A Brief History of String Theory
by Emil Martinec
Professor at the University of Chicago
Lời nói đầu. Dưới đây là nguyên bản tiếng Anh bài Lược sử lý thuyết dây của Giáo sư vật lý Mỹ Emil Martinec ở Viện Enrico Fermi, Đại học Chicago, và là giám đốc của Trung tâm vật lý lý thuyết Kadanoff của Đại học này, dành riêng cho phiên bản tiếng Việt Tại sao lý thuyết dây? của tác giả Joseph Conlon vừa được nxb Trẻ cho ra mắt (xem giới thiệu: Lời dẫn nhập cho sách Tại sao Lý thuyết day?). Do có nhu tham khảo bản gốc nên chúng tôi xin đăng tải lên.
GS Emil Martinec (1958-)
Emil Martinec từng là thành viên của nhóm được mệnh danh là ‘Bộ tứ đàn dây Princeton’ (Princeton string quartet) của Đại học Princeton gồm có David Gross, Jeffrey Harvey, Ryan Rohm và ông. Họ đã xây dựng heterotic string theory năm 1985 và đã thúc đẩy cuộc cách mạng siêu dây thứ nhất những năm 1980. Đó là một loại dây kín (closed loop) lai ghép giữa một siêu dây và dây boson.
Tiểu luận soi sáng sâu sắc về nhiều mặt từ kinh nghiệm của một người trong cuộc. Nó mô tả sống động và súc tích sự ra đời, sứ mệnh, ảnh hưởng và tương lai của lý thuyết dây. Lý thuyết dây không ra đời từ óc tưởng tượng thuần túy của con người, mà tình cờ từ các hiện tượng thực nghiệm vật lý được quan sát trong máy gia tốc hạt.1
Xin chân thành cám ơn vinh dự và sự ưu ái mà ông đã dành cho độc giả Việt Nam. Rất mong có ngày GS Martinec sẽ đến thăm Việt Nam và nói chuyện về lý thuyết dây tại Trung tâm ICISE Quy Nhơn.
Về GS David Gross trong nhóm ‘Bộ tứ đàn dây Princeton’: Ông từng là khuôn mặt nổi bật của vật lý hạt, đoạt giải Nobel vật lý hạt năm 2004 chung với Frank Wilczek (học trò ông) and David Politzer cho khám phá ‘tự do tiệm cận’, tính chất rất đặc biệt của lực mạnh, còn gọi là sắc động học lượng tử (Quantum Chromodynamics, QCD). Ông đã từng đến sinh hoạt với Hội nghị khoa học thế giới tại Trung tâm ICISE Quy Nhơn của GS Trần Thanh Vân và Lê Kim Ngọc. NXX
A core aspect of our humanity is our desire to make sense of our place in the world, to understand what laws govern the universe – and if not to find the answer to why we are here, then at least to find the answer to how we are here. Physics is that part of science devoted to the investigation, at the most basic level, of the constituents of matter and the forces that govern their interactions. Down through the centuries, physics has successively delved deeper into microscopic realms, further back in time and farther out in space in pursuit of that quest. Our most powerful “microscopes” are particle accelerators, in which high-energy collisions of subatomic particles teach us about the structure of matter and interactions on the smallest scales. Our most powerful telescopes teach us about the largest structures in the cosmos, which had their origins in the big bang that began our universe. Remarkably, because the big bang was so hot and energetic, the same processes we now study in our particle accelerators were happening on a grand scale back then, and left their imprint on the distribution of galaxies we observe today with our telescopes. Over the last few decades, a remarkable synthesis has applied our understanding of fundamental particles and their interactions to calculate, with remarkable precision, how the universe began, how the matter in it came to be and how it coalesced to form galaxies such as our Milky Way, and so on down to stars such as our Sun.
A remarkable theme emerging from that journey is the parallel development of physics and mathematics, the language we use to express the laws of nature. For instance, Newton developed the calculus in order to formulate his laws of gravity and the motion of bodies; and Einstein borrowed ideas from geometry in order to re-formulate gravitation in a framework consistent with his principle of relativity. In recent decades, string theory has emerged as the next step on this journey. It holds the promise of unifying the fundamental forces of nature, and at the same time has deepened the connection between mathematics and theoretical physics. In particular, it has enriched and deepened the connection between geometry and matter in new and unexpected ways.
