Gravitational Wave Detectors: How They Work

Simplified gravitational wave detectors

It’s official: this Thursday, February 11, at 10:30 EST, there will be parallel press conferences at the National Press Club in Washington, D.C., in Hannover, Germany, and near Pisa in Italy. Not officially confirmed, but highly probable, is that people running the LIGO gravitational wave detectors will announce the first direct detection of a gravitational wave. The first direct detection of minute distortions of spacetime, travelling at the speed of light, first postulated by Albert Einstein almost exactly 100 years ago. Nobel prize time.

Time to brush up on your gravitational wave basics, if you haven’t done so! In Gravitational waves and how they distort space, I had a look at what gravitational waves do. Now, on to the next step: How can we measure what they do? How do gravitational wave detectors such as LIGO work?

Recall that this is how a gravitational wave will change the distances between particles, floating freely in a circular formation in empty space: How distances change when a simple gravitational wave passes through a ring of particles. This is what gravitational wave detectors need to measure.The wave is moving at right angles to the screen, towards you. I’ve greatly exaggerated the distance changes. For a realistic wave, even the giant distance between the Earth and the Sun would only change by a fraction of the diameter of a hydrogen atom. Tiny changes indeed.

How to detect something like this?

The first unsuccessful attempts to detect gravitational waves in the 1960s tried to measure how they make aluminum cylinders ring like a very soft bell. (Tragic story; Joe Weber [1919-2000], the pioneering physicist behind this, was sure he had detected gravitational waves in this way; after thorough analysis and replication attempts, community consensus emerged that he hadn’t.)

Afterwards, physicists came up with alternative scheme. Imagine that you are replacing the black point in the center of the previous animation with a detector, and the rightmost red particle with a laser light source. Now you send light pulses (represented here by fast red dots) from the light source to the detector; let’s first look at this with the gravitational wave switched off:Simplified gravitational wave detector without gravitational wave

Every time a light pulse reaches the detector, an indicator light flashes yellow. The pulses are sent out regularly, they all travel at the same speed, hence they also reach the detector in regular intervals.

If a gravitational wave passes through this system, again from the back and coming towards you, distances will change. Let us keep our camera trained on the detector, so the detector remains where it is. The changing distance to the light source, and also the changing distances between the light pulses, and some of the changes in distance between light pulses and detector or source, are due to the gravitational wave. Here is what that would look like (again, hugely exaggerated): The same simplified gravitational wave detector, but now with a gravitational wave passing through.

Keep your eye on the blinking light, and you will see that its blinking is not so regular any more. Sometimes, the light blinks faster, sometimes slower. This is an effect of the gravitational wave. An effect by which we can hope to detect the gravitational wave.

“We” in this case are the radio astronomers working on what are known as Pulsar Timing Arrays. The sender of regular pulses are pulsars, rotating neutron stars sweeping a radio beam across our antennas like a cosmic lighthouse. The detectors are radio telescopes here on Earth. Detection is anything but easy. With a single pulsar, you’d need to track pulse arrival times with an accuracy of a few billionths of a second over half a year, and make sure you are not being fooled by various other sources of timing variations. So far, no gravitational waves have been detected in this way, although the radio astronomers are keeping at it.

To see how gravitational wave detectors like LIGO work, we need to make things a little more complex.

Interferometric gravitational wave detectors: the set-up

Here is the basic set-up: Two mirrors, a receiver (or “light detector”), a light source and what is known as a beamsplitter: Basic setup for an interferometric gravitational wave detector

Light is sent into the detector from the (laser) light source LS to the beamsplitter B which, true to its name, sends half of the light on to the mirror M1 and lets the other half through to the mirror M2. At M1 and M2, respectively, the light is reflected back to the beam splitter. There, the light arriving from M1 (or M2) is split again, with half going towards the light detector LD, the other half back in the direction of the light source LS. We will ignore the latter half and pretend, for the sake of our simplified explanation, that all the light reaching B from M1 or M2 goes on to the light detector LD.

(To avoid confusion, I will always refer to LD as the “light detector” and take the unqualified word “detector” to mean the whole setup.)

This setup, by the way, is called a Michelson Interferometer. We’ll see below why it is a good setup for gravitational wave detectors.

In what follows, we will assume that the mirrors and the beam splitter, shown as being suspended, react to the gravitational wave in the same way freely floating particles would react. The key effects are between the mirrors and the beam splitter in what are called the two arms of the detector. Arm length is huge in today’s detectors, running to a few kilometers. In comparison, light source and light detector are very close to the beamsplitter; changes of the distances between these three do not signify.

