It is a small step from this, perhaps, to suggest that the total energy of the entire universe might be precisely zero. Such a universe with total energy equal to zero is attractive, because it allows for a universe that began from nothing. All matter and energy we might see could have arisen from a quantum mechanical fluctuation (including a gravitational quantum mechanical fluctuation in space itself). While Feynman speculated on this possibility, the current best model for the evolution of the universe, called inflation, is based on this very idea. The originator of the idea of inflation, Alan Guth, has said that in this case the universe is the ultimate example of a “free lunch.”
Interestingly, a universe with zero total gravitational energy is spatially flat—that is, on large scales it behaves like a normal Euclidean space where light travels in straight lines. There is now very good evidence that the universe is flat by direct measurements of its geometry on large scales, one of the most exciting developments in cosmology in recent times. As early as 1963, however, Feynman suggested this was likely to be the case because the fact that gravitationally bound galaxies and clusters of galaxies—the largest bound objects in the universe, tens of millions of light-years across—do exist implied that the positive kinetic energy of the expansion of the universe was roughly balanced by the negative gravitational potential energy in these systems. Hewas right.
There was one application of his quantum field theory arguments to gravity where he seemed to have departed from his normal sensible physical intuition, however. In his work on QED he, as well as others, had shown that virtual particles not only exist but also are necessary in order to understand the properties of atoms. Thus, empty space is not empty but is a boiling brew of virtual particles. The laws of quantum mechanics tell us that the smaller the scale one wants to consider, the higher the energy the virtual particles that can briefly exist can have. Feynman once referenced this by saying that in the space in the closed palm of a hand, virtual particles existed with enough energy to power our entire civilization. Unfortunately advocates and crackpots have used this statement to express their desire to develop devices that exploit the energy of the vacuum to do precisely this, and solve our energy problems.
What Feynman somehow forgot, and what the Russian physicist Yakov Zel’dovich made clear in 1967, is that all energy gravitates, even the energy of empty space. If empty space had as much energy as Feynman argued, the gravitational forces would be so great as to blow up the earth, because according to general relativity, when energy is put into empty space, the resulting gravitational force is repulsive, not attractive. Therefore, the energy of empty space cannot be, on average, orders of magnitude larger than the energy of all matter, or the resulting repulsive force would be so large that galaxies would never have formed.
Nevertheless, Feynman was not completely wrong. The most astounding discovery in the last fifty years, if not longer, has been the discovery that empty space does contain energy—far less than Feynman imagined, but enough so that the energy of empty space is currently dominating the expansion of the universe, causing it to accelerate. We currently have no understanding of why this is the case, and why empty space possesses both energy and an amount of energy that is comparable to the total energy contained in all galaxies and matter in the universe. It is probably the biggest mystery in physics, if not all of science.
Feynman’s mistake aside, if the idea of inflation—an early period of exponential expansion that would have resulted in a flat universe today and could have generated all of the structures currently observed—is correct, then there is an exciting implication that hearkens back to Feynman’s original calculations. If gravitons are elementary particles like photons, then one can calculate that the same quantum mechanical processes which operated during inflation (to eventually produce matter density fluctuations that would have collapsed to form all of the galaxies and clusters we see today) would have also generated a background of gravitons in the early universe, which would today be observable as a background of gravitational waves. This is indeed one of the core predictions of inflation, and an area of physics I have personally been exploring. Most exciting, we may soon be able to detect such a background with satellites that have been sent up to probe the large-scale structure of the universe. If such a background is observed, it will imply that the calculations Feynman performed when he decided to approach gravity like any other field theory allow a prediction that can be compared with observations, meaning at the very least that the apology he offered for thinking about esoteric and undetectable effects in quantum gravity was not necessary.
IT IS APPROPRIATE to conclude this chapter on Feynman’s fascination with gravity by once again returning to the apology with which he began his first paper on the subject. Feynman was attracted to quantum gravity because it was off the beaten path. By the same token he realized that that was the case because the only calculations one might perform would result in predictions of effects that were potentially forever unmeasurable, because gravity is so weak. And so as he began his formal discussion of quantum gravitational effects, he stepped back and said, “It is therefore clear that the problem we are working on is not the correct problem; the correct problem is what determines the size of gravitation?”
A more prescient statement could not have been made. The real mystery that has been driving theoretical particle physicists is the question of why gravity is forty orders of magnitude weaker than electromagnetism. Almost all of the current efforts toward unifying forces, including string theory, are directed toward addressing this puzzling and fundamental question about the universe. It is likely that scientists will not have a full and complete understanding of either gravity or the other forces until they are able to answer this question.
This is characteristic of perhaps the single most remarkable feature of Feynman’s lasting legacy. Even as he may have failed to resolve the answers to many of nature’s fundamental mysteries, he nevertheless unerringly shed light on the very questions that have continued to occupy the forefront of science to this day.
CHAPTER 16
From Top to Bottom
The game I play is a very interesting one. It’s imagination, in a tight straitjacket.
