What I personally find very promising about string theory, despite its issues in giving us a realization of our four-dimensional world, is that it naturally encodes the emergence principle. As we saw earlier, we start with a theory of strings with no gravity and it spits out the equations of general relativity and gauge theories (albeit in ten dimensions) when the string experiences quantum corrections. Also, there are realizations in string theory (and loop quantum gravity) from which gravitational theories emerge holographically. Holography, invented by Gerard ‘t Hooft and Leonard Susskind, is the idea that physics is encoded in a space that is one dimension less than what is experienced. According to the holographic principle, our experience of this four-dimensional world is actually an illusion, and the information is encoded in a three-dimensional screen that lives at the boundary of the four-dimensional world. A good geometric realization of this is to imagine that our world is a three-dimensional, solid ball. Then by holography all the information about the ball is actually encoded in the two-dimensional surface of the ball, not in its volume. Exactly how the three-dimensional world is reconstructed depends on the exact nature of the model at hand. Hints of the holographic principle were inspired by laws of black hole mechanics discovered by Jacob Bekenstein and Stephen Hawking. They found that for a black hole the entropy did not depend on the volume within the black hole but the area of the horizon. The information of the relevant degrees of freedom was found to be residing on the two-dimensional surface of the black hole and did not care about its interior.
String theory realizes holography by using the correspondence principle that Niels Bohr argued for in quantum mechanics. In string theory there are two theories that are complementary, or dual, to each other. One theory is a nongravitational conformal field theory (CFT), actually a theory of gauge fields, like the ones found in the standard model but with a lot more symmetries. This gauge theory is complementary to a theory of gravity with maximal symmetry with a negative cosmological constant, known as AdS. Since its inception the duality has passed every nontrivial test of the equivalence between these two theories. Taken literally, this means that gravity emerges from the dynamics of the CFT—which itself has no gravity. The string theoretic realization of holography was invented by the Argentinian physicist Juan Maldacena. I remember when Maldacena, at the time a postdoc at Princeton, wrote this first groundbreaking paper in 1997 now referred to as the AdS/CFT correspondence. The AdS/CFT duality relates a gravitational theory to a space-time called AdS, which is a homogenous and isotropic space-time with a negative cosmological constant. I was a graduate student and wondered how he set up the string theory calculation to realize the holographic duality. The paper quickly got international acclaim, and up to today its impact and number of citations by others have not lost steam.
Whatever quantum gravity will look like, I’m convinced that holography will play a key role.
12
THE COSMIC BIOSPHERE
The total number of minds in the universe is one.
—ERWIN SCHRÖDINGER
I have always wanted to know the answer to three questions: Why is the universe expanding? Is there life elsewhere in the universe? And if so, how did it originate? These might seem like distinct concerns, but I have come to the opinion that the questions about the expanding universe and life are linked. The last several chapters have focused on the first question. Now I want to deal with the second.
The thing is, I don’t think they are first and second questions. While we don’t have complete answers to these questions, I think origination and persistence of living systems are traced back to the expansion of the universe. You should be at least astonished by this claim, so let’s dive into it.
When we look at cosmic history, we see the chain of events that unfolded to generate the large-scale structure in the universe. Cosmic time unfolded with primordial quantum vibrations that blossomed into a hierarchy of stellar environments called galaxies and planets that are now part of an interwoven cosmic structure. When we look at the large-scale structure of galaxies, we assume that life and the universe that it inhabits, borrowing from the famed biologist Stephen Jay Gould, are “non-overlapping magisteria.” To most cosmologists, complex systems like life are of little consequence to the problems we are trying to solve, such as the big bang singularity and the parameters of the standard model. To my biologist friends, life is housed in a biosphere that is decoupled from the happenings of the universe out there. But what evidence do we have that life and the universe are truly decoupled? Conversely, what evidence do we have that life and the universe are coupled?
My dance with biology and physics started way back in college, when I considered majoring in biology. I have never been able to shake the biology bug, and my current research in cosmology has reawakened my biological questions, even though I put formal plans aside for a while. At my college, bio majors were required to have a year of organic chemistry, so laziness got the best of me. Still, my musings in biology persisted throughout my studies in physics. In the middle of my second year of graduate school I decided that I wanted a change, so I approached my field theory professor Gerry Guralnik, a codiscoverer of the Higgs boson, and confided to him that I was interested in biophysics and was considering leaving physics altogether. Then Guralnik said, “Let me call my former PhD adviser Wally now and get his advice.”
