This video is sponsored by Magellan TV.
Welcome back to Launch Pad, I'm Christian Ready your friendly neighborhood astronomer.
And as you may have heard, the Arecibo radio telescope collapsed on December 1st, 2020
after 57 years of operation. At a whopping 306 meters - 1000 feet across - it was the
largest single-aperture telescope in the world for most of its life. In fact, it wasn't until
2016 when China's 500-meter Aperture Spherical Telescope - or FAST - finally broke Arecibo's
aperture record.
But unlike most radio telescopes, Arecibo was equipped with a radar. That allowed the
telescope to work in reverse and bounce radio signals off of planets and asteroids and analyze
what came back.
That allowed Arecibo to discover that Mercury wasn't tidally locked to the Sun as previously
thought, but instead orbits the Sun in a 3:2 spin resonance.
Later, it was used to make radar maps of asteroids, such as Bennu. These observations helped to
plan ahead for OSIRIS-REx's mission to capture a sample.
But the most extraordinary use of Arecibo's radar was back in 1974, when it transmitted
a message, announcing humanity's existence in the cosmos. Even though it's racing away
from our solar system at the speed of light, it won't reach its destination for at least
another 22 thousand years.
So, while we're waiting for a reply, I thought we'd talk about the story of the Arecibo message,
decode it, and discuss its potential implications. But first, I'd like to thank Magellan TV,
who are very kindly sponsoring today's video.
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In 1974, the Arecibo telescope was upgraded just in time for its 10th anniversary. Astronomer
Frank Drake thought to commemorate the occasion by using Arecibo's radar to send a message
to an extraterrestrial civilization.
Drake pioneered the Search for Extraterrestrial Intelligence, or SETI, and is the author of
the famous Drake Equation which aims to describe the number of detectable civilizations within
our Galaxy. He collaborated with Carl Sagan in designing the plaques on the Pioneer spacecraft,
and oversaw the creation of the Golden Records on Voyager.
Now, like the Pioneer plaques and Voyager records, the Arecibo message was a symbolic
gesture; it was designed to show that we could do it and to show off the incredible power
of the Arecibo telescope.
However, it was also an opportunity to really think about how to actually construct a message
that could be interpreted without any expectation of a common spoken language.
Drake designed the message with the assistance of scientists at Cornell University as well
as Arecibo. Now despite popular myth, Carl Sagan wasn't involved in helping to design
the message, though he certainly supported and helped promote the effort.
And so, on November 16 1974, a message was beamed to the globular cluster M13.
It consists of roughly 100,000 stars and lies about 22 thousand light-years away in the
constellation Hercules. It was chosen because they wanted the message to reach as many stars
as possible. Ideally, they would have sent the message to the Galactic center. Had that
been the case, the signal could have been intercepted by at least a billion stars, but
the Galactic center never rises overhead at Arecibo's latitude in Puerto Rico. So, M13
was selected instead.
The message was sent with Arecibo's megawatt transmitter. A megawatt of energy isn't a
lot if it's omnidirectional; that is, if it radiates in all directions. But when concentrated
into a narrow beam, it outshines the Sun by a factor of a million! That's powerful enough
to be detected by another Arecibo-sized antenna on the other side of the Galaxy!
Now, there's no reason to assume that an alien civilization would have the same concepts
of language as we do. But Drake was confident that anyone intelligent enough to receive
the message would, by necessity, have to have an understanding of mathematics and science.
That's why the signal was broadcast at a frequency of 2,380 MHz with occasional pulses shifted
by 10 MHz.
Digitizing the signal produces a string of bits with two different values. By convention,
we treat each bit as either a 1 or 0 but they could just as easily be either an "on" or
"off", whites and blacks, or...whatever these are.
The message was 1679 bits in length. That's a special kind of number called a semi-prime.
that means it can be factored into two prime numbers. In particular, 1679 factors into
the primes 73 and 23. If the bits are arranged into 23 rows and 73 columns, the message is
completely unintelligible. But if is instead arranged into 73 rows and 23 columns, an unmistakeable
message emerges.
The message is broken down into seven parts. The top part of the message are the binary
representations of the numbers 1-10. It's meant to act as a kind of rosetta stone for
the rest of the message.
Now if you already know how binary works or don't care to know how binary works, feel
free to skip ahead to the next part of the video. Otherwise, let's walk through how to
read the first 10 numbers.
The bits at very bottom of each number are intended as markers to indicate the start
of the column for the number in question. Each bit in the column above represents an
increasing power of 2, starting from 0 and counting upwards.
The value of the number the bits represent is calculated by adding up the values of the
bits that are switched on.
So for example, in the first column, the bit above the marker is "on", but the rest are
off, so the value of this column is 1 + 0 + 0, or 1.
In the next column, the bottom bit is off but the one above it is on, followed by a
bit turned off, so it's 0 + 2+ 0, or 2.
The third number is 1+ 2 + 0, or 3.
Four is simply 4.
Five is 1+0+4. Six is 0+2+4, and seven is 1+2+4
The numbers 8-10 are a little tricker, because only the first four rows could be used to
express these first 10 numbers, so these numbers are represented by two columns. This is why
the bottom marker is there to indicate where to start reading the numbers from. So the
positions represent 1, 2, 4 as above, followed by positions 8, 16, and 32.
Now writing numbers this way may not be obvious to anyone who manages to arrange the message
correctly, even if they understand binary. But if they do figure this part out, there's
a good chance they can figure out the rest of the message.
