Lecture 41: Science Fact or Science Fiction?

Intelligent Life in the Universe

Four Opinions:

1. It is highly likely that intelligent life has arisen elsewhere in the Universe.

2. There is no evidence of extraterrestrial visits to the Earth, now or in the past.

3. Lack of visits may be explained by the extreme difficulty of interstellar travel.

4. If we do make contact, it will be by receiving radio signals.

Basic Requirements for Life

Energy

Warmth to permit liquid water (liquid methane?)

Energy to fuel chemical reactions

Complex chemistry

Elements heavier than H and He

Carbon as building blocks for complex molecules

Protection from harmful UV light

Mutations inhibit emergence of complex life

Ozone layer, underwater, or underground

Extreme Life on Earth

Dark Life

Bacteria that thrive many kilometers beneath the Earth or deep in polar ice.

Hot Life

Microbes surviving in boiling geysers, pools and deep ocean thermal vents

On the Moon:

Strep bacteria survived 3 years on the lunar surface!

Life in the Solar System?

Mars

May have liquid water and a thicker atmosphere in the past

Europa

Liquid water ocean warmed by tides

Protected by outer shell of ice

Titan

Methane chemistry

Complex molecules present

Each may satisfy the basic requirements for life to develop

What do we mean by “Intelligent”

This usually means:

A highly advanced technological civilization

Capable of communicating across interstellar distances

Capable of interstellar travel by spacecraft

Interested in finding and communicating with other intelligences.

In other words: life like us.

Do we qualify as “Intelligent”?

Just barely:

Only had radio communications technology for ~100 years

Only had limited (short-duration) manned spaceflight for ~40 years.

Only sent robotic spacecraft to the edges of our Solar System in the last decade

May or may not yet have sufficiently sensitive radio reception technology

Sheer Weight of Numbers

The primary reason I think life must have arisen elsewhere is the sheer number of stars in the visible Universe:

~200 billion galaxies in the visible Universe

~100 billion stars per galaxy

Total of ~2x10²² (20 billion trillion) stars

Even one chance in a trillion would yield more than 20 billion possible sites for life.

Planetary Requirements for Life

Long-lived, stable star

Good: F, G, &K stars: last > 3 Gyr

Bad: O, B & A stars: short-lived, high UV output (damaging to organic molecules)

Bad: M stars: small & dim, powerful flaring

Stable orbital environment

Excludes more binary star systems

Metals=chemically evolved environment

Need metals to make rocky planets

Need carbon for complex molecules

The Drake Equation

N=number of advanced civilizations in the Galaxy

R_*=rate at which Sun-like stars form

f_p=fraction of stars with planetary systems

n_e=number of Earth-like planets per system

f_l=fraction of Earth-like planets with life

f_i=fraction where intelligent life has evolved

f_c=fraction with communication technology

L=lifetime of an advanced civilization

Measurement and Conjecture

Only the first 3 terms of the Drake Equation can be measured by astronomers:

Star formation rate:

R_*~ 1 per year (F, G, & K stars)

Fraction of Stars with Planets

f_p~ 0.1-0.2 from recent planet searches

n_e number of Earth-like planets per system may be measurable in the next few years

The rest are purely conjectural

Shameless Optimism

One very optimistic view:

n_e=0.1 (1 in 10 solar systems have earths)

f_l=1 (if earth-like, life is inevitable)

f_i= (if life, intelligence is inevitable)

f_c= (if intelligence, technology is inevitable)

L=100 years (we made it this far….so far….)

Extraterrestrial Visitations? No.

“Extraordinary claims require extraordinary proof”

No extraordinary proof has been offered:

Fuzzy photographs

Anecdotal accounts of visits & abductions

Claims of government conspiracies

These do not count

There are unexplained sightings, but failure to explain them does not justify a leap to a truly wild explanations.

Where are they?

The extreme difficulty of interstellar travel is a plausible explanation of a lack of visits.

The distances between stars are enormous:

Need very large amounts of time or

Extremely large amounts of energy

The fastest spacecraft: Voyager 1&2

Outward bound at 15 km/sec (0.005% c)

Need 80,000 years to reach the nearest stars

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 acceleration time (use more fuel at first)

50% max efficiency for matter/anti-matter fuel

But the production efficiency is extremely low

Possible given a sufficiently advanced technology?

Talk is Cheap! (and travels at the speed of light)

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

What wavelengths to use?

Microwaves 1000-10000 MHz is a region of relatively low cosmic background “noise”

Lasers at visible to 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)… is this a good thing?

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.

The Search…

SETI:

Search for Extra-Terrestrial Intelligence

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

Phoenix project of the SETI Institute

Optical & Microwave SETI at Harvard

Various smaller projects

What are we looking for?

Signals that appear “artificial”

Very narrow “bandwidth” (<300 Hz, the narrowest natural maser sources)

Pulsed signals (common way to encode information)

Highly polarized signals (another encoding scheme)

Very little frequency “drift”

So far, no detections…

Pioneer 10 detected. Nice to know we can detect our own artificial signals from deep space.

What if we detect something?