by Shane L. Larson
When you and I were in elementary school, we were taught “The Scientific Method” (which we’ve talked about before) and told that it was how scientists do science. In a similar vein, I was once told that cooking is just stirring stuff together and heating it up. I’m sure that is how chefs do cheffing. But as any chef knows, the art of cooking is not so easily distilled to a pithy soundbite easily told to 4th graders. So it is with science. Astronomy in particular often defies the rigid, black and white portrayal of science given by The Scientific Method.
For the most part, astronomy is not a bench science. Astronomers do not have controlled experiments that can be repeated and studied. Instead, astronomy is a spectator science — all we can do is look at the Cosmos, and see what it has thrown up on the vast canvas we call the sky. Some things persist, and can be looked at over and over and over again. Other things happen only rarely, and if we aren’t looking or aren’t prepared to look closely, the information washes across our small planet then vanishes forever. Because of the unpredictability of Nature and because of the paucity of many astronomical events, one of the most important aspects of astronomy is squeezing every possible bit of information out of the signals we receive, so that we may best understand what created those signals. Our primary partner in this endeavour has for most of the history of modern science, been light.
It’s not surprising that light provides one of our main connections to the Cosmos. We are creatures that have evolved to depend on our sight as one of our primary senses. In addition, light is readily generated by atoms, which make up all the stuff that the stars and planets and interstellar gas are made of. There are both obvious and subtle ways that information is encoded in and carried by light, most of which are useful in astronomy.
The foremost and most obvious role light plays in astronomy is that of signal beacon — light is shed from an object, and makes its way across the Cosmos to say to the astronomers of Earth, “I’m here! Look at me!” In the case of stars, that light is rather pointlike and fixed in space; often it is steady and unchanging, at least over timescales that are readily noticed. Many stars have color. Consider the constellation of Orion, one of the most recognized constellations the world over. This deep sky image shows both unincorporated gas as well as stars. The three belt stars lie in an almost perfect line, and are roughly the same brightness, shining with a blue-white light. They are called (from east to west): Alnitak, Alnilan, and Mintaka. Orion’s eastern shoulder is a baleful red star known as Betelgeuse; the right knee is a brilliant blue star called Rigel. These names have been passed to us down through the ages, from a time before we knew what the stars were, what they were made of, or what their colors meant. But today, we know far more than those who first named these beacons in the sky. Both Rigel and Betelgeuse are supergiant stars, and both are about 10-20 times the mass of our Sun, but their similarities end there. Their colors were different — one blue, one red — and it was later discovered their sizes were different: Betelgeuse is about 1000x the diameter of the Sun, whereas Rigel is about 74x the diameter of the Sun.
We’ve talked before about how the fingerprints of atoms impress themselves in light (read that post here) by making spectral lines, and how the color of a star is a direct measure of its temperature (read that post here). Together these two properties allow us to say much about the stars without ever visiting them up close — this is one of the most important discoveries about light that has ever been made. In the case of Rigel and Betelgeuse, it has allowed us to know that Rigel is blue because it is far hotter than red Betelgeuse. Why the color discrepancy? Because Rigel is young, and Betelgeuse is elderly.
If all we ever learned about the Cosmos was the temperature and composition of the stars, we would consider those discoveries triumphs of the human intellect. But is there more that light can tell us? Of course there is! But all observations of Nature require us to be careful observers. Light is a tempestuous partner in the game of astronomy. Like messages on the internet (how science propagates on the Internet was famously described here), the news carried by light might be very interesting, but is subject to distortion and confusion because during the long journey to Earth, light is easily influenced. Any matter the light encounters during transit has the potential to change the light, to corrupt and distort the news it carries from the distant corners of the Cosmos. There are many such effects that distort the stories told by light, but let me tell you two of them.
