Ready, SETI, Go!
Dr. Sofia Sheikh, an expert in exoplanet detection and astrobiology, works at the forefront of the Search for ExtraTerrestrial Intelligence (SETI) research by observing and analyzing radio wave signals that have potentially been emitted by intelligent extraterrestrial civilizations! She recently got her PhD from Penn State in Astronomy & Astrophysics with a dual-title in Astrobiology. Along with her work at the Berkeley SETI Research Center, Dr. Sheikh is a member of the Breakthrough Listen project, the largest SETI collaboration in existence.
Table of Contents:
Who's Out There?
Are we alone? Ever since humanity realized that the Earth is not unique as far as planets are concerned, the idea that intelligent species may exist has fascinated (and frightened) us. Unfortunately (or maybe fortunately, depending on the friendliness of the aliens) no starships have arrived to publicly reveal themselves to humanity.
Since humanity currently doesn’t have the technology to visit other solar systems (they are just too far away!), if we wanted to detect any alien civilizations, we would have to wait for them to send us a message. But there’s no guarantee that a species exists with advanced enough technology to contact us. Even if they did, there’s no guarantee that they would be able to find us in the vastness of the cosmos. So instead, scientists involved in the Search for ExtraTerrestrial Intelligence (SETI), are working hard to look for other signs that extraterrestrial life is out there, which is called indirect detection.
Thinking About Distances
One of the triumphs of Albert Einstein’s Theory of Relativity was the discovery of a cosmological speed limit, the speed of light (c). Objects with mass can never reach the speed of light, and even massless objects cannot exceed it. This makes traveling to other star systems incredibly time consuming.
For instance, it takes light from our sun over four years to reach our sun’s closest stellar neighbor, Proxima Centauri. But the fastest objects we can make are nowhere near as fast as light! Our fastest probe, Parker Solar Probe, would take over 8,500 years to reach Proxima Centauri, then another 8,500 years to make it back to Earth!
Searching for Clues: How to Detect an Alien Civilization
If we can’t send a spacecraft to a distant planet, and they’re not sending one to us, how could we discover an alien civilization? It can be useful to put ourselves in the aliens’ shoes: how would an alien civilization with similar technology to our own detect life on Earth?
The answer is kind of simple: our civilization on Earth emits a ton of signals (radio waves, microwaves, etc.) that an alien civilization could detect with their telescopes! Any time you use wireless technology (internet, cellular services, radio), you send and receive signals that are potentially detectable by receivers outside of Earth, or even outside our solar system. We call these signals technosignatures (short for technology signatures), as they indicate the existence of technologically advanced alien life.
SETI vs METI
Searching for alien civilizations can be done either actively or passively. METI (Messaging Extraterrestrial Intelligence), is a type of active search for extraterrestrials- it involves broadcasting signals out into space, with the hope that it might attract the attention of alien civilizations. On the other hand, SETI is a passive search, which involves listening for signals produced by other civilizations without intentionally trying to attract their attention. Due to debates over the risk of messaging extraterrestrial civilizations, modern SETI research is almost entirely passive.
Delayed Signals
Because of the finite speed of light, all types of signals, including radio waves propagating in space, take time to travel to a receiver. This can significantly delay sending messages to distant sources, which limits how quickly we can transfer information through space. Think of sending a radio signal like sending a letter to a friend. Once the letter is sent, you cannot add more information to it, so when your friend reads the letter they can only learn about events that occurred before the letter was sent.
When we communicate with each other on Earth, the finite speed of radio waves is practically unnoticeable due to the (relatively) short distances involved. However, it becomes relevant when dealing with distances between stars. To understand these vast distances, astronomers sometimes measure distances in "light years", which is the distance that light can travel in a year. So an alien civilization located 10 light years away would just now be receiving signals produced by Earth ten years ago, in 2012. But a civilization located 500 light years away has not received any signals from Earth yet, since humanity was not producing radio waves in 1522. That civilization wouldn’t be able to see signs of us until 2399, which would be 500 years after the first radio waves were produced in 1899. Hopefully we’ll still be around to answer them if they decide to send a signal back to say hello!
Alien Listening
Because humans produce detectable signals, it is very likely that alien civilizations do as well. But what type of signals should SETI researchers look for? It costs time and money to point your telescope at a particular spot in the sky, collect data, then analyze the data for potential signals.
