A Brief History of Time

by Stephen Hawking

in an infinite static universe nearly every line of sight would end on the surface of a star. Thus one would expect that the whole sky would be as bright as the sun, even at night.

But in 1929, Edwin Hubble made the landmark observation that wherever you look, distant galaxies are moving rapidly away from us. In other words, the universe is expanding.

Their existence can be ignored because it would have no observational consequences.

Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory.

there are the laws that tell us how the universe changes with time.

initial state of the universe.

Today scientists describe the universe in terms of two basic partial theories—the general theory of relativity and quantum mechanics.

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. Quantum mechanics, on the other hand, deals with phenomena on extremely small scales, such as a millionth of a millionth of an inch.

they cannot both be...

incorporate them both—a quantum theory of gravity.

The nonexistence of absolute rest therefore meant that one could not give an event an absolute position in space, as Aristotle had believed. The positions of events and the distances between them would be different for a person on the train and one on the track,

eclipses of Jupiter’s moons appeared later the farther we were from Jupiter. He argued that this was because the light from the moons took longer to reach us when we were farther away.

substance called the “ether” that was present everywhere, even in “empty” space.

They compared the speed of light in the direction of the earth’s motion with that at right angles to the earth’s motion. To their great surprise, they found they were exactly the same!

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.

the universe could not be static, as everyone previously had thought, but is in fact expanding; the distance between the different galaxies is growing all the time.

it predicts a number of different possible outcomes and tells us how likely each of these is.

Brownian motion—the irregular, random motion of small particles of dust suspended in a liquid—could be explained as the effect of atoms of the liquid colliding with the dust particles.

the temperature might drop below the critical value without the symmetry between the forces being broken.

one must measure time using imaginary numbers, rather than real ones. This has an interesting effect on space-time: the distinction between time and space disappears completely.

“The boundary condition of the universe is that it has no boundary.” The universe would be completely self-contained and not affected by anything outside itself. It would neither be created nor destroyed. It would just BE.

Even though the universe would have zero size at the North and South Poles, these points would not be singularities,

This might suggest that the so-called imaginary time is really the real time, and that what we call real time is just a figment of our imaginations.

a scientific theory is just a mathematical model we make to describe our observations: it exists only in our minds. So it is meaningless to ask: which is real, “real” or “imaginary” time? It is simply a matter of which is the more useful description.

Why do we remember the past but not the future?

First, there is the thermodynamic arrow of time, the direction of time in which disorder or entropy increases. Then, there is the psychological arrow of time. This is the direction in which we feel time passes, the direction in which we remember the past but not the future. Finally, there is the cosmological arrow of time. This is the direction of time in which the universe is expanding rather than contracting.

The collapse of a star to form a black hole is rather like the later stages of the collapse of the whole universe.

no boundary condition causes disorder to increase and the conditions to be suitable for intelligent life only in the expanding phase.

One is in the interior of a rotating black hole. Another is a space-time that contains two cosmic strings moving past each other at high speed.

if you can travel faster than light, the theory of relativity implies you can also travel back in time, as the following limerick says:

when time travelers go back to the past, they enter alternative histories which differ from recorded history.

the rocket would remain in the same space-time and therefore the same history, which would have to be consistent.

one could regard it as an antiparticle traveling back in time and coming out of the black hole.

when space-time is warped enough to make travel into the past possible, virtual particles moving on closed loops in space-time can become real particles traveling forward in time at or below the speed of light.

basic objects are not particles, which occupy a single point of space, but things that have a length but no other dimension, like an infinitely thin piece of string.

“How much choice did God have in constructing the universe?”

If we find the answer to that, it would be the ultimate triumph of human reason—for then we would know the mind of God.

politics is for the present, but an equation is something for eternity.”