Warp drives may or may not be possible,  but if they are then could a distant alien   civilization’s warp fields produce  gravitational waves that we could   see here on Earth?

Well according to  a recent study that may be the case.

We are entering a golden era of gravitational  wave astronomy.

Over the past nine years LIGO -   the laser interferometer gravitational wave observatory has found around 90 mergers between black  holes or neutron stars, with sensitivity   increasing with each new observing run.

Using  everything we’ve learned from LIGO, we’re now   planning the next generation of detectors,  like Cosmic Explorer, the Einstein telescope,   and the newly approved Laser Interferometer  Space Antenna LISA.

We have a good idea of   some of what these detectors will see—from  spinning white dwarfs to merging supermassive   black holes.

But what will they find that we  aren’t expecting?

Whenever we open a new window   to the universe we find unexpected phenomena.

Maybe in one of these windows there be aliens.

Specifically, if there are advanced  alien civilizations out there,   and if these civilizations travel the  universe with warp technology, could we   detect signatures of the massive distortion in  spacetime caused by those warp bubbles?

Well,   we wouldn’t be doing this episode if the answer  wasn’t at least a remote maybe.

Specifically,   a new study finds that the collapse of  a warp bubble of moderate size within   our galactic neighbourhood should be visible  as gravitational waves for future detectors.

Let’s pick this apart to see if and when  we’ll discover our first alien warp bubble.

First a recap.

Warp drives have long been a  useful tool in science fiction to evade the   pesky speed limit of the universe: lightspeed.

And then, back in 1994, Mexican physicist Miguel   Alcubierre found a solution for the superluminal  warp field within the classical framework of   general relativity.

Obviously we covered  this in a previous video.The loophole that   Alcubierre used is that, while it’s impossible  to travel through space faster than light,   there’s no limit to the speed with which a patch  of space can move relative to another patch.

We know for sure that this superluminal relative  motion of patches of space occurs.

For example,   two sufficiently distant regions of the universe  are moving away from each other faster than light   due to the expansion of the universe.

And we can  think of the space beneath the event horizon   of a black hole as moving at superluminal  speeds relative to the distant exterior.

The Alcubierre warp field works by  expanding space behind a patch of   space and contracting it in front, which ends  up pushing this bubble of space forward.

And   because general relativity allows it, there’s  no fundamental limit to the speed that bubble   can reach.

Now just put a spaceship in  the bubble and you have a warp ship.

So why don’t we have our own warp drives  yet?

Well, because they may be impossible,   and even if they’re possible they  have some pretty insane requirements.

The Alcubierre warp field is a valid  solution to the Einstein equations.

Those equations relate the geometry of space  with the mass and energy that space contains.

The geometry is captured by the metric on the  left side of the Einstein equations.

Almost any   imaginable spacetime geometry can be described by  the metric, including the Alcubierre warp field.

The equations of general relativity then tell you  what distribution of mass and energy space needs   to have in order to be warped into your chosen  metric—that’s the stress-energy tensor on the   right.

But not every imaginable metric corresponds  to a physically possible mass-energy distribution.

The Alcubierre metric looks to be one such  impossible configuration.

For one thing,   it violates something called the  null energy condition.

Put simply,   to curve space as the warp metric demands,  the corresponding stress-energy tensor   demands a space containing an enormous  amount of negative mass.

Unfortunately,   this so-called exotic matter is almost certainly  not possible.

Workarounds have been discussed,   like using the Casimir effect.

Erik Lentz even  came up with a warp solution that may not require   negative mass.

Although, even this suffers the  other big problem with the Alcubierre field—the   amount of energy required to move even a  small vessel capable of carrying people   is estimated to be the mass-energy equivalent of  anything between a large moon to an entire star.

We have videos on these recent alternative  proposals, but the new paper by Katy Clough,   Tim Dietrich and Sebastian Khan goes back to the  OG Alcubierre field.

They make no claims about the   physical or practical possibility of constructing  this field in the first place.

Instead, they ask   what really happens to a warp bubble after  the fact, assuming it somehow gets made.

