because only those values allow for life to exist on earth. Ironically, this means that all of the planets and stars that pepper the night sky are part of an intricate system that is required for the universe to exist and for there to be life on earth. This is a paradoxical reversal of people’s normal reaction to the universe’s largess. Staring up at the night sky, people typically see the vast universe teeming with galaxies, realize their own comparative size and unimportance, and muse on the possibility for life on other galaxies. The weak anthropic principle reverses the lens to view the universe’s amazing structure as a requirement for life to exist.
Cosmologists have long sought to explain the inherent order in the universe by assuming a series of successively more powerful underlying principles that reduce to just a few core mathematical equations; the Grand Unifying Theory—GUT. GUT deals with the fine-tuning dilemma by proposing that there is an underlying, and currently unknown, meta-law that exists and explains why the Big Bang would trigger a series of coincidences leading to this current world. GUT holds the potential for explaining many of the cosmic coincidences in terms of a simpler, fundamental theory. If successful, the GUT would provide a complete description at the physical level. Molecular interactions, forces, and particle properties would be fully understood and predictable. Despite the name, however, a successful GUT will still not provide sufficient detail to predict a person’s every move and thought.
Developing a GUT is enormously complex. One promising approach that focuses on the intrinsic, minute structural details of atoms is string theory. String theory envisages very small particles held together by an attraction akin to that of a string of spaghetti. These very small strings resonate in many dimensions, giving rise to the properties of atoms, and are so small they are unobservable. Previously, the existence of these fundamental particles has been inferred from the detectable paths left by a particle’s interaction with a photographic plate or an electronic detector. One of the difficulties with current string theory is that of experimental proof. One estimate posits that the equipment required for proving string theory would be at least ten light years in length. The point at which string theory leaves predictive science and becomes an exercise in mathematics or philosophy is a difficult question.
Physics has been extremely successful at illuminating the intricate physical relationships that govern the world’s existence. Assuming that a GUT can be found, the existence of the few core principles might have arisen through a chance event that then led to the unfolding of the universe. Science can potentially uncover the underlying structure of the universe and maybe even the ultimate laws of nature. Two unanswered questions will remain: why do the laws have great structure, beauty, and elegance? And, how did the universe’s structure arise?
The strong anthropic principle claims that humanity had to exist and therefore the universe had to be fine-tuned. A helpful analogy to understand the difference between the weak and strong anthropic principles is to imagine a person standing in front of a firing squad. One hundred sharpshooters all fire but as the smoke dissipates the person is alive. One interpretation, corresponding to the weak anthropic principle, is that the person was just incredibly lucky. An alternative interpretation, corresponding to the strong anthropic principle, is that the person had to survive; the marksmen’s intention was to ensure that the person would live in just the same way that the fine tuning of the universe exists to allow life to develop.
A particularly ingenious way of requiring this universe to exist is to assume a multitude of universes. The multiverse theory views the 1 chance in 1060 as looking like incredibly good luck but with an infinite number of possible universes the chance becomes reasonable. If there exist an infinite number of universes then there must be a universe having exactly the character of our universe. The multiverse theory suffers from several unprovable assumptions, many which raise philosophical questions. Why are there random rather than non-random universes? Why are there an infinite number of universes? Furthermore, unlike most scientific theories, the multiverse theory is not testable.
The Habitable Zone
As stars die and explode they disperse their mass as the proverbial “dust of the stars.” Subsequent accretion leads to concentric rings of increasingly dense particles that collide, stick, and fragment like breadcrumbs in a kitchen mixer. Over time the “feeding zone” generates particles ranging in size from dust grains to small planetesimals. Eventually these coalesce to form planets. Each feeding zone consists of a specific mixture of elements, with the lighter, more volatile elements being increasingly found further from the central star like ash driven from a campfire. Paradoxically, nitrogen, hydrogen, carbon, and oxygen are light elements that are more prevalent on Mars and Jupiter than earth but are essential to life on the blue planet. Had earth formed closer to the sun there would be even less of these essential elements, whereas further from the sun there would be no earth, only a planet drowned in water.
Fortunately, the accretion process generated a delightful habitat for intelligent life on earth—politicians notwithstanding! Remarkably, the earth continues to reap 40,000 tons of interstellar compost annually. Most interstellar debris is small, but occasionally large meteors penetrate the atmosphere and arrive on earth’s surface. In all of earth’s bombardment by meteors, one stands out; an impact 4.5 billion years ago with an accretion the size of Mars. The seemingly chance event was essential for several of earth’s unique properties: the tilt axis of earth that’s responsible for the seasons, the length of the day, the spin direction, and most importantly the formation of an exceptionally large moon.
Gauging the precise requirements needed for a habitable planet is difficult because there is only one vantage point in the universe: earth. From this biased perspective earth seems ideally—even providentially—positioned for life. Astrobiologists have coined the phrase “habitable zone” to describe the distance a planet needs to be from a central star for life to exist. Just like toasting marshmallows, the main issue is one of temperature: a planet too close to a sun will be fried, whereas one too far away will remain frozen. Overlaid on top of this requirement is the change of the star’s luminosity over the extended periods of time required for complex life to develop. At the time of earth’s formation, the sun is estimated to have been about 30 percent fainter than at present so that as the sun ages the habitable zone drifts further away from the sun. As a reference point, complex life on earth has arisen only during the last 10 percent of the earth’s existence. Life can exist outside habitable zones in the same way that astronauts can exist on the moon, but this is not favorable for complex life to develop. A relatively wide habitable zone exists for microbes, which tolerate a much wider range of conditions than higher life forms, with an ever narrowing concentric habitable zone in moving up to plants and animals. Complex life, minimally animal life, requires a habitable zone where the distance of an Earth-like planet from the central star maintains an ocean of liquid water and an average global temperature less than 50 °C. Of all animal life on earth only a few extremophiles would be able to survive outside these conditions.
Maintaining an optimal temperature depends on the distance of a planet from the sun, which, for life to evolve, must coincide with habitable conditions on the planet’s surface to support life. An aging sun releases more heat in a mad dash to use any available fuel, which moves the habitable zone further outward. The more massive the star, the faster the star brightens and the narrower the habitable zone. Although earth’s sun is often viewed as a typical star, the earth’s sun is larger than 95 percent of all known stars—anything but typical. The most common star in the Milky Way has only 10 percent of the mass of the earth’s sun which requires a planet to be much closer to be in the habitable zone. At this close distance the gravitational tidal effect from the star induces synchronous rotation of the planet with the star so that the planet rotates with the same face toward the star. Just as with earth’s moon, this synchronicity leads to excessive heating on one side of the planet while the other side freezes. Life is confined to a narrow band between the two zones.
In the same way that the earth lies in the habitable zone mapped out by the sun, the sun lies in a “galactic habitable zone” within the Milky Way. Earth’s sun is about 25,000 light years from the center of the Milky Way and located between
