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November 16, 2010 > TechKnow Talk: Measuring the infinite: yardstick on the universe

TechKnow Talk: Measuring the infinite: yardstick on the universe

For as long as people have existed, we have gazed into the clear night sky and wondered, "How far, how fast, how big, how old?" The answers to these questions have remained unknown through the course of human history, but science and technology have now begun to provide some preliminary answers.

The key to many astronomical measurements is light, so let's begin there. Historically, light was thought by most to travel instantaneously from one place to another. However, during the Renaissance, scientists begin to take seriously the possibility that it possessed a finite speed. Early attempts to prove it, by opening a lantern and observing it in a distant mirror, for example, failed.

But the invention of the telescope allowed observation of objects much farther away, such as Jupiter's moons. Astronomers detected a slight difference in the orbital period of these moons, based on whether the Earth was moving toward or away from them. Such observations were advanced as proof of the finite - though extremely fast - speed of light as early as the 17th century.

A variety of more sophisticated methods were employed to refine the estimate of light's speed over the next 200 years, and by the early 20th century, the speed of light had been calculated to within one-tenth of one percent of the currently accepted value. Today, very precise measurements of light speed are based on laser interferometry techniques, which involve splitting a laser beam into two paths of unequal length, then recombining them and studying the resulting interference patterns.

It is a mind-boggling speed. In one second, light travels about 300,000 kilometers (186,000 miles), or more than seven times the circumference of the Earth. No wonder early researchers were unable to notice a delay in a mirror on the next hill!

When the Apollo astronauts were on the moon, communications were delayed by 1.3 seconds in each direction - the speed of light over the intervening 240,000 miles. When Mission Control in Houston asked a question, they had to wait about three seconds for a response.

As we peer deeper into the universe, the speed of light helps us understand the vastness of even our local neighborhood. Light from the sun reaches Earth in about eight minutes, but it travels more than four years to arrive here from the next nearest star.

The distance light travels in one year is called a "light year," so the distance to the nearest star is about four light years. Our Milky Way galaxy is about 100,000 light years across and most other galaxies are millions of light years away. The human mind is utterly incapable of imagining such enormous distances.

As the term implies, since light from distant objects has taken so long to reach us, we are in effect looking back in time when we observe them. For example, a nova (or exploding star) observed in 2006 was about 5,000 light years away. So we can think of that event taking place some centuries before the great pyramids were constructed in Egypt. A supernova visible on Earth in 1987 was about 170,000 light years away, having occurred when our remote ancestors were using crude stone tools to hunt African game.

How are these incredible distances measured? Again light plays a key role. As a siren approaches a stationary observer, the sound waves are compressed, increasing the pitch. As the source of the sound passes and recedes, the sound waves expand, reducing the pitch. Fortunately for astronomers, the same phenomenon occurs with light.

Light from objects moving away from us is shifted to a longer wavelength. The faster the objects are receding, the more pronounced this "red shift." When astronomers measure red shifts, they find that almost every object in the universe is moving away from us! The further away a celestial body is, the faster it is receding.

How can this be? It is because the universe is expanding. Imagine marking some dots on a deflated balloon. As the balloon is inflated, the dots all move away from each other. Another common analogy is a loaf of raisin bread. As the loaf is baked it expands, and the raisins all move away from one another, at a rate proportional to their separation.

Thus the distance of a luminous object can be determined by measuring its red shift. Such measurements are confirmed by observing the brightness of certain types of nearby objects, then measuring the brightness of similar objects far away. The brightness decrease is proportional to the inverse square of the distance. Certain types of variable stars, which behave like cosmic lighthouses, are also useful in confirming distance measurements.

Cosmology, the study of the nature and evolution of the universe, seeks answers to "big questions" that sometimes blur the boundaries between science and philosophy. It is a field of considerable current research.

Observing distant objects and measuring how fast they are racing away from us allows cosmologists to calculate the expansion rate of the universe. Simplistically, if there is enough mass in the universe, it will eventually cease to expand and gravity will draw it all back together. If there is insufficient mass to arrest the expansion, it will continue expanding forever.

Current thinking is that the expansion rate may be increasing and there is just about the right amount of mass, called critical density, to eventually stop it, or nearly so. But that is at best an informed guess and is complicated by the apparent presence of so-called dark energy and dark matter-so named because forces we cannot see seem to be influencing the expansion rate.

Looking back in time instead, can the expansion be simulated "backward," to pinpoint the time when the universe began? Such calculations, based on several assumptions, yield an age of about 13.7 billion years. The orbiting Hubble Space Telescope has recently imaged objects as far away as 13 billion light years that, if these calculations are accurate, provide a view of the universe in its infancy.

Is the universe finite or infinite? The short answer is that no one knows. Infinity is as much a philosophical concept as a scientific one. Cosmologists tend to think of an infinite universe as one that expands forever. But that begs the question of whether such a universe is infinite always, or only after an infinite period of expansion. Into what, or where, does an infinite universe expand? Are such questions even meaningful?

And what if the three dimensional space we perceive is merely a cross-section or intersection of higher-dimensional universes, as some cosmologists believe? Can a three dimensional infinity exist inside a "larger" space of four, five, six, or more dimensions? This is probably a good time to cue the theme from Twilight Zone. The more we learn about the universe, the more evident it becomes that fact is far stranger than science fiction.

As with most scientific explorations, each cosmological answer has given rise to a host of new questions. For example, will two parallel lines extended into space stay parallel forever, or will they eventually intersect? This simple question has profound implications regarding the structure of the universe, its governing physical laws, and its ultimate fate.

The knowledge we have gained of the cosmos is nothing compared to the depth of our remaining ignorance. Opportunities abound for future generations to discover answers to our most ancient questions and to shape our understanding of the fundamental nature of the universe.


After five years and 58 TechKnow Talk columns, the TechKnow Guy is retiring. Many thanks for your questions and comments. Stay curious and never stop exploring!

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