The search for extraterrestrial intelligence (SETI) is a collective term for any scientific searches for intelligent extraterrestrial life.

It is done by monitoring radio signals for signs of transmissions from civilizations on other planets.

Topics

The history of SETI

Where could other forms of life exist in our solar system?

Where could other forms of life exist in our galaxy?

How likely is it that life would exist? The Drake Equation

What exactly is our galaxy?

How could we detect sings of intelligent life from outside of our solar system?

scanning radio waves

Why don’t any Earthly organisms detect radio waves?

so what are radio waves, and how do we detect them?

The Water hole: What radio frequencies should we listen to?

Misconceptions about listening with radio telescopes

How can we differentiate between natural or artificial (intelligent) signals?

scanning infrared for signs of Dyson spheres or other megastructures

so what is IR, and how do we detect it?

Where might we find life?

Goldilocks Zone/Circumstellar habitable zone – single star systems

Habitable zones for binary star systems

Atmosphere of brown dwarf stars

surface of neutron stars (very speculative)

Could we realistically ever travel to other star systems? physics of interstellar travel

Where would other forms of intelligent life exist?

There may be other forms of life even here in our own solar system, but almost certainly that would be only primitive, single celled organisms.

That being said, the number of worlds in our own solar system where life may exist, even right now, is larger than more people think. For a variety of reasons, scientists believe that there is a possibility of life existing on

Europa, a moon of Jupiter

https://europa.nasa.gov/why-europa/ingredients-for-life/

NASA Europa Clipper expedition

Europa: A World of Ice, With Potential for Life. NASA

NASA Europa in depth

Enceladus, a moon of Saturn

https://solarsystem.nasa.gov/missions/cassini/science/enceladus/

https://solarsystem.nasa.gov/resources/17649/ingredients-for-life-at-enceladus/

https://solarsystem.nasa.gov/moons/saturn-moons/enceladus/in-depth/

Mars

https://mars.nasa.gov/news/8863/searching-for-life-in-nasas-perseverance-mars-samples/

https://en.wikipedia.org/wiki/Life_on_Mars

https://www.nature.com/immersive/d41586-021-00321-7/index.html

https://mars.nasa.gov/science/goals/

https://www.smithsonianmag.com/science-nature/life-on-mars-78138144/

NASA Viking mission: Evidence of Life on Mars in the 1970s

Jupiter – ideas about how life could exist in its upper cloud layers.

Carl Sagan, Cosmos. Possibility of life on Jupiter. Video

Particles, environments, and possible ecologies in the Jovian atmosphere.. Carl Sagan

https://www.centauri-dreams.org/2009/02/25/edwin-salpeter-and-the-gasbags-of-jupiter/

What exactly is our solar system? See our resource the Solar system.

When we talk about SETI, we’re not looking for life in general, but we’re looking for very complex forms of life that have evolved intelligence and the ability to communicate with the electromagnetic spectrum.

Such life could exist on other planets, or large moons, around other stars in our galaxy, the Milky Way.

At this point we should take a look at what we mean by “galaxy”.

Here is a view of our galaxy as seen from Earth, New Hampshire.

This is what our galaxy would look like if we were above the galactic center, looking down at it.

There are approximately 100 billion stars in our galaxy, with perhaps one trillion planets and large moons, each of which has existed for billions of years. Many scientists believe it likely that life has evolved on many of these worlds.

For a variety of reasons, we have reason to believe that many of these worlds would in many ways be Earth-like, some of them larger than Earth. These are often called super earths.

Current – and even any other potentially feasible – technology is unable to let us detect SETI signals from life on planets in other galaxies. If we were to consider other galaxies, the odds of intelligent life existing somewhere out there is considered near certain.

Observations with the Hubble Space Telescope reveal that there are about two trillion galaxies in the observable universe – each of these galaxies likely having billions of planets and large moons.

This photo shows the Sombrero galaxy, m104. The other points of light around it are not stars, but entire galaxies!

At this point we should stop and clarify precisely what we mean by the word “universe.” – The universe

What are radio waves?

Radio waves are just a part of the EM (electromagnetic) spectrum.

That sounds dandy, except, what exactly is the “electromagnetic spectrum”?

