Archives For astrophysics

black hole accretion disk

Falling into a black hole is a confusing and complicated business, rife with paradoxes and weird quantum effects to reconcile. About a month ago, we looked at black holes’ interactions with the outside world when something falls into them, and today, we’re going to look into the other side of the fall. Conventional wisdom holds that inside a black hole gravity is exponentially increasing until time, space, and energy as we know it completely break down as the singularity. Notice I’m not talking about matter at all because at such tremendous gravitational forces and with searing temperatures in the trillions of degrees, matter simply can’t exist anymore. Movies imagine that singularity as some sort of mysterious portal where anything can happen, while in reality, we’re clueless about what it looks like or even if it really exists. We don’t even know if anything makes it down to the singularity in the first place. But what we do know is that somewhere, whatever is swallowed by the black hole should persist in some weird quantum state because we don’t see any evidence for black holes violating the first law of thermodynamics. Enter the fuzzball.

Quantum fuzzballs aren’t really objects or boundary layers as we know them. Instead, they’re a tangle of quarks and gluons made up of the matter that gave rise to the black hole and what it’s been eating over its lifetime. They don’t have singularities, just loops of raw energy trapped by the immense gravitational forces exerted on them. On the one hand, thinking of a black hole as just a hyper-dense fuzzball eliminates the anomalies and paradoxes inherent in descriptions of singularities, but on the other, simply making a problem go away with equations doesn’t mean it was solved. And that’s the real problem with quantum fuzzballs. They appear as exotic math in general relativity being extended deep into a realm where its predictive powers begin to fail, so while it’s entirely possible that we identified in what direction we need to explore and what we’d expect were we to look into a black hole, it’s equally likely that the classic idea of their anatomy still holds. Unless we drop something into one of those gravitational zombies nearby, we won’t know if the current toy models of what lies inside of it are right. All we have is conjecture.

supernova flare

FRBs just can’t seem to catch a break this month. First, they were an alien signal. Then just as quickly as they were attributed to aliens because the Daily Fail decided to get creative with two out of context words and no one seemed to bother to fact check them, the bursts were called a false signal caused by microwave interference. Not just any microwave interference mind you, but the kind in which you warm up leftovers according to a widely quoted story for which, again, reporters decided that reading the actual paper is for chumps. Popular Science seems to have been the only mainstream publication to actually read the whole thing and point out that no, it’s not open microwave doors creating FRBs, but an extraterrestrial source. While the bursts seen by Parkes and mislabeled as a potential alien communication may have been coming from the kind of interference generated by a prematurely open microwave door by the media are likely just interference from cell towers or another source emitting as the same frequency, there is a batch of FRBs that came to us from as far away as 3 billion light years.

Hold on though, how are some FRBs a case of mistaken identity and others are coming all the way from intergalactic space all from the same telescope? Well, the first study deliberately took what were thought to be 11 signals deserving extra attention and processed their distribution to see if they could find any patterns that would give us a clue as to their origins. Unlike you were told by just about everyone, it probably was not aliens, or even microwaves, since there was a string correlation between signal distribution and a constant we use to sync equipment placed across the world. What exactly emitted the signals we don’t know, but it’s likely fairly humdrum communications equipment. The second study tried to figure out if they could generate a fake signal with microwave ovens, which they could, and then used the data they collected to ferret out whether the FRBs they tracked matched these control perytons.

This is where the story gets interesting. After the second team found matches between the two in terms of frequency, CNET and most others called it a day and told the world that those goofy scientists think aliens were contacting them because they couldn’t wait for their nachos to warm up, adding their inability to fact check to their inability to read an entire paper. But when taking a close look at the distributions form their perytons and genuine FRBs, the researchers found key differences pointing to the bursts coming to us from deep space. Unlike the perytons, FRBs did not have predictable clustering when all candidate signals were included in the analysis, mostly did not line up with the position of the stars in our own galaxy, and one could not match any of their control signals to such an extent that it would be impossible to mistake it for a peryton. So this means that FRBs are indeed extragalactic signals from violent cosmic events and SURONs along with exotic events like neutron star collisions and quakes, are back on the table.

