As an astrophysicist, I am always struck by the fact that even the wildest science-fiction stories tend to be distinctly human in character.
No matter how exotic the locale or how unusual the scientific
concepts, most science fiction ends up being about quintessentially human (or
human-like) interactions, problems, foibles and challenges.
This is what we respond to; it is what we can best
understand. In practice, this means that most science fiction takes place in
relatively relatable settings, on a planet or spacecraft.
The real challenge is to tie the story to human
emotions, and human sizes and timescales, while still capturing the enormous
scales of the Universe itself.
Just how large the Universe actually is never fails to
boggle the mind.
We say that the observable Universe extends for tens
of billions of light years, but the only way to really comprehend this, as
humans, is to break matters down into a series of steps, starting with our
visceral understanding of the size of the Earth.
A non-stop flight from Dubai to San Francisco covers a
distance of about 8,000 miles (12,900 km) – roughly equal to the diameter of
the Earth. The Sun is much bigger; its diameter is just over 100 times Earth's.
And the distance between the Earth and the Sun is
about 100 times larger than that, close to 100 million miles.
This distance, the radius of the Earth's orbit around
the Sun, is a fundamental measure in astronomy; the Astronomical Unit, or AU.
The spacecraft Voyager 1, for
example, launched in 1977 and, travelling at 11 miles per second (17 km/s), is
now 137 AU from
the Sun.
But the stars are far more distant
than this. The nearest, Proxima Centauri, is about 270,000 AU, or 4.25 light
years away. You would have to line up 30 million Suns to span the gap between
the Sun and Proxima Centauri.
The Vogons in Douglas Adams's The Hitchhiker's
Guide to the Galaxy (1979) are shocked that humans have not travelled
to the Proxima Centauri system to see the Earth's demolition notice; the joke
is just how impossibly large the distance is.
Four light years turns out to be about the average
distance between stars in the Milky Way Galaxy, of which the Sun is a member.
That is a lot of empty space!
The Milky Way contains about 300 billion stars, in a
vast structure roughly 100,000 light years in diameter.
One of the truly exciting discoveries of the past two
decades is that our Sun is far from unique in hosting a retinue of planets:
evidence shows that the majority of Sun-like stars in the Milky Way have
planets orbiting them, many with a size and distance from their parent star
allowing them to host life as we know it.
Yet getting to these planets is another matter
entirely: Voyager 1 would arrive at Proxima Centauri in 75,000
years if it were travelling in the right direction – which it isn't.
Science-fiction writers use a variety of tricks to
span these interstellar distances: putting their passengers into states of
suspended animation during the long voyages, or travelling close to the speed
of light (to take advantage of the time dilation predicted in Albert Einstein's
theory of special relativity).
Or they invoke warp drives, wormholes or other as-yet
undiscovered phenomena.
When astronomers made the first definitive
measurements of the scale of our Galaxy a century ago, they were overwhelmed by
the size of the Universe they had mapped.
Initially, there was great skepticism that the
so-called 'spiral nebulae' seen in deep photographs of the sky were in fact
'island universes' – structures as large as the Milky Way, but at much larger
distances still.
While the vast majority of science-fiction stories
stay within our Milky Way, much of the story of the past 100 years of astronomy
has been the discovery of just how much larger than that the Universe is.
Our nearest galactic neighbour is about 2 million
light years away, while the light from the most distant galaxies our telescopes
can see has been travelling to us for most of the age of the Universe, about 13
billion years.
We discovered in the 1920s that the Universe has been
expanding since the Big Bang.
But about 20 years ago, astronomers found that this
expansion was speeding up, driven by a force whose physical nature we do not
understand, but to which we give the stop-gap name of 'dark energy'.
Dark energy operates on length- and time-scales of the
Universe as a whole: how could we capture such a concept in a piece of fiction?
The story doesn't stop there. We can't see galaxies
from those parts of the Universe for which there hasn't been enough time since
the Big Bang for the light to reach us. What lies beyond the observable bounds
of the Universe?
Our simplest cosmological models suggest that the
Universe is uniform in its properties on the largest scales, and extends
forever.
A variant idea says that the Big Bang that birthed our
Universe is only one of a (possibly infinite) number of such explosions, and
that the resulting 'multiverse' has an extent utterly beyond our comprehension.
The US astronomer Neil deGrasse Tyson once said: 'The
Universe is under no obligation to make sense to you.'
Similarly, the wonders of the Universe are under no
obligation to make it easy for science-fiction writers to tell stories about
them.
The Universe is mostly empty space, and the distances
between stars in galaxies, and between galaxies in the Universe, are
incomprehensibly vast on human scales.
Capturing the true scale of the Universe, while
somehow tying it to human endeavours and emotions, is a daunting challenge for
any science-fiction writer.
Olaf Stapledon took up that challenge in his
novel Star Maker (1937), in which the stars and nebulae, and
cosmos as a whole, are conscious.
While we are humbled by our tiny size relative to the
cosmos, our brains can none the less comprehend, to some extent, just how large
the Universe we inhabit is.
This is hopeful, since, as the astrobiologist Caleb Scharf of
Columbia University has said: 'In a finite world, a cosmic perspective isn't a
luxury, it is a necessity.'
Conveying this to the public is the real challenge
faced by astronomers and science-fiction writers alike.
This article was originally published at Aeon and has been republished under Creative Commons.