by Shane L. Larson
People often ask me why I became a physicist. I often respond with stories about my great mentors (some of whom I’ve written about, like Doc or David Griffiths). But more often than not I simply tell them the truth: physics was the easiest science I could major in! This declaration is usually met with gasps of horror, looks of incredulity, or laughter. In response to these varied expressions of disbelief, I quip, “Have you ever taken organic chemistry?” Quick, look to your nearest organic chemist — they are nodding knowingly. There is a LOT to remember in a science like chemistry or biology; I have much respect for my colleagues in those fields (and sympathy; their brains must be unbearably full). All fun and games aside, there is some truth to my statement — physics is exceedingly fundamental, and that makes it easy in ways other science cannot be.
There is an internet comic circulating from one of my favorite web comics, Abstruse Goose. In the upper frame it shows an idyllic sketch of a park with a cute fuzzy little bunny enjoying a meal of a carrot near a babbling brook on a warm spring day. In the lower frame, it shows the same picture, overlaid with a great many mathematical equations that represent the processes going on in the picture. The caption reads, “This is how scientists see the world.”
Many who look at this image may shake their heads at the perceived gobbledygook, perhaps they suppress a shudder as their psyche ripples with a memory of a long-ago science class for which they hold few fond memories. But scientists like myself, we look at this image and we nod sagely. Yep, that’s what makes everything go. But I don’t think it is how scientists see the world. I think it is how we explain the world. The Cosmos is a great machine that obeys a set of rules that govern how it works — what it does each moment, how all the parts interact in harmony, what happens if something changes, and how the past influences the future. Scientists have a name for the rules that govern the great machine of the Universe — they are called The Laws of Nature.
Physics is a science that seeks to understand the most basic, the most fundamental Laws of Nature. Why do apples fall to the ground? What is light? Why does my Dr. Pepper get warm and why does my coffee cool off? What is the smallest thing that other things are made of? Why does the Earth spin, and why do distant galaxies seem to be running away from us? These are all deceptively simple but intensely probing questions about the nature of the Cosmos. Surprisingly, physics has discovered that there are answers to all of these questions! From a set of only perhaps 10-20 fundamental equations that summarize the basic Laws of Nature, all of the basic behaviours of the Cosmos can be deduced; all of these questions can be answered.
Now anyone who has ever picked up a physics textbook is probably scratching their heads right now. There are a LOT more than 10-20 equations in most textbooks. But the point is there are only a few fundamental rules, what physicists call “First Principles,” from which all the others are derived. It is a bit like alphabets. In modern English, there are only 26 letters, but from those 26 letters we can create, literally, a billion words! And we are making more all the time (I’m pretty sure Abraham Lincoln did not know the word “email”). Physics is the same way; we take a few basic ideas, we mix them together in a variety of ways that brings out depth, richness, and sophistication that is awe-inspiring in its scope, and breathtaking in its beauty.
One of the most important truths of science is that the Laws of Nature are not the story; they are the words of the story, not the tale itself. Knowledge and wisdom come from reading the book of Nature, and realizing that knowledge and wisdom are not immutable — they evolve with each reading of the book. Let me tell you a story about one chapter of Nature’s book that we’ve managed to puzzle our way through.
The beauty of the Laws of Nature is they are everywhere, and they are the same everywhere. The beauty of our brains is that we have managed to learn what some of those Laws are, and have discovered how to use them and apply them far afield from how we discovered them. Consider the following interesting factoid: statistics suggest that roughly three quarters of the population needs corrected vision; that means a great many of you reading this blog right now are looking through glasses or contacts; perhaps some of you are still using old fashioned chatelaine glasses or monocles (historical investigations have suggested that the first glasses appeared in Pisa, Italy around the end of the 13th Century). The physics of how glasses help you see, indeed the physics of how your eye collects and focuses all light, is called optics.
Imagine looking at an object. What you see is light traveling from each point on the object to you. In most cases it is reflected light, but in some cases (like with a flame, a light bulb, or the Sun) the light is generated by the object itself. In either case, what you see is a consequence of light flying from the object to you — a little packet of light from every part of the object. To represent this idea, physicists draw little arrows from the object to you, representing the light that you are seeing.