Now that we have the science sorted out, I’d like to turn back to the media for just a moment to humbly ask what the hell is wrong with those who take anything the Daily Mail says and rush to publish something, anything, no matter how poorly researched, distorted, or outright full of crap it happens to be as long as they can publish it quickly enough to ride the Google Trends waves to some extra views. Yes, the media was always awful at reporting science, but this is a rather remarkable low. As mentioned above, reporters who couldn’t be bothered to read whatever the paper they’re covering said made up some alien contact theories no one entertained, said that experiments to rule out human interference with results was in fact proof that the “aliens” were microwave ovens, and proceeded to cast scientists who were just trying to study an interesting phenomenon as the lab-coat wearing version of the Keystone Kops. Your readers deserve real news, written by people who know how to research stories. They deserve better than what you throw at them without a second thought as you rush to the next SEO-dictated topic.

See: E. Petroff, et. al. (2015). Identifying the source of perytons at the Parkes radio telescope arXiv: 1504.02165v1

icy void

Remember the anomalous Cold Spot, the bizarre, low temperature area spotted in the maps of the Cosmic Microwave Background Radiation, or CMBR for short, the echo the Big Bang which gives us a very high level overview of the structure of our universe? Cosmologists bristled at an anomaly stretching some 1.8 billion light years and seemingly violating what we thought was a universal rule that our cosmos is isotropic and homogeneous, i.e. expanding similarly in every direction and with roughly the same density of galaxies from end to end. And so they analyzed the map using different means and some were able to rule it out as an artifact in the data. Still, the question of whether it was really there never went away because every time you figure out some way of erasing something from your data set because it seems weird, you haven’t gotten rid of it, and sure enough, it appeared yet again on Planck’s CMBR map and was now stuck for good. This left scientists with a dilemma. Why was there a cold spot so large and so cold?

Well, the answer to that is a distinct lack of galaxies which makes the Cold Spot about 20% less dense than the typical patch of the sky. This has of course given pop sci headline writers cover to call it The Great Void, a grandiose moniker which overstates the shortfalls in density for this area of the universe, and when billed as the answer to why The Cold Spot is so cold, oversells the effect it has on the background temperature in this patch of the sky. In fact, just 10% of the temperature drop can be linked back to the lack of density while the rest is still very much open to debate. To give credit where credit is due, virtually all iterations of this story did mention this somewhere along the line, but since it’s a fact that people usually read just the first half of most articles, I thought I’d put my disclaimers and conditionals in the top half of my post, rather than towards the bottom as the articles in question because my feeling is that a lot of people will be convinced that the Cold Spot mystery is solved when in fact, it actually deepened.

While you can find anything in the CMBR you want if you stare hard enough, seeing the spot in both the WMAP and Planck results shows that it’s a persistent feature, unlike Roger Penrose’s proposed echoes of past Big Bangs, a hypothesis he was never sufficiently able to explain, and evidence for which strongly depends on how you process the data. And while it’s not really the biggest structure in the known cosmos since that title belongs to a group of quasars more than twice as large if we get nitpicky, as much of the media claims, it’s still a really important feature. When combined with some other weird observations, it hints at something under the surface of our cosmological framework. If you take the so-called Dark Flow discovered several years ago, and add it to the Cold Spot, as well as galactic superclusters which challenge the cosmological principle, one of the odd but still plausible explanations that ties all of them together, is that our universe is being bumped by other universes, essentially giving us evidence of a multiverse we think should exist to explain inflation and making the Cold Spot a cosmological bruise.

Of course now the big question is how we can validate that hypothesis because we steer right into the horizon problem, which puts other universes out of our reach and any attempt to even create a census of what occupies the multiverse is fraught with problems for which we have no existing solutions. Frustratingly, if the colliding universe explanation is in fact the right one, we’ll have to hold off on giving out the Nobel Prize for it because it would remain just out of reach to our instruments, tantalizing us through anomalous patterns in the CMBR and mysterious flows hinting at bizarre mechanics just beneath the fabric of space and time we can observe, but not study in enough depth to come to a solid conclusion. Even a few years ago, we would’ve simply defaulted to Occam’s Razor and ruled what we’re seeing as artifacts from data processing, but the fact that the anomalies keep showing up pretty much rules out that explanation. Now some of our more exotic cosmological theories may well have to be put to the test.

