Walking through Weinberg's 'Dreams of a final theory'

For much of the past half century, Steven Weinberg has been a scientist at the forefront of our quest for a deeper understanding of the most fundamental aspects of physics. In 1992, at a moment of crisis for the physics community and the scientific mission at large, Weinberg penned 'Dreams of a final theory' to communicate the burning questions in particle physics and the challenge ahead for mankind.

Since the SSC was gutted, the physics community has waited for two decades to receive experimental feedback in this quest. Riding on the LHC we've found the Higgs, but that's just another step forward in the quest.

Most questions Weinberg posed are as relevant today as they were back then, and we've uncovered some exciting new directions in the intervening years. In this talk, I will try to articulate those questions, and what we've learned since then. Along the way, I hope to give you a perspective on some of the broader questions driving research in fundamental physics today.

Transcript

1. Walking through Weinberg’s ‘Dreams of a final theory’ Sivaramakrishnan Swaminathan 3 July ’15
2. Steven Weinberg “Our knowledge of the principles that determine these and other notions is at the core of phyical science and a precious part of our civilization.”
3. Before we get started Disclaimer: This is only my understanding of the content. I apologize for any possible misrepresentations. If you find this interesting, I encourage you to read the book. You needn’t read all the quotations in this presentation. I couldn’t resist putting in excerpts; Weinberg is very articulate.
4. Chapter 1: Prologue
5. Chapter 2: On a piece of chalk
6. Let’s play a child’s game Pushing the thread of “understanding” Any question that we can ask about it ultimately leads to the quantum mechanical and relativistic theory of ‘elementary’ degrees of freedom aka the Standard Model of particle physics.
7. Why? Why? Why? “Why? Why? Why? [. . . ] The word ‘why’ is notoriously slippery [. . . ] Explanation, unlike deduction, carries a unique sense of direction [. . . ] This ‘because’ does not have to do with our ability actually to deduce anything but reflects our view of the order of nature.”
8. All roads lead to Rome Caveats to the notion of an ultimate theory and its power: Historical accidents (initial conditions) Complexity (sensitive dependence to initial conditions) Emergence (more is different)
9. Chapter 3: Two cheers for reductionism
10. In deference to the times There is an undeniable sense of hierarchy among the scales in physics. After all, the value we attach to “elementary” is somewhat sentimental. He uses the word fundemantal/elementary in (merely?) this way. We are all reductionists.
11. Chapter 4: Quantum mechanics and its discontents
12. Chapter 5: Tales of theory and experiment
13. The ‘art of science’ Many meanings of the word ‘theory’ Science is messy! How science is not done Hypothesis −→ Experiment −→ Conclusion
14. What would experiment be without theory? “[. . . ] half-serious maxim attributed to Eddington: One should never believe any experiment until it has been confirmed by theory.”
15. But why? Because experiments are hard! “There is nothing in any single disagreement between theory and experiment that stands up and waves a flag and says, ‘I am an important anomaly’ [. . . ] It took theory to explain which were the important observations.” Never in the past century has experiment overthrown a well-accepted theory. Proven incomplete and provided new regimes, yes – but never overthrown as invalid.
16. Physicists’ taste for the truth General relativity QED and renormalization of infinities Electroweak theory FTL neutrinos
17. A pearl of wisdom “It appears that anything you say about the way that theory and experiment may interact is likely to be correct, and anything you say about the way that theory and experiment must interact is likely to be wrong. [. . . ] I think that one should not hope for a science of science, the formulation of any definite rules about how scientists do or ought to behave, but only aim at a description of tha sort of behaviour that historically has led to scientific progress – an art of science.”
18. Chapter 6: Beautiful theories
19. Simplicity and elegance Occam’s razor Eg: Symmetry principles “The symmetries that are really important in nature are the not the symmetries of things, but the symmetries of laws.” Spontaneous breaking
20. What do we seek? “We do not want to discover a theory that is capable of describing all imaginable kinds of force among the particles of nature.” This is somewhat unlike other (more applied) areas! “not only is our aesthetic judgement a means to the end of finding scientific explanations and judging their validity — it is part of what we mean by an explanation.”
21. Another pearl of wisdom “Deducing the consequences of a given set of well-formulated physical principles can be difficult or easy, but it is the sort of thing that physicists learn to do in graduate school and that they generally enjoy doing. The creation of new physical principles is agony and apparently cannot be taught.”
22. On why we have this sense of beauty. . . 1. We’ve been trained (evolved) by nature to find it beautiful. 2. Scientists tend to choose problems that have beautiful solutions, eg. calculating critical exponents -vs- calculating the Curie temperature. 3. (Through our education) We’re used to discovering a simpler and more beautiful explanation on peeling back each layer. “Sometimes when our sense of beauty lets us down, it is because we have overestimated the fundamental character of what we are trying to explain.”
23. Chapter 7: Against philosophy
24. “I raised in the previous chapter the problem of what Wigner calls the ‘unreasonable effectiveness’ of mathematics; here I want to take up another equally puzzling phenomenon, the unreasonable ineffectiveness of philosophy.”
25. Positivism: Mach, Kaufmann, Einstein (Relativity vs QM), Pickering (quarks) “No one would give a book about mountain climbing the title Constructing Everest.” Against Kuhn’s take on the absence of objective truths “The danger they present to science comes from their possible influence on those who have not shared in the work of science but on whom we depend, especially on those in charge of funding science and on new generations of potential scientists.”
26. Chapter 8: Twentieth century blues
27. Lots of symmetry, but broken spontaneously Nature has more symmetry than apparent
28. What were/are the driving questions in particle physics? Quantum mechanical description of gravity (BH information paradox) Something different about the strong force What triggers electroweak symmetry breaking? (We found the Higgs! 2012) Baryogenesis GUTs, and the (big) hierarchy problem, SUSY
