Class notes
ASTRO101: Gravity and light
- Course
- ASTRO101 (ASTRO101)
- Institution
- University Of Alberta (UofA)
Understanding gravity and light
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The relationship between light and gravity is fundamental toour understanding of black holes. In order to tie these ideas
together, we will have to relearn the concepts of gravity,
space, time, and light that we've been presented in our
favorite movies, TV shows and other great works of art.
Popular culture doesn't always get the science behind these
concepts right. But it's surprising how scientific accuracy
informs spectacular effects in movies like in Interstellar. The
scientific foundation for black holes has been built by
hundreds of scientists contributing ideas throughout history.
But one unmistakable scientist was responsible for the
biggest developments, Einstein.
Einstein's theory of gravity, describes the gravitational attraction
between massive objects, such as a star or a black hole, by
associating the force of gravity with an equivalent curvature of
space, sort of like this curved sheet. All physical objects including
rocketships and individual particles of light called photons, travel on
curved spacetimes similar to the curved path that these marbles
follow. In this simple demonstration, we have an up and down
direction defined by the earth's gravity, which gives us the effect of
the marbles orbiting around the central depression. In outer space,
near a star or a black hole, we don't have earth's gravity holding
the marbles down on the curved sheet. Instead, we recognize
gravity as curvature of space and time. This curved sheet provides
valuable insight into how we can imagine the warping of spacetime.
A black hole is a region of space where the force of gravity
becomes so strong that the curvature of spacetime prevents light
from escaping. In this curved sheet model, imagine that a marble
represents light. If the marble is aimed away from the center and
tossed with a high enough speed, the marble is able to escape from
the depression in the center. The region outside of a black hole or
a normal star is like this. Light that is emitted from objects in the
neighborhood of a black hole are still able to escape. But there is a
special spherical boundary surrounding the black hole, which
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scientists call the event horizon. Once a marble falls within the
event horizon, it must continue falling inwards. You could try to aim
a marble outwards, but you'd never be able to make the marble go
fast enough that it can escape.
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