# What would harnessing of gravitational waves look like?

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We have harnessed and learned to generate and control sound waves, light waves, water waves, and electromagnetic waves in general. What would be needed for science to be able to generate and use gravitational waves in the same way? What would be the outcome?

The mass of fundamental particles, like the proton or electron is very very small in comparison to the charge. This means that the motion of electrons in atoms can produce significant amounts of high frequency electromagnetic radiation.

Quantum effects stop electrons from falling into the protons they orbit.

To produce and control gravitational radiation in significant amounts you would need something very small, but very massive: a microscopic pair of black holes.

Such things would be rather hard to manage. In short it is beyond any conceivable current or future technology.

To put this in perspective, consider a fifty-plus year old human whose main source of exercise for the last twenty-plus years has been walking back and forth between work and the parking spot where the person parks his or her car. Let's strap that person to a bicycle connected to a generator. The energy output of that feeble source of energy easily exceeds the feeble 200 watt gravitational waves produced by the Earth's orbit about the Sun.

Gravitation is extremely feeble compared to electromagnetism. The electrostatic repulsion between a pair of electrons is 1045 times stronger than is the gravitation attraction between a pair of electrons. Gravitational waves are in turn extremely feeble compared to gravitation itself. I mentioned the Earth's orbit about the Sun. The total mechanical energy of that orbit is about 1034 times greater than the paltry amount of energy lost due to gravitational waves.

Imagine a Kardashev level III civilization, a civilization that has learned to harness the equivalent of the energy output of an entire galaxy. Aside: Humanity isn't even at Kardashev level I. That's another few hundred years in the future. Also note that the Kardashev scale is logarithmic. Karadshev level I is near future science fiction. Kardashev level II is also in the realm of science fiction. Kardashev level III? That's beyond science fiction.

Even that Kardashev level III civilization would not harness gravitational waves. It would instead go out of their way to avoid them. A Kardashev level III civilization might, for example, intentionally feed matter to a supermassive black hole at the heart of a galaxy to make that black hole become an active galactic nucleus. The energy output of an AGN is immense, exceeding that of a normal galaxy. The gravitational waves produced by an AGN are minuscule compared to the electromagnetic output, by a factor of about 10-80 or so.

## Riding the Wave

Syracuse student and faculty researchers have contributed to groundbreaking discoveries in astronomy.

Over the past decade, there has been a revolution in observational astronomy spearheaded by the Laser Interferometer Gravitational-wave Observatory (LIGO). The LIGO scientific collaboration has over a thousand members from institutions across North America, and, together with the European Virgo and Japanese KAGRA collaborations, operates four gravitational wave detectors across the globe. Until 2015, information from space came overwhelmingly from measuring different light waves, a method of observation which has been used by astronomers since Galileo first pointed his homemade telescope skyward over 400 years ago. On September 14, 2015, the two detectors of the LIGO collaboration made the first observation of gravitational waves&mdashripples in space itself&mdashcreated by merging black holes.

Stefan Ballmer, associate professor of physics

By using lasers to precisely measure the travel time of light over large distances, LIGO has been able to directly detect gravitational waves warping space. Historically, it has been challenging for astronomers to directly observe black holes with telescopes since, unlike stars, black holes do not produce light. However, by observing how black holes warp space, astronomers can now throw back the veil hiding these objects and start to create a population map of black holes in the universe. &ldquoIf you&rsquore doing something new, a new wavelength, or a completely new way of observing the universe, there&rsquos going to be surprises,&rdquo says Stefan Ballmer, associate professor of physics in Syracuse University&rsquos College of Arts and Sciences. &ldquoIn fact, there were surprises from the very first source of gravitational waves.&rdquo

In the years since the first detection of gravitational waves, LIGO has observed over 40 binary black hole mergers and two binary neutron star mergers. A neutron star is an incredibly dense remnant of a normal star created when stars of certain masses explode in a supernova. LIGO and Virgo detected the collision of two neutron stars through their gravitational waves in 2017. This observation allowed astronomers to point telescopes at the collision and detect the aftermath of the collision in all wavelengths of light, from gamma rays to radio waves. Thus the event came to detectors and telescopes through the different messengers of light and gravitational waves. The era of multi-messenger astronomy has begun, and the students and faculty of the Syracuse University Department of Physics are at the forefront of this emerging field.

This graphic shows the masses of black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and neutron stars detected through gravitational waves (orange). [Image credit: LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

## Gravitiational Waves Give Us A New Way To Look At The Universe

Imagine that instead of the Sun, Moon, planets and stars in the sky, all you had ever seen was clouds. Not the puffy white ones silhouetted against a blue sky, but the thick gray, expansive stratus clouds that are the hallmarks of a dreary winter. But unlike the winter clouds that last for weeks or months at worst, these lasted for all of human history. Yet someone devised the means to part the clouds one night, just for a short while, and allowed us to glimpse the Universe beyond our atmosphere, ever so briefly. Imagine that there was just one point of light that shone through, perhaps a planet, with incredible detail on it: rings, bands, colors, and maybe even moons. How dramatically would your conception of the Universe change from that moment? Now that the results are in -- the LIGO collaboration has indeed detected gravitational waves from two merging black holes -- we can recognize that we've just had exactly that type of moment in astronomy.

Image credit: screenshot from the LIGO press conference announcing the discovery of gravitational . [+] waves.

For the first time, one of the oldest unconfirmed predictions of Einstein's greatest accomplishment, the general theory of relativity, has been successfully put to the test. Two black holes in a far-off galaxy, some 1.3 billion light years away, orbited one another in a cosmic death-spiral, radiating their gravitational energy away until they at last merged, releasing three solar masses worth of material into ripples in the fabric of space itself, via E = mc^2, in the form of gravitational waves. These waves travel outwards through the Universe, causing everything the pass through to compress and expand like a racquetball being squished in one direction, then the perpendicular direction, and so on, traveling forever and ever at the speed of light.

