Computer simulation shows a view of the multiverse, in which each colored ray is another expanding cosmos.
Courtesy Andrei Linde
A sublime cosmic mystery unfolds on a mild summer afternoon in Palo Alto, California, where I’ve come to talk with the visionary physicist Andrei Linde. The day seems ordinary enough. Cyclists maneuver through traffic, and orange poppies bloom on dry brown hills near Linde’s office on the Stanford University campus. But everything here, right down to the photons lighting the scene after an eight-minute jaunt from the sun, bears witness to an extraordinary fact about the universe: Its basic properties are uncannily suited for life. Tweak the laws of physics in just about any way and—in this universe, anyway—life as we know it would not exist.
Consider just two possible changes. Atoms consist of protons, neutrons, and electrons. If those protons were just 0.2 percent more massive than they actually are, they would be unstable and would decay into simpler particles. Atoms wouldn’t exist; neither would we. If gravity were slightly more powerful, the consequences would be nearly as grave. A beefed-up gravitational force would compress stars more tightly, making them smaller, hotter, and denser. Rather than surviving for billions of years, stars would burn through their fuel in a few million years, sputtering out long before life had a chance to evolve. There are many such examples of the universe’s life-friendly properties—so many, in fact, that physicists can’t dismiss them all as mere accidents.
“We have a lot of really, really strange coincidences, and all of these coincidences are such that they make life possible,” Linde says.
Physicists don’t like coincidences. They like even less the notion that life is somehow central to the universe, and yet recent discoveries are forcing them to confront that very idea. Life, it seems, is not an incidental component of the universe, burped up out of a random chemical brew on a lonely planet to endure for a few fleeting ticks of the cosmic clock. In some strange sense, it appears that we are not adapted to the universe; the universe is adapted to us.
Call it a fluke, a mystery, a miracle. Or call it the biggest problem in physics. Short of invoking a benevolent creator, many physicists see only one possible explanation: Our universe may be but one of perhaps infinitely many universes in an inconceivably vast multiverse. Most of those universes are barren, but some, like ours, have conditions suitable for life.
The idea is controversial. Critics say it doesn’t even qualify as a scientific theory because the existence of other universes cannot be proved or disproved. Advocates argue that, like it or not, the multiverse may well be the only viable nonreligious explanation for what is often called the “fine-tuning problem”—the baffling observation that the laws of the universe seem custom-tailored to favor the emergence of life.
Physical laws clamor for life: the universe knew we were coming.
“For me the reality of many universes is a logical possibility,” Linde says. “You might say, ‘Maybe this is some mysterious coincidence. Maybe God created the universe for our benefit.’ Well, I don’t know about God, but the universe itself might reproduce itself eternally in all its possible manifestations.”
Taking on Copernicus
Linde is lying in bed, recovering from a bad fall off a bicycle that broke his left wrist. His left hand, bound in a cast, rests on a pillow. Linde is sturdily built, with thick gray hair that flops down over his forehead; you wouldn’t necessarily pick him out as a man who spends much of his time lost in thought about the distant universe. Right now he is ignoring his injury, reciting a long list of some of the cosmic coincidences that make life possible.
“And if we double the mass of the electron, life as we know it will disappear. If we change the strength of the interaction between protons and electrons, life will disappear. Why are there three space dimensions and one time dimension? If we had four space dimensions and one time dimension, then planetary systems would be unstable and our version of life would be impossible. If we had two space dimensions and one time dimension, we would not exist,” he says.
The idea that the universe was made just for us—known as the anthropic principle—debuted in 1973 when Brandon Carter, then a physicist at Cambridge University, spoke at a conference in Poland honoring Copernicus, the 16th-century astronomer who said that the sun, not Earth, was the hub of the universe. Carter proposed that a purely random assortment of laws would have left the universe dead and dark, and that life limits the values that physical constants can have. By placing life in the cosmic spotlight—at a meeting dedicated to Copernicus, no less—Carter was flying in the face of a scientific worldview that began nearly 500 years ago when the Polish astronomer dislodged Earth and humanity from center stage in the grand scheme of things.
Carter proposed two interpretations of the anthropic principle. The “weak” anthropic principle simply says that we are living in a special time and place in the universe where life is possible. Life couldn’t have survived in the very early universe before stars formed, so the universe had to have reached a certain age and stage of evolution before life could arise.
The “strong” anthropic principle makes a much bolder statement. It asserts that the laws of physics themselves are biased toward life. To quote Freeman Dyson, a renowned physicist at the Institute for Advanced Study in Princeton, the strong anthropic principle implies that “the universe knew we were coming.”
A Wild Profusion
The anthropic principle languished on the fringes of science for years. Physicists regarded it as an interesting idea, but the real action in the field lay elsewhere. And in the late 1970s, Linde, then a professor at the prestigious Lebedev Physical Institute in Moscow, was in the thick of that action. At the time, he wasn’t interested in the anthropic principle at all; he was trying to understand the physics of the Big Bang. Linde and other researchers knew that something was missing from the conventional theory of the Big Bang, because it couldn’t explain a key puzzling fact about the universe: its remarkable uniformity.
Strikingly, the temperature of space is everywhere the same, just 2.7 degrees Celsius above absolute zero. How could different regions of the universe, separated by such enormous distances, all have the same temperature?
In the standard version of the Big Bang, they couldn’t. The universe as a whole has been cooling ever since it emerged from the fireball of the Big Bang. But there’s a problem: For all of it to reach the same temperature, different regions of the universe would have to exchange heat, just as ice cubes and hot tea have to meet to reach the uniform temperature of iced tea. But as Einstein proved, nothing—including heat—can travel faster than the speed of light. In the conventional theory of the Big Bang, there simply hasn’t been enough time since the universe was born for every part of the cosmos to have connected with every other part and cooled to the same temperature.
