## On the nature of time & quantum mechanics

… there are known knowns: there are things we know we know. We also know there are known unknowns: that is to say we know there are some things we do not know. But there are also unknown unknowns — the ones we don’t know we don’t know.” — Donald Rumsfeld.

I’m doing a popular talk on “The Nature of Time & Quantum Mechanics” tomorrow at Balticon. I’m deliberately not including anything from my paper “Quantum Time“.

Instead I look at a couple of areas at the intersection of time & quantum mechanics. There are too many such areas for one talk. In accordance with my father’s rule of three (you can only get three points across in any one talk) I selected three of them, one from each of Donald Rumsfeld’s categories.

- The delayed choice quantum eraser. I find this amazing: if you try to see which slit the particle went thru in the double slit experiment, it becomes a single slit experiment. But if you do something that should tell you which slit it went thru — and then deliberately erase your knowledge — the single slit experiment turns back to a double slit experiment & we recover the interference pattern. And this is the case even if we do the probe/erase after the particle has gone thru the two slits! Weird but well understood & tested.
- The time symmetric formalism of Aharonov, Bergmann, & Lebowitz. They formulated quantum mechanics in a time symmetric way, demonstrating that it is not essentially asymmetric in time. It’s just usually drawn that way, as Jessica Qubit might put it. There has been some speculation that their formalism could imply retro causation. I doubt it myself but this would be a known unknown.
- The competition between the inflationary universe model & the ekpyrotic (cyclic) model of the universe. The inflationary model now has a bit of competition in the ekpyrotic model of Steinhardt & Turok (see their book Endless Universe for a popular treatment). Colliding branes, bouncing universes, & decaying dark energy oh my! We have no idea what about the start, expansion, & finish of the universe we don’t know. We don’t even know if the terms start & finish make sense, universe-wise.