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Lecture 3

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McGill University
PHYS 183
Tracy Webb

th Lecture 3: January 11 , 2013  use artifacts as evidence for what happened in the past  can look at the history of the universe because light times a certain amount of time to get from one place to another  speed of light is the fastest thing in the universe, nothing can travel faster (it is the upper limit)  light is emitted by the sun and we see it 8 minutes later  when there is a delay between reporters in different locations on the news, it is a light delay essentially  the pictures we have of Andromeda are 2.5 million years old due to the speed of light o there could be life on those planets today but we won’t know for 2.5 million years  can use telescopes as time machines in a way that archeology can’t  2 million years is a very small time in astrophysics  sun and Milky Way have barely changed  universe is about 14 billion years old  can’t see galaxies that are 15 billion light years away because light has not yet had time to travel that distance  we would be looking back to a time before the universe existed  light year: the distance that light travels in one year (about 10 trillion km)  unit of distance, not time 1.2 The Scale of the Universe  if the sun was the size of a grapefruit, the earth would be the size of a ball point on a pen o Mercury would be 5 m from the sun, Earth is 10 m, Jupiter is 100 m and Neptune is 320 m o nearest star would be in Vancouver (Alpha Centuri is about the same size as our sun) o we are in a very dense galaxy  most of the universe is empty space  if you counted one star per second it would take you a few thousand years to count them all (100 billion) 
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