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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 7: SETI, Interstellar Travel, and Colonization

Key Ideas

Radio wavelengths may be the best place to look for extraterrestrial intelligence
1420 MHz (or some mathematical multiple) may be the most natural frequency to search
No detections so far, although searches are ongoing
We are sending out signals as well
--Some deliberate
--Some not-so-deliberate
Traveling to other systems is very difficult
--Stars are far away
--Speed of light is a fundamental limit
--Enormous energy required
Colonization of the Galaxy can take place in a reasonable amount of time


We can try to 'skip over' some steps in the search for life to find intelligent civilizations directly
Success depends on number of intelligent civilizations
Where do we look?
How do we look?
What are we looking for?

How many neighbors?

An Optimistic Guess

N*=100 billion
L=5 billion years (Age of the Sun)
Age=10 billion years
N=2.5 billion

Average Density

~0.01 per cubic parsec

Average Distance

~5 parsecs

Talk is cheap!

If you really want to bridge interstellar distances, use light to send messages.
Messages travel at the speed of light
Very low energy cost per message

How do we look?

Where to look?
What wavelengths to use?
Microwaves: 1000-10,000 MHz is a region of relatively low cosmic background "noise"
--Relatively cheap
--Less sensitive to interstellar dust
Lasers at visible or IR wavelengths: very few natural lasers in the sky to cause confusion

Earth is already on-the-air

We have been inadvertently beaming radio signals into space for the last 80 years:
--Radio broadcasts from the 1920s onward
--Television broadcasts from the 1950s onward.
--We could detect these with current technology
Episodes of "I Love Lucy" will have already reached most solar neighborhood stars (~40 light years)

If our optimistic estimate is correct
There is at least one civilization within our 'horizon'
They may already be sending a return message
Very likely very advanced!


The Search?

--Search for Extra-Terrestrial Intelligence
A relatively inexpensive search strategy to look for radio signals from extraterrestrial civilizations.
Various kinds:
--"Piggy back"

Potentially Habitable Planets

Don't actually know.
Depends on distribution of rocky planets.
Optimistic: n_Earth=1

Examples of Current Searches

--UC Berkeley
--'Piggy Back'

Allen Telescope Array
--SETI Institute
--Targeted Survey
--350 Antennas

What are we looking for?

Signals that appear "artificial"
--Very narrow "bandwidth" (<300Hz, the narrowest natural sources)
--Pulsed signals (common way to encode information)
--Very little frequency "drift"
Where to look?

Natural Frequency

Hydrogen offers a natural frequency
"Spin-flip" occurs at 1420 MHz (21 cm)

Mathematical multiples

Hydrogen is the most common element in the Universe
Natural place to look
Might want to avoid this directly because of natural emission
Some mathematical multiple, i.e.
"Hydrogen times pi = 4461 MHz"

So far, no detections.

WOW signal

SETI project at the Big Ear radio telescope at Ohio State
On August 15, 1977, a strong, narrowband radio signal was detected
Had properties expected from a non-terrestrial and non-solar origin
Lasted 72 seconds
Has not been detected again.
The perils of long distances
Signals get much weaker with distance
Limits the range of stars we can probe.

Sending Signals?

Increasing radio silence
Earth's radio brightness has been decreasing:
--Introduction & spread of cable TV.
--Increased use of "directed" communication (e.g., fiber optics, beamed satellite, etc.).
Sufficiently advanced civilizations may emit less "waste radio" & become radio quiet:
--If a civilization wants to be found, it may have to deliberately broadcast its presence.

Interstellar Travel Getting there?

Objects have mass
Accelerating masses requires energy
The more the acceleration, the more energy
The distances between stars are enormous:

Need very large amounts of time
Extremely large amounts of energy

Current technology?

The fastest spacecraft: Voyager 1 & 2
Outward bound at 15 km/sec (0.005% c)
Need 80,000 years to reach the nearest stars.
Difficult to do much better!

Facts of Life

Problems with bringing your own fuel:
--Acceleration requires fuel, which has mass
--The more mass, the more energy required to accelerate\
--The more energy required, the more fuel you need
The "Rocket Equation"
Faster velocities require exponentially more mass

Relativistic Starships

Accelerate a starship to near-light speeds:
--Need 0.1c to reach nearest star in 50 years.
Energy costs are enormous:
--Amount of fuel increases exponentially with the velocity
Fuel Sources
--Max efficiency for matter/antimatter fuel

Other Fuel Sources?

Interstellar Hydrogen?


Travel at 0.1c
50 years to nearest star system
150 wait time
Inhabited region grows at 1% speed of light
Colonization of entire galaxy in 10 million years!!!

Travel at 0.01c
500 years to nearest star system
5,000 wait time
Inhabited region grows at 0.1% speed of light
Colonization of entire galaxy in 100 million years

Colonization times are small compared to the age of the Galaxy for reasonable assumptions

Where is everybody?

See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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