277 | Cumrun Vafa on the Universe According to String Theory

/ Physics / 9 Comments

String theory, the current leading candidate for a theory of quantum gravity as well as other particles and forces, doesn't connect directly to the world we see. It's possible that there is a large landscape of possible states of theory, with the hope that one of them represents our universe. The existence of a landscape implies the existence of a corresponding swampland -- universes that are not compatible with string theory. I talk with Cumrun Vafa, a respected physicist and originator of the swampland program, about how we might use constraints on what kinds of physics are compatible with string theory to make predictions about cosmology and other experimental regimes.

Here is a research paper on Swampland phenomenology. In the conversation we refer to a famous diagram representing different ten-dimensional string theories, as well as 11-dimensional M-theory, as different limits of an underlying fundamental theory. Here is the diagram:

cumrun vafa

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Cumrun Vafa received his Ph.D. in physics from Princeton University. He is currently Hollis Professor of Mathematicks and Natural Philosophy, and Chair of the Physics Department, at Harvard University. He has done fundamental work on the dynamics of superstrings, the entropy of black holes, F-theory, and other topics. Among his awards are the Breakthrough Prize in Fundamental Physics, the Dirac Medal, and the Dannie Heineman Prize for Mathematical Physics. He is a member of the American Academy of Arts and Sciences and the National Academy of Sciences. He is the author of the book Puzzles to Unravel the Universe.

9 thoughts on “277 | Cumrun Vafa on the Universe According to String Theory”

  1. Marcel

    I would put forward the conjecture that this episode has the largest guest-to-host ratio for speaking time. (This is an empirical question that can be answered by analyzing the transcripts of all the episodes, if anyone wants to write a script to do that.)

  4. Jake Browning

    Can anyone give me some feedback on this idea: The act of measuring 2 entangled particles at the same time rules out ANY hidden variable theory (local or non-local).

    For example, if there is some hidden variable X shared by both particles, once particle A is measured, it hasn’t heard from particle B yet, so it chooses some spin based on X and the fact that B hasn’t been measured. Similarly for B.

    So measuring the particles at the same time is equivalent to a local hidden variable theory which by Bell cannot exist (assuming measurement independence).

  5. Howard Gary

    I found the short video posted below: ‘String Theory Explained- What is the True Nature of Reality?’ (1 May 2018) helpful in trying to grasp at some intuitive level what string theory is all about and how it might offer a guide to understanding the true nature of reality, or at least as close as humanly possible to understanding the true nature of reality.

    https://www.youtube.com/watch?v=Da-2h2B4faU

  6. Howard Gary

    Jake, if I understand your argument correctly, you’re suggesting that measuring the 2 entangled particles at the same time is equivalent to a local hidden variable, which by Bell cannot exist. But unless the 2 particles were at the same location how could you ensure they were being measured at the same time? Whether 2 events at separate locations are simultaneous depends on the motion of the observe relative to the location of the events.

  7. Nathan Argaman

    Jake, you can’t take the argument too far, because standard quantum mechanics works, and you can think of it as a non-local hidden-variable theory, where the wavefunction psi is the non-local variable X. Of course, this type of theory can’t be ruled out.
    In your case, a joint measurement, both X and the two measurement settings joinly determine the probability distribution of both outcomes. Bohmian mechanics also works in this manner. The variables in the model “communicate” at a distance, but this remains hidden in the sense that the outcomes never carry any nonlocal signals.

    (Howard, it is actually standard to consider simultaneous distant measurements; of course that requires choosing a preferred reference frame, but that’s legitimate and is not a problem.)

    By the way, note that Bell’s theorem also assumes causality. Bell did state that “locality” is an abbreviation of “local causality,” but never studied the possibility of models violating causality internally (with the outcomes never carrying any retrocausal signals, of course). Such models have distinct advantages; see, e.g., the book Time’s Arrow and Archimedes’ Point / Huw Price.

  8. Howard Gary

    Nathan, do you agree with this flowery description of the so-called “Block Universe”, where the past remains fixed, like a fossilized record etched into the block, it’s as real as the present, but the future, like an uncarved section of the block, awaits its turn to emerge. – A realm where past, present, and the future coexist in a harmonious dance across the cosmic stage.

    Ref: Microsoft CoPilot

  9. Nathan Argaman

    Howard, your question is sort-of philosophical, and I don’t know the answer. I do use a variant of the block universe approach in looking at models where you can’t evaluate the internal variables (aka hidden variables) for time t_1 until you specify all the necessary inputs, including inputs at time t_2 later than t_1. But to make things easier, I assume that all of this occurs in the past. And I think the model is only an approximation of what actually happens in nature. We’re nowhere near achieving a full understanding of what is “actually going on” in quantum physics.

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