Weird Life: The Search for Life That Is Very, Very Different from Our Own

by David Toomey

  Specifically, this asymmetry might be evidence that most universes would be inhospitable—short-lived, courtesy of not enough dark energy, or mostly empty space, courtesy of too much—and evidence that within the subset of universes that allowed for life, conditions in the vast majority (ours included) would be met by the slimmest of margins. In other words, evidence that we are somewhere very near the outer edge of the dartboard, exactly where the odds say we should be.* In fact, in 1987 Steven Weinberg, using anthropic reasoning, had predicted a narrow range of values for the cosmological constant, simply by choosing values that would allow a universe with life.

  LIFE IN THE MULTIVERSE

  Anthropic reasoning was premised on the assumption that the values of the Standard Model and cosmological parameters were the only ones possible for a universe that allowed life. In time the assumption became a given. Many papers appeared showing that a small tweak of a value or a parameter would turn a life-allowing universe into a barren one. But in 2005, three physicists—Roni Harnik, Graham Kribs, and Gilad Perez—wondered if there might be a counterexample—that is, a value of the Standard Model or cosmological parameter that, if changed, might still yield a universe that allowed life. As to the particular value or parameter to change, the obvious candidate was the weak force.

  The weak force governs the process of radioactivity. It also governs nucleosynthesis, the process that occurred in our universe’s first three minutes and produced the hydrogen and helium that later formed stars. Contra its name, the weak force is very strong, some 1032 times stronger than gravity. And like the cosmological constant, the actual, detected value is far stronger than the calculated value, and so technically unnatural. The most favored explanation for the actual value involves theoretical particles that may be discovered with the Large Hadron Collider at CERN near Geneva, Switzerland, but at present no one knows why the weak force is so strong, and many physicists—revisiting that calculated value—question why there’s a weak force at all. Harnik, Kribs, and Perez turned that question into a thought experiment. They didn’t imagine merely a universe with a slightly stronger or weaker weak force; they imagined a universe without one.8

  Most physicists who made adjustments in values and force strengths made one adjustment at a time. Certainly, as mentioned earlier, a small tweak of the strength of the weak force—a tweak in either direction—would mean a lifeless universe. What Harnik, Kribs, and Perez quickly realized was that if they wanted a universe with no weak force that still allowed life, like an audiophile seeking a stereo sound with no distortion, they would have to make several adjustments at once.

  THE NECESSITY OF STARS

  If the physics here was so speculative as to be fringe, the biology was decidedly conservative, especially by the standards of some of the ideas described in previous chapters. It was biology that needed carbon.

  Although the fundamental needs of life based in chemistry are known, the stages a universe needs to go through to produce that life are not. Nonetheless, most scientists would concede that at the very least, that universe must have stars.

  Stars in our universe are made of hydrogen and helium, and both were created a few seconds after the big bang by nucleosynthesis, a process, enabled by the weak force, involving reactions that turn protons into neutrons and neutrons into protons. Without a weak force, nucleosynthesis in our universe would be impossible, hydrogen and helium could not be created, there would be nothing to make stars with, and the game would be over before it began. But Harniz, Kribs, and Perez realized that a universe with no weak force might still have stars—if certain initial conditions early in that universe’s history were different from the initial conditions in our universe.

  One such condition was the ratio of matter to antimatter. In our universe, the heat radiation released after the big bang created enormous amounts of matter and antimatter, and slightly more matter than antimatter. As the universe cooled, most of the antimatter and matter annihilated each other, leaving an excess of matter. If a universe with no weak force had begun its existence with a different matter–antimatter ratio, it might still manage nucleosynthesis shortly after its big bang. It would, however, be nucleosynthesis of a different sort. It would produce not the common form of hydrogen (with a nucleus made of a single proton) but the form called hydrogen 2 (with a nucleus made of a proton and a neutron). Hydrogen 2 could go to make stars, but stars different from those we know. Sun-like stars in our universe fuse hydrogen nuclei (protons) to make helium 4 nuclei (two protons and two neutrons). Stars in a universe without a weak force would fuse hydrogen 2 nuclei with a proton to produce helium 3, with a nucleus of two protons and a neutron. Such stars would be smaller and cooler than stars like our Sun, but they would nonetheless be capable of warming any planets and moons orbiting them. Moreover, they could burn for 7 billion years—long enough, if life on Earth is any guide, for organisms to arise on those planets and moons. And they would have internal processes to forge elements like oxygen, carbon, and nitrogen, the elements necessary to familiar life.

