Download
1 / 88
880 likes | 996 Views
Extra-terrestrial Civilizations. Are we alone? Contact …. Direct contact through traveling to the stars and their planets Will be a challenge because of the vast distances involved and the (slow) speeds we can travel. Are we alone? Contact ….
E N D
Are we alone? Contact … • Direct contact through traveling to the stars and their planets • Will be a challenge because of the vast distances involved and the (slow) speeds we can travel
Are we alone? Contact … • Radio communication more likely possibility for contact • Electromagnetic radiation travels at the speed of light.
Civilizations • Will life always develop technology? Some societies on Earth have not developed the means to communicate with ETs. • Will a society want to communicate? A society may develop the means to search for ET but elect not to attempt to reach out.
Consider ... • How many intelligent civilizations exist? • How long on average do they last? • How does communication proceed?
Drake Equation • One possible way to estimate the number, N, of civilizations. • N = Ns x fs x ps x ls x lc x L
Stars in the Galaxy, Ns • The number of stars in the Milky Way galaxy … about 300 billion.
Suitable stars (fraction), fs • Star must be old enough to allow life to develop: spectral types F, G, K • Star must have enough heavy elements to form planets … 0.005
Suitable planets in a Solar System, ps • To date, extra-solar planets have been ‘hot Jupiters’ • Planets to sustain life need to be in the habitable zone around a star … 1.0
Fraction of planets suitable for life, ls • Very speculative … sample of 1 only to date (Earth) • If a planet is suitable for life, good reason to think life will develop • Conservative approach suggest: Earth and Mars could produce life … 0.5
Life develops a civilization, lc • Again, very speculative. • Simple life started on Earth nearly 3.5 billion years ago. • Extinction level events common … for example 250 and 65 million years ago.
Life develops a civilization, lc • As long as some form of life exists after an extinction event occurs, natural selection should continue and life redevelops. • Assuming life develops then a case can be made that a form of civilization is inevitable … 0.33
Lifetime of a civilization, L • Firstly, the age of our Milky Way galaxy is 10 billion years. • How long have we had the ability to communicate with ET … about 50 years. • How many times have we sent a communication … not many! • Radio telescope, Pioneer and Voyager
Drake Equation Result • Substituting into N = Ns x fs x ps x ls x lc x L • N = 300x109x0.005x1x0.5x0.33xL/10x109 = L/40 • Large numbers top and bottom tend to cancel out.
Range of answers … • Depending upon your optimism or pessimism, N can vary significantly … • From 10L (Carl Sagan,1978) to a very optimistic 120L to a pessimistic L/10 billion • If civilization survives for 100s or 1,000s of years then N could be very large indeed.
Survival lifetimes • Dinosaurs lived for 150 million years … can we survive for longer thus increasing L substantially? • Some species of life have lived for over 200 million years on Earth. • Humans are living ‘outside’ the laws of Natural Selection … may well reduce L. • Upper limit based upon life of a star … 10 billion years.
More than the Milky Way … • Ours is not the only galaxy in the universe
Why communicate at all? • Curiosity • The urge to talk and listen! • The hope to learn/gain knowledge • The need for resources and/or living space • Because we can!
Why not? • Fear (enslavement, destruction, etc) • Inertia … happy as we are • Economics … expensive to try and need to deploy resources appropriately. • Of course, contact may happen by accident … leakage of radio and TV signals.
How far away is a civilization? • Even assuming optimistic values for the Drake Equation, the closest civilization maybe 100s of light years away! • Average stellar separation in the outskirts of a galaxy … 5 to 10 light years. • Two way communication then becomes a problem.
People or Photons? • People have mass and that requires enormous amounts of energy to accelerate. • People have needs (food, water, air, etc) which means more mass to transport! How much mass per person to take? • Space ships travel very slowly • Photons are mass-less and travel at the speed of light!
Current spaceship technology • Spacecraft travel at speeds much less than 100,000 km per hour • At this speed, travel to the nearest star would take 46,500 years!
Photons • Sending a signal has its own energy challenges • Signal strength drops off as the square of distance.
Photons … • Thus for any given signal strength, sending it say one million times further requires (one million)2 times as much energy … that is, one trillion. • This is technically possible (bigger transmitters, shorter messages, etc) but is not cheap. It is cheaper than sending people in spacecraft though.
Space Travel • (12) Humans have gone to the Moon • Machines have traveled in our Solar System out to Neptune and en route as we speak to Pluto • As a species we have the urge to explore and colonize.
