Astronomy

Could information be transmitted from inside a black hole using gravitational waves?

Could information be transmitted from inside a black hole using gravitational waves?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Could you use gravitational waves to communicate from inside the event horizon of a black hole to someone who was outside?


A black hole is defined as a region of spacetime where the gravitational effects are so strong that the escape velocity within the event horizon is greater than the speed of light. According to relativity, nothing can propagate faster than the speed of light, so at most gravitational waves would propagate at this speed. According to particle physics, mediation of gravity would be controlled by massless particles called gravitons, which, by virtue of being massless, would travel at the same speed as photons, i.e. the speed of light. Suffice to say gravitational waves (analogous to electromagnetic waves) would not be able to escape from inside the event horizon.

The question of information, in regards to black holes is open however. The No-Hair Theorem would seem to imply the destruction of information as the black hole consumes matter. However, the holographic principle, if correct would imply that information regarding the matter that formed the black hole would be encoded on the 2-dimensional boundary of the event horizon. This information could very well be exhausted by Hawking radiation or gravitational waves. Since this information was outside of the event horizon this would be fully consistent with the formulation of black holes.


Seeds of black holes could be revealed by gravitational waves detected in space

Gas and stars in a slice of the EAGLE simulations at the present day. The intensity shows the gas density, while the color encodes the gas temperature. Researchers used the EAGLE simulations to predict the rate at which gravitational waves caused by collisions between supermassive black holes might be detected. Credit: The EAGLE project/Stuart McAlpine

Scientists led by Durham University's Institute for Computational Cosmology ran the huge cosmological simulations that can be used to predict the rate at which gravitational waves caused by collisions between the monster black holes might be detected.

The amplitude and frequency of these waves could reveal the initial mass of the seeds from which the first black holes grew since they were formed 13 billion years ago and provide further clues about what caused them and where they formed, the researchers said.

The research is being presented today (Monday, June 27, 2016) at the Royal Astronomical Society's National Astronomy Meeting in Nottingham, UK. It was funded by the Science and Technology Facilities Council, the European Research Council and the Belgian Interuniversity Attraction Poles Programme.

The study combined simulations from the EAGLE project - which aims to create a realistic simulation of the known Universe inside a computer - with a model to calculate gravitational wave signals.

Two detections of gravitational waves caused by collisions between supermassive black holes should be possible each year using space-based instruments such as the Evolved Laser Interferometer Space Antenna (eLISA) detector that is due to launch in 2034, the researchers said.

13.8 billion years of evolution of the gas in the EAGLE simulations. The intensity shows the gas density, while the colour encodes the gas temperature. Researchers used EAGLE simulations to predict the rate at which gravitational waves caused by collisions between supermassive black holes might be detected. Credit: The EAGLE project/Stuart McAlpine

In February the international LIGO and Virgo collaborations announced that they had detected gravitational waves for the first time using ground-based instruments and in June reported a second detection.

As eLISA will be in space - and will be at least 250,000 times larger than detectors on Earth - it should be able to detect the much lower frequency gravitational waves caused by collisions between supermassive black holes that are up to a million times the mass of our sun.

Current theories suggest that the seeds of these black holes were the result of either the growth and collapse of the first generation of stars in the Universe collisions between stars in dense stellar clusters or the direct collapse of extremely massive stars in the early Universe.

As each of these theories predicts different initial masses for the seeds of supermassive black hole seeds, the collisions would produce different gravitational wave signals.

This means that the potential detections by eLISA could help pinpoint the mechanism that helped create supermassive black holes and when in the history of the Universe they formed.

13.8 billion years of evolution of the dark matter in the EAGLE simulations. The intensity shows the density of dark matter. Researchers used EAGLE simulations to predict the rate at which gravitational waves caused by collisions between supermassive black holes might be detected. Credit: The EAGLE project/Stuart McAlpine

Lead author Jaime Salcido, PhD student in Durham University's Institute for Computational Cosmology, said: "Understanding more about gravitational waves means that we can study the Universe in an entirely different way.

"These waves are caused by massive collisions between objects with a mass far greater than our sun.

"By combining the detection of gravitational waves with simulations we could ultimately work out when and how the first seeds of supermassive black holes formed."

Co- author Professor Richard Bower, of Durham University's Institute for Computational Cosmology, added: "Black holes are fundamental to galaxy formation and are thought to sit at the centre of most galaxies, including our very own Milky Way.

"Discovering how they came to be where they are is one of the unsolved problems of cosmology and astronomy.

"Our research has shown how space based detectors will provide new insights into the nature of supermassive black holes."

Gravitational waves were first predicted 100 years ago by Albert Einstein as part of his Theory of General Relativity.

