Using the GAVRT Radio Telescope: The SETI Project Lesson 4: Probability of Discovering a Signal Questions? Please contact Carolyn Donelan @ carolyn.donelan@richlandone.org.

Make a Prediction! Complete this sentence: “My prediction on the chances of us finding an extraterrestrial signal is __________.” You may make your prediction in a ratio, percentage, or just a statement about the chance of finding such a signal.

What is statistics? “Statistics is a branch of mathematics dealing with the collection, analysis, interpretation, and presentation of masses of numerical data.” Source: http://www.merriam-webster.com/dictionary/statistics

What is Probability? “Probability is the branch of mathematics that deals with calculating the likelihood of a given event’s occurrence, which is expressed as a number between 1 and 0.” Source: http://whatis.techtarget.com/definition/probability

Some Vocabulary An experiment has results, called outcomes. An outcome is the results of a single trial. (For example, a single run of the telescope). An event is one or more outcomes. (For example, more than one class discovering the same signal during a session.) Probability is the measure of how likely that event is.

So what is the probability of discovering a signal from an extraterrestrial civilization? To calculate that probability, we need some more data…

How Many Stars Are There? The Universe has about 300,000,000,000,000,000,000,000 stars. (say “300 sextillion stars”!) By NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI) - http://hubblesite.org/newscenter/archive/releases/2014/27/image/a/ (image link), Public Domain, https://commons.wikimedia.org/w/index.php?curid=33189266

How Many Stars Are There? The Milky Way has between 200,000,000,000 and 400,000,000,000 stars. (say “between 200 billion and 400 billion stars”!) Image credit: http://www.nasa.gov/images/content/178493main_sig07-009-hires.jpg

The Galactic Plane Our galactic center holds about 60% of our galaxy’s stars. GAVRT “sees” most of our galactic plane. In the galactic plane, there are pulsars, quasars, turbulent pockets of interstellar gas. These all make “noise”, which cover a wide band of frequencies. (So we will search in a narrow band of frequencies, less than 300Hz) Image credit: https://www.sbnature.org/content/690/galeq.PNG

N = R* * fp * ne * fl * fi * fc * L Drake’s Equation Astronomer Frank Drake developed a formula for evaluating the possibility of hearing an extraterrestrial signal. N = R* * fp * ne * fl * fi * fc * L N represents the number of extraterrestrials that use radio technology. R represents the rate that sun-like stars are formed in the Milky Way. fp represents the fraction of those stars that have planets. ne represents the number of planets in a star system that would be able to support life. fl represents how many Earthlike planets are there on which life has arisen. (This number is uncertain.) fi represents the number of planets with intelligent life. (This number is uncertain.) fc represents the number of planets with intelligent life that have the ability to communicate with radio waves. (This number is uncertain.) L represents the length of time than an intelligent civilization continues to exist. (This number is uncertain.)

Drake’s Prediction Frank Drake predicted that the final number of stars with possible planets with intelligent life that communicates with radio waves in our galaxy to be about 2,000. As we learn more (such as finding more exoplanets with the Kepler Telescope), we can revise our prediction. (So, what percentage of 200 billion is 2,000?)