And,   of utmost importance, does a warp bubble produce  gravitational waves that we could detect?

So the Alcubierre metric describes a bubble of space  that can travel faster than light.

When in motion   at a constant velocity, the warp bubble should  not produce any gravitational waves.

It should   produce them when accelerating or slowing down or  perhaps when forming or switching off.

In short,   when the warp field is undergoing some evolution  over time beyond constant motion.

Unfortunately,   the Alcubierre solution doesn’t say anything  about how the metric should evolve over time.

And this is what the researchers explore in detail in  their paper—the natural evolution of an Alcubierre   bubble according to general relativity.

This  is no easy calculation.

Understanding the time   evolution of a metric and its corresponding  mass-energy distribution requires something   called an equation of state.

In the paper,  the authors point out that there is no known   equation of state that would sustain the  required metric over time.

That means it’s   not enough to just have exotic matter, you  also need a yet-unknown behavior of that   matter to lead to the also-yet-unknown  equation of state to sustain the bubble.

But let’s assume some advanced civilization  figures out how to make exotic matter and   contain it so that it holds together for  the journey across the galaxy.

Again,   at constant velocity spacetime would be perfectly  flat far away, meaning there wouldn’t be any   observable gravitational waves.

But we also  know that the warp bubble does not hold its   shape if the delicately engineered drive stops  constraining exotic matter with the required   equation of state.

So what happens when the drive  is switched off?

Or what happens if it fails?

In the latter case it’s probably very bad for the  aliens, but may be very good for us because it   could produce observable gravitational waves.

So how do we calculate what happens when you   burst a warp bubble?

It’s not easy.

While Einstein’s equations are very elegant,   calculations can quickly become horrendously  messy.

After all, we’re trying to evolve complex   distributions of matter through 4-D spacetime.

For this reason the researchers used numerical   relativity simulations—which means calculating  each time step based on the previous time step.

To do this they needed to split 4-D spacetime  into slices of 3-D space—or hypersurfaces—at   consecutive times.

The aim is a sort of flip-book  in which each page is a 3-D spatial volume,   and flicking through the pages gets  us to the evolution of that space and   so reconstructs a 4-D spacetime  containing an evolving metric.

So they rewrite the Alcubierre metric in terms  of a three dimensional metric that lives on the   hypersurface and introduce a few functions that  tell us how to translate between different ways   of slicing—different hypersurfaces—and  how they embed into the overarching 4D   spacetime.

The starting metric on the first  page of the flipbook has a corresponding   initial configuration of matter, and that  matter should evolve as the metric evolves.

So far so good.

Up until now, this has all been  pretty standard practice for simulating general   relativity.

This is the same framework we’d use  to tackle more realistic, dare I say mundane   problems, like the merger of black holes.

But  simulating warp drives is not such smooth sailing.

Remember that we need this equation-of-state thing  in order to evolve the Alcubierre metric and the   corresponding mass-energy distribution over  time.

And remember also that we don’t know of   an equation of state that allows this metric to be  stable.

But that’s OK, because we’re trying to see   what happens when the warp bubble fails.

This  means its possible to come up with a workable   equation of state for a failing warp bubble with,  admittedly, a few speculative assumptions about   things like the pressure, timescales of the decay,  and the resulting matter after the collapse.

Even if these assumptions about the  equation of state of non-existent   matter are about as accurate as assuming a  spherical Klingon in a frictionless vacuum,   the point is that even with sensibly imperfect  assumptions we can still learn something about   the physics.

As well as progress with  the calculation, which is always nice.

So, we’ve set up the first page of our  flipbook—one of our hypersurfaces with   a matter configuration, which in turn gives us  our desired Alcubierre metric.

We’ve specified   the equation of state, and so we’re ready to  simulate.

By specifying these initial data,   Einstein’s equations and the equation  of state take over and the computer can   evolve the geometry of spacetime and  the distribution of matter from one   hypersurface to the next.

Once we reach  the final hypersurface flipbook page   we can stitch together spacetime again and  watch as it evolves through the simulation.

So, how exactly does it evolve?

