Simulating the Whole Universe 326
Roland Piquepaille writes "An international group of cosmologists, the Virgo Consortium, has realized the first simulation of the entire universe, starting 380,000 years after the Big Bang and going up to now. In 'Computing the Cosmos,' IEEE Spectrum writes that the scientists used a 4.2 teraflops system at the Max Planck Society's Computing Center in Garching, Germany, to do the computations. The whole universe was simulated by ten billion particles, each having a mass a billion times that of our sun. As it was necessary to compute the gravitational interactions between each of the ten billion mass points and all the others, a task that needed 60,000 years, the computer scientists devised a couple of tricks to reduce the amount of computations. And in June 2004, the first simulation of our universe was completed. The resulting data, which represents about 20 terabytes, will be available to everyone in the months to come, at least to people with a high-bandwidth connection. Read more here about the computing aspects of the simulation, but if you're interested by cosmology, the long original article is a must-read."
Umm, Paradox? (Score:2, Interesting)
Basically, you'd end up infinitely short on processing power. The faster you make the computer, the faster you need the computer to be. It's like working out so that you can get strong enough to pick yourself up by the bootstraps. The stronger you get, the more you weigh and you make the impossible less possible.
LK
Shortcuts (Score:1, Interesting)
Somehow I don't see how adding a few more shortcuts to the assumptions made matters, since they convienently decided to "reduce" mass to particles a billion times the mass of our sun. Really, what kind of useful calculations can you make when you vary that significantly from your target system.
Or did they assume the mass of each scientist's brain was also a billion times the mass of our sun?
Re:Kind of useless? (Score:3, Interesting)
They learn how large-scale structures formed in the universe
It is, if you don't care about how the universe as we see it came to be. If you do, it isn't.
People have been developing and enhancing codes like this for decades. They're already extremely well-tuned.
Pulling a figure out of your ass
As far as structure formation goes, all you need to know is that there's a chunk of mass.
Largely irrelevant to the effects they're trying to model. Obviously, you aren't a topcoder.
Re:Obligatory question (Score:1, Interesting)
Yes, and it also includes simulated scientists simulating simulated scientists simulating the universe. But no more levels of scientists after that --- from there on, it's turtles all the way down.
MOND (Score:2, Interesting)
At this resolution (Score:5, Interesting)
Are they modeling any of the physical (star formation, etc) interactions of matter or just the gravitational interaction. It seemed like the latter, but the article did mention the apparent non-interaction of dark matter.
Re:Umm, Paradox? (Score:1, Interesting)
you cannot observe every single part of an open system, it is beyond you.
You can see, cophrehend any closed system, if you have enough time.
this is why physics can be so interesting, and frustrating, your dealing with half-assed solutions all the time, becuase there is no way, ever, you can create a fool-proof answer system. its always your beset guess, becuase you can't do trial and error for ever.
Speed of Gravitational attraction ? (Score:3, Interesting)
As these mass points get further and further apart this would have a huge effect on the results. Unless of course Gravity is instentaneous across any distance opening the door to some interesting possibilities. Namely the ability to communicate across large distances without delay. Perhaps even FTL travel.
While I find this excercise interesting I also find it a tad ridiculose. So many simplifications have to be made to even attempt it and the whole thing is based on some assumptions that are not necesarrily cold hard fact... such as the mass of the universe. Theory says one thing, observation says another. Dark matter was invented to close the gap. Don't get me wrong, there are a lot of smart people that have come up with an awful lot of observation which seems to confirm its existence, but it could be that our point of veiw is insufficient. After all by all observations the Ptolemaic model of the movement of the heavens was accurate and it had all sorts of added rules for handling what was observed.
Also there is the issue of the N body problem where N is greater than 2. Did you know we cannot accurately model our solar system just using keplers laws ? We have to create stabilising factors in the system to keep the planets paths from becoming unstable in their orbits. And yet here they are attempting to simulate an N body problem where N = 10 billion.
http://www.lactamme.polytechnique.fr/Mosaic/image
That link shows what happens with a pure Keplerian system of equations for 9 bodies.
Thus introducing such things as mass simplification for objects farther away ( creating groupings etc ) and the tree approach for close objects all creates an introduction of error into the equation. Further more they have to use some means of stabilizing the equations similar to solar system models which is a value based on observation but with no understanding for what really controls it ( if they don't do this then the system of equations can't model our own solar system much less 10 billion mass points expanding since 380k years after the big bang ). This is all chance for more error to creep into the equation. Then with all of this they run a simulation for a simplified mass points using simplified interactions with an unkown stabilizing force over the course of billions of years and then expect people to believe that what they wind up with has any significant correlation to reality.
Do not be decieved by impressive things like 4 teraflops and 20 terabytes of information. To me this seems an interesting intellectual excercise, but the chances of the results being meaningful are pretty slim.
Re:Speed of Gravitational attraction ? (Score:2, Interesting)
P.S.- It is my personal belief that gravity is transmitted at the speed of light, if only because of what I have read and gravity being instantaneous making no sense to me.
Great thinking though. Reply to me on anything you disagree with.
Re:Speed of Gravitational attraction ? (Score:5, Interesting)
On the other I know about the increased accuracy from higher fidelity time samples but all that does is postpone the inevitable chaos in the equations. Most solar system models don't even use keplers equations. They use the information determined from solving them via a 2 body problem ( planet and the sun ) and then assume that orbital period is more or less sacrosacnt. This creates a stable model which accurately represents what we have observed... but does not allow for the chaos that creeps in when we try to replicate observed motions using Keplers laws to atempt to model all interactions. If your really interested (or already know alot about it) a fascinating subject based in reality is orbital mechanics... ie how do you accurately rendesvous with other planets when you are traveling in an N body problem where N is greater than 3 over periods of time that are too great to be able to avoid the chaos ? The simple answer is you make small corrective burns along the way based on observation to recalibrate the route. But the significance there is that you can't use Keplers equations for more than a rough estimation for navigating in space at N > 2 ( like landing the martian rovers ).
Keplers laws work almost flawlessly for 2 bodies which is why they are so powerful. However I think that is the problem. They work flawlessly for N=2 even when there is no real world true N=2 problem to solve. Essentially to solve the N = 2 problem for any planet you assume the attraction from anything other than the sun is insignificant. This works amazingly well and is what led to the discovery of the last two or three planets if memory serves.
But as accurate as that is there is no getting around the chaos of the 3 body equation no matter how fine grained your time samples are. This is not true of the 2 body problem.. IE it dosn't matter what your time sample is, the 2 body problem works. If it dosn't its because there is another source of significant gravitational attraction at work. However over a great enough time span my guess is even the 2 body equation has inherent chaos in reality.. IE a pure theoretical 2 body equation is perfect, but for the earth and the sun sooner or later what is deemed insignificant in the 2 body problem for practical purposes will become significant over a long enough time frame.
All in all it reminds me of the old parallax problem that led the Greeks to dismiss a Heliocentric model of the solar system and choose Ptolemy's view of a an earth centered model. I think our frame of refference is such that the inherrent error in Keplers laws are not readily observable just the same as the greeks frame of refference was insufficient to observe parallax.
A mass a billion times that of our sun. (Score:1, Interesting)
Whew! (Score:3, Interesting)
Well, at least we know that we will be around for a few months. Do we have to download the whole bloody thing to find out when the world ends?