The route to the discovery and development of string theory has seen several surprising twists and turns, and we still have far to travel. It began as an attempt to make sense of the results being seen in particle accelerators in the 1950’s and 60’s. More powerful accelerators showed a proliferation of new strongly-interacting particles, which were shown qualitatively to fit the spectrum of a vibrating string whose tension would be set by the size and mass of the proton; the different vibrational modes were supposed to describe the different particles being observed. Eventually it was realized that such a string was merely a phenomenological approximation to the actual theory underlying strong nuclear forces, known as quantum chromodynamics (QCD). Attention of physicists turned to working out the details and predictions of QCD, as well as other aspects of particle interactions; string theory development dwindled to the work of just a few researchers by the mid-1970’s. But in a remarkable reassessment, they made the bold proposal that rather than being a theory of just one of the fundamental forces, that string theory made more sense as a theory of all the forces, including gravity. According to this proposal, the scale of the string tension was not the size of the proton, but rather the scale of quantum gravity – a size some twenty orders of magnitude smaller, and thought to be the smallest length scale in nature.
It took another decade for this idea to ignite the interest of the broader community of theoretical physicists. By then, the “Standard Model” of particle physics, as it was now called, had been thoroughly explored, and proved wildly successful; and so physicists turned their attention to unifying it with Einstein’s theory of gravity. This led to a proliferation of new results sometimes called the “first superstring revolution”, showing how string theory could encompass the Standard Model, as well as resolve some major issues arising from attempts to merge Einstein’s general relativity with quantum theory.
The work of Stephen Hawking and others in the 1970’s provided one of these puzzles. They showed that a black hole in quantum gravity must have an enormous number of possible internal states, while Einstein’s general relativity gave no clue as to what these internal configurations might be. For two decades, the internal structure of black holes remained a mystery. Then in the mid-1990’s, string theorists showed how to characterize the internal structure of a special class of black holes. Remarkably, their answer relied in an essential way on the structure of “extra dimensions” in string theory. It had been something of an embarrassment that the mathematical consistency of string theory requires there to be six or seven additional dimensions of space, beyond the three that are readily apparent in the world around us. In order to be compatible with observation, these extra dimensions must be extraordinarily small, perhaps as small as the quantum gravity scale; so small that they are unobserved. But these extra dimensions still have a structure. Supposing that the black hole is merely a large, heavy object built out of the fundamental constituents of string theory – strings, membranes, etc. – it was found that the number of ways that one could wrap those extended objects around the extra dimensions precisely accounts for the black hole’s internal configurations. Tiny extra dimensions (and their intricate structure) turned out to be necessary ingredients to explain the internal states of these particular black holes.
This landmark calculation was one of the great successes of the “second superstring revolution”, where we began to understand what happens when strings interact strongly with one another under the crushing forces inside a black hole. The implications of this result are still being worked out. Even though we can count the number of black hole states, we still don’t understand exactly how string theory deals with the extraordinary forces at play at the spacetime singularity inside a black hole. That singularity bears some similarity with the big bang at the beginning of the universe, and so one may hope that understanding the structure of black holes may teach us about the early universe. In a related vein, observations have shown that the universe is mostly made up of matter and energy in forms quite different from those of everyday experience. A detailed accounting of the nature of this “dark matter” and “dark energy” has yet to be achieved, and remains a major challenge for string theory.
Thus string theory represents a remarkable achievement of the human intellect, one discovered almost by accident. It builds upon decades of progress in understanding nature at the deepest level, and involves some of the most profound concepts in mathematics. At the same time it is still very much a work in progress. It has proven remarkably difficult to extract concrete experimental predictions from the theory, yet its achievements in encompassing both the Standard Model of particle physics and a quantum theory of gravity, and providing new insights into the structure of black holes, provide tantalizing evidence that we are on the right track.
1. Xem thêm Tại sao lý thuyết dây?, mục 5.2, hay trong Roger Penrose, The Road to Reality. The complete guide to the laws of the universe. Jonathan Cape, 2004, mục 31.5, trang 884 và tiếp theo. Yoichiro Nambu, nhà vật lý Mỹ gốc Nhật, giải Nobel vật lý năm 2008, có lẽ là người đầu tiên vào đã đề xuất ý tưởng dây năm 1970 để giải thích một số hiện tượng quan sát được.