Let’s say an advanced alien species takes  a trip to see their cosmic neighbours.

On   this occasion they allow the new recruit to  pilot the warp drive.

Everything’s going great   until they miss a warning light coming from the  matter containment field and… the warp bubble   bursts, destroying the ship and crew in  the process.

We’ve all been there.

Or,   depending on exactly how this stuff works,  maybe this was a controlled dispersion   of the warp bubble once they reached  the destination.

But the catastrophic   version is likely to make bigger waves, and also  better represents the simulation in this paper.

The first thing that happens is that  the warp bubble collapses inwards,   presumably obliterating the ship, and then  expands outwards, all at the speed of light.

That outward expansion is accompanied by  a series of gravitational waves—ripples   that follow the massive fluctuations in  spacetime curvature of the ruptured bubble.

So what would these waves look like  to us?

The signal shape would be very   different to the black hole or neutron  star mergers we’ve seen.

In fact,   the researchers likened the signal to the  head-on collision between black holes,   but without the characteristic ringdown at the  tail end as the merged black hole settles down.

That means, if we could detect an event, it would  stand out as not being from a natural source.

But could we detect these gravitational waves  in the first place?

The study calculates that   the collapse of a 1km radius warp bubble traveling  at 10% light speed would produce a signal at the   Earth with a strain of 10^-21 if it happened one  megaparsec away - that’s around 3 million light   years - or a bit further away that the Andromeda  galaxy.

A strain of 10^-21 means lengths here   would be shifted back and forth by a factor  of one over a billion trillion.

Sounds tiny,   but that’s exactly the strain limit that LIGO  can detect.

So can LIGO detect bursting warp   bubbles in Andromeda?

Not quite.

The frequency  of this 10%-lightspeed kilometer-radius warp   bubble’s gravitational waves are 300kHz, which  is way higher than LIGO’s sensitivity range.

While high frequency detectors operating in the  Mega to Giga hertz range are being considered, focus for   now is on much lower frequency sources—the larger  slower natural phenomena that are much more likely   to be real.

So, for the time being, don’t expect  headlines saying we’ve detected the gravitational   wave signature of a stalled hyper-advanced  extraterrestrial warp drive.

But it’s pretty exciting that the detector capable  of seeing such a thing is within our technological   grasp.

And although any signal is well outside  LIGO’s frequency, it’s possible that the   gravitational waves from a burst warp bubble will  appear even stronger than the 10^-21 strain.

For example,   if it’s from within our own galaxy then it’s  much closer than 1 megaparsec and so will be   “louder”.

If the warp bubble is traveling faster  than 0.1c then it emits more intense gravitational   waves on bursting.

But 10% lightspeed requires a  mass-energy equivalence of 1% the mass of the sun,   so let’s not get greedy.

I should also add that,  while superluminal speeds are technically possible   for warp bubbles, in the words of the authors  would likely lead to pathologies—aka physics   breaking—and anyway require computational  resources beyond this study to investigate.

There’s some other good news: if we do  detect the gravitational wave signal of   a bursting warp bubble, we may see more than  just those waves.

After the bubble collapses,   the exotic matter escapes—waves of positive  and negative energy, which might lead   to an electromagnetic counterpart  to the gravitational wave signal.

We call such signals multimessenger  events, and we routinely search for   the other messengers—typically photons and  neutrinos—for every gravitational wave we   detect.

We routinely do see the electromagnetic  counterpart of neutron star mergers for example.

Even if we never detect an alien warp  signal—perhaps because they don’t exist—this   type of research is still interesting.

Warp drives  may have their issues, but they are motivated by   the extremely well-tested theory of general  relativity.

It’s instructive to poke at the   edges of such theories to test the limits  of their validity and to come to a better   understanding of what’s actually possible,  and what’s not.

We can add new intuition and   techniques to our toolbox for understanding  the possible universe.

Or—far less likely,   but who knows—we build a high-frequency  gravitational wave detector and immediately   witness a gravitational pop, pop, pop—the  unmistakable bursting of bubbles warped spacetime.