Jim Baen's Universe

Quarks to Quasars:The Science of Science Fiction

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Galactic Geobiology

Written by Ben Bova

Last time we asked “Where is everybody?” and found that it’s not enough to seek extraterrestrial intelligence per se: we ought to be looking for ETs who are more or less at the same level of technology that we are.

But the problem is that we haven’t any evidence that intelligent aliens of any kind exist. After nearly half a century of searching for intelligent radio signals, not one peep has been heard.

Why?

Let’s take a look at the physical parameters. Our Sun is part of the Milky Way galaxy, a huge pinwheel-shaped assemblage of more than 100 billion stars. There are more than a billion other galaxies in the observable universe, many of them smaller than the Milky Way, some of them much larger.

Out of billions and billions of stars, could ours be the only world on which intelligence has arisen? As a friend of mine often said, the mind bagels.

Let’s look at the lay of the land. The distances between stars are truly mind-numbing.

For distances within our solar system, astronomers use a unit called the Astronomical Unit (AU), which is the average distance between the Sun and the Earth: 93 million miles, a distance that light travels in a little more than eight minutes. But the AU is much too small a yardstick when it comes to the distances between stars.

Imagine a map in which the 93-million-mile AU is shrunk down to one inch. On such a map, the Earth would be one inch from the Sun, Jupiter about five inches, distant Pluto 40-some inches, more than three feet from the Sun.

On that scale of one inch equaling one AU, how far away would the nearest star to our solar system be?

Four and a third miles.

The difference between the distances inside the solar system and the distances to the stars is the same as the difference between inches and miles.

Astronomers use a different yardstick when they deal with interstellar distances: the lightyear.

Light travels at about 186,000 miles per second. There are 31,536,000 seconds in a 365-day year. Therefore light trav­els some 5,865,696,000,000 miles in a year—just a tad less than six trillion miles. That distance is called a lightyear. Note that this is a measure of distance, not of time. [See the sidebar at this link for more information on interstellar distances]

It is one of those odd coincidences that there are just about the same number of AUs in a lightyear (63,072) as there are inches in a mile (63,360).

The nearest star to our solar system is Alpha Centauri, at 4.3 lightyears ^[1] Most of the stars that you can see in the night sky are within a few hundred lightyears, although Polaris, the North Star, is 1000 lightyears away.

Our pinwheel-shaped Milky Way galaxy is about 100,000 lightyears across its diameter. Our Sun and its retinue of planets lies off to one side of the galaxy, some 30,000 lightyears from its core.

Astronomers feel that such vast distances make it unlikely (some say impossible) to build space vehicles for interstellar travel. That’s why they are seeking radio or laser signals instead of searching for starships. Signals travel at the speed of light: nothing in the universe goes faster, according to Einstein.

Science fiction writers are not so pessimistic. They write stories in which starships ply the heavens, often using futuristic technology to break the lightspeed barrier, Einstein notwithstanding.

Is this possible? Will we be able to go star-roving some day? Will malevolent aliens come to invade us?

Definitely maybe.

Einstein showed that if you could travel close to the speed of light, say 90 percent of lightspeed or better, time would slow down for the ship and its occupants. Clocks aboard such a ship would tick more slowly than clocks on Earth. The crew—and the ship’s machinery—would age much more slowly.

The passage of time is relative. It depends on how fast you’re moving. This is the basis of the famous Twin Paradox. One twin goes star roving while the other stays on Earth. The roving twin ages much more slowly than the stay-at-home. When the rover returns to Earth, his twin is much older than he.

A trip to Polaris, 1000 lightyears away, could be accomplished in a subjective time of a few years if the ship could go at nearly the speed of light. But when the ship returned to Earth, two thousand years would have elapsed. The ship and its crew would be only a few years older, but Earth and the people on it would have aged two thousand years.

Which means that star voyages will be one-way trips. You can’t go home again. Or, if you do, your home will have aged enormously. Your friends and family will be long dead. Even if future technology keeps them alive for centuries or millennia, they will certainly have changed over all the years you were away. Or they’ll be the biggest bores in the galaxy.

You can’t go home again once you start roaming the stars.

What if you could travel faster than light? You’d still be aging at a different rate than the folks who stayed home.

So if intelligent aliens visit our world in starships they will be on one-way journeys. Scientists, perhaps, living forever in their starships as they explore the universe. Or tradespersons, exchanging information and artifacts from one world to another. Or marauding aggressors spreading death and destruction across the starways. Or religious evangelists, come to save our souls.

Whatever their motivation, star rovers will be on one-way journeys, traveling across the galaxy forever, their home world long abandoned.

