Jan 5 th Test tomorrow on light Posters due January 15th Summative Jan 20 th and 21 st Final exam January 29 th 9:15-11:15 Today: The myth of simultaneity

In the late 1800s, scientists thought that light travelled through a medium called the luminiferous aether. According to this theory, the speed of light should change depending on your direction of motion relative to the aether. Many experiments (Michelson-Morley Interferometer) were performed but failed to detect any ‘aether wind’.

Einstein put together two main ideas to develop his special theory of relativity: All inertial reference frames are equally valid. You can still play tennis on a moving cruise ship. The laws of physics are the same, no matter if you are moving or not. The speed of light is constant in all reference frames. If you run with a flashlight, the beam still travels away from you at the speed of light. If I am run towards you, I still see the beam traveling towards me at the speed of light.

Simultaneity We are used to thinking of events as occurring at a certain time, as though time were something that all observers could agree on. The special theory of relativity shows that this is not true. Events that are simultaneous to one observer might not be simultaneous to another.

Simultaneity Diagram The observer on the ground says “I see the light at the same time. The light travelled equal distances at equal speeds. The events must be simultaneous” Diagram The observer on the train says “I see the light at different times. The light travelled equal distances at equal speeds. The events must not be simultaneous” Simultaneity Video

Both observers are correct. Events that are simultaneous in one inertial reference frame are not necessarily simultaneous in another inertial reference frame.

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