As previously mentioned, in Newtonian cosmology space and time were considered to be fixed entities. I use the word entity because both space and time are qualities of the universe whose existence, at least for us, is defined by these two terms. They are two things that are experienced by each of us all of the time, at least while we're awake. They form the very fabric of what we exist in, but defy clear definition. When we use words like holes in space, or tears in the fabric, even warping space—as you'll see when we get to general relativity—we have to ask, what exactly is being torn or warped?
The space-time continuum is made up of four dimensions: three of space—length, width, and depth, and the fourth, time. It allows any event in our world and the universe to be located. We are normally aware of moving through space, but don't often consider time as one of the dimensions we operate in—it's more of a commodity or separate thing. However, if you want to meet someone for lunch or see a movie, this involves a location in space as well as time. You really can't separate one from the other.
Time is easy enough to measure, we have clocks for that purpose. But all they do is measure time. What is time? How fast is it going by? If time stops at the speed of light, as Einstein's theory of special relativity explains, (and which we'll get to in the next section) that means that it has to be moving by right now … from moment to moment. Well? Exactly how fast is it going by? As you can see, the space-time continuum, as it is referred to, has aspects of it that are conceptually difficult to grasp.
Classical physics regarded space as a continuous expanse, extending in all directions. And as previously discussed, was found not to contain the ether that electromagnetic waves moved in. That was still a big puzzle as long as light was considered to be a wave—if it wasn't, then no big deal. Once light was shown to also behave as a particle, there was no need for any medium like ether for it to move in. But in any case, space was still considered to be unchangeable. And for now we'll leave it at that.
It's important to point out that the speed of light is constant at 670 million miles per hour in a vacuum, like outer space. The speed of light does vary when traveling through different mediums such as air or water. But the decrease in speed is negligible so, for our purposes and discussion, the speed of light is considered to be a constant.
Time was also considered to be an unchanging thing. Regardless of how time may seem to change for us internally, outside of us clocks tick away time at a constant and even pace. But all of this was about to change. The one thing that scientists weren't absolutely sure of was whether or not light always traveled at the same speed. Maybe it slowed down or sped up at times. As it turned out, the only constant in the universe was the speed of light. Let's look at why this is so.
A good way to understand the constant speed of light is to use one of Einstein's favorite tools, a thought experiment. It's exam time at Star Fleet Academy, and for your final exam you and a friend (let's call him Al) have to set up an experiment to find out if the speed of light is constant, regardless of its source. You both go to the nearest space station, and while you remain on the station, Al rents a shuttlecraft from Stan's Cosmic Wrecks. You're making the final adjustments to very accurate measuring instruments, and Al is on the shuttlecraft setting up an ancient projectile weapon from the twentieth century.
A number of experiments have been conducted to prove that the speed of light is constant. One test was conducted in 1913 when the speed of light received from a stationary star was compared to the light received from a moving star. In both instances the speed of light was the same. Another test was made in 1955. In this case the speed of light was measured from the opposite sides of a rotating sun. While one side is turning toward you the other side is turning away. You would expect the light from the edge coming toward you would be faster than the speed of light from the edge turning away from you. Nope, light has the same velocity from both edges.
Al takes off in the shuttlecraft and flies past the space station. You measure his speed as 50,000 mph. He then stops right out in front of you so that he's not moving, and he fires his high velocity cannon. You measure the speed of the cannonball at 10,000 mph. Now he flies by again, and fires the cannon while he flies by. You measure the speed of the cannonball and find that it's traveling at 60,000 mph. Where does that speed of the cannonball come from? It's equal to the combined speed of the shuttle-craft added to the speed of the cannonball. Okay, now in setting up the next experiment everything is the same except that you've replaced the ancient projectile weapon with a laser gun.
You already know the speed of the shuttlecraft but you want to be sure the instruments are calibrated correctly to measure the speed of light that is emitted from the laser gun. So Al stops in front of the space station and fires the laser gun. Sure enough the speed of light is 670,000,000 mph. He then flies by and shoots the laser gun. Based on the results of the first experiment you would expect that the speed of light from the laser gun should be the combined speed of the shuttlecraft plus the speed of light, or 670,050,000 mph. But to your amazement, the speed of light measured from your instruments is just 670,000,000 mph. The speed of the shuttlecraft has not added any more speed to the light emitted from the laser gun. You turn your final exam results into Star Fleet Academy and pass with an A. You've shown that the speed of light is constant regardless of its source.
An important clarification to be made about any discussion of relative motion is that the type of motion we're talking about is constant velocity motion. That means that whatever speed you're traveling at, that speed remains constant. It's also called force-free motion. Because as soon as you introduce a force such as acceleration, or some kind of push or pull, that can be felt and you now know that something has changed. You become aware of movement.
It doesn't matter whether you are moving toward a source of light or receding from the source. Your movement has no impact on the speed at which light travels from its source. In Newtonian physics this doesn't make sense. It even goes against common sense. If you chase after someone running three miles an hour and you're running ten miles an hour, you'll catch up to him or her. If you take off after a light wave you would think you should be able to catch up to it. You would think that the light wave would appear stationary; the light wave would stand still. But there is no such thing as a stationary light wave. This thought intrigued young Albert Einstein. So much so that he spent 10 years of his life trying to understand the nature of light and its impact on space and time.
Excerpted from The Complete Idiot's Guide to Theories of the Universe © 2001 by Gary F. Moring. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.