To answer
that question requires doing some numbers (you can read through this quickly if
quantitative arguments cause your brain to idle).
First, we
reverse the situation, just to see if tuning in remote TV makes sense. Imagine
that there are alien couch potatoes 55 light-years away who, bored with their
own Fall lineup, have constructed a LOFAR-style antenna in hopes of picking up
“I Love Lucy’s” debut. Hunky TV transmitters on Earth belch out a few hundred
thousand watts of power. That energy is not beamed in all directions equally;
most of it is aimed around the horizon (which, of course, is where the audience
is). Because of this slight beaming, the effective transmitter power is a bit
more: let’s say a million watts, to keep the math simple.
OK, how
strong is that signal by the time it reaches our putative alien audience at 55
light-years distance? Not very. The megawatt broadcast washes over ET’s world
with a power density of about 0.3 million million million million millionths of
a watt per square meter, which is not exactly a scorching signal. Actually,
only about a third of that transmission power is in the “carrier” – the part of
the broadcast that’s very narrow in frequency and easily detected. So knock
that piddling power density down by another factor of three if you want to know
the strength of the easily detectable part of the transmission. (Of course, if
they only find the carrier, they won’t get the picture and sound. But Lucy’s
jokes might not appeal to aliens anyway.)
Could their
LOFAR-style antenna find that carrier, thereby indicating that a program was on
the air? Well, engineers have computed that at the frequency of VHF television,
LOFAR will have an effective collecting area similar to that of the 305-meter
diameter Arecibo antenna in Puerto Rico.
That’s
big. That’s brawny. But not brawny enough. In our SETI experiments at Arecibo, we could find a signal if it were
about 0.1 million million million millionths of a watt per square meter. That
number, you will notice if you count up the words, is a million times bigger
than the “I Love Lucy” carrier at 55 light-years. The aliens’ LOFAR would be inadequate
to detect the broadcast by a factor of a million, a not entirely negligible
amount. Simply stated: LOFAR couldn’t hear it.
So here’s
the bottom line: LOFAR would only be able to find TV signals comparable to ours
from a distance of much less than one light-year! Turning this around,
the mother of all rabbit ears couldn’t pick up the Alien Broadcasting Network
at the distance of even the nearest star.
Disappointing,
but you might argue that the extraterrestrials will have much, much more powerful
TV transmitters than we do. In fact, their broadcasts would have to be
millions of times more powerful to even produce a blip on LOFAR, which seems a
bit silly and likely to set alien roofs on fire.
Now Loeb
points out that LOFAR and other large telescopes now being planned can stare at
the same spot on the sky for months or years. That allows a signal to build up,
making even weak transmissions visible. After a year of staring, LOFAR’s
sensitivity will be several hundred times better than our work at Arecibo. That’s the good news. The bad
news is that it’s still inadequate to hear TV transmitters similar to our own.
But there’s
another point, and one that’s possibly of greater significance. The whole idea
of television broadcasting may be passé for the aliens. Consider: how do you
pick up your daily dose of boob-tube fodder? With a rooftop antenna strapped
to your chimney? Probably not. You most likely get your TV via a cable, fiber
optic, or direct broadcast satellite dish. The powerful television
transmitters on the hills outside town are going to vanish in the next few
decades. They likely vanished long ago on ET’s world.
So while
it’s certainly an excellent idea to look for cosmic signals at low frequencies,
it’s unlikely that any will resemble the type of entertainment we still loft
into the skies here on Earth. And some would say that’s reassuring.
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