why space opera won't fly (long)

I've posted a little article
http://www.lightandmatter.com/article/spaceopera.html
about the hard scientific constraints on interstellar space opera.
The part near the end is basically just a rehash of some of the
ideas I posted in March in the thread "realistic aliens and the
necessities of storytelling," but the main focus is more on the
fundamental physical reasons why you can't fly around the galaxy
in spaceships. Back then, many rasfcians came up with creative
and interesting ideas about how to write about aliens without doing
violence to basic science, and my intention here isn't to make it
sound as if I'm rejecting all their cool ideas and simply reiterating
what I said before. I just wanted to collect some of these ideas in
one place on the web. They're fairly complicated to explain, and I
want to be able to point to one place where I've tried to lay them
out fairly coherently for people who aren't physicists.

Of course I'll be interested in any comments!

Below is a sloppy cut-and-paste of the text of the article. Some
of the math won't come through very clearly, but I know some people
prefer to keep their usenet on usenet and their web on the web.

-------------------------------------------------------------------

Why Space Opera Won't Fly

My seven-year-old daughter came home from school last week with a
homework assignment that called for her to estimate the number of
crackers in a box. 1? 10? 100? 1000? "How am I supposed to know?" she
complained. I love what her school is doing, but the sad truth is
that most people -- even many highly educated people -- never really
develop a gut feeling for the wildly varying magnitudes of numbers,
or an ability to reason about them.

A good example is the way some science fiction writers try to
compress cosmic scales of time, space, and energy to make them
conform to the human experience. The basic motivation seems to be a
lack of creativity. We're all familiar with the earthbound tropes
represented by Horatio Hornblower, Captain Hook, or Stanley and
Livingstone, so why not just translate all those tired old storylines
into outer space?

Well, there are a lot of good reasons why not. Let's start with
energy scales. The U.S.S. Enterprise of Star Trek fame is about the
same size and tonnage as the Queen Elizabeth 2, so if it was moving
at half the speed of light, its kinetic energy would be something
like 10^24 joules. That's equivalent to about a hundred billion Saturn
V rockets, or about a thousand times the total megatonnage of the
world's nuclear arsenals. In other words, the Enterprise is the
ultimate weapon of mass destruction. If you accidentally crash it
into a planet (didn't that happen in one of the movies?), it's more
than enough to destroy everything alive.

==FTL: only time-traveling gods need apply.==

But then, then Enterprise doesn't just go at half the speed of light,
it goes faster than that! In some episodes, the ship has traveled
across the entire galaxy in a matter of hours, which means it went
something like 10^8 times the speed of light. How big is its kinetic
energy at that speed? If you plug in to Einstein's equation
mc2[(1-v2/c2)-1/2-1], you get an imaginary number, and that's becase
according to relativity you can't just keep accelerating an object
until it goes past the speed of light. That's not speculation, it's
an everyday fact of life for physicists working at particle
accelerators. Science fiction writers (including, I admit, some of my
own favorites) have spent an enormous amount of effort trying to work
around this inconvenient fact. Actually, general relativity doesn't
prohibit faster than light (FTL) travel quite as clearly as the more
restricted theory of special relativity. Knowledgeable physicists
have published plenty of papers in refereed journals about some of
the possible methods (e.g., wormholes, or the Alcubierre drive), but
due to the basic structure of relativity, all of these methods have
certain things in common that make them poor plot devices for stories
about human beings:

* They all require the manipulation of matter and energy on a
godlike scale. For instance, one estimate says that an Alcubierre
drive for a spaceship would expend something like 10^47 joules of
energy. That's equivalent to about a billion stars going nova.

* By the logic of relativity, any technology for FTL is also
automatically a technology for time travel.

Since these are generic facts that are directly built into the
structure of relativity, you can really only have your characters
driving aroung the galaxy in FTL spaceships if they're gods -- and
there's not going to be much drama, because any time anything goes
wrong, they can go back in time for a do-over. It's getting a little
warm in here.

So if we really are restricted to slower than light (STL) travel,
what kinds of stories can we tell while staying within the bounds
imposed by the laws of physics? The next pesky problem comes from the
laws of thermodynamics: it turns out that even if the Galactic
Federation wasn't worried about your STL spaceship's awesome
potential as a weapon of mass destruction, you'd still get roasted
alive by your own engines. Here's how it works. If a spaceship is
going to travel at an appreciable fraction of the speed of light,
then the amount of energy it needs to carry along is on the same
order of magnitude as Einstein's E=mc2, and this is the amount of
energy you could produce by converting the entire ship's mass into
pure energy. In other words, the only fuel that's going to have a
high enough energy density is a supply of antimatter, which you can
annihilate slowly as you go along. That means your ship's drive falls
into a very broad category of devices known as heat engines --
devices that turn heat into mechanical work. The laws of
thermodynamics place strict limits on the efficiency of heat engines.
These are not just technological limits that might be surpassed by
clever engineers someday, they're fundamental limits that come from
the basic laws of physics.

