"A Brief History of Time," Vocabulary from Foreword-Chapter 2

Learn this list of words used by physicist Stephen Hawking to explain theories of the universe.

Here are links to our lists for the scientific text: Foreword-Chapter 2, Chapters 3-5, Chapters 6-8, Chapters 9-12, Einstein-Newton
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Activities for this list:

definitions & notes only words
  1. 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.
  2. 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.
  3. 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.
  4. theory
    a well-substantiated 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).
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. relativity
    the theory that space and time are not absolute concepts
    The general theory of relativity describes the force of gravity and the large-scale 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 twenty-four zeros after it) miles, the size of the observable universe.
  10. 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.
  11. 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.
  12. indicate
    give evidence of
    Galileo’s measurements indicated that each body increased its speed at the same rate, no matter what its weight.
  13. basis
    the fundamental assumptions from which something is begun
    Galileo’s measurements were used by Newton as the basis of his laws of motion.
  14. 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.
  15. 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.)
  16. 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.
  17. 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.
  18. 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.
  19. 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.
  20. 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.
  21. 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.
  22. 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.
  23. predict
    indicate by signs
    Maxwell’s theory predicted that radio or light waves should travel at a certain fixed speed.
  24. 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.
  25. 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.
  26. 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.
  27. 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.
  28. 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.)
  29. 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).
  30. 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 space-time.
  31. 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.
  32. 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.
  33. specify
    define clearly
    It is often helpful to think of the four coordinates of an event as specifying its position in a four-dimensional space called space-time.
  34. 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 space-time is not flat, as had been previously assumed: it is curved, or “warped,” by the distribution of mass and energy in it.
  35. orbit
    the path of a celestial body in its revolution about another
    The mass of the sun curves space-time in such a way that although the earth follows a straight path in four-dimensional space-time, it appears to us to move along a circular orbit in three-dimensional 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.
  36. 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.
  37. 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.
  38. 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.
  39. 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 space-time affects the way in which bodies move and forces act.
  40. 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.

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