A very brief history:
100 years ago, physicists thought that there was a sub observable medium called Ether, also spelled Aether,
in which all matter and waves moved within.
James Clerk Maxwell used the Ether idea to explain electromagnetic fields and waves.
His field equations are valid yet today.
Another physicist named Lorentz had his own notion of
the Ether known as The Lorentz Ether Theory, or LET for short. He modified his original thinking after an experiment
was performed by Michelson and Morely where it was showed that light was not slowed in speed by the Ether.
Lorentz and another physicist named Fitzgerald concluded, independently, that the length of an inferometer arm,
which is part of the apparatus of the Michelson and Morely experiment, contracted in length by a factor of:
sqrt(1-v^2/c^2)
The faster the objects speed in Ether, the more its length contracts.
Lorentz also concluded that time must be different between the frame of reference of the Ether and the frame of the apparatus.
This time he coined as local time.
So we have, regarding objects in relative motion:
1. The speed of light is measured c (300,000 km/s) in all frames of reference.
2. The length of objects contract.
3. Time is different.
Enter Einstein and his Special Relativity Theory.
The experimental results of Einstein's Special Relativity, SR for short, is identical to those of LET.
However, Einstein's Theory concludes no Ether is necessary.
It is space and time that explain length contraction as well as time dilation.
Many experimental results during the past 100 years supports SR. Hence, SR is the widely accepted theory in mainstream physics today over LET.
A difference between SR and LET:
Yes, there is a major difference between these two theories with
respect to length contraction.
LET requires an Ether in order to contract matter.
SR requires merely coordinates of a system.
Imagine a rod of 1 meter long (its length measured in its frame), moving relatively. It's length is parallel to its direction of motion.
On each end of the rod is a widget.
While both SR and LET will claim equal amounts of length contraction, SR requires no rod in-between the two widgets,
the coordinates between the front and rear widget will length contract.
This would not occur in LET since the Ether contracts matter and not empty space or the Ether itself.
Length contraction reality:
Do object's length's really contract?
I say no, and offer a thought experiment below, where its results conclude no contraction will be observed.
I must point out here that all events in different frames of reference will be observed by all other frames of reference.
The differences being, times and coordinates of relative frames.
As an example, someone is bouncing a ball on a moving bus.
The man bouncing the ball is not moving in his frame, the road and you are moving, this is relative motion.
Both you and the man see the ball bouncing but only you see it moving down the road.
The man does not.
Hence, different coordinates.
It takes time for the light reflected from the ball to strike you and the man.
Hence, different times.
Thought experiment:
Consider two systems with observers in relative motion.
One system will be stationary.
Another system will move at a velocity with respect to the stationary system.
Consisting of a light clock who's proper length's (as measured in their own frame), are as follows.
A pulse laser, where a mirror in the shape of a parallelogram is placed 1 light second directly vertically above the laser.
And a detector placed directly horizontally a distance of .3 light seconds away from the laser in the direction of the motion as viewed by the stationary system.
It is the intent of the mirror to reflect the light towards the detector.
Pictures are provided below.
