To demonstrate the complexity of the task of observer, we use an experiment to demonstrate that length contraction and length expansion are a visual illusion.
The experiment consists of a train of line segments of each length l, which move towards the right.
Length contraction is observed when an object moves away from an observer. Length expansion is observed when an object moves towards an observer.
Length contraction is observed because the object moves away from the observer and light towards the observer.
Length expansion is observed because both the object and light move both towards the observer.
These observations are an illusion because the actual length does not physical change.

Fig 1 shows two trains of line segments moving towards the right at t0 (bottom) and t2 (top) Each line segment is indicated with 1 arrow. The arrow > indicates the front.  indicates the back. The two blue line segments between x1 and x0 and between b0 and b1 are used to demonstrate length contraction. The two most important points are x0, which shows the position of the observer at t0 and b0 at t2 The front of the red segment is shown at t0 positions x0 and at t2 position b1. That means the line (x0  b1) shows the front of the line segment. The line (x0  b2) shows a light signal emitted from the point x0. The length are selected that the speed of light c is twice the speed v of the line segment. At t0 the observer position x0 sees the front of the red segment instantaneous. At t2 the observer position b0 sees the end of the line segment instantaneous. At t2 the observer at position b0 cannot see the front of the red line at b1. To know what the observer at (b0,t2) can see in the past you have to follow the line (x2  b0). The line (x0  b1) shows the movement of the front of the line segment. The point r, which is at the crossing point of both lines, shows when the observer at position b0 can see the front of the line segment. This is at t1.6 The length line segment qr is the distance of the front of the segment at t1.6. The reality is the length l. A simple calculation shows that the length of the line segment (qr) is 2/3*l. That means the observer observes length contraction at (b0,t2) 

Length Contraction experiment  complexFig 2 shows a line segment moving towards the right, with the lengt l, demonstrating length contractionThe border line is the line x0, a0, b0. To the right of this line the observer sees length contraction. To the left of this line shows length expansion. indicates the front and the arrow < indicates the back. The front of the segment is shown at the positions a1, x0 and b1. What the Observer at x0 sees at t0 is indicated at the two lines under the angle of 45 degrees. Both lines indicate events in the past. If there is a light flash at t1 at position x1 than the observer at x0 will see this flash at t0 If there is a light flash at t1 at position x1 than the observer at x0 will also see this flash at t0 If there is a light flash at t2 at position x2 than the observer at x0 will see this flash at t0 The two lines at 45 degrees, at b0 and at t2 show similar events. If there is a light flash at t0 at position x2 than the observer at b0 will see this flash at t2 If there is a light flash at t0 at position x2 than the observer at b0 will see this flash at t2 At t2 the actual length l of the line segment is the line indicated by the points b0 and b1. In that case the point r, shows the front of the line segment, that is observed at t2 by the observer at x0 i.e. b0, while the actual position is the line b0, b1. Mathematical the length of the observed length is 2/3 times l, while the actual length is l. That means length contraction At t0 a similar situation arises with the back of the line segment. In that case the back of the train is at the point x1 i.e. at a distance l. However what is observed is the event or point a2 at t2. This event is at a distance 2 times l from the observer. That means length expansion > 
From a physical point of view, it is important that the actual physical length of each segment does not change. What is observed is in some sense a visual illusion. This comes because all what we see at a distance happened in the past (earlier). That means what moves towards us, happened futher away, and what moves away, happened closer.
What you read in literature is only? about length contraction. Length expansion is as important.
A similar effect is also at work when the movement of objects is discussed like in Newtons Mechanics. The major difference is that the force of gravity is not based on the speed of light, but the speed of gravity. Using Newtons Mechanics the effect is real.
This teaches us one more lesson: we should be carefull to interprete what is observed, as a true image of the physical reality.
Fig 1 is valid for all 3 dimensions.
To be continued. >
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