The waves are concentric ripples caused by violent events in the Universe that squeeze and stretch the fabric of space time but most are so weak they cannot be detected.

LIGO detected gravitational waves using ground-based instruments, called interferometers, that use laser beams to pick up subtle disturbances caused by the waves.

eLISA will work in a similar way, detecting the small changes in distances between three satellites that will orbit the sun in a triangular pattern connected by beams from lasers in each satellite.

In June it was reported that the LISA Pathfinder, the forerunner to eLISA, had successfully demonstrated the technology that opens the door to the development of a large space observatory capable of detecting gravitational waves in space.


Astronomers Discover New Information about Black Hole Mergers

Merging black holes ripple space and time in this artist’s concept. Pulsar-timing arrays — networks of the pulsing cores of dead stars — are one strategy for detecting these ripples, or gravitational waves, thought to be generated when two supermassive black holes merge into one. Image Credit: Swinburne Astronomy Productions

While searching for gravitational waves generated by supermassive black holes merging, astronomers using the Parkes Radio Telescope have discovered new information about the frequency and strength of black hole mergers.

Throughout our universe, tucked inside galaxies far, far away, giant black holes are pairing up and merging. As the massive bodies dance around each other in close embraces, they send out gravitational waves that ripple space and time themselves, even as the waves pass right through our planet Earth.

Scientists know these waves, predicted by Albert Einstein’s theory of relativity, exist but have yet to directly detect one. In the race to catch the waves, one strategy — called pulsar-timing arrays — has reached a milestone not through detecting any gravitational waves, but in revealing new information about the frequency and strength of black hole mergers.

“We expect that many gravitational waves are passing through us all the time, and now we have a better idea of the extent of this background activity,” said Sarah Burke-Spolaor, co-author of a new Science paper published October 18, which describes research she contributed to while based at NASA’s Jet Propulsion Laboratory in Pasadena, California. Burke-Spolaor is now at the California Institute of Technology in Pasadena.

Gravitational waves, if detected, would reveal more information about black holes as well as one of the four fundamental forces of nature: gravity.

The team’s inability to detect any gravitational waves in the recent search actually has its own benefits, because it reveals new information about supermassive black hole mergers — their frequency, distance from Earth and masses. One theory of black hole growth to hit the theorists’ cutting room floors had stated that mergers alone are responsible for black holes gaining mass.

The results come from the Commonwealth Scientific and Industrial Research Organization’s (CSIRO) Parkes radio telescope in eastern Australia. The study was jointly led by Ryan Shannon of CSIRO, and Vikram Ravi, of the University of Melbourne and CSIRO.


A video about the new Parkes findings from Swinburne University of Technology in Melbourne, Australia. Credit: Swinburne Astronomy Productions

Pulsar-timing arrays are designed to catch the subtle gravitational waves using telescopes on the ground, and spinning stars called pulsars. Pulsars are the burnt-out cores of exploded stars that send out beams of radio waves like lighthouse beacons. The timing of the pulsars’ rotation is so precise that researchers say they are akin to atomic clocks.

When gravitational waves pass through an array of multiple pulsars, 20 in the case of the new study, they set the pulsars bobbing like buoys. Researchers recording the radio waves from the pulsars can then piece together the background hum of waves.

“The gravitational waves cause the space between Earth and pulsars to stretch and squeeze,” said Burke-Spolaor.

The new study used the Parkes Pulsar Timing Array, which got its start in the 1990s. According to the research team, the array, at its current sensitivity, will be able to detect a gravitational wave within 10 years.

Researchers at JPL are currently developing a similar precision pulsar-timing capability for NASA’s Deep Space Network, a system of large dish antennas located around Earth that tracks and communicates with deep-space spacecraft. During gaps in the network’s tracking schedules, the antennas can be used to precisely measure the timing of pulsars’ radio waves. Because the Deep Space Network’s antennas are distributed around the globe, they can see pulsars across the whole sky, which improves sensitivity to gravitational waves.

“Right now, the focus in the pulsar-timing array communities is to develop more sensitive technologies and to establish long-term monitoring programs of a large ensemble of the pulsars,” said Walid Majid, the principal investigator of the Deep Space Network pulsar-timing program at JPL. “All the strategies for detecting gravitational waves, including LIGO [Laser Interferometer Gravitational-Wave Observatory], are complementary, since each technique is sensitive to detection of gravitational waves at very different frequencies. While some might characterize this as a race, in the end, the goal is to detect gravitational waves, which will usher in the beginning of gravitational wave astronomy. That is the real exciting part of this whole endeavor.”

