"Tell me what happened!"

"The tree came apart.

"Why?"

"Maybe the fire set it off, but it was ready. Clave, everythiiing in the Smoke Ring has some way of getting around. Some way to stay near the median... middle, where there's warater and air. Where do you think jet pods come from?" The hanand relaxed a little, and the Grad kept talking. "It's a plant's way of gettmg around. If a plant wanders out of the median, t too far into the gas torus region--"

"The what?"

Alfin asked, "What on Earth is going on?"

'Clave wants to know what happened. Alfin, can you steer this thing and pick up some more of us? Here—" He passed across his store of jet pods.

Alfin took them. He took his time deciding what to do with them, and the Grad ignored him while he lectured. "The Smoke Ring runs down the median of a much bigger region. That's the gas torus, where the molecules... the bits of air have long meanfree-paths. The air is very thin in the gas torus, but there's some. It gets thicker along the median. That's where you find all the water and the soil and the plants. That's what the Smoke Ring IS living thing wants to stay."

"Where it can breathe. All right, go on."

"Everything in the Smoke Ring can maneuver somehow. An¬ imals mostly have wings. Plants, well, some plants grow jet pods. They spit seeds back toward the median where they can grow and breed, or they spit sterile seeds farther into the gas torus, and the reaction pushes the plant back toward the median. Then there are plants that send out a long root to grab anything that's passing. There are kites—"

"What about the jungles?"

"I... I don't know. The Scientist never—"

"Skip it. What about the trees?"

"Now, that's really interesting. The Scientist came up with this, but he couldn't prove it—"

The hand tightened. The Grad babbled, "If an integral tree falls too far out of the median, it starts to die. It dies in the center. The insects eat it out. They're symbiotes, not parasites. When the center rots, the tree comes apart. See, half of it falls further away, and half of it drops back toward the median. Half lives, half dies, and it's better than nothing."

[W]e might expect intelligent life and technological communities to have emerged in the universe billions of years ago. Given that human society is only a few thousand years old, and that human technological society is mere centuries old, the nature of a community with millions or even billions of years of technological and social progress cannot even be imagined. ... What would we make of a billion-year-old technological community?

In the Next Generation episode “Galaxy's Child,” the Enterprise stumbles upon an alien life-form that lives in empty space, feeding on energy. Particularly tasty is radiation with a very specific frequency 1420 million cycles per second, having a wavelength of 21 cm.

In the spirit of Pythagoras, if there were a Music of the Spheres, surely this would be its opening tone. Fourteen hundred and twenty megahertz is the natural frequency of precession of the spin of an electron as it encircles the atomic nucleus of hydrogen, the dominant material in the universe. It is, by a factor of at least 1000, the most prominent radio frequency in the galaxy. Moreover, it falls precisely in the window of frequencies that, like visible light, can be transmitted and received through an atmosphere capable of supporting organic life. And there is very little background noise at this frequency. Radioastronomers have used this frequency to map out the location of hydrogen in the galaxywhich is, of course, synonymous with the location of matterand have thus determined the galactic shape. Any species intelligent enough to know about radio waves and about the universe will know about this frequency. It is the universal homing beacon. Thirty-six years ago, the astrophysicists Giuseppe Cocconi and Philip Morrison proposed that this is the natural frequency to transmit at or listen to, and no one has argued with this conclusion since.

During the filming of 2001, Kubrick, who obviously has a grasp for detail, became concerned that extraterrestrial intelligence might be discovered before the $10.5 million film was released, rendering the plot line obsolete, if not erroneous. Lloyd's of London was approached to underwrite an insurance policy protecting against losses should extraterrestrial intelligence be discovered. Lloyd's of London, which insures against the most implausible contingencies, declined to write such a policy. In the mid-1960s there was no search being performed for extraterrestrial intelligence, and the chance of accidentally stumbling on extraterrestrial intelligence in a few years' period was extremely small. Lloyd's of London missed a good bet.

The virtue of thinking about life elsewhere is that it forces us to stretch our imaginations. Can we think of alternative solutions to biological problems already solved in one particular way on Earth? For example, the wheel is a comparatively recent invention on the planet Earth. It seems to have been invented in the ancient Near East less than ten thousand years ago. In fact, the high civilizations of Meso- America, the Aztecs and the Mayas, never employed the wheel, except for children's toys. Biology – the evolutionary process – has never invented the wheel, in spite of the fact that its selective advantages are manifest. Why are there no wheeled spiders or goats or elephants rolling along the highways? The answer is clearly that, until recently, there were no highways. Wheels are of use only when there are surfaces to roll on. Since the planet Earth is a heterogeneous, bumpy place with few long, smooth areas, there was no advantage to evolving the wheel. We can very well imagine another planet with enormous long stretches of smooth lava fields in which wheeled organisms are abundant. The late Dutch artist M. C. Escher designed a salamander-like organism that would do very well in such an environment.

The fact that this chain of life existed [at volcanic vents on the seafloor] in the black cold of the deep sea and was utterly independent of sunlight—previously thought to be the font of all Earth's life—has startling ramifications. If life could flourish there, nurtured by a complex chemical process based on geothermal heat, then life could exist under similar conditions on planets far removed from the nurturing light of our parent star, the Sun.

The idea that we shall be welcomed as new members into the galactic community is as unlikely as the idea that the oyster will be welcomed as a new member into the human community. We're probably not even edible.

Though one might imagine "living" organisms such as intelligent computers produced from other elements, such as silicon, it is doubtful that life could have spontaneously evolved in the absence of carbon. The reasons for that are technical but have to do with the unique manner in which carbon bonds with other elements. Carbon dioxide, for example, is gaseous at room temperature, and biologically very useful. Since silicon is the element directly below carbon on the periodic table, it has similar chemical properties. However, silicon dioxide, quartz, is far more useful in a rock collection than in an organism's lungs. Still, perhaps lifeforms could evolve that feast on silicon and rhythmically twirl their tails in pools of liquid ammonia. Even that type of exotic life could not evolve from just the primordial elements, for those elements can form only two stable compounds, lithium hydride, which is a colorless crystalline solid, and hydrogen gas, neither of them a compound likely to reproduce or even to fall in love. Also, the fact remains that we are a carbon life-form, and that raises the issue of how carbon, whose nucleus contains six protons, and the other heavy elements in our bodies were created.