Human scales

The universe is often an obstacle to science fiction.

Our astronomer is collecting his notes on advice he has given to science fiction writers, and sometimes adds advice he hasn’t given yet but maybe should soon.  Just now he is dealing with time.

Not the billions of years of cosmological time or low-mass stellar evolution.  Nor the mind-bending strangeness that happens around black holes, and the switch of time and space that happens at the event horizon.  Nor even the dilation that comes from speeds comparable to that of light, and gives cosmic-ray mesons an unusually long path through the atmosphere.  He is contemplating the much more ordinary timescales for stories limited to, say, the Solar System, and travel within it.

The year is by definition how long it takes the Earth to circle the Sun.  (We will leave aside the various technical definitions of the year.)  Mercury and Venus take shorter times; the Martian year is just short of two Earth years.  As we go outward, times get much longer: a Jupiter year is 12 Earth years, Saturn 30, Uranus 84 and Neptune 165.  In fact it was only a little over a decade ago that Neptune had gone around the Sun once since it had been discovered.  Think of that, since it is relevant: at its discovery, railroads were new and ships were powered by sails; a Neptune-year later, everyone had a smartphone.

It’s not just a matter of Uranus-birthdays.  It takes something over half an orbital-time for a spaceship to get from Earth to an outer planet, using a standard lowest-energy maneuver.  There are tricks that use less energy, but require specific alignments of planets and often take longer.  In principle one could travel faster, but that requires more energy, and making the trip to Saturn in just a couple of Earth years would take far more powerful engines than anything we can seriously contemplate.  It’s not physically impossible, but perhaps similar to making a sailing ship fly.

We’ve had eras of long voyages on Earth, of course.  Getting a European ship to and from the spice-island colonies was a matter of a year or two, even under good conditions.  Stories could be written (and have been written) under delay-times of that order, and indeed they can be important elements of the plot.  But a voyage-time of six years imposes a major disconnect between the crew and those left at home.  And a three-decade trip to Saturn and back means a change of more than a generation.

(At the other extreme time is beyond human limits also.  Speeds in orbit, even just in orbit around the Earth, are measured in kilometers per second.  That means (unless you’ve carefully matched trajectories) the other spaceship you see passing you now is kilometers away the next second.  You are not going to see the enemy fighter-ship in time to raise your space-rifle and take even one shot.  In space, things happen over years, or milliseconds.)

All this can get in the way of a good story.  At the very least it can be tedious to explain that the trading voyage Grandfather sent out won’t be back for a decade yet,