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Einstein's idea

Last lesson we learned how the classical theories of physics were trying (and failing) to explain the behaviour of light.  It was not until 1905 when Albert Einstein proposed a new revolutionary theory that was able to explain this behaviour.

Albert Einstein

Here is the introduction of his article “Electrodynamics of a moving body”. Read, discuss and answer the questions below:

It is well known that Maxwell's electrodynamics—as usually understood at present—when applied to moving bodies, lead to asymmetries that do not seem to be inherent in the phenomena.

Examples of this sort, together with the unsuccessful attempts to detect a motion of the earth relative to the ‘light medium,’ lead to the conjecture that not only the phenomena of mechanics but also those of electrodynamics have no properties that correspond to the notion of absolute rest. Rather, the same laws of electrodynamics and optics will be valid for all coordinate systems in which the equations of mechanics hold good. We shall raise this conjecture (whose content will hereafter be called the “principle of relativity”) to the status of a postulate and shall also introduce another postulate, which is only seemingly incompatible with it, namely that light always propagates in empty space with a definite velocity c that is independent of the state of motion of the emitting body.

1. What does Einstein means with “Maxwell's electrodynamics, when applied to moving bodies, lead to asymmetries”

2. What is the ‘light medium’ to which Einstein is referring to?

3. Which ones the following reasons does Einstein provide to support his theory? Choose the two right answers:

  1. Maxwell equations were wrong.
  2. Maxwell equation should not change when we change the frame of reference.
  3. Maxwell equations fail when a body is moving with respect to the light medium.
  4. The Earth has no motion with respect to the light medium.
  5. No experiment has proved the existence of a light medium.

4. Which ones of the following are the main postulates of its theory? Choose the two right answers:

  1. The laws of electromagnetism are only valid when the laws of mechanics are right.
  2. The speed of light is constant through the light medium.
  3. All laws of physics –electromagnetism, optics and mechanics- are valid in all frames of reference.
  4. The speed of light is always the same, independently of the relative motion between source and receptor.
  5. The speed of light depends of the movement of the emissor.

5. Taking into account all of the above, write a summary of the text.

Einstein's theory has consequences that seem unveleivable. Let's explore them in the following unit by pressng the button "...and its consequences"

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