First, a word about homework 3 and measurement errors.

Those of you that did the measurements carefully, e.g. waited for sufficient orbits to build a histogram, produced nice looking results.

In which there is a well defined main sequence, and a single detached red star which is the one red giant in this sample.

Those of you that did this in a hurry or didn't care at all produced these kinds of results

The digrams are a mess and therefore it would be difficult to even answer the questions.

An average of all the data also looks pretty bad, but it effectively demonstrates the problem with measuring errors. A good question for the final exam, indeed.

Things to note:

• The smallest errors are down at the very bottom of the main sequence. This is because these stars are the closet, and therefore the fixed measurement error is a smaller percentage of the true measurement.

• The largest errors tend to be for the hot and luminous stars ( upper end of the main seqeunce) These stars tend to be rare so there are very few nearby examples.

• There is only one bonafide red giant in this sample of 62 stars. If you made a sloppy HR diagram, you wouldn't know this and you would conclude that the population of red giant stars is much larger than it really is.

Moral:

• If you take poor data you can't learn anything

• If you rush your way through life, you won't learn anything.

So, speaking of life, how much of it is there, out there in the Universe?

Is there Intelligent Life in the Universe?

A Statistical Estimate for the How Many Are Out there

We can write down a statistical equation to estimate the number of civilizations which currently exist in our Galaxy .

This equation has three parts:

• An astronomical part which basically predicts the number of planets that exist in the galaxy which have a liquid surface
• A biological part which basically estimates the probability that intelligent life will evolve on those planets which have a liquid surface
• A socio-economic part which determines the survivability of the civilization on that planet

Remember, we seem to be part of this Process Our choice now is which divergent path do we choose? - the long term one or the short term one?

So, here is the equation:

• The Astronomical part is the first three terms
• The Biological part is the next two
• The socio-economic part are the final two terms

• The uncertainty associated with each term grows as you move down the right hand side of the equation

So what should the estimates be?

Well today's in class exercise will find out.

Using This Worksheet let's fill out each value one by one having some dicussion about range of plausibility for the values.

All parameters denoted as F(x) are probabilities and have values between 0 an 1.

Therefore, if you think the probablity is 1 in 10000 for one of these value you would need to enter that as .0001

The value for L, lifetime, is in years. So if you think communicating civilizations live for 10,000 years then you put in 10000 for L.

The value for N(e) is just a number. For our solar system, N(e) = 1. If you think that earth like planets exist in only 1 out of every 10 solar systems then you would input 0.1 as the value for N(e).

After each value is entered, press publish to global view so we can see the results build up.

A preferred Answer?

• The Astronomical Part: Basically, observations suggest that planetary formation is a natural consequence of star formation. Nearby stars exhibit dusty disks in orbit about them which are likely solar systems in the process of formation. For every star of some temperature, there is a zone around it called the habitable zone in which the temperature is right for the condensation of water vapor. The width of the habitable zone depends on the temperature of the star. If you make planets via the acretion of small grains then it is likely that one of the acreting planets will land in this habitable zone. In our case, if the sun were a cooler star we would have this class on the second planet from the sun, if it were hotter, we would be Martians. Hence, I regard the formation of planets with a liquid surface as ubiquitous.

• The Biological Part: I believe that given enough time, chemistry, mixing in the liquid surface, local concentration of organic molecules, etc, will eventually lead to a self-replicating system. That process took a billion years to happen on the earth so maybe all you need is time. But, since the planet is subject to random catastrophe, there are all kinds of problems that species have to overcome, I rate the probability of reaching intelligence as only 10% (of course it could be much smaller!).

• The Socio-Economic Part: Okay, does intelligence begat Technology? Is it the manifestation or the asshole of Intelligence? Does the capacity to destroy our planet mean that we can never really be Intelligent? And what about our lifetime as a technical civilization, will it be short or long? I assume again only a 10% probability of intelligent civilizations becoming technological (maybe most are smarter than US?). So overall, I have assumed that on only 1% of all planets with a liquid surface does a civilization which builds radio telescopes arise.

  For the lifetime I adopt the following argument:

  • 99% of all technological civilizations extinct themselves after 100 years
  • The remainin 1% survive, in harmony with their planetary resources for 1 billion years.
  • This leads to an average lifetime of 10 million years (1% of 1 billion)

• Notice, that the number of civilizations which currently exist is strongly correlated with your estimate of the average lifetime.

• Multiply it all together to get one million civilizations which now exist. This is actually a small number.

Why one million is a small number

What is the Moral: Life, All Life is Rare whether its yours, your neighbors or your neighbors pet slug.