Does this explain why we haven’t been visited? I don’t think so. I think the real explanation lies in the “geography” (for lack of a better word) of the Milky Way galaxy.

Our Sun is about 4.5 billion years old. The galaxy is at least twice that age. There are billions of stars that are much older than we. Presumably, many of those stars harbor planets on which life and intelligence could arise. Intelligent species arising on such worlds would be far older, and presumably far wiser, than we.

The central regions of the Milky Way are thick with very aged stars. The newer stars lie out in the periphery, where our solar system is located. Does this mean that the old science fiction concept of a vast interstellar empire, ruled by ancient races that are far older and smarter than we, is backed by the astronomical facts?

Unfortunately, no.

The oldest stars were built out of the original elements created in the Big Bang: hydrogen, helium, and a smattering of lithium. The three lightest elements. Any planets that they harbor would be built of the same materials. Such worlds could not be sites where life could arise. You can’t make living organisms out of nothing but hydrogen, helium and a smattering of lithium.

But stars are pressure cookers in which heavier elements are created. In the core of a star pressures and temperatures are so high that hydrogen nuclei are fused together to form helium. That’s what makes stars shine: the energy released by nuclear fusion reactions. Our Sun, for example, converts four million tons of hydrogen into helium every second. The energy released by this fusion reaction we see as sunshine. The Sun has been doing this for more than four billion years, and has enough hydrogen to continue “burning” four million tons of hydrogen every second for another four or five billion years.

As the star ages, though, it uses up most of the hydrogen in its core. The gets hotter and denser. Eventually helium nuclei begin to fuse into carbon, oxygen and neon. And when the helium runs low, the star starts “burning” these heavier elements to create still-heavier ones. Like a narcotics addict looking for a fix, the star turns to constantly-heavier elements to keep shining.

But it’s on a journey to oblivion. Once the star begins to produce iron nuclei in its core, the game is up. When iron nuclei fuse together they don’t produce extra energy; they consume energy.

Suddenly the energy that’s kept the star going winks out. The star collapses. If it’s a very massive star the collapse triggers a spectacular supernova explosion; for a few days the star will outshine the entire galaxy. Then it collapses into a neutron star or perhaps a black hole.

Our Sun isn’t massive enough for such a extravagant demise. It will merely blow off some of its outermost layers and shrink down to a white dwarf state, about the size of Earth. Of course, that relatively quiet death rattle will evaporate Earth’s atmosphere and oceans. But that won’t happen for another four or five billion years. Plenty of time for us to either get out of the neighborhood or figure out a way to rejuvenate the Sun and prevent its demise.

What’s this got to do with finding intelligent extraterrestrials? Everything.

The oldest stars could not support life. They didn’t have the oxygen, carbon, nitrogen and other elements essential to living organisms. Without oxygen, for example, you can’t even produce water. And water is essential to all the life forms on Earth.

Astronomers refer to any element heavier than helium as a “metal.” The earliest stars had no “metals” in them. Nor could their planets have “metals.” But those stars cooked up some “metals” in their interiors and, when they died, spewed those elements into interstellar space, where they became the building blocks for newer stars.

The galaxy is a big recycling project, of sorts. Stars are born out of interstellar gas clouds; they manufacture heavier elements in their cores and then blast those elements into space where they form the building blocks for new stars.

And planets. And life, once there’s enough of the heavier elements available.

This means that the oldest stars could not produce life. And even the next generation of stars probably didn’t have enough “metals” in them to produce intelligent civilizations. Civilization requires real metals. Our history rings with the clang of the Bronze Age, the Iron Age, the ages of steel and uranium. We had to have copper to learn about electricity and magnetism. We need aluminum to build airplanes—and spacecraft.

Our Sun is a “metal-rich” star, in astronomers’ parlance. The Sun is about 98% hydrogen and helium, two percent “metals.” We shouldn’t expect stars with lower “metal” content to produce a civilization capable of high technology.

The vision of an incredibly older and wiser interstellar empire is undoubtedly nothing but a dream. It’s not backed by the facts. It just might be that we are the oldest and wisest civilization in this neck of the woods.

And it gets worse.

The core of the Milky Way galaxy (and many other galaxies) harbors a monstrous supermassive black hole that is eating up stars by the thousands. And spewing out hard radiation. Even if those oldest stars at the galaxy’s heart could have produced life, they’re drenched in so much radiation that such living creatures would have been fried long ago.

That cloud of deadly radiation is spreading. We’re about 30,000 lightyears from the galaxy’s core, so we’re safe for the time being. But that expanding cloud of radiation is heading our way.