The ground-based LIGO observatory is based in Louisiana and Washington. It is a joint project of Caltech and the Massachusetts Institute of Technology, Cambridge, Massachusetts, with funding from the National Science Foundation. The European Space Agency is developing the space-based LISA Pathfinder (Laser Interferometer Space Antenna), a proof-of-concept mission for a future space observatory to detect gravitational waves. LIGO, LISA and pulsar-timing arrays would all detect different frequencies of gravitational waves and thus are sensitive to various types of merger events.

Publication: R. M. Shannon, et al., “Gravitational-Wave Limits from Pulsar Timing Constrain Supermassive Black Hole Evolution,” Science 18 October 2013: Vol. 342 no. 6156 pp. 334-337 DOI:10.1126/science.1238012


How to observe a 'black hole symphony' using gravitational wave astronomy

Shrouded in mystery since their discovery, the phenomenon of black holes continues to be one of the most mind-boggling enigmas in our universe.

In recent years, many researchers have made strides in understanding black holes using observational astronomy and an emerging field known as gravitational wave astronomy, first hypothesized by Albert Einstein, which directly measures the gravitational waves emitted by black holes.

Through these findings on black hole gravitational waves, which were first observed in 2015 by the Laser Interferometer Gravitational-Wave Observatories (LIGO) in Louisiana and Washington, researchers have learned exciting details about these invisible objects and developed theories and projections on everything from their sizes to their physical properties.

Still, limitations in LIGO and other observation technologies have kept scientists from grasping a more complete picture of black holes, and one of the largest gaps in knowledge concerns a certain type of black hole: those of intermediate-mass, or black holes that fall somewhere between supermassive (at least a million times greater than our sun) and stellar (think: smaller, though still 5 to 50 times greater than the mass of our sun).

That could soon change thanks to new research out of Vanderbilt on what's next for gravitational wave astronomy. The study, led by Vanderbilt astrophysicist Karan Jani and featured today as a letter in Nature Astronomy, presents a compelling roadmap for capturing 4- to 10-year snapshots of intermediate-mass black hole activity.

"Like a symphony orchestra emits sound across an array of frequencies, the gravitational waves emitted by black holes occur at different frequencies and times," said Jani. "Some of these frequencies are extremely high-bandwidth, while some are low-bandwidth, and our goal in the next era of gravitational wave astronomy is to capture multiband observations of both of these frequencies in order to 'hear the entire song,' as it were, when it comes to black holes."

Jani, a self-proclaimed "black hole hunter" who Forbes named to its 2017 30 Under 30 list in Science, was part of the team that detected the very first gravitational waves. He joined Vanderbilt as a GRAVITY postdoctoral fellow in 2019.

Along with collaborators at Georgia Institute of Technology, California Institute of Technology and the Jet Propulsion Laboratory at NASA, the new paper, "Detectability of Intermediate-Mass Black Holes in Multiband Gravitational Wave Astronomy," looks at the future of LIGO detectors alongside the proposed Laser Interferometer Space Antenna (LISA) space-mission, which would help humans get a step closer to understanding what happens in and around black holes.

"The possibility that intermediate mass black holes exist but are currently hidden from our view is both tantalizing and frustrating," said Deidre Shoemaker, co-author of the paper and professor in Georgia Tech's School of Physics. "Fortunately, there is hope as these black holes are ideal sources for future multiband gravitational wave astronomy."

LISA, a mission jointly led by the European Space Agency and NASA and planned for launch in the year 2034, would improve detection sensitivity for low-frequency gravitational waves. As the first dedicated space-based gravitational wave detector, LISA would provide a critical measurement of a previously unattainable frequency and enable the more complete observation of intermediate-mass black holes. In 2018, Vanderbilt physics and astronomy professor Kelly Holley-Bockelmann was appointed by NASA as the inaugural chair of the LISA Study Team.

"Inside black holes, all known understanding of our universe breaks down," added Jani. "With the high frequency already being captured by LIGO detectors and the low frequency from future detectors and the LISA mission, we can bring these data points together to help fill in many gaps in our understanding of black holes."

The work was funded in party by NASA (grant 80NSSC19K0322) and the National Science Foundation Grant PHY-1806580, PHY-1809572 and PHY-1708212). Work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration.


Seeds of black holes could be revealed by gravitational waves detected in space

Scientists led by Durham University's Institute for Computational Cosmology ran the huge cosmological simulations that can be used to predict the rate at which gravitational waves caused by collisions between the monster black holes might be detected.

The amplitude and frequency of these waves could reveal the initial mass of the seeds from which the first black holes grew since they were formed 13 billion years ago and provide further clues about what caused them and where they formed, the researchers said.

The research is being presented today (Monday, June 27, 2016) at the Royal Astronomical Society's National Astronomy Meeting in Nottingham, UK. It was funded by the Science and Technology Facilities Council, the European Research Council and the Belgian Interuniversity Attraction Poles Programme.

