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Olber's Paradox
The Problem
We see stars all around, so why doesn't their
combined light add up to make our night sky--and
surrounding space, for that matter--bright? German
physicist Heinrich Wilhelm Olbers put the same
puzzle this way in 1823: If the universe is infinite
in size, and stars (or galaxies) are distributed
throughout this infinite universe, then we are
certain to eventually see a star in any direction we
look. As a result, the night sky should be aglow.
Why isn't it?
In fact, the answer is far more profound than it
appears. There have been many attempts at explaining
this puzzle, dubbed Olbers' Paradox, over the years.
One version implicated dust between stars and
perhaps between galaxies. The idea was that the dust
would block the light from faraway objects, making
the sky dark. In reality, however, the light falling
on the dust would eventually heat it up so that it
would glow as brightly as the original sources of
the light.
Another proposed answer for the paradox held that
the tremendous red shift of distant galaxies--the
lengthening of the wavelength of light they emit due
to the expansion of the universe--would move light
out of the visible range into the invisible
infrared. But if this explanation were true,
shorter, wavelength ultraviolet light would also be
shifted into the visible range--which doesn't
happen.
Solution
The best resolution to Olbers' Paradox at present
has two parts. First, even if our universe is
infinitely large, it is not infinitely old. This
point is critical because light travels at the
finite (though very fast!) speed of about 300,000
kilometers per second. We can see something only
after the light it emits has had time to reach us.
In our everyday experience that time delay is
minuscule: even seated in the balcony of the concert
hall, you will see the conductor of the symphony
raise her baton less than a millionth of a second
after she actually does.
When distances increase, though, so does the time
delay. For instance, astronauts on the moon
experience a 1.5-second time delay in their
communications with Mission Control due to the time
it takes the radio signals (which are a form of
light) to travel round-trip between Earth and the
moon. Most astronomers agree that the universe is
between 10 and 15 billion years old. And that means
that the maximum distance from which we can receive
light is between 10 and 15 billion light-years away.
So even if there are more distant galaxies, their
light will not yet have had time to reach us.
The second part of the answer lies in the fact that
stars and galaxies are not infinitely long-lived.
Eventually, they will dim. We will see this effect
sooner in nearby galaxies, thanks to the shorter
light-travel time. The sum of these effects is that
at no time are all of the conditions for creating a
bright sky fulfilled. We can never see light from
stars or galaxies at all distances at once; either
the light from the most distant objects hasn't
reached us yet, or if it has, then so much time
would have had to pass that nearby objects would be
burned out and dark.
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