Recreating Our Galaxy in a Supercomputer • Hacker News Board

Re: Recreating Our Galaxy in a Supercomputer

#31

Post by dr_zoidberg » Fri, Sep 09, 2016, 3:00 PM UTC

privong wrote: post #25

adekok wrote: post #14

Earlier quoted context omitted.

> Can somebody who knows about this tell me how much of the novelty in this is CS and how much physics? It's not so much a novelty as it is pushing the envelope. The main problem with these kind of simulations is that they're O(N^2). You can't get away with modelling each star, you have to model the interaction of each star with every other star. And then add in interstellar dust, which has non-neglible mass. And the…

> The main problem with these kind of simulations is that they're O(N^2). You can't get away with modelling each star, you have to model the interaction of each star with every other star. Summary of what's below: The type of simulations described in the article do not scale like O(N^2) but more like O(N log N). A small correction here.. The gravitational force calculation for N-body simulations are only O(N^2) if th…

Some simulations also use Smoothed Particle Hydrodinamics (see here[0] for a simplified example), like the Earth-Theia impact Moon simulations[1]. However I'm not in the field and can't really tell how much this kind of code has been used lately, but you did remind me of it when you talk about this simulations being monte carlo sampling of stellar mass distribution, because SPH has some concepts like that "inside" the particles.

Also, for everyone to enjoy simulations results, check the simulations by Canup from the giant impact hypothesis[2]. Another thing to consider, the supercomputer used for this specific simulation had about the same computer power as a high end GPU from today.

[0] https://benedikt-bitterli.me/bbw.html

[1] http://lasp.colorado.edu/~espoclass/ASTR_5835_2013_Files/201...

[2] http://www.boulder.swri.edu/~robin/moonimpact/

Re: Recreating Our Galaxy in a Supercomputer

#32

Post by web007 » Fri, Sep 09, 2016, 3:02 PM UTC

sillysaurus3 wrote: post #12

Le_SDT wrote: post #9

Earlier quoted context omitted.

I'm curious. What do you mean by "Human vision is completely overlooked"? Are you talking about color perception? 3d rendering? Thanks :)

There's pretty much no way to avoid writing a large comment in response to this. So, with apologies... It was my focus for a long time to achieve perfectly photorealistic rendering. I come from a gamedev background, and I've been fascinated since around age ten about how to get a computer to paint pictures. By 17 I was writing game engines mostly equivalent to quake, and used this portfolio to get into the industry.…

2 points:

1. "Take a picture and make it look right" is exactly what people have done for things like POVRay since at least the 90s. I can't find it now, but at one point someone set up a glass ball on a checkerboard and used a point light and a camera to confirm the diffraction and distortion models were correct because someone claimed they didn't look right or were doing the wrong thing. The math that's there for rays, shapes, diffraction, diffusion, caustics, etc. is accurate, and necessary but not sufficient. Which brings me to

2. CG realism has generally hit the Uncanny Valley by now. It's so close to real that we think "that's pretty good", but it's far enough away that we still know "something's wrong". It's the difference between a dummy, a corpse and a live person.

A couple of examples I remember from the past decade are laser + milk and better skin rendering on one of the NVIDIA demos a while back. There wasn't (isn't?) a good model to simulate the diffraction and subsequent diffusion of a laser shining into a glass of milk. Actual lasers with actual milk don't do the things we expect of modeled lasers in simulated milk. Some component is missing, but all the existing math is right for lots of other cases. The NVIDIA skin thing was adding 3 or 4 layers to an existing model to simulate subsurface scattering and reflection that happens in skin, vs old models that treat skin as paint. The old stuff was right, just not enough.

All of that aside, there are decent photorealistic rendering options for some materials today, but at the cost of CPU hours of render time. If you can do better then please do, even if it's just for one material or one physics action.

Re: Recreating Our Galaxy in a Supercomputer

#33

Post by nsxwolf » Fri, Sep 09, 2016, 3:06 PM UTC

I thought the Milky Way was a barred spiral.

Re: Recreating Our Galaxy in a Supercomputer

#34

Post by nonbel » Fri, Sep 09, 2016, 3:13 PM UTC

dekhn wrote: post #23

JabavuAdams wrote: post #15

Earlier quoted context omitted.

Molecules are tricky. They're obviously machines, but you're dealing with Quantum Mechanics if you go to first principles.

