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A century of Earth's radio transmissions
has now washed over thousands of other star systems,
carrying with it some of our greatest broadcast
masterpieces, as well as our worst reality TV.
What are the chances that alien civilizations have detected
and even decoded these signals?
We are a young technological civilization.
An advanced alien race monitoring our planet,
even from the nearest neighboring star,
would have seen nothing, radio quietness until only around
a century ago.
Then, beginning with the faint sporadic buzz
of the first experiments with wireless transmission,
the radio brightness of this small planet
would have bloomed into a continuous planet-wide
cacophony of TV and radio broadcasts, satellite relays,
and radar pulses.
This bubble of chatter is, as I speak,
spreading out into the galaxy at the speed of light,
the unmistakable signature of an emerging technological power.
The outer edge of this bubble is now over a hundred light years
away.
And it carries with it the first transatlantic radio
transmission of Marconi himself.
That edge may be hopelessly diffused,
but it's followed closely by the much brighter
80-light-year shell, on which rides the footage of the 1936
Berlin Olympics, episodes of the "Lone Ranger,"
and Orson Welles' ill-fated radio adaptation of "War
of the Worlds."
That shell has washed over several thousand star systems.
Based on Kepler Space Telescope observations,
that also means thousands of potentially habitable planets,
lots of chances for hypothetical civilizations
to have learned of our existence.
So the question is, who could have seen us?
What manner of civilization would be powerful enough
to sense this bubble?
What tech level would be needed to actually decode
the signal it carries?
If aliens arrive on our doorstep tomorrow, would they have been
alerted to our presence by Nikola Tesla's
first experiments, broadcast footage of the Moon landing,
the "Phantom Menace?"
To answer the question of who can see us,
it's helpful to first ask who could we see?
After all, humanity remains the one example
of a technological civilization, for now.
Our best astronomers have been engaged
in the search for extraterrestrial intelligence,
SETI, in earnest since the '60s, when Frank Drake first
peeked at the stars Tau Ceti and Epsilon Eridani.
Drake peered into the so-called water hole, a narrow frequency
range in the radio spectrum between a pair of H and OH
emission spikes, which itself is within the broader
window from 1 to 10 gigahertz.
There, the natural universe is especially quiet.
Surely, this is the window where aliens
would choose to broadcast their messages welcoming us
into the galactic community.
And, of course, Drake saw nothing.
Decades of SETI programs followed,
utilizing several of the world's great radio telescopes,
like Arecibo in Puerto Rico and the Parkes radio telescope
in Australia.
These searches often focused on the water hole
or the surrounding quiet frequency window.
But with the exception of some one-off oddities,
the search was not successful.
The 1977 Wow! signal is the most compelling,
a narrow-frequency radio blast detected
in the water hole, that still has no broadly accepted
natural explanation.
But no one ever saw it again.
Apparently, no one has tried particularly
hard to get our attention.
And it's important to understand that these searches could only
have been successful if there were
deliberate powerful beacons emitted
by an alien civilization.
The radio leakage produced by their own internal broadcasts
would be much harder to detect.
Our best-targeted search so far, the SETI Institute's Project
Phoenix, scanned 800 stars within 200 light years.
At that range, it could have picked up a gigawatt beacon
beamed directly at us.
But it would only have picked up leakage from a civilization's
internal broadcasts from within a couple of light years,
if those broadcasts with the same strength as our own.
In that range, there is exactly one star, ours.
An alien civilization at exactly our current tech level
would never have seen our TV bubble.
We have sent out more powerful beacons,
like the stream of 10,000 tweets and celebrity
videos that Arecibo blasted in the direction of the Wow!
signal.
But that's a long shot, and perhaps for the best.
I'm not sure we want our first contact
to be a typical slice of the twitterverse.
One of the reasons it's hard to spot unintentional radio
leakage is that a distant civilization's radio bubble
is likely to overlap in frequency
with our own transmissions.
This is another advantage for searching for alien signals
in and around the water hole.
At around 1500 megahertz, it's in the ultrahigh frequency
(UHF) band.
Most TV broadcasts air at the tens
to hundreds of megahertz of the merely very-high frequency
(VHF) band.
VHF works better for over-air broadcasts
because it carries further and is less
blocked by buildings and such.
So we often search the UHF to avoid the thick soup
of our own VHF chatter.
But surely aliens are working with the same physics
that we are.
Perhaps we miss each other because we can't easily
peer through our own local VHF soups.
