Whenever we open a new window on the universe, we discover things that no one expected.
Our newfound ability to measure ripples in the fabric of spacetime—gravitational
waves—is a very new window, and so far we’ve seen a lot of wild stuff.
We’ve observed black holes merging, and their oddly high masses challenges our understanding
of black hole formation and growth. We’ve seen colliding neutron stars that have
forced us to rewrite our ideas of how many of the elements of the periodic table get made.
But what else might be hiding in the ripples’ of spacetime? Oh, I know: how about the
gravitational wakes caused by planet-sized alien spacecraft accelerating to near light speed.
The Laser Interferometer Gravitational Wave Observatory—LIGO—was built with
some very particular natural phenomena in mind—in particular merging black holes and neutron stars,
which we’ve now seen, and supernova explosions which we haven’t yet.
Gravitational waves are produced whenever an object with mass is accelerated, and so
we’re constantly bathed in complex network of spacetime ripples. LIGO can only see the most
powerful of these—caused by very massive objects undergoing extreme acceleration—and those only
within a certain frequency range. Still, it’s worth asking what else might LIGO be able to see.
For example, could an alien spacecraft be massive enough and accelerate rapidly enough to produce
observable gravitational waves? That was the question asked by Luke Sellers and team. Their
recent paper introduces the RAMAcraft—the rapid and massive accelerating spacecraft—a ship that
creates a wake of detectable spacetime ripples. But just how massive and how rapidly accelerating
and how close would a RAMAcraft have to be for LIGO to be sensitive to it? Let’s find out.
Let me add a couple of notes of caution. First, this paper is not yet passed through
the peer review process, though you can get it on the preprint archive.
Second, we have NOT observed signals like this—at least as far as anyone has noticed. The paper is
extremely speculative, and in a way it’s just a bit of fun. But beneath the fun premise,
the paper asks a serious question: given LIGO’s capabilities and known sensitivity,
what kind of other signals might it be capable of detecting?
First, though, a brief reminder about gravitational waves and LIGO.
We’ve covered this stuff before, of course. Einstein’s general theory of
relativity predicts that accelerating masses create self-propagating ripples in spacetime,
just as accelerating electric charges make ripples in the electromagnetic field—also known as light.
Gravitational waves are waves in the fabric of space and time—they change
lengths and durations when they pass by. LIGO spots gravitational waves
by detecting the tiny variations in the relative lengths of its 4-km-long arms.
Those arms alternately grow and shrink as a wave passes by, and from the pattern of
those changes we can identify the nature of the gravitational wave source. The two
LIGO detectors in Washington and Lousianna in the US, and with the help of their cousins, European
VIRGO and Japanese KAGRA, we can triangulate location of the source of a gravitational wave.
Now, it’s super easy to make gravitational waves- just flap your arms up and down,
and you’re emitting gravitational waves. Making detectable gravitational waves is a different
story. Gravity is by far the weakest force, so while the electromagnetic signature of your little
chicken dance is all too observable, you’re effectively invisible in gravitational waves.
To make detectable gravitational waves you need really big masses and you need to wiggle
them really really hard. For example, a pair of black holes in a binary pair are accelerating
towards each other, and so generate gravitational waves. These waves carry energy away, causing the
black holes to sink closer together. In a tighter orbit they move faster,
and the stronger acceleration results in stronger gravitational waves at higher frequencies.
If a pair of black holes the mass of our Sun were to spiral together, then just before merger they’d
be moving at roughly half the speed of light. And the power they radiate in gravitational
waves is more than the power emitted in light by all the stars in the observable universe.
And this is what LIGO was designed to be sensitive to- the most powerful
burst of waves emitted right at the end of such an inspiral.
I made a point of emphasizing that it’s not the speed of the black holes that
creates gravitational waves, but rather the acceleration.
More precisely it’s the change in “quadrupole moment of the mass distribution”—but let’s just
think about acceleration because that’s what we need for today. A black hole moving at an
insane but constant speed in a straight line wouldn’t emit waves. But orbiting black holes
do emit waves, because they’re accelerating towards the center of mass of the binary
system. But there are potentially other types of acceleration besides orbits. For example,
we have linear acceleration—just good-ol’ speeding up in a straight line.
