Cosmos Quest for Students, “Light, Time & Gravity”

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cosmosCOSMOS: A Spacetime Odyssey, Episode 4, “A Sky Full of Ghosts,” is an exploration of how light, time and gravity combine to distort our perceptions of the universe.

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Episode Summary
We eavesdrop on a series of walks along a beach on summer nights in the year 1809.
William Herschel, whose many discoveries include the insight that telescopes are time
machines, tells bedtime stories to his young son, John, who will grow up to make some
rather profound discoveries of his own. A menacing stranger lurks nearby. All three of
them figure into the funhouse reality of tricks that light plays with time and gravity.

We take the Ship of the Imagination to cosmic landmarks, observing the light travel time to Earth for each and the events that were unfolding then… and to the beginning of space
and the end of time.

We are, each of us, born in a gravity well. A toddler struggles against an invisible force to
stand triumphantly for the first time. A beautiful young woman hops on a motorbike and
rides off, experiencing the joys of the Italian countryside and the phenomena related to
relativistic travel throughout the cosmos. What would happen if the speed of light were not finite?

We keep re-adjusting the gravity in New York City to see its effect on the unflappable

We meet the shadowy figure from the 1780’s who first realized that black holes must exist. We venture to the event horizon of a black hole in the Ship of the Imagination, only to be drawn over the edge and into it.

And we travel back in time to the day in Neil’s youth that Carl Sagan spent with him.

Discussion Topics
• What would the personal implications be if the speed of light were infinite? How
would the sky look? Would we be able to judge the age and size of the universe?
How would that impact our ability to communicate with other intelligent beings in the
• How do do light and time interact with space to make it possible for a black hole to
contain another universe?

• You’re on your speed-of-light motorbike. You turn on the headlight. How fast is the
light from your your headlight traveling?
• Telescopes are time machines. If you had a telescope powerful enough to see what
was happening on Earth from anywhere in the universe, where would you take it and
what do you think you might see?
• In a hypothetical thought experiment, consider what would happen if Earth’s gravity
was suspended. What would happen to the earth’s atmosphere?

Proposed Activities
• Turn on the television so that it’s not on any actual station. Some of what you’re
seeing in the noise of the signal is a remnant of the creation of the universe. TV
receivers are sensitive to microwaves and pick up some of the background radiation
left over from the big bang.

Relevant Scenes from COSMOS
• Act One: William Herschel and his son discuss “a sky full of ghosts”
• Act Two: Gravity’s Empire
• Act Three: The Speed of Light
• Act Four: Weightless New York
• Act Five: Black Holes
• Act Six: An Intergalactic Subway System
For a deeper dive, more subjects touched on in Episode 4: 
• Speed of light
• The illusions of sunrise and horizon
• The light year as a unit of measure
• Physics of pulsars
• The distance from Earth to the center of the Milky Way galaxy
• Events on Earth when that light left the center
• 13.2 billion year old light
• Cosmic background radiation
• The illusion of centrality in the universe
• Cosmic evolution
• Gravity
• Binary star systems
• Newton and the mystery of “action at a distance”
• Faraday and electro-magnetic fields

• Maxwell equations
• Einstein and relativity
• Rotational and orbital velocities of the Earth, Sun, and galaxy
• Physical consequences of infinite light speed
• Physics of black holes
• Invention of photography
• Time travel

Written by Ann Druyan
Produced by Cosmos Studios

All rights reserved, Cosmos Studios. For distribution to FOX affiliates to accompany the original broadcast distribution of COSMOS: A SpaceTime Odyssey.

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