parameters16

"Let's see the very thing and nothing else."
- Wallace Stevens, "Credences of Summer"

It is a more serious question than most people
realize, I think, to ask: How can we travel the length
and breadth of our galaxy? I don't want to say there are
no solutions. Science fiction has conceived warp drive,
subspace communications, the tesseract, suspended
animation, and starship colonies journeying for
generations from one star to another, and surely the
human imagination has the potential to act, somehow,
on whatever it conceives. But presently our most
advanced space technologists are still grappling with
the problems of sending human beings to Mars, which
at closest approach is only a few light minutes from
Earth. Given the size of just the Milky Way galaxy
alone, trekking from star to star seems like a
far-fetched notion. It seems doubtful, at the moment,
that human beings will ever actually, physically visit
and explore other planetary systems. You can't get
there from here, apparently.

a mile or 25,000 miles, we generally speak of how long
it takes to travel those distances. A person walking
relatively briskly can cover a mile in about fifteen
minutes. It is well within the range of human beings to
walk across North America, some 3,000 miles, over a
period of several months. It would take years to walk
(and sail) around the 25,000 mile circumference of the
Earth, but there is time enough for any healthy human
being to do it. Still, when we think of walking, 25,000
miles seems like a long way.
Of course these things have been said before. But
that fact does not mitigate the strangeness of time and
space: light, whose constant speed is about 186,000
miles per second, travels more than seven
Earth-circumferences in one second. This alone is
essentially unimaginable; it's one of those things we
can talk about rationally enough, but which we cannot
possibly comprehend. Yet it's possible to say
meaningful things with it, such as the time it takes
light to travel from the Sun to the Earth (about 8
minutes), or the time it took Voyager 2's radio signals
to reach Earth from Neptune (about 4 hours).
As far as anyone knows, the speed of light is a
physical constant of the universe which has obtained
since the moment of creation, whatever that means,
and which will obtain until the end of the universe,
whatever that means. After Neptune there is Pluto,
and after Pluto there is the Oort cloud of comets, and
then interstellar space. The nearest star to our solar
system is Alpha Centauri, a triple star system in which
the nearest of the three (Proxima Centauri) is
approximately 4.24 light-years away. The idea of
physical "distance" begins to become unraveled in
one's mind even at this relatively nearby location. A
photon of light that covers more than seven
Earth-circumferences in 1 second, covers the distance
between Proxima Centauri and Earth in a little over 4
years.
It's possible to make a scale model in your mind, to
make this distance more or less recognizable. If the
distance from the Earth to the Sun was six inches (six
inches equals 93 million miles), then the distance to
Proxima Centauri would be about 26 miles. Altair, the
bright star in the constellation Auriga, is about 16.5
light-years distant; on the six-inch scale it would be
about 100 miles away. These stars are said to be
"nearby."
Of the brightest stars in the sky, many are not nearby
in comparison to Proxima Centauri and Altair.
Betelgeuse, the topmost star in the familiar figure of
Orion, is some 520 light-years away; on the six-inch
scale, Betelgeuse would be over 3,000 miles distant.
Rigel, at Orion's foot, is some 900 light-years from
Earth, more than 5,000 miles away on the six-inch
scale. And six inches, just to be clear, is equivalent to
about 93,000,000 miles.
The whole Milky Way galaxy, containing the Sun and
its neighbors, and perhaps as many as 300 billion other
stars of various sizes and temperatures, is thought to
be about 100,000 light-years in diameter. The six-inch
scale will not help, at this point, because even on this
scale the galaxy itself is well over half a million miles

The problem, of course, is that the distances between
objects in the galaxy prohibit the journeys. Even the
nearest stars, like Alpha Centauri, Barnard's star, or
Sirius, are so far away their distances are
incomprehensible, simple as they seem. The main
parameters restricting travel among the stars are first,
the physical distances themselves, which in a
Newtonian sense are pretty much constant, at least as
we now perceive interstellar space, and second our
understanding of those immense distances, which is to
say, the properties of galaxies themselves. The
problem of deep-space travel, that is, is a problem of
understanding and overcoming immense distances.
The sense of this immensity does not lend itself to a
single context or remark or group of remarks. To gain
even a hazy understanding of the distances between
visible objects in a galaxy, and even beyond galaxies,
let me come down, again, to some basic facts, by way
of numbers, and then discover what the numbers
imply. For example, to understand small distances like