The study combined simulations from the EAGLE project - which aims to create a realistic simulation of the known Universe inside a computer - with a model to calculate gravitational wave signals.

Two detections of gravitational waves caused by collisions between supermassive black holes should be possible each year using space-based instruments such as the Evolved Laser Interferometer Space Antenna (eLISA) detector that is due to launch in 2034, the researchers said.

In February the international LIGO and Virgo collaborations announced that they had detected gravitational waves for the first time using ground-based instruments and in June reported a second detection.

As eLISA will be in space - and will be at least 250,000 times larger than detectors on Earth - it should be able to detect the much lower frequency gravitational waves caused by collisions between supermassive black holes that are up to a million times the mass of our sun.

Current theories suggest that the seeds of these black holes were the result of either the growth and collapse of the first generation of stars in the Universe collisions between stars in dense stellar clusters or the direct collapse of extremely massive stars in the early Universe.

As each of these theories predicts different initial masses for the seeds of supermassive black hole seeds, the collisions would produce different gravitational wave signals.

This means that the potential detections by eLISA could help pinpoint the mechanism that helped create supermassive black holes and when in the history of the Universe they formed.

Lead author Jaime Salcido, PhD student in Durham University's Institute for Computational Cosmology, said: "Understanding more about gravitational waves means that we can study the Universe in an entirely different way.

"These waves are caused by massive collisions between objects with a mass far greater than our sun.

"By combining the detection of gravitational waves with simulations we could ultimately work out when and how the first seeds of supermassive black holes formed."

Co- author Professor Richard Bower, of Durham University's Institute for Computational Cosmology, added: "Black holes are fundamental to galaxy formation and are thought to sit at the centre of most galaxies, including our very own Milky Way.

"Discovering how they came to be where they are is one of the unsolved problems of cosmology and astronomy.

"Our research has shown how space based detectors will provide new insights into the nature of supermassive black holes."

Gravitational waves were first predicted 100 years ago by Albert Einstein as part of his Theory of General Relativity.

The waves are concentric ripples caused by violent events in the Universe that squeeze and stretch the fabric of space time but most are so weak they cannot be detected.

LIGO detected gravitational waves using ground-based instruments, called interferometers, that use laser beams to pick up subtle disturbances caused by the waves.

eLISA will work in a similar way, detecting the small changes in distances between three satellites that will orbit the sun in a triangular pattern connected by beams from lasers in each satellite.

In June it was reported that the LISA Pathfinder, the forerunner to eLISA, had successfully demonstrated the technology that opens the door to the development of a large space observatory capable of detecting gravitational waves in space.

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.


When black holes meet—inside the cataclysms that cause gravitational waves

Binary black holes come in a variety of forms, but they are all astounding. Credit: NASA, ESA, and G. Bacon (STScI)

It has long been predicted that when two black holes merge, they ought to give out a staggering amount of energy in the form of gravitational waves.

To put the breathtaking scale of this outburst into perspective, it's been caclulated to be equivalent to the power output of 10 23 of our suns. That's 100,000,000,000,000,000,000,000 suns!

Most of this stupendous burst of gravitational energy is given out in the last few orbits, as the black holes merge into a single rotating hole.

So binary black holes are like gravitational time bombs. They announce their existence in a pure gravitational explosion. The countdown timer for the explosion is set by the initial spacing of the two black holes. And only gravitational wave astronomy can reveal their existence.

Black hole pairs can be formed in a few different ways.

The first pathway to a binary black hole starts with pairs of stars born together. This is not uncommon between one third and one half of the stars in the universe are members of binary pairs.

These stars will evolve together, and if they are massive enough, they will live fast and die young. In only a million years, both stars will have evolved, exploded and collapsed, leaving behind a pair of black holes.

If the stars are massive enough, they could collapse into black holes. Credit: NASA/JPL-Caltech

Spinning around each other like gravitational egg beaters in the sky, binary black holes tend to clear away the stars around them. Their masses could be 20 to 100s of times the mass of our sun. We call these systems co-evolved binary black holes.

Co-evolved binaries are likely to be tidally locked, meaning the spin of each star is matched to its orbital rotation, causing the pair of black holes to have their spin axes lined up like most of the planets in the solar system.

Co-aligned spin is the key signature of binary black holes that were born together. The signature can be measured in gravitational wave signals.

A binary black hole system can form in another way. Two black holes, born individually in a relatively dense cluster of stars, can capture each other.

The sling-shot effect, which space agencies use to take energy from planets to sling spacecraft out of the solar system, plays a crucial role here.

