Copernicus's theory was the first hint that perhaps the nature of reality depended on the position of the observer. Galileo thought this to be true as well and developed the first ideas of relative motion. In fact, he proposed the idea that we could not tell by experiment alone whether we were moving or not. Here's his basic idea: If you were riding in the closed cabin of a steadily moving ship, without looking out of the porthole, you would not be able to tell whether you were moving or not. Allow your pet parrot to fly around the room outside of his cage. Watch fish swim around in their bowl. Toss objects back and forth. Notice how things fall to the ground. No matter how many experiments you perform, Galileo said, “You shall not be able to discern the least alteration in all the forenamed effects, nor can you gather by any of them whether the ship moves or stands still.”
One of the most frequently used clichés about Einstein's theory of relativity is that it shows that everything is relative. The statement that, “everything is relative” really has no meaning. It's like saying everything is bigger. And if you ask, bigger than what? Apply that same question to everything being relative. In order to be relative, you have to be relative to something.
So it looks as though you can't tell the difference between standing still or moving if you don't have reference to something outside of you. You may be moving relative to the world outside of the ship, but not moving to the world inside it. Motion is relative because it depends on your point of view.
Let's say you ask your friend who is standing on the dock to tell you whether or not you're moving as you sail by in your ship. Your friend looks at you strangely and says, “Well duh, of course you are moving, can't you see the dock moving past you?” And of course you reply, “But how do I know that I'm moving and not the dock?” And then your friend says, “Or you could say that you are moving relative to my dock, but as long as you stand still in your cabin, you are not moving relative to your ship.” And then you answer with the aura of wisdom that permeates all those who read The Complete Idiot's Guide books, “Cool, so I guess everything is relative.”
You may wonder why examples like the speed of light or relativity take place in outer space. It's a great place to explain how some theories work because there are two things that impact examples on earth. In space there is a vacuum, so objects move with no resistance and exhibit Newton's laws of motion perfectly. That's one of the reasons why satellites stay in orbit for as long as they do. There is very little to slow them down. The second is that there is no gravity to act as a force on anything. At least not in the regions of space the examples take place. There are some more examples coming up over the next few sections that will also take place in outer space. It's just a great place to conduct thought experiments. Providing you have enough oxygen!
This question would be a little more difficult for an outside observer to answer if you wanted to know whether the earth was moving or not. Where would you put the dock for your friend to stand on? How could you find a frame of reference where your friend would be standing still? The earth is whizzing around the sun at almost 20 miles per second. Our solar system is moving with respect to the center of our galaxy at 150 miles per second, and our galaxy—the Milky Way—is rushing toward our neighboring Andromeda galaxy (at least from the point of view of the Andromeda galaxy) at 50 miles per second. And if you look at the earth from a far-off quasar, you might see us speeding away at 165,000 miles per second, which is close to the speed of light.
As you can see, the earth moves relative to the sun, our solar system moves relative to the galaxy, and the galaxy moves relative to the rest of the universe, and by current theories the universe is moving, too. But what is it relative, too? Other universes? Parallel universes? God? It goes on and on.
If you don't quite have the concept of relativity down yet, here's one more thought experiment that may help clarify it for you. Imagine that you're floating in outer space. There's no stars or planets or space ship for you to get your bearings or to give you a frame of reference. You just have a red distress light blinking on and off on your backpack. How you got in this situation is unknown to you. Off in the distance you see a yellow blinking light coming toward you. As it comes closer you recognize a friend of yours who seems to be in the same predicament that you are. Unfortunately, because neither one of you has a jet pack you can't maneuver toward each other. So you wave to you friend as she drifts by.
The unusual thing about watching your friend float by is that from her perspective she's not moving, you are. Let's turn it around. Your friend is floating in space and sees a red blinking light off in the distance coming closer. She now recognizes that it's you and waves to you as you go drifting by. From her point of view you are moving toward her. Each of you thinks that you are stationary while the other is moving. Each perspective is correct. And that is the fundamental principle of relativity. Motion is relative.
In this last section I want to close with some thoughts on the impact Einstein had on classical physics and prepare the ground for his special theory of relativity. That's coming up in the next section.
Einstein extended Galileo's concept of relativity from motion to also include space and time. He looked inside the cabin of Galileo's moving ship (except that for him it was a light beam) and was deeply impressed with the fact that the laws of nature stayed exactly the same. Jump up and gravity pulls you down in the same way whether or not you're standing still or moving. Clocks tick, water flows, raindrops fall, and electricity works the same whether you are moving or at rest. Relativity established that the laws of nature are, in a sense, absolute and do not depend on the motion of the system. It's because they are absolute that you can't tell whether you are moving or not.
It's almost an oxymoron to label relativity as an absolute since the meaning of each is the total opposite of the other. The essence of absolutism in science is defined by a quality that is fixed, unchanging, complete, and whole. While at the heart of relativity is change and differing interpretations. But sometimes the best way to explain a concept is to put it in terms that describe it, paradoxically, by its opposite. This is a method that will be used more than once when we enter into the quantum universe.
But this is only true in the everyday conventional world. What Einstein discovered was that the usual concepts of physics embodied in Newton's laws simply don't work at very high speeds or under conditions of extreme gravity or in many other situations. Newton's laws do not hold true in all frames of references, so the laws of nature depend on whether you are moving and what system you are in. The laws of nature depend on your point of view. This may seem beyond the realm of possibility, but remember what happened when we were looking at the dual nature of light? When the scientist looked for a particle, they found a particle. And when they looked for a wave, they found a wave. The very act of observation changes the outcome of the experiment, so maybe it does really depend on your point of view.
Galileo and Newton knew that motion was relative, but insisted that space and time were absolute. Einstein saw that space and time, and energy, and mass as well were also relative. However, all of this was true only because of the absolute nature of other universal constants, among them the absolute speed of light. That speed, as you know based on your Star Fleet Academy final exam, is an absolute 186,000 miles per second (or 670,000,000 mph) from any point of view, from any frame of reference, moving or not. The only reason “everything is relative” is that the speed of light and the laws of nature are not. And since light itself is really nothing more than the motion of magnetic and electric fields relative to each other, forces which are behind everything from the structure of matter to all of our processes including perception, Einstein clearly found a very fundamental “absolute” frame in which to construct his relative universe. And with that, it's time to look at just how special the special theory of relativity is.
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.