Light pulses in a gravitational wave detector

Next, let us see how light pulses run through this detector. Here is the same setup, seen from above: Simple interferometric gravitational wave detector, seen from aboveLight source LS, the two mirrors M1 and M2, the beamsplitter B and the light detector LD: all present and accounted for.

Next, we let the light source emit light pulses. For greater clarity, I will make two artificial and unrealistic changes. I will send red and green pulses into the detector, representing the light that goes into the horizontal and the vertical arm, respectively. In reality, there is no distinction, just light apportioned at the beamsplitter. Light running towards M1 will be offset a little to the left, light coming back from M1 to the right, for better clarity. Same goes for M2. This, too, is different in a real detector. That said, here come the light pulses: Simplified interferometric gravitational wave detector with light running through both armsLight starts at the light source to the left. Light that has left the source together, travels together (so green and red pulses are side by side) until the beam splitter. The beam splitter then sends the green pulses on their upward journey and lets the red pulses pass on their way towards the mirror on the right. All the particles that arrive back at the beamsplitter after reflection at M1 or M2. At the beamsplitter, they are directed towards the light detector at the bottom.

In this setup, the horizontal arm is slightly longer than the vertical arm. Red particles have to cover some extra distance. That is why they arrive at the detector a bit later, and we get an alternating rhythm: green, red, green, red, with equal distances in between. This will become important later on.

Here is a diagram, a kind of registration strip, which shows the arrival times for red and green pulses at the light detector (time is measured in “animation frames”): Arrival times at the light detector of a simplified gravitational wave detectorThe pattern is clear: red and green pulses arrive evenly spaced, one after the other.

Bring on the gravitational wave!

Next, let’s switch on our standard gravitational wave (exaggerated, passing through the screen towards you, and so on). Here is the result: Simple interferometric gravitational wave detector with a gravitational wave passing throughWe have trained our camera on the beamsplitter (so in our image, the beamsplitter doesn’t move). We ignore any slight changes in distance between beamsplitter and light source/light detector. Instead, we focus on the mirrors M1 and M2, which change their distance from the beamsplitter just as we would expect from the earlier animations.

Look at the way the pulses arrive at our light detector: sometimes red and green are almost evenly spaced, sometimes they close together. That is caused by the gravitational wave. Without the wave, we had strict regularity.

Here is the corresponding “registration strip” diagram. You can see that at some times, the light pulses of each color are closer together, at others, farther apart: Arrival times for light pulses in a gravitational wave detector

At the time I have marked with a hand-drawn arrow, red and green pulses arrive almost in unison!

The pattern is markedly different from the scenario without a gravitational wave. Detect this change in the pattern, and you have detected the gravitational wave.

Running interference

If you’ve wondered why detectors like LIGO are called interferometric gravitational wave detectors, we will need to think about waves a bit more. If not, let me just state that detectors like LIGO use the wave properties of light to measure the changes in pulse arrival rate you have seen in the last animation. To skip the details, feel free to jump ahead to the last section, “…and now for something a thousand times more complicated.”

Light is a wave, with crests and troughs corresponding to maxima and minima of the electric and of the magnetic field. While the animations I have shown you track the propagation of light pulses, they can also be used to understand what happens to a light wave in the interferometer. Just assume that each of the moving red and green dots in the detector marks the position of a wave crest.

Particles just add up. Take 2 particle and add 2 particles, and you will end up with 4 particles. But if you add up (combine, superimpose) waves, it depends. Sometimes, one wave plus another wave is indeed a bigger wave. Sometimes, it’s a smaller wave, or no wave at all. And sometimes it’s complicated.

When two waves are in perfect sync, the crests of the one aligning with the crests of the other, and the troughs aligning, too, you indeed get a bigger wave. The following diagram shows at which times the different parts of two light waves arrive at the light detector, and how they add up. (I’ve placed a dot on top of each crest; that is what the dots where meant to signify, after all.) Constructive interference of light wavesOn top, the green wave, perfectly aligned with the red wave (which, for clarity, is shown directly below the green wave). Add the two waves up, and you will get the (markedly stronger) blue wave in the bottom panel.

