Gravitational Lensing

Gravitational lensing is one of my favorite physical phenomena. To introduce it to you, I'll first need to present two concepts that make gravity and energy a little easier to think about.

1) We often think of gravity as being caused by mass distorting something we call spacetime. Imagine a sheet stretched tightly and perfectly level. If you were to roll an extremely small marble across the sheet, it would travel in a perfectly straight line without interruption. However, if you put a bowling ball in the middle of the sheet and rolled the marble past (not even necessarily towards) the bowling ball, it's quite easy to imagine that the

marble would roll towards the bowling ball. This is a three dimensional simulation of gravity. In our world, space is a three-dimensional "sheet" on which large masses (stars, planets, etc.) are resting. They distort otherwise "straight" space, causing other masses to "roll" towards them.

2) A handful of scientists including Einstein (See? Now you believe me.) discovered that light and matter were, in fact, the same thing. It's kind of the same way that thunder and lightning are two different manifestations of the same thing. You see one and hear another at distinctly different times, but it was all caused by only one event. Thus, the path of light can be bent by gravity just like the path of a baseball is.

Around 1900, Einstein predicted that his model of gravity (discussed above) could be seen if we could observe a star the was directly behind the Sun during a total solar eclipse. They picked a star that would be directly behind the sun during the eclipse and attempted to observe it during the time of maximum darkness. Sure enough, they were able to see the star, the path of its light having been bent around the Sun towards Earth.

However, now we use it to sound the deepest, oldest reaches of the universe. By pointing a telescope at extremely massive clusters of galaxies, we can resolve several smudges or blurry images in the field, such as the blue objects scattered around the field pictured here. Each blue image is actually the same galaxy which lies behind the cluster in the center, but is being bent around it in several different directions. This phenomenon, called strong lensing, is useful in determining the shape and extent of our universe, which is currently calculated at approximately 14.5 billion light years across (about 90 billion trillion miles).

Closer to home, an effect called microlensing helps us solve yet another mystery of the universe. Currently, we can account for only about 4% of the mass that is "supposed" to be in it. Numerous factors (which require their own post) make it plainly obvious that all of the matter that we can see isn't even close to the amount that there needs to be to make it behave the way it does. We speak of dark matter, which is simply a (very mysterious and cool) name for "stuff that we can't see". And it isn't even all that far away. Dark matter governs the behavior of even our own galaxy, the Milky Way. But just because we can't see it doesn't mean that we can't see what it does. Occasionally, a hunk of dark matter crosses in front of a star, causing it to quickly change its apparent light output. These hunks are called MACHOS (Massive, compact halo objects, of course. . . what were you thinking of?) and are frequently observed. They give us enormous insight into what dark matter could actually be composed of, and its participation in our conception of the universe.

Incredibly, a simple concept like "gravity bends light" can be used to produce observable data that helps us to understand mysteries as complex as the age, size, dimension and composition of the universe.