PART 4: EXTRATERRESTRIAL INTELLIGENCE,  INTERSTELLAR TRAVEL, FERMI PARADOX

Read book chapters: 12 and 13.

What are the odds of finding intelligent life in the Milky Way? The "solution" was formulated in the form of an equation by Frank Drake: The Drake "equation".

Let's first do a more familiar problem: How many piano tuners are there in Chicago?

Assumptions:

• Every piano tuner can tune 200 pianos/yr

• A piano needs tuning every year.

  So we need one piano tuner for every 200 pianos that get tuned every year.

• Number of people in Chicago and its suburbs: 5,000,000

• Number of family homes: 30% of that or a fraction of 0.3.

• Most pianos are owned by families with school-age children: 50% of that, so fraction = 0.5.

• How many such families make enough money to own a piano? Let's say one in three, so 33% or fraction = 0.33.

• In how many such families do the children actually decide to play piano? Let's say one in ten, so 10% or fraction = 0.1.

• How many such families then decide to actually have the piano tuned every year? Let's say one in four, so 25% or fraction = 0.25.

  So how many piano tuners do we need?

  5,000,000 X 0.3 X 0.5 X 0.33 X 0.1 X 0.25 = 6188 pianos, so we need about 30 piano tuners.
  
  
  

Now, with that example we can try the same equation for the Milky Way to estimate the number of advanced civilizations that may be present:

Number of civilizations present today in the Milky Way Galaxy =

(number of stars present today in our galaxy suitable to have planets in habitable zones for sufficiently long time), times

       (fraction of those stars that actually have habitable planets), times
                   
       (fraction of habitable planets that have life), times
      
       (fraction of habitable planets with life on which intelligent civilization capable of interstellar communication has developed at some point in the planet's history), times
      
       (fraction of planets that have intelligent civilizations now)

Example.

- Suppose that every one in ten stars has a planet that is habitable,
- that one in 100 of those develops life on it,
- that one in 1000 of those has at some point developed intelligent life,
- and that one in 10000 of those has intelligent life form present now.

If our Galaxy has 100,000,000,000 stars with habitable zones, then:

• 10,000,000,000 would have habitable planets,

• 100,000,000 would have planet(s) with life on it,

• 100,000 would have at some point have had intelligent life forms on it,

• and 10 would have a present day civilization capable of communicating with us.

Obviously, it is all in the numbers! We don't know the odds of any of the terms in the equation yet, although we are making good progress with the first couple of items on the list.

The BIG questions:

- odds for finding planets with life elsewhere?
- odds for developing intelligent life?
- how long will an intelligent civilization last?

Important aspect: idea of "convergent evolution". This is related to the question of what are the odds of developing advanced life if life is already present in more primitive forms. The idea of convergent evolution is that even if life evolves through different paths for different species, many share characteristics anyway, even if they followed different evolutionary (in a biological sense) paths. Under this scenario, the odds of developing advanced life forms may be large. See the graph in the book on brain mass versus body mass. Can you think of arguments in favor of this notion and against this notion?

The reality of the Drake equation: with the example of only one planet with advanced life forms known, it cannot give us any reliable answer as to the number of civilizations that might be out there. 

So, let us move on to a more active initiative:

How to find evidence of extra-terrestrial intelligence?

Past claims that we have been "visited" are highly spurious at best, and outright lies at worst. There is a lot of money to be made selling books about pseudo science, as any trip to a bookstore will tell you!

We are currently left with one option:

Search for signals from outer space that cannot be explained by natural phenomena (remember Ockham's razor!). This is done by project SETI, the Search for Extraterrestrial Intelligence.

Considerations:

We have already been broadcasting our existence to the outside world; they may be watching our soap operas as we speak. There are about 2500 stars within 50 light years from Earth. And we have been "leaking" t.v. and radio waves into space for longer than that...

Think about this: if aliens were listening for our radio signals, only those within about 100 light years would even know we exist. This region spans a tiny, tiny section of the entire MW, so most alien civilizations, if they exist, can not know about us from listening in....

