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Albert Einstein in 1921

Einstein's Original Theory - The Postulates of Special Relativity

Special relativity (also know as the special theory of relativity) is the description of the relationship between space and time, and one of Albert Einstein's two elements of his theory of relativity (the other being general relativity).Special relativity follows two postulates as the core of the theorem. The first postulate, the principle of relativity, dates back to Galileo. and maintains that "the laws of physics don’t change, even for objects moving in inertial (constant speed) frames of reference." . "An inertial frame of reference is a reference frame in which a body at rest remains at rest and a body in motion moves at a constant speed in a straight line unless acted on by an outside force". In essence, "if two people who are moving in different directions both measure the speed of light to be the same, then either of the two properties that speed relies on - time or distance - must differ between observers. Einstein’s theory of special relativity shows that both of these parameters can vary according to perspective." "For example, a car’s motion is measured relative to its starting point or the road it is moving over, a projectile’s motion is measured relative to the surface it was launched from, and a planet’s orbit is measured relative to the star it is orbiting around". "The second postulate references the speed of light, which is a constant, and independent of the relative motion of the source". The speed of light does not change for any observer, regardless of how they are moving in relation to the source of the light.

The First Postulate

The first postulate of special relativity helps us understand how and why our environment reacts the way it does. If you are sitting in your living room and toss an apple straight up into the air, it will return right to your hand, and you will see it travel straight up and down. If you were to do the same thing sitting on a moving bus, it will travel the same way, and look exactly the same to you. Therefore, even though you are in two different reference frames, you will observe the same results when performing identical experiments. This demonstrates that absolute motion has no impact, which is what the first postulate is arguing. However, the path of the apple would look different to an observer standing on the street watching you throw the apple on the moving bus. That idea is related to postulate two. Postulate one is used to identify objects that either remain at a constant motion or don't move at all.

Invariance of the speed of Light

The Second Postulate

The second postulate says that the speed of light is constant for everyone observing it, regardless of how they are moving relative to the light source. For example, "arrows shot from a moving train (A) and from a stationary location (B) will arrive at a _target at different velocities—in this case, 300 and 200 km/hr, respectively, because of the motion of the train. However, such commonsense addition of velocities does not apply to light. Even for a train traveling at the speed of light, both laser beams, A and B, have the same velocity: c". One may argue that this is the first postulate in another form. If the laws of physics apply equally to all frames of reference, then light must travel at the same speed regardless of the frame.

The Relativity of Simultaneity

A result of our understanding of special relativity is the relativity of simultaneity. Even though things might seem to be simultaneous in our frame of time, the might not be so in a different frame of time. "If, for example, we place a light source between two observers that emits a beam of light in each direction, then the observers will think that the beams were emitted simultaneously if they reach each observer at the same time. If the observers are moving, however, then the beams will not reach both observers at the same time, and so they may conclude that they were not emitted simultaneously. Galileo’s relativity shows that both views are correct. This means that what we perceive as the present only corresponds to what is occurring simultaneously to us, in our reference frame".

relativity of simultaneity

How does the first postulate help explain phenomenon and why is it important?

This theory is the reason we can explain the relationship between energy and matter. It helps us compare observations made by individuals in different locations. Special relativity is able to compare observations from entirely different locations by looking at the speed of the observer and object. For example, astronauts in different spaceships moving at the same distance have comparable observations. This was previously thought to be untrue. Scientists used to look at an idea called ether, which was a theoretical substance that acted as a medium for light waves to pass through. Einstein made this idea completely obsolete by ignoring it in his theory and having it still remain true. Einstein was the first to remove the ether entirely and just trust the laws of physics would remain the same at an equivalent distance. Before Einstein, there was no definite theory that equated space and time and interpreted how they affect each other. This link between space and time also led to the idea that the universe can been seen in three dimensions, up & down, left & right and forward & backward, or the space-time continuum.