Globular clusters can also be the birthplace of binary black holes. Credit: ESA/Hubble & NASA, Acknowledgement: Judy Schmidt

Stars passing near the black holes get random sling-shots as they drift through the cluster. Black holes from the early universe, which are normally expected to be at least 20 times as massive as normal stars, tend to lose energy to the stars they pass, and so they slowly sink towards the centres of their star cluster.

Over billions of years, as more massive black holes sink towards the centre of globular clusters, the density increases until the typical spacing between stars and black holes is about as close as the distance between the sun and Pluto.

In these super-dense conditions, black holes can capture other black holes. Once a black hole pair has formed, it again acts like an egg beater, which transfers energy to passing stars.

Each interaction tends to cause the black hole binary to shrink, while the whole binary simultaneously gets a forward kick, which is usually strong enough to throw the binary right out of the cluster into intergalactic space.

These "capture binaries" have two significant differences when compared to co-evolved binaries: their spin axes will be randomly oriented, because the black holes themselves were born separately. These signatures can also be measured in gravitational waves.

Gravitational waves can give direct evidence of the existence of black holes. Credit: Alain Riazuelo, IAP/UPMC/CNRS, CC BY-SA

Sling-shot interactions with other stars can also take energy from widely spaced binaries, so as to reduce the time to coalescence, and also can create black hole binaries near the centres of galaxies.

But galaxies have much stronger gravity than globular clusters. This means it's much less likely the black holes will be flung into interstellar space.

In these different ways, black holes born from the first stars end up as binary pairs: some captured near the centres of galaxies some still near their birthplace and others drifting through empty space for billions of years.

These are gravitational time bombs. They are all spiralling together towards coalescence. The time setting depends on their proximity.

Billions of binaries across the universe will be creating a random background of gravitational waves, ripples on a cosmic sea of space-time. But when each finally merges, they emit a vast explosion of gravitational energy, triggering a cosmic tsunami.

Countdown to coalescence

The gravitational wave emission from black hole binaries is like waves created by a moving ship. They take away energy, causing the binary to spiral inexorably towards merger.

In the emptiness of interstellar space, they can only emit electromagnetic waves if they encounter gas or comets, which could trigger weak X-ray emission. They are so small and so distant that conventional astronomy is unlikely to ever be able to detect them.

Each black hole system is like a countdown timer. Each one set to a different time according to its starting conditions. In the chaotic conditions of a collapsing gas cloud we would expect a range of time settings.

Likewise all other formation scenarios will create binaries with various time settings. Some will have times set longer than the age of the universe. Others will coalesce in an moment of cosmic time.

Only those binaries with their gravitational countdown timer set to match our place and time in the universe are useful to us. These are like cosmic time capsules that release their data in the form of a vast explosion of gravitational energy, detectable by the LIGO gravitational wave detectors.

Are there enough of these binaries, with their correct time clock setting that we can detect these gravitational explosions? Today we know that the answer is yes. See the remaining articles about the historic detection of gravitational waves by LIGO to find out more!

This article was originally published on The Conversation. Read the original article.


Ask Ethan: Do Merging Black Holes Create An Information-Loss Paradox?

A computer simulation, utilizing the advanced techniques developed by Kip Thorne and many others, . [+] allows us to tease out the predicted signals arising in gravitational waves generated by merging black holes. The question of what happens to the information encoded on the surfaces of the event horizons, though, is still a fascinating mystery.

Do merging black holes lose information? They absolutely must, according to General Relativity and the known laws of physics. Take two black holes, merge them together, and they lose mass. For the ten black hole-black hole mergers LIGO and Virgo have seen so far, each one has lost mass in the process: about 5% of the total, on average. So where does the information that was encoded by that mass go? That's what our Patreon supporter Pierre Fransson wants to know, asking:

When black holes merge they [lose] energy through gravitational waves. Does this pose the same problem as Hawking radiation does, with respect to loss of information? Or is the information on what has gone into the black hole somehow encoded into the gravitational wave? And if it is could we someday hope to decode what went into the black hole using gravitational waves?

Let's take a look at black hole information in general, and then let's examine what happens when they merge.

A still image of a visualization of the merging black holes that LIGO and Virgo have observed so . [+] far. As the horizons of the black holes spiral together and merge, the emitted gravitational waves become louder (larger amplitude) and higher pitched (higher in frequency). The black holes that merge range from 7.6 solar masses up to 50.6 solar masses, with about 5% of the total mass lost during each merger. The frequency of the wave is affected by the expansion of the Universe.

Teresita Ramirez/Geoffrey Lovelace/SXS Collaboration/LIGO-Virgo Collaboration

Black holes used to present a tremendous puzzle for astrophysicists when it came to the idea of information. No matter what it is that you make your black hole out of – whether it's stars, atoms, protons, electrons, antimatter, heavy elements, or exotic particles – there are only three things that matter for the properties a black hole possesses: its total mass, electric charge, and angular momentum.