Questions:

• should we be concerned about announcing our existence into space?

• what is the strongest signal we have been sending into space?                     

PROJECT SETI and here is how you can help: SETI at home

What to search for?

Radio waves makes most sense: lowest energy electromagnetic waves, cheap to produce, very good for communication since they are not blocked by atmosphere, interstellar dust, etc. Radio waves are part of the electromagnetic spectrum, just as visible light, and travel at the speed of light in vacuum.

Challenge: the radio spectrum is vast. What frequency ("color") of waves to search for, and what type of signal? The signal has to be artificial, e.g. restricted to very narrow band width (but not a known spectral line),  and likely periodic or distinctly repetitive. Maybe the signal would be close to a well-known frequency, such as that of the element hydrogen, which has a distinct spectral line at 21-cm wavelength in the radio spectrum. There is also some prominent OH lines lines around 18 cm, so some astronomers talk about looking in the "water hole" of the spectrum, between wavelengths of 18 and 21 cm.

If the signal is not sent on purpose, but e.g. just leakage of radio waves into space, like our radio, t.v., and radar signals, then we might be able to detect a periodic variation in the signal of such a planet due to its rotation and the location of its major cities/centers.

False alarm: pulsars.
These are fast-spinning neutron stars which emits beams of radiation, much like a lighthouse, that were at first mistaken for alien signals. E.g. the "Crab pulsars" spins 30 times per second and emits pulses of radation at that rate. It is the neutron star left over from the star that exploded in 1054 AD as a supernova (the explosition was recorded by the Chinese).

Where to search?

Well, it makes sense to start with local stars, since the signals will quickly get fainter with distance (as the inverse of distance squared). Also, pick stars like Sun or somewhat later in spectral type. There are plenty to chose from. Recent results on extra-solar planet detections might help to focus the search on particularly interesting stars.

Challenge: there are many many stars to chose from, and it is quite possible the nearby ones won't have anything interesting: if they did, why aren't "they" here?

How close would they be? If there are 20,000 civilizations in the Milky Way, randomly distributed through stars in space, the closest one, on average, would be 1000 light years away. Signals get pretty faint from that far away!

How to search? See above section on radio waves.

Challenge: establishing contact is by no means trivial. Remember, we need to decode their signal, then send a response back. If they are 50 lightyears away from us, and it takes us 5 years to figure out the message and send a response, it will take another 50 years for our signal to get to them. The larger the distance to the star, the harder "communication" will be!

How might signal be decoded? Hopefully they use "binary notation" like we do in all our computers and messages in the "digital age".

E.g. number 12,345 = 1x10⁴ + 2x10³ + 3x10² +
                                     4x10¹ + 5x10⁰

Likewise, we can write any number in binary notation, pick a simple one: 45 which equals 101101 in binary notation:

101101 = 1x2⁵ + 0x2⁴ + 1x2³ + 1x2² + 0x2¹ + 1x2⁰
            (= 32    + 0       + 8      + 4       + 0       + 1)

Every number can be written using the binary system, as can letters, even pictures (see book Figure 11.6; the squares should be only be black or white!):

Arecibo message



INTERSTELLAR TRAVEL: Dreams and realities...

Challenges for manned space flight we discussed in class:

- extra development cost
- safety
- additional weight of missions
- need to return
- time limit to space travel, while unmanned missions can keep going as long as there is no failure or loss of power.

Why is space travel expensive, even "local" (close to Earth and in solar system)?

- Every object launched from Earth needs to get a speed that is of order the escape speed from Earth. That figure is 11 km/s. To put something in close Earth orbit (like the spac shuttle, which orbits at about 200-300 miles above Earth) requires a speed of about 7 km/s. In class, we compared these speeds with typical every day speeds:

walking - 1.5 m/s
car - 35 m/s
airplane - 220 m/s

The amount of energy required to accelerate something from zero velocity to a velocity v scales with the velocity squared and with the mass of the object. This is called "energy of motion" or "kinetic energy".