Not so if the two waves are maximally misaligned, the crests of each aligned with the troughs of the other. A crest and a trough cancel each other out. The sum of a wave and a maximally misaligned wave of equal strength is: no wave at all. Here is the corresponding diagram: Destructive interference of light wavesRecall that this was exactly the setup for our gravitational wave detector in the absence of gravitational waves: Red and green pulses with equal spacing; troughs of the one wave perfectly aligned with the crests of the other. The result: No light at the light detector. (For realistic gravitational wave detectors, that is almost true.)

When a gravitational wave passes through the detector, the situation changes. Here is the corresponding pattern of pulse/wave crest arrival times for the animation above: Interference pattern for a gravitational wave passing through the simplified gravitational wave detectorThe blue pattern, which is the sum of the red and the green, is complex. But it is not a flat line. There is light at the light detector where there was no light before, and the cause of the change is the gravitational wave passing through.

All in all, this makes a (highly simplified) version of how gravitational wave detectors such as LIGO work. Whatever the scientists will report this Thursday, it is based on light signals at the exit of such an interferometric detector.

And now for something a thousand times more complicated

Real gravitational wave detectors are, of course, much more complicated than that. I haven’t even started talking about the many disturbances scientists need to take into account – and to suppress as far as possible. How do you suspend the mirrors so that (at least for certain gravitational waves) they will indeed be influenced as if they were freely floating particles? How do you prevent seismic noise, cars or trains in the wider neighborhood and so on from moving your mirrors a tiny little bit (either by vibrations or by their own gravity)? What about fluctuations of the laser light?

Gravitational wave hunting is largely a hunt for noise, and for ways of suppressing that noise. The LIGO gravitational wave detectors and their kin are highly complex machines, with hundreds of control circuits, highly elaborate mirror suspensions, the most stable lasers known to physics (and some of the most high-powered). The technology has been contributed by numerous group from all over the world.

But all this is taking us too far, and I refer you to the pages of the detectors and collaborations for additional information:

LIGO pages at Caltech

Pages of the LIGO Scientific Collaboration

GEO 600 pages

VIRGO / EGO pages

You can find some further information about gravitational waves on the Einstein Online website:

Einstein Online: Spotlights on gravitational waves

Update: Gravitational Waves Discovered

Gravitational Waves and How They Distort Space

Gravitational waves distort space in a rhythmic fashion. These simple animations show how.

It’s official: on February 11, 10:30 EST, there will be a big press conference about gravitational waves by the people running the gravitational wave detector LIGO. It’s a fair bet that they will announce the first direct detection of gravitational waves, predicted by Albert Einstein 100 years ago. If all goes as the scientists hope, this will be the kick-off for an era of gravitational wave astronomy: for learning about some of the most extreme and violent events in the cosmos by measuring the tiny ripples of space distortions that emanate from them.

Time to brush up on your gravitational wave knowledge, if you haven’t already done so! Here’s a visualization to help you – and we’ll go step by step to see what it means: Visualization of a simple gravitational wave. Gravitational waves distort space in a rhythmic fashion.

Einstein’s distorted spacetime

In the words of the eminent relativist John Wheeler, Einstein’s theory of general relativity can be summarized in two statements: Matter tells space and time how to curve. And (curved) space and time tell matter how to move. (Here is a slightly longer version on Einstein Online.)

Einstein published the final form of his theory in November 1915. By spring 1916, he had realized another consequence of distorting space and time: general relativity allows for gravitational waves, rhythmic distortions which propagate through space at the speed of light.

For quite some time, physicists weren’t sure whether these gravitational waves were real or a mathematical artifact within Einstein’s theory. (For more about this controversy, see Daniel Kennefick’s book “Traveling at the Speed of Thought and  this article.) But since the 1980s, there has been indirect evidence for these waves (which earned its discoverers a Nobel prize, no less, in 1993).

Gravitational waves are emitted by orbiting bodies and certain other accelerated masses. Right now, major international efforts are underway to detect gravitational waves directly. Once detection is possible, the scientists hope to use gravitational waves to “listen” to some of the most violent processes in the universe: merging black holes and/or neutron stars, or the core region of supernova explosions.

Just as regular astronomy uses light and other forms of electromagnetic radiation to learn about distant objects, gravitational wave astronomy will decipher the information contained within gravitational waves. And if you go by recent rumors, gravitational wave astronomy might already have kicked off in mid-September 2015.

What do gravitational waves do?

