Theories of the Universe: The Concept of Time
Of all of the theories of the Universe, the one of time is the most enigmatic and compelling. In this guide, we will look at the meaning and perception of time, and its different categories.
The Meaning of Time
We all accept time almost unthinkingly. But is the universe only a great big clock? Is time not related to anything else but itself? Every idea of time that we can possibly think of is deeply connected to a concrete physical event: the swing of a pendulum, the vibrations of a quartz crystal, the orbit of the earth, the quantum leaping of atoms, the motions of magnetic and electric fields, the lives of suns, your lunch with your friend, and on and on. Without such events, what would time consist of? You can’t have time in a void because there would be nothing to relate it to. Time makes sense only when it’s connected to something.
Here’s another thought experiment. Imagine that a rock is the only inhabitant of the universe, and ask yourself these questions:
- What position does it have? Does this question have any meaning? No, because position can only be defined with respect to another position or thing.
- How about what size is it? Again we have nothing to compare it to.
- Does it continue to exist? Well, it doesn’t change, there’s nothing that’s going to come along and impact it.
- So how can we tell the passage of time? You can’t. Both space and time have no meaning at all.
That’s a rather extreme scenario but one that illustrates the fact that it’s the interaction of things, objects, people—all of the things that exist in the universe that provides a framework in which time and space have meaning.
The Perception of Time
Of course, we must also ask if time has any meaning for anything except us. Is time meaningful to animals, plants, or gods?
Time, most importantly, is a form of perception. We often speak of our “sense” of time. Just as there is no such thing as color without the eye to discern it, so an instant or an hour or a day is nothing without an event to mark it. We go about marking and discerning in a variety of ways. Perceptions of time change from country to country, from person to person, and even within us, from time to time. “A watched pot never boils” is a statement about the relativity of subjective time. Because we measure time by events that mark it, it should not be surprising that our sense of time is intimately influenced by the nature of the events themselves. Time also changes with perspective. This can be best explained by looking at the different ways in which time is categorized.
Atomic Time
The world of subatomic particles expresses extremely short, precise periods of time. The atomic year of hydrogen, for example, is 10 to the minus sixteenth power (that’s one divided by the number 10 with 16 zeroes after it). This is infinitely short compared to an earth year. Yet the atomic year is incredibly long compared to the life span of many nuclear particles, which are millions of times shorter. The time scale at this microcosmic level is often counted in nanoseconds. And another area where time is this short is inside of a computer. For example, it may take a signal 12 nanoseconds to get from one place to another, which in some cases is a long time, but very short compared to snapping your finger.
Radioactive Time
Radioactivity is another form of atomic time. There are certain forms of atoms that are unstable and after a while decay into more stable forms. For example, the nucleus of a carbon atom contains six protons and six neutrons and is known as carbon-12.
There is also another form or isotope of carbon, carbon-14, whose nucleus contains six protons and eight neutrons. During the process of radioactive decay, one of the extra neutrons emits a negatively charged electron and becomes a positively charged proton. Presto chango, abracadabra, the unstable carbon nucleus had changed into a stable nitrogen nucleus with seven protons and seven neutrons. What’s really cool about this is that every 5,700 years, exactly half of a given number of carbon-14 nuclei will decay into nitrogen-14 nuclei. And every year after that the amount will be half of the year before. In other words, if you started with one million carbon-14 atoms, 5,700 years later you would have 500,000 carbon-14 atoms. And 5,700 years after that you would have 250,000 carbon-14 atoms. That period of time is called the half-life of carbon-14 atoms.
Geological Time
In geological time, an instant can be 10 million years because that amount of time is just 1/450 of the Earth’s history. A thousand years is an interval so short that it has little meaning to geologists and is only a passing moment. Geology breaks down the timeframe on earth based on principle physical and biological features. The four eras, which are the largest spans of time, covering hundreds of millions of years, are as follows:
- Precambrian—4,500,000,000 to 600,000,000 years ago
- Paleozoic—600,000,000 to 280,000,000 years ago
- Mesozoic—280,000,000 to 135,000,000 years ago
- Cenozoic—135,000,000 to 12,000 years ago
Each of these eras is broken down into periods or systems, and each period is further divided into epochs or series. The divisions are based on changes in the Earth, formation of landmasses, glaciations, and the development of different species and the extinction of others. It’s not that dissimilar from biology in the way that everything is classified and categorized according to certain chief features.