  Which brings us to the remaining requirement for a life-bearing universe. The elements forged inside stars must have a means to be dispersed through space, so that they might make landfall on planets and other bodies hospitable to complex chemistry. In our universe, the elements forged within stars are dispersed when those stars explode as supernovas, and most supernovas result from collapsing stars. The explosions are caused by shock waves from the star’s core, and the shock waves are sustained by neutrinos produced by the weak force. With no weak force, many massive collapsing stars would simply fizzle out, keeping their synthesized elements inside. But our universe has another kind of supernova. Its explosions, which are thermonuclear and triggered by accretion, can also disperse those elements. Such supernovas and their attendant explosions require no weak force, and they could occur in a universe without one.

  Harnik, Kribs, and Perez admitted that the life in a universe without a weak force would have to make accommodations rather unlike the accommodations life has made to our universe. The traditional habitable zone of a planet orbiting the smaller, cooler stars of such a universe would be nearer those stars. In addition, the stars of a universe with no weak force could synthesize only traces of elements heavier than iron, and because they couldn’t synthesize even small amounts of very heavy elements like uranium (one source of our planet’s internal warmth), a planet’s internal heating and plate tectonics would need to arise from a process other than radioactive decay. But as we’ve seen, other processes are available to heat a planet from within. As for the uranium, life probably wouldn’t miss it. Biochemistry like that we know would have all the chemicals it needed to make simple organisms that would metabolize, reproduce, and evolve.

  Harnik, Kribs, and Perez hypothesize that such organisms might even evolve into intelligent beings. Such beings might discover that their universe is governed by three fundamental forces, posit a multiverse, and reason that their universe is typical of the subset of universes that allow life. They would not think of themselves as fundamentally weird. (It’s a good bet that no one thinks of himself or herself as fundamentally weird.) But they would think of us—living in a universe with a fourth force that is both technically unnatural and not necessary to life—as very weird indeed. And unless there’s an explanation for the strength (and very existence) of our universe’s weak force, they would have a point.

  MORE ADJUSTMENTS, MORE UNIVERSES

  A few years after Harnik, Kribs, and Perez published their work on a universe without a weak force, another team of physicists—Robert Jaffe, Alejandro Jenkins, and Itamar Kimchi—performed a similar thought experiment. By this time it had become nearly a given that any change in the masses of quarks (and hence the masses of protons and neutrons) would make for a universe hostile to life. Jaffe, Jenkins, and Kimchi wondered whether they could find an exception—that is, a change in the masses of quarks that could also yield a universe that allowed life.

  Changin
g quark masses, even in theory and even for theoretical physicists, is no simple matter, and it’s not surprising that the paper in which Jaffe, Jenkins, and Kimchi describe their thinking is thirty-three pages sprinkled liberally with discussions of Yukawa couplings and Higgs vacuum expectation values. Nonetheless, its conclusion was straightforward and intelligible to anyone who made it through the first week of high school chemistry. In our universe, neutrons are about 0.1 percent heavier than protons. It seemed that if protons were only slightly heavier than neutrons, you could still have hydrogen 2 and helium 3—and both could play roles in an organic chemistry that, in broad outlines at least, would be like the one used by familiar life.

  Quarks come in six varieties that physicists call flavors. The “up” and “down” flavors are particularly important to the commonplace chemistry of our universe. Two up quarks and a down quark make a proton, and two down quarks and an up quark make a neutron. The four other flavors of quarks are heavier and unstable, and they quickly decay into up and down quarks. But the team found that if you could bring them into play, you could make rather more radical changes, and still have a universe that allowed life. The strange quark is the lightest of the heavier, unstable quarks. If you reduce its mass enough, to about the mass of the up quark, and at the same time make the down quark a good deal lighter, then you might make atomic nuclei not with protons and neutrons (the components of atomic nuclei in our universe), but with neutrons and a particle called Σ–, or “sigma minus.” A universe with atoms that had such nuclei could have stable forms of hydrogen, carbon, and oxygen, the elements necessary for organic chemistry.

