What was your biggest recent surprise?

I was surprised that it is possible to apply simple(?) signal processing techniques to extract subtle signals from a video, e.g. somebody's heartbeat.

Surprise levels:

1) I never thought of that (that there could be useful hidden signals in standard video). Their paper references a few other attempts at this.
2) If I had thought of it, or someone had mentioned the idea, I would have guessed that those signals are not strong enough to be extracted by any method.
3) And, even if there were a signal, I would have thought it would take very powerful techniques and many assumptions (like manually annotating where you expect to see the heartbeat, etc.) to make it work. This is required less than I'd expect. From the paper:

we auto- matically select, and then amplify, a band of temporal frequencies that includes plausible human heart rates.

A very salient moment of surprise was when I realized that my mental model of a simple three-quark proton was deeply (or simply) wrong:

You may have heard that a proton is made from three quarks. Indeed here are several pages that say so. This is a lie — a white lie, but a big one. In fact there are zillions of gluons, antiquarks, and quarks in a proton. The standard shorthand, “the proton is made from two up quarks and one down quark”, is really a statement that the proton has two more up quarks than up antiquarks, and one more down quark than down antiquarks. To make the glib shorthand correct you need to add the phrase “plus zillions of gluons and zillions of quark-antiquark pairs.” Without this phrase, one’s view of the proton is so simplistic that it is not possible to understand the LHC at all.

http://profmattstrassler.com/articles-and-posts/largehadroncolliderfaq/whats-a-proton-anyway/

What still surprises me, whenever I think of it, is how we live in a such a big world, even on the smallest scales we are able to probe. And also that things like nuclei happen to be stable over long enough timescales for things like chemistry and life to occur.

Not really related to any explicit field of study, but...

Most recently, I was surprised by the extent to which the Japanese still use faxes.

Before that, I was really surprised by the whole Planetary Resources thing. My model of the world claimed that aside for some relatively minor stuff like space tourism and such, plausible pushes to actually do something new and non-trivial in space simply do not happen, and that there would be essentially no real progress in any kind of space exploration before the Singularity. At best, there would be a new private space station in orbit, or NASA would announce a manned Mars mission that would get quietly killed by budget cuts a few years later. Having a bunch of billionaires announce a real effort to actually mine asteroids was something that made it slightly easier for me to alieve in the Singularity happening some day. Before, both asteroid mining and the Singularity used to belong to the mental category of "things that I intellectually acknowledge as possible, but which would be such huge changes to the current paradigm that on a gut level, I don't really grasp either of them happening".

I learned that I'm not crazy for having been confused by the double-speak I was taught in college about "observation" in Quantum Mechanics and that maybe there's a community where I can get straight answers to things.

According to RolfAndreassen:

'Observation' is a shorthand (for historical reasons) for 'interaction with a different system', for example a detector or a human; but a rock will do as well. I would actually suggest you read the Quantum Mechanics Sequence on this point, Eliezer's explanation is quite good.

According to Douglas_Knight,

I advocate in place of "many worlds interpretation" the phrase "no collapse interpretation."

Thanks Less Wrong.

That there is reason to believe that it is "relatively easy" (say if we survive x-risk and get a good singleton within a million years) to colonize billions of galaxies. That makes the expected (ignoring possibility of discovering new useful physics, creating universes etc) hedonic utility of x-risk reduction up to some 9-orders of magnitude greater than I had previously thought.

I think my biggest recent surprise was the notion that AI didn't have to be sentient to be worth taking seriously. Seemed like such a simple idea to overlook but it totally changed my worldview.

Not very recent, but...

I was surprised way back when I learned that we had already located some neurons which seem to encode the expected utility of possible actions. ('Utility' here isn't meant in the philosophical sense but in the neuroeconomic sense.)

I also remember being amused 1+ years ago when I did some more studying in AI and decision theory and learned that all currently described AI agents are Cartesian dualists. (This is old news 'round these parts, I know.)

Even though the composition of two rotations is a rotation on a standard sphere, the same is not true for higher dimensional spheres. Possibly even weirder, on a sphere the composition of two periodic rotations is not necessarily periodic.

Just figured something new out, based on my original post here.

The energy/time version of the uncertainty principle says that virtual particles of any given energy can spontaneously appear - but the bigger the energy, the shorter they last. This explains why the strength of electromagnetism falls off at a distance - virtual photons with high energies last for short times and thus travel short distances, while virtual photons with low energies can last for longer times and travel longer distances. All straight from the book.

But I just recalled that other forces, the strong and weak, are described as having a range limitation. I've always read about that range-limit existing - but since no reason was given for it, and I couldn't figure it out, I just shrugged my shoulders with an assumption of 'quantum weirdness'. But now I have an idea /why/ that range limit exists: with a minimum amount of energy in any given virtual particle for those forces, in the form of those particles' rest mass, the uncertainty principle thus also imposes a maximum lifespan, and thus a maximum range.

It's been such a long time since I've had a chance to figure out something about physics that I wasn't simply directly told, it's a surprisingly pleasant experience. :)

(Now, I'm wondering if this particular idea implies that since gravity's range is infinite, that implies that if gravity is transmitted by force-particles rather than space-curvature (assuming that that's a distinction with meaning), then the virtual gravity force-carrying particles have to be able to have arbitrarily small energies, and thus no significant rest mass...)

I just had a big "update".

EDIT: I'm a little less sure now. See the end.

I found something to teach programming on an immediate level to non-programmers without knowing they are programming, without any cruft. I always wished this was possible, but now I think we're really close.

If you want to get programming, and are a visual thinker, but never could get over some sort of inhibition, I think you should try this. You won't even know you're programming. It may not be "quite" programming, but it's closer than anything else I've seen at this level of simplicity. And anyway it's fun and pretty.

The important thing about this "programming" environment is that it is completely concrete. There are no formal "abstractions," and yet it's all about concrete representation of the idea formerly known as abstractions.

Enough words. Take a look: http://recursivedrawing.com/

[I was excited because to me this seems awfully close to the untyped lambda-calculus, made magically concrete. The "normal forms" are the "fixed points" are the fractals. It's all too much and requires more thought. It only makes pictures, though, for now. However, I can't see anything in it like "application" so... the issue of how close it is seems actually quite subtle. Somehow application's being bypassed in a static way. Curious. I'm sure there's a better way to see it I just haven't gotten yet.]

PS: Blue! Blue! Blue! (**)

** This is a joke that will only make sense if you've read The Name of the Wind: Rothfuss. If you prefer to spoil yourself, here, but buy the book afterward if you like it.

cross-posted here [I'm not sure about the etiquette, but I think this idea deserves not to be lost in an old thread.]