Science, Physics, Relativity, Faq's and Feedback

Historical Overview #3

Roughly 19 years ago I started with "My Homepage" 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 Historical Overview. To read it go to: Historical Overview #1
6 Years ago I wrote my second Historical Overview. To read it go to: Historical Overview #2
And this one is my third Historical Overview.


1. Understanding

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 what they all agree.

2. Physics

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.
Physics is about earthquakes, about the pendulum of Leon Foucault, about clocks, positions, speed, accelerations, humans, statistics, elementary particles and measuring devices. 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 because not always all physical implications are taken into account. This is the case when rods are considered rigid which physical do not exist.

3. How do we unravel the laws of Nature. How do we perform "Physics"

The primary way to do science is by performing experiments under, as many as possible, different conditions.
For example you use meter sticks to measure and observe how a ball behaves when you drop the ball from different heights. By using a clock you can measure the speed. When you do this accurate enough you can establish the relation between speed and height. A step further is the relation between acceleration and height.
In many cases you can use apparent different conditions to study the same laws.
For example you can use water to unravel how water waves behaves and using the same mathematics you can study how monochromatic light behaves. Monochromatic is light with the same frequency, which implies that the light waves have the same length, which is the physical starting point to observe interference patterns.
This is important to understand the Michelson-Morley experiment.

4. Coordinate system and rest frame

In order to study physics, specific the movement of objects, we have to assign positions to the objects involved. What we need is a coordinate system. A coordinate system is not physics, it is mathematics.
The standard coordinate system is a 3D system in three dimensions. The best coordinate system is the largest possible.
The earliest coordinate system used the Earth as the centrum. That is okay if you want to describe locally what is happening, but from a more global point of view not very practical because everything rotates around the Earth.
To take the Sun as the centrum is only practical if you study our Solar system. A better one is the centrum of our Milky way except that you should be carefull that the cordinate system itself does not rotate.

A general accepted way is to study physics from the view point of a frame at rest or a rest frame. As such the surface of the earth is considered at rest. A train standing still is than considered at rest with a speed v = 0. A moving train has a speed v > 0.
The physical issue to answer is: My understanding is that there can only be one frame which you can really call at rest. IMO for proper understanding we should call this the real (or absolute) rest frame.
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 Microwave Background radiation our real rest frame.

My understanding is that only in this real rest frame the speed of light is the same in both directions.
My understanding is that you can not specify two rest frames which both are considered real (absolute) rest frames.
You can not define:
both trully at rest.
As such that:
  1. The speed of light in both frames is the same in both directions.
  2. Two events observed simultaneous by one observer in one train cannot be observed simultaneous by the other observer. and that both observers claim that they are simultaneous in their own reference frame (using each there synchroneous clocks).
  3. The speed of a light flash measured in one frame cannot have the same speed as measured in a moving frame (over the same distance) except if the clocks in the moving frame count at a lower rate.
In reality only one of the frames can be called at rest. And even if that is the case is a Question mark. In principle the frame with the moving train can be the only one frame which is truelly at rest.

5. The speed of light

The speed of light is the speed of photons through space.
From a physical point of view the speed should be identical in all directions. This is the case from the view point of an absolute restframe.

6. Simultaneity

One important issue in order to understand the evolution of the universe is the concept of simultaneity. Simultaneity implies that there are simultaneous events. Simultaneous events are all the events that happen at the same moment during the evolution of the universe.
To observe simultaneous events is more difficult. There are two important issues. Consider two simultaneous events.
For you to observe these events simultaneous does not depend about the speed of these two events nor about the speed of the observer.
For you to observe these events simultaneous you have to be at the right position at the right moment.
The right position is the plane perdendicular to the line that connects the two events, specific the point half way at that line.
The right time is the duration of the distance of a point on that plane and and the position of one event, divided by the speed of light after the event.

