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Date: February 23, 2011

Title: More Things in Heaven and Earth

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<Organization: Quest University Canada

Links: My home page : http://www.questu.ca/academics/faculty/rob_knop.php

Description: Basic physics and astronomy have a long history of informing each other. The first concrete example was Newton’s Law of Universal Gravitation, describing the orbits of planets and the dropping of apples as the same thing. Some atomic elements were first discovered in astronomical objects. More recently, observations of astronomical objects have pointed to exotic properties about fundamental particles known as neutrinos. And, there are pointers to other things we may learn about basic physics as a result of observations originally made in astronomy.

Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. He now teaches physics the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Greg Dorais, and is dedicated to the Chabot Space And Science Center in Oakland California, home of Bill Nye’s Climate Lab, Space Explorers Summer Camp, and so much more. At Chabot Space And Science Center, the universe is yours to experience. Set amid 13 trail-laced acres of East Bay parkland, with glorious views of San Francisco Bay and the Oakland foothills, Chabot is a hands-on celebration of sights, sounds, and sensations. Find out more about the Chabot Space And Science Center at www.chabotspace.org.

Transcript:

More Things in Heaven and Earth

“There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.” When Hamlet spoke that line, it was because he had just seen a ghost. He was talking to the rational and hard-headed Horatio– the closest there is to a “natural philosopher” in Shakespeare’s play. However, I’m not going to talk about ghosts today. Instead, I’m going to talk about some of the things that natural philosophers– what today we call scientists– have learned about our world by observations of the heavens. I am Rob Knop, professor of physical science at Quest University Canada. Thank you for listening to 365 Days of Astronomy!

Ancient scientists believed that the imperfect realm of Earth, and the perfect realm of what we see in the skies, were governed by different laws, or at least that they were in different categories. Things are complicated down here on Earth. Moving objects seem to come to rest; rocks, mountains, lakes, all have knobby and irregular shapes; over time things decay. In contrast, if you look at the sky, you see things stay the same for a very long time. Stars rise and set in their regular pattern every day like clockwork– no, not like clockwork, better than clockwork! Ancient astronomers developed models for the operation of astronomical objects based on these seemingly obvious observations.

At the outside of everything was what was called the Celestial Sphere. This was a sphere, rotating once every 24 hours; a firmament that bounded our Earth from the glorious light of heaven beyond. The stars were pinpricks in this sphere, allowing the light of heaven to shine through. Inside this were other large spheres, riding on which were the objects observed to be moving through the skies: the Sun, the Moon, and the planets. (To the ancient astronomers, the Sun and Moon were themselves planets, and a planet was a “wandering star”. Next time somebody tells you that Pluto should be grandfathered in as full planet status, remember that the Sun and the Moon have a much earlier claim for that grandfathering!)

Today, we understand that the same laws of physics that govern the interaction of everyday objects here on Earth, as well as of atoms and particles, are the laws that govern the motion of astronomical objects. This knowledge, however, is only a few hundred years old, and can be traced back to Isaac Newton. It was his law of universal gravitation that unified the motions of objects in the Solar System with the motion of objects falling on Earth. Decades earlier, Johannes Kepler had produced his empirical laws that gave descriptions of how the planets move. Newton explained them with his theory of gravity, and showed that the same theory of gravity could explain why objects on Earth fell at the same rate regardless of their mass. The true nature of Newton’s gravity is very difficult to observe if you only have the motion of objects near the surface of the Earth to measure. However, by looking at how planets and comets orbit the Sun, you can figure out how gravity changes as distance between objects change. This was perhaps historically the first quantitative case of astronomy providing the data that allowed us to figure out how basic physics works. It was far from the last case, however.

Helium is the second lightest element, after Hydrogen. You’re probably familiar with its use in balloons. As it’s lighter than air, helium balloons are buoyant and float upwards unless confined with a string. If you breathe in some Helium– not the most healthy of things, but many of us have done it– your voice acquires a high-pitched squeak, as a result of the higher speed of sound in Helium compared to in air. Have you ever thought about the name of this element though? Have you noticed that it sounds very much like Helios, the personification of the Sun from ancient Greek mythology? Indeed, it turns out that Helium was first discovered not here on Earth, but in the Sun. Astronomers can observe the signatures of specific atomic elements in astronomical objects through a technique called spectroscopy. In the Sun, you can see the signatures of a lot of elements. Hydrogen, of course, but also oxygen, sodium, iron, and others. The spectroscopic signature of Helium was first observed in prominences on the Sun during a solar eclipse in the 19th century. This signature didn’t correspond to any element known on Earth, and so it was named after the Sun. It was only later that we isolated Helium on the Earth and recognized that it was the same thing that had originally been identified on the Sun.

Another proposed element first observed in astronomical objects turned out not to be quite as new as originally thought. The element Nebulium was introduced in the 19th century to explain some blue-green wavelengths of light observed from nebula, but which had not been observed from any earthly element. It would turn out that this spectroscopic signature was not from a new element after all, but rather from an unusual form of everyday Oxygen. In the nebulae where these wavelengths show up, the gas is bathed in plentiful ultraviolet light. The photons in this ultraviolet light have enough energy to ionize the elements– that is, to rip off one or more of their electrons. What was identified as Nebulium turned out to be Oxygen that had had two of its electrons ripped off. There’s more to the story than that, though. It turns out that most of the time on Earth, even if you manage to doubly ionize Oxygen, you still don’t see these particular wavelengths. In order to see them, you need not only to ionize Oxygen, but it has to be at a very low density– a density that you only find in space.