The Wally that Gerry was referring to is Walter Gilbert, a theoretical particle physicist who had also caught the biology bug and ended up winning the Nobel Prize for a key discovery in genetics—the nature of stretches of DNA known as introns and exons—which, among other things, led to the human genome project and gene therapy. Guralnik made the call and within minutes Gilbert invited me to visit his lab at Harvard. I spent three hours talking with Gilbert and he gave me an in-depth tour of his lab. He also made a recommendation for me to work in Harvard’s biophysics program. That summer, thanks to Gilbert’s recommendation, the renowned biophysicist Jim Hogle offered me a job in atomic resolution virus structure determination using X-ray crystallography. From that experience, I learned some new tools and gained a deep appreciation of the complexities involved in biology and the inapplicability of physical reductionism in attempting to comprehend life’s processes. It also comforted me to learn that I wasn’t alone: other physicists and mathematicians—including many of the greatest—had explored their own biological musings. People including John von Neumann, Eugene Wigner, Claude Shannon, Norbert Wiener, and Roger Penrose, to name a few. I especially think that we can take some lessons and inspiration from the story of Erwin Schrödinger, whose audacious speculations and predictions in biology have been hugely influential. Physics has undergone exponential growth since Schrödinger’s time, and in this “naive” spirit, I will playfully venture into ways in which biology may inform some mysteries in physics and vice versa. But first let’s learn from Schrödinger’s scientific audacity and ingenious contributions to biology.
Schrödinger moved to Ireland to escape the Nazis in 1938, and continued his work in Dublin. In 1943 he gave a series of lectures at Trinity College that would eventually revolutionize biology. In 1944 they were published in a tiny book called What Is Life? In it, Schrödinger speculated on how physics can synchronize with biology and chemistry to explain how life can emerge from inanimate matter. Schrödinger proposed his central question by asking, “How can the events in space and time which take place within the spatial boundary of a living organism be accounted for by physics and chemistry?”
In other words, how is it that the same laws of physics that describe a star account for the intricate processes of metabolism within the living cell? Schrödinger quickly admits that the physics of his time was insufficient to explain some of the ingenious experimental findings that his predecessors and contemporaries had made about the living cell. He even considered the possibility that physics as it was known then may not be enough, when he said, “One must be prepared to find a new physical law prevailing in it.” Despi
te this, Schrödinger marches ahead, using the physics of his time to make some auspicious predictions that inspired the discovery of the double helix structure and functioning of DNA.
Quantum mechanics plays a key role in life, since it is the quantum that is necessary for the stability of atoms and the bonding rules to construct the plethora of molecules found in living matter. (As to whether the more “exotic” quantum properties, such as entanglement or quantum tunneling—where a wave function can actually pass through a barrier—may play a role in life, this remains an open research issue. We’ll get to that.) Schrödinger opens his argument by conjuring quantum mechanics as the starting point to understand the difference between nonliving and living matter. For example, the bulk properties of a piece of metal, such as its rigidity and ability to conduct electrons, require an emergent long-range order, as we saw when I first raised the emergence principle. These properties should be a result of the bonding mechanism and the collective effects of the quantum wavelike properties of electrons in the metal’s atoms. Schrödinger then describes how the atoms in inanimate matter can organize themselves spatially in a periodic crystal, before making a daring leap. Life clearly is more complicated and variable than a piece of metal, so periodicity isn’t going to cut it. So Schrödinger makes a bold proposal: that some key processes in living matter should be governed by aperiodic crystals. More astonishing, Schrödinger postulates this nonrepetitive molecular structure—which will turn out to be a great description of DNA—should house a “code-script” that would give rise to “the entire pattern of the individual’s future development and of its functioning in the mature state.”
Before Schrödinger’s time biologists had the idea of the gene, but it was a formless unit of inheritance, with much that was left unknown. When Schrödinger proposed his idea about how genetic material should work, it was completely unanticipated in anyone else’s work. Today, of course, the idea that genes are governed by a code, similar to a computer code, which could program the structures and mechanisms of the cell and determine the fate of living organisms, might just seem to be common sense. While I say this, exactly how this is accomplished at a molecular level is still a rich research enterprise in biology. What is remarkable is that Schrödinger used reasoning stemming from quantum mechanics to formulate his hypothesis. Schrödinger was an outsider to biology, and this naturally made him a hidden deviant and ripe for making a paradigm-shifting contribution. Whether quantum mechanics was operating in some subtle way for life’s processes was not central to Schrödinger’s argument, but his new line of reasoning provided a completely new set of concepts to explore novel mechanisms. It’s worth noting that no one took Schrödinger’s “code-script” seriously, even after biologist Oswald Avery’s 1944 publication of a paper that gave strong evidence for DNA as the carrier of genetic material. Of course, Avery’s work itself wasn’t immediately embraced: as described by science historian Matthew Cobb, part of the reason for this lack of excitement over Avery’s discovery was that DNA was thought to be a “boring” molecule with a repetitive structure—exactly what Schrödinger had said a gene could not be. Nevertheless, Schrödinger’s quantum reasoning led to a prediction that the aperiodic structure of DNA carried a code that could program life.