Example with two Questions.

 |           | B'
 |           .   .  /
 |     B   .       4
 |   ./  .   |   ./
 | . / .   . | . /
 2  /.       3  /
 | 1       . | /
 |/  .   .   |/
     Figure 1
    Figure 1 shows:
  • One Observer A at the center of the train at rest. This observer will emit a light signal which will reflected by two mirrors at equal distance of A. Observer A will see both reflections simultanious at B.
  • One Observer A' at the center of a moving train. This observer will emit a light signal which will reflected by two mirrors at equal distance of A'. Observer A' will see both reflections simultanious at B'.
  • The Observers A and A' are initially at the same "position". That means there is physically only one light signal.
  • Both observers will each see two simultaneous reflections. The Question is are these reflections simultaneous? Are the reflections 2 and 3 simultaneous and or are the reflections 1 and 4 simultaneous?
  • Accordingly to Figure 1 only 2 and 3 are simultaneous. But is this sketch correct?
    Observer A' could claim that he is at rest but than A is moving. This means that the events 1 and 4 are simultaneous and not 2 and 3.
  • What this means that the two observers A and A' will agree that their respectivily reflections cannot both be simultaneous. Only one. The most obvious answer is that none of the reflection pairs are simultaneous.
This same issue resolves a related issue:
Using the same methode A can synchronise a set of identical clocks.
A' in a moving frame can do the same. As such you get to sets of synchronised clocks: A and A'. The problem is that only one set can be called trully called synchronised. It is either A or A'. And this is not even sure. The set of synchronised clocks A'' with the fastest moving clocks, is the winner

7. Special Relativity

One of the ingredients of Special Relativity is that the laws of physics must have the same form in every inertial frame. (See: Was Einstein right? by Clifford M Will at page 263). (The problem is what are the laws of physics). Because laws are discriptions of physical processes and because the difference between inertial frames is a difference in (constant) speed this implies that all processes should be indepent of speed. I doubt this specific if you approach the speed of light.
Standing in a train at rest is rather simple. Standing in a moving airplane is also simple.

It should be understood that acceleration is not part of SR. As such standing in a moving train, when it starts or stops is not considered. However these processes are important because when different constant speeds are studied accelerations are implicable linked as part of the experiments.
Standing in a moving train also involves gravity. Also that is not considered. All of that raises an important issue about the importance and practicallity of Special Relativity.
There are two major areas to study in SR, called: Time Dilation and Length Contraction
There are two important issues:
  1. is Time Dilation a physical effect, a mathematical concept or more a thought experiment.
  2. is Length contraction a physical effect, a mathematical concept or etc.
To establish this (part of) you have to perform independent experiments to demonstrate what is involved. At least experiments performed in one reference frame.
For example to consider rods and or disc as rigid is not allowed, because rigidity is not a physical concept. Rigid rods do not exist. See Rigidity

8. 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 such a clock runs behind compared with a clock with stayed at home. This behaviour (if it can be demonstrated) 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.

In reality you should exclude acceleration. To do that you need clocks which constant speeds. One for each speed to demonstrate.
The problem even when acceleration is not directly involved in the experiment the whole explanation for time dilation can be in the fact that the behaviour of clocks is affected by acceleration.

9. 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 the simplest way is in one coordinate sytem using a rod at rest and one in movement.
IMO you can use a video camera to demonstrate this.
 A1--------------A2 A
      ------->      B
     Figure 2
    Figure 2 shows:
  • Train A is the train at rest. The letters A1 and A2 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 Physical Length Contraction involved if in one frame of the camera you can see both lights and train B is also inbetween the two lights.
A much more complex way is to use clocks.

10. Length Contraction and Terrell rotation

The document: The Physics Classroom explains in the sub-sections "Einsteins Theory of Special Relativity" and "Length" the concept: Relativistic Length Contraction.
The document claims: The issue is how is the length measured. The details are not supplied.
In order to demonstrate "length contraction" the document shows a simulation with 4 frames/speeds. IMO this demonstration is wrong.
Frame 1 shows a demonstration with a spaceship which moves at 10% of the speed of light. The observed length is 199 feet and constant in the demonstration. However that is wrong. The demonstration should include Terrell Rotation.

What is important specific only when the spaceship passes the finish line it becomes possible to measure the length of the space ship. In order to do that you need at least one timer to measure the time it takes to go from top to bottom of the spaceship over the finish line.
If you perform your experiment in such away that the finish line is also the start line and that you take care that the length of the path line (going from start to finish) each time is the same and is known than it is possible to calculate the length of the spaceship for different values of v of the spaceship.
In reality this is very difficult to perform accurately.
See also: Comments "Terrell rotation" in Wikipedia

11. Rigidity

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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