ZirkMan said:
I try to get the gist of the Special and General theories of relativity for more than two years now. And I still don't understand if the geometrical explanation really explains or just describes what is physically going on when talking about all the relativistic effects (like gravitation, time dilation and space contraction).
This is what I mean by the difference between "explains" and "describe":
Are the theories of relativity physical theories, in a sense that the mechanism of the relativistic effects can be explained by energy transfer or interaction (in whatever form), like I believe any physical theory should and does?
A related question is if spacetime itself is a form of energy and for example gravitational attraction can be exlained as an energy interaction between objects and curved spacetime or the curved spacetime is just an abstraction that let us model the outcome of some other physical (in the defined sense) processes but is not the physical process itself that is responsible for the relativistics effects?
I wonder the same things. I've learned to do the math and come up with the right answer, but that doesn't mean I understand it. Here is my best shot as of today, for better or worse.
The reason relativity seems so strange is the environment in which our form of life evolved. There are plenty of natural clocks, and the idea of "distance" is also perfectly natural. The concepts of time and distance are very basic and useful to us. Electronic clocks at sea level all run at the same speed. So no wonder we got the idea that time was the same everywhere. But this is not the norm, just a peculiarity of the surface of a planet.
Surfaces of planets are very exceptional environments. The norm is some place in space surrounded by unblinking stars. Nothing changes. Everything is moving, the constant and repeatable distances of Earth don't exist. If distance has little meaning and utility, then velocity has little meaning as well. If you are using hydrogen wavelengths as a clock, then the observation that all clocks run differently would be fundamental. That's what they do.
In such an environment what DOES matter is energy, rest mass, and wavelength. This you would have. You will notice that if you define quantities in these terms then things work naturally. Expend twice as much energy to move yourself someplace, and you get there twice as fast. This simple relationship always holds. Use a hydrogen wavelength from a local star as a clock: that will give you some idea of relative velocity and clue you in on the effects of gravity So...what's the problem?
The problem is that distance, time, and velocity are derivative concepts of the fundamentals. Using them is a lot like having no concept that the Earth is a sphere and navigating using a Mercator projection. Why do I have to make all these screwy corrections? (Clearly no force is at work making the shortest path between two points a curve on the map.)
One can learn to think of the special relativity corrections as four-dimensional rotations around an imaginary axis. Sort of. That is, it is better than the Mercator style, but I can't claim that I visualize this. I've learned what to do in the common cases, that's all. It seems to me that there is the same problem: we are forcing the distance and universal time concepts on there, and it just isn't a comfortable fit.
What really is fundamental are certain second order differential equations. That's what is maintained, and our time and distance concepts are solutions to those equations. That means in order to get numbers you have to do two integrals, and those are not easy. But that is what time and distance really are: solutions to non-linear second order differential equations. When you look at it that way, then perhaps you no longer expect to get simple results. I am told that in general relativity that even relative velocity is not well-defined: it's path dependent. If we expect it to behave in a simple way, we will be disappointed.
So if time and distance are derivative concepts falsely assumed to be simple, why do we use them? Easy: we need them to design lab equipment, and to measure and express the results. It's part of our language. Time and distance may not be simple, but they certainly are useful here on Earth. Not only that, all of our pre-1900 physics was built around these concepts and they work very well down here. We aren't going to throw that away for something that is much harder to use and doesn't necessarily come up with useful numbers.