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Unit 7: Astrophysics Page 33
Textbook Reading
Lesson 3: Relativity
Space and Time equator), then the speed of the
light can be added to the speed of
Most people use common sense the earth’s rotation, and the two
when they interact with their speeds will add and can be
world. They know that if they are measured against a beam of light
traveling 60 mph and an oncoming aimed toward the north pole
car travels also at 60 mph, the (where the rotation speed does not
impact will feel like 120 mph. But factor into the light speed). This
you’ll have to leave all that behind was a very plausible idea.
as we step into the world of special
relativity. This about this: If you are
traveling in a car at 60 mph, and
Problems with Newtonian you poke your head out the
Physics sunroof to toss a ball at 20 mph
out of the car, how fast is the ball
In the late 19th century, a famous going?
experiment by Michelson and
Well, it depends. Imagine the car is
Morely was conducted to detect the
passing a gas station right when
Earth’s motion through the ‘ether’.
you throw the ball.
The ‘ether’ was something the
scientists thought up to explain
If you toss the ball in the same
how electromagnetic waves
direction that the car is traveling
traveled through space. Initially,
in, the ball speed will be seen by
they did not believe that light could
an observer standing at the gas
travel through a vacuum like
station would be 80 mph.
space. Today, we know that the
ether does not exist and light really
If you toss the ball perpendicular
does travel through a vacuum.
to your line of travel (for instance,
directly to the person at the gas
The experiment went something
station), then the ball speed is 60
like this: if we shine a beam of
mph in the forward direction and
light in the same direction that the
20 mph in the sideways direction.
earth is rotating (along the
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