Thereís no Such Thing as Gravity:
The World Just Sucks!
A slightly different approach to general relativity
for high school physics students
By Mark Sowers
Table of Contents
Almost everyone would agree that sight is the most important of our senses. It gives us knowledge about our environment from a distance. We donít need to be in direct contact with that house over there to know that it is red. Yes sound and to some extent smell also does this, but sight reaches the farthest. Sight tells us something about our environment even if that environment includes a galaxy many billions of miles away. It is light that makes this possible.
Light is something that everyone is familiar with. And yet it is something that many of us donít really understand.
So Iíll start with the big question. What is light? Some say light is a particle of energy, called a photon. Some say light is a wave, similar to the waves in a pond. But in reality light is neither a particle nor a wave. Light is light. I know, how profound is that? But itís true. Light is like nothing else that we know of in our daily lives. It is so completely different, so completely foreign, that itís difficult to conceptualize.
Light behaves like a particle. And light behaves like a wave. But just because light behaves that way, doesnít mean it is that way.
Light is an electromagnetic Ďwaveí. Notice I put the word wave in quotes because I just got done saying that light isnít a wave. The problem is that scientists donít have a good term for exactly what light is, so we use something familiar, like Ďwaveí. From this point forward Iíll leave the quotes off, but understand that the word wave just doesnít do light the justice it deserves.
The radio waves that send music to my stereo are electromagnetic waves. The x-rays that the doctor used to find my broken bone are electromagnetic waves. The ultraviolet light that gave my grandfather skin cancer are electromagnetic waves. And as I mentioned before, light is an electromagnetic wave. The only difference among all these waves is their frequency. We canít see radio waves, x-rays, or ultraviolet light, but they are all exactly the same thing; they are all forms of light. So when I refer to light, I donít mean just the tiny fraction of electromagnetic waves that we can see. I mean all electromagnetic waves. To me, all these waves are light.
So why use such a complicated term like 'electromagnetic'? Long ago people thought electricity and magnetism were two completely different things, so they gave them different names. But scientists started to notice a relationship between them and merged the names. Now we understand that they are not just related, they are exactly the same thing; just in two different forms.
If you take a magnet and move it past a wire, you will create electricity in that wire. This is how a generator works. A generator is simply lots of wires moving quickly past a magnet.
If you take a wire and move electricity through it, you will create a magnetic field that a compass can detect. This is how an electromagnet works.
Notice in both of these examples I said moving. If the magnet were just lying still next to a wire, even though there would be a magnetic field, there would be no electricity. Only a moving magnetic field generates electricity, and only moving electricity generates a magnetic field.
But what if you moved a magnet and there wasnít a wire nearby? As it turns out, the wire is irrelevant. The moving magnetic field still generates an Ďelectric fieldí. All the wire does is take that electric field and turn it into electricity. And the opposite is also true: a moving electric field generates a magnetic field, whether or not there is a wire or anything else nearby.
So letís try a thought experiment. Imagine you pick up a magnet hold it in the air. The magnet emits a magnetic field out into the air. Now suppose you move that magnet through the air. Not near any wires, youíre just waving it around in the air. What is happening? You are moving a magnetic field. And what happens when you move a magnetic field? You generate an electric field nearby. Just by waving that magnet around, you generate an electric field.
Now hereís the fun part. You have just created an electric field where there wasnít one before. Creating an electric field or changing an electric field is the same as moving an electric field. So not only do you have an electric field, you have a changing electric field. And a changing electric field creates a changing magnetic field nearby. This of course creates a changing electric field nearby, which creates a changing magnetic field nearby. Once you start this process, it continues on its own forever. It propagates.
In 1864 Scottish mathematical physicist James Maxwell calculated how fast a changing electric field would create a nearby magnetic field, and vice versa. He found that it happens extremely fast, that it changes from an electric field to a magnetic field and back again millions of times every second. He also calculated how far away each new field is from the one that created it. Putting these numbers together, he realized that these changing waves of electricity and magnetism propagate at a speed of about 186,000 miles each second, the speed of light.
So by waving that magnet around in the air, you have created forever-changing waves of electricity and magnetism, which means you have just created light.
Light travels fast. Really fast. It travels at 186,000 miles each second. In one second, it can go all the way around the world 7-and-a-half times. It can go from the Earth to the moon in just a few seconds. It can go from the Earth to the Sun in about 8 minutes.
To be clear, 186,000 miles per second is the speed of light in a vacuum. As long as there is nothing else around to interfere with it, these changing waves of electricity and magnetism will propagate at 186,000 miles per second. But if light has to pass through something, like air or glass, it slows down. Itís like driving down a dirt road instead of a highway. You try to go fast, but you canít because of all the stuff around you slowing you down. The more dense the material, the more that material slows light.
|Material||Speed of Light (mph)|
Since air is not very dense, light passes through it almost as quickly as it does a vacuum. But a diamond, which is very dense, is able to cut the speed of light by more than half.
It is this property, the slowing down of light that leads to many marvelous effects. Imagine driving your car rather quickly down the highway, and you drift slightly onto the gravel berm (shoulder). The wheels on the left side of the car are suddenly slowed down by the gravel while the wheels on the right side are unaffected. This forces the left side of the car to slow down. And when one side of the car moves more slowly than the other side, the car turns: its path is bent.
The same thing happens to light as it passes from one material to another: it is bent. I see better when looking through a pair of glasses because the glasses bend light. Different frequencies of light (i.e. different colors of light) are bent to different degrees. This is how a prism splits white light into many colors (Figure 1). Itís also how raindrops can create a rainbow. The greater the difference in speeds between the materials, the more light is bent. This is why a diamond sparkles more than glass.
We most often think of this transition from less-dense to more-dense materials as being a sharp boundary like the edge of the diamond or the surface of the water. But it can also be very gradual. As the sun heats the blacktop of a highway, the air next to the road gets heated and becomes less dense, while the air several feet above the road remains cooler and more dense. The density of the air changes gradually the higher you go. This is known as a Ďgradientí. This density gradient slowly bends light from the sky in an upward curve towards an observerís eyes.
As a result the sky appears in a different location from where it actually is. It appears as if it is on the road. This is called a mirage. Your mind tries to explain this unexpected observation by assuming the road is wet and that the image of the sky is reflected off the wet surface.
It is the speed of light in a vacuum that will concern us for the rest of this paper. And from now on when I refer to the speed of light, I mean specifically the speed of light in a vacuum. I bring up these other materials because it is important to understand how light behaves when it is forced to slow down. Its path is bent. And just like your car, its path is bent towards the berm: towards the region of slower travel.