All parts of the EM spectrum – radio, visible light, etc. – are oscillating electric and magnetic fields.

For details see Light is an electromagnetic field.

How are radio waves different from other parts of the EM spectrum?

They’re made of the same thing, behaving in exactly the same way.

The only difference is that radio waves are hundreds of meters to thousands of meters long.

Other parts of the EM spectrum have longer or shorter waves.

What creates radio waves?

With a radio receiver we can hear radio waves coming from all around us. They are naturally produced, and comes from all over the Earth, and outer space.

Radio waves are naturally created by:

* Wind whipping over a surface, creating static electricity

(Here’s a more mundane example of static electricity.)

* Lightning

* atoms trapped in the magnetic fields around the Earth, and around all other planets as well.

* The Sun (it puts out all frequencies of EM radiation!)

* All stars

* Ionized interstellar gas surrounding bright, hot stars

* Supernovas

* There are also more complex radio waves that are naturally generated. See Natural and man-made terrestrial electromagnetic noise

By the late 1800’s humans had learned not only how to receive radio waves, but how to generate them.

Today we artificially create radio waves for all sorts of purposes, including

• Traditional, over-the-air, radio stations (AM and FM radio)

• Traditional, old-fashioned, TV (television)

• Wi-Fi

• Bluetooth

• Cellphone communication (cell towers and the phones)

Cornell.edu: Observational-astronomy. SETI and extraterrestrial life

Kaiserscience – All about the electromagnetic spectrum

What is a radio telescope?

The technology of how we detect radio waves.

http://abyss.uoregon.edu/~js/glossary/radio_telescope.html

How does an antenna pick up radio waves?

“If we place a conducting material on the path of such a wave, the passing wave will create an oscillating electric field inside the material; and that field will accelerate charges back and forth through the conductor.”

https://en.wikipedia.org/wiki/Antenna_(radio)

http://spiff.rit.edu/classes/ast613/lectures/radio_ii/radio_ii.html

The history of SETI

This section has been adapted from “Search for extraterrestrial intelligence.” Wikipedia, The Free Encyclopedia. 4 Mar. 2019

There have been many earlier searches for extraterrestrial intelligence within the Solar System. In 1896, Nikola Tesla suggested that an extreme version of his wireless electrical transmission system could be used to contact beings on Mars. He conducted an experiment at his Colorado Springs experimental station.

In the early 1900s, Guglielmo Marconi, Lord Kelvin and David Peck Todd also stated their belief that radio could be used to contact Martians, with Marconi stating that his stations had also picked up potential Martian signals.

On August 21–23, 1924, Mars entered an opposition closer to Earth than at any time in the century before or the next 80 years. In the United States, a “National Radio Silence Day” was promoted during a 36-hour period from August 21–23, with all radios quiet for five minutes on the hour, every hour.

At the United States Naval Observatory, scientists used a radio receiver, miles above the ground in a dirigible, to listen for any potential radio messages from Mars.

A 1959 paper by Philip Morrison and Giuseppe Cocconi first pointed out the possibility of searching the microwave spectrum, and proposed frequencies and a set of initial targets.

In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named “Project Ozma”, after the Queen of Oz in L. Frank Baum’s fantasy books. Drake used a radio telescope at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani.

Soviet scientists took a strong interest in SETI during the 1960s and performed a number of searches. Soviet astronomer Iosif Shklovsky wrote the pioneering book in the field, Universe, Life, Intelligence (1962), which was expanded upon by American astronomer Carl Sagan as the best-selling book Intelligent Life in the Universe (1966).

In the March 1955 issue of Scientific American, John D. Kraus described an idea to scan the cosmos for natural radio signals using a radio telescope. Ohio State University soon created a SETI program.

In 1971, NASA funded a SETI study that involved Drake, Bernard M. Oliver of Hewlett-Packard Corporation, and others. The resulting report proposed the construction of an Earth-based radio telescope array with 1,500 dishes known as “Project Cyclops”. It was not built, but the report formed the basis of much SETI work that followed.

Why don’t any organisms detect radio waves?

Much life on earth can see in visible light. Some organisms can see IR (infrared) or UV (ultraviolet) light – but as far as we know none can see the radio part of the EM spectrum. Why not?

