Physics in 100 years

Here I indulge in wide-ranging speculations on the shape of physics, and technology closely related to physics, over the next one hundred years. Themes include the many faces of unification, the re-imagining of quantum theory, and new forms of engineering on small, intermediate, and large scales. When Leon Cooper, on behalf of Brown University, asked me to contribute to their 250th anniversary by giving a talk about the next 250 years of physics, I of course accepted immediately. Then I thought about it. I soon realized that I’d taken on a task that is way beyond me, or (I suspect) anyone else. The most impressive attempt of this kind I know of is the set of Queries that Isaac Newton attached to his Opticks in 1718. The Queries contain many important ideas and suggestions, but by 1818, after 100 years, they were showing their age1. By that time Lavoisier’s scientific chemistry was already well along, the wave theory of light was being established, Oersted and Faraday were just over the horizon, and most of the Queries had been answered, or transcended. So as a first step I renormalized 250 → 100. On further reflection, I also came to realize that the point of this talk should not be to predict future history accurately, as in a test or a business plan. I interpret my assignment, rather, as an opportunity to exercise disciplined imagination. For the exercise to have intellectual value, beyond mere entertainment or hype, it should be grounded in judgements of three kinds • What are the weak points in our current understanding and practices? • What are the growth areas in technique and capability? Unlike Brown University, which is self-renewing. 1 ar X iv :1 50 3. 07 73 5v 1 [ ph ys ic s. po pph ] 2 6 M ar 2 01 5 • Where are the sweet spots, where those two meet? Here, in that spirit, I’ve assembled a baker’s dozen of Query-like miniinquiries. They are loosely structured: Some overlap with others, and some range over several sub-topics. Here and there, by way of summary, I will venture a few provocative predictions. Those are italicized below. 1 A Diversity of Unifications Our first seven items concern different sorts of unification. On one occasion, following a public lecture about the possibility of a unified theory of the four forces, an audience member posed me this disarming question: Why do physicists care so much about unification? Isn’t it sufficient just to understand things? It’s a profound question, actually. It is not at all self-evident, and may not be true, that going after unification is an efficient way to advance knowledge, let alone to address social needs or to promote human welfare. I can offer two answers: • Historical: It is apparent, in retrospect, that many of the greatest advances from the past have been unifications of disparate subjects. In most of these cases, when the unification took place each side of the separate subjects was already well developed on its own, and the “value added” by unification was not immediately obvious. But in every case the unification itself proved extremely fertile, in unanticipated directions: – Unification of algebra and geometry (Descartes)2 – Unification of celestial and terrestrial physics (Galileo, Newton) – Unification of mechanics and optics (Hamilton) – Unification of electricity, magnetism, and optics (Maxwell) – Unification of space and time (Einstein, Minkowski) – Unification of wave and particle (Einstein, de Broglie) I’ve attached names to the developments not to assign credit, but as shorthand to recall some central contributions.