Biological Time
You may have heard some women refer to their biological clock ticking away. This usually means that they are ready to have children or that time is running out to have children. This analogy can be applied to other things in nature, too. (I’m sure nature doesn’t have that on its mind.) There are all kinds of biological clocks. All living things need to tell time to survive, to coordinate their internal functions with the clocks of the outside world, to know when to hibernate, fly south, sprout, shed, or grow a winter coat. Our hearts need to know when to pump blood and our lungs when to breathe. Different organs in the same body may keep different times to different kinds of clocks, releasing chemicals in concert with communications from a central brain. Stomachs, livers, and sleep centers, may all tick to different, yet coordinated, times.
The limits of our ability to perceive time intervals even determine how we see the world. If we could sense intervals shorter than one twenty-fourth of a second, which we can’t, you would see the dark gaps between the frames of a movie. If we could perceive much longer intervals of time, on the other hand, we could actually be able to watch plants or children grow.
Our experience of time definitely seems to change as we grow older. Some people think that this quickening sense of time depends upon the diminishing percentage of a lifetime that each hour or year takes up. To a year-old baby, a year is a lifetime—an eternity. To a 10-year-old, a year is but a tenth of his or her lifetime, and each hour is proportionately shorter. When we reach 50, time is passing five times faster still, and a year is only 2 percent of our lives. And if we reach 100, a year is only 1 percent.
Astronomical Time
The time interval we call a year is marked by a single revolution of the Earth around the Sun. Our day is a single spin of the Earth around its axis and, long before we showed up on Earth, the month was very likely matched by the orbit of the Moon around the Earth. It doesn’t any longer because the Moon’s orbit is continually changing as the Moon moves further away. None of these astronomical measures could be considered absolute because they are constantly changing. Some 500 million years ago, our day was only 20 and a half hours long.
Let’s leave our earth perspective for a moment and consider the concept of time on the planet Mercury. There the day is longer than the year. In other words, it takes Mercury longer to revolve on its axis than it takes for it to orbit once around the Sun. Talk about a confusing birthday party! We get so accustomed to thinking of days as the natural division of years into 365 parts that it’s easy to forget that the day, like the year, just happens to define time relative to our planet’s unique position in relationship to the Sun.
An important point to discuss before I finish this section on astronomical time is the concept of distance. In mathematics, there is a formula that defines the relationship between distance, speed (or rate), and time. Distance equals rate times time (d = r × t). Any time you move from one place to another, a certain amount of time expires. For example, I can walk from my house to the library. If I walk at three miles per hour and it takes me two hours to get there, I’ve covered a distance of six miles. So distance is the product of my rate, or the speed at which I travel, times the duration of time it takes for me to get from one place to another. This is all directly related to space and time through the absolute speed of light. Let me explain.
In order for us to locate ourselves in the universe in relation to all of the other billions of celestial bodies around us, we need to know how far away they are. Powerful astronomical telescopes and radio telescopes can locate planets, stars, black holes, quasars, and other galaxies. By locating their point of origin, or source of light, and other electromagnetic radiation, the distance can be calculated by how long it takes for the light to reach us. The speed of light is the measuring tool used to find out how far away these celestial bodies are. For example, it takes the light from the sun approximately eight minutes to reach us on earth. The light from some of our neighboring planets can take weeks and months to get to us. After that, distances and time are measured in light years or the distance that light travels in one of our earth years. That’s pretty far away. Our closest star, Proxima Centauri, is four light years away, or 24,000,000,000,000 (twenty-four trillion) miles.
As you can see, the concept of time is nearly as vast as the Universe itself—and twice as precious!
From The Complete Idiot’s Guide to Theories of the Universe by Gary F. Moring, M.A.