But what do gravitational waves do? For that, let us look at a simplified, entirely hypothetical situation. (The following are variations on images and animations originally published here on Einstein Online.) Consider particles drifting in space, far from any sources of gravity. Imagine that the particles (red) are arranged in a circle around a center (marked in black): A ring of particles floating in space in a circle

If a simple gravitational wave were to pass through this image, coming directly at the reader, distances between these particles would change rhythmically as follows: How distances change when a simple gravitational wave passes through a ring of particles

Note the distinctive pattern: When the circle is stretched in the vertical direction, it is compressed in the horizontal direction, and vice versa. That’s typical for gravitational waves (“quadrupole distortion”).

It’s important to keep in mind that this animation, and the ones that will follow, exaggerate the gravitational wave’s effect quite considerably. The gravitational waves detectors such as aLIGO hope to measure are much, much weaker. If our hypothetical circle of particles were as large as the Earth’s orbit around the Sun, a realistic gravitational wave would distort it by less than the diameter of a hydrogen atom.

Gravitational waves moving through space

The animation above shows what could be called a “gravitational oscillation.” To see the whole wave, we need to consider the third dimension.

We talk about a wave when oscillations propagate through space. Consider a water wave: At each point of the surface, we have an oscillation, with the surface rising and falling rhythmically. But it’s only the fact that this oscillation propagates, and that we can see a crest moving over the surface, that makes this into a wave.

It’s the same with gravitational waves. To see that, we will look not at a single circle of freely floating particles, but at many such circles, stacked one behind the other, forming the surface of a cylinder: Circles of particles, stacked so as to form a cylinder

In this image, it’s hard to see which points are in front and which in the back. Let us join each particle to its nearest neighbors with a blue line, and let us also fill out the area between those lines. That way, the geometry is much more obvious:  The previous cylinder, with neighboring particles joined with lines.

Just remember that neither the lines nor the whitish surface is physical. On the contrary, if we want the particles to be maximally susceptible to the effect of the gravitational wave, we should make sure they are truly floating freely, and certainly they shouldn’t be linked in any way!

Now, let us see what the same gravitational wave we saw before does to this assembly of particles. From this perspective, the wave is passing from the right-hand side in the back towards the left-hand side on the front: A gravitational wave passing through a 3d cylinder of particlesAs you can see, the wave is propagating through space. For instance, the point where the vertical distances within the circle of particles is maximal is moving towards the observer. The wave nature can be seen even more clearly if we look at this cylinder directly from the side: The action of a gravitational wave on an assembly of particles, seen directly from the side

What the animations show is just one kind of simple gravitational wave (“linearly polarized”). Here is another kind (“circularly polarized”): Action of a circularly polarized gravitational wave

This, then, is what the gravitational wave hunters are looking for. Except that they do not have particles floating in free space. Instead, their detectors contain test masses (notably large mirrors) elaborately suspended here on Earth, with laser light to detect the minute distance changes caused by gravitational waves.

More realistic gravitational wave signals, which contain information about merging black holes or the bulk motion of matter inside a supernova explosion, are more complicated still. They combine many simple waves of different frequencies, and the strength of such waves (their amplitude) will change over time in a characteristic fashion.

In these animations, gravitational waves look a bit like wriggling space worms. But these space worms could become the astronomers’ best friends, carrying information about the cosmos that is hard or even impossible to obtain in any other way.

[Don’t miss the sequel: Gravitational wave detectors: how they work]

Update: Gravitational Waves Detected

Weekly Space Hangout – Jan. 22, 2016: Dr. Stuart Robbins

Host: Fraser Cain (@fcain)

Special Guest: Dr. Stuart Robbins, Research Scientist at Southwest Research Institute (SwRI); Mars Impact Craters, Science Lead on Moon Mappers and Mercury Mappers.

Guests:
Morgan Rehnberg (cosmicchatter.org / @MorganRehnberg )
Kimberly Cartier (@AstroKimCartier )
Dave Dickinson (@astroguyz / www.astroguyz.com)
Jolene Creighton (@futurism / fromquarkstoquasars.com)
Pamela Gay (cosmoquest.org / @cosmoquestx / @starstryder)
Brian Koberlein (@briankoberlein / briankoberlein.com)
Continue reading “Weekly Space Hangout – Jan. 22, 2016: Dr. Stuart Robbins”

Spacecraft Launches to Test the Hunt for Ripples in the Fabric of Spacetime

LISA Pathfinder Liftoff

The European Space Agency successfully launched the LISA Pathfinder, a spacecraft designed to demonstrate technology for observing gravitational waves in space. The launch took place at Europe’s spaceport in Kourou, French Guiana on a Vega rocket, at 4:04 GMT on December 3, (10:04 pm EST Dec 2), 2015.

