The Physics of Paradox — Followup

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

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.

A Very Short Introduction to Nothing

Was there a creation or was there always something? Could there even be nothing if there were no one to know there was nothing? The more I tried to understand these enigmas, the more I felt that I was at the edge of either true enlightenment or madness. — Frank Close

I’ve just finished the concise & entertaining “Nothing: A Very Short Introduction” by Frank Close. It’s part of the “Very Short Introduction” series from Oxford University Press. They are generally reliable. The obvious trap is for the author to talk more about his own views/work than his subject in general, but of the 20 I’ve read, only two have made this mistake (Hume & Ancient Warfare, if you must know).

Frank Close, who is a big name in nothing, in the physics of nothing that is, does a nice, very short job of introducing it to us, starting with the Rigveda’s Creation Hymn:

There was neither non-existence nor existence then.

There was neither the realm of space nor the sky which is beyond.

What stirred? Where?

up through the Higgs vacuum, the idea that the vacuum is not empty but is pervaded by the Higgs fields, which is responsible for giving particles mass. CERN was built partly to check this out & the cernistas are now hot on the trail of the Higgs.

I’m suspicious of the Higgs particle myself; it has a slightly kludgy feel to it, at least to my taste. I think particles have had a good run for their money over the last century & and now it is time for emergent phenomena to have a go. For instance, only a few percent of the mass of the protons & neutrons comes from the masses of their constituent quarks; most of their mass is really from the energy (via the familiar mass = E/c-squared) of the quantum dance of those quarks. If most mass comes from the energy stored in quantum interactions, could all mass be the result of such? Certainly an interesting question & and would leave us with one less variable to explain, with a slightly less massive problem.

In fact, I’d go further myself: space and time are difficult to understand, what if they are merely averages over the quantum wave function of the rest of the universe? and all of our universe is merely the friction of one part of the quantum wave function of the universe against another part. No mass, no space, no time, no vacuum, nothing but interactions.

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.

Next step for time & quantum mechanics

“When you come to a fork in the road, take it!” — Yogi Berra, Quantum Philosopher

I’ve been wondering what to do with quantum time.  I’ve gotten a certain amount of feedback on the original paper, ranging from “hard but interesting” to “interesting but hard”.
There are really two directions I would like to take this project at this point:

1. Do the calculations in a more transparent way, to leave us, hopefully, just with “interesting”.
2. Extend the ideas to the multi-particle case, which is needed for the analysis of all but the most trivial cases. For instance, we need this even to compute bound state wave functions.

In the spirit of quantum mechanics, it seems best to do both. I look at each in turn.

I noticed when I talked in Baltimore last month that the animations were really the most transparent part of the talk.  But the only way to develop them is to use numerical methods.  I’ve done a bit of numerical work in the past, mostly to calculate charged particle orbits around a black hole (when I was a grad student at Princeton).  From this I learned two things:

1. Numerical calculations are tricky.  I learned this the hard way. I had thought — ah youth — that the smaller you make the step size, the more accurate the results. But I found this was true only up to a point; below a certain step size the calculations would produce obvious nonsense: at small enough step sizes, round-off errors dominated the results, sending the particles either into the black hole or out into space. [No real particles were harmed in the course of this experiment.] Ultimately, I had to completely rewrite the equations in a non-linear but stabler way to get something meaningful.
2. If you don’t have a reliable source of physical intuition, tricky can quickly escalate into nonsense.  With anything involving time this is particularly a problem, largely because our usual intuition about time is so compelling that it is hard to move past it.  And if we do move past it, where do we get a “reliable source of physical intuition”?

Consequently I’ve been a bit chary of doing numerical work.  But while researching my Baltimore talk, I came across a work, Advanced Visual Quantum Mechanics, by Bernd Thaller, where the problem was solved, at least for low dimensional cases. Bernd Thaller worked primarily with Mathematica, a higher level language, but used a C program written by Manfred Liebmann for the low level numerical work.  This was a dissertation paper by Liebmann. A quick scan of the table of contents was enough to confirm my intuition that the problem is non-trivial.

I’ve spent a few hours with Liebmann’s dissertation.  It is written in German but apparently my high school German, Google translate, and a fair knowledge of the subject area [plus checking the references as I go] is enough to let me stumble thru it. The basic approach is essentially path integrals done a step at a time, in such wise as to minimize the numerical error at each step. This I can manage. Approximate proudly!

The second problem is how to extend quantum time to the multi-particle case.  The main problem here is how to generalize the single particle results to the multi.  After some mulling, and in the spirit of “approximate proudly” I’ve decided that using the usual Feynman rules but with the standard propagator replaced by the slightly fuzzier quantum time propagator is a reasonable first step.  When we are only looking for first order corrections, we don’t need an elaborate theoretical framework.

What to use for the “slightly fuzzier” is a bit of a question. Our single particle action is:

The most obvious generalization to the field theoretical case looks like:

This won’t do.  It is dimensionally wrong.  We need to multiply τ by something with dimensions of mass. But we can’t use the mass of any specific particle, as that would be to prefer one over another. We will instead insert a factor κ, defined as something with dimensions of mass/energy:

This will give us [insert hand-waving here] a propagator looking like:

If we are looking at a Feynman diagram we will wind up convoluting over the laboratory time:

Which makes most of our integrals look like products of the Laplace transform of the propagator:

Compare to the usual Feynman propagator:

Modulo an overall dimensional factor of κ [the sort of thing that comes out in the wash], they look much alike — in the limit of small κ.  As small κ corresponds loosely to large τ and as we expect to get standard quantum theory back in the long time limit of quantum time, this is fine.

The next question is where did κ come from?  We don’t need to sort that out entirely up front, but we do need to know we have at least one viable answer.

If we want to take an aggressively Machian view of quantum mechanics, then there is nothing to the universe but its wave function:  space and time are mere ensemble averages over the wave function of the rest of the universe.  κ then can be a measure of how much energy stored in the local vacuum fluctuations, small but not zero:

So, that is the plan for the multi-particle case:  use the κ-ified propagator with the Feynman rules, require we get standard quantum theory back in the large τ/small κ limit, see the testable inferences in re quantum time/multiple particle case as the first order corrections due to non-zero κ and/or small dispersions along the time dimension.

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