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.