Gravitational waves are ripples in the fabric of spacetime, which were predicted by Albert Einstein in his General Theory of Relativity. So far, because they are extremely tiny and incredibly faint, gravitational waves have proved to be elusive. The technology needed to detect them is highly sensitive and therefore has been difficult to conceive, plan and build.
Continue reading “Spacecraft Launches to Test the Hunt for Ripples in the Fabric of Spacetime”

Weekly Space Hangout – May 29, 2015: Dr. Bradley M. Peterson

Host: Fraser Cain (@fcain)
Special Guest: This week we welcome Dr. Bradley M. Peterson, whose research is directed towards determination of the physical nature of active galactic nuclei.
Guests:
Jolene Creighton (@jolene723 / fromquarkstoquasars.com)
Charles Black (@charlesblack / sen.com/charles-black)
Brian Koberlein (@briankoberlein / briankoberlein.com)
Dave Dickinson (@astroguyz / www.astroguyz.com)
Morgan Rehnberg (cosmicchatter.org / @MorganRehnberg )
Alessondra Springmann (@sondy)
Continue reading “Weekly Space Hangout – May 29, 2015: Dr. Bradley M. Peterson”

It Turns Out Primordial Gravitational Waves Weren’t Found

Last March, international researchers from the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) telescope at the South Pole claimed that they detected primordial “B-mode” polarization of the cosmic microwave background (CMB) radiation. If confirmed, this would have been an incredibly important discovery for astrophysics, as it would constitute evidence of gravitational waves due to cosmic inflation in the first moments of the universe. Nevertheless, as often happens in science, the situation turns out to be more complicated than it initially appeared.

In a joint analysis of data from BICEP2/Keck Array in the South Pole and the space-based Planck telescope, scientists from both collaborations now have a more complete picture and argue that the interpretation of the evidence is muddier than they had previously thought. Their paper will appear in the arXiv pre-print server in a few days and is submitted for publication in the journal Physical Review Letters. [Update: the paper is now available on the arXiv.] The European Space Agency issued a press release about the paper on Friday after a summary of it was leaked and briefly posted on a French website.

Courtesy: Jet Propulsion Laboratory
Courtesy: Jet Propulsion Laboratory

According to inflationary theory, the universe expanded for a brief period at an exponential rate 10-36 seconds after the Big Bang. As a result, models of inflation predict that this rapid acceleration would create ripples in space, generating gravitational waves that would remain energetic enough to leave an imprint on the last-scattered photons, the CMB radiation, approximately 380,000 years later. The CMB spectrum, the “afterglow of the hot Big Bang,” has rich structure in it and has been measured to a “ridiculous level of precision,” according to Professor Martin White (University of California, Berkeley), who gave a plenary talk on cosmology results from Planck at the recent American Astronomical Society meeting.

The twists in the polarization signal of the CMB, known as B-modes (shown below) and quantified by a nonzero tensor-to-scalar ratio r, would be evidence in favor of inflation but they are much more difficult to detect. Scientists are trying to decipher a signal on the level of parts per trillion of ambient temperature, mere fractions of a nano-degree! Since inflation would explain why the universe appears to have no overall curvature, why it approximately appears the same in all directions, and why it has structures of galaxies in it, BICEP2’s result last year—the first claimed detection of cosmic inflation—excited physicists around the world. But last summer, Planck scientists presented a map of polarized light from interstellar dust grains and argued that the polarization signal BICEP2 detected could be due to “foreground” dust in our own Milky Way galaxy rather than due to primordial gravitational waves in the distant universe. The hotly debated controversy remained unresolved and led to the new joint analysis by scientists from both teams.

Courtesy: BICEP2
Courtesy: BICEP2

BICEP2 is sensitive to low frequencies (150 GHz) while Planck is more sensitive to higher ones (353 GHz). As Professor Brian Keating (University of California, San Diego), a member of the BICEP2 collaboration, puts it, “it’s as if you’re listening to an opera, but BICEP2 could only hear the tenors and Planck could only hear the sopranos.” Unfortunately, the joint analysis produced only an upper limit to the value of r, meaning that the evidence for B-mode polarization due to inflation remains elusive for now. “It’s probably at best an admixture of Milky Way dust and gravitational waves,” says Keating. [Full disclosure: until last year, Ramin Skibba was a research scientist in the same department but in a different field as Keating at UC San Diego.]