Which is why, for nearly all simulations, you use Born-Oppenheimer approximation and molecular dynamics using force fields derived from QM, but without the full explicit QM treatment. This is fine because with only rare examples (as far as modern science knows), QM treatment is not necessary for most molecular machines.

I didn't realize that. It sounds like the same situation with astronomical simulations. General Relativity is too complicated so everyone uses Newtonian mechanics plus a fudge factor.

I think it is odd that the two most cherished scientific theories are not actually used in practice to compare with data.

Also, this got me wondering how they did this simulation, which lead me to this site: http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html

There is a lot of code.. but after a quick look at it I see Netwonian mechanics. From analytic_gravity.h:

  void GravAccel_StaticIsothermalSphere()
  {
      double ISO_M200=95.21;
      double ISO_R200=160.0;
      double ISO_Eps=0.1;
      double ISO_FRACTION=0.9;
      double r, m, dx, dy, dz; int i;
      
      for(i = FirstActiveParticle; i >= 0; i = NextActiveParticle[i])
      {
          dx = P[i].Pos[0]; dy = P[i].Pos[1]; dz = P[i].Pos[2];
          r = sqrt(dx * dx + dy * dy + dz * dz);
          
          if(r > ISO_R200)
              m = ISO_M200;
          else
              m = ISO_M200 * r / ISO_R200;
          
          m *= ISO_FRACTION;
          if(r > 0)
          {
              P[i].GravAccel[0] += -All.G * m * dx / r / (r * r + ISO_Eps * ISO_Eps);
              P[i].GravAccel[1] += -All.G * m * dy / r / (r * r + ISO_Eps * ISO_Eps);
              P[i].GravAccel[2] += -All.G * m * dz / r / (r * r + ISO_Eps * ISO_Eps);
          }
      }
  }

Re: Recreating Our Galaxy in a Supercomputer

#35

Post by danielvf » Fri, Sep 09, 2016, 3:19 PM UTC

sillysaurus3 wrote: post #24

formula1 wrote: post #21

Earlier quoted context omitted.

I appreciate you probably put a lot of effort out but your post largely sums up to "They are wrong, I've done extra curricular research to prove it, I have no alternative" While noble, I cant help but wanting to understand what alternatives you were leading yourself done. As an example, light can arguably simplified to intersecting cylinders and spheres that bounce off surfaces to create new 3d shapes. Each shape als…

We can come up with hundreds of simplified lighting models. Have you tried the one you mentioned? Does it correspond to photographs, video? That's the hard work, and it's why I listed none. Here's something specific: What did you mean by "multiply"? You cannot "multiply" colors. Not unless you concede that your model has nothing whatsoever to do with physical reality. And at that point, why not use a photo of nature…

I've been diving to light, color, and color perception the last couple weeks as part of a fun work project. I've got a diffraction grating spectrometer on my desk that measures light in half nanometer wavelengths. From the papers that I've read, and the stuff I've messed with so far, it seems that human perception of colour - on average - is well studied, and has been since the 1930's. We know how light gets into the eye, and how much each wavelength of light will excite the average eye's individual color specific sensors.

Once the info leaves the receptors, now that's a another story...

Re: Recreating Our Galaxy in a Supercomputer

#36

Post by scott_s » Fri, Sep 09, 2016, 3:30 PM UTC

sillysaurus3 wrote: post #24

formula1 wrote: post #21

Earlier quoted context omitted.

I appreciate you probably put a lot of effort out but your post largely sums up to "They are wrong, I've done extra curricular research to prove it, I have no alternative" While noble, I cant help but wanting to understand what alternatives you were leading yourself done. As an example, light can arguably simplified to intersecting cylinders and spheres that bounce off surfaces to create new 3d shapes. Each shape als…

We can come up with hundreds of simplified lighting models. Have you tried the one you mentioned? Does it correspond to photographs, video? That's the hard work, and it's why I listed none. Here's something specific: What did you mean by "multiply"? You cannot "multiply" colors. Not unless you concede that your model has nothing whatsoever to do with physical reality. And at that point, why not use a photo of nature…

Perhaps a way of stating it is that you want to create computer programs which do the same image processing that our brains do. That, I think, is what you're getting at: we don't see "reality", we see images processed by our brain, from signals received by our eye, mixed with a bunch of learned and instinctual models already existing in our brain. Accurately modeling the signals entering our eye is not enough. We also need to perform - at least approximately - the some processing that our brain does.

