Lecture 36: Where is Everyone
Astronomy 101/103
Terry Herter, Cornell University
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Lecture
Topics
  • Why don't we see them?
  • Limits to growth
  • Starflight

Where is
Everyone
  • If life is common in the Universe and we are typical, then eventual exploration and colonization seem likely.
  • How long to colonize the galaxy?
  • If traveling at 1/100th speed of light, about ten million years.
  • The galaxy is ~10 billion years old, so colonization should have happened long ago.
  • We should be descendants of some ancient settlers!
  • We certainly are not!!

Which assumptions are wrong?


Possibilities
  • Civilizations don't live very long?
    • Overpopulation
    • Self-destruction (war, eco-disaster, ...)
  • Peaceful bliss? (Sociological Changes)
  • Interstellar travel is too hard
    • Yet we don't have to travel but just communicate!
  • We are alone.

Overpopulation?
  • ~ 6.5 billion people in the world.
  • Every minute:
    • 247 babies are born & 107 people die
  • The world gains 140 people each minute.
    • 8,400 per hour
    • 202,000 per day
  • About 74 million people per year!

Source: US Census Bureau

Population of
Growth


Year (A.D.)
Population
Doubling Time (years)
0
100,000,000
-----
1000
200,000,000
1000
1500
400,000,000
500
1800
800,000,000
300
1900
1,600,000,000
100
1965
3,200,000,000
65
1979
4,500,000,000
-----



Limits to
Growth
  • If left unchecked, problems will arise with:
    • Energy production
    • Food production
    • Maintaining healthy ecology
    • etc.

Earth's
Population
History

Historical Population

Data from US Census Bureau

Earth's
Population
History

Historical Population (Inverse Plot)

Data from US Census Bureau

Future
Population
Projection

Projected Future Population (Inverse Plot)

Data from US Census Bureau

Population
Growth Rate

Population Growth Rate

Data from US Census Bureau

Food
Limit

Suppose:

  • All land used for production of wheat.
  • Production of 800 tons/square mile.
  • Wheat yields 3.7 Calories/gram.
  • One person needs 2500 Calories/day.
  • Final density of 3140 people/square mile.
  • Total earth population 180 x 109.
  • 45 times greater than current population.
  • If growth rate stops at:
    • 2.0 % / year => this situation reached in 172 years
    • 1.0 % / year => this situation reached in 342 years
    • 0.5 % / year => this situation reached in 682 years

Energy
Limit
  • Limit due to temperature balance of the earth.
  • If allowed maximum energy production is 1% of solar input = 2 x 1015 Watts.
  • Present consumption ~ 1013 Watts for the world (about 4 times USA).
  • Projected (based on US) to exceed allowed limit in
    • 500 years for 1% growth
    • 153 years for 2% growth

 

U.S
Power
Consumption



 

Growth in U.S
Power
Consumption

Plot show yearly change in US Power consumption

Space
Flight
  • The galaxy is many light-years across ~100,000 ly
  • To travel great distances, we need to go fast, that is, as near to the speed of light as possible.
  • It still takes a long time to travel between the stars (from Earth's perspective).

Practicalities
  • Interstellar flight is not easy!
  • The first starships are on their way.
  • 4 spacecraft are on interstellar voyages:
    • Pioneer 10 and 11
    • Voyager 1 and 2
  • Voyager 1 will approach within 1.64 ly of another star in 40,300 years!!

How do
Rockets
Work?
  • Newton's Third Law:
    • For every action there is an equal and opposite reaction.
    • Conservation of momentum
  • Rockets are self-contained.

  • They do not "push against" the outside world.
  • A spring or explosive pushes the two blocks apart.
  • The blocks go in opposite directions.
  • The smaller block moves faster.
  • In a rocket, the burning of fuel pushes material away from the rocket at high speed.
  • The "payload" moves (accelerates) in the opposite direction.
  • The faster the exhaust material, the greater the acceleration.

Populsion
Methods
  • Classical rockets carry their own fuel, so they must push it too (until it is "burned up").
  • Chemical rockets won't work.
  • Nuclear propulsion may be possible.
  • Alternatives are "beamed" energy (lasers, pellets), interstellar ramjet, and interstellar solar sails.

How Long?
  • For flight to Proxima Centauri (4.3 ly) Fly-Through mission at 0.05c.
    • About 86 years.
  • For a standard rocket, the mass of the rocket would be 1000 to 1,000,000 times the mass of the payload, for the "best" impulse fusion vehicles.

Patience...
  • It is not necessary to travel at relativistic speeds, but long range planning is necessary.
  • The first star travelers will be unmanned probes/robots.

Von Neuman
Probe
  • Build a smart probe which travels between stars and replicates itself.
  • It will fill the galaxy in a short time (millions of years) with such probes.
  • Where are they?
    • too hard to build? (unlikely)
    • no interest (by all civilizations?)
    • we are alone?
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