodies move and forces act.

notion
a general inclusive concept
The old idea of an essentially unchanging universe that could have existed, and could continue to exist, forever was replaced by the
notion of a dynamic, expanding universe that seemed to have begun a finite time ago, and that might end at a finite time in the future.