See: Szapudi, et. al. (2015). Detection of a supervoid aligned with the cold spot of the cosmic microwave background MNRAS, 450 (1), 288-294 DOI: 10.1093/mnras/stv488

black hole accretion disk

Apologies for the sudden hiatus everyone. In the last several weeks, life has interfered with any possibility of writing and when there has been time for anything, it’s been occupied by Project X which actually does concern this blog and will be detailed in the future. But I’m finally back, and back with an astronomical bang, or FRBs to be exact. You see, recently astronomers have been puzzled by extremely energetic bursts that last for fractions of a second and vanish forever. It’s like a GRB, the birth cry of a newly born black hole, but it all happens in less than the blink of an eye rather than depending on the size of the cataclysm. These bursts are currently called FRBs and no one is really sure what they are, where they generally originate in the night sky, and how much energy they’re really emitting, repeating the original dilemma with GRBs when they were first discovered. Now we have our first theoretical contender called SURONs, or Supramassive Rotating Neutron Stars, the end result of supernovae that should have created black holes but didn’t, not yet at least. They’re essentially ticking black hole time bombs floating in space.

When our sun will die, it will slowly pulsate and cool into a white dwarf because its mass is below the Chandrashekhar limit, the point at which a star becomes too heavy not to collapse on itself as a supernova. There are some objects that challenge exactly where this limit comes into play, but it seems to be about 1.44 solar masses. Stars heavier than that produce iron in their cores during the last stages of their lives and the unique thing about iron is that fusing it produces no net energy output. Bascially, the strong nuclear force’s interactions with iron’s nucleons create a point of diminishing returns on the nuclear binding energy and the tightly wound nuclei of iron is the first element from which a nuclear reaction can’t extract anything worthwhile. No matter how much iron is being fused, there’s just not enough energy to keep its outer layers from collapsing inward and detonating as a supernova. This is when another important astronomical limit comes into play, the Tolman-Oppenheimer-Volkoff limit. (Yes, that Oppenheimer.) If a neutron star left after a supernova is about two solar masses, it will collapse on itself as a black hole.

Although "will" is kind of a strong word really, a better one would be "should." And this is exactly where the SURONs come into play. Neutron stars are made of degenerate matter, or particles in such a high density environment that the only thing keeping them from falling into each other is, well, each other. Compressing them any more shatters matter as we know it and creates chaotic maelstroms of energy that flow into each other. Degenerate matter at the core of neutron stars can be so hot and dense that it’s basically a weird quantum fluid with no viscosity already, so it’s not going to take all that much to turn it into a black hole. In fact, SURONs are just over the limit and the pressure of its outer layers should’ve triggered a collapse but the particles in their cores were given a brief reprieve. Stars spin and whatever momentum is left after their fiery death has to transfer to the pulsar left behind. Because the star was well over a million miles across and a typical pulsar is tens of miles across, that energy sends the little pulsar spinning wildly arouns its axis, sometimes as fast as 1,122 times per second. This releaves just enough pressure to keep the core from imploding and leave the SURON a neutron star spinning wildly through space.

But there’s a catch. SURONs have extremely strong magnetic fields and those fields will interact with the nebula left behind as will the interactions between its radioactive death beams and gas and dust. Over thousands of years, this will all put a brake on how quickly the neutron star spins which means that at a certain point, the pressure on its core will start building back up until the inevitable happens and the degenerate matter swallows itself and becomes a black hole. Since the SURONs is relatively puny, this collapse happens in a fraction of a second. Its fearsome and powerful magnetic fields will be severed from the just formed event horizon and re-connect very, very violently just outside of it, generating a potent and very short radio pulse. An FRB. This is a nice and tidy explanation because SURONs would be roughly the same size and the event would be pretty much uniform, almost like a Type Ia supernova used as a standard unit for measuring the rate of the universe’s expansion. We don’t know if these neutron stars ticking away into new black holes really do dot the sky and this is not the only possible explanation of FRBs, but it is a pretty good one and it seems quite solid. And that’s often as good as it gets in astronomy…

See: Falcke, H., Rezzolla, L. (2013). Fast radio bursts: the last sign of supramassive neutron stars. Astronomy & Astrophysics arXiv: 1307.1409v1

black hole accretion disk

While many news outlets were reporting a new paper showing that a black hole’s accretion disk can accelerate gas to nearly the speed of light by the event horizon’s distortion of the very fabric of time and space around it, they missed something quite important. Yes, knowing for a fact that gas is screaming around the event horizon at relativistic speeds and beaming out violent X-rays we can see for millions of light years is definitely awesome. Every time the universe does such amazing things, just being able to witness it, document it, and understand how it happens tends to be a huge feat. But there’s more to what this tells us than just how the event horizon works or how relativistic frame-dragging is affecting the flow of gas in the accretion disk. It also tells us a little about what must have happened to the black hole to make it as large as it is.