  All the life that developed from such chemistry might look a lot like familiar life. But that appearance would be deceiving. In a decidedly fundamental way, it would be weird. Is this then as weird as life can get? Perhaps not.

  WEIRDER STILL

  In 1997, Max Tegmark argued, à la Plato’s realm of ideas, that mathematical structures are real. His reasoning was that on a daily basis, each of us employs a simple test of a thing’s reality. We know something is real because someone else can see it too. When we apply that test to mathematical structures like geometrical theorems, they pass. Mathematical structures, said Tegmark, “satisfy a central criterion of objective experience: they are the same no matter who studies them.”9 He then carried the claim to a vastly larger scale, noting that many theoretical physicists suspect that the reason mathematics describes the universe so well is that the universe is inherently mathematical. It necessarily follows, he said, that an equation representing the theory of everything would not merely describe reality; on the most fundamental level it would be reality.

  We have no such equation (at least not yet), but suppose that at some time in the future, theoretical physicists do discover one. It would immediately present us with another question: Why this particular equation, and not others?10 Taking anthropic reasoning to another level, Tegmark offers an answer. He calls it the “ultimate-ensemble” theory. As the multiverse implies that the strengths of forces, physical constants, and dimensionality in our universe are set by chance and require no further explanation, so this theory implies that the (as yet undiscovered) equation that describes the multiverse—or as Tegmark would prefer, is the multiverse—was likewise set by chance and requires no further explanation. Thus, Tegmark can imagine other multiverses with other theories of everything. There might be, he suggests, a multiverse without quantum effects and a multiverse in which time is not continuous.

  With only a little imagination we can add to the list. There might be a multiverse in which time flows not evenly but fitfully, a multiverse whose time flows backward, a multiverse without relativistic effects, and so on. Although Tegmark does not say so explicitly, it follows that there would be life in some of them. As with the life-allowing universes in the many-worlds multiverse and the inflationary multiverse, they would no doubt be a small subset of the whole. But they would be interesting. Their life would be weirder in a more fundamental sense even than the hypothetical life in universes without a weak force and the hypothetical life in universes whose quarks have masses different from the masses of “our” quarks.

  We are rather far out on a speculative limb here, a place many theoretical physicists see no reason to venture. Some would say that Tegmark’s answer to the question “why this equation?” is too easy. As Brian Greene notes, the hypothetical multiverses associated with quantum mechanics and inflationary cosmology are much more than the product of anthropic reasoning. They arose quite independent of such reasoning, from unanticipated consequences of quantum mechanics and inflationary cosmology, and so are on firmer ground. Greene has a second reservation. Theoretical physicists can hypothesize origins for the multiverses—a wave evolving via the Schrödinger equation for the many-worlds multiverse and a fluctuating inflation field for the inflationary multiverse. So far, though, no one has proposed a way that Tegmark’s ultimate ensemble might have come into being.

  But as long as we’re on this limb, we might as well have some fun. It so happens that others have been here already, carrying anthropic reasoning still further. There are ideas of a multiverse that subsumes even the ultimate ensemble, and they proceed from the question, Why should any given universe, or for that matter any given multiverse, be fundamentally mathematical? Why not, for instance, define subsets of universes that are good or bad, or universes that are beautiful or ugly?11 Tegmark would say that such descriptions can have no objective reality and are not scientifically meaningful. But they might nonetheless have a place within a multiverse that has been proposed, on various grounds, by philosopher Robert Nozick, philosopher David Lewis, and physicist John Barrow.12 Each of the three has argued that our universe may be part of a multiverse that includes every possible universe.*

  Recall that the traditional universe, because it contains a finite number of particle arrangements but an infinite number of particles and an infinite amount of space, must also contain all the weird life described in previous chapters, no matter how improbable, as long as it does not violate some natural law. But natural laws vary across universes. And a multiverse that contains all possible universes with all possible laws, logically, must also contain all possible life. We might reasonably expect that such a multiverse contains all the life hypothesized for the traditional universe, the many-worlds multiverse, the inflationary multiverse, and the ultimate ensemble. It must also contain a good deal more, including all the weird life of science fiction, all the animals in Robinson’s Fictitious Beasts and all mythology and fantastic literature, as well as the vastly greater number of beings that are possible but have never been imagined.

