Story posted September 03, 2009
"A Curious Universe"
Professor of Physics Thomas W. Baumgarte
Bowdoin College's 208th Convocation was held Wednesday, September 2, 2009, in Pickard Theater, Memorial Hall. Following is the text of Professor Thomas Baumgarte's address.
Thank you, President Mills, for your very kind and flattering introduction. It is a great honor to present the convocation address, and to have this opportunity to welcome the Class of 2013.
Perhaps, of course, you are already sick and tired of being welcomed, because you had days and weeks of welcomes during pre-orientation and then orientation. So perhaps convocation comes as a relief; as we are marking the official opening of the academic year, you are finally entering the post-orientation part of your Bowdoin education.
This is an exciting occasion for many of us, and an exciting time of the year. I hope it is exciting for you: you have arrived at college, tomorrow you will start doing what you came here for, and I hope you are looking forward to your four years here at Bowdoin. Presumably you all have some hopes and expectations for your time here, and I hope they will all come true.
Many of my colleagues will agree that this is an exciting time for us, too. A new cohort of students has arrived on campus, bright-eyed and bushy-tailed, tomorrow you will be in our classes, perhaps you will be as excited about our subjects as we are, and perhaps you will end up completing a wonderful project or honors thesis. Just as you have hopes and expectations from Bowdoin, we have hopes and expectations from you. One if the characteristics that we hope you bring with you is curiosity. An intellectual curiosity in the world and the universe, and a desire to learn about literature, music, society, the humanities and the arts and sciences, to find out how it all works, how it fits together and how to make it better.
For a faculty member, this raises an interesting question: what in my field could spark the curiosity of such an incoming student. As a physicist I have to ask: what makes physics interesting and fascinating? In fact, there were times when some considered physics a nearly complete subject, with nothing new to explore, and therefore not worth pursuing. Albert Michelson, who would later become the first American to win a Nobel Prize in physics, is quoted as saying in 1894 that "the grand underlying principles have been firmly established ... further truths of physics are to be looked for in the sixth place of decimals." In other words: boring... Around the same time, Max Planck was advised by Prof. Dr. Philipp von Jolly, a physics professor not to study physics, arguing that "in this field, almost everything is already discovered, and all that remains is to fill a few holes."
But Planck ignored that piece of advice, of course, and both Planck and Michelson went on to make discoveries that laid the foundations for two completely new and mysterious underlying principles, quantum mechanics and relativity. A whole new generation of physicists came in, revolutionized the field, and firmly established what we call 20th century physics.
But now, you realize, we live in the 21st century, and you wonder: are there any new underlying principles to be established? Any more physics revolutions to look out for? Having mentioned quantum mechanics and relativity, I could point out that physicists are still struggling to unify the two theories, to develop a theory of quantum gravity. That would be a revolution. Instead, let me tell you about something different; a truly unexpected and remarkable discovery that, just like the discoveries of Planck and Michelson, may well open the flood gates for another completely new understanding of the universe.
Until about 10 years ago or so, cosmology was not what you'd call an exact science. Cosmology is the study of the universe as a whole; cosmologists try to understand the structure of the universe, its past, including the "big bang," the cosmic explosion that was the beginning of it all, and also its future: will the universe expand forever, or will it "re-collapse," and end in a "big crunch?"
Actually, it's pretty easy to give you a little demonstration of how these two scenarios could happen. Imagine I throw something into the air, say, a marshmallow. If I throw that into the air, you know that it will keep slowing down, on its way up, until it turns around at some maximum height, and then comes back. Now imagine the Earth is one galaxy, the marshmallow another one; then you see them flying apart for a while, slowing down, or "decelerating" in the process, until the mutual gravitational attraction makes them turn around and re-collapse in a big crunch. Now, if I throw the marshmallow harder, it will climb higher. At some point it will hit the ceiling, but imagine there is no ceiling. In fact, in principle I could throw the marshmallow so hard that, while it would keep slowing down, it would never turn around and come back. That's what would happen if the marshmallow's speed exceeded the so-called "escape speed."
