Back To The Future: An Inquiry of the History and Nature of Time

I. Introduction

Time is a two-sided topic. First, time can be considered quantatatively, in the physical sense. An exploration of time from this perspective entails questions such as the age of the earth and the universe, and how these can be ascertained. In this vein, we hope to offer a survey of the geologic aspect of time including stratigraphy and radiometric dating. The second aspect of time we shall address bridges the gap between the scientific and philisophical paradigms. To this end, we aim to shed light on the age and origin of the universe, the physical workings of time, and the nature of time.

II . The Age of The Earth

A. Early Theories: The Church and Time

Early studies of time are characterized by two theories. The first asserts that time is vectorial, meaning that time proceeds in a uniform manner. In geological terms this means that the formation of the earth occurred in successive periods. Each period leaves behind a unique sedimental layer. The layers accumulate, one on top of the other, becoming a chronological record of time. The second theory, in opposition to the first proposes a cyclical model of time, where time does not proceed in distinct successive periods. Rather, time unfolds in repeating identical periods. Each period produces several types of rock through varying episodes of heating and cooling. This cycle repeats explaining the variety of rock forms observable in the earth's crust . This theory troubled the religious because it raised the possibility that the earth is of almost infinite age. This division between ideas supporting and challenging the church defined the study of geology for many years.

B. Changing With the Times: Geology Separates From Religion

With the dawn of the industrial revolution in England, geology made major advances. The field of stratigraphy, a categorization of the earth's sedimentary strata through fossil remains, blossomed. Advances in stratigraphy standardized geological research enabling its emergence as a science separate from religion. The theory of catastrophism is a result of these advances. The theory is based on the observation that strata alternate in fossiliferous and non -fossiliferous layers. Catastrophism reasons that the non fossil layers were the result of catastrophes which interrupted the cycles proposed by the cyclical model of time. The theory of Uniformity refutes Catastrophism by arguing that the non-fossilierous layers were not the result of catastrophes, but rather the residual effects of time. The theory is labeled Uniformity because it assumes time to be uniform. Catastrophism and Uniformity are monumental in that they take the clash between cyclical and vectorial time out of the realm of religion, and into the world of science. While Catastrophism and Uniformity never preicted a given age for the Earth, the theories new aims and techniques set the stage for modern progress.

C. The Efforts of Stratigraphy

After a considerable lull in the study of time through geology, great progress began to be made in the field of stratigraphy. The first stratigraphic cross section was developed by relating fossil remains to the rocks they are found in. By comparing the fossil remains in different areas it became possible to label strata containing similar fossil remains. In this way many strata were labeled and a map of the strata began to form . It was clear that a strata of certain thickness represented a corresponding division of time, however there was no way to measure the time.

D. Radiometric Dating and the Age of the Earth

The first step in radiometric dating was the discovery that uranium emits radioactive rays. The second step in the revolution of radioactive dating was the realization that radioactive decay, the property that causes certain elements to emit radioactive rays, is an unvarying constant, and therefore can be harnessed to tell time (see figure 4). However, even if we can measure the amount of Uranium in a given sample, to tell time we must know how much Uranium existed originally. The residual method proved to be the solution to this problem. Since uranium produces eight helium atoms for every atom that decays you can add the number of helium atoms divided by eight to the number of existing uranium atoms to calculate the original uranium content. A simple analogy can be made to an hourglass. An observer can determine how much time has passed by subtracting the amount of sand at the bottom of the hourglass to the original amount of sand. The original amount of sand can be ascertained through the residual method by adding the amount of sand in the bottom of the glass, which represents the radioactive decay Helium in the analogy, to the amount of sand at the top of glass, which represents the amount of the remaining parent isotope Uranium. The residual method was applied to several samples. One sample gave a date of 1.5 billion years which was then accepted as a minimum age for the earth.

The next progress in radiometric dating was the discovery that the ultimate product of the decay of uranium is lead. Measuring specimen containing lead, a two billion year-old sample was discovered, and so, a new minimum age for the earth was discovered. However, more important than this discovery, was the exploration of the uranium lead cycle. Using the mass spectrometer, a device capable of measuring the isotope proportions of a given sample element, scientists uncovered the workings of Uranium's radioactive decay. They discovered two different uranium isotopes, and four lead isotopes. One Uranium isotope, 235u, disintegrates into the Lead isotope 207Pb at twenty times the rate that a second Uranium isotope, 238u, breaks down to a second Lead isotope 208Pb. This enabled scientists to take more accurate measurements of the age of a specimen, and also to check their results. To extend the hourglass analogy geologists now had two hourglasses to measure the age of the earth instead of just one. A third method of radiometric timetelling was established with the information that 207Pb accumulates a twenty times the rate of 208Pb. This discovery made it possible to measure the age of a specimen without calculating the original Uranium content. To refer again to the hourglass scientists could measure time passed without regard for the total amount of sand in an hourglass. Instead, by using a ratio between two hourglasses they could detrmine how much time has passed. Three methods of radiometric dating, each one validating the next, enabled extremely accurate measurements of the age of geologic samples.

