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Simulating the Whole Universe
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."
How can you accurately simulate the computer that is simulating the entire universe?
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
"Hi. This is my friend, Jack Shit, and you don't know him." - Lord Kano
"the computer scientists devised a couple of tricks to reduce the amount of computations..."
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?
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.
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.
Someone had to ask: wonder if anyone's simulated the universe using MOND. How did the researchers account for all this dark matter that's supposed to be around? It's far more likely that we got the force law wrong. Do these dark matter guys still believe in Santa Claus? BTW has anyone successfully simulated a galaxy and produced results that correspond to observations? I think this problem is still open...
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.
Bleh!
Last I heard there was some question as to the speed of gravitational attraction. IE if the effect of gravity is only as fast as that of light then the earth is being acted on by the gravity from the point the sun was at 8 minutes ago or some such while the sun is similarly being affected by the earths poistion from 8 minutes ago.
s /NCOR.11.16.D/display.html
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.
I don't ask you to be me. I only ask you not expect me to be you.
So they are simulating a universe full of black holes?
"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."
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?