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Recreating Our Galaxy in a Supercomputer

caltech.edu

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Re: Recreating Our Galaxy in a Supercomputer

#51

Post by semi-extrinsic »

dekhn wrote: post #38
nonbel wrote: post #34

Earlier quoted context omitted.

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,…

Fourier methods are good, but only when your potential is decaying slowly. If it decays fast, like van der Waals interaction which are plentiful in MD, you just have to do the plain n-body approx.

Re: Recreating Our Galaxy in a Supercomputer

#52

Post by Zenst »

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"?

Indeed, and with that it will hopefully expose details in models missing, after all how accurate are weather forecasting models and how quickly do those nuances propagate and yield week ahead forecasts that are totally different.

Though if they can run a model and end up billions of year later approximating aspects of today then they must be getting more right than wrong.

Re: Recreating Our Galaxy in a Supercomputer

#53

Post by dredmorbius »

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"?

There's little difference between the two phrases you're using.

More accurate models offer stronger explanatory or predictive capabilities. They answer questions earlier models couldn't.

Re: Recreating Our Galaxy in a Supercomputer

#54

Post by tizzdogg »

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 work in film vfx, and if you think we dont compare our rendered images to real photographs and video constantly, then you dont really know how modern graphics are produced. We shoot reference for everything, and use things like gonioreflectometers to sample real world BSDFs. Yes, lighting models are always necessarily simplified from the real world physics. But the thing is that 99% of the time that doesnt matter, because most common types of surfaces are able to be reproduced accurately enough to fool the majority of people. I would wager that most of the CG things you see these days on TV or in movies you have no idea was CG.. it's only the bad stuff (or obviously impossible) that stands out.

I actually am a bit confused what you are arguing here.. all of these problems you're mentioning have been well-understood by graphics researchers for the past couple decades.

Re: Recreating Our Galaxy in a Supercomputer

#55

Post by VikingCoder »

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"?

"answered questions"

Yes. Like, "Given everything we understand, how could it be that in the physical world, there aren't more dwarf galaxies?"

And there was no answer to that question, until now.

Now they come up with a new model, and run it, and they end up with a number of dwarf galaxies that much more closely matches the physical world. No one had done that before, so we didn't know how it could be.

This has answered the question, "HOW COULD IT BE?"

The answer is, "If reality matches this new model."

Re: Recreating Our Galaxy in a Supercomputer

#57

Post by a_throwaway3838 »

VikingCoder wrote: post #55
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"?

"answered questions" Yes. Like, "Given everything we understand, how could it be that in the physical world, there aren't more dwarf galaxies?" And there was no answer to that question, until now. Now they come up with a new model, and run it, and they end up with a number of dwarf galaxies that much more closely matches the physical world. No one had done that before, so we didn't know how it could be. This has answ…

Saying "it has been answered" is a strong definitive statement, though. It's rather arrogant to say "Yes, thanks to this model, we absolutely know ." Are we absolutely certain that this model is correct? Are we absolutely certain that what we can extrapolate from this model accurately applies to the physical world?

Granted, yes, questions were answered... I guess. But saying that "The simulation solves a decades-old mystery" is uncomfortably confident and counter to how scientific process works.

It's definitely catchy and clickbaity, though

Re: Recreating Our Galaxy in a Supercomputer

#58

Post by dekhn »

semi-extrinsic wrote: post #51
dekhn wrote: post #38

Earlier quoted context omitted.

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,…

Fourier methods are good, but only when your potential is decaying slowly. If it decays fast, like van der Waals interaction which are plentiful in MD, you just have to do the plain n-body approx.

In MD, it's a combination of VDW and electrostatic. Typically VDW is just a cutoff (VDW forces beyond 8A are neglible), and electrostatic is handled by particle-mesh ewald. I worked on DNA and RNA, which have strong electrostatic interactions and my thesis work showed PME was critical for stability.

Re: Recreating Our Galaxy in a Supercomputer

#59

Post by dekhn »

Florin_Andrei wrote: post #46
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.

> QM treatment is not necessary for most molecular machines Still largely true, but more exceptions are being found all the time.

Do you mean, to simulate the active site of an enzyme, or that people are trying to explicitly model known quantum biology, like photosynthetic centers? I don't know that anybody is claiming it's necessary for protein folding or DNA/RNA simulations.

Re: Recreating Our Galaxy in a Supercomputer

#60

Post by a_throwaway3838 »

dredmorbius wrote: post #53
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"?

There's little difference between the two phrases you're using. More accurate models offer stronger explanatory or predictive capabilities. They answer questions earlier models couldn't.

It might seem small, but I'd argue that the difference in phrases is pretty stark.

Saying we "have a better model of"-something leaves room for further exploration and interpretation. It's admission that there's room for different possibilities, and its not taking an /absolute/ position on something.

But saying something "has answered questions" comes off as absolute and final. Is 1+1=2? Yes. Answered. Case closed. If you're going to say that "The simulation solves a decades-old mystery", then you'd better be damn sure that your simulation is correct, your interpretations are correct, your simulation correctly reflects physical reality, etc.

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