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Originally Posted by Ken G
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Originally Posted by Nereid
I've read this, several times, and I'm sorry to say I still don't understand what you're saying.
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I guess what I'm really saying is that giant simulations have two main purposes-- one is to try and guarantee that the key physics is included (the "fishing net" philosophy of fishing), and the other is to try and obtain highly quantitative results when the situation is very well constrained by the physics and idealized assumptions are either not necessary or well supported in fact. But in neither case are we "finished" when the simulation achieves these ends-- yet that is often how they are treated. |
I think one thing I'm getting stuck on is "
giant simulations" ...
Let's take the re-analysis of the HIPPARCOS data that was published (fairly) recently*.
The original results involved number-crunching that excluded certain behaviours (or events), even though these were known to have occurred, and even though the physics was at least somewhat understood: the satellite going in and out of the Earth's shadow, and micrometeorite hits (among others); the re-analysis attempted to model these and remove their impact from the data, to produce more accurate proper motion and parallax estimates (as well as better error bars).
Certainly both analyses involved quite intricate modelling and lots of CPU hours; too, they both seem to have had both of the purposes you mention.
However, in each case we are (were) "finished" ... the objective of the mission^ was to produce parallax and proper motion estimates (together with robust estimates of uncertainty)!
In both cases, there certainly are other physical behaviours or events that have not been simulated or estimated, and sometime in the future someone may develop an even more complex simulation that uses even more CPU hours, and so produce an even better set of outputs. In fact, the known inaccuracies (etc) of the original analysis was one motivation for doing the re-analysis; however, diminishing returns have probably set in, not least because GAIA is likely to fly before too long.
Is this example even within the scope of your original post? If so, in what way(s) do you consider astronomy to be be unfinished (wrt this mission and the analyses)?
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In the first case, the work has just begun-- we have insured that the dominant physics is present in each situation the simulation covers, but we have not yet identified what that physics is. More often than not, in my experience, giant simulations can be broken down into smaller pieces where in each subset there is actually something quite simple happening, something that does not require the full simulation to understand (though it may have required it to find). It is often the case that in hindsight, the simple physics makes perfect sense and feels like it should have been anticipated prior to the simulation (though in practice it often is not). But my point is, if this followup analysis never occurs, as is all too often the case in the literature and at meetings, then this great promise is never actualized. Instead, people think the simulation has accomplished its goals, and nothing more is needed from it. It is a "complete" simulation-- but is it completed?
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Maybe a concrete example might help?
Perhaps simulations of the SgrA* SMBH and its accretion disk? or of SMBH accretion disks in general? They involve a mix of highly non-linear interactions, physical processes and mechanisms from a great many parts of the physics textbook, etc, etc, etc yet the phenomenology ("observables") is all wrapped up in photons from point sources (well, except for certain ingenious attempts to get at it indirectly, like footprints of past flares).
If so, then I doubt anyone would say that the simulations are complete, or completed ... the results serve mainly to show how little is actually understood and as pointers to which of the myriad things not yet considered needs to be worked on next.
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In the second case, where quantitative accuracy is the motivation for the complexity of the simulation, there is no simpler subset that can achieve that end. However, we are still not done, because even when quantitative accuracy is the goal, there is still a role for approximation. One role is in anticipating how the results will vary as you change the parameters. The "kitchen sink" approach to that issue is a brute-force variation of the input parameters (a so-called "grid of models"), and then interpolate to any actual desired situation.
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Sounds a bit like "google science"!
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But a simplified approximate understanding of the dominant physics also informs the process of understanding the sensitivity to input parameters, and the inaccuracy introduced by taking that approach is well compensated by the insight gained. Once the approximate dependences are understood, one can anticipate what combination of parameters will achieve some desired end, and then a full simulation can be run for the new parameters as a final check on that prediction. But the simple fact is, astronomers are forever undertaking calculations that are more accurate in form than they are in substance-- they are grinding out that third decimal place, yet invariably some new physics discovery comes along and changes the first decimal place. Examples are countless.
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Critique understood; examples please!
But not a countless number of them; just three would suffice ...
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The Millennium Simulation: huge simulation of a CDM-dominated universe, using GR, which aimed to learn something about the growth of large-scale structure (among other things). One of the many research programmes it (or rather its predecessors and previous analytic work, it just 'shrank the error bars') kicked off was a search for (an OOM more) CDM-dominated dwarf galaxies and other research into dwarfs (merger histories, starburst histories, ...).
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Granted, this is certainly a good example of how a "complete simulation" can make predictions that observers can then attempt to test, as a check on the assumptions of the simulation. But what I'm saying is, does this really complete the relationship that the theorists and observers should be having?
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Assuming this is not a rhetorical question, goodness gracious me no!
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Is it enough for theorists to say "I predict this, never mind why, you can picture this cartoon if you like, but what matters is that you look for it"? Put differently, can you tell me (in physical terms, not cartoon terms-- we get plenty of cartoons) why the Millennium Simulation produces so many dwarf galaxies? What is the key aspect of that simulation that gives you that, and once you understand that key aspect, how could you have obtained that same result without the simulation?
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Maybe this is a bad example ... the excess of predicted dwarf galaxies (over what had been observed up till then) was known well before the Millennium Simulation was run, and how these vast numbers of dwarf galaxies formed was also known (as in, what physics was at work, and why); what was not known (and still isn't, due to the limitations of the Millennium Simulation) was the bounds on the hierarchical clustering; crudely, should the MW have ~100, or ~1000 dwarf satellite galaxies?
And there are plenty of simulations which seek to answer this (and other) question, using a range of techniques and 'nulling out' a range of different physical processes (all already known to exist).
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As I said, hindsight is better than foresight, which is the main purpose of simulations if you ask me, so I'm not saying we never needed the full simulation. I'm merely saying it should not be viewed as the completed pinnacle of the theoretical effort.
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OK, bad example then; I doubt that anyone views the Millennium Simulation as "
the completed pinnacle of the theoretical effort"!
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Exoplanets: do the doppler programmes qualify as elaborate simulations? After all, to find the n-th planet, you first have to nail down the parameters of the first n-1 ones!
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In some situations, the accurate quantitative analysis is all that has value, because you are not trying to understand how something is working-- you already know how it works, and you are pressing for greater and greater detailed accuracy in your model inputs. Such situations are pretty rare in astronomy! But this is indeed one.
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Or not ("
pretty rare in astronomy"); maybe it's more a matter of degree/point of view?
Anyway, that helps me understand your point, thanks.
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Also, the use of microlensing to find planets may be described as hot (and full of models), even though it gives only one shot at each planet. And what of transit searches? and the models used to infer something about the atmospheric composition of the transiting planets (once they're actually identified)?
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Transit searches are observational, I'm talking about the interaction between theorists and observers. So your last point is more relevant-- about what we can theoretically infer about the atmospheres. But this is a perfect example of what I'm talking about-- when we get information about the atmosphere of a transiting planet, do we need a giant "black box" simulation of all the things that might possibly be happening in that atmosphere? Do we really expect to be able to anticipate the possibilities so completely? I'd say we need a simplified understanding of what kinds of observed features map roughly and approximately into what general kinds of physical phenomena, long before we'll be in any position to apply meaningfully a black box simulation to making quantitative predictions (indeed we may never be in that position-- we can hardly even do that in our own atmosphere, just witness global climate simulations).
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Again, this helps ... AFAIK no one has tried to infer something about transiting exoplanet atmospheres from giant simulations that are meta-GCMs!
(to be continued)
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if anyone would like a reference, just ask.
^
of course, the mission also produced other results, such as discovery of new binaries and variables