Category: Announcement

Physics of Paradox

This talk — scheduled for the Library of Congress & for Capclave next week — is now up.

It was a lot of fun to put together:  I discuss time in relativity & quantum mechanics, kinds of time, some possible time machines, the three kinds of paradox (grandfather, bootstrap, & freewill), the Hawking & Novikov consistency conditions for avoiding paradox, some ways to implement those conditions, paradox noise, what the world might look like if paradox avoiding time travel were possible, and of course why this is likely.

I’ve got the talk on line as Keynote (for Mac users), PowerPoint (for PC users), PDF in slides-only and also annotated forms.

I’m doing a practice run on the talk in two days at the Radnor Memorial Library in the Winsor room from 6pm to 8pm (when we have to be out).  I start the actual talk about 6:30pm.  This is a dry run (well more of a wet run really) for the talks next week.

If you are not too far from Wayne, PA & have an interest in time & paradox (but then if not why are you reading these words?) please feel free to come!

Time and quantum mechanics at the Chestnut Hill Book Festival

Spoke at noon yesterday (July 10th, 2010) at the Chestnut Hill Book Festival; in spite of heavy rain a nice crowd.
This was my Balticon Time & Quantum Mechanics talk, adjusted for a general (rather than a science fictional) audience.  I covered over a hundred years of physics in less than an hour — a lot — but the audience survived & even seemed to prosper, asking some good questions!
I’ve uploaded the power point and keynote versions of the talk so you can see the animations of the double slit experiment, if you have power point and/or keynote.  You may have to tell your browser how to handle .ppt and/or .key files, for all parts to work with maximum smoothness. I’ve also uploaded the pdf and html versions.
The references — several asked after them — are on slide 36.  Enjoy!
I’d like to thank Oz Fontecchio for organizing this, Ferne Welch for moral & practical support, Bob Rossberg (sp?) for critical help on the AV, & the Chestnut Hill Book Festival for providing the venue!

Put your minds in full upright position

I’ve been asked to do a talk on Time & Quantum Mechanics talk at the Library of Congress, as part of their What If series. This is Thursday, October 21st, 2010. Presumably I’ll do something involving both Time and Quantum Mechanics. But what?

At least I have the opening sentence ready:

Prepare for take off. Fold your assumptions away. Put your minds in full upright position.

Time & quantum mechanics at Chestnut Hill Book Fair

I’ll be speaking on Time & quantum mechanics at the Chestnut Hill Book Fair, Philadelphia, July 10th at noon.

This will be basically a reprise of my talk at Balticon except that as the audience is a general one, rather than a science fiction crowd, I’ll focus more on the basics, why time is a problem, why quantum mechanics is a problem, and why the two together are really a problem.

Time & quantum mechanics talk done

Did the talk Saturday evening as planned.  Very sophisticated audience: almost everyone there had heard of the double slit experiment!

The talk went over well:  audience enthusiastic; lots of good questions.  Post talk I was asked if I would do similar presentations for Capclave & for the Library of Congress.  Leaning in favor.

I think that to really get across quantum mechanics, especially in a short time, nothing beats animations; the animations of the single & double slit were probably the most effective bits.  In future talks I shall be more animated.

The animations were courtesy of Bernd Thaller’s Advanced Visual Quantum Mechanics. He provides a useful kit of Mathematica functions for building such; looks a good starting point.

I’ve uploaded the power point & keynote versions of the talk so you can see the animations of the double slit experiment, if you have power point and/or keynote.  You may have to tell your browser how to handle .ppt and/or .key files, for all parts to work with maximum smoothness.

The pdf & html versions are still present, of course.  These are the talk as delivered, slightly different from the version previously posted (I added several more slides on the quantum eraser).

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.

I can no other answer make, but, thanks, and thanks.

