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

Siva Swaminathan
July 03, 2015
230

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.

Siva Swaminathan

July 03, 2015
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Transcript

  1. Walking through Weinberg’s
    ‘Dreams of a final theory’
    Sivaramakrishnan Swaminathan
    3 July ’15

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  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.”

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  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.

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  4. Chapter 1: Prologue

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  5. Chapter 2: On a piece of chalk

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  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.

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  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.”

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  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)

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  9. Chapter 3: Two cheers for reductionism

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  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.

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  11. Chapter 4: Quantum mechanics and its
    discontents

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  12. Chapter 5: Tales of theory and experiment

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  13. The ‘art of science’
    Many meanings of the word ‘theory’
    Science is messy! How science is not done
    Hypothesis −→ Experiment −→ Conclusion

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  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.”

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  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.

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  16. Physicists’ taste for the truth
    General relativity
    QED and renormalization of infinities
    Electroweak theory
    FTL neutrinos

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  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.”

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  18. Chapter 6: Beautiful theories

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  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

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  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.”

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  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.”

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  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.”

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  23. Chapter 7: Against philosophy

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  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.”

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  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.”

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  26. Chapter 8: Twentieth century blues

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  27. Lots of symmetry, but broken spontaneously
    Nature has more symmetry than apparent

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  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

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  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)

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  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

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  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.”

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  32. Chapter 9: The shape of a final theory

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  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.”

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  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?”

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  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.

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  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.”

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  37. Chapter 10: Facing finality

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  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.”

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  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.”

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  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.”

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  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.”

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  42. Chapter 11: What about God?

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  43. “The theologian Paul Tillich once observed that among
    scientists only physicists seem capable of using the word
    ‘God’ without embarrassment.”

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  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.

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  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.”

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  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.”

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  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.”

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  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.

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  49. Chapter 12: Down in Ellis county

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  50. Afterword: ‘The crisis of big science’

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