7.3. REFERENCES 101
to the floor and shattered does not pull itself together and jump back to the table, even if given
enough energy to do so. is basic principle is related to the concept of entropy—and it seems
to be a basic law that it always increases.
Gravity on the other hand seems to work in the opposite sense; it pulls a random cloud of
gas and dust into a star, for example. And so gravity appears to impose an order on the chaos.
e formation of a star by gravity does not really defy the laws of statistics and thermodynamics,
but from our perspective it seems to in important ways. Intense light is produced coming from a
small place, for example, when a star is formed. And this allows for the possibility of life [Penrose,
2004, Sec. 27.7].
But this is only temporary. As we shall see in Chapter 9, the stars eventually exhaust all
possible means for generating energy, and so go dark in the form of some compact gravitational
concentration of mass—either a white dwarf, a neutron star or a black hole. It is possible that
the universe could eventually be a dark and lonely place—dark, compact points of mass that no
longer interact with each other, increasingly separated by an accelerated expanding universe.
But it is important to remember that our best prediction for the future of the universe—
the
M
D 0:3,
ƒ
D 0:7 curve in Figure 7.1—is the projection of a model. It is the simplest
model that fits the observations, but other more-complex possibilities are imaginable that would
also agree with current observations. And even if the universe does eventually become dark and
uninteresting, projections of the
M
D 0:3,
ƒ
D 0:7 model suggest it would take an almost
unimaginably long time to do so—on the order of
10
14
years. So it is still important to take out
the trash now and then.
Long before the universe itself becomes unrecognizable, enormous changes are certainly
in store for Earth. e Sun for example—absolutely necessary for life on Earth—will very much
turn against us in roughly 5 billion years. It will become brighter and larger, cooking Earth and
eventually swelling so large that Earth will be consumed.
But remember our cosmic calendar. Five billion years is also roughly the age of Earth; and
so we could say that Earth is, literally, middle-aged. If we place the history of the universe in a
calendar of one year, then 5 billion years occupies roughly the last four months. But the whole
record of humanity, by comparison, occupies only the last 8 seconds of December 31. So it is
important to keep a sense of proportion!
7.3 REFERENCES
Roger Penrose. e Road to Reality: A Complete Guide to the Laws of the Universe. Vintage Books,
2004. 101
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