You see, after analyzing the spectrum of X-rays from iron spinning around the event horizon of the supermassive black hole at the center of the galaxy NGC 1365, physicists can rule out the idea that instead of spinning around the event horizons, the gas was just obscuring what really went on around the black hole’s maw. There was too much distortion for gas to be in the way of the incoming radiation. But the measurements also show that the event horizon itself is zipping around its own axis that the “surface” of the event horizon, the point where tidal forces around the singularity are so strong, a particle would have to be traveling faster than the speed of light to escape, is traveling at nearly the speed of light itself. Not only is that astonishing, but there is only one way it could be spinning that quickly. Collisions with other giant black holes.

If a supermassive black hole gained its mass simply by sipping matter of stars and gas, it would have a low spin because the angular momentum of these objects is small. But merging with one or two other black holes could do the trick because these objects can spin as quickly as 1,000 times per second when they’re at stellar mass. When they collapse, the initial sin of the star that formed them is conserved and all that energy needs to go somewhere, so the black hole begins to church around its axis faster and faster. And when two fast-spinning black holes merge, they impart their energy tp each others’ spin, making the resulting object travel even faster. The sum of the mechanics involved is mind-boggling because the collision is between superheated quark and gluon streams of energy with zero viscosity, so there are few real world analogies we could refer to when talking about it, but the general mathematical results give us something not unlike the monster at the heart of NGC 1365 which shows us that we’re standing to understand more about how supermassive black holes feed on each other and shape the galaxies around them.

See: Risaliti, G., et al. (2013). A rapidly spinning supermassive black hole at the centre of NGC 1365 Nature, 494 (7438), 449-451 DOI: 10.1038/nature11938

[ illustration by NASA/JPL ]

insignificance

Since the dawn of modern cosmology there’s been an implicit assumption that no particular spot in the universe was supposed to be any more special than the rest. On the biggest scales of all, scales at which galaxies are treated like tiny particles, the universe is supposed to be isotropic and homogeneous i.e. more or less uniform in composition and its expansion from the Big Bang. For decades, simulations and observations seemed to show that this was really the case, but as a newly published paper argues, this might no longer be the true because lurking at the dawn of the universe was a group of quasars stretching for nearly 4 billion light years and tipping the very large metaphorical scales at 6.1 quintillion solar masses. That’s a big enough cluster to shatter the theorized limit on how big cosmological structures should be able to get by a factor of four. It looks as if the cosmological principle might need some refining unless it turns out that data from the Sloan Digital Sky survey is wrong and this cluster is much, much smaller than it appears.

Here are the basics on the fancifully named Huge Large Quasar Group, or Huge-LQG for short. It’s made up of 73 quasars arranged like a Y chromosome that was been shot right through the center with a high speed projectile. The upper, crescent-shaped branch is 56 quasars and the remaining 17 cluster tightly right underneath it. It’s about eight times the width of the Great Wall, which was once considered such an enormous cluster of galaxies that it too was once billed as a discovery that would challenge the cosmological constant. But simulations showed that it simply wasn’t big enough and that clusters as wide as 1.2 billion light years still leave the cosmos more or less uniform and isotropic. And this is the major issue with Huge-LQG. It’s almost four times wider and there’s no explanation for how a structure this big could exist without being torn apart by gravity and the expansion of space-time long before it gets anywhere near that size. Now, we can’t exactly toss the cosmological principle away yet, but we at least have to refine it.

