The
universe is not limited by our imagination. It stretches for a dozen
more orders of magnitude in either direction, towards the immense
as well as towards the infinitesimal. Even to record such a range
of size requires us to fall back on exponential or power-of-ten
notation, in which 1,000,000,000 or one billion becomes 10,
and one billionth or 0.000000001 becomes 1/10
or 10.
The diameter of the Earth is 1,300,000,000 cm, or 1.3 x 10
cm, and the diameter of the observable universe has been estimated
to be 1.7 x 10
cm. The Earth fits into this universe like a single small bacterium
fits into the entire solar system. At the other extreme, the diameter
of an atomic nucleus is only 0.0000000000001 cm, or 10
cm. Such vast size ranges are nearly beyond our comprehension.

The pages of this chapter show a gallery of objects of different
sizes: one typical object for each order
of magnitude or ten-fold change
in length, from 10
cm (the universe) down to 10
cm (an atomic nucleus). These are the boundaries of reality as we
know it.

Each object is 10 times the length of the object just below it,
and 100 times the length of the object two places below. The first
eleven orders of magnitude, from 10
cm to 10
cm, describe astronomical bodies outside our solar system: stars,
galaxies, and galactic clusters.