To solve this problem, Dr. Sheikh developed a list to help us figure out if the place we want to look for clues is a good one. She considered nine factors, listed below:
1. Observation Capacity
Do we have the resources and technology that scientists would need to make this observation?
Good Proposal: We have the correct type of telescope currently built.
Bad Proposal: We would need to build a new telescope before we could make this observation.
2. Cost
How much does the proposed search cost?
Good Proposal: The proposed search is cheap.
Bad Proposal: The proposed search is expensive.
3. Extra Benefits
Can the data we get from this observation be used for purposes other than SETI?
Good Proposal: The data could be used to study, for example, formation of a certain class of stars.
Bad Proposal: The data is useless outside of SETI research.
4. Detectability
Will researchers be able to get any signal, or is there too much noise nearby from other astronomical sources?
Good Proposal: The signal would be clear and stronger than any noise around.
Bad Proposal: The signal could be lost in the noise.
Example 1: Signal-to-Noise Ratio (SNR)
Signal-to-Noise ratio (SNR) is a measure of how clearly the signal can be seen through the present noise. Say you want to send the signal displayed in Figure 1a to your friend who’s located on the moon. If the signal has high SNR, it would be easy to see the signal through the noise (Figure 1b). But if the signal has low SNR, it would be really hard to figure out what the initial message was (Figure 1c). Even if the signal has low SNR, scientists have developed methods to extract a signal out from the noisy data, which we’ll talk about in Example 2.
5. Duration
How long will the signal last?
Good Proposal: The signal lasts a long time, so it’s easier to observe and gives us more data.
Bad Proposal: The signal lasts only a short time.
6. Ambiguity
Can natural processes mimic the signal of interest, or are aliens the only explanation?
Good Proposal: There are no known natural origins of the signal.
Bad Proposal: Natural processes can produce the same signal, such as pulsars, a special type of star whose signals were once thought to be caused by extraterrestrial intelligence.
7. Extrapolation
Are scientists making assumptions about the alien civilizations when proposing that a certain type of signal should be observed? Do the signals require technology far beyond that of current humans?
Good Proposal: The signal doesn’t require any assumptions about the history of the alien civilization, and could also be produced with current (human) technology.
Bad Proposal: Either the signal requires a human-like history of development, or couldn’t be produced by current (human) technology.
8. Inevitability
How likely is it that any given alien civilization would someday produce this type of signal?
Good Proposal: All alien civilizations should be able to produce this signal if they become advanced enough.
Bad Proposal: Only a few alien civilizations would likely be able to produce this signal.
9. Information
What would we learn?
Good Proposal: The signal would tell us a lot of information about the aliens, beyond just their existence.
Bad Proposal: The signal would only alert us to the existence of the alien civilization.
Dr. Sheikh’s research focuses on identifying radio signals from extraterrestrial civilizations, as they score well on this list of questions! They’re easy to perform, relatively cheap, nonnatural, producible with current (human) technology, and should contain information beyond the mere existence of an alien civilization. Radio signals are also pretty easy to detect. Because they’re such good candidates, much of the current research in the Breakthrough Listen project has involved radio waves.
Example 2: Noisy, Noisy Data
Say you want to send a signal to your friend who’s located on the moon. Your signal is a wave, which we can mathematically describe as
f( t ) = 10 sin( 2t ) (Fig. 2a).
In a perfect world, your friend would receive the exact, clean signal that you sent. However, when your friend measures the signal and plots it, they find that they received some noisy points instead (plotted in 2b). Oh no! Your signal has been affected by the bane of scientists everywhere: noise!
“Noise” in data refers to random variations in the predicted. Noise could come from other signals interfering with yours, your detector not being perfect, etc. But not to worry! Scientists have spent decades figuring out how to extract signals from noisy data.
One way is a method of “moving averages” (MA), which smooths out our data and removes noise. This method works by assuming that noise should be random, so it will “average” out. Using our noisy data, we define a new dataset such that, if our original dataset had values
{a, b, c, d, e..., n},
then our new data set will have
{(a+b+...+n)/n, (b+c+...+m)/n, ...}
So that the numerator of each term has exactly n number of terms. If n=2, then
{(a+b)/2, (b+c)/2, (c+d)/2, ...}.
