Physics explains how the universe works. It models the behavior of the universe as mathematical laws. The only help it can offer why one law is true is that it can be verified by experiment, or that it can be derived from another law. As in mathematics, this leads back to more fundamental laws until you reach "axioms." At this point, why can only be answered "because that is the way the world is."
But I expect the question isn't really "Why is the universe thus?" It is "Why is the universe different from what everyday experience leads me to expect? Why the apparent contradictions of special relativity? Why is correct physics so counter-intuitive?" This can be at least made more plausible by relating it to familiar situations.
Time is hard to understand because it has some of the same properties as space. We don't notice this because we move so slowly.
The speed of light is about $3 * 10^8 m/s$. We are comfortable at everyday speeds around $3 m/s$.
Consider a world where the fastest things moved at $3 * 10^{-8} m/s$. This works out to about $1 m/year$, the speed of a glacier. It is not much faster than $1 cm/year$, the speed of continental drift. What kind of physics would a glacier world physicist come up with? What would he think of everyday physics?
For one thing, he sees a fundamental difference between time and space. Objects move in time, but are frozen in space. The position of an object is an unchangeable property of the object. The vector distance from one object to another is an unchangeable constant.
In the everyday world, I stand on the side of a road and watch a car drive by. At $t_0$, the car starts at $x_0$. At $t_1$, the car arrives at $x_1$. On the other hand, the driver thinks the car is fixed. At $t_0$, the car seat starts at $x'_0$, right under him. At $t_1$, it arrives at $x'_0$, still right under him.
If two cars drive past me in opposite directions, we might all agree that I start at the same place, $x = 0$. One driver might think I arrive at a position to the south of the start. The other would think I arrive at a northerly position. I think I don't move.
None of us find anything remarkable in these disagreements about whether two events at two different times occur at the same place or not. It is a simple consequence of motion.
A glacial physicist would find these disagreements very confusing. He might accept that I can be in different places at different times. But he might wonder how I can arrive at multiple places at the same time.
The root of all differences between glacial and everyday physics is that time and space are more alike than he expects. You move in both time and space. He does not see that because he moves so slowly.
We have exactly the same problem when we think about relativistic physics. Time and space are more alike than we expect, but we move too slowly to see it. All of the differences between everyday physics and special relativity follow from this.
We see many counter intuitive effects at high speeds. It is customary to start with one, the constant speed of light, and derive all the other effects of special relativity from it. Einstein started this convention because it quickly leads to all the important math with only one new postulate. But it is certainly possible to start somewhere else.
We will start with some observations about cause and effect in classical physics. In quantum mechanics, cause and effect is not so simple. We will ignore quantum mechanics. Gravity also changes things. We will ignore gravity.
Suppose two events occur at the same place, but at different times. The earlier event might be the cause of the later one. Suppose two events occur at the same time, but different places. Neither event can be the cause of the other.
Suppose two events occur at different times and places. We can reduce this to one of the simpler cases above. Suppose somebody can drive at constant velocity from one event to the other. From his point of view, they occur at the same place and different times. This driver has proved that one can be cause and the other effect. Everybody else can rely on his proof.
In the everyday world, we expect time to be universal. We do not expect one observer to see the two events at different times and another to see them at the same time. We see no need to have separate descriptions of time for stationary and moving observers.
Never the less, we can state it this way. If one driver at constant velocity sees that two events at different places occur at the same time, then he has proved for everybody that neither event is the cause of the other.
So what is cause and effect in an everyday, classical world where we ignore gravity and quantum mechanics? For our purposes, pretty much everything can be reduced to one of these cases.
- Cause: particle 1 travels to particle 2 and collides. Effect: particle 2 recoils or something.
- Cause: particle 1 exerts an electrical or magnetic force on particle 2. Effect: particle 2 accelerates.
- Cause: particle 1 exerts a strong or weak force on particle 2. Effect: particle 2 accelerates.
This glosses over a lot. (E.G. Particle 2 explodes or annihilates particle 1.) But we are only interested in one essential feature: For an effect to occur, something must travel from the time and place of the cause to the time and place of the effect.
We won't consider strong or weak forces further. In this particular point, they behave like electromagnetic forces, but the full description quickly gets into quantum mechanics.
Classical electromagnetic forces are described by fields. If you wiggle particle 1, particle 2 doesn't feel the effect until the field travels from 1 to 2. That is, the electromagnetic field is the agent of cause and effect. For this case, cause and effect travel at the speed of the electromagnetic field. Or more famously, at the speed of light.
The situation is similar for particles. Nobody has been able to accelerate any particle with mass up to the speed of light. If we dip into quantum mechanics, photons are massless particles that travel at the speed of light. Cause and effect are again limited by the speed of light.
Hopefully this much makes it clear that the speed of light is more important than you might have expected. Perhaps it is a little bit reasonable that the speed of cause and effect should be the same for everyone, no matter how they move.
The standard derivation of special relativity still leads to the very counter-intuitive result that time is different for a moving observer. A space time diagram is the best way to see this.
It turns out that time and space are more alike than we expect. Every strange new feature of time is just like an everyday feature of space. We are confused by each feature of time in exactly the same way that a glacier world physicist is confused by the corresponding feature of space.
Confusing to a glacier world physicist: I see two events separated by a short distance and long time. A driver can choose a velocity less than the speed of light that changes how he sees positions. He makes the points occur at the same place from his point of view by driving from one to the other. Though I still see them at different places.
Confusing to me: I see two events separated by a long distance and short time. A driver can choose a velocity less than the speed of light that changes how he sees time. He makes the points occur at the same time from his point of view. (He cannot drive through them both.) Though I still see them at different times.
This doesn't make it any more intuitive. But I hope it helps to understand why simply going fast produces counter-intuitive results.
This is still pretty sketchy. Pictures would help. But I have run out of steam for tonight.