Obviously, something is missing and if we were to simply adjust and say that 4 billion light years should now be the new limit on quasar groups, we would be missing why that’s the case. Letting go of the cosmological principle opens us to new models of galactic and cosmic evolution and exciting new ideas. However, it’s not really that simple because we’d also have to explain how an anisotropic early universe became the mostly isotropic, homogeneous mature one we see today while working in the confined space of a finite cosmos. One easy way to stick with homogeneity could be to declare that the known universe must be much bigger than we think because if your scale is big enough, anything can become small enough to be homogenized into your structure, but without being able to see beyond 13 billion light years or so, super-sizing the universe is an extremely questionable proposition. Either way, Huge-LQG leaves us with a dilemma that really gives the status quo a run for its money, and that’s how the really exciting breakthroughs can be made, fascinating new science gets done, and Nobel Prizes are eventually earned…

See: Clowes, R., et al. (2013). A structure in the early Universe at that exceeds the homogeneity scale of the R-W concordance cosmology MNRAS DOI: 10.1093/mnras/sts497

primordial black hole

Usually a new discovery in deep space tends to further complicate our picture of the universe, almost as if the cosmos says "oh yeah, you think you have a good idea of how this works?" and throws a monkey wrench into the works, or sometimes, the whole screaming, angry monkey. So when it comes to phenomena as complex and exciting as black holes, surely there can’t be any data that makes them easier to understand. But this time, when physicists wanted to figure out if jets from black holes followed the same patterns as the mass of the objects went up, nature was willing to cooperate. As it turns out, the powerful jets of material shot from the accretion disks of black holes of 20 solar masses and 20 million solar masses follow the same mechanism. How do we know that? By plotting their strength against the mass of the black hole. If the data follows a linear trend, we know that the physics don’t require a new process to explain the numbers.

So what exactly is happening around black holes? As you may already know, black holes aren’t the cosmic vacuum cleaners far too many sci-fi movies made them out to be. They simply stay where they were very violently born and their immense tidal forces accelerate anything straying nearby into their maws. But black holes are tiny on an astronomical scale and only eat so much at a time. Whatever doesn’t fall directly into their event horizons is whipped around them until it heats up into a glowing accretion disk we can detect. And some of this material gets trapped in the powerful magnetic fields around the black hole and is launched into deep space at 99.9% of the speed of light in the form of highly energetic jets which produce powerful gamma rays. This process seemed to be the same for every black hole observed, but there’s no way to be sure if the black holes affected the jets beyond kinetic energy unless you start comparing gamma ray bursts to one another and plotting them along a trend line.

If the trend is exponential, that means new physics are needed to explain the sudden surges in power as we go up in the jet’s energy and vice versa. But the observed trend between kinetic energy of the jets and the power of the gamma ray bursts is linear, which means that it’s rather likely that the process behind forming the jets is the same across the entire spectrum of known black holes. The black hole’s mass affects how much is can swallow at a time and how powerful the jets it emits could be. The power of the jets affects the observed gamma ray bursts when a new black hole is formed and when it’s in the middle of a large meal consisting of stars and gas floating through interstellar space. So if we know that when we up the jets’ power, we also make the GRB stronger in a predictable way, that tells us that we can more or less confidently scale up what we learn about smaller black holes to their immense siblings, and estimate black holes sizes based on the GRBs’ strength. And that’s very useful for learning more about these prolific and extremely influential gravitational ghosts of giant stars.

See: Nemmen, R., et al. (2012). A universal scaling for the energetics of relativistic jets from black hole systems Science, 338 (6113), 1445-1448 DOI: 10.1126/science.1227416

quasar

As we discussed many times on Weird Things, black holes are the most amazing and terrifying things in the universe we know, and they’re not shy about gathering every law of physics we’re sure we understand, then laughing at them and doing something completely different. Well, not completely different per se, but the incredible heat and gravity of these objects makes time and space flow in ways they can’t anywhere else. One of these extreme phenomena is time dilation induced by gravity. We talked about the extreme effects of time dilation at relativistic speed over the last few years and mentioned that you could technically cross the entire universe in a human lifetime if you were traveling at 99% the speed of light. And the same effect applies when you’re exposed to an extremely gravitationally powerful object. Time would continue normally for you if you’re falling into a black hole, but to an outside observer, you’d be frozen in time and he won’t see you spaghettified and turned into quark-gluon soup past the event horizon.

Or at least that’s the theory which one physicist says might be wrong. According to his view, any particle falling into the black hole would never actually cross the horizon because the dilation is so extreme as to keep it falling until the black hole evaporates. Unfortunately for him, this really doesn’t sound even remotely right since that would prevent black holes from accreting mass. We know they do exactly that. If particles could never fall into a black hole, there would be absolutely no accretion and black holes would have the same mass with which they were created. It could be possible but it would make explaining hypermassive 10 billion solar mass beasts very, very difficult. You’d need a significant portion of a galaxy to implode in on itself just right, circling into itself over eons without enough gravitational nudges and tugs from the various stars and solar systems inside to maintain some semblance of equilibrium. And that just doesn’t sound right. It’s a lot more straightforward to assume that supermassive black holes are born maybe a thousand or so orders of magnitude smaller and work their way up through galactic collisions, gaining most of their mass during massive cataclysms rather than steady feedings.

