66 4. TOOLS FOR UNDERSTANDING TIME
Figure 4.3: My personal nanosecond, cut to 30:0 cm from a polycarbonate rod and proudly
displayed on its oak stand. Tail of cat (Tobias) is shown for scale.
is look-back time—extremely tiny for the distances of our ordinary experience—has pro-
found implications for astronomy. It means that when we look out into space, we look back in
time. If we express astronomical distances in light years, then the look-back time is simply that
number of years. e star ˛ Centauri A is 4.3 light years distant, and so its look-back time is
simply 4.3 years.
is begs an obvious question: since we see only the past, when we look up at the stars in
the night sky are they—right now—markedly different from how we see them? For our naked-
eye view, the answer is likely to be, “not all that much.” Yes, we see the stars not as they are, but
rather how they were years, dozens of years, centuries and even thousands of years in the past.
But as we will see in Chapter 9, even a thousand years is a blink of the eye for the vast majority
of stars.
Look-back time allows us to survey the history of the universe, simply by looking farther
and farther into space. And so it is a crucial tool of cosmology. But because of the finite age and
expansion of the universe, and the effect matter has on the geometry of space and time, the rela-
tion between look-back time and distance is more complex for extremely distant objects [Ryden,
2017, p. 98].
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