10 How the World Began
197
sun has set. However, if the system is in thermal equilibrium, the dust will
have had an eternity to reach the same temperature as the incoming radiation, and will emit at the same frequency as is incident upon it. Again, the
sky should be as bright as the surface of a star.
Another possible objection is: “light takes time to travel from the source
to our eyes, so the photons from the farthest stars simply have not yet arrived
on earth”. This remark implicitly assumes an origin of time, i.e. a beginning.
If the universe is eternal, there will always be light arriving, no matter how
far away the sources are located. After all, the light has forever to make the
trip.
So we see that far from being naïve, the question: “why is the sky dark at
night? ” challenged the prevailing 19th Century view of the universe as homogeneous, eternal and infinite. (Any glance at the night sky will reveal that it
is not really homogeneous, being full of constellations of stars, nebulae and
dust clouds. However, we are considering here a much bigger picture, where
these local concentrations are averaged out.) If we trust our logic, at least one
of these three assumptions must be wrong, for our logic leads us to conclusions that are contradicted by observation. Despite all of the above objections,
nineteenth century scientists did not appear to be overly troubled, and in the
early times of Einstein, the stationary, uniform and infinite universe was the
common wisdom, shared by Einstein himself.
In the next two Sections, we will see which of the 19th Century assumptions were erroneous, but we will also learn that the sky is indeed bright
at night. It is simply that the “light” arriving from the depths of space has
been red-shifted to frequencies far too low to be visible to our eyes, and
must be detected by sophisticated equipment designed for that purpose. This
discovery was one of the major achievements of 20th Century astronomy.
10.4 The Expanding Universe—Theoretical
Framework
It is now time to advance to the Twentieth and Twenty-first Centuries, which
saw an incredible expansion of our knowledge and interpretation of the world
about us. This was brought about by the development of powerful new
instrumentation, and by the revolutionary ideas of Quantum Mechanics and
the Special and General Theories of Relativity. We have discussed these theories in Chaps. 5, 6 and 7 of this book, and we shall now see their importance
in the understanding of the universe.
197
sun has set. However, if the system is in thermal equilibrium, the dust will
have had an eternity to reach the same temperature as the incoming radiation, and will emit at the same frequency as is incident upon it. Again, the
sky should be as bright as the surface of a star.
Another possible objection is: “light takes time to travel from the source
to our eyes, so the photons from the farthest stars simply have not yet arrived
on earth”. This remark implicitly assumes an origin of time, i.e. a beginning.
If the universe is eternal, there will always be light arriving, no matter how
far away the sources are located. After all, the light has forever to make the
trip.
So we see that far from being naïve, the question: “why is the sky dark at
night? ” challenged the prevailing 19th Century view of the universe as homogeneous, eternal and infinite. (Any glance at the night sky will reveal that it
is not really homogeneous, being full of constellations of stars, nebulae and
dust clouds. However, we are considering here a much bigger picture, where
these local concentrations are averaged out.) If we trust our logic, at least one
of these three assumptions must be wrong, for our logic leads us to conclusions that are contradicted by observation. Despite all of the above objections,
nineteenth century scientists did not appear to be overly troubled, and in the
early times of Einstein, the stationary, uniform and infinite universe was the
common wisdom, shared by Einstein himself.
In the next two Sections, we will see which of the 19th Century assumptions were erroneous, but we will also learn that the sky is indeed bright
at night. It is simply that the “light” arriving from the depths of space has
been red-shifted to frequencies far too low to be visible to our eyes, and
must be detected by sophisticated equipment designed for that purpose. This
discovery was one of the major achievements of 20th Century astronomy.
10.4 The Expanding Universe—Theoretical
Framework
It is now time to advance to the Twentieth and Twenty-first Centuries, which
saw an incredible expansion of our knowledge and interpretation of the world
about us. This was brought about by the development of powerful new
instrumentation, and by the revolutionary ideas of Quantum Mechanics and
the Special and General Theories of Relativity. We have discussed these theories in Chaps. 5, 6 and 7 of this book, and we shall now see their importance
in the understanding of the universe.