Whether you made a black hole out of ten solar masses of oxygen atoms, uranium atoms, or antiprotons-and-positrons should be completely irrelevant to what you find. Quantities like baryon number, lepton number, isospin, and a slew of other particle properties don't play any role in the physics of a black hole. Once you fall inside, that information should be lost forever.

At least, that's what happens in General Relativity all by itself.

The mass of a black hole is the sole determining factor of the radius of the event horizon, for a . [+] non-rotating, isolated black hole. For a long time, it was thought that black holes were static objects in the spacetime of the Universe, and General Relativity assigned them an entropy of zero. This, of course, cannot be the case.

The story changes, however, if you start to consider things like thermodynamics and quantum physics. Without those considerations, General Relativity tells you what a black hole's entropy is: zero.

That should set off alarm bells in your head. Obviously, that cannot be right. Everything that has a temperature, energy, and particle properties has a non-zero entropy, and entropy can never decrease. If the matter that you made black holes out of had a non-zero entropy, then by throwing that material into a black hole, entropy would have to go up or stay the same it could never go down. A black hole must have a finite, positive, and non-zero entropy to account for all the matter that falls into it.

Black holes are not isolated objects in space, but exist amidst the matter and energy in the . [+] Universe, galaxy, and star systems where they reside. They grow by accreting and devouring matter and energy, but also lose energy over time due to the competing process of Hawking radiation. The second law of thermodynamics implies, since matter falls into these black holes, that they must have an entropy which grows as their mass increases.

NASA/ESA Hubble Space Telescope collaboration

While we conventionally think of entropy as something like "information content" or "disorder," neither one of those definitions truly encapsulates what it physically is. Instead, it's better to think of entropy as the number of possible configurations that a quantum state could theoretically possess.

Whenever a quantum particle falls into a black hole's event horizon, it has a number of particle properties inherent to it, including spin, charge, mass, polarization, baryon number, lepton number, and many others. If the singularity at a black hole's center doesn't depend on those properties, there must be some other location that stores that information. John Wheeler was the first person to realize where it could be stored: the event horizon. By considering what an outside observer would see as a quantum particle (or a set of particles) fell into a black hole's event horizon, we can understand how entropy – or information, if you like – gets encoded.

When a mass gets devoured by a black hole, the amount of entropy the matter has is determined by its . [+] physical properties. But inside a black hole, only properties like mass, charge, and angular momentum matter. This poses a big conundrum if the second law of thermodynamics must remain true.

Illustration: NASA/CXC/M.Weiss X-ray (top): NASA/CXC/MPE/S.Komossa et al. (L) Optical: ESO/MPE/S.Komossa (R)

From far away, something falling in would appear to asymptotically approach the event horizon, spaghettifying in the process. Its apparent color would turn redder and redder due to the effects of gravitational redshift, and the amount of time to cross the horizon would asymptote to infinity, as relativistic time dilation took effect. The information from anything that falls into a black hole must appear to be encoded along the surface of the event horizon.

Since a black hole's mass determines the size of its event horizon, this gave a natural place for the entropy of a black hole to exist: on the surface area of the event horizon. As a black hole grows, its event horizon grows, accommodating the additional entropy and information of whatever falls in.

Instead of zero, the entropy of black holes would be enormous, based on the number of quantum bits that could be encoded on an event horizon of a particular size.

Encoded on the outermost surface of the black hole, the event horizon, can be bits of information. . [+] Each bit can be encoded on a surface area as small as the Planck length squared (

10^-66 m^2), where the entire amount of information that can be encoded is proportional to the event horizon's surface area.

T.B. Bakker / Dr. J.P. van der Schaar, Universiteit van Amsterdam

And that brings us to the problem of merging black holes. We now have two of them, in orbit around one another, with a tremendous amount of entropy encoded on their surfaces. Let's imagine we have two black holes of roughly equal masses, which more-or-less corresponds to the black hole mergers LIGO and Virgo have seen. Black hole #1 has a certain mass (M) and a amount of entropy: let's call it S. Black hole #2, if it's the same mass (M) as #1, also has S for its entropy.

Now, let's imagine them merged together. In the end, the new black hole will have almost (but not quite) double the original mass its new mass will be the sum of both black hole #1 and black hole #2, minus about 5%. All told, its total mass will be 1.9M, assuming each black hole lost 5% of its mass. This means there's a set of gravitational waves traveling through the Universe carrying that missing energy: 0.1Mc 2 , where mass is converted into energy by Einstein's famous rule.

