Science is in the questions,
and not in the answers
Invited Lecture: Henry Joseph Science Fair, Albany Academy for Girls, April 26th, 2002
I would like to congratulate all of you for choosing to complete a science fair project this year. By doing this, you have opened a small window into the world in which I work -- the world of science and of discovery. There is a big difference between the lessons you are learning in your classrooms, and the lessons I hope you have learned in the process of working through your science fair projects. Most of the lessons you learn in school direct you to learn the correct answers, which are well known and accepted. 2+2=4. George Washington was the first president of the United States of America. Plants produce energy from sunlight through the process of photosynthesis. The word "bargain" is spelled b-a-r-g-a-i-n. These are all well-accepted facts.
However, you will find as you grow older that not everything you learn in school and in books is correct. When I was your age, I read books about a dinosaur called a Brontosaurus, which was discovered and named by a scientist in 1879. Scientists discovered in the late 1970's that the Brontosaurus in all of the pictures in my books had the body of an Apatosaurus and the head of a Camarosaurus. For a hundred years, the museums had the wrong head on their dinosaur skeletons! It takes a scientist of strong will and clear thinking to challenge a mistake made repeatedly for a hundred years.
It is a habit of good scientists to question everything that they hear or read. We figure out for ourselves, by comparing information from different sources and from our own observations, which information we accept as the truth. When new information contradicts something that we thought was true, we try to decide which information is correct. We demand that facts come with proof to back them up. And now we are back to the place I started.
Science is about the questions, not the answers. To me a question which has a known answer is not an interesting question. It is the questions for which do not have answers that capture my attention. Here are some examples from my own field of astronomy: How did the Universe begin, and will it ever end? How did galaxies form? What is really inside a black hole? How did life begin? Is there other life in the Universe besides that which we know about on the Earth? The answers to these questions are not written in the back of the book. They cannot be researched in the library. Your teachers do not know. I do not know, though in all of these cases, we know something about what the answer might be. The answers will come only from the combined efforts of many people, all working for truth, each checking the work of the others.
Astronomy is a fascinating and humbling field of research. Let me tell you some things that are known in astronomy.
Our Earth is a planet which orbits a star called the Sun. It is one of nine planets of the Sun. Scientists have recently argued over whether Pluto should really be called a planet, since it is thought that it was not created in the same way that the other planets were.
The Sun is an ordinary star, just like all of the other stars we see when we look up at the night sky. The Sun is orbiting around the center of the Galaxy, in a similar way to the way the planets orbit around the Sun. All of the stars in the Galaxy are moving around the black hole in the center of the galaxy. Just as the positions of the planets in our sky change as the planets orbit the Sun, the positions of the stars in the sky change as the stars orbit the center of the Galaxy. The motions of the stars are slower, though, and without modern equipment and satellites, one would have to watch hundreds or thousands of years to see the constellations in the sky change. From the motions of the stars, we know that 90% of the matter in the Galaxy is dark; it does not emit light like stars do. But we do not know what the dark matter is. There are things called brown dwarfs and planetoids, which are made up of the same type of stuff as our Sun is, but are not big enough to shine like a star. Is this the dark matter? Or is it some exotic kind of matter that we never knew about before?
We know that our Milky Way galaxy is one galaxy among billions of galaxies (or more) in the Universe. By the way, that has only been known for the past hundred years. When your great grandparents were born, we had no idea that the Sun was in a galaxy at all, or that other galaxies existed out there. All of the galaxies are moving. All of the galaxies formed at some time in the past. Sometimes two galaxies run into each other and merge to form one bigger galaxy. The stars in each galaxy change as they grow older over billions of years. But we do not have a good model for how galaxies formed in the first place. Did the stars form first, and they get pulled together by gravity to form galaxies? Or did the galaxies form first as large clouds of dust and hydrogen gas which only later formed stars within them?
We know that every galaxy in the Universe is racing away from every other galaxy in the Universe. We believe that this is because the Universe itself is getting bigger and bigger, as the distances between galaxies become ever larger. When I was a graduate student, everyone assumed that the rate of expansion was decreasing with time. The pull of gravity would act to try to pull galaxies towards each other and slow the expansion -- just as the pull of gravity slows the speed of a rock thrown up into the air, until it stops, and begins to fall back towards the Earth. So, the big question was: "Is the rate of expansion decreasing fast enough so that eventually the Universe would begin to shrink back down in size and the galaxies would move closer to each other again?" Recently, scientists have discovered that the rate of expansion is not getting slower at all. It is speeding up. Although we can write down mathematics to describe what is happening, we do not really know what pushes the Universe apart at an ever increasing rate.
We know that some galaxies have black holes in their centers. What is a black hole? I want to go back to that rock example. Suppose I threw a rock up in the air. It would go up, and then fall back to the Earth. If I picked it up, and threw it harder, it would go faster up into the sky, and go higher before it falls back down to the Earth. If I put it in a slingshot, I could make it go even faster than I could throw, and make it go even higher up into the sky. But it would still fall back down to the Earth. It takes the power of a rocket lift-off to make a rock go fast enough that it goes up, flies out into space, and does not come back. That is with the gravity on Earth.