Using the observed data (Fig. 2b.), your friend applies the moving average formula for different values of n (see Fig. 3). By increasing the value of n, the original signal can be almost perfectly recovered, even if we did not know what the signal was supposed to look like (Figure 4). Wow!
Of course, the reconstructed signal is not perfect. But this method is one of the simplest tools we have to extract signals from data. Scientists normally use much more complicated methods to almost perfectly recreate the signal.
Earth Transit Zone
Using her nine factors above, Dr. Sheikh and her collaborators have started a new project that will look for signals from civilizations that might exist in what’s known as the “Earth Transit Zone” (ETZ). The “transit” in the ETZ comes from the transit method, which is one way that scientists can detect planets outside of our solar system, called exoplanets. The transit method involves tracking the changes in the brightness of other stars in search of periodic, regular changes. This is because if the star has an exoplanet orbiting it, that’ll cause a periodic dimming of that star’s brightness as the exoplanet passes in front. With observations of the changes in the light coming from the star, we can also learn some information about the planet, like its size and how fast it is orbiting. This method has been incredibly successful for finding new planets-- of the ~4,000 exoplanets we know of, 3755 were discovered with the transit method!
However, there are some limitations to the transit method. Detection of the exoplanet requires that the planet is orbiting at a particular angle with respect to the telescope you are using. We can’t detect planets that don’t pass in front of the star from the perspective of our telescopes. So if an alien on a planet inside the Earth Transit Zone were looking through a telescope at our Sun, they would be able to see the Earth transiting in front of the star. But if the alien was on a planet outside the ETZ, then it would be much harder for them to detect our tiny little planet.
With this knowledge in mind, Dr. Sheikh and colleagues decided to look for radio signals coming from inside the ETZ. If other civilizations in the ETZ are looking for extraterrestrial life like we are, they’d be able to detect technosignatures from Earth and learn some things about our planet, like that our Earth has water. A preliminary survey through the ETZ for radio signals didn’t produce any promising signals. But there are still tons of stars in the ETZ for Dr. Sheikh to explore!
False Alarms
So far, we’ve never detected extraterrestrial life, but it's not for lack of trying. There’s been a lot of times where potential signals were observed, only to be later explained as coming from astrophysical (or human) sources. One of these potential signals was observed on April 29th, 2019. The Breakthrough Listen Center was using the Parkes Radio Telescope in Australia to observe Proxima Centauri, the closest star to the Sun and home of the closest known exoplanet. When analyzing the resulting data for technosignatures, scientists (including Dr. Sheikh), discovering a single candidate “signal-of-interest.” This signal was over five hours long and puzzled the scientists, as they didn’t think any natural phenomena could produce it.
This signal produced a buzz in the media when its discovery was revealed. However, science takes time, and it would take Dr. Sheikh and her collaborators over two years from the initial detection to fully analyze the system. Based on their analysis, the signal turned out to be from Earth-based noise – a disappointing, but not surprising, result.
The Truth is Out There
While the Proxima signal may have been a false alarm, Dr. Sheikh and other SETI researchers remain committed to hunting for clues of alien civilizations. Dr. Sheikh is currently preparing to begin a new technosignature observing project at Hat Creek Radio Observatory using the Allen Telescope Array. This new observing project will focus on another region of space where extraterrestrial civilizations could observe the Earth orbiting the sun. Maybe some day Dr. Sheikh will find evidence of extraterrestrial civilizations, hidden in signals from the stars.
Photo courtesy of SETI Institute
Written by Jackie Lodman and Cayla Dedrick
Edited by Oriel Humes, Caroline Martin, Manasvi Verma, and Sofia Sheikh
Cartoons by Weilu Shen
Primary sources:
Breakthrough Listen Initiative
Dr. Sofia Sheikh Awarded National Science Foundation Fellowship for Research & Education at the SETI Institute’s Allen Telescope Array from the SETI Institute
Nine axes of merit for technosignature searches by Sofia Sheikh in International Journal of Astrobiology
Analysis of the breakthrough listen signal of interest Blc1 with a technosignature verification framework by Sofia Sheikh et al. in Nature Astronomy
Additional resources:
Was That a Dropped Call From ET? from The New York Times
Aliens could be talking to us right now. This scientist is listening. from Popular Science
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