Welcome back, Witten!


My apologies for the lack of honorifics, but I wanted to pay homage to that truly great TV sitcom, “Welcome Back, Cotter.” So long as you’re here, I have a question prompted by recent discussions with Lee Smolin. He classified string theory as a “background dependent” theory. I think I know what that is, and I think he’s prolly right about that. But he also said that string theory is ‘like general relativity in more dimensions’. That seems not quite right to me. For one thing it seems like a good thing to let the number of dimensions flap in the breeze. [Although, why we’re stuck on the integers is a bit cryptic what with fractals and all.] Anomaly cancellation seems like rather heavy machinery for fixing the dimension, but it does seem a lot better than a guess*. Writing the metric of the embedding space in the action does assume diffeomorphism invariance? But I don’t see y’all assuming the Equivalence Principle. If the Equivalence Principle becomes a theorem, then I’d have to say you’d really done something beyond GR. So what’s the story?

Maybe it’s a good thing you were on a plane when I put this up. It’s been too long since then young Dr. Sean [Now Professor Sean and inheritor of Feyman’s desk] taught me how to calculate spin connections. Google(assumptions of GR) -> inconclusive {so far}, but I’m thinking EP is assumed in ST. Not that that’s a bad thing. The big problem** with GR is that you can’t quantize it. If you take the assumptions of GR except allow dimension to float then fix it due to quantum mechanics, it IS conservative relative to other approaches, but often in science, that’s a good thing.*** It’s hard to make these judgements when the experimental air is thin. I feel dizzy.
*You could callthe choice of 3+1 dimensions the result of observation, but there are psychological experiments that cause me to question whether it is obvious.

**The assumption that the fundamental object of the theory has finite extension seems less important with the re-emegence of point particles in the form of D0 branes.
***I won’t bring Martha Stewart into this having already slightly abused Josh Lapan with my fractional wit.

Ed Witten, who some people regard as the smartest person in the world*, is one of the leading physicsts working in string theory. I ran into him yesterday in the basment of the Science Center. There is evidence that suggests that in one regard at least he is mortal. On the other hand, today when I gave him the score of The Game, I had to explain it.

Middle-aged Professor Sean, has wriiten the definitive review of “The Trouble with Physics.” I’m not the first to feel that the overall concerns of the book are wothy of attention, but the supporting examples are not the best. Loop Quantum Gravity is probably an especially poor choice. Also, Sean Pf. Diddy Cosmos** points out [though I suspect it was on the grapevine] that just because String Theory is “background dependent now” doesn’t mean it always will be. His recommendation? Worh a read with a critical eye.

*This assumes that “smarter than” is a transitive relation.

**Okay. It needs work.

The Trouble with Physics?
An Honor to Serve with You

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