This result must seem disappointing to BICEP2 scientists, but science often works this way, especially for such a difficult phenomenon to study. The signal is strong, but the interpretation is more complicated than it first appeared. On a positive note, the analysis shows that CMB researchers are faced with a foreground challenge rather than one due to the Earth’s atmosphere or to their detectors.

Illustration by Andy Freeberg, SLAC / South Pole Telescope photo by Keith Vanderlinde
Illustration by Andy Freeberg, SLAC / South Pole Telescope photo by Keith Vanderlinde

Although Planck will have additional polarization measurements and more assessments of systematic uncertainties in a later data release, they will not be able to settle this debate for now. But new experiments will come online soon, including a BICEP3, and they will produce more precise measurements that could effectively remove the contribution from dust. The signal is tractable, and scientists are looking forward to the day when they can declare with strong statistical significance that they have finally discovered evidence of inflation.

What Will We Never See?

Thanks to our powerful telescopes, there are so many places in the Universe we can see. But there are places hidden from us, and places that we’ll never be able to see.

We’re really lucky to live in our Universe with our particular laws of physics. At least, that’s what we keep telling ourselves. The laws of physics can be cruel and unforgiving, and should you try and cross them, they will crush you like a bug.

Here at Universe Today, we embrace our Physics overlords and prefer to focus on the positive, the fact that light travels at the speed of light is really helpful. This allows us to look backwards in time as we look further out. Billions of light-years away, we can see what the Universe looked like billions of years ago. Physics is good. Physics knows what’s best. Thanks physics. And where the hand of physics gives, it can also take away.

There are some parts of the Universe that we’ll never, ever be able to see. No matter what we do. They’ll always remain just out of reach. No matter how much we plead, in some sort of Kafka-esque nightmare, these rules do not appear to have conscience or room for appeal.

As we look outward in the cosmos, we look backwards in time and at the very edge of our vision is the Cosmic Microwave Background Radiation. The point after the Big Bang where everything had cooled down enough so it was no longer opaque. Light could finally escape and travel through a transparent Universe. This happened about 300,000 years after the Big Bang. What happened before that is a mystery. We can calculate what the Universe was like, but we can’t actually look at it. Possibly, we just don’t have the right clearance levels.

On the other end of the timeline, in the distant distant future. Assuming humans, or our Terry Gilliam inspired robot bodies are still around to observe the Universe, there will be a lot less to see. Distance is also out to rain on our sightseeing safari. The expansion of the Universe is accelerating, and galaxies are speeding away from each other faster and faster. Eventually, they’ll be moving away from us faster than the speed of light.

What would you see at the speed of light/
What would you see at the speed of light/

When that happens, we’ll see the last few photons from those distant galaxies, redshifted into oblivion. And then, we won’t see any galaxies at all. Their light will never reach us and our skies will be eerily empty. Just don’t let physics hear a sad tone in your voice, we don’t want to spend another night in the “joy re-education camps”

Currently, we can see a sphere of the Universe that measures 92 billion light-years across. Outside that sphere is more Universe, a hidden, censored Universe. Universe that we can’t see because the light hasn’t reached us yet. Fortunately, every year that goes by, a little less Universe is redacted from the record, and the sphere we can observe gets bigger by one light-year. We can see a little more in all directions.

Finally, let’s consider what’s inside the event horizon of a black hole. A place that you can’t look at, because the gravity is so strong that light itself can never escape it. So by definition, you can’t see what absorbs all its own light. Astronomers don’t know if black holes crunch down to a physical sphere and stop shrinking, or continue shrinking forever, getting smaller and smaller into infinity. Clearly, we can’t look there because we shouldn’t be looking there. They’re terrible places. The possibility of shrinking forever gives me the heebies.

Artistic view of a radiating black hole.  Credit: NASA
Artistic view of a radiating black hole. Credit: NASA

And so, good news! The chocolate ration has been increased from 40 grams to 25 grams, and our physics overlords are good, can only do good, and always know what’s best for us. In fact, so good that gravity might actually provide us with a tool to “see” these hidden places, but only because “they” want us to.

When black holes form, or massive objects smash into each other, or there are “Big Bangs”, these generate distortions in spacetime called gravitational waves. Like gravity itself, these propagate across the Universe and could be detected.It’s possible we could use gravitational waves to “see” beyond the event horizon of a black hole, or past the Cosmic Microwave Background Radiation.

The problem is that gravitational waves are so faint, we haven’t even detected a single one yet. But that’s probably just a technology problem. In the end, we need a more sensitive observatory. We’ll get there. Alternately we could apply to the laws of physics board of appeals and fill in one of their 2500 page application forms in triplicate and see if we can be granted a rules exception, and maybe just get a tiny little peek behind that veil.