—RICHARD FEYNMAN
In December of 1959 Feynman gave a lecture at the annual meeting of the American Physical Society, which that year was being held at Caltech. Once again, a desire to strike out in new and uncrowded directions was clearly on his mind, as he began the lecture with a quote I used earlier: “I imagine experimental physicists must often look with envy at men like Kamerlingh Onnes, who discovered a field like low temperature, which seems to be bottomless and in which one can go down and down. Such a man is then a leader and has some temporary monopoly in a scientific adventure.” The lecture, published the next year in Caltech’s Engineering and Science magazine, was titled “There’s Plenty of Room at the Bottom.” It is a beautiful and fanciful discussion of a whole new world of possibilities that had nothing to do with particle physics or gravity but were firmly grounded in phenomena with direct applications.
In spite of the esoteric field of particle physics that he had chosen to focus upon, Feynman never lost his interest or his fascination with the physics of the world we can see and touch. And so the opportunity to present this lecture represented for Feynman a chance to let his imagination wander over a domain that had always fascinated him, looking for new fodder for his next scientific adventure—somewhere he might have a monopoly. It also represented his own fascination with the remarkable possibilities of physics in two areas that had captured his imagination since he was a child, through to his time at Los Alamos: mechanical devices and computing.
The lecture was a milestone, and has often been reprinted, because it basically outlined a whole new field of technology and science, or rather a set of fields, related to what is now called nanotechnology but not restricted to that realm. Feynman’s central point was tha
t in 1959 when most people were thinking of miniaturization, they were being timid—that there was a universe of space between the size of human-scale machines and the size of atoms. By exploiting this space, he imagined we could not only change technology but also open up whole new domains of scientific inquiry that were then beyond the reach of scientists. And these domains were not like quantum gravity but were domains that might be exploited in his own time if people seriously thought about the remarkable universe under their noses. As he put it, “It is a staggeringly small world that is below. In the year 2000, when they look back at this age, they will wonder why it was not until the year 1960 that anybody began seriously to move in this direction.”
Feynman began his lecture by saying that some people were impressed by a machine that could write the Lord’s prayer on the head of a pin. That was nothing. He envisaged first writing the entire Encyclopaedia Britannica on the head of a pin. But, he argued, that was nothing, because one could easily do that with regular printing by simply shrinking the area of each dot used in half-tone printing by a factor of 25,000. As he argued, even then each dot would contain about 1,000 atoms. No problem, he imagined.
But even that was timid, he argued. What about writing all of the information in all of the books in the world? He performed an estimate for doing so that is amusingly similar to one that I did when I tried to consider how much information would be required to store a digital copy of someone for transporting, in The Physics of Star Trek. He argued that it would be easy to store one bit of information (that is, a 1 or 0) using, say, a cube of 5 atoms on a side, containing 125 atoms. He also estimated there were about 1015 bits of information in all of the books in the world, which at the time he estimated to be about 24 million volumes. In that case, to store all of the information in all of the books in the world would take merely a cube of material less than one-hundredth of an inch on a side—as small as the smallest speck of dust visible to the human eye. Okay, so you get the picture.
Feynman wanted to explore the possibilities of exploiting matter at the atomic level, the range being, as he described, almost unfathomable. Moreover, true to his first love (as he described her in his Nobel lecture), the most exciting thing of all was that once scientists started engineering at this level, they would have to directly confront the realities of quantum mechanics. Instead of building classical machines, they would have to start thinking about quantum machines. Here was a way to merge the quantum universe with the universe of human experience. What could be more exciting?
I was struck, when rereading his lecture, by his remarkable prescience. Many of the possibilities he described have since come to pass, even if not exactly as he imagined, and usually only because he didn’t have the necessary data within his straitjacket at the time to imagine properly. Once again, while he might not have directly and personally solved all of the problems, he asked the right questions and isolated the developments that have become the very forefront of technology, a half century later, as well as imagined the principles that might form the basis of technology in the next fifty years. Here are a few examples:
(1) Writing All of the Books on Earth on a Dust Speck: How far have we come toward putting all of the books on a speck of dust? When I taught at Yale University in 1988 I bought what was then the largest hard-drive in the physics department. It was 1 gigabyte, and it cost $15,000. Today I own a paperclip-sized memory stick I keep on a keychain. It holds 16 gigabytes and cost me $49. I have a 2-terabyte (or 2,000-gigabyte) portable external hard-drive for my laptop that cost me $150, so I can now buy 2,000 times as much storage for one-one hundredth as much money. Feynman’s estimate of 1015 bits for all of the books in the world equals about 100 terabytes, or about fifty portable hard-drives. Of course, most of the space in these drives is not for storage, but for the read mechanism, the interfaces with the computer, and power supplies. Moreover, no one has made any effort to miniaturize the storage size beyond that which fits comfortably next to a laptop. We are not yet able to store large amounts of information on atomic-size scales, but we are now only off by a factor of about a thousand.
