Theories of the Universe: The Relative Nature of Space and Time

The Relative Nature of Space and Time

Einstein's theory of special relativity is based on two fundamental principles. They were both covered in the last section. Can you identify what they are? Here's a hint. One has to do with light and the other with motion. Tick, tick, tick … time's up. Without saying what they are yet, I can tell you that they are both intimately connected with time. That was also touched upon in the last section, but we'll go into it in much more detail now.

Okay, I'm sure you know what the two principles are, but just in case, let's go over them. Light, it has been determined, always travels at a constant speed, regardless of its source or the frame of reference. The other is the principle of relativity, which basically states that motion is relative. The combination of these two principles show that time is also relative. And as previously discussed, you can't separate time from space. So if time is relative, space must be also. How is it that time is relative? You'll know that by the end of this section.

It's About Time

Isaac Newton wrote in his Principia:

• Absolute space, in its own nature, without relation to anything external, remains always similar and immovable. Absolute, true, and mathematical time, is itself, and from its own nature, flows equably without relation to anything external.

This concept of time seems natural enough. We all accept it 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. And, 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. Or “time's fun when you're having flies,” oops, I mean “time flies when you're having fun” is another. 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. Here's a list of just some of the modes of expression for time:

• Atomic time
• Geological time
• Biological time
• Astronomical time

Let's see how different each one of these are.

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.

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.