The next set of numbers are 1, 6, 7, 8, and 15. These are the atomic numbers of hydrogen,
carbon, nitrogen, oxygen, and phosphorous. These are the elements that ultimately make
up DNA.
The chemical groups that make up DNA nucleotides are defined using the same convention.
So, for example, 7 hydrogen atoms, 5 carbon atoms, 0 nitrogen, 1 oxygen, and 0 phosphate
atoms form the compound Deoxyribose - the "D" in "DNA".
This is followed by descriptions of Adenine, Thymine, and Deoxyribose
Then two phosphate molecules are presented on either side of the message, representing
their locations on the sides of the DNA double helix.
Then a row of Deoxyribose, Cytosine, Guanine, and Deoxyribose,
Followed by another set of Phosphates.
The long double column underneath represents the number 4 billion, 294 million, 441 thousand,
822. In 1974, this was thought to be the number of nucleotide base pairs in the human genome.
Today, we know that number is closer to 3.2 billion. Oops.
Surrounding this number is a graphical representation of a double helix. Hopefully, whomever receives
the message will recognize the shape and understand that the message is trying to convey DNA information.
The double helix converges onto a depiction of a human being. This could be the most recognizable
part of the entire message, assuming whomever receives it is bipedal.
On the left is a horizontal binary depiction of the number 14. This number, multiplied
by the wavelength of the message (126 mm), gives the height of the average American male.
For example, the wavelength of the message is 126 mm. Multiplied by 14 gives 1.764 meters
or 5 feet, 9.4 inches or...whatever units the aliens would like to measure with.
To indicate that this number represents height, vertical bars appear above and below this
number. I admit, I wouldn't have guessed this one, either.
On the right is the binary representation of the number 4,292,853,750 which at the time
was the best estimate of the number of human beings on planet Earth.
The next section represents the solar system, with the third planet from the Sun raised
to indicate where the message came from. And yes, Pluto is included here as well because
at the time, we didn't know there was a larger Kuiper belt out there.
Below that is a graphical depiction of the Arecibo dish, with a large "M" depicting the
path of the radio beam going from the transmitter to the dish and out into space.
Underneath is a the binary expression of the number 2,430. When this number is multiplied
by the wavelength of the message, the telescope's diameter is revealed. Horizontal dots on either
side of the number hint that this number is used to describe the telescope's diameter.
Now, would anyone be able to actually decode this? Well, the story goes that Drake anonymously
mailed an early version of the message to several scientists around the world. He sent
each of them a single piece of paper that only had a string of 1’s and 0’s printed
on it. The paper arrived in an otherwise unmarked envelope, with no explanation or instructions
of any kind. However, every single recipient managed to decode the string into the message!
After Drake constructed the final Arecibo message, he showed the decoded version to
several scientists and asked them to interpret it. He found that nobody could interpret the
entire message. But each scientist recognized the part relevant to their discipline. In
a 2007 interview with Astrobiology magazine, Drake said that Carl Sagan got the numbers
and planetary information correct, but didn't figure out the DNA chemistry.
So who knows? Maybe if the receiving civilization has enough subject matter experts looking
at it, they'll eventually be able to figure it out.
However, it's hard to imagine there's anyone in M13 to read the message. M13 is a globular
cluster made of older stars that formed when the Universe was much younger. As a result,
they don't possess a high abundance of the heavy elements needed to form complex life.
Besides, the radiation environment in a globular cluster is pretty nasty, so even simple life
wouldn't have the best shot at evolving into something more complex.
However, a reply mysteriously appeared in a crop circle next to the Chilbolton radio
telescope in 2001. Now, I'm not saying it's artists...but it's artists.
Of course, the Arecibo message sparked a debate as to whether or not it's a good idea to directly
message aliens to begin with. After all, what if an alien civilization interprets our message
as hostile or worse, they recognize it as friendly but choose to mount an invasion to
enslave us? Personally, I'm really not losing any sleep over it. I mean, M13 is 22 thousand
light-years away. The earliest we could anticipate a reply is 44 thousand years from now. So
I'm sure by then we'll have figured out a way to hold Charleston.
Besides, if they do try to respond, they might find it increasingly hard to find us. We used
to broadcast signals into space all the time. The surface of our planet was covered with
omnidirectional TV transmitters. Each transmitter broadcast with up to 10 megawatts of power,
with as much as half its energy escaping into space.
But now the majority of our transmissions come from satellites sending narrow beams
to the ground. Instead of megawatt transmitters, a typical satellite sends HBO to the ground
at just 20 watts. Less than a single watt escapes into space at any given time.
Maybe that's why we haven't detected any extraterrestrial signals. Maybe they all got cable and stopped
broadcasting.
Or maybe they know we're here but they're leaving us alone following a kind of Prime
Directive. Or...maybe we're alone.
In any event, we won't be sending another message with Arecibo. It's destroyed and I
have no idea if it will ever be rebuilt. But that one message will continue to M13 and
far beyond. It's interesting to think that in a way, Arecibo's ultimate legacy will continue
to propagate through the Universe long after we're gone.
Meanwhile, the Breakthrough Listen project may - and this is a big "may", mind you - may
have detected a signal that seems to be coming from somewhere in the region around Proxima
Centauri. Now, I'm not holding my breath, it may turn out to nevertheless be a step
forward in the search for techno-signatures. We're still waiting for the science team to publish
their results, I made a video about what we do know about it so far, so if you haven't
seen it yet, I'll see you over there when we're done here.
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