Challenges to travel to the stars • Distances involved are enormous and will take us time to traverse • The energy requirements are equally immense and very difficult to satisfy (even if we are willing to pay the price).
Power for the trip • Chemical combustion is our current form of energy in rockets … very inefficient. • Solar power works well near stars but is also inefficient • Nuclear power for both on-board power (to live, etc) as well as thrust is possible with our technology. • Matter and anti-matter … more efficient certainly but also beyond our means at present.
Exotic power • Interstellar Ramjets … • Ion propulsion … prototypes already tested. • Warp drive … dilithiunm crystals anyone?
Time Dilation • As you travel faster, your own clock (in your frame of reference) slows down from an outside perspective. • Traveling at a significant fraction of the speed of light means you experience a smaller passage of time compared to an Earth based observer
Relativity • T = T0 / Sqrt (1 –v2/c2) • where T0 is the time elapsed in the moving frame of reference • where T is the time elapsed in the stationary frame of reference • where v is the speed you are moving relative to the stationary observer.
A solution? Perhaps traveling at high speed will allow people to survive interstellar treks.
Time dilation example • You and your friend synchronize your watches. • You remain on Earth and your friend ‘flies off’ at 99% the speed of light. • Your friend returns when 1 hour of time has elapsed according to their watch. • You have waited approximately 7 hours for your friend to have returned!
One more danger .. • At higher speeds for our spacecraft, the particles in the ISM are now moving at enormous velocities relative to you. • If your spaceship is moving at 99% the speed of light, the kinetic energy of a particle in the ISM will seem like a very energetic bullet and could do serious damage to the spacecraft … shields anyone?!
Automated Messengers • Instead of people in spaceships, send automated messengers. • Pioneer and Voyager spacecraft already carry messages from Humanity
Von Neuman machines • Build an automated robotic spacecraft and send it to a distant star/planet. • When there, let it mine resources and replicate itself, sending copies of itself to other stars/planets. • In short order, such robots could be everywhere! • So where are they? … the Fermi Paradox (later)
Radio contact: A test? • If civilizations are common, then why have we not yet ‘heard’ them? • To find the signals from ET may involve solving technology not yet known to us. • Is the search for contact a test in itself … are we worth talking to?
Consider … • You can see a cell phone but cannot ‘hear’ what it hears. • Electromagnetic signals pass through your body all the time and you cannot detect them. • Thus the human body is limited to what information it can process as is the cell phone.
Direct or Accidental signals • Realizing that signals from ET may well be very weak, where should we look? … what frequency? • We may be lucky and detect signals not beamed at us … eavesdrop on ‘Star Trek’, ‘Friends’ ,etc. • What type of signal should we look for? • What direction/star (planet) should we listen to?
Where to look • Closer civilizations if they are sending signals will presumably have the strongest signals and be easier to detect. • Signal strength drops off as the square of distance.
Type of Stars • As discussed, stars like our Sun first targets. • In the Milky Way galaxy, stars with similar spectral types (F, G, K) constitutes 10% or more of all stars (30 billion or more). • Double, multiple, very luminous (and thus short lived) stars not suitable targets. • Specialization regarding how many planets contain technologically advanced civilizations.
What frequency to choose? • Recall our discussion about electromagnetic radiation and the multitude of frequencies associated with it.
Because of its electric and magnetic properties, light is also called electromagnetic radiation • Visible light falls in the 400 to 700 nm range • Stars, galaxies and other objects emit light in all wavelengths
Familiar Frequencies • AM dial … radio stations tuned in with frequencies 500 – 1500 KHz • FM dial … radio stations tuned in with frequencies 88 – 110 MHZ • TV channels with frequencies 70 – 1,000 MHZ
ET listens to … CBC? • How to decide what frequency ET will listen to? • Is there a galactic, common hailing frequency? • We assume that a civilization technologically advanced enough to send/receive radio signals will know the language of science.
Considerations • Economical to send a radio photon than say, a (visible) light photon. If we are sending to many stars, cost needs to be controlled (low). • The selected frequency must be able to traverse significant distances without interference or loss.
Problems during transmission • Photons of energy at the wrong frequency will be absorbed … you cannot see through a brick wall but your phone can pick up a signal through the same wall. • Long wavelength radiation can travel further with less absorption … best for sending/receiving signals
Natural background • The galaxy is quote noisy … stars would wash out a visible light signal (even if it could travel a long way through the dust). • The cosmic background radiation is an echo/hiss left over from the Big Bang (high frequency cutoff). • Charged particles (mostly electrons) spiral around the magnetic field lines producing synchrotron radiation (low frequency cutoff).