Astronomy and physics continued their exchange of information in the early twentieth century when the discovery of nuclear fusion explained a mystery that had been developing about the Sun. It’s been less than a hundred years that we knew what it was that allowed the Sun to shine and keep shining. The answer is in fact nuclear fusion; at the core of the Sun there’s a furious, ongoing thermonuclear explosion, producing all of the energy that we ultimately see shining out from the surface of the Sun. Before fusion was discovered, however, astronomers’ best model for what powered the Sun was gravitational contraction.

When you drop something, energy is released. Objects that are farther apart from each other have gravitational potential energy, and that gravitational potential energy can be converted into other forms if the objects are allowed to come closer together. On earth, you see that most obviously when you drop an object; just before it hits the ground, it’s moving much faster than just after you dropped it. The gravitational potential energy was converted into energy of motion. The model for energy production in the Sun was that the Sun was slowly shrinking. In doing so, its gravitational potential energy heated up the gasses of the Sun, allowing it to stay hot as it gave off energy in the form of light. By the twentieth century, however, this model was in trouble. The problem was that the known mass and size of the Sun wasn’t enough for it to provide sufficient energy for a long enough time to power it for as long as we knew it had to have been around. Time scales from geology and evolutionary biology– as well as from other areas of astronomy– told us that the Sun and other stars needed to be shining longer than could be supported under this model. However, we didn’t have any answer for how the Sun could be shining.

Nuclear fusion solved that problem. Today, we know that a star the size of the Sun has a lifetime of about 10 billion years given the fuel for nuclear fusion it has at its core. Indeed, we know that our Sun is just about halfway through that life span.

One of the products in nuclear fusion are particles called neutrinos. These are elusive particles that almost never interact with regular matter. Indeed, they were first proposed to explain energy that seemed to be missing from reactions between basic particles, such as in the decay of the neutron. Physicists were loathe to introduce an unseen, and possibly unseeable, particle in order to explain seeming violations of conservation of energy, momentum, and angular momentum. However, it turned out that neutrinos were real. They do in fact interact with regular matter, it’s just that they do so very infrequently. If you hold up your thumb, about 60 billion neutrinos from the Sun are passing through it every second. Have you ever noticed? That you haven’t is a testament to how unlikely it is for a neutrino to interact with that thumb-matter. The vast majority of neutrinos that hit the Earth pass right through it as if it weren’t even there.

It was physics that informed astronomy when the discovery of fusion provided a mechanism whereby we might understand how the Sun powers itself. And, indeed, this produced the prediction that there should be neutrinos coming from the Sun. These were first observed at the Homestake Mine in South Dakota in the 1960s. It turned out there was a problem, though. Only about a third as many neutrinos were observed coming from the Sun as there ought to have been. This discrepancy was known as the “solar neutrino problem”, and wouldn’t be solved for a few decades. The eventual solution would turn out to be another case of observations of astronomical objects informing basic physics.

There are three kinds of neutrinos, called the electron, muon, and tau neutrinos. Electron neutrinos are associated with electrons, a familiar particle, and are what is produced inside the Sun. However, it turns out that when a neutrino is traveling from one place to another, it isn’t able entirely to maintain its identity about what sort of neutrino it can be. An electron neutrino may turn into a tau neutrino, or a muon neutrino, by the time it is observed on Earth (if it is one of those very rare ones that does get observed). The distance from the Sun to the Earth is enough that neutrinos are able to completely lose their identity, and have a one third chance of becoming any of the three types of neutrinos. The original measurements of neutrinos were only sensitive to electron neutrinos. This is why they only saw a third as many neutrinos as they should have; two thirds of them had turned into something else by the time they reached the Earth. Because of these observations of the Sun, we now know that neutrinos are not massless as once thought, but have a very tiny mass. We also learned something very extraordinary about neutrinos themselves– that they can turn into other types of neutrinos.

How will astronomy inform fundamental physics in the future? There are a couple of ways we can guess. Astronomical observations have made it assured that an exotic form of dark matter exists. Physicists haven’t directly discovered the particles that make up this dark matter, but it is assuredly only a matter of time. Also, around the same time that the solar neutrino problem was solved, it was discovered that the expansion of the Universe is accelerating. “Dark energy” is the name given to whatever it is driving this acceleration. We don’t know what that is, and it’s not clear how particle physics can even address what it might be, but it’s a pointer to some sort of new physics. On another front, we expect that there ought to be gravitational waves. Our theory of gravity predicts these ripples in space, but we haven’t yet observed them. When we do, almost certainly they will be waves that resulted from merging black holes in the distant universe. And, of course, there are the things that nobody has even thought of. Given the long history of astronomy giving evidence for new things in physics, it’s only reasonable to suppose that it will happen again in ways we haven’t imagined.

End of podcast:

365 Days of Astronomy
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