History of Quantum Physics (2)
Is light particles? Waves?
Left: the particle theory of light
Right: the wave theory of light
As we mentioned last time, in the year of 1887, Hertz proved in his experiment the existence of electromagnetic wave. He also proved that light is in essence a kind of electromagnetic wave.
What is light?
In ancient Greece, people guessed that light was something shot out by our eyes, and when it reached something, the thing was seen by us. For example, Empedocles thought that world consisted of four elements - water, fire, air and earth. And Aphrodite the goddess light up human eyes with fire. And when the fire element coming out of human eyes reaches the object, we can see the object then.
But how can it explain that we can’t see anything in the dark?
A more sophisticated hypothesis was introduced. For example, Plato thought there were 3 different lights, coming from the eyes, from the object, and from the light source. Vision then was a result of the overall result of the three.
In the age of Rome, Lucretius proposed that light reached human eyes directly from the light source. But his idea was not accepted. Until round 1000 AD, Islam scientist al-Haytham generalized the correct cognition of the imaging from light: the reason why we can see an object is just because light reaches our eyes after it is reflected by the object. His Arabic book later was translated and introduced to the West and Roger Bacon continued the research. That’s the foundation of modern photology.
Based upon the assumption that light always travels in straight line, Euclid in his Catoptrica (Reflective Photology) had some research about the reflection of light. Ptolemy, Haytham and Johannes Kepler all did some research about the deflection of light. And the Dutch physicist Willebrord Snell generalized the law for the deflection of light in 1621. Finally, all the properties of light were summarized by Pierre de Fermat, who was also known as “the king of amateur maths”, in to one simple rule, “light always travel in the shortest route”. Photology as a physical subject has finally formally established.
However, when people have known so well about all the behaviors of light, still, the most fundamental is remaining unsolved, “light, in it’s essence, what is it?”
People in Ancient Greece always tended to regard light as a thin string of particle flow. In other words, light consists of very small “light atoms”. This is what we call the “particle theory” of light. The particle theory can explain well why light travels in straight line, and why it strictly reflects, even the deflection phenomenon can be explained by the differed speed of light in different mediums. But apparently, it also has some difficulties: Why don’t two strings of light collide and scatter? Where do the particles hide before we switch on the light? Can their number reach infinity?
After the dark Medieval Age, people gained a further understanding about the physical world. Wave motion phenomenon was researched into a new depth. It was widely accepted sound is a kind of wave. People began to suspect if sound could be a wave, then why couldn’t light be? At the beginning of the 17thcentury, Rene Descartes in hisDiscours de la méthodefirst proposed this possibility: light is a pressure and travels in medium. Shortly after, an Italian maths professor called Francesco Maria Grimaldi did an experiment. He let a string of light travel through two small holes and reach the screen in the dark chamber. He found an image of the projection with its edge of dark-light stripes. Grimaldi immediately recalled the diffraction of water waves and he proposed: light is possibly a wave like water wave. This is the earliest“light theory”.
The light theory states that light is not a physical particle, but a wave caused by the vibration of the medium. Let’s think about the“human wave”of the spectators on the football court: although every watcher just stands and sits down, instead of running everywhere. But that tide does moves around the whole court. This tide is a wave. The water wave in a pond is also the same. It is not, in essence a transmission, but the result of water surface’s up-down vibration along the route. If light is also a wave, we can easily understand why there are stripes in the image of projection and it’s also easy to explain why lights travel through each other without interruption. As for reflection and straight-traveling, people later learned that the wavelength for light is short, then under most circumstances, the behavior of light is like that of classic particles. But wave theory also has a difficulty: So that wave in essence is the vibration of the medium, then it must transmit in some medium, like sound’s travel in air, water even solids but not in vacuum. If there is not a person in the court, the“human wave”is never attainable.
But light is not this case. It seems that it can move ahead without any medium. Here’s a simple example. The light from the star travels all the way through the galaxy and is then seen by us. But the universe is almost vacuum. It seems to be a great threat to the wave theory. But wave theory evades this question by assuming an intangible medium to make the transmission possible. This medium has an impressive name, called“Aether”.
Thus, the wave theory of light stepped on the stage of history. As we shall soon see, the new-born power was destined to fight against the particle theory over centuries. And in the mid-17th century, the darkest time right before the dawn of science, nobody could foresee a blazing fire would be caused by the two sparks.