So, per unit mass, it takes (11,000/220) squared = 2500 times more energy to launch something in orbit than have it go up in a commercial airplane. This does not include any considerations of forces acting to slow down the object once it is being accelerated (e.g. friction by air and the pull of gravity). This energy requirement explains why in any launch from Earth, most of the space craft is taken up by rockets and fuel, not scientific or human payload.

If we want to leave the solar system or travel from Earth to other planets, the gravitational pull from the Sun becomes important too. To leave the solar system from Earth's location requires a speed of 42 km/s. Since we are already going at 30 km/s around the Sun, we need to gain additional speed to make it to 42 km/s.

So, if we launch a space craft from Earth in a direction that takes it out of the solar system and it attains just the velocity of escape from the solar system by the time most of the fuel is gone, what will happen?

First, the craft experiences no friction in space, so it is not being slowed down by friction. It is being slowed down by the gravitational pull of the Sun. If it moves at just escape speed, it will continue to slow down and never go fast by the time it reaches large distances from the Sun. So, instead we accelerate it further on its way out of the solar system, using the gravitational "assist" from other planets. By cleverly using the knowledge of orbits and dynamics in the solar system, we can actually increase the speed of space craft greatly by having them "swing by" other planets. Thus, we can increase the speed from that required to leave Earth to higher speeds and send the craft on its way.

Still, there are enormous hurdles to get far at the typical speeds that can be reached. For example, going at 100 km/s if we could reach that, it would still take about 12,000 years to reach the closest star to us. We would not need to power the space craft much with rockets, but it would require energy to send signals back to us. And those signals would take longer and longer to reach us, and get fainter and fainter as the space craft gets further away. Once far away from the Sun, the craft cannot be powered by solar energy, so it must have its own internal power source; most likely, with current technology, this would be a nuclear reactor, using radioactive elements to generate energy.

A manned mission out of the Solar System is clearly completely beyond our technological capabilities. Will it always be?

The space craft has to be self-sufficient, it has to provide food, medical care, energy, life support systems etc to a large crew; if it were to travel for generations in terms of human life times, it would have to be very large and take along a lot of people. With conventional fuels and thinking we will not get very far. Several exotic ideas have been proposed in the past, e.g. using nuclear bombs behind the space craft to accelerate it to high speed. Other ideas include using the hydrogen in space as a source of fuel (nuclear fusion!) by having the space ship collect the hydrogen gas as it moves at high speed through the space between stars. The density of the hydrogen is very low and you would need to move fast and have a very large funnel to collect the hydrogen to make this work...

How fast could we travel? Einstein's theory of special relativity sets a hard limit on the highest possible speed; nothing with mass can reach the speed of light. If we keep accelerating an object by giving it more energy it will still not exceed the speed of light, instead it will act as if its mass is increasing and since the energy of motion goes as product of mass and velocity squared, the gain in velocity will be less and less as we approach the speed of light. The relativistic effects become most noticeable only at speeds close to the speed of light, so in principle we could travel at e.g. 25% speed of light without too much non-Newtonian effects.

There is one important advantage however if you could travel at close to the speed of light; for the travellers, the time will slow down compared to an observer at rest, and the distances will be foreshortened. This peculiar effect, called time dilation and length contraction, are the result of Einstein's postulate that the speed of light is the same for every observer in any frame of reference moving at constant speed or at rest.

The implications of Einstein's postulates are:

• clocks slow down inside fast moving space craft compared to Earth (this has been measured). At the extreme limit of the speed of light, no time passes. In other words, for a light beam traveling through space there is no change in time if a clock could come along (which it can't, since it has mass!).

• astronauts could in fact reach the nearest stars in their life time if we could reach speeds close to the speed of light. Many generations might have passed on Earth but not in the space craft.  The challenges would be enormous to manage such "expeditions", you would not find your relatives if you could ever come back on Earth.

We discussed: twin paradox. More on the twin paradox than you ever wanted to know.

Are there better ways to travel? Can we beat Einstein's limit to speed?