Re: Recreating Our Galaxy in a Supercomputer

#37

Post by privong » Fri, Sep 09, 2016, 3:31 PM UTC

dr_zoidberg wrote: post #31

privong wrote: post #25

Earlier quoted context omitted.

> The main problem with these kind of simulations is that they're O(N^2). You can't get away with modelling each star, you have to model the interaction of each star with every other star. Summary of what's below: The type of simulations described in the article do not scale like O(N^2) but more like O(N log N). A small correction here.. The gravitational force calculation for N-body simulations are only O(N^2) if th…

Some simulations also use Smoothed Particle Hydrodinamics (see here[0] for a simplified example), like the Earth-Theia impact Moon simulations[1]. However I'm not in the field and can't really tell how much this kind of code has been used lately, but you did remind me of it when you talk about this simulations being monte carlo sampling of stellar mass distribution, because SPH has some concepts like that "inside" th…

SPH has been fairly common in galaxy simulations over the past 20 years or so. But there's some glaring issues with "classic" SPH formulations, to do with fluid instabilities. As a result, lots of people are transitioning to moving mesh codes. I put a few links in my other comment (https://news.ycombinator.com/item?id=12461336). I'm pretty sure the simulation in the press release is also using a moving mesh for the hydrodynamics.

Re: Recreating Our Galaxy in a Supercomputer

#38

Post by dekhn » Fri, Sep 09, 2016, 3:36 PM UTC

nonbel wrote: post #34

dekhn wrote: post #23

Earlier quoted context omitted.

Which is why, for nearly all simulations, you use Born-Oppenheimer approximation and molecular dynamics using force fields derived from QM, but without the full explicit QM treatment. This is fine because with only rare examples (as far as modern science knows), QM treatment is not necessary for most molecular machines.

I didn't realize that. It sounds like the same situation with astronomical simulations. General Relativity is too complicated so everyone uses Newtonian mechanics plus a fudge factor. I think it is odd that the two most cherished scientific theories are not actually used in practice to compare with data. Also, this got me wondering how they did this simulation, which lead me to this site: http://www.tapir.caltech.edu…

Yes, both astrophysics codes and MD are basically "n-body approximations" https://en.wikipedia.org/wiki/N-body_simulation although if I understand correctly universal simulations do explicitly model general relatively

Personally I find the "particle mesh". If you model the N-body calculation this way, you can solve for the field (potential) by converting to fourier space, doing some FFTs (which computers are really, really good at), then converting back to real space. This works in both astro and molecular simulations, really well.

Re: Recreating Our Galaxy in a Supercomputer

#39

Post by distrill » Fri, Sep 09, 2016, 4:02 PM UTC

hossbeast wrote: post #27

I'm always a bit uncomfortable when I read about findings from these simulations. To say they have "answered questions" about galactic evolution seems to strong a statement. How about, "created a more accurate model"?

The article does not mention answering questions about anything. It pretty much just says that they created a more accurate model.

Re: Recreating Our Galaxy in a Supercomputer

#40

Post by hypertexthero » Fri, Sep 09, 2016, 4:02 PM UTC

sillysaurus3 wrote: post #12

Le_SDT wrote: post #9

Earlier quoted context omitted.

I'm curious. What do you mean by "Human vision is completely overlooked"? Are you talking about color perception? 3d rendering? Thanks :)

There's pretty much no way to avoid writing a large comment in response to this. So, with apologies... It was my focus for a long time to achieve perfectly photorealistic rendering. I come from a gamedev background, and I've been fascinated since around age ten about how to get a computer to paint pictures. By 17 I was writing game engines mostly equivalent to quake, and used this portfolio to get into the industry.…

The recommendation to get a camera is a very good one. A camera used for an extended period of time is a very good teacher, and not only of seeing.

To go even further, learn to draw. Use your hands and other senses instead of only thinking. Read Drawing on the Right Side of the Brain by Betty Edwards and The Hand by Frank R. Wilson.

I'm curious what your favorite games are, visually, @sillysaurus3?

A couple of mine are Far Cry 2 and [Elite: Dangerous][1] Horizons. I also love the sound in both of these which is to me a seemingly inseparable element to great screen work.

[1]: http://simongriffee.com/notebook/elite-dangerous-education/