My friend Streen (man, I should get peoples’ real names ;)) wanted this image, so I thought I’d post it, but also include the text surrounding it.
A spacecraft cruising through space may gain or lose speed, even if its engines are shut off, because gravity can accelerate it forward or backward. However, an electromagnetic wave traveling through space never changes its speed. Not because gravity doesn’t act on light, for it does. Gravity can change the frequency of light or deflect light — but it can’t change the speed of light. What keeps light moving always at the same, unvarying speed in empty space? The answer has to do with electromagnetic induction and energy conversion.
If light were to slow down, its changing electric field would generate a weaker magnetic field, which, in turn, would generate a weaker electric field, and so on, until the wave died out. But, what would happen to the energy in the fields? If the fields faded away with no means of transferring energy to some other form, energy would be lost. That would be incompatible with the law of energy conservation, so the light can’t slow down.
Figure: The electric and magnetic fields of an electromagnetic wave are perpendicular to each other and to the direction of motion of the wave.If light were to speed up, a similar argument prevails. The changing electric field would generate a stronger magnetic field, which, in turn, would generate a strong electric field, and so on, a crescendo of ever-increasing field strength and ever-increasing energy, clearly a no-no with respect to energy conservation. at only one speed do mutual induction continue indefinitely, with neither a loss nor a gain in energy. From his equations of electromagnetic induction, Maxwell calculated the value of this critical speed and found it to be 300,000 kilometers per second. In his calculation he used only electric and magnetic fields. He didn’t use the speed of light. He found the speed of light!
This book has some grammar issues (as can be seen above) but it also has some excellent diagrams, and is written at a level such that anyone with no background in physics could understand the content.
In case you are interested, the book is Conceptual Physics, Eighth Edition, by Paul G. Hewitt, published by Addison-Wesley. The ISBN is 0-321-00971-1, and using that number you can easily buy it at wonderful online book retailers.