The root of the problem with this paper lies with its author seemingly forgetting that dilation has an observable effect from the outside while time for the object in question continues as if nothing happened. Were the test particle in the paper see another particle going at the speed of light right next to it, it wouldn’t keep pace with it; the other particle would seem as if it was flying away from its point of reference at the speed of light. He achieves his result by removing a metric he doesn’t seem to have any grounds to remove, and while describing how a black hole accretes a good amount of matter, then evaporates over time due to Hawking radiation, he says that a test particle will just fall until the black hole unraveled into nothingness. These flows of events seem to contradict each other, unless I’m missing something crucial, and since the paper describes opposite outcomes to the same process, methinks it’s staying put on arXiv. The whole point of a black hole’s event horizon is that it must eventually be crossed and nothing can escape it, and once something crosses the event horizon, it’s effectively inside the black hole. If your paper doesn’t get the definition of this critical juncture right, it’s pretty much bound to be flawed.

varies starship concept

Well ladies and germs, it appears that when I tried to calculate how much effort it would take for an alien civilization to create a warp drive, I may have been wrong and so were the theoreticians on whose work I based my numbers. And that’s a good thing because the latest buzz from the DARPA sponsored 100 Year Starship Symposium is that warp drives are many, many orders of magnitude more feasible than initially assumed. Rather than requiring the mass energy of all of Jupiter to jump start, it would require just 67.8 exajoules, which translates to roughly 755 kilos of material. Considering that just a few decades ago, the first theoretical basis of warp drives was considered to be impossible because it seemed like it would take more than the energy of the entire cosmos to create a space-time bubble, the new requirement lowers the bar to interstellar travel down to almost nothing. Yes, there’s the matter of how we can create a burst of energy approaching 68 exajoules, but we certainly have ideas involving large and powerful lasers.

Hold on though, how did we go from having to turn Jupiter into a spark plug to less than one ton of matter to kick-start a warp bubble? By fine tuning the warping of space and time required. In the classical scenario, we’d need a spherical bubble containing the ship, and aside from causing a number of rather nasty side effects, this arrangement turns out to be very energy-demanding since there’s so much space to warp. The first downgrade came from changing how the energy was applied. Rather than blasting out a space-time bubble, you’d basically implode space and time around you to manipulate the cosmological constant, or the Λ in Einstein’s equations, also known as dark energy. This downgrade in energy requirements does away with the warp bubble and proposes an oblong doughnut shape in which the ship is propelled in an area of normal and stable space-time being moved faster than light. For all intents and purposes, the spaceship will stand still as the universe moves around it. It sounds like a sci-fi cliché, but it may just work.

From what I’ve read on the subject, I could speculate that entirely possible that there would be a leak of Hawking radiation or a high-energy halo from the warp field, but these may not be big obstacles to warp travel. If anything, we may want to use powerful magnetic fields to channel all this energy into acceleration and really put the pedal to the metal when traveling to very distant stars. We’ll need to do a lot of experiments to know for sure and those experiments are already starting as a small NASA lab is trying to create space-time disruptions on an atomic scale with laser beams. When it can do that reliably, it can start scaling up to real-world objects and see if space and time will cooperate. If it does, we may be on our way to becoming the kind of space-faring species we only read about in sci-fi novels and space exploration will become a lot easier and more important. But at the same time, we have to stay realistic and understand that this is a tentative first baby step towards warp drives and into barely charted territory in which the laws of physics may cooperate with us just as easily as they might hinder us…

[ illustration by Adrian Mann ]