Suppose, in the interest of exhibiting the Laws of Nature, I go skydiving while you watch. You, and all your friends, can see me while you stand on the ground. I can show how the light gets to you by drawing little arrows that represent the light flying (in physics-speak we say “propagating”) from the skydiver to your eyeball.
In reality, every piece of the skydiver is sending light in every direction. That is why you can see the skydiver no matter where you are standing. When your eye (or your camera) looks toward him, it gathers all the little pieces of light that are arriving at that point in space, and it makes an image.
There are subtleties to be sure, but this is the basic law of Nature that forms the foundation of optics: light propagates in a straight line (a ray) from the point where it was emitted. Whether it is a skydiver, the words you are reading in this blog, or the light from a distant star or planet, the light got to you flying on a straight line.
But what about your glasses or contact lenses? That is what started this conversation. When light passes through materials, like water, or glass, or plastic, the direction it travels is changed — the ray bends. You can see an excellent example of this at home by filling up a glass with water. Here I’ve filled up a wine glass, and if I look through it, everything looks upside down! That’s because the water in the glass bends the light. The direction it bends it depends on where the light hits the wineglass, and the result is to make an upside down picture when you look through it. Try it! As you get older, the lens in the front of your eye changes its overall composition, generally hardening so the muscles in your eye cannot flex the lens to send all the rays of light to the proper place on the back of your eyeball. When this happens, we can help your eye out by bending the light before it even shoots inside your head. All the machines and diopters and numbers used by your eye-doctor are about how much to bend the light before it gets in your eye.
So now we have a new addition to our law of Nature: light bends when it flies through an object of different material. We didn’t know this part when we started. We just drew some rays to explain how both you and I could see the skydiver from different places. We had to add to our rule about light when we started thinking about light flying through curved glasses of water. This is the nature of how we understand the Cosmos — we write down the laws of Nature that explain everything we have seen. But when we discover something new, we either rewrite the law of Nature to be more inclusive, or we write down a new law of Nature! Science is a constantly evolving process. It represents, to the best of our ability, our understanding of how the great machine of the Cosmos works.
One of the most important pieces of the history of discovery in optics was its application to astronomy. The obvious application was in the invention of the telescope, but let’s pass on that for a moment and think about how light travels to us from the most distant places in the Cosmos. In 1979, a team of scientists using the 2.1 meter telescope at Kitt Peak discovered a pair of quasars now known to astronomers as QSO 0957+561 A/B, in the constellation of Ursa Major. They are colloquially known as the Double Quasar. At the time, the discovery was remarkable because the two quasars were very close together on the sky. If you hold a dime at arms length, then both quasars would fit behind Roosevelt’s eye! Most things in the sky change with time, so astronomers like to periodically go back and observe objects over and over again. This kind of persistent habit revealed something spectacular about the Double Quasar: if quasar A suddenly got brighter, then quasar B would follow suit 417 days later. It was like clockwork — anything quasar A did, quasar B mimicked 417 days later. How could this be? Quasars are energetic galaxies, powered by black holes, billions of light years away. They are giant, insentient objects in the Cosmos, not annoying siblings engaged in a game of copycat! What was going on?
As it turns out, the answer had been discovered by Albert Einstein more than 60 years before. If gravity is strong enough, it can bend the path that light travels on, just like a lens or a glass of water. The Double Quasar is really just two images of the same quasar. The gravity of a galaxy between Earth and the quasar is bending the light, like a lens! Like a bad pair of glasses though, the gravitational lens (a giant elliptical galaxy, in this case) is not perfect, and it doesn’t bend all the light toward Earth so it arrives at the same time. The result is we see two images of the quasar, delayed by 417 days.
The same laws of physics — light, rays, optics — but with gravity playing the role of the lens. In this case, the discovery was made long after a physicist had predicted that Nature would permit such behaviour. This too is the way science works — we have ideas about how Nature might work, about how the different pieces of Nature might work together in harmony. Some of those ideas might be right, and some of those ideas might be wrong. All we can do is look for a sign, look for a clue to tell us whether we are on the right track or not!
This post is part of an ongoing series, celebrating the forthcoming science series, Cosmos: A Spacetime Odyssey by revisiting the themes of Carl Sagan’s classic series, Cosmos: A Personal Voyage. The introductory post of the series, with links to all other posts may be found here: http://wp.me/p19G0g-dE