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Unit 7: Astrophysics Page 40
heading toward them (blue This is one of the strangest parts of
headlight) or traveling away from relativity: events are not
them (red headlight). conserved. This means that two
different people can see two
different things happen, and also
observe they occur at different
time intervals.
Simultaneity is Relative
When we first look at time dilation
there seems to be a big problem
with it: “moving clocks run slow”.
But which clock is moving?
If you’re inside the train, you
would see a white headlight. You might be tempted to say that
the moving clock (the one with our
In both cases the light travels at
time traveling twin) ran slow. Does
186,000 miles per second.
this mean that the Earthbound
twin’s clock ran fast? Actually, the
Suppose you were inside the train,
twin in the rocketship will say that
traveling near the speed of light
the Earthbound twin’s clock runs
and you have two light bulbs wired
slow, too!
up to the same switch.
This seems like a paradox, but it
One bulb is at the front of the car,
really isn’t. The traveling twin can
the second at the back of the car.
claim she’s at rest and the
Earthbound twin is moving, so his
When you flip the switch, the light
clock is running slow. It all
from both light bulbs hit your eyes
depends on your frame of
at the same time, just as it you
reference.
weren’t moving. Is that what a
stationary observer at the train
Take another look at that star trip:
station would see? Nope…
the distance between Earth and the
star must be less than 10 light-
Someone sitting at the train station
years as measured by the space
would see one light bulb turn on
ship’s reference frame.
ahead of the other, and they would
be different colors, even if they
were wired to the same switch.
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