For the real black holes that exist or get created in our Universe, we can observe the radiation . [+] emitted by their surrounding matter, and the gravitational waves produced by the inspiral, merger, and ringdown. Where the entropy/information goes during this merger is not yet determined.

LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

But here's where we run into the big conundrum that demonstrates how difficult it is to answer the question of where the entropy (or information) goes when black holes merge. You can imagine three possible solutions:

  1. The information from both black holes remains entirely encoded on the event horizon of the new, larger-mass black hole. The gravitational waves carry none.
  2. The maximal amount of information possible gets encoded onto the gravitational waves: these energy-carrying waves are also entropy-carrying waves, leaving the merger remnant with the least amount of entropy possible.
  3. The information gets split in some non-maximal way between the new event horizon and the gravitational waves themselves.

Unfortunately for all of us, all three possibilities are allowed.

LIGO and Virgo have discovered a new population of black holes with masses that are larger than what . [+] had been seen before with X-ray studies alone (purple). This plot shows the masses of all ten confident binary black hole mergers detected by LIGO/Virgo (blue). Note that the total post-merger mass gives a black hole that is

361% the surface area of either progenitor.

LIGO/VIrgo/Northwestern Univ./Frank Elavsky

Remember what we said about the amount of entropy that a black hole can possess: it's proportional to the event horizon's surface area. But that surface area is proportional to the mass squared, which means that if black hole #1 had an entropy of S and black hole #2 had an entropy of S, then a black hole with 1.9 times the mass of #1 and #2 would have an entropy of

3.6S, enough to easily hold the information of both progenitor black holes. This is the Bekenstein-Hawking entropy.

On the other hand, gravitational waves can carry entropy, too, just like any wave can. And it's not like we can just calculate how much quantum information is in those waves like we can for photons without an understanding of the underlying quantum (gravitational) processes at play, we are limited in how much we can say about the entropy carried by gravitational waves from merging black holes.

Inspiraling masses, such as in binary pulsar systems, exhibit orbital decay consistent with the . [+] emission of gravitational radiation in General Relativity. The change in the curvature of spacetime must correspond to the radiation carried away by gravitational waves.

NASA (L), Max Planck Institute for Radio Astronomy / Michael Kramer

But we can say something of great importance here: the gravitational waves must carry some entropy themselves. During the inspiral phase preceding the merger, these two event horizons are practically unchanged, yet the system is losing mass and energy as the two massive black holes approach one another in space. The gravitational waves carry that energy away, and must also carry the information-and-entropy associated with that energy change with them.

Throughout the entirety of the merger, these gravitational waves are being generated by the changes in curved space itself, and the energy for those waves comes from the changing configuration of the matter-and-energy distribution of the fabric of space. But how much of the information from either of the two event horizons makes it out and into the waves, though, is a question we cannot answer at present, either theoretically or observationally.

The event horizon of a black hole is a spherical or spheroidal region from which nothing, not even . [+] light, can escape. Although conventional radiation is emitted from outside the event horizon, it is unclear where, when, or how the entropy/information encoded on the surface behaves in a merger scenario.

NASA Jörn Wilms (Tübingen) et al. ESA

Information doesn't get lost when two black holes merge, since the final state is known to have a greater entropy than either initial state, so it's not the same as the problem of Hawking radiation. But we cannot say with any certainty how the entropy encoded on those two black hole event horizons gets transferred into the new event horizon and outgoing gravitational wave system we wind up with in the end.

Observationally, we have no way of extracting any sort of entropic or informational signal from gravitational waves at present. Nor can we measure the entropy encoded on an event horizon. We have every reason to believe that information is preserved, and that most of the information from the progenitor black holes winds up in the merged product. But until we find a way to measure and quantify the entropy in black holes and gravitational waves, we must confess to our own ignorance.


Two at once

Astronomers were already working in overdrive when they spotted the potential black hole&ndashneutron star merger. At 08:18:26 UTC on April 25, another train of waves hit the LIGO&rsquos detector in Livingston, Louisiana, and Virgo. (At the time, LIGO&rsquos second machine, in Hanford, Washington, was briefly out of commission.)

That event was a clear-cut case of two merging neutron stars, Hanna says&mdashnearly two years after the first historic discovery of such an event was made in August 2017.

Researchers can usually make such a call because the waves reveal the masses of the objects involved objects roughly twice as heavy as the Sun are expected to be neutron stars. Based on the waves&rsquo loudness, the researchers also estimated that the collision occurred some 150 megaparsecs (500 million light-years) away, says Hanna. That was around three times farther than the 2017 merger.