Schrödinger’s unappreciated insight came about from an auspicious set of events. In 1943 the US Scientific Research and Development Committee hired some scientists to study information from radar for antiaircraft purposes. Among the scientists were Claude Shannon, the pioneer of information theory, and Norbert Wiener, who found connections between control systems in machines and biological life and coined his findings cybernetics. Both fields have surged into prominence today, especially in the form of quantum information theory, as well as machine learning and artificial intelligence, but they were hugely influential in our understanding of the theoretical and computational underpinnings of life.
As important as Shannon and Wiener are, the game really changed when computer science pioneer John von Neumann argued that a gene was the carrier of information. Von Neumann imagined a gene as a tape that could program an organism. He made an analogy between self-replicating machines and cellular replications. In this case, in order for a machine to replicate itself, there must be an underlying mechanism present to copy the information that specifies the machine itself. In a 1951 conference proceeding, the godfather of developmental biology Sydney Brenner said “[Von Neumann] divided the machine—the automaton as he called it—into three components: the functional part of the automaton; a decoding section which actually takes a tape, reads the instructions and builds the automaton; and a device that takes a copy of this tape and inserts it into the new automaton.” It wasn’t immediately obvious what von Neumann was onto, at least not to Brenner: “I think that because of the cultural differences between most biologists on the one hand, and physicists and mathematicians on the other, it had absolutely no impact at all. Of course, I wasn’t smart enough to really see then that this is what DNA and the genetic code was all about.”1 Brenner’s observations about how cultural differences between biologists, physicists, and mathematicians were in part responsible for biologists missing von Neumann’s earth-shattering insight about information, genes, and replication remain relevant today. The silos of scientific disciplines and scientific social orders still limit how scientists work.
There were two more insights that Schrödinger posited between life and physics. I will leave it for you to read his landmark book What Is Life? about his reasoning that living things have to negate the second law of thermodynamics, which describes the fact that closed systems evolve to maximize disorder, or entropy. Schrödinger called this negentropy: “What an organism feeds upon is negative entropy. Or, to put it less paradoxically, the essential thing in metabolism is that the organism succeeds in freeing itself from all the entropy it cannot help producing while alive.” Living entities have to stabilize their structure and function over their lifetime against the fundamental tendency for disorder, since entropy always increases. Other biologists, mathematicians, and physicists have further developed Schrödinger’s idea into rigorous mathematical statements about how this works.
His third insight has to do with consciousness, and we’ll come back to that in the conclusion of the book. First I want to ask, how would Schrödinger revise What Is Life? with the new developments in modern biology and cosmology?
Over the years I have developed a pattern: I have chosen to make friends with biologists. Having those friendships means that I’m always asking about what they think is cool in biology. In the past several years I’ve had frequent conversations with my friend Salvador Almagro-Moreno, a molecular biologist. Sometimes meeting over a drink, we exchange ideas: I tell him my latest musings in cosmology research and he tells me about his musings in biology. It probably won’t surprise you to learn that Salvador is a proud owner of a first edition of Schrödinger’s What Is Life? He and I share a vision that I think many of our fellow biologists and physicists would find too deviant, and even repugnant. We don’t let that stop us, and there have been many times that we stayed up late talking about this fascination. There was definitely some strategy here: in part our conversations were an exercise in deliberately generating an outsider perspective, hopefully to benefit each other’s research insights. But it wasn’t all so calculating: it was a lot of fun, and just one of the things that make our jobs as scientists so rewarding. What is interesting to me about our discussions is not just the ideas themselves, but how our conversations generated new questions in both our zones of inquiry.
In 2014, I held the E. E. Just chair in natural sciences at Dartmouth College. Ernest Everett Just was a pioneering Black developmental biologist who cemented the early field of epigenetics. His remarkable life story, scientific contributions, and legacy are majestically described in Black Apollo of Science by Kenneth Manning. As the E. E. Just professor, I had the responsibility of running a university-wide science program for promising student
s from underrepresented groups interested in science. Salvador was awarded the E. E. Just postdoctoral fellowship and helped me with the program by leading its mentoring and teaching component. Nowadays, Salvador is a biology professor at the University of Central Florida, one of the largest Hispanic-serving institutes in the United States, and is leading similar efforts as the E. E. Just program. One day we were discussing some program business when the conversation, as usual for us, evolved. I started telling my friend about a research idea I had in cosmology. The idea was a new mechanism to understand what is called the fine-tuning problem: as with the value of the cosmological constant, there are other constants of nature that have values that are just the way they need to be so that life could happen. A hugely important set of constants, known as coupling constants, that determine the strength of the force—such as electromagnetism or the weak force—are involved in interaction between particles. The idea I told Salvador about was that, if the universe went through a large number of cycles of collapse and expansion, the big bang phase would provide an opportunity to reset the values of the coupling constants. Most cycles of expansion might not host life, but eventually the universe would hit the jackpot. Salvador asked me very precise questions about my project, and the next day, in a state of elation, he told me that he was able to implement the cyclic-universe idea into the development of a theory based on some experiments he completed involving genetic evolution in biology.
Fear of a Black Universe Page 14