There is nothing known within physics today that contradicts Einstein's results. There is speculation of being able to travel by short cuts through "worm holes" associated with black holes, or other exotic ways. If the Universe is as we understand it today, 4-dimensional (3 space and 1 time coordinate) such short cuts are unlikely. However, there is much we don't know about the Universe. It may be possible that there are "hidden" dimensions that allow for short cuts and make travel possible. If we have ever been visited by aliens, they more than likely would have used such shortcuts.

Here is an excellent primer on black holes and the effects that happen there. Note that gravity slows down the light speed too and hence time goes slower near a black hole. Actually, you can think of the "slowing down of light" more like a distortion in space-time so that the light has to travel larger distances near the massive object than if the object were not there. The space time is also distorted, such that there is a gravitational Doppler shift to the wavelength of the light.
One way to see the effect of gravity on light is through gravitational lenses, which we discussed before:
Hubble picture of lensing galaxy.
Explore here: Black holes, gravity's relentless pull
Make sure that you explore several topics in the black hole web page, including:

• travel to a black hole

• seeing what happense to a clock that is dropped into a black hole

• seeing how the orbits are different around a black hole compared to Kepler's elliptical orbits

• how black holes form

• how we know black holes exist

LAST SECTIONS

THE FERMI PARADOX AND CONTACT

Read book chapters: rest of chapter 13

The Fermi Paradox.

Who was Enrico Fermi?  Famous 20th century physicist, working on elementary particle physics, discoverer of the "Fermi-exclusion principle", involved with the Los Alamos Manhattan project in the 2nd world war.

The Fermi paradox: Where is everyone?

If life is common and present on many other planets in the Milky Way, then surely some civilization would be far more advanced than we are today, and they should have been able to colonize our Galaxy, even if interstellar travel proceeded at slow pace. They could have hopped from star to star and colonized large sections of the Milky Way in millions of years, even travelling well below the speed of light (see book).

Even if the civilizations themselves could not travel, they would have been able to design self-replicating robots that could have traveled across the Milky Way. Where are they?

Motives for colonization:

- if our history on Earth is any indication...
- avoid extinction: send out groups of an advanced species to settle new colonies. Not everyone has to go!
- threats of star ending its life or "impending" impacts.

Non-motives for colonization:

- Cannot solve population growth problem on Earth.
- Conquest?
- Bring natural resources back to Earth. Seems too expensive.


Possible explanations for the Fermi paradox:

1. We are in fact the most advanced civilization. In this case, SETI will not find a signal for a long time...

Seems unlikely: our most rapid advance has happened in a relatively short period (see "cosmic calendar argument" in the book) and our solar system was formed 8 billion years after the Milky Way began to form so there are many stars and presumably planets out there that preceded us by many eons. While the very early MW might have lacked heavier elements, successive generations of stars would have created them and so there would be ample opportunity for life to develop elsewhere.

Also: everything we have learned since Copernicus has shown us that Earth is not the special place it was once thought to be. We are not the center of the solar system, there are billions of stars like the Sun, we are not near the center of the Milky Way, and there are billions of galaxies like ours.

Proponents of this idea: "Rare Earth Hypothesis". Many conditions may have conspired to make Earth more special than we think.

2.  Civilizations are not interested or able to colonize other solar systems.
     a. Technological challenges?
     b. sociological considerations?
     c. self-destruction

3. They exist but don't reveal themselves to us.

Why would we be interesting to them? The "Zoo hypothesis" (or "wildlife refuge").
The "sentinel" hypothesis. They are monitoring our progress and are waiting for us to advance more.

What is the most popular explanation in your eyes?

Or have we been visited?

Roswell
"Alien abductions"

The true answer :)
SETI fiction: They are made out of meat.

BACK TO EARTH: LET'S PLANT OUR FEET IN SOLID SOIL AND MAKE A BALANCE OF WHAT WE DO AND DON'T KNOW AND WHAT SITUATION IS FROM SCIENTIFIC PERSPECTIVE

Scientific results thus far:

THE GOOD:
Billions and billions:

• Sun is a normal star of which there are many in the Milky Way.