  OTHER-DIMENSIONAL LIFE

  In a multiverse of all possible universes and all possible natural laws, is there any kind of life that is impossible? British mathematician Gerald Whitrow suggested that the category of life that cannot exist is life in other spatial dimensions.* In 1955, Whitrow performed a set of thought experiments demonstrating the unique fitness of three dimensions for living things. It was, as several have noted, an anthropic argument. Much as Steven Weinberg’s prediction constrained our universe to a specific range for the cosmological constant, Whitrow’s anthropic reasoning constrained our universe to three dimensions. Whitrow noted that if space had one more dimension and gravity were unchanged, the inverse square law would be an inverse cube law, and planets would spiral into the Sun. Scaling down seemed likewise unfeasible. In a universe with two dimensions, waves could not propagate and deflect properly, and a universe with one dimension (obviously) would severely limit movement. A few years later Whitrow suggested that a universe of two dimensions would not allow the evolution of neural networks and intelligence. “In three or more dimensions,” he wrote, “any number of [nerve] cells can be connected with [one another] in pairs without intersection of the joins, but in two dimensions the maximum number of cells for which this is possible is only four.”13

  A. K. Dewdney’s The Planiverse (1984), which de
scribes the physics and biology of a two-dimensional universe, goes some distance in challenging Whitrow, noting that if nerve cells are allowed to fire nerve impulses through “crossover points,” they can form flat networks as complex as any in three-dimensional space. Two-dimensional minds would operate more slowly than three-dimensional ones, because in the two-dimensional networks the pulses would meet more interruptions, but they would nonetheless work. Dewdney also answers the common objection that an intestinal tract in a two-dimensional being would split it apart. He imagines a two-dimensional fish that has an external skeleton and gains nourishment by the internal circulation of food vesicles. As long as a cell is isolated, food can enter it through a membrane that can have only one opening at a time. If the cell is in contact with other cells, it can have more than one opening at a time because the surrounding cells can keep it intact.

  To all such ideas you might respond that none of this matters, simply because neither we nor our descendants are likely ever to see such organisms. Whether things that we can never see or touch, and whose very existence we cannot prove, are important or unimportant is a subject with philosophical and theological implications that reach far beyond our focus here. In any case, there is a category of such life that we (or our descendants) may be able to see, touch, and measure. It is life that inhabits a specialized subset of all possible universes: universes that are simulated.

  SIMULATED LIFE

  In the several decades since John Conway introduced his “Game of Life,” software developers have created many software programs that simulate organisms individually and collectively, and they’ve done so with increasing verisimilitude. Some are pure games, some are educational tools, and some, like those that ecologists use to model populations, are scientific instruments. Are simulated organisms living? If we accept the NRC report’s provisional definition of life—that is, a “chemical system capable of Darwinian evolution”—the answer is no, but perhaps only because they are not a chemical system. Whether they meet the second part of the definition turns on what may be some fairly subtle distinctions. Those who claim that simulated organisms are living would say they are fully capable of Darwinian evolution; many literally are programmed for it. Moreover, they’d say, simulated organisms do all the things real organisms do: grow, consume, metabolize, reproduce, and die. In contrast, those who claim that simulated organisms are not living argue that they merely mimic these behaviors, that all the growing, consuming, and evolving—however impressive to you and me watching it happen on a screen—is virtual, not real. But still others have argued that the difference between virtual and real is increasingly indistinct. Once again, a definition of life eludes us, and it eludes us in a new way.