Brief Answers to the Big Questions

Life organizes itself to survive and reproduce. The first thing humans invented that fits this definition is the computer virus. Most life forms that we know of, though, are largely carbon, which must exist in its present form to allow for life. Significant changes to carbon’s characteristics—its electric charge, how its nucleons are constructed, and other properties—would preclude the existence of life as we know it.

When the universe began, only protons and neutrons existed. A hydrogen atom contains a single proton, but in the intense heat of the first second, some neutrons combined with protons to form helium. Hydrogen and helium gases slowly cooled until, after about two billion years, clumps of gas formed into galaxies of stars. Stars convert hydrogen into helium and some of the bigger atoms. Some stars are so big that they explode, spreading heavier atoms across space, which condense in nearby clouds to form new stars surrounded by planets.

About one in five nearby stars have Earth-like planets that might contain life. On Earth, molecules made of carbon, nitrogen, and other elements formed into ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), the atoms of which act like a code for assembling proteins. These molecules, which form fairly easily and can split and reproduce, are the building blocks of life on Earth.

Once our planet cooled enough, life on it arose fairly quickly. It took a while—more than two billion years—for life to evolve from single-celled creatures to multicellular forms. Animals and plants evolved for several hundred million years until humans appeared. They, in turn, developed language—another way, besides DNA, to reproduce information. Writing accelerated this process; today, the internet contains millions of times more information than our DNA.

All this knowledge, including weapons technology, has evolved quickly, while the human brain, with its hostile traits, has barely evolved at all. We’re now able to destroy civilization and, through more recent developments, alter our own genes, perhaps to make ourselves stronger or more intelligent. Whether people with such new traits will dominate or extinguish the less fortunate remains to be seen.

One way to spread out the risk is to colonize other planets or star systems. The speed of light, though, limits such travel—it would take at least eight years to fly to the nearest star and back—and faster-than-light travel is unlikely, or we’d have tourists from the future visiting us. We can send machines to do the exploring; these would build more machines for further exploration. Eventually, they’d become a new life form, perhaps replacing us.

If other planets evolved intelligent life and visited us, it would have been obvious by now and likely “unpleasant.” Perhaps we’re the only high-tech life form in our galaxy, or intelligence is a rare and mostly unnecessary trait. It took billions of years on Earth to go from DNA to spaceships, and perhaps most planets don’t evolve intelligence before their suns die—or they do evolve intelligence, but it’s wiped out by an asteroid strike or self-destruction.

The author participated in the 2015 launch of Breakthrough Listen, to date the largest effort “aimed at finding evidence of civilisations beyond Earth” (85). A related initiative, Breakthrough Message, aims to design a signal to send to planets with aliens; this project makes Hawking nervous since such beings might not be friendly.