So, in our marshmallow model of the universe we could decide whether the universe will expand forever, or end in a big crunch, by measuring the marshmallow's speed and figuring out whether or not this speed is greater than the escape speed. Simple enough. As it turns out, though, the universe is slightly more complicated. You see, the escape speed depends on the properties of the Earth. On the Moon, for example, the escape speed is a lot smaller than it is here, because the gravitational attraction is weaker. We don't really know what the gravitational attraction between different galaxies is, so we need one more piece of information. That piece of information is the rate at which the expansion is slowing down, the so-called "deceleration." For the marshmallow, we could measure the speed and the deceleration, and from those two pieces of information we could decide its fate without any information about the mass or radius of the Earth. Similarly, if we can measure the expansion and the deceleration of the universe, we can figure out its age, and its fate: will it expand forever, or end in a big crunch?
We know that the universe is expanding, because far-away galaxies move away from us. The further the galaxy is away, the faster it's moving away from us — that's what Edwin Hubble discovered 80 years ago in 1929, and that's what you'd expect if the whole universe is expanding. Measuring the rate of this expansion proved to be quite difficult, though, and it remained impossible to establish an accurate value for the next 70 years.
Only about 10 years ago did high-precision experiments become available, that allowed astronomers to determine the rate of the universe's expansion very accurately. From a number of complementary experiments, we now know that the universe's current rate of expansion is approximately 72 km/s/Mpc. That's the famous Hubble constant.
Now that we know the universe's rate of expansion, we would like to know the rate at which this expansion slows down, i.e. the universe's deceleration. That's the grand prize: once we know both the expansion and the deceleration, we can figure out how old the universe is, whether it will end in a big crunch, if so, when, whether there is any point in paying the credit card bills — all of that. So several groups of astronomers attempted to measure the rate of deceleration, and found something truly remarkable: the universe's expansion is not slowing down, it is speeding up! Going back to our marshmallow analogy, it's as if the marshmallow would not only not turn around and come back, it would keep speeding up as it's moving away from us. Just as counterintuitive as that seems to you, that's how counterintuitive the accelerating expansion of the universe is to physicists. The gravitational attraction between galaxies is supposed to slow down the universe, but apparently there is something else going on.
What have we learned from that? Well, we have learned that the universe will never recollapse; instead it will continue to expand forever, at an ever-increasing rate. We also know the age of the universe — about 13.7 billion years. But, perhaps most importantly, we have learned about the extent to which we do not understand the universe. If the universe is accelerating, despite the gravitational attraction between galaxies, that means that some other force is at play, or that the universe is filled with some matter with some very unconventional properties.
Cosmologists describe the situation in terms of different components of the universe. There are things like photons, for example, light and radiation — but in today's universe they play only a very minor role. Then there is ordinary matter, anything made from atoms, the stuff that we are familiar with — our bodies, the Earth, stars, etc. — but that makes up only about five percent.
The remaining 95 percent are completely unknown. Even though we have no idea what this stuff is, we can give it a name. In fact, there are two different kinds of stuff, which behave differently. Astronomers have called one "dark matter," and the other "dark energy." Dark matter behaves, in terms of gravitational forces, as ordinary matter. Therefore it is only mildly mysterious, but still we do not know what it is. Perhaps some elementary particles? Perhaps black holes? Whatever it is, it makes up about 25 percent of the universe. The rest of the universe, a whopping 70 percent, is what cosmologists now call "dark energy." Dark energy is the really weird stuff; it is responsible for the acceleration of the universe, and its properties are very different from anything you can buy at a corner store. What it is, we have no idea. This is truly unsettling, if you think about it long enough: we understand only about five percent of the matter in the universe; the remaining 95 percent are completely unknown. We certainly live in a strange and curious universe.
It almost seems like hardly anything has been discovered, and clearly there is much more than "just a few holes" to be filled. Perhaps a whole new set of underlying principles is waiting to be found. Perhaps it is time for a new generation of physicists to establish the principles of 21st century physics. That might be you!
I have talked about physics and astronomy, because that's what I am most familiar with, and because they are dear to my heart. But what I would like to say is intended much more broadly: every field of study has its cutting edges, with exciting discoveries just waiting to be made. And every field will benefit from a new generation of scholars, bringing a new and fresh perspective. It will probably be hard to get to those cutting edges; it may take years of study, lots of work and even more persistence. But it all starts with curiosity.
So, I hope that you will let your curiosity guide you, and I hope that you will find a subject that will spark your enthusiasm and will lead you to its cutting edges. And should you figure out the workings of the universe in the process, please do drop me a note.
Thank you for listening, and again: welcome to Bowdoin.