But suppose the pull of the Earth's gravity were stronger. Then, maybe even our current rockets would not be going fast enough to send them out into the solar system; they would just fall back to the Earth. Suppose the pull of gravity on the Earth were so strong that even if we threw the rock up at the speed of light, it was not fast enough to send the rock out into space. Light itself could not leave the atmosphere. Then, we would be inside a black hole. Planets, stars, galaxies - even light - can fall into a black hole, but nothing can ever make it back out, because nothing can go faster than the speed of light. I guess this is the ultimate "roach motel." Nothing can make it back out to tell us what it was like in there.
As a physicist, I observe the world around me, and from those observations, I try to understand the basic reasons why the world, or the Universe, or this rock, is the way it is. Since I have never been able to see the inside of a black hole, I can only guess what it is like in there by comparing the conditions in there to the places we have seen, that are the most similar to the conditions in there. The thing is, there is no place we have ever seen that is like the inside of a black hole. So, we can really only guess what it is like in there.
You might have noticed that I cannot tell you anything I know, without telling you something I do not know. That is the nature of a scientist. I have never discovered the answer to any question without the answer generating more questions than I had before. It is the same with all of my colleagues. We are the people for whom every answer only generates another question. So you see for us, the science really is in the questions. I believe that children are really great scientists, since children see the world as a long list of questions, just like I do. One of the interesting things about being a mother and a scientist is that there have actually been time when my children got tired of asking me "why" before I ran out of energy to answer them. Because those questions are important to encourage, even if the answer is "I don't know, and what's more, neither does anyone else."
I want to tell you about one thing I discovered a couple of years ago, just to let you see what one astronomer does, and how scientists think. Most of you have probably seen a picture of a galaxy, which shows the beautiful spiral arms. That is more or less how the Milky Way galaxy looks. What most people don't know is that all of the stars in those spiral arms are inside a big ball of dimmer stars. This big ball of dimmer stars is called the halo of the galaxy. When I was in college, I learned that the halo of the Milky Way galaxy is composed of stars which are as old as the Milky Way galaxy itself. In the beginning, there was a big ball of hydrogen gas. As the Milky Way formed, this big ball of gas flattened out into a disk, which then formed the spiral arms we see today. The stars in the halo were stars that were formed before the galaxy flattened out, so those stars were left behind in a big spherical distribution.
Five years ago, I started studying this ball of stars to try to understand how the stars in that part of the galaxy were distributed in the sky. When I began, I imagined that they were pretty evenly distributed, so that one would see about the same number of these stars in any direction, just as I had been told in my college classes.
What my collaborator and I discovered is that the stars were not evenly distributed, or even randomly distributed. Much to everyone's surprise, we found enormous groups of stars that were all clumped together, way out in the outer parts of our galaxy. Of course, the first thing we thought is that something was wrong with our data. Maybe all those things that we thought were stars were not really there. Maybe there was something wrong with the equipment or the programs which identified them as stars. It turns out that another colleague of mine was looking at similar star data, and independently found one of the clumps we had been puzzling over. It was then that we realized that the clumping we were seeing was real. It was as if a switch had been thrown, the lights had come on, and we knew what our data was telling us. There were two big groups of stars, way out at the edge of the galaxy. Clusters and groups of stars are common in our galaxy. But a group of this many stars, spread across a region of the galaxy as big as the distance from our Sun to the center of the galaxy, was unheard of. We thought we had found what was left of two small galaxies which had come too close to the Milky Way galaxy, and were in the process of being ripped apart by the force of our galaxy's gravity.
After we published the paper, another scientist pointed out some facts that we had not known about. He had found a small galaxy, called the Sagittarius dwarf spheroidal, which had come too close to the Milky Way, and was in the process of being ripped apart. It is true that there are small galaxies which are actually inside our galaxy! He had calculated where the stars from this galaxy would be spread in the sky, and they exactly matched the places we had found groups of stars. We had not found two separate groups of stars. We had found two pieces of the same great stream of stars, which went all the way around the Milky Way galaxy. The stars in the stream had originally been part of the Sagittarius dwarf galaxy, but they were torn away and now orbited around the center of our own Milky Way galaxy.
Since we published that paper, we have looked at the positions of many more stars in the halo of the Milky Way galaxy. It seems to us that wherever we look, we find more clumps of stars, and not the even distribution we had originally expected.
By combining our results with the results of other astronomers who are also finding groups of stars moving together as if they had a common origin, our picture of how the Galaxy formed is changing from the one I learned in college. At least some of the stars in the outer parts of our galaxy formed first in other galaxies, which then fell into our galaxy. They did not form in that original ball of hydrogen gas as it was flattening out into a disk which contains spiral arms. Maybe all of the stars in the outer parts of the Milky Way were formed in other galaxies and then later fell into our galaxy. As we look at more and more stars, we will try to figure this out.
So you see, every discovery creates more questions. How many galaxies have fallen into our galaxy? Were any stars in the Milky Way halo formed as our galaxy was forming, or are they all initially from other galaxies? Did any of the dark matter in our galaxy fall in after the spiral arms were formed, and can we measure the dark matter by looking at these star streams? Can we find more star streams by looking at more stars? How long ago did the Sagittarius dwarf fall into the Milky Way, and how long will it last?
And now, I am ready to answer any questions that you might have, as best I can. But remember that I only have answers because scientists like myself have concentrated on the questions.