ET Life #37kretke/astr3300/lectures/lecture37.pdfThe monolith in Arthur C. Clarke’s “2001: A Space Odyssey” ... the nearest star is like the difference between walking across

ET Life #37

May the fourth be with you!

Reminders

• All extra credit must be in to me by today (email by midnight)

• Paper Revisions & Response to Peer Reviewer Due Monday 5pm (turnitin)

• Final Exam 7:30am-10:00am Thursday

Fermi Paradox

• The apparent size and age of the universe suggest that many technologically advanced extraterrestrial civilizations ought to exist

• However, we haven't found any confirmed evidence of aliens

The Universe Is Old, So…

The Cosmic Calendar

(Book says Aug. 14)

• First civilizations might have arisen ~5 Byr after galaxy formed:– Mid-June on CC.

• If 1/1,000,000 stars has civilization, a new civilization arises each 60,000 yr!– Each 3 min on CC.

Colonizing with Von Neumann Machines

Self-replicating machines.

1. Travel to and explore other worlds.2. Dig up resources.3. Use those resources to build more robots.4. Disseminate and colonize additional worlds.

Would spread from star system to star system.

Requires technology only slightly more advancedthan our own.

How Long Would It Take To Colonize The Galaxy?

If:-you could travel at 10% the speed of light, 0.1 c (3 x 107 m/sec)

And:The average distance between stars is 5 light years (50 years)

And:After 150 years you can spread to the next system, sending newcraft to one or two other systems.

Then:You could colonize the entire galaxy in 10 million years if youstart at the edge of the galaxy.

If you travel at 0.01 c, and it takes 5,000 years between hopsit would only take 100 million years to colonize the entire galaxy.

The Coral Model of Galactic Colonization

• Coral Model:– Advanced beings

(or self-replicating machines) are sent from home planet to build colonies;

– Each colony sends out more colonies.

• Entire galaxy is colonized “quickly”:– ~105 yr at 0.9 c– ~107 yr at 0.1 c– ~108 yr at 0.01 c

Possible Solutions to the Fermi Paradox

• We are alone / we are the first• Civilizations are common, but no one has

colonized the galaxy– Technological difficulties (interstellar travel is hard)– Is our desire to expand unusual?– Do civilizations destroy themselves?

• There is a galactic civilization– Maybe it is actively avoiding us– Maybe we haven't looked hard enough (or in the

right way)

The Prime Directive / The Zoo Hypothesis

Other civilizations are aware of our presence, but they havedeliberately kept us in the dark about their existence.

Or they have kept our solar system “off limits” to leave us alone.

The Sentinel Hypothesis:The monolith in Arthur C. Clarke’s “2001: A Space Odyssey”ETs leave a device that sends a signal when our civilization becomes sophisticated enough.

Interstellar Travel

Is it possible? Why is it so difficult?

• Interstellar travel might be possible, but it would be very difficult because stars are so very far away and space is quite empty.

• The difference between going to the Moon and the nearest star is like the difference between walking across town and walking to the Sun! – Walking across town at 1.3 m/s for 10 km takes

about 2 hours.– To get to the Sun (149.6 x 106 km) at this rate

would take: 3,646 years

• It would be something like 100,000,000 times farther than we’ve ever traveled.

Is it possible? Why is it so difficult?

• Contrasting various kinds of propulsion– Why do cars, ships, planes and balloons

move forward? How do they stop?– Why doesn’t this principle work in

space? – Stuff has to be taken to push against.– How does a rocket work?– What determines the speed of a rocket?

What determines a rocket’s speed?

• How much propellant is used

• The velocity of the propellant

• In space, you can reach the same top speed by taking either a lot of propellant and throwing it slowly, or by taking a little bit of propellant and throwing it fast.

Specific Impulse (Isp

)

(How much thrust / unit of fuel)

Thrust

(How muchpower, howquickly you can accelerate)

Needed to get out of gravity well

Would eventually get you there, but slow acceleration

Specific Impulse turns out to be a huge problem

How much propellant would it take to get to the nearest star in 1000 years?

Chemical

• High thrust, low specific impulse Isp ~500

• Massp = 10137 kg… more than all the mass in the Universe

• All ‘traditional’ rockets: V2, Saturn V, Space Shuttle, ICBMs, etc.

Saturn V

Nuclear Thermal

• Still low Isp ~1000

• still more than all the mass in the Universe

• Tested in the 1960s for a post-Apollo Mars mission (scrapped; ATT). Great for interplanetary travel

Fission and Fusion

• Isp ~5000 – 10,000

• Massp = 109 fission or 103 fusion supertankers

• Basic designs have been made, but no working models

Ion Drive

• Isp ~5000 – 50,000

• Massp = 10 equivalent railroad tanker cars of Xe propellant

• Actually built and flown! DS1 xenon ion drive in 1999, exhaust velocities ~100,000 km/s but need another source of power (nuclear, solar) to run engine

Matter/Antimatter• Isp ~105 - 107

• Might work at ~100% efficiency, mass required is reasonable

• No real designs, many enormous hurdles (containment, production, radiation, efficiency)

• 100 billion dollars for 1 mg!

Specific Impulse (Isp

)

(How much thrust / unit of fuel)

Thrust

(How muchpower, howquickly you can accelerate)

Needed for

interstellar travel

Needed to get out of gravity well

Would eventually get you there, but slow acceleration

RECALL

• This is for a 1000 year, one-way voyage to the nearest star.

• Are there ways to do this without carrying the propellant with you?

• Space is not quite empty

Relativity and other weirdness

• No matter who you are or where you are, how fast you’re moving and in what direction, the speed of light in vacuum always appears the same to you.

• Whoa, but this has been verified many times

• You can’t go more than 3 x 108 m/s

• The faster you move, the slower time passes

• The faster you move, the more massive you get

To a person on earth, the journey takes the “normal” amount of time...

to the fast moving traveller, time seems shorter

Intelligent Life

• When will humans find convincing evidence of intelligent extraterrestrial life?

a) In the next 20 years.b)In the next century.c) In the next millennium.d)Never--there is probably no other

intelligent life out there to find.

http://zenpencils.com/comic/42-neil-degrasse-tyson-the-most-astounding-fact/

Course overview• You have thought deeply about the possibility of “life out there” one of

the oldest questions of humanity• You have learned how astronomers, geologists, biologists, planetary

scientist, etc. are able to scientifically address these big questions• You can look at a planet or moon and understand how a scientist might

determine if life is plausible on that world• You understand that everything we know about life is based on one

singular origin of life on one singular world, and know that everytime we look elsewhere in the universe (or on our own planet) we have seen things that have astonished us and made us re-evaluate our assumptions

The End

Thanks for a great class!

Questions you may want to Consider

• How has humanity's ideas about extraterrestrial life changed with time? What scientific advances/evidence have lead to these changes?

• What is the different ways that life has been defined? Which of these are the the most useful when we look for life elsewhere?

• What are the basic requirements for life? Why might these not be enough for complex life? What else does Earth have that seems to have been useful for the development of complex life?

Questions you may want to Consider

• How big is the universe? The galaxy? The solar system? How does this impact our ability and strategies for searching for life elsewhere?

• How do we think life emerged on Earth? How does this relate to our ideas on how life may have formed elsewhere?

• Why do we think all life on Earth is related? What implications does that have for the emergence of life?

Questions you may want to Consider

• Where are the best places to look for life in the solar system? Why? How are scientists looking to understand the plausibility of life?

• How do we look for extrasolar planets? What are the challenges of finding Earth-like planets?

05/04/12