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

Listening?

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

fplanet=0.5

nEarth=1

flife=1

fintelligent=0.1

fcommunicate=1

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!

SETI

The Search?

SETI:

--Search for Extra-Terrestrial Intelligence

A relatively inexpensive search strategy to look for radio signals from extraterrestrial civilizations.

Various kinds:

--Targeted

--"Piggy back"

Potentially Habitable Planets

Don't actually know.

Depends on distribution of rocky planets.

Optimistic: n_Earth=1

Examples of Current Searches

SERENDIP

--UC Berkeley

--Arecibo

--'Piggy Back'

--SETI@home

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

--Chemical

--Fission/Fusion

--Max efficiency for matter/antimatter fuel

Other Fuel Sources?

Interstellar Hydrogen?

Sails?

Colonization

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.

[ Return to the Astronomy 141 Main Page | Unit 4 Page ]