Say that somewhere out there is a species of space-faring aliens which have relativistic rockets or warp drive technology that lets it travel between solar systems. Considering the sheer size of the universe, it’s probably a good bet that at least one exists. And as these aliens are tooling around, their spacecraft will likely leave what we could call a wake in the fabric of space and time, a wake that we could observe under the right conditions, when the stars align. This is the main gist of an arXiv paper which considers that despite the possibilities of a successful detection of an alien craft’s fly-by being almost nil, we could still try just in case we do get lucky. To start a long term survey, we just need to find star pairs close to each other and aligned with the Earth at about the right angle to give us a good view of the space between them. Then we just look and wait for something to show up, ideally a smear of light magnified by the relativistic wake of the spacecraft we’re trying to detect. It’s a neat idea and the authors readily acknowledge that we may just be too far away to notice alien travelers, or be in a region of space where there are no civilizations capable of interstellar travel, which keeps them grounded when discussing such a lofty SETI approach. But there is one thing they may want to explore a little further…

When we last discussed the Icarus project, did you notice the sheer size of the probe being considered? Go and have a look at that monstrosity and note that the Empire Stare Building does not look all that much bigger by comparison. That’s not because Icarus’ designers have a thing for really large spacecraft, it’s because this craft will have to carry so much fuel and have giant engines to accelerate. Any future interstellar craft designed to support humans, would be even bigger than Icarus to carry all the essentials across trillions and trillions of miles. Let’s say that at some point, we’ll actually decide to build a ship able to ferry humans between the Sun and Alpha Centauri at relativistic speeds, and equip it with a brand new, state of the art artificial black hole engine which should get us up to relativistic speeds very, nicely, shaving the travel time down to only a couple of years instead of several millennia. We’d need to build something much like the Burj Khalifa tower in Dubai to house all the things necessary to comfortably support and house our crew, then get another pair of similar structures and devote them to being engines and fuel tanks, and at least another one to function as a backup tank and to securely house all the shuttle craft that will let the crew go down to the surface of their target world because that giant assembly is simply never going to be able to land. It’s far too huge and heavy. And keep in mind that these estimates are probably erring on the small side, relying on a radical propulsion system.

Now, our imaginary spaceship which we’ll call something inspiring, say, The Really, Really Huge, would have an approximate mass of 2 million tons empty and without the micro black hole suspended between the giant engines armed with nuclear lasers and fuel. The black hole would add at least another million tons and all of its fuel, all the relevant supplies, and supporting spacecraft would bring the total mass of our interstellar craft to something in the neighborhood of 4 million tons. Depending on its configuration, it could be close to 1,000 or so meters long which is just about two thirds of a mile, and about a quarter of a mile across. Sounds huge and very, very expensive, doesn’t it? And this baby goes from zero to ~0.5c in just 6.3 months! How could alien astronomers not notice something like that screaming through the voids of space, warping the photons from the sunlight behind it and leaving a high speed smear in the spectrum of our sun on its way out? Well, for the size and speed of this thing, you have to remember that its traveling through space and as such is tiny if we’re going to compare it to the kind of objects telescopes can actually resolve. We have trouble imaging gas giants in other solar systems, gas giants which are 50,000 times bigger than our hypothetical ship. Sure, its wake is going to affect how the spectrum of a star looks but the warping would be so tiny that it may not even be visible as an artifact of the imaging process, the tiniest fraction of a pixel across, smaller than an exomoon.

And that’s the real gotcha in an otherwise interesting plan. Even if you’re lucky enough to catch an alien ship in the middle of crossing between two nearby solar systems and snap that one in a quadrillion shot, how exactly do you prove that this microscopic smudge in the spectrum is the trail of an extraterrestrial spacecraft? What says it wasn’t dust in the air or atmospheric fluctuations at the time of the shot? Even if you take a picture with an orbital telescope to avoid having a stray air particle from blotting out a snapshot of a relativistic craft, there’s still the potential of a microscopic speck of space debris or a wandering electron to mess with the shot. If the alien species in question build a ship the size of Mercury and flies past our solar system, we’d probably have some chance of catching their relativistic wake by happenstance. Otherwise, the ship will be just too small for a proper identification, if would even register in the image in the first place. Likewise, if we set our sights on a few dozen nearby stars floating close to each other, we wouldn’t necessarily boost our odds of seeing aliens traverse between them since we have no guarantee that they would evolve and thrive in those systems, just a vague estimate of probability that a planet supporting life in general may exist there. It seems that if we’ll ever catch ET mid-flight, it would’ve had to buzz our telescopes on its way to planets unknown…

See: Garcia-Escartin, J.C., et al. (2012). Scouting the spectrum for interstellar travelers arXiv: 1203.3980v1