Aliens.
We did a prior episode about whether it makes sense
to believe they exist.
But for today, let's assume that they do exist
and that we want to meet some.
How would we let them know we're here?
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Let's leave aside for the moment the fact
that signaling aliens might not be a good idea.
If we wanted to do it, how we go about it?
Today, I'm going to summarize three
different signaling methods that have
been discussed in the literature along with their pros and cons.
I make no claim that this list is comprehensive.
If I've missed something major, you
can let me know in the comments.
But if you're going to bring up probes, which you can,
you should know that I'm deliberately
ignoring that option.
It's too slow, it's too hard to do in bulk,
and it's too relying on aliens being able to decipher
the content of the probe.
I want to discuss signaling without the added
complications of messaging.
Let's start with the radio.
Contrary to popular misconception,
it could be very difficult for aliens
to pick up the radio and TV transmissions
that we've been leaking into space since around World War
II.
The growing consensus is that, even if nearby aliens have
space-based radio receivers with the total area equivalent
to a large city pointed right at Earth full time,
our leakage would be hard to detect, let alone decipher.
You can see the links in the description for papers
that have analyzed this.
So what would it take to have a radio
beacon that operated fairly continuously
and could be detected?
Well, a 2010 paper linked below by Benford, Benford,
and Benford-- it's three physicists all
related-- analyze the physics and the economics
of this question.
Now, I'm simplifying a bit here.
But they suggested that the optimal solution
would be a phased array of many radio telescopes sending
narrow pulses of high-powered microwaves
at frequencies of 10 gigahertz or higher,
sweeping the whole sky about once a year or so.
Now, depending on the available and affordable transition
power-- and we're talking gigawatts here
minimum-- an array like this could
be detected from a few thousand light years away.
But the cost would be enormous.
100 billion US dollars just to get
an omnidirectional transmitter built or only about 10 billion
if you aimed it in the galactic plane
alone, where, for reasons I can't get into here,
many people think life is more likely.
Now, ongoing cost for operation, personnel, and empower
would add billions more dollars.
So while large distances with microwave transmission
are possible, it would require a massive ongoing investment
of humanities resources to go loud enough and last
long enough to go beyond the mere cosmic blade.
So let's move on to method two, lasers.
Nowadays, lasers can produce high-power ultra-short pulses
that can outshine the sun for less than a nanosecond.
Nothing natural pulses with stellar brightness
for a nanosecond at a time, so laser pulses
would look distinctly artificial to our Vulcan friends.
They're also detectable.
Even we can notice pulses that short with photomultiplier
tubes, although you need repeated flashes to know
that you don't have a false positive from a cosmic ray
or something.
Attenuation from interstellar dust
makes the range for optical lasers
less than that for microwave beacons.
You only get about 1,000 light years or so
before it becomes too dim.
But at those distances, a laser's spot size
would only grow to around the size of Jupiter's orbit.
So you could target the pulse to individual star systems
and avoid wasteful transmission into empty space.
With a lot of lasers, you could even
flash several star systems per second, re-aim,
and repeat over and over.
Now, we could probably implement this in a few decades.
But as with radio, the continuous energy usage
would be very expensive.
And that is the major overhead of the options
we've discussed so far.
You have to keep transmitting for a very long time
to increase the odds that someone will see the signal
or there's not much point.
But there's another signaling option
that wouldn't require us to transmit anything at all.
In a recent video, we discussed the transit method
for finding exoplanets.
Using something like NASA's Kepler satellite,
astronomers stare at 150,000 or so stars at once
continually for a few years, looking
for the tiny dips in starlight that
occur when a planet transits in front of its host star
along our line of sight.
This is actually how the majority of exoplanets
have been identified.
Now, if Kerbals are doing astronomy--
and that's our target audience-- they're
probably also looking for planets.
And they're probably using, among other things,
a device like Kepler to do it.
Why?
Because even though there are other ways to detect
exoplanets, the transit method is
the only one that scales well.
With a few Kerbal Keplers, the Kerbal astronomers
could look at hundreds of thousands or millions of stars
at once.
All right, here's where it gets interesting.
What if, instead of a planet, the object
that's transiting in front of its star looks like this?
Or like this?
Before Kepler was even launched, French astronomer Luke Arnold
wrote a paper-- link below-- showing
that the transits of objects like these
could be distinguished from those of round planets.
And a sequence of louvered slits would
look especially distinct and artificial,
all without any transmissions or energy requirements.
Now, these objects wouldn't need to be
the same mass as a planet.
But they would need to be the same size.
So I'm picturing an ultra-thin, ultra-lightweight, opaque
material that we could launch into solar orbit
and then unroll like a sail.
That's not doable today.
But in a few hundred years, maybe.
Now, we would need to tweak the orbit occasionally
due to solar wind and radiation pressure.
But if that could be automated or if space maintenance
missions inside the solar system become easier for us
in the future, then this setup could
serve as a really long-lasting billboard.
Now, like the lasers, billboard transits
could only be seen within a few light years of earth
or maybe a couple thousand if the Kerbal Kepler
instrument is really sensitive.
So to signal aliens very far away near the galactic center,
you'd still need high-powered microwaves.
But all in all, a billboard seems
like a distinct, cheap, ongoing signal
that aliens could notice just by doing exoplanets astronomy,
even if they aren't looking for other aliens.
I want to finish with a comment.
Suppose we conclude that billboards
are the most efficient, most noticeable way to signal
aliens, or at least one of them, that aliens might apply
Princess Bride poison logic.
They might think that we'd think that they'd
think that billboards are a good signal so that we
would go looking for them.
In which case, in order to catch our attention,
maybe alien have put up billboards.
To my knowledge, no one has actually
analyzed exoplanet transit data to look for these signals.
But the Kepler data set is public.