So what speeds up in a straight line? People trying to get somewhere.
So what if those people are aliens yeeting themselves through the cosmos? In the paper
we’re looking at today the team does a pretty straightforward general relativity calculation
to figure out what the gravitational radiation should look like for a massive body accelerating
at a constant rate. And, importantly, in what circumstances LIGO could detect it.
Gravitational waves in four-dimensional spacetime are hard to visualize, so let’s start with an
analogy: waves on the surface of water. An inspiraling orbit is a bit like stirring the
water in circles, a linearly accelerating object makes something more like the wake of a boat.
And it’s this wake that would wash over the earth, and LIGO, and we could potentially detect.
In order to figure out what types of spaceship wake LIGO could detect,
we need to work backwards. We need to figure out the mass and acceleration rate that would
produce a gravitational wave with the right intensity and frequency to be detected by LIGO.
Let's start with frequency, just like with telescopes, LIGO is optimized for a specific
range of frequencies. It’s sensitive to gravitational waves between a few tens
of hertz up to a few thousand hertz, with the sweet spot somewhere in the middle of that range.
Outside that range, the ‘noise’ gets too loud and tends to drown out real signals.
The frequency of a gravitational wave is roughly equal to the frequency of the phenomenon that
produced it, or the inverse of the timescale of that phenomenon—which would be the orbital
period in the case of inspiraling objects. LIGO is designed to be most sensitive to the frequency of
inspiralling black holes just before they merge, and particularly to the mass range expected for
black holes produced in the deaths of massive stars. It’s just a coincidence that the resulting
frequencies are the range of human hearing—as long as you convert the gravitational wave to
a sound wave first. …
So what ‘frequency’ is a spaceship wake?
It’s a little more complicated than the frequency of a black hole merger. The gravitational wave
signal ends up being a mix of frequencies, some high, and some low. But crudely speaking,
larger acceleration means more energy is emitted in higher frequency waves,
while lower accelerations have stronger low-frequency waves. For an accelerating
spacecraft it’s nicer to think about this in terms of time spent accelerating. If we assume that the
aliens want to reach a certain maximum speed—say, a certain fraction of the speed of light—then the
acceleration is determined by the amount of time taken to reach that speed. The longer
the acceleration time to reach a given speed, the stronger the low-frequency waves will be.
We can also think about this in terms of wavelength. Low frequency means long wavelength.
And the longer our craft spends accelerating, the longer the gravitational wave it can emit.
Based on LIGO’s frequency sensitivity and assuming acceleration to
30% the speed of light, this team of scientists figures that they’d need to
reach that speed from standstill in just a few tens of seconds in order to be detectable by LIGO.
And that's really slamming the foot - or pseudopod or plasma tentacle or whatever on the gas pretty
hard. But how massive and how nearby would such a craft need to be in order to be detectable by
LIGO? The more massive the craft, the stronger its gravitational wake. Now LIGO is a very sensitive
instrument, capable of detecting changes in the lengths of its arms by a thousandth the
width of a proton. That lets us spot black holes merging a billion light years away.
In fact, LIGO could also detect an acceleration starship an entire universe away… if that starship
had the mass of the the entire Sun. That’s a solar mass accelerated to 30% light speed in a fraction
of a second. Not too plausible perhaps. Although we are talking about the hundreds of billions of
galaxies each with hundreds of billions of stars, and we just need one civilization
in all of that to reach this frankly absurd technological power. Still, it sounds pretty iffy.
But thinking more modestly, how massive would the craft have to be in order for
an acceleration inside the Milky Way to be visible? Well, from the other side of the
galaxy it would have to be the mass of Jupiter. Moving a Jupiter mass to a good fraction of light
speed in less than a second is also pretty far out, and still doesn’t seem likely that
such insane levels of tech exist in our galaxy, given we don’t see any other evidence for it.
If this were to happen closer to Earth - say, within 30 light years, or our local stellar
neighbors, then we could potentially detect a moon-mass accelerating ship.