Now let's say your spaceliner, with a mass of 100,000 tons, is going
to spend ten years accelerating up to one tenth of the speed of
light, a speed at which it would take most of a human lifetime to get
to the nearest star. That means your engines have to have a power of
10^11 horsepower, or about ten times the output capacity of entire
U.S. energy infrastructure. If your engines are fifty percent
efficient, then half of that energy isn't going into propelling your
ship, it's going into heating it -- and remember, there's no air in
outer space, so you can't use a fan to blow air over a radiator. Your
ship will melt down in a fraction of a second from its own waste
heat. What if we make the engine more efficient? The theoretical
maximum efficiency of a heat engine according to the laws of
thermodynamics is given by 1-Tc/Th, where Tc is the cold temperature
of the environment into which the engine can dump its waste heat, and
Th is the temperature at which the heat is produced by burning the
fuel. Letting Th be a million degrees Kelvin (and assuming we can
contain something that hot!), and letting Tc be room temperature, we
get a theoretical maximum efficiency of 99.97%. That still leaves
0.03% as waste heat, and that waste heat is still enough to kill the
crew faster than you can say "well done."

==Time isn't on my side.==

If we're limited to speeds much less than the speed of light, then
it's going to take an extremely long time to get around -- probably
many thousands of years to get to the nearest star, and many millions
to get to the other side of the galaxy. That rules out any plausible
economic motive for interstellar travel, and it also makes
colonization of the galaxy seem pretty silly, since these timescales
are similar to the time it took human beings to evolve in the first
place -- DNA evidence seems to show that humans didn't evolve speech
until about 40-100 thousand years ago. If our ancestors 100,000 years
ago didn't have speech, what are the chances that our descendants
100,000 years from now are going to be interested in seeing our
snapshots from our galactic tour? And that's assuming that the rate
of evolution stays constant. In reality, humans have reached the
level of technology where we can intentionally manipulate our own
DNA. For all I know, my grandchildren will be wallpapering the living
room with their cerebral cortexes.

If economics and colonization are out, then the only remaining
plausible motive for interstellar exploration seems to be to make
contact with aliens, and unless those aliens are really hung up on
handshakes and face time, we can talk to them much more easily with
radio signals rather than physical travel.

But even then, there's a problem with that whole idea of making
contact with aliens, and it has to do with time scales again. Let's
look at the history of our species' evolution on this planet. Here's
how it went:

* ~3x10^9 years -- Bacteria were the only form of life.

* ~5x10^8 years -- Earth had animals, but no intelligent life.

* ~3x10^6 years -- Hominids walked on two feet and made stone tools,
but had no language.

* ~10^5 years -- Hominids had language.

* ~10^2 years -- Humans developed modern technology such as radio.

For comparison, if the age of the earth was compressed to one year
beginning on January 1, then modern human technology would have
occurred on December 31 at 11:59:59 pm. Let's imagine that
intelligent life evolves on Alpha Centauri. That could have happened
any time within the last few billion years. The chances that the
Centaurians are currently anywhere near our level of technological
development are essentially zero. What's much more likely is the kind
of scenario portrayed in Clarke's 2001: A Space Odyssey. If there are
other species in our galaxy who have gone high-tech, the odds are
that they did it hundreds of millions of years ago, and are now so
far beyond our own level that they would seem like gods to us. It's
not even remotely plausible that we would make first contact with a
species that was within a hundred years of our level of development,
as portrayed in Star Trek (the Vulcans) or Larry Niven's Known Space
(the Kzinti).

==Creativity==

I posted some of these ideas in March 2007 on the usenet group
rec.arts.sf.composition, and the thread ("realistic aliens and the
necessities of storytelling") eventually accumulated over seven
hundred comments. What was fun about the discussion was that it
unleashed so much creativity. People discussed a vast number of ways
of making stories work despite the limitations imposed by science.
Despite the intentionally provocative title of this article, I'm not
acutally out to spoil everyone's fun. I'd just like to read more SF
that finds creative ways of working within the bounds of scientific
possibility, rather than taking the lazy way out and translating
Horatio Hornblower into outer space. In 1940 it was reasonable to
write science fiction with Martian princesses, tailfinned spaceships,
and so on. Today, written SF seems to be losing the battle for young
people's hearts and minds, and I think part of the reason may be that
some of the tropes seem worn out and quaint to them. Even if they
don't know the kind of science I'm presenting here, they do know that
crewed space travel is ridiculously hard -- so hard that we've blown
up two space shuttles, and haven't been back to the moon since 1972.
They realize that Star Wars and Star Trek are fantasy, not science
fiction, so maybe it's not so surprising that they're buying so much
Harry Potter, and when they do buy an SF novel, it's likely to be
something like Scott Westerfeld's stories, which are about body
modification rather than space travel.

==Wiggle room==

There's still plenty of wiggle room here, of course. Here are a few
loopholes in the ideas I've discussed -- some of them suggested by
people on rec.arts.sf.composition.

An STL spaceship capable of near-relativistic speeds could
conceivably have some extremely efficient method for radiating away
its waste heat, or it could somehow contain the a matter-antimatter
reaction at some incredibly high temperature, so that the maximum
thermodynamic efficiency was very close to unity.

A spaceship doesn't have to carry its energy supply with it. This is
the idea behind Robert L. Forward's starwisp concept, and there is a
variety of other beamed propulsion ideas out there.

The technology required for interstellar travel is godlike, but that
doesn't mean your characters have to be gods. They can use alien
technology they don't understand, as in 2001, or Frederik Pohl's Gateway.

Even though it's ridiculously unlikely that our first contact with an
alien species would just randomly happen to be with one that was near
our level of technology, it's possible that there are many, many
intelligent species in our galaxy, so some of them may be near our
level. It's also possible that the social dynamics of the galaxy work
out so that there are many species constrained to be near one level,
e.g., everything could be managed behind the scenes by powerful
aliens. Homo sapiens has already had a couple of first contacts, by
the way -- with Homo neanderthalensis and Homo erectus.
.

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