But there is a way to peer straight through our own radio
noise as though it wasn't there.
The answer is interferometry.
Two radio telescopes separated by a large enough distance
can filter out local transmissions.
A signal that appears in one, but not the other,
must be an earthly transmission.
Now, that's not the main purpose of interferometry.
That is to increase the spatial resolution
of your observations.
You get the resolution of a telescope
as large as the separation of the component
telescopes, also super-useful.
Interferometry has been around for a while.
But we're only now building an interferometer facility
with the sensitivity to potentially see an alien TV
bubble to many light years and that is large enough
to filter out local noise.
It's the Square Kilometer Array or SKA.
The SKA is actually many, many telescopes.
Arrays of thousands of radio dishes
will be built in Africa and hundreds of thousands
of antenna installed in Australia.
When connected up, it'll effectively
form a giant radio telescope, with over a square kilometer
surface area.
But as an interferometer, it has the resolution of a telescope
thousands of kilometers across.
Its sensitivity and pinpoint resolving power
will be vastly greater than any telescope of any type
that we have ever built.
The SKA is not being built to search for aliens.
One of its primary purposes will be
to catch the radio emission from hydrogen gas in the extremely
early universe.
That emission, produced at 21 centimeters wavelength, or 1420
megahertz frequency, is, by definition,
one of the boundaries of the waterhole.
But if such radio waves travel to us
from the earliest of times, then they
become stretched out as they travel through an expanding
universe.
That redshifted early hydrogen emission is now found slap
in the middle of the noisy lower frequency part of our own TV
broadcast spectrum.
To spot these radio photons, we need a truly gigantic
interferometer, both for extreme sensitivity
and to eliminate our own radio buzz.
That's the SKA.
Such a machine also just happens to be perfect for spotting
alien television bubbles.
According to the calculations of Harvard University's Avi Loeb
and Matias Zaldarriaga, the SKA should be sensitive enough
to spot our own TV bubble from a hundred or more
light years away.
So will we soon be picking up extra terrestrial sitcoms?
Not quite.
Loeb and Zaldarriga's numbers assume
pointing SKA at a target star system for an entire month
and adding up all of the radio emission over that time.
For an artificial radio source, that
would look like emission over a narrow-frequency range
that doesn't correspond to any natural process.
These emission spikes may also shift back and forth
in frequency due to Doppler shift,
as the distant technologically advanced planet
orbits its star.
So that's enough to identify a technological source,
but not to actually decode the signal.
An alien civilization with their own SKA
certainly couldn't watch Earth TV.
To do that, they would need to achieve
SKA's one-month sensitivity in a tiny fraction of a second.
That means compensating for the difference in exposure
or integration time with the sheer size of the telescope.
Let's figure it out.
What would be needed to, say, tune
in to the first season of the original "Star Trek" series?
That signal is now 50 light years away.
Assume a bit rate of 10 million per second,
for a miserable 30 frames per second, at 200 by 200
resolution, and only 8-bit color.
Forget audio, which is a shame because the original series
dialog is awesome.
The aliens would need a radio telescope trillions
of times the SKA's surface area and equivalent to a dish
around three times the radius of the Moon's orbit.
This is perhaps conceivable for a Type II civilization.
Although it's exceedingly unlikely
that there's one of those within our radio bubble.
We may even be a little optimistic
in thinking that the SKA will detect alien TV.
As the SETI Institute's John Billingham,
along with James Bedford, point out,
that one month integration time requires a very consistent
narrow frequency signal, which may not
be consistent with typical broadcasts.
But remember, humanity is young as a radio-noisy civilization.
Simple probability says that any other civilization
is likely to be ahead of us on the curve.
It's not hard to imagine them building a much larger SKA,
without going full-Type II.
Our SKA will cost a few billion dollars
by the time it's done in 2030.
That's a significant investment, but only a tiny fraction
of the global GDP.
For a slightly more advanced civilization,
that's several Moore's law doublings ahead of us
in various technologies, and accounting for moderate GDP
growth over a century or two, a super-SKA,
with several hundred square kilometers of collecting area,
would constitute the same degree of investment
as our own one-square-kilometer array.
In that case, Earth's TV bubble would
be spotted within hours, or even minutes,
of aliens pointing the facility to our solar system,
even if they weren't looking for us.
Surely, any advanced civilization
is curious about its own origins.
That redshifted 21-centimeter hydrogen emission
really is one of the most important keys to understanding
the very early universe.