Lately it appears to me what a long, strange trip it’s been.
— Robert Hunter of the Grateful Dead
We are all travellers in the wilderness of the world, and the best we can find in our travels is an honest friend.
— Robert Louis Stephenson
I thank my long time friend Jonathan Smith for invaluable encouragement, guidance, and practical assistance.
I thank the anonymous reviewer who pointed out that I was using time used in multiple senses in an earlier work.
I thank Ferne Cohen Welch for extraordinary moral and practical support.
I thank Linda Marie Kalb and Diane Dugan for their long and ongoing moral and practical support.
I thank my brothers Graham and Gaylord Ashmead and my brother-in-law Steve Robinson for continued encouragement.
I thank Oz Fontecchio, Bruce Bloom, Shelley Handin, and Lee and Diane Weinstein for listening to a perhaps baroque take on free will and determinism. I thank Arthur Tansky for many helpful conversations and some proofreading. I thank Chris Kalb for suggesting the title.
I thank John Cramer, Robert Forward, and Catherine Asaro for helpful conversations (and for writing some fine SF novels). I thank Connie Willis for several entertaining conversations about wormhole physics, closed causal loops and the like (and also for writing several fine SF stories).
I thank Stewart Personick for many constructive discussions. I thank Matt Riesen for suggesting the use of Rydberg atoms. I thank Terry the Physicist for useful thoughts on tunneling and for generally hammering the ideas here. I thank Andy Love for some useful experimental suggestions, especially the frame mixing idea. I thank Dave Kratz for helpful conversations. I thank Paul Nahin for some useful email. I thank Jay Wile for some necessary sarcasm.
I thank John Myers and others at QUIST and DARPA for useful conversations.I thank the participants at the third Feynman festival for many good discussions, including Gary Bowson, Fred Herz, Y. S. Kim, Marilyn Noz, A. Vourdas, and others. I thank Howard Brandt for his suggestion of internal decoherence.
I thank the participants at The Clock and The Quantum Conference at the Perimeter Institute for many good discussions, including J. Barbour, L. Vaidman, R. Tumulka, S. Weinstein, J. Vaccaro, R. Penrose, H. Price, and L. Smolin.
I thank the participants at the Third International Conference on the Nature and Ontology of Spacetime for many good discussions, including V. Petkov, W. Unruh, J. Ferret, H. Brown, and O. Maroney.
I thank the participants at the fourth Feynman festival for many good discussions, including N. Gisin, J. Peřina, Y. S. Kim, L. Skála, A. Vourdas, A. Khrennikov, A Zeilinger, J. H. Samson, and H. Yadsan-Appleby.
I thank the librarians of Bryn Mawr College, Haverford College, and the University of Pennsylvania for their unflagging helpfulness. I thank Mark West and Ashleigh Thomas for help getting set up at the University of Pennsylvania.
I thank countless other friends and acquaintances, not otherwise acknowledged, for listening to and often contributing to the ideas here.
I acknowledge a considerable intellectual debt to Yakir Aharonov, Julian Barbour, Paul Nahin, Huw Price, L. S. Schulman, Victor J. Stenger, and Dieter Zeh.
I thank Balticon for having me speak on this.  And I thank Chris Heimark and the other members of my Macintosh Programming SIG for inviting a talk on quantum time.
Finally, I thank the six German students at the Cafe Destiny in Olomouc who over a round of excellent Czech beer helped push this to its final form.
And of course, none of the above are in any way responsible for any errors of commission or omission in this work.

Finally

Quantum Time now up on the physics archive.

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.

The Other Shoe Drops

I’ve just finished the cross checks on my dissertation “Quantum Time“.

The dissertation asks “what happens if measurements in the time dimension are fuzzy, just as we know they are in the space dimensions”?  Another way to put this is “what if particles are spread out in time, not just fixed in the present instant, but extending a bit into future and past”?

The question is motivated by relativity:  From relativity we know that time and space are interchangeable.  Even if a particle should happen to be flat in one frame, with no extension into past or future, in another frame it will have such an extension.  Therefore it is simplest to assume any particle is always extended a bit in time, just not so much that we notice it.

But to work out specific predictions from this is tricky:  how do you give an experimentalist something to chew on?

I was able to work out the rules for this by rewriting quantum mechanics in a way that doesn’t favor space over time.  But even with that part done, it was still tricky to apply these rules to specific experimental cases and be confident that I had used the rules consistently and correctly.

Eventually what I hit on is a set of principles (aside from being as careful as possible about the algebra!):  when the extension in time goes to zero, we should get exactly the standard result.  And every experiment should morph smoothly into its neighbors.  For instance, if we are working on a double slit experiment, and we separate the two gates far enough, the results should look like those for two separated single gates.

I’ve spent the last two months working on this & as of yesterday the analysis seems complete.

In general, of course, everything gets fuzzier in time.  If you send a particle through a “chopper”, a gate in time, then the pattern it leaves in time at the detector will be more spread out if time is fuzzy.

There were some surprises of course.  For instance, the classic double slit experiment normally produces an oscillating comb-like pattern at the detector.  If time is fuzzy, not only does each tooth of the comb get wider (we knew that was coming) but the teeth get more spread out.  And shorter.  So there are three different effects to look for.

All three effects are subtle, so it is possible that the effects of quantum time have already been seen, but racked up to experimental noise.

I’m letting the latest version of the paper cool off for a week, then giving it a quick double check & submitting it to the physics archive next weekend.

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