Yesterday’s colloquium was entitled “String Theory and Cosmology”, usually a sign that I can safely spend that hour in the lab trying to get my qubits to work. If I had known that the speaker would be giving the talk from handwritten transparencies I definitely would have stayed away, figuring that the talk was so overly technical that Powerpoint couldn’t handle it, and the speaker would be running through some incomprensible morass of equations and text that had been lifted from the Necronomicon and then translated a couple times by Babelfish.
But fortunately I did go to the colloquium, which turned out to be pretty accessible. The speaker, Shamit Kachru, was very good and able to give sort of a hand-wavy outline of what string theorists are up to. String theory is a very difficult and jargon-heavy subject, and there was no way for him to get very technical without losing 95% of the audience (myself included), so I can’t say that I gained much understanding of what string theories are actually about. However, I did at least grasp where the boundaries of knowledge are in this field, which I think can best be classified using the epistemological scheme invented by philosopher/poet Donald Rumsfeld:
- Known knowns: The Standard Model of particle physics, which describes the behavior of particles in certain regimes (i.e. the experimentally accessible ones) to very high accuracy. And general relativity, which describes gravity in observable regimes.
- Known unknowns: What gravity does at energies where it’s comparable to the other three forces (it’s normally much weaker). Also, various mathematical quirks and inconsistencies in the Standard Model.
- Unknown knowns: Various string theories generate universes that look sort of like this one. But it’s unknown whether any of them do describe the actual universe, because they only make interesting predictions at energies much higher than could possibly be tested. (I believe the number cited in the talk was 1017 GeV; the best accelerators run at 103 GeV.) The connection to cosmology in the talk was in trying to explain the origins of the universe using string theory; out of all of the potential verieties of theories, a few do make testable predictions on observable phenomena like the cosmic microwave background due to how they address the Big Bang. So if we’re lucky enough to live in one of these universes, we could confirm it with certain astrophysical experiments.
- Unknown unknowns: And then there’s the possibilty that string theory isn’t the right answer, but rather something no one’s thought of yet. As Douglas Adams noted, “There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.
There is another theory which states that this has already happened.” (String theory always reminds me of that quote.)
Now, for serious string theory blogging you should be reading Cosmic Variance, since I don’t really know much at all about the field beyond what can be communicated in an hour-long colloquium. However, I’m starting to understand why it’s interesting. (Also, it turns out that the guys shambling down the halls around here muttering about “braaaaanes” aren’t zombies but overworked string theorists. Oops.)
I just discovered that there are videos of the colloquia on the physics department website, here, so you can actually watch this talk if you’re interested. (It hasn’t been posted yet but probably will be within a week.) Another good one from this semester was “Cycles in Fossil Diversity” by Rich Muller, which was a study of what causes species to thrive or die out at apparently regular intervals in Earth’s history.