Some of you may remember that warp fields do require the entire mass-energy of a star or
a Jupiter or a moon depending on how you calculate it. Now maybe that’s a way to produce the stupid
accelerations required, while at the same time protecting the RAMAcraft from the cataclysmic
g-forces from that acceleration. Unfortunately, the known warp field solutions don’t produce the
same space-time wakes as simply accelerating a large mass. But perhaps there are undiscovered
warp solutions that can do the job. But however they do it, the aliens are going to need to
access an unthinkable amount of energy. Remember that a single merging black hole binary produces
more power than emitted by all the stars in the universe for an instant. Scale that down to a mere
planet-mass craft, and we’re still talking entire galaxies or at least many stars worth of power.
This all sounds pretty out there.
But this isn’t a fool’s errand. There are a couple of good reasons that we might want to
think of weird sources of gravitational waves, whether RAMAcraft or otherwise.
We mostly think about looking for aliens through electromagnetic radiation, whether their radio
transmissions, their planet-covering city lights, or the light blotted out by their Dyson spheres.
But the problem with electromagnetic waves is that they dim very quickly with distance. The
intensity of the light drops as the square of its distance. Double your distance from a light source
and it’s a quarter the strength. 10 times further away and it’s a factor 100 fainter. In addition,
electromagnetic observatories - especially traditional telescopes but also to some
extent radio observatories - tend to be limited to relatively small patches of
the sky at a time, which further slows the process of looking for this stuff.
Due to this, our searches for artificial electromagnetic signals are pretty much
limited to nearby stars in the Milky Way. For example, even the most ambitious search to date,
Breakthrough Listen, has an ongoing goal of observing a million stars and a handful of
exotic candidates requiring literally years worth of observing telescope time on some of the biggest
radio dishes in the world. That’s a fraction of a percent of the stars in the Milky Way.
In contrast, the “strain”—or stretching and squishing of space caused by gravitational
wave weakens not as an inverse square law, but just as an inverse law.
Being ten times farther only means ten times weaker in strain, versus being 100 times weaker.
Another convenient fact about gravitational waves is that they pass through anything. Photons are
easily blocked by a bit of dust between the stars, or by our atmosphere. But gravitational waves are
literal spacetime ripples. All the matter they pass through just wibble-wobbles a little bit,
leaving the wave largely unaffected. So we’re able to monitor gravitational waves coming from
all directions constantly, and to very large distances. So if that gigatech civilization
is out there with its stupidly large spaceship, we will detect it as long as it’s close enough.
Now there are other gravitational wave observatories that can or will do better
than LIGO in some respects. The upcoming LISA- the Laser Interferometer Space Antenna - will
have arms a million times longer than LIGO. And that means it’s sensitive to
frequencies almost a million times lower. It’s a little complicated to make a direct
comparison of frequency sensitivity, but adding the fact tha LISA is able to monitor
uninterrupted for many days due to the lack of gravitational noise out there in space,
means it should be able to detect RAMAcraft that take several days to accelerate. The
Pulsar Timing Array - a gravitational wave observatory consisting of pulsars scattered
across the Milky Way - could be sensitive to craft that accelerate over months or years.
They still need to be of Jupiter-ish masses for us to spot them from across the Milky Way,
but now the aliens don’t have to get whiplash from the g-forces in order to be seen.
We probably are not going to see RAMAcraft - they’re a bit too outlandish to be likely.
On the other hand, we would not want to miss them if they exist. And on the other, other hand, we
may well see signals with LIGO and its successors that look like linearly accelerating masses,
and this paper does a good job of laying out what the physical conditions of such phenomena would
have to be for us to spot them. For example, we could have orbiting black holes with highly
elongated orbits, or maybe a star or giant planet accelerating as it plummets into an enormous black
hole, or a weird type of supernova where an intact chunk of the core gets exploded out.
The universe does weird stuff all the time and it's good to know what to look for,
as long as we don’t immediately conclude that it’s aliens. Because it’s never aliens.
But it would be cool to find out that there are planet-sized spacecraft hooning around on the
other side of the galaxy, leaving us sloshing around in their wakes of rippling spacetime.