It's pretty natural to want to scan the heavens
at exactly the frequencies where we earthlings are the noisiest.
If there's a slightly more technologically advanced
civilization within our radio bubble,
they've probably seen us.
However, our steady broadcasts have only
washed over a few hundred solar-type star
systems and only a few thousand stars total.
Technological civilizations would
need to be extremely common throughout our galaxy--
and I mean tens of millions of them across the Milky Way--
for there to be any chance that there's
another one so close to us, But we just don't know.
If anyone else inhabits this very local region
of the galaxy, then they may have been
aware of us for some time now.
Any civilization within 40 to 50 light years
could have sent a return signal that will reach us any day.
Their fleets of welcome ships will
be a little further behind, but no doubt
carry only the best wishes for their noisiest neighbors
in nearby spacetime.
Hey, everyone.
I just wanted to say thanks again
for those of you sponsoring us on Patreon.
I want to give a very special shout-out
to Luna IT Solutions, who are sponsoring us
at the quasar level.
Thanks a ton.
It really is a huge help.
A couple of weeks ago, we looked back into black holes
and studied the nature of the event horizon.
You guys had some excellent and quite tricky questions.
Luca$sino asks what we would really see if we followed
the monkey directly through the event horizon?
Would it appear ahead of us despite previously appearing
frozen on the event horizon?
Well, sort of, yes.
The light from the instant of the monkey's crossing
of the event horizon is eternally trapped
at that horizon.
It's trying to walk upwards on a downward escalator
of spacetime.
But you're traveling down that same escalator
and so you pass this light.
And so you see the instant of the monkey's crossing.
The escalator speeds up as you descend.
And so light that the monkey emitted from below that point
is trying to go up, but is actually going downwards.
And yet, you're traveling downwards faster.
So you overtake some of that light,
which means you see the monkey ahead of you.
Douglas Oak asks a great question
that I've heard asked before.
If time dilation approaches infinity,
do you reach the singularity before the black hole
evaporates?
OK.
So a distant immortal observer, with a ridiculously good
telescope, will detect photons from the falling
monkey at all future times.
Eventually, those photons will come billions, even trillions,
of years apart from each other and be hugely redshifted.
For an eternal non-leaking black hole, there is no last photon.
But real black holes decay.
And so the final burst of Hawking radiation
as a black hole evaporates will be accompanied
by all of the remaining light emitted
by everything that fell into it as it crossed the event
horizon.
But the infalling stuff isn't saved.
Although we never saw the moment of its crossing,
that did happen.
Its matter became part of the black hole and re-emerges
as that Hawking radiation.
Grim Reaper Of Trolls is curious about taking their warp
drive into a black hole.
Well, I would encourage it.
You can escape a black hole by traveling faster
than the speed of light.
In principle, you can do that with a warp drive.
The probably impossible technology of the warp drive
allows you to cause a patch of space
to move at really any speed you have the energy to reach.
So you could resist the faster-than-light flow of space
within the black hole.
The event horizon itself just wouldn't
apply within your warp bubble.
You know, there's probably a Penrose diagram for this.
I'll look into it.
Some people complained about my use of the expression
"escape velocity greater than the speed of light"
at the beginning of the episode.
First, let me note that I gave that statement
in a list of popular examples of oversimplifications
about black holes.
Second, it's not actually an incorrect statement.
Despite the derision it receives in nerd circles,
it's accepted to talk about the escape
velocity at the surface of the Earth
because we're used to Newtonian approximations there.
But it's no less accurate near a black hole.
At the event horizon, you need to travel at the speed of light
relative to the black hole's stationary frame
of reference as recorded by a distant observer.
That's the velocity you need to escape.
The reason that people poo-poo the expression "escape
velocity" is that it reminds us of a strange coincidence.
You happen to get exactly the right size for the event
horizon if you use Newtonian gravity to calculate
the distance at which the escape velocity reaches
the speed of light.
The correct way to get this number
is by using general relativity to find the point where
the flow of spacetime reaches the speed of light.
But the concept of escape velocity at the event horizon
is still as meaningful as it is from the surface of the Earth.
It's a useful classical simplification in both cases.
Cornerrecord asks about that thing when
you're in a black hole and time becomes
space and space becomes time?
Well, that's probably a topic for an entire video.
So for now, I'll just quote what's his name.
The inward direction acts like the future direction.
So once inside the horizon, avoiding a singularity
is like avoiding next Tuesday.
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