Bench Press

I have always been impressed with the work of astronomers. Unlike biologists and chemists who can, for a wide array of topics, actually touch and feel what they are studying, astronomers have to make conclusions with only careful observations conducted with powerful telescopes and computers informed by understanding the laws of physics (quantum and relativity included) and backed up by complex computational models.

One phenomena which astronomers can use to better explore deep space is an effect predicted by Einstein’s theory of general relativity called gravitational lensing. General relativity predicts that the path of light can be bent by gravitational fields; the most dramatic example of this would be a black hole, where gravity is so strong that light “falls” back into it. The same effect, on a less dramatic scale, could result in the path of light being bent on its way to Earth by the gravity of another object. The term “gravitational lens” refers to the fact that this bending is similar to the bending of light by a telescope lens.

Now, for the layperson, the fact that light can bend is probably just a cool effect which has no practical importance. But, to a well-trained astronomer, the knowledge of how gravity works lets them use the phenomena of gravitational lensing to understand both the objects that are emitting light (because the lensing effect allows us to see objects which are so far away that they are blocked by another object) and the “lens” itself (understanding the mass, structure, and position of what is bending the light).

Take a look at the pictures (HT: Wikipedia) below of Einstein rings. These occur when the line of sight to a bright, faraway object is being blocked by another object. However, because of the gravitational lens effect, the light from that faraway object can bend around the closer object, resulting in a ring which gives scientists a chance to study not only the faraway object but also understand the structure of the intervening space.

There are countless other examples of the application of gravitational lensing in the study of astronomy, but one of the most clever that I heard about recently was the study of dark matter. The theory in a nutshell: the universe is believed to be mostly dark matter – matter which does not reflect or emit any light whatsoever. Because it doesn’t seem to emit or reflect electromagnetic radiation, there has been no direct observational way to study it. However, dark matter does have mass. This means it has gravity and can thus bend light as a gravitational lens!

Researchers were able to took astronomical survey data from around the world and, using sophisticated computer algorithms and programs, compile a picture of gravitational lensing due to dark matter. From that, they were then able to digitally put together a picture of the structure of the dark matter in (at least part of) the universe and get a sense for how it’s evolved over time (the further from Earth you look, the further back in time):

And with this they made a striking conclusion – we all have dark matter to thank for the existence of the stars and the galaxies:

Our results are consistent with predictions of gravitationally induced structure formation, in which the initial, smooth distribution of dark matter collapses into filaments then into clusters, forming a gravitational scaffold into which gas can accumulate, and stars can be built.

Awesome.

(Image credit) (Image credit)

The N-body problem is a classic question in physics and mathematics. It is interesting, because while it is very simple to state, it results in a wide array of interesting behavior and very deep insights into how the universe works.

The setup is as follows: if we have N objects in space, with N masses, initial positions, and speeds, where and how fast will each of those N objects be moving if they are subject only to each other’s gravity (and the laws of classical physics – e.g. Newton’s Laws of Motion, etc)?

For a situation with 2 objects (N=2) and some of the cases with 3 objects (N=3), the problem has been well-discussed (the former leads to elliptical orbits, the latter results in the dynamics of the moon orbiting around the earth while subject to the sun’s gravity). But for problems more complex, the solution is anything but pretty. Behavior can emerge which may seem regular and predictable but degenerate into pure chaos. In fact, Poincare’s study of the N=3 problem eventually served as the foundation for the study of chaos theory, or the rise of chaotic, seemingly random behavior (like turbulence) out of “orderly” equations and behavior.

But, the challenge of studying these problems and formulating/testing hypotheses becomes increasingly more challenging as N increases. After all, how do you test an idea for how the N body problem plays out when the problem is that its so difficult to figure out how the N body problem plays out?

Enter the age of the computer simulation. Mathematicians/physicists/astronomers now have the tools to test their ideas (or just pass the time watching simulations) using computers to simulate the behavior of complex systems of N bodies.

But, the fun doesn’t stop with just a handful, or even hundreds, of particles. For an N-body simulation to be sufficient for as astronomer trying to study the structure of the universe, it would have to be scaled up to model billions of particles over distances in the billions of light years.

These super simulations are staggering to comprehend. The Millennium Simulation, run in 2005 to test our understanding of quasars and dark matter, simulated ~10 billion particles (with each “particle” representing a mass about a billion times larger than our sun) across a mind-boggling 8 trillion quadrillion cubic light years expanse of space containing over 20 million galaxies.

But even the Millennium simulation is mere child’s play compared to Project Horizon, which aimed to model “half the observable universe with enough resolution to describe a Milky Way-like galaxy with more than 100 dark matter particles”.

And, probably, this is merely the tip of the iceberg for what such super simulations may be capable of. How the future of this will shape out, I believe, is dependent on the following N=3 (pun intended) questions:

1. It is relatively simple (although computationally challenging) to create a toy simulation to validate or disprove one’s theory. It is much more challenging to build a simulation which can reveal testable/actionable (e.g. not simply if one’s theory is right or wrong) conclusions to further our understanding of the system of interest. For example, will future models of the human metabolome reveal genes or regulatory pathways of interest, or are they simply to validate existing models (something which may bias the model design away from revealing more interesting behavior)? Answering this question in the affirmative is essential, or else these simulations becomes vanity toys – something to use up research funds on without driving real value for the underlying science.
2. Will Moore’s Law/alternative processors/computer science keep up with demands for greater precision and computational power? Otherwise, these super simulations will hit constraints like energy consumption or the introduction of calculation error.
3. Will scientists become more adept at communicating the value of these models? This is potentially the most important question as the investment necessary to run these simulations are significant, not only in terms of cost, but in terms of manpower and energy consumption. With the current financial crisis and a potential future energy crisis, demonstrating value to the public and to more “traditional” scientists/doctors/engineers who rely on non-computational techniques will become more and more important for these future simulations to survive.

For now, I’ll leave you with a video summarizing the gorgeous results of the Millennium simulation:

No they haven’t actually, although Eric’s girlfriend threw me for a loop by sending me an IM with that headline.

However, my disappointment quickly melted by the website she linked me to and the adorable particles therein:

The Higgs Boson - found!

They’re brought to you by “The Particle Zoo”, which was inspired when an aspiring physicist realized that each particle seemed to have its own “personality” — why not make them into little plushies? I happen to think the Gluon and Dark Matter are especially cute:

Particle Zoo promises the release of “anatomically correct” particles (so probably breaking out some of the particles into their quark “components”) and some sort of “quantum duck” in 2009.

The real question now is, who will buy me one for the holidays?