[MUSIC PLAYING] 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. [MUSIC PLAYING]
Their fleets of... "welcome" ships will be a little further behind.
Love it.
Note: Moore's Law is not a law. It's a marketing guideline that we're nearly at the end of. If we shrink transistors anymore we'll get quantum interference with electrons regularly tunneling to places they shouldn't. This and other limits will see an end to Moore's "Law" relatively soon.
Impossible