One example is called gravitational lensing. Einstein taught us that mass and energy are equivalent. What does that mean? There are many consequences, but the important thing for this conversation is this: energy feels the pull of gravity, just like mass does. That means light, a form of energy, should feel the pull of gravity as it is speeding along through the Universe. This is exactly what would happen to you or me. Suppose we are drifting along in our starship, and pass a large object to our left, say a galaxy. The gravity from the galaxy would tug on us, and pull our course inexorably to the left. Once we were past the galaxy, the gravitational pull would fade with distance, and we’d continue to travel on a straight line, but it would be a different straight line than the one we started on!
This is exactly the same thing that happens to light! Suppose light is speeding through the Cosmos, and passes a large object on its right, say a galaxy. The gravity from the galaxy tugs on the light, and pulls its course inexorably to the right. Once it is past the galaxy, the gravitational pull fades with distance, and the light continues to travel along a straight line, though it is now travelling a different direction than it started out in — this has important consequences for astronomy? Why? Because as astronomers, we often make the default assumption that when I see a light ray from the Cosmos, it came directly from its origin — we like to think that if we simply extend the direction all the way back through the Universe, we can determine where the light came from! But the deflection of light by gravity has made that extended line point somewhere else!
There is a remarkable consequence of this gravitational deflection that was predicted by Russian physicist Orest Chwolson in 1924: an “Einstein Ring.” Light from a distant source doesn’t just fly to one side of a deflecting galaxy — it flies on every side. If you are sitting in the right place, the gravitating galaxy that is bending the light will act like a cosmic magnifying glass, making a ring out of light that goes around every side of the galaxy!
In 1936, Einstein famously panned the idea of seeing a ring, thinking the chances of a perfect alignment would be small, and even if an alignment occurred, our telescopes would not be up to the task of seeing the small ring. But as is often the case, technology outstrips our most pessimistic predictions. In 1988 radio astronomers discovered the first Einstein Ring; many more have been found, like the one shown below, LRG 3-757.
For our second tale, remember where this conversation started last time — with Feynman diagrams. A Feynman diagram shows us how light and matter (specifically electrons and positrons) interact. One of the things we learned by drawing Feynman diagrams is that you can predict new effects, then go look for them. A classic example is called “pair production” where two photons collide and create an electron and a positron. This effect was first seen in the lab in 1997 (read a NY Times article about the result here), and scientists are currently working on the design of a “photon collider” that will make pair creation a regular event in the laboratory.
But this begs an obvious question: could we use this effect to do astronomy? Is there some way we could observe this effect, and use it to answer some deep question about the Cosmos? The answer of course is yes!
Consider a blazar — an active galaxy, billions of lightyears away. A blazar is characterized by the emission of extremely high energy light, created by the jet of a super-massive black hole that is pointing almost directly at the Earth. The light from blazars must travel through most of the known Universe to reach us here on Earth. During that time, it will pass through ever increasing amounts of starlight generated by the generations of stars that have come into being, evolved, and perished over the ages of time. Gamma ray astronomers call this “the Cosmic Fog” because it acts just like real fog on Earth. If you shine your flashlight out into a fog bank, you can only see it from a certain distance away, and that distance depends on how thick the fog is. Starlight is the same way — the more starlight the gamma rays have to pass through, the more likely it is that they will encounter a photon of light from a star. And what can happen when two photons meet? They produce an electron-positron pair, and the photons vanish (their energy was used to make the particles)! The net result is that when viewed from Earth, the blazars don’t have as much high-energy gamma ray light as we expected — the gamma rays are absorbed by the Cosmic Fog of Starlight.
The most important thing about this is: scientists can measure how much of the gamma ray light has gone missing. That tells us how much starlight was encountered. Since we know something about how much starlight a star puts out (based on its temperature and size), this allows us to estimate the number of stars in the Universe whether we can see them directly in the telescope or not!
That is an astounding feat, and one that should impress upon you one thing: that we are an exceedingly clever species, and with science as a tool, we can figure out just about anything.
This post is the second and final part in a sequence talking about clever ways to use starlight to understand the Cosmos. The first part was here.