We live an amazing Universe, most of which we’ll never be able to see. But that’s okay, there’s enough we can see to keep us busy until infinity. What law of physics would you like to be granted a special exception to ignore. Tell us in the comments below.

“Spotters Guide” for Detecting Black Hole Collisions

A supermassive black hole has been found in an unusual spot: an isolated region of space where only small, dim galaxies reside. Image credit: NASA/JPL-Caltech

When it comes to the many mysteries of the Universe, a special category is reserved for black holes. Since they are invisible to the naked eye, they remain visibly undetected, and scientists are forced to rely on “seeing” the effects their intense gravity has on nearby stars and gas clouds in order to study them.

That may be about to change, thanks to a team from Cardiff University. Here, researchers have achieved a breakthrough that could help scientists discover hundreds of black holes throughout the Universe.

Led by Dr. Mark Hannam from the School of Physics and Astronomy, the researchers have built a theoretical model which aims to predict all potential gravitational-wave signals that might be found by scientists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors.

These detectors, which act like microphones, are designed to search out remnants of black hole collisions. When they are switched on, the Cardiff team hope their research will act as a sort of “spotters guide” and help scientists pick up the faint ripples of collisions – known as gravitational waves – that took place millions of years ago.

X-ray/radio composite image of two supermassive black holes spiral towards each other near the center of a galaxy cluster named Abell 400. Credit: X-ray: NASA/CXC/AIfA/D.Hudson & T.Reiprich et al.; Radio: NRAO/VLA/NRL
X-ray/radio composite image of two supermassive black holes spiraling towards each other near the center of Abell 400 galaxy cluster. Credit: X-ray: NASA/CXC/AIfA/D.Hudson & T.Reiprich et al.; Radio: NRAO/VLA/NRL

Made up of postdoctoral researchers, PhD students, and collaborators from universities in Europe and the United States, the Cardiff team will work with scientists across the world as they attempt to unravel the origins of the Universe.

“The rapid spinning of black holes will cause the orbits to wobble, just like the last wobbles of a spinning top before it falls over,” Hannam said. “These wobbles can make the black holes trace out wild paths around each other, leading to extremely complicated gravitational-wave signals. Our model aims to predict this behavior and help scientists find the signals in the detector data.”

Already, the new model has been programmed into the computer codes that LIGO scientists all over the world are preparing to use to search for black-hole mergers when the detectors switch on.

Dr Hannam added: “Sometimes the orbits of these spinning black holes look completely tangled up, like a ball of string. But if you imagine whirling around with the black holes, then it all looks much clearer, and we can write down equations to describe what is happening. It’s like watching a kid on a high-speed spinning amusement park ride, apparently waving their hands around. From the side lines, it’s impossible to tell what they’re doing. But if you sit next to them, they might be sitting perfectly still, just giving you the thumbs up.”

Researchers crunched Einstein's theory of general relativity on the Columbia supercomputer at the NASA Ames Research Center to create a three-dimensional simulation of merging black holes. Image Credit: Henze, NASA
Researchers crunched Einstein’s theory of general relativity on the Columbia supercomputer at the NASA Ames Research Center to create a three-dimensional simulation of merging black holes. Credit: Henze, NASA

But of course, there’s still work to do: “So far we’ve only included these precession effects while the black holes spiral towards each other,” said Dr. Hannam. “We still need to work our exactly what the spins do when the black holes collide.”

For that they need to perform large computer simulations to solve Einstein’s equations for the moments before and after the collision. They’ll also need to produce many simulations to capture enough combinations of black-hole masses and spin directions to understand the overall behavior of these complicated systems.

In addition, time is somewhat limited for the Cardiff team. Once the detectors are switched on, it will only be a matter of time before the first gravitational wave-detections are made. The calculations that Dr. Hannam and his colleagues are producing will have to ready in time if they hope to make the most of them.

But Dr. Hannam is optimistic. “For years we were stumped on how to untangle the black-hole motion,” he said. “Now that we’ve solved that, we know what to do next.”

Further Reading: News Center – Cardiff U

Three Supermassive Black Holes Tango in a Distant Galaxy, Marking a Huge Discovery

In a galaxy four billion light-years away, three supermassive black holes are locked in a whirling embrace. It’s the tightest trio of black holes known to date and even suggests that these closely packed systems are more common than previously thought.

