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It's About Time

Poster art by Karen Teramura.

How have various cultures throughout history measured time? How has the theory of relativity affected our understanding of time? How do we know the age of Earth and the Universe? Four University of Hawaii at Manoa faculty members explored the concept of time from its cultural beginnings through our current scientific understanding at the IfA's Frontiers of Astronomy Community Event, "It's About Time," on December 3.

Bob Joseph
Photo by K. Teramura.

Bob Joseph (IfA) explained that modern Westerners tend to see time as a linear progression, but that was not always so. Many early civilizations, as well as some non-Western ones today, saw time as a recurring cycle, generalizing from various well-known astronomical cycles, including phases of the Moon and the annual cycle of seasons. Christianity, which sees history as a series of unique events beginning with the Creation and progressing through the life of Christ to a final union with God, brought the idea of a linear progression of time into Western thought. The rise of the merchant class and the circulation of money, along with awareness that time is a valuable commodity that can slip away, further supported the idea of a linear flow of time. Another occurrence that furthered this trend was the invention and use of mechanical clocks in the Middle Ages. With clocks placed in central locations, such as on church steeples, "people became more aware of the continual passing of time," said Joseph. He also noted the importance of the definition of absolute time by the physicist and mathematician Isaac Newton (1643-1727), "Absolute, true and mathematical time of itself and from its own nature, flows equably without relation to anything external." This concept of time persists to the present day. Joseph closed by quoting 20th century physicist Richard Feynman: "Time is what happens when nothing else does."

John Learned
Photo courtesy UH.

John Learned (Department of Physics & Astronomy) picked up the story in 1905, when Albert Einstein published the special theory of relativity, which says that the speed of light is constant throughout the Universe and is independent of the motion of the observer and of the object emitting the light. Learned illustrated the relativity of time and space with the following example: Suppose that you left Earth on a spaceship that accelerated continuously at the gentle rate of 1 g. In about five years time (as perceived on the spaceship), you would be traveling at almost the speed of light. After about 11 years of spaceship time, you would have traveled 27,000 light-years, the distance to the center of the Milky Way galaxy. Back on Earth, 27,000 years would have passed. "Clocks do run faster or slower depending on your system motion: events in a moving system take longer to occur from the perspective of a system at rest," explained Learned.

Klaus Keil
Photo courtesy UH.

Klaus Keil (Hawai'i Institute of Geophysics and Planetology, HIGP) explained how we know the precise age of the solar system. Chondritic meteorites, which are fragments of primitive asteroids, contain so-called calcium-aluminum-rich inclusions (CAIs). These consist of minerals that are rich in elements with high boiling points and so must have condensed from the solar nebula at high temperatures. Thus, they were the first solid materials that formed and, hence, their age dates the beginning of the solar system's formation. Ages of such rocks are determined by measuring the amount of a radioactive element, such as uranium (the parent), and the element it decays to, in this case, lead (the daughter). Since scientists know the half-life of the radioactive parent (the time it takes for half of the parent to decay into the daughter), they can calculate the age of rocks containing these elements. At HIGP, a complex instrument called an ion microprobe is used to determine the decay products of short-lived radioactive elements such as aluminum with a mass of 26 (26Al), which has a half-life of 730,000 years and decays into magnesium (26Mg). These data, combined with absolute ages determined by the uranium-lead method, show that the CAIs, and hence the solar system, are 4.5672 billion years old.

Gareth Wynn-Williams
Photo by K. Teramura.

Gareth Wynn-Williams (IfA) explained how astronomers have estimated the age of the Universe and the implications for the future of time. Since light takes time to arrive at Earth, and since the light from more distant objects takes longer, scientists are able to see the Universe as it was at different ages by looking at objects at various distances. They have found that the Universe was denser in the past than it is today, and by working backwards have figured out when the Universe began. The expansion of the Universe validated the Big Bang theory and eliminated the possibility that the Universe is in a steady state (always the same size). Scientists theorized that after the Big Bang the Universe would continue to expand indefinitely unless it contains sufficient matter--and, therefore, gravity--to cause it to stop expanding and start contracting, which eventually would result in the Big Crunch. In the early 1990s, they were very surprised to find that not only is the Universe expanding, but that the rate of expansion is accelerating, and therefore the Universe--and time--will likely continue forever.