See Why don’t any organisms detect radio waves?

How difficult will this be?

It is very difficult to pick up Earth’s radio waves from another solar system. As such, we can imagine that it would  very difficult to pick up radio waves here, from some other solar system.

That’s why we aren’t really looking for random radio waves that happen to escape out into space. Rather, the current projects are looking for much more powerful signals, that we hope would be sent out on purpose.

https://io9.gizmodo.com/are-we-screwing-ourselves-by-transmitting-radio-signals-493800730

“That’s a rather extraordinary claim, so I spoke to SETI expert and scifi novelist David Brin about it — and he’s not convinced detection is this easy. He told me that, even if an ETI had a one square kilometer array, they would have to point it a at Earth for the duration of an entire year. “

Because it would take that long,” he told io9. “But why stare if you don’t already have a reason to suspect?”

Like SETI Institute’s Seth Shostak, Brin believes that Earth is not detectable beyond five light years. “With one exception: Narrow-focused, coherent (laser-like) planetary radars that are aimed to briefly scan the surfaces of asteroids and moons,” he says, “

And not to be confused with military radars that disperse.””

How can we differentiate between natural or artificial (intelligent) signals?

Consider listening to the sound of radio static. Compare that to the sound of a song, or a person giving a speech. Both are sounds – how are they similar? How are they different?

Come up with ideas on how we could differentiate between natural or artificial (intelligent) signals.

“Humanity has received some odd signals in the past. We’ve also sent out some signals ourselves. How could we determine that a signal we’d received was artificial in origin? Or of course inversely, how could an extraterrestrial civilization determine a signal we had sent out was was artificial?”

Space.stackexchange.com – What-evidence-would-be-needed-to-determine-a-signal-was-artificial-in-origin?

Listening for Extraterrestrial Blah Blah: At the cosmic dinner party, intelligence is the loudest thing in the room. By Laurance R. Doyle, Illustrations by Tianhua Mao

Listening for Extraterrestrial Blah Blah

The Water hole: What radio frequencies should we listen to?

Tba

Hailing Frequencies Open, Captain! What is the “water hole”?

Misconceptions about listening with radio telescopes

Radio signals diminish in strength very rapidly with distance – they decrease according to the inverse square law. What does that mean?

Consider cooking oil that is sprayed. The cooking spray hits a piece of toast and deposits an even layer of butter, 1 mm thick.

When the butter gets twice as far, it becomes only 1/4 as this.

If it travels 3 times as far, it will spread out to cover 3 x 3, or 9, pieces of toast.

So now the butter will only be 1/9th as thick. (1/9 is the inverse square of 3)

This pattern is called an inverse-square law.

The same is true for a can of spray paint: as the paint travels further, it covers a wider area, so the paint per area is inversely less thick.

The same pattern of spreading out and weakening, the inverse square laws, is true for radio waves.

Animations of the inverse-square law – animated clip: Inverse-square law for light

Okay – so by the time that radio signals reach even the next solar system they would be unbelievably weak. The radio signals would be even millions of times weaker by the time they travelled across even 1% of the galaxy.

Our radio telescopes could never pick up such radio signals.

So if that’s the case, what then are SETI researchers listening for?We are looking for a civilization that wants to be known, one that has deliberately built a high power radio beacon, aimed in one direction at a time.

A tightly beamed signal would be millions of times stronger – if by chance we happen to be in its path.

Do SETI researchers believe that someone out there is deliberately sending a signal to us here on Earth specifically? No. However, we know that there are billions of stars, and tens of billions of planets. Many of these planets might support life.

Therefore, at any given time there could be many thousands of other worlds with intelligent life. The hope is that some of them would want to communicate, sending a tight, beamed radio signal out into space. If so, then one day we might intercept such a communication.

Article: Is there anybody out there? Jason Davis, October 25, 2017, The Planetary Society Planetary.org – Is there anybody out there?

Goldilocks Zone/Circumstellar habitable zone

This section from evolution.berkeley.edu, A Place for Life: A special astronomy exhibit of Understanding Evolution

From the known properties of stars and of the chemistry of water, astronomers can define “habitable zones” around stars where liquid water (and hence life) could exist on the surface of planets.

