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Unit 7: Astrophysics Page 36

the requirement that the motion are different. Einstein’s second idea

must be in a straight line and at was to reject the notion that time
constant speed. and space are absolute.

One of the predictions that comes Time and Space are Not

out of this idea about all viewpoints Constant
being equal is that all
electromagnetic waves, including The speed of light is the same
light, travel with speed c (186,000 in all reference frames. The only
miles per second), even though way for this to be possible is if the
different observes may move increments of time and space are
relative to each other! different for different observers.

Imagine you’re standing at a gas
station and you have a device that
can measure the speed of light. It
has a very fast clock inside that will
measure how long it takes light to
travel along the meter stick inside.
Your friend drives by in a car at 70

mph with an identical device. You
both see a traffic light flash up
ahead, and you both measure the

exact same speed for light.
Common sense tells us that clocks
Worse yet, another friend with an
tick at regular intervals, and that a
identical device in an airplane
meter stick does not change from
traveling at 600 mph measures the
one day to the next. Our common
speed of light from the same flash
sense comes from our experience
and gets the same answer for c.
in the real world, which happens to
be very limited in terms of speed.
A fourth friend traveling at half the
If we had learned as a baby to
speed of light in a rocketship
crawl at 90% the speed of light,
repeats this same experiment and
then these ideas about space and
also gets the same value for c.
time would make intuitive sense to
How can this be?
us.

It’s possible only if our
Since we don’t travel anywhere
measurements of time and space
near the speed of light, we often

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