28.4 The Search for 28.4 The Search for Extraterrestrial IntelligenceExtraterrestrial Intelligence

For the sake of argument…For the sake of argument…

(and this section) we will assume that we (and this section) we will assume that we have intelligent, communicative neighbors.have intelligent, communicative neighbors.

Meeting Our NeighborsMeeting Our Neighbors

We assumeWe assume– The average lifetime of a technological The average lifetime of a technological

civilization is 1 million yearscivilization is 1 million years

We further assumeWe further assume– The average distance between civilizations is The average distance between civilizations is

30 parsecs (pc) or 100 light years30 parsecs (pc) or 100 light years

As a result, any two-way communication As a result, any two-way communication will take at least 200 yearswill take at least 200 years

The fastest space probes today…The fastest space probes today…

Travel at 50 km/s.Travel at 50 km/s.

This means that to go to the star nearest This means that to go to the star nearest our solar system, Alpha Centauri, it would our solar system, Alpha Centauri, it would take about 50,000 years.take about 50,000 years.

To our nearest assumed technological To our nearest assumed technological neighbor would take 600,000 years.neighbor would take 600,000 years.

Speeding up to light speed would help. Speeding up to light speed would help. We just can’t do it yet.We just can’t do it yet.

What have we already done?What have we already done?

Pioneer 10Pioneer 10, launched in the mid-1970s, is , launched in the mid-1970s, is now beyond the orbit of Pluto contains a now beyond the orbit of Pluto contains a plaque with Earth info.plaque with Earth info.

VoyagerVoyager has similar information. has similar information.

Is it a good idea to contact ET?Is it a good idea to contact ET?

Any civilization that contacts us would Any civilization that contacts us would likely be more advanced than us.likely be more advanced than us.From your book, “If extraterrestrials From your book, “If extraterrestrials behave even remotely like human behave even remotely like human civilizations on Earth, then the most civilizations on Earth, then the most advanced aliens may naturally try to advanced aliens may naturally try to dominate all others.”dominate all others.”Maybe not all intelligent species do this! Maybe not all intelligent species do this!

Radio CommunicationRadio Communication

Long-wave radio radiation may be the best Long-wave radio radiation may be the best form of energy to use to communicateform of energy to use to communicate– Light and other high frequency radiation is Light and other high frequency radiation is

scattered while moving through dusty scattered while moving through dusty interstellar spaceinterstellar space

Likely we would listen for radio signals Likely we would listen for radio signals from other civilizations.from other civilizations.

This is underway for some stars.This is underway for some stars.

Radio Communication ProblemsRadio Communication Problems

Our success is contingent upon whether or Our success is contingent upon whether or not ET is broadcasting radio signals.not ET is broadcasting radio signals.

Some stars give off radio waves, as do Some stars give off radio waves, as do some interstellar gas clouds.some interstellar gas clouds.

Even Earth gives off radio waves!Even Earth gives off radio waves!

Why? Because of all of the radio stations!Why? Because of all of the radio stations!

Only FM signals leave Earth (AM Only FM signals leave Earth (AM broadcasts are trapped in our ionosphere).broadcasts are trapped in our ionosphere).

Radio Communication ProblemsRadio Communication Problems

The radio waves the Earth gives off are The radio waves the Earth gives off are not constant. Why?not constant. Why?There are pockets of radio activity on There are pockets of radio activity on Earth (eastern US, western Europe).Earth (eastern US, western Europe).If any sufficiently advanced civilization If any sufficiently advanced civilization resides on a planet orbiting any of the resides on a planet orbiting any of the thousand or so stars within 70 ly of us, we thousand or so stars within 70 ly of us, we have already broadcast our presence to have already broadcast our presence to them!them!

The Water HoleThe Water Hole

What frequency do we aim for? What frequency do we aim for? At or near 20 cm. This is near the At or near 20 cm. This is near the frequency of hydrogen, and the simple frequency of hydrogen, and the simple molecule hydroxyl (OH).molecule hydroxyl (OH).These two together form water.These two together form water.The range of 18-21 cm is called the The range of 18-21 cm is called the water water holehole, or oasis, where all advanced galactic , or oasis, where all advanced galactic civilizations would gather civilizations would gather electromagnetically.electromagnetically.

Project PhoenixProject Phoenix

Also known as “SETI” – Search for Also known as “SETI” – Search for Extraterrestrial Intelligence.Extraterrestrial Intelligence.

Began in the late 1990s.Began in the late 1990s.

Large radio antennas are used.Large radio antennas are used.

Computers “listen”. Humans listen only if Computers “listen”. Humans listen only if the computer shows something the computer shows something interesting.interesting.

No signs have been detected yet.No signs have been detected yet.

Arecibo Radio Telescope – Puerto Arecibo Radio Telescope – Puerto RicoRico

Arecibo googledArecibo googled

VLA – Socorro, NMVLA – Socorro, NM