Too close to the star, and water will boil; too far, and it will freeze. This so-called Goldilocks zone, where the temperature is just right, depends on both the distance from the star and the characteristics of the star itself.

The habitable zone around luminous giant stars is further from the star than the habitable zone around faint dwarfs.

Of course, as noted previously, life may also exist outside these zones, for example in subsurface oceans on icy moons heated from the moon’s interior.

We know that there are around 200 billion stars in our Galaxy. Recent research has revealed that most of them have planets, and that tens of billions of these planets are likely similar in size to Earth, made of rock, and orbit in their stars’ habitable zones.

The question that remains to be answered is what fraction of those potentially habitable worlds host life.

https://www.e-education.psu.edu/astro801/content/l12_p4.html

https://en.wikipedia.org/wiki/Circumstellar_habitable_zone

https://www.nasa.gov/content/kepler-multimedia

Habitable Zones of Different Stars. NASA/Kepler Mission/Dana Berry.

https://www.nasa.gov/ames/kepler/habitable-zones-of-different-stars

Habitable zones for binary star systems

What about planets in a solar system with two stars?

Most stars in the Galaxy have at least one stellar companion—binary or multiple star systems. Stars like our Sun with no stellar companion are in the minority.

It would probably be difficult for there to be stable, only slightly elliptical planet orbits in a binary or multiple star system.

Complex life (multi-cellular) will need to have a stable temperature regime to form so the planet orbit cannot be too eccentric.

Simple life like bacteria might be able to withstand large temperature changes on a planet with a significantly elliptical orbit but complex life is the much more interesting case.

Suitable binary stars would be those systems where either:

(a) the binary stars orbit very close to each other with the planet(s) orbiting both of them at a large distance (called a “circumbinary planet”)

or (b) the binary stars orbit very far from each other so the planet(s) could reside in stable orbits near each of the stars—the one star’s gravity acting on a planet would be much stronger than that of the other star.

image from https://www.astronomynotes.com/lifezone/s2.htm

A cool article on this subject:  I Built a Stable Planetary System with 416 Planets in the Habitable Zone

Strong magnetic fields may be necessary

Earth has a strong magnetic field.

It turns out that this might be necessary on a planet if complex life is to evolve.

Why does Earth have such a strong magnetic field? Earth’s core is still hot and molten. Metal still moves inside it, and moving metal has moving free electrons.

Electrons moving around – by definition – are an electrical current. And it turns out that electrical currents create their own magnetic field!

Inside the earth

This field protects the Earth’s atmosphere from some of the Sun’s radiation.

Without such a field most of a planet’s atmosphere is likely to be blown away into space, as happened to Mars.

Mars now has very little atmosphere, and its surface is constantly irradiated by solar radiation.

What else may be necessary for life to evolve?

How do tectonic plates make Earth hospitable to life?

The Drake Equation

An equation named after Frank Drake, who first summarized the things we need to know to answer the question, “how many possible extraterrestrial civilizations are out there?”

The equation breaks this complex question into small parts. The Drake Equation

Matthew Bobrowsky says “I introduce the Drake Equation not to actually estimate the number of technological civilizations in the Galaxy, but to provide an indication of the kinds of things to consider when deciding about the likelihood of finding life on another world.”

“It’s also interesting to note that the most uncertain factor in the Drake equation is the average lifetime of a technological civilization. We have no idea how long we (humans) will last, but I have an interesting discussion with students about the various ways — both natural (e.g., an asteroid impact) and by our own hands (e.g., global nuclear war) that our species could become extinct at any time.”

Astrobiology: Life elsewhere in the universe

Here we will link to lessons about the realistic possibility of life existing elsewhere in our galaxy, The idea is far more mainstream than most people realize.

Why would anyone think that it is likely that life would also evolve elsewhere in the universe?

(A) Why not? We have no data to assume otherwise.

(B) We do have enormous amounts of data available on what life on Earth is made of, and how it evolved over time. We have enormous amounts of data available on biochemistry and organic chemistry. enormous amounts of data available on how stars work and how planetary systems form.

The most common chemical elements in life are the most common elements in the universe

Discussions on how atoms naturally form molecules, including precursors to the organic molecules here on Earth

Evidence that this same chemistry happens elsewhere in the galaxy

Class discussion: What conditions would life need to evolve on another world?