• Stars like the sun can shine for billions of years due to the nuclear fusion in their centers. Our solar system is 4.5 billion years old, less than the age of the universe and the age of the Milky Way (12-14 billion years).

• The Milky Way is a normal spiral galaxy of which there are hundreds of billions in the Universe.

• The Earth is one of 4 terrestrial planets in the solar system. The other planets are like us in size and composition, but lack water (either liquid or all together) and have very different atmospheres.

• Planets around other stars are being found in large numbers. We cannot yet find planets of Earth mass easily but so far this is a technological limitation. There is no reason to claim that planets like Earth don't exist elsewhere. In fact, there are probably (you guessed it) billions and billions of them...

• Our solar system is not located in any special place in the Milky Way.

• Our Milky Way Galaxy is not located in any special place in the Universe.

• The Earth and its inhabitants are made out of atoms that are equally common elsewhere in the universe.

• Living beings are made out of a subset of those atoms.

• There is strong evidence that early life on Earth was primitive and that complexity of species increased over time due to evolution. Biological evolution is the result of mutations in genetic material upon reproduction, with natural selection as the "guiding mechanism" to push evolution in particular directions depending on the local and global conditions.

• There is overwhelming evidence for biological evolution occurring on earth, including evolution of species, mass extinction of species, increases in intelligence among some species with time,  advances in use of tools and technological discoveries.

All of this taken together, in my mind, makes the existence of life elsewhere a likely prospect.

THE (as of now still) BAD:
The uncertain or unknown:

• Science argues for the spontaneous generation of life in some primitive form on Earth, but we have not been able to replicate this in the laboratory.

• We have not yet found evidence of life on other places in the solar system. The likeliest places to find such life may be on Mars and on some of the major moons near the giant planets. Our guiding principle for establishing likely presence of life is the need for liquid water to exist. This establishes the "habitable zone" around other stars and planets.

• The Earth's moon seems rather special and its formation a fluke of nature. How essential has it been for development of advanced life on Earth?

THE UGLY....?
The perhaps impossible:

• The physical laws that we have discovered place severe limits on our ability to travel to other stars in reasonable amounts of time from the perspective of earthlings. The speed of light in vacuum is faster than any speed an object that has mass can attain. If we could travel at close to the speed of light, time dilation would allow the astronauts to reach the other stars, but we have no way of generating the amounts of energy required to do this in a spaceship. We have no scientific evidence that these same limitations would not apply to other civilizations.

• The jump from current technology in space travel to traveling through the Milky Way to other stars is an enormous one. It will require completely different technology than "conventional rocket" engines. It will require self-sustaining space ships in areas far from direct energy sources. It is a jump that is very much larger than the one from cars to airplanes, or airplanes to current space flight capabilities.

• We can in principle communicate with other civilizations through electro-magnetic waves which travel at the speed of light. It seems plausible that radio waves are the most likely wavelength range of electromagnetic waves to use for such communication. We are searching for signals of artificial nature that may be coming to us from other civilizations. We have not found any such signals but have only started looking recently and over a limited range in wavelength and search space. Given the weakening of signals with distance traveled and the enormous distances between stars, absence of signals will perhaps never imply evidence of absence of alien civilizations.

We don't have scientific evidence to support:

• Past or present visits by aliens

• Descent from aliens

• Monitoring by aliens

• Advice from aliens

• Materials from aliens

• Contacts from aliens

While absence of evidence is not evidence of absence, we should be guided by scientific principles to make conclusions, and with no evidence or theoretical supporting arguments, at this point absence of evidence at least implies no scientific case for support of the mysterious. Sorry, Captain Kirk.

All in all though, the universe is a big and miraculous place. Predictions about what we can and cannot do in the future have always been wrong. We should therefore not be guided by pessimism, but by the search for the unknown. It is only in this way that increases in our knowledge and technological improvements have come about. Enjoy the journey.