We put a link in the description.
Everyone has access, including SETI.
So besides looking for radio signals and laser pulses, which
SETI does now, should SETI also be
looking for geometric alien billboards
orbiting nearby stars.
Let us know what you guys think in the comments.
As always, I'll report back on the conversation
on the next episode of Space Time.
Two weeks ago, we asked whether gravity was an illusion.
We followed it up last week with a challenge about a helium
balloon in free fall.
I'm going to answer comments from the gravity episode
first, then give you the answer to the challenge question.
First off, Blackmark52 and Gary Oak
both pointed out that the frames of reference video that we
linked to is from University of Toronto and not MIT.
I knew that.
It was a brain hiccup.
We'll put up a correction.
My apologies to all of Canada.
Several of you asked whether Earth is actually
expanding outward, and that's not what I meant at all.
What I meant to get across is that, since Earth's surface
obviously is not accelerating outward
in the ordinary sense of the word,
then we wouldn't be able to call falling frame inertial
and Earth's surface accelerated unless we reexamined what
we even mean in the first place or think
we mean by the terms "acceleration"
an "inertial frame."
That means reexamining Newton's first and second laws
and so forth.
Something's going to have to give.
Now, in the last episode, I was just
trying to give the gist of Einstein's early thinking
about gravity, which evolved and to give you
a general flavor of the seeming tension
between the equivalence principle
and common sense that can only be resolved once we change
our concept of the geometry of the world,
including the fact that time has a geometric character.
More details about this will come in a future episode.
Other questions that a lot of you asked
were similarly noting these points of the parent tension.
For instance, Euan Johnson, Josh Mullins, and danx505
all asked how two frames of reference falling
on opposite sides of Earth can both the inertial when
they're clearly accelerating with respect to each other.
And Agen0000 pointed out that you
can distinguish ordinary celebration
from real Newtonian gravity, because real gravity will
vary in strengths slightly at both ends of a falling box.
Now, these are exactly the right things to point out.
But the last episode wasn't intended to address them.
All these tensions end up being resolved by,
among other things, the facts that, one, acceleration does
not mean what you think it means and, two,
once we get the meaning of acceleration clearer, the rule
that inertial frames of different locations
can't be accelerated with respect to each other breaks
down.
But I know it's frustrating when I say they we'll get there.
And I know that the answers I'm giving at this stage
aren't satisfying or even really answers,
and that's somewhat unavoidable.
I'm trying to take material that is inherently mathematical
and that it takes most professionals
years to understand and make it sensible to non-professionals
without using any math.
That's really difficult. I don't want
to end up just throwing out buzzwords
that aren't fully explained or recycling analogies
that, at some level, are a little misleading.
So to do justice to the concepts,
I need to break them down into smaller chunks
and build on one another.
And remember, each of these smaller chunks
is still super weird.
So I know what you're looking for,
and I'm trying to do a good job for you.
Just bear with me.
We'll get there soon.
Lexis lex asked whether all frames are inertial in GR
or even if the concept of inertial frame
is an incoherent in general relativity.
And that is a great question that we'll also be discussing
in upcoming episodes.
But here's a short answer.
Inertial frames exist in GR.
They're just really falling frames.
But global inertial frames do not.
You can't describe all events everywhere
with a single inertial frame, only ones that
are relatively close to the origin of your frame
in both space and time.
Finally, Pedro Gusmao, Stephan W, and George Mpantes all
noted for different reasons that maybe "illusion"
isn't the best word to use when describing gravity.
I don't disagree.
I also don't expect people to take our titles
and our phrasing ultra-literally, which I
think those viewers understand.
But we still need some kind of concise phrase
that's not terribly misleading, and "illusion" is close enough.
This is also why I "air-quote" things so much.
And now the moment you've all been waiting for-- the answer
to the challenge question.
If I take a van that has a balloon in it filled
with helium that's attached to the floor by a string
and I simultaneously drop the van off a cliff
and cut the string relative to the interior of the van,
the balloon does not move.
There are several ways to understand
why that's the answer.
I think the easiest is to invoke the equivalence principle.
The frame of the falling van should
be equivalent to a frame that's an intergalactic space away
from gravity.
Now, if you put a helium balloon,
even in a van filled with air in intergalactic space, how would
it know which way to go?
All directions are equivalent in the absence
of some kind of real gravity in the Newtonian sense picking out
some direction.
So the balloon couldn't possibly go anywhere,
because there's no direction that's picked out.
Now, how do you reconcile this with the fact
that helium is still less dense than air?
Shouldn't it have to float in some direction?
Yeah, but buoyancy only exists in the presence
of a supposed gravitational field.
Fluids get less dense as you rise in the opposite direction
of a gravitational field, because the fluid that's
lowered down has to hold up the weight of all the fluid that's
above it.
Once you make everything weightless
by letting it free fall or alternately
going out to intergalactic space,
the air below doesn't have to hold the air above anymore.
There isn't even a distinction between below and above.
So all the air pressure equalizes and the balloon
doesn't go anywhere.
Now, technically, it will take a few milliseconds
for the pressure to equalize, during which there
will be some slight bobbing of the balloon.
But it basically stays where it is.
Now, if the explanation that I just gave
is not entirely satisfying to you
now that we've given the answer, I
encourage you guys to start talking amongst ourselves
in the comments.
We got hundreds of submitted responses,
somewhere between a quarter and a third of them were right.
If you see your name scrolling down below,
you were one of the people who got
it right with the correct answer and were among our five
randomly selected winners.
Congratulations.
If you see your name, send us an email--
pbsspacetime@gmail.com-- with your full name,
your mailing address-- it could be
outside the US-- and your preferred t-shirt size,
and we will send your prize.
Again, congratulations.
And I hope the rest of you participate
in future challenges.
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