The question “Why is the speed of light constant?” is often asked by those trying to understand physics. Google has 2760 links to that question. Yet the answer is so simple that a 10-year-old can understand it, that is, if you accept Quantum Field Theory.
Take the ten-year-old to a lake and drop a stone in the water. Show her that the waves travel through the water at a certain speed, and tell her that this speed depends only on the properties of water. You might drop different objects at different locations and show her that the waves travel at the same speed, regardless of the size of the object or location of the water.
Then tell her that sound travels through air with a fixed velocity that depends only on the properties of air. You might wait for a thunderstorm and time the difference between the lightning and the sound. Tell her that a whisper travels as fast as a shout. I think a ten-year-old can grasp the concept that water and air have properties that determine the speed of these waves, even if she doesn’t know the equations.
Anyone who can understand this can then understand why the velocity of light is constant. You see, in Quantum Field Theaory space has properties, just as air and water have properties. These properties are called fields.
As Nobel laureate Frank Wilczek wrote, “One of the most basic results of special relativity, that the speed of light is a limiting velocity for the propagation of any physical influence, makes the field concept almost inevitable.”
Once you accept the concept of fields (which admittedly is not an easy one), that’s all you have to know. Light is waves in the electromagnetic field that travel through space (not space-time) at a speed dictated by the properties of space. They obey fairly simple equations (not that you need to know them), just as sound and water waves obey simple equations. OK, the Quatum Field Theory equations are a bit more complicated, but quoting Wilczek again, “The move from a particle description to a field description will be especially fruitful if the fields obey simple equations… Evidently, Nature has taken the opportunity to keep things relatively simple by using fields.”
However this question can have a different meaning: “Why is the speed of light independent of motion?” This fact was first demonstrated by the famous Michelson-Morley experiment, in which light beams were timed as the earth revolved and rotated. The surprising result was that the speed of light was exactly the same regardless of the earth’s motion.
As I wrote in my book (see quantum-field-theory.net): That the speed of light should be independent of motion was most surprising… It makes no sense for a light beam – or anything, for that matter – to travel at the same speed regardless of the motion of the observer… unless “something funny” is going on. The “something funny” turned out to be even more surprising than the M-M result itself. In a nutshell, objects contract when they move! More specifically, they contract in the direction of motion. Think about it. If the path length of Michelson’s apparatus in the forward direction contracted by the same amount as the extra distance the light beam would have to travel because of motion, the two effects would cancel out. In fact, this is the only way that Michelson’s null result could be explained.
However the idea that objects contract when in motion was just as puzzling as the Michelson-Morley result. Why should this be? Once again the explanation is provided by Quantum Field Theory. Quoting again from my book:
We must recognize that even if the molecular configuration of an object appears to be static, the component fields are always interacting with each other. The EM field interacts with the matter fields and vice versa, the strong field interacts with the nucleon fields, etc. These interactions are what holds the object together. Now if the object is moving very fast, this communication among fields will become more difficult because the fields, on the average, will have to interact through greater distances. Thus the object in motion must somehow adjust itself so that the same interaction among fields can occur. How can it do this? The only way is by reducing the distance the component fields must travel. Since the spacing between atoms and molecules, and hence the dimensions of an object, are determined by the nature and configuration of the force fields that bind them together, the dimensions of an object must therefore be affected by motion.
It is important to understand that it is not just Michelson’s apparatus that contracted, it is anything and everything on earth, including Michelson himself. Even if the earth’s speed and the consequent contraction were much greater, we on earth would still be unaware of it. As John Bell wrote about a moving observer:
But will she not see that her meter sticks are contracted when laid out in the [direction of motion] – and even decontract when turned in the [other] direction? No, because the retina of her eye will also be contracted, so that just the same cells receive the image of the meter stick as if both stick and observer were at rest. – J. Bell (B2001, p. 68)
In conclusion, for those who want to understand physics, I say use Quantum Field Theory and: WAKE UP AND SMELL THE FIELDS.