Iair Arcavi, an astrophysicist at Tel Aviv University who works on the Las Cumbres Observatory, one of GROWTH&rsquos competitors, was in Baltimore, Maryland, to attend a conference called Enabling Multi-Messenger Astrophysics (EMMA)&mdashthe practice of observing these events in multiple wavelengths. The alert of the April 25 event came at 5:01 A.M. &ldquoI set it up to send me a text message, and it woke me up,&rdquo he says.

A storm of activity swept the meeting, with astronomers who would normally compete with each other exchanging information as they sat with their laptops around coffee tables. &ldquoWe&rsquore losing our minds over here at #EMMA2019,&rdquo tweeted astronomer Andy Howell.

But in this case, unlike many others, LIGO and Virgo were unable to significantly narrow down the direction in the sky that the waves came from. The researchers could say only that the signal was from a wide region that covers roughly one-quarter of the sky. They narrowed down the region slightly the day after.

Still, astronomers had well-honed machines for doing just this type of search, and the data they collected the following night should ultimately reveal the source, Kasliwal says. &ldquoif it existed in that region, there&rsquos no way we would have missed it.&rdquo

In the 2017 neutron-star merger, the combination of observations in different wavelengths produced a stupendous amount of science. Two seconds after the event, an orbiting telescope had detected a burst of gamma rays&mdashpresumably released when the merged star collapsed into a black hole. And some 70 other observatories were busy for months, watching the event unfold across the electromagnetic spectrum, from radio waves to x-rays.

If the April 26 event is not a black hole&ndashneutron star merger, it is probably also a collision of neutron stars, which would bring the total detections of this type up to three.


Gravity Waves and Black Hole Collisions

Scientists led by Durham University's Institute for Computational Cosmology ran the huge cosmological simulations that can be used to predict the rate at which gravitational waves caused by collisions between the monster black holes might be detected.

The amplitude and frequency of these waves could reveal the initial mass of the seeds from which the first black holes grew since they were formed 13 billion years ago and provide further clues about what caused them and where they formed, the researchers said.

The research is being presented today (Monday, June 27, 2016) at the Royal Astronomical Society's National Astronomy Meeting in Nottingham, UK. It was funded by the Science and Technology Facilities Council, the European Research Council and the Belgian Interuniversity Attraction Poles Programme.

The study combined simulations from the EAGLE project - which aims to create a realistic simulation of the known Universe inside a computer - with a model to calculate gravitational wave signals.

Two detections of gravitational waves caused by collisions between supermassive black holes should be possible each year using space-based instruments such as the Evolved Laser Interferometer Space Antenna (eLISA) detector that is due to launch in 2034, the researchers said.

In February the international LIGO and Virgo collaborations announced that they had detected gravitational waves for the first time using ground-based instruments and in June reported a second detection.

As eLISA will be in space - and will be at least 250,000 times larger than detectors on Earth - it should be able to detect the much lower frequency gravitational waves caused by collisions between supermassive black holes that are up to a million times the mass of our sun.

Current theories suggest that the seeds of these black holes were the result of either the growth and collapse of the first generation of stars in the Universe collisions between stars in dense stellar clusters or the direct collapse of extremely massive stars in the early Universe.

As each of these theories predicts different initial masses for the seeds of supermassive black hole seeds, the collisions would produce different gravitational wave signals.

This means that the potential detections by eLISA could help pinpoint the mechanism that helped create supermassive black holes and when in the history of the Universe they formed.

Lead author Jaime Salcido, PhD student in Durham University's Institute for Computational Cosmology, said: "Understanding more about gravitational waves means that we can study the Universe in an entirely different way.

"These waves are caused by massive collisions between objects with a mass far greater than our sun.

"By combining the detection of gravitational waves with simulations we could ultimately work out when and how the first seeds of supermassive black holes formed."

Co- author Professor Richard Bower, of Durham University's Institute for Computational Cosmology, added: "Black holes are fundamental to galaxy formation and are thought to sit at the centre of most galaxies, including our very own Milky Way.

"Discovering how they came to be where they are is one of the unsolved problems of cosmology and astronomy.

"Our research has shown how space based detectors will provide new insights into the nature of supermassive black holes."

Gravitational waves were first predicted 100 years ago by Albert Einstein as part of his Theory of General Relativity.

The waves are concentric ripples caused by violent events in the Universe that squeeze and stretch the fabric of space time but most are so weak they cannot be detected.

LIGO detected gravitational waves using ground-based instruments, called interferometers, that use laser beams to pick up subtle disturbances caused by the waves.

eLISA will work in a similar way, detecting the small changes in distances between three satellites that will orbit the sun in a triangular pattern connected by beams from lasers in each satellite.

In June it was reported that the LISA Pathfinder, the forerunner to eLISA, had successfully demonstrated the technology that opens the door to the development of a large space observatory capable of detecting gravitational waves in space.