Science, Physics, Relativity, Faq's and Feedback

Historical Overview #3

This homepage is now roughly 19 years old and again it is time to give an overview of what I have done.
This is not my first Historical Overview.
13 Years ago I wrote my first. To read it go to: Historical Overview #1
6 Years ago I wrote my second. To read it go to: Historical Overview #2


This homepage is all about understanding. Understanding of the physical processes that take place throughout the Universe day by day. What their origin was, how they evolve and what will happen in the next coming billion of years.
In order to understand we read about what other people do, we perform experiments, we observe the present, we ask our self questions and we discuss as much as possible.
When you observe the Universe or better what happened near you you will find that many things are more or less the same. There are the seasons which repeat itself year after year but also the planets in the solar system move in a rather regular pattern. Isaac Newton was impressed and interested why an apple falls from a tree (any apple) and as a result (simple claimed) he invented what is now called Newton's Law. Newton's Law describes the behaviour of moving objects. Newton's Law is not the cause that objects move the way they behave i.e. move relative of each other.

In order to observe the universe we use light. That means the processes and the events we observe always happened in the past. The question is how important is light i.e. photons in order to understand the behaviour of the moving objects. The same question can also related for humans. To call blackholes black because we humans can not observe them is a misnomer.

Both documents indicate that not all scientist agree with each other. The issue is to unravel about which they all agree.


As said before this document is about understanding physical processes. Its about the evolution of the universe, its about chemistry, its about celestial mechanics and astro physics.
Its also about laws. Each law is a description of a set of processes which are considered physical identical. Its about the parameters that influence these these laws and the experiments involved to demonstrate.
It is not so much about mathematics. Mathematics becomes important if accurate measurements are available.
It is not about thought experiments. Thought experiments are discussions about experiments based on a certain theory. Thought experiments become problematic as long as not all physical constraints are taken into account. Special Relativity also to a large extend can be considered a thought experiment as long as physical considerations are not taken into account. This is the case when concpets like rigid rods are considered which physical do not exist.

The (real) rest frame

There are two questions:
  1. What is a real frame at rest.
  2. Can there be more than one rest frame.
IMO for the whole of the universe there can only be one frame which you can really call at rest.
Only in that frame the speed of light in both directions is identical.
You could the frame centered around the Sun at rest. But because the sun moves throughout the Milky way the center of the Milky way is a better frame to call at rest. You can also call the frame of the Cosmic Micro wave Background radiation our rest frame.
This rest frame is the only frame in which all events at any instant in time are happening simultaneous.
This is the only one frame in which clocks show the Universal time or the age of the Universe.

The speed of light

One central issue is the speed of light in two directions. This issue is important because light is the basic of almost all our observations in space, so it is very important what light (photons) physical mean.
When you have a short physical event in space (a light flash) this event propagates in space in all dimensions symetrical. This implies that the speed is the same in both directions but it does not guarantee that the speed is constant. The frame from which you make this claim has to be at rest. This does not mean that the source of the event has to be at rest also.

Special Relativity

Special Relativity descibes processes which have a constant speed. This is called relative because the speed is always measured relative to something else in movement. In practice this is the speed of light.
That means in the language of SR that the movement between frames (i.e moving observers) is realative. That is true except if you consider that there is only one rest frame
There are two major effects in SR, called: Time Dilation and Length Contraction
An important question is: if both effects are physical, are they both real.
For example a rigid rod and a rigid disc are not considered physical objects. See Rigidity

Time Dilation, Clock Dilation

A clock when moved along a closed path from a starting point and back towards the same point is supposed to under go Time dilation.. That means that this clock runs behind compared with a clock with stayed at home. If that is the case this behaviour is called Time Dilation.
The problem with demonstrating this effect by means of an experiment is that time dilation should clearly be demonstrated as a function of a constant speed v of the clock and not as a result of acceleration. In principle you start the experiment only after the speed is constant and terminate before you start to reduce its speed. As such demonstrating Time Dilation by means of an air plane which flies around the earth could be rather misleading, because such an airplane is constantly undergoing acceleration.
A different problem is that you should Time Dilation only demonstrate in one direction.
That means when you demonstrate Time Dilation using a horse race track you should start (continue) the experiment at the beginning of a straight leg and stop (hold) the experiment at the end of that leg and continue only when this same leg is repeated.

Length Contraction

Length Contraction is the effect that the length of a rod is shortened when in movement. That means relative to a rod at rest. The rods considered should not be rigid. See Rigidity
The problem is how is this length contraction measured.
IMO only in one frame using a rod at rest and one in movement.
IMO you can use a video camera to demonstrate this.
  L--------------L A
      ------->     B
     Figure 1
    Figure 1 shows:
  • Train A is the train at rest. The letters L are two lights to identify the front and the back of the train.
  • Train B is the moving train. The length of this train at rest should be larger than the length of train A with the two lights.
  • VC is the video camera. There is Length Contraction involved if in one frame of the camera you can see both lights and train B is also inbetween the two lights.


Rigidity is a very difficult subject. Rigidity literally means that a rigid object is not subject of deformations. Rods in general are supposed to undergo length contraction when in movement. The question is if rigid rods are also supposed to undergo length contraction when in movement.
A different problem is what exactly is rigid material and if there are different non-rigid materials.

Rigidity is even more difficult when rotating discs are considered. The question is if rotating discs undergo Length Contraction when in movement i.e. are rotating.
Also here there are two possibilities i.e. if the disc is rigid or non-rigid.
The problem is that many people assume that rotating discs always are subject of stress and deformation as such SR does not apply.

Created: 4 March 2015

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