What conditions would life need to evolve into advanced forms of life?

Even if advanced forms of life evolve, would they necessarily be able to develop technology?

Example: Dolphins and whales on earth have near human-like intelligence, awareness, and emotions, but they don’t have hands. They can’t manipulate their environment; can’t mine minerals or metals, and this can’t develop technology.

needs for energy – what possible energy sources?

needs for a solvent – water seems to be the only likely solvent, although we can investigate other options

protection from radiation (from stars, supernovas, et.)

how long would it take for life to evolve? How long are the lives of stars?

Given this, what kinds of stars could we expect to possibly have intelligent life?

(likely not around O-type stars, they burn out too quickly)

What kind of biochemistry would exist?

• probably carbon based

• we can investigate other possibilities

What kind of conditions could life thrive under? Consider conditions necessary for human life in specific, then animal life in general. Then compare to range of conditions that archaea can survive in.

Potential habitats for life

Terrestrial planets

Super Earths

Worlds like Europa, Enceladus

Atmosphere of brown dwarf stars

Alien life could thrive in the clouds of failed stars: cold brown dwarf stars

Cold brown dwarf star no hotter than a summer’s day

Atmospheric Habitable Zones in Y Dwarf Atmospheres, Jack S. Yates, Paul I. Palmer, Beth Biller, Charles S. Cockell , The Astrophysical Journal

Megastructures (Dyson swarms, etc.)

Evidence for ET intelligent life

None currently exists

UFO fads in the late 1800s, 1950s, and today. Extremely low quality photographs, evidence is considered at best very poor.

The WOW signal and other somewhat more realistic events that could be interpreted as evidence

Possible significant downside to contacting ET intelligence

TBA

Could we realistically ever travel to other star systems?

Here we learn about the potentially realistic physics of interstellar travel

Learning Standards

Common Core, English Language Arts Standards » Science & Technical Subjects

CCSS.ELA-LITERACY.RST.9-10.1 – Cite specific textual evidence to support analysis of science and technical texts, attending to the precise details of explanations or descriptions.

CCSS.ELA-LITERACY.RST.9-10.2 – Determine the central ideas or conclusions of a text; trace the text’s explanation or depiction of a complex process, phenomenon, or concept; provide an accurate summary of the text.

CCSS.ELA-LITERACY.RST.9-10.4 – Determine the meaning of symbols, key terms, and other domain-specific words and phrases as they are used in a specific scientific or technical context.

CCSS.ELA-LITERACY.RST.9-10.5 –  Analyze the structure of the relationships among concepts in a text, including relationships among key terms (e.g., force, friction, reaction force, energy).

CCSS.ELA-LITERACY.RST.9-10.6 – Analyze the author’s purpose in providing an explanation, describing a procedure, or discussing an experiment in a text, defining the question the author seeks to address.

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-PS2-5. Provide evidence that an electric current can produce a magnetic field and that a changing magnetic field can produce an electric current.

6.MS-PS4-1. Use diagrams of a simple wave to explain that (a) a wave has a repeating pattern with a specific amplitude, frequency, and wavelength, and (b) the amplitude of a wave is related to the energy of the wave.

HS-PS4-1. Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling within various media. Recognize that electromagnetic waves can travel through empty space (without a medium) as compared to mechanical waves that require a medium.

HS-PS4-5. Communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy.

Next Generation Science Standards: Science & Engineering Practices
● Ask questions that arise from careful observation of phenomena, or unexpected results, to clarify and/or seek additional information.
● Ask questions that arise from examining models or a theory, to clarify and/or seek additional information and relationships.
● Ask questions to determine relationships, including quantitative relationships, between independent and dependent variables.
● Evaluate a question to determine if it is testable and relevant.
● Ask and/or evaluate questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of the design

Common Core Math Standards (Inverse-square law)

CCSS.Math.Content.7.RP.A.2a ( Grade 7 ): Decide whether two quantities are in a proportional relationship, e.g., by testing for equivalent ratios in a table or graphing on a coordinate plane and observing whether the graph is a straight line through the origin.

CCSS.Math.Content.7.RP.A.2c ( Grade 7 ): Represent proportional relationships by equations.