29. Effective field theories EFTs (modern perspective on renormalizability) are consistent with tests done so far, but are an anathema since they break down when extrapolated to high energies. Like a hound on the scent of a trail, this seems like an avenue to dig deeper and come to terms with more fundamental physics (since fundamental physics must always give physically sensible answers)
30. My take on major markers in the intervening decades (~20 years) The LEP paradox and the little hierarchy problem (now the LHC null surprise) String theory Holography** (implications for quantum gravity) The expanding universe Inflation
31. On new students entering particle physics [Weinberg:] “It is a tribute to the fundamental importance of elementary particle physics that very bright students continue to come into the field when so little is going on.” [Me:] “To pursue a PhD in theoretical high-energy physics, one needs to be half retarded and half advanced – half-advanced for wanting to capture the excitement of the past, and half retarded for seeing a future in it.”
32. Chapter 9: The shape of a final theory
33. String theory: the first plausible candidate [Strings, TaDa!] “It is not that someone suddenly had an inspiration that matter is composed of strings and then went on to develop a theory based on this idea; the theory of strings had been discovered before anyone realized that it was a theory of strings.”
34. The sociology of string theory “A fair fraction of today’s young theoretical physicists are working on string theory [. . . ] so far no detailed quantitative predictions have emerged that would allow a decisive test of string theory [. . . ] impasse has led to an unfortunate split in the community of physicists. String theory is very demanding; few of the theorists who work on other problems have the background to nuderstand technical articles on string theory, and few of the string theorists have time to keep up with anything else in physics, least of all with high-energy experiments. Some of my colleagues have reacted to this unhappy predicament with some hostility to string theory. I do not share this feeling. String theory provides our only present source of candidates for a final theory – how could anyone expect that many of the brightest young theorists would not work on it?”
35. The dreaded anthropics “The idea of the anthropic principle began with the remark that the laws of nature seem surprisingly well suited to the existence of life [. . . ] The energies of nuclear states depend in a complicated way on all the constants of physics, such as the masses and electric charges of the different types of elementary particles. It seems at first sight remarkable that these constants should take just the values that are needed to make it possible for carbon to be formed[..]” It’s a cop-out; it means we give up and go home.
36. Russian reversal, a Soviet response “A Soviet emigre physicist told me that a few years ago a joke was circulating in Moscow, to the effect that the anthropic principle explains why life is so miserable. There are many more ways for life to be miserable than happy; the anthropic principle only requires that laws of nature should allow the existence of intelligent beings, not that these beings should enjoy themselves.”
37. Chapter 10: Facing finality
38. “Once again I repeat: the aim of physics at its most fundamental level is not just to describe the world but to explain why it is the way it is.” “Perhaps there is a final theory [. . . ] but humans are simply not intelligent enough. [. . . ] A far more pressing worry is that the effort to discover the final laws may be stopped for want of money.”
39. Our best hope “I do not mean to suggest that the final theory will be deduced from pure mathematics [. . . ] so rigid that it cannot be warped into some slightly different theory without introducing logical absurdities like infinite energies.” “The problem seems to be that we are trying to be logical about a question that is not really susceptible to logical argment: the question ofw hat should or should not engage our sense of wonder.”
40. Implications of a final theory (1) “Of course a final theory would not end scientific research, not even pure scientific research, nor even pure research in physics. Wonderful phenomena, from turbulence to thought, will still need explanation whatever final theory is discovered [. . . ] A final theory will be final in only one sense – it will bring to an end a certain sort of science, the ancient search for those principles that cannot be explained in terms of deeper principles.”
41. Implications of a final theory (2) “we may regret that nature has become more ordinary, less full of wonder and mystery [. . . ] There will be endless scientific problems and a whole universe left to explore, but I suspect that the scientists of the future may envy today’s physicists a little, because we are still on the voyage to discover the final laws.”
42. Chapter 11: What about God?
43. “The theologian Paul Tillich once observed that among scientists only physicists seem capable of using the word ‘God’ without embarrassment.”
44. Occam’s razor The only way science can proceed is to assume there is no divine intervention, and see how much purchase you get.
45. On science and religion “For those who see no conflict between science and religion, the retreat of religion from the ground occupied by science is nearly complete.”
46. On why humans seek refuge in religion “the pain of confronting the prospect of our death and the deaths of those we love impels us to adopt beliefs that soften the pain. If we are able to manage to adjust our beliefs this way, then why not do so? [. . . ] I can see no scientific or logical reason not to seel consolation by adjustment of our beliefs – only a moral one, a point of honour [. . . ] The honour of resisting this temptation is only a thin substitute for the consolations of religion, but it is not entirely without satisfactions of its own.”
47. On religion (from elsewhere, not this book) “With or without religion, you would have good people doing good things and evil people doing evil things. But for good people to do evil things, that takes religion.”
48. “The more the universe seems comprehensible, the more it also seems pointless.” As we hone our understanding of nature, it seems more like nature by itself is not lending any purpose for humanity – that seems to be up to us.
49. Chapter 12: Down in Ellis county
50. Afterword: ‘The crisis of big science’
51. Thank you!