Lesson 5.1 - Implications of the Speed of Light
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UNANSWERED QUESTIONS
At the beginning of the 20th century there were some unanswered questions and problems in our physical understanding of the universe.- What medium does light travel through?
- Almost all physicist though that they needed to look for this unknown 'aether' that light propagates through.
- Why doesn't an atom collapse into itself?
- The charged electron spinning around the nucleus should give off EM radiation, losing energy and quickly crash into the nucleus.
- Why does a blackbody not emit high frequency radiation?
In the following unit we will try to answer these questions through the development of our modern physical understanding.
LUMINIFEROUS AETHER
It was accepted that there was some medium permeating the universe called the "luminiferous aether" and light propagates through it.
If the aether permeates the whole universe then due to earths motion there must be some detectable "aether wind". Assuming that the speed of light would change in different direction relative to this "aether wind", It should be possible to measure the motion of the aether.
The light is split into two paths of equal length. Any deviation from an equal path will generate a change in the diffraction pattern on the screen. (Waves will be out of phase)
If there was an aether, when the earth is travelling with it light should get a small boost in speed. Alternatively when the earth is travelling against the aether light should slow down a little.
This difference in speed will cause one of the beams to arrive in front or behind the other generating a change in the diffraction pattern.
BUT THAT DOESN'T MAKE SENSE!
Consider two cars approaching each other at 80km/h
An outside observer will see the two cars approaching a central point each travelling 80km/h.
An observer in one of the cars will see the other approaching at 80km/h + 80km/h = 160km/h
Two rockets approaching each other at 0.8c
An outside observer will see the two rockets approaching a central point each travelling 0.8c
An observer in one of the rockets will see the other approaching at 0.8c + 0.8c = 1.6c
ENTER ALBERT EINSTEIN
Einstein proposed a theory to remove the problems of aether and the speed of light all together
POSTULATES OF EINSTEIN'S SPECIAL THEORY OF RELATIVITY
- All laws of physics are equivalent in all inertial frames (constant velocity) of reference.
- The speed of light is the same in all inertial frames of reference regardless of the velocity of the observer or the velocity of the light source.
From different inertial reference points, it is impossible to answer questions of simultaneity.
Not only is there no absolute spatial frame of reference, as we have seen throughout this course, there is no absolute temporal frame of reference.
The only constant is the speed of light