E. Radiometric Dating and the Age of the Earth

Despite the great advance in radiometric dating the problem remained that it could not be used to determine anything more than a minimum age for the earth. Since the earth is an active planet its rocks are constantly being recycled. Geologists do not have easy access to rock samples that are as old as the earth. To circumvent this problem researchers measured the Lead content of marine sediments, which are considered to be an average sample of the earth's crust. Assuming that all 206Pb or 207Pb is the product of Uranium they obtained a maximum age of four billion years for the earth. Other scientists were able to make more accurate readings by studying meteorites, which are not part of a living system. Studying meteorites, scientists obtained an age of 4.6 billion years for most meteorites, and so it is now accepted that the planets of our solar system formed 4.6 billion years ago!

III. The Age of Our Universe

A. Radiometric Dating and the Age of the Universe

The age of the earth is merely one chapter in the history of time. Radiometric dating can be harnessed to give a picture of the full history of time, i.e. the age of the universe. Because there are no available samples from the formation of the universe, scientists must take an indirect path to predict the age of the universe. Instead of measuring the ratio of parent and daughter isotope in a sample (the hourglass method), scientists studying the age of the universe compare present day isotope abundance ratios to the computed abundance ratios for red giants to determine when the heavy elements were formed. This approach is based on the idea that because two different isotopes have different half lives, their ratios will change with time. However, scientists can not use this approach and derive a number from a simple ratio because this would imply that all the heavy elements were created in one super-nova explosion. Instead, heavy elements are constantly being formed in super -nova explosions. If we assume that super-nova explode at a regular rate through time, the abundance of a heavy element increases with time until it is counteracted by radioactive decay. At this point the isotope abundances reach equilibrium. Three isotopes 235u, 238u, and 232Th are used to apply this principle. The ratios at the formation of the planets 100, 345, and 818 respectively, are compared to the steady state numbers of 100, 410, and 2460. The 235u is 100 in both cases demonstrating that it reached equilibrium at the formation of the planets 4.6 billion years ago. 238u has a ratio of 345/410 demonstrating that it is about 80% of the way to equilibrium, whereas 232Th has a ratio of 818/2460 showing it is roughly 30% of the way to steady state. By plotting the ratios for these two heavy elements against 235u from time 0 up to the formation of the universe, the time elapsed between the start of the universe and the formation of the planets can be measured. In both cases the time elapsed is roughly ten billion years. This gives an age of 15 billion years for the universe.

B. The Red Shift and Hubble

Another process that gave way to modern estimates at the age of our universe is how the red shift is observed in stars and galaxies. To understand this, we must realize that when a source is moving towards us and sends out a signal, the frequency is higher (the time between wave crests is smaller). When a source moving away sends a signal, the frequency of the waves we receive is lower. Because the lowest frequencies appear at the red end of the spectrum, the red shift implies that objects (such as stars) moving away from us will have their spectra shifted toward the red end of the spectrum.

During the 1920's, Edwin Hubble observed this phenomenon while studying the spectra of galaxies. He found that all galaxies were red-shifted, meaning that they were all moving away from us. He also found that the red shift or speed moving away from us and the distance the galaxy was from us were directly proportional. This discovery in 1929 shocked the scientific community and the world alike because since early developments in science the universe was thought to be static. In addition to dating the universe, Hubble's discovery provided a new theory about the origin of the universe, known as the "Big Bang."

C. The Big Bang

Centuries before the big bang theory was proposed, there were conflicting views about the origin of the universe and the makeup of outer space. In 340 B.C. Aristotle believed that the planets, the sun, the moon, and the stars all revolved around the earth. Aristotle, a non-believer in creationism, wrote that humans and the universe had existed and would exist forever. It wasn't until the 16th Century that Nicholas Copernicus theorized that the earth and planets orbited the sun, which he claimed was the center of the solar system. Moreover, in the Judeo-Christian world, traditions stated that the universe had a finite, not so distant beginning.

