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.

1. 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.
2. 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.
3. 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.

I’ve put the slides for the talk up as a pdf & as html.

Dissertation complete

I’ve finished re-checking the dissertation:  629 equations, 188 references, 110 pages, 83 input files, 48 lists, 36 footnotes, 28 quotes, 17 figures, 6 chapters (counting the appendix), 5 requirements, 1 idea.  It should be up on the physics archive in a day or two.

Why quantum time?

Why quantum time?

A few years ago I was looking for an interpretation/formalism for quantum mechanics which would be manifestly symmetric between time and space. The first question I had was:

Is time already quantized?

Is time treated using the same quantum rules as space is? can quantum mechanics be written in a way which is manifestly covariant?
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The Block Universe

‘Clearly,’ the Time Traveller proceeded, ‘any real body must have extension in four directions: it must have Length, Breadth, Thickness, and–Duration. But through a natural infirmity of the flesh, which I will explain to you in a moment, we incline to overlook this fact. There are really four dimensions, three which we call the three planes of Space, and a fourth, Time. There is, however, a tendency to draw an unreal distinction between the former three dimensions and the latter, because it happens that our consciousness moves intermittently in one direction along the latter from the beginning to the end of our lives.’

– H. G. Well’s The Time Machine

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This is still the best single explanation of the idea of the  ”block universe”, though Well’s Time Traveller does not use that term. As Julian Barbour puts it his The End of Time: “The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the life line of my body, does a section of this world come to life as a fleeting image in space which continuously changes in time.”
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References on Time & Time Travel

Updated 12/2/2009

Recommended popular books on time. All of the authors know their physics; none are mortal enemies of the English language.  Enjoy:
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