In 1965, Gordon Moore, the co-founder of Intel, proposed a “law” that the available storage and speed of computers would double about once every twelve months or so. Over the past forty years this goal has been met or exceeded as technology has continued to keep pace with demand. Thus, given the fact that 1,000 = 210, we might be a decade away from Feynman’s goal, not just of writing but also of reading all of the books in the world on the head of a pin.
(2) Biology on the Atomic Scale: As Feynman put it in 1959,
What are the most central and fundamental problems of biology today? They are questions like: What is the sequence of bases in the DNA? What happens when you have a mutation? How is the base order in the DNA connected to the order of amino acids in the protein? What is the structure of the RNA? Is it single-chain or double-chain, and how is it related in its order of bases to the DNA? What is the organization of the microsomes? How are proteins synthesized? Where does the RNA go? How does it sit? Where do the proteins sit? Where do the amino acids go in? In photosynthesis, where is the chlorophyll? How is it arranged? Where are the carotenoids involved in this thing? What is the system of the conversion of light into chemical energy? It is very easy to answer many of these fundamental biological questions; you just look at the thing!
Could he have enumerated more clearly and precisely the frontiers of modern biology? At least three Nobel Prizes have already been awarded for research that allows the sequence of molecular base pairs in DNA to be read at essentially the atomic level. Sequencing of the human genome has been the holy grail of biology, and the ability to determine genetic sequencing has been improving at a rate that far outstrips Moore’s law for computers. What cost over a billion dollars to achieve the first time, less than a decade ago, now can be done for several thousand dollars, and it is expected that within the next decade people will be able to sequence their own genome for less than the cost of a good dinner at a restaurant.
Reading out molecules is important, but the real key to advances in biology is determining three-dimensional molecular structures at the atomic scale. Protein structure determines function, and determining how the atomic components of proteins fold up to form a working mechanism is currently one of the hottest topics in molecular biology.
However, as Feynman also anticipated, the ability to probe biological systems at the atomic level is not merely a passive enterprise. At some level, if scientists can read the data, they can write the data—they can build biological molecules from scratch. And if they can build biological molecules, they can ultimately build biological systems—that is, life—from scratch. And if we can build these systems from scratch and understand what makes them function the way they do, then we will be able to design life-forms that don’t currently exist on earth, perhaps life-forms that extract carbon dioxide from the atmosphere and make plastic, or algae that produce gasoline. If this sounds far-fetched, it isn’t. Biologists like George Church at Harvard, and Craig Venter, whose private company helped first decode the human genome, are working on these challenges right now, and Venter’s company recently received $600 million from Exxon for an algae-to-gasoline project.
(3) Observing and Manipulating Single Atoms: In 1959 Feynman bemoaned the sorry state of electron microscopy, which itself was a relatively new field. Because electrons are heavy (compared to light, which is massless), the quantum mechanical wavelength of electrons is tiny. This means that while light microscopes are limited by the wavelength of visible light, about 100 to 1,000 times the size of atoms, electrons can be manipulated by magnetic fields to magnify and produce images of far smaller objects from which they scatter. Yet in 1959 the possibility of imaging individual atoms seemed remote, as the energies involved suggested that the systems would have to be disrupted in order to observe them.
How things have changed. Using the very properties of quantum
mechanical systems, as Feynman had again anticipated, new microscopes called scanning-tunnelling microscopes and atomic force microscopes are allowing images of single atoms in molecules to be made. Moreover, Feynman predicted, “The principles of physics, as far as I can see, do not speak against the possibility of maneuvering things atom by atom. It is not an attempt to violate any laws; it is something, in principle, that can be done; but in practice, it has not been done because we are too big.” Sure enough “atomic tweezers” have now been developed using techniques similar to those used in the new microscopes, and intense lasers have been created allowing researchers to regularly manipulate and move individual atoms. Once again, three different Nobel Prizes have been awarded for this work.
Scientists can now not only resolve single atoms in space but can also do so in time. Laser technology has allowed the production of laser pulses that last femtoseconds (10–15 seconds). This is comparable to the timescale over which chemical reactions between individual molecules occur. By illuminating molecules with such short pulses, researchers hope to observe the sequence of events, at an atomic level, by which these reactions take place.
(4) Quantum Engineering: What most excited Feynman about atomic-scale machines and technology was the realization that once one is working at these scales, the strange properties of quantum mechanics become manifest. Understanding this, one might then hope to design materials with specific, and sometimes exotic, quantum mechanical properties. Once again, from his paper:
What would the properties of materials be if we could really arrange the atoms the way we want them? They would be very interesting to investigate theoretically. I can’t see exactly what would happen, but I can hardly doubt that when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have, and of different things that we can do. When we get to the very, very small world—say circuits of seven atoms—we have a lot of new things that would happen that represent completely new opportunities for design. Atoms on a small scale behave like nothing on a large scale, for they satisfy the laws of quantum mechanics. So, as we go down and fiddle around with the atoms down there, we are working with different laws, and we can expect to do different things. We can manufacture in different ways. We can use, not just circuits, but some system involving the quantized energy levels, or the interactions of quantized spins, etc.
Quantum Man: Richard Feynman's Life in Science Page 23