“What remains extraordinary to me is that these black holes, which are at the very extreme of Einstein’s Theory of General Relativity, are orbiting one another at 300 times the speed of sound on Earth,” said lead author Roger Deane from the University of Cape Town in a press release.

“Not only that, but using the combined signals from radio telescopes on four continents we are able to observe this exotic system one third of the way across the Universe. It gives me great excitement as this is just scratching the surface of a long list of discoveries that will be made possible with the Square Kilometer Array.”

The system, dubbed SDSS J150243.091111557.3, was first identified as a quasar — a supermassive black hole at the center of a galaxy, which is rapidly accreting material and shining brightly — four years ago. But its spectrum was slightly wacky with its doubly ionized oxygen emission line [OIII] split into two peaks instead of one.

A favorable explanation suggested there were two active supermassive black holes hiding in the galaxy’s core.

An active galaxy typically shows single-peaked narrow emission lines, which stem from a surrounding region of ionized gas, Deane told Universe Today. The fact that this active galaxy shows double-peaked emission lines, suggests there are two surrounding regions of ionized gas and therefore two active supermassive black holes.

But one of the supermassive black holes was enshrouded in dust. So Deane and colleagues dug a little further. They used a technique called Very Long Baseline Interferometry (VLBI), which is a means of linking telescopes together, combining signals separated by up to 10,000 km to see detail 50 times greater than the Hubble Space Telescope.

Observations from the European VLBI network — an array of European, Chinese, Russian, and South American antennas — revealed that the dust-covered supermassive black hole was once again two instead of one, making the system three supermassive black holes in total.

The VLBI network. Image Credit: Deane
The VLBI network. Image Credit: Roger Deane

“This is what was so surprising,” Deane told Universe Today. “Our aim was to confirm the two suspected black holes. We did not expect one of these was in fact two, which could only be revealed by the European VLBI Network due [to the] very fine detail it is able to discern.”

Deane and colleagues looked through six similar galaxies before finding their first trio. The fact that they found one so quickly suggests that they’re more common than previously thought.

The inner pair of black holes of the triple system as seen by the European VLBI Network (EVN). Contours show radio emission at 1.7 GHz, the colour scale show radio emission at 5 GHz frequency. Credit: R.P. Deane et al.
The inner pair of black holes of the triple system as seen by the European VLBI Network (EVN). Image Credit: R.P. Deane et al.

Before today, only four triple black hole systems were known, with the closest pair being 2.4 kiloparsecs apart — roughly 2,000 times the distance from Earth to the nearest star, Proxima Centauri. But the closest pair in this trio is separated by only 140 parsecs — roughly 10 times that same distance.

Although Deane and colleagues relied on the phenomenal resolution of the VLBI technique in order to spatially separate the two close-in black holes, they also showed that their presence could be inferred from larger-scale features. The orbital motion of the black hole, for instance, is imprinted on its large jets, twisting them into a helical-like shape. This may provide smaller telescopes with a tool to find them with much greater efficiency.

“If the result holds up, it’ll be very cool,” binary supermassive black hole expert Jessie Runnoe from Pennsylvania State University told Universe Today. This research has multiple implications for understanding further phenomena.

The first sheds light on galaxy evolution. Two or three supermassive black holes are the smoking gun that the galaxy has merged with another. So by looking at these galaxies in detail, astronomers can understand how galaxies have evolved into their present-day shapes and sizes.

The second sheds light on a phenomenon known as gravitational radiation. Einstein’s General Theory of Relativity predicts that when one of the two or three supermassive black holes spirals inward, gravitational waves — ripples in the fabric of space-time itself — propagate out into space.

Future radio telescopes should be able to measure gravitational waves from such systems as their orbits decay.

“Further in the future, the Square Kilometer Array will allow us to find and study these systems in exquisite detail, and really allow us [to] gain a much better understanding of how black holes shape galaxies over the history of the Universe,” said coauthor Matt Jarvis from the Universities of Oxford and Western Cape.

The research was published today in the journal Nature.

Watch: New Documentary Follows the Hunt for Gravitational Waves

A newly released documentary brings you behind the scenes in the hunt for gravitational waves. The 20-minute film, called “LIGO, A Passion for Understanding,” follows the scientists working to create one of the most powerful scientific tools ever made: the Laser Interferometer Gravitational-Wave Observatories (LIGO). You can watch the documentary above.
Continue reading “Watch: New Documentary Follows the Hunt for Gravitational Waves”