Although it took the big bang theory some time to catch on (and still has opponents), its fundamentals led researchers to clear great hurdles in knowing the origin of our universe. At the point just before the big bang, estimated to be about 15 billion years ago, all objects in the universe were infinite, and all at the same place. All the laws of science, as well as all ability to predict the future, were non-existent. It is important to state that events before the big bang have no consequences and are undefined and, most importantly, that the beginning of time occurred at the big bang.

We assume that just after the big bang the universe was dense, hot and glowing white. If this was true, we should be able to see this early light today due to the distances needed to arrive at our planet. The light, however, is so greatly red-shifted that it appears as microwaves, which provided the missing piece in the puzzle. This also was further evidence of how red shift showed that the universe is expanding. Since all galaxies are moving away from each other, there is no way to determine the center of the universe. This means that the universe must look the same in every direction, as seen from any other galaxy.

IV. Physical Variables in Our Universe

A. Space, Time, and Relativity

In our universe, events occur in space-time. To simplify this phrase, we shall briefly discuss dimensions. Space refers to the dimensions length, width and height. Combined with the concept of time, space-time is the four-dimensional space whose points are events. It has been proven that no such concepts as absolute space and absolute time exist. Einstein's theory of relativity ended the idea of absolute time, concluding that each individual must have their own measure of time. Simply stated, it says that the laws of science should be the same for all freely observers, no matter what their speed. Most importantly, nothing may travel faster than the speed of light (186,000 mi./sec). As an object accelerates, more mass is added which requires more energy for accelleration of the mass(mass=energy). By the time the object nears the speed of light, its mass is infinite. So, there needs to be an infinite amount of energy to propel it. However, since there can be no infinite amount of energy in our practical physical universe, this entire concept of absolute time is blasted!

V. The Nature of Time

A. Black Holes

A black hole is the modern term given to a collapsed star in space-time with tremendous gravitational force. Black holes are fascinating because anyone or anything that enters will never leave, not even light. The boundary is called the event horizon, the one-way "membrane" into the black hole. The interesting thing about the fate of the objects that enter the event horizon is that, as Hawking says, "...they will soon reach the region of infinite destiny and the end of time."

B. Arrows of Time: Distinguishing the Past from the Future

An arrow of time distinguishes the past from the future, giving time a direction. The three arrows of time philosophically rationalize why time is moving how it is, as well as how we know that the universe is still expanding. The first is the thermodynamic arrow in which disorder, or entropy, increases over time. The second is the psychological arrow in which we remember the past and not the future. Last is the cosmological arrow of time in which the universe is expanding rather than contracting. The thermodynamic arrow is related to the psychological arrow when we process information in our brains. We remember the past and store it, but the process of remembering uses energy, which is converted to heat. Therefore order is changed into disorder. The thermodynamic arrow is related to the cosmological arrow because in order for life to survive, disorder follows order during digestion. Humans must convert food into energy into heat, following the thermodynamic arrow. The thermodynamic arrow is unsuitable in the contracting phase, so we wouldn't be alive if this was the case. It now is apparent that, by comparison, the three arrows are pointing in the same direction of time in which our lives progress.

VI. Conclusion

Through studying the quantitative as well as the qualitative aspects of time, it is evident that we are very fortunate to be living on the verge of the Twenty-First Century. Our knowledge about physical properties as well as positive proof of philosophies and theorems would surely be enough to knock Newton's socks off! The challenge presented to the layman in regards to the specifics of time is not a simple one, but the deeper understanding of why our planet and universe is here, existing the way it does, is fascinating. We can only hope that through the inspiration of past minds and the passion of the thinkers of today, the answers to our greater questions will be provided.

Glossary

Absolute Time- The notion that time is constant.

Active Planet- A planet in which new geologic formations are being created.

Big Bang- The theory that the universe began at one point in space time.

Blach Hole- A collapsed star in space time that contains matter so densely packed that nothing, not even light, can escape its gravitational field

Catastrophism- The theory that geologic history proceeds in vast spans of rest periods interrupted by catastrophic events.

Daughter Isotope- A new isotope created by the radioactive decay of another atom.

Event Horizon- The one way membrane into a blackhole.

Isotope- Atoms of the same element with the same number of protons but a different number of nuetrons.

Parent Isotope- An atom in which radioactive decay produces new isotopes.

Radioactive Decay- The natural decay of atoms with unstable nuclei.

Space- The dimensions length width and height.

Space Time- The four dimensional space with time being the fourth dimension whose points are events in the universe.

Stratigraphy- The study of layerings of sedimentary rock.

Super-nova- An exploding star that releases energy.

Uniformity- A theory of geologic time explaining phenomena through the effects of time alone.