The thing is, experiments like LIGO aren't the only types of gravitational wave detectors we can build, merging black holes aren't the only things we can detect, and more generally, astronomical objects aren't the only things we can use gravitational radiation to learn about! The reason we saw inspiraling black holes first is because LIGO, the cheapest gravitational wave detector we can build that's capable of seeing these waves as the Universe produces them, is sensitive to those types of waves. But in reality, there are all sorts of things to look for, which fall into four different classes.

Image credit: NASA, of an inspiral and merger of two neutron stars illustration only.

1.) Compact, super-fast-moving objects. This is the class that includes what LIGO saw, where small (less than 1,000 solar mass) black holes merge together. Merging neutron stars will also produce gravitational waves, as will individual pulsars, and as will supernovae of both main varieties. LIGO will see the more massive, equal mass black holes first, and is expected to see a handful of them per year. Remember, the detector only came online in September of 2015, and the announced signal came from September 14, 2015. There's likely to be many more black hole mergers in the coming few years, particularly as LIGO's sensitivity improves and its search range gets extended farther and farther into the deep Universe. The big thing that determines which objects fall into this range is their frequency, or how many times-per-second these objects emit a wave. LIGO can detect objects from about 1-to-10,000 Hz, which means objects that emit waves more than once per second!

Image credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI, of the supermassive black hole, . [+] Sagittarius A*, at the Milky Way's center.

2.) Slower and/or more massive objects. These won't have fields that are quite as strong as the objects LIGO sees, but there are many more objects like this out there in the Universe for us to examine. At the heart of nearly every galaxy -- including our own -- is a supermassive black hole, with millions or more times the mass of the Sun inside. A detector whose arms are far larger than Earth, like a giant space antenna in the form of LISA (or eLISA), can locate these. Binary stars, binary white dwarfs, supermassive black holes eating other objects, and highly unequal-mass mergers will all emit gravitational waves of much lower frequencies, where they take minutes, hours or even days to emit gravitational waves. We can't see them with LIGO, but a much larger interferometer in space would be sensitive to them. If NASA decides to invest in it (and even if it doesn't, ESA will), we can fly our first detectors for these objects sometime in the 2030s.

Images credit: Ramon Naves of Observatorio Montcabrer, via . [+] http://cometas.sytes.net/blazar/blazar.html (main) Tuorla Observatory / University of Turku, via http://www.astro.utu.fi/news/080419.shtml (inset).

3.) Ultra-massive black hole orbits and mergers. Ever heard of a quasar, or of an active galactic nucleus? These billion-solar-mass black holes at the cores of active galaxies had to get that big somehow, and it most likely came from gigantic mergers. There's even one such system, OJ 287, where a 100 million solar mass black hole orbits an 18 billion solar mass black hole, that it's known must emit a tremendous amount of gravitational waves. These have orbital periods on the order of years, and the corresponding incredibly low frequencies to go along with it. Using conventional laser-based detectors is impractical for this, but using an array of pulsars -- and seeing how their timing is affected -- would do the trick. This is something the NANOgrav collaboration, just starting out, will be working to make happen over the coming decades.

Image credit: National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — . [+] Funded BICEP2 Program modifications by me.

4.) Relic gravitational wave radiation from the Big Bang. And why stop with astrophysical sources? These fluctuations from the birth of the Universe would show up in the polarization of the leftover light from the Big Bang, and are being looked for right now! You'll remember that BICEP2 erroneously announced the discovery of these waves back in 2014, only to discover that the foreground dust from our own galaxy accounted for that polarization signal. But these gravitational waves should exist, and they should exist at all frequencies. Depending on what we find -- what the amplitude and spectrum of these waves are -- we can potentially reconstruct exactly what the earliest moments of our Universe, and what the end of inflation, was really like.

Image credit: Minglei Tong, Class.Quant.Grav. 29 (2012) 155006, via . [+] http://arxiv.org/pdf/1206.2109.pdf.

In addition, it isn't just that gravitational waves come from these sources, it's that each one of these sources can potentially teach us a tremendous amount about the Universe. Yes, there's astrophysics involved, but the more sensitively we can measure each of these things, the more we can learn about:

• the types of gravitational waves emitted by each of these classes of sources,
• the physics of the critical, final moments of mergers, supernovae, and other cataclysmic events as viewed through gravitational waves,
• and with the potential, at high enough sensitivities, to look for quantum gravitational effects that may depart from general relativity.

There are proposed future observational missions looking to observe a great many of these at sensitivities outclassing all of the missions enumerated above, like NASA's Big Bang Observer, which would probe all the sources in classes 1, 2 and 4 to better accuracy than any other proposed mission. An array of six interferometers near Earth in orbit, with three at each of the L4 and L5 Lagrange points, could improve our sensitivities over LISA and LIGO by many orders of magnitude, allowing us to measure the leftover gravitational waves from inflation directly.

Image credit: Gregory Harry, MIT, from the LIGO workshop of 2009, LIGO-G0900426, via . [+] https://dcc.ligo.org/public/0002/G0900426/001/G0900426-v1.pdf.

In addition, the possibility of correlating optical astronomy with gravitational wave astronomy can give us multiple views of the same objects, teaching us more about the Universe than we'd ever known. You might have wondered if two merging black holes would emit some sort of electromagnetic radiation, like gamma rays?

Well, even though we've only got one event in gravitational radiation, there was a very suspicious coincidence of a gamma-ray burst detected by NASA's Fermi satellite just 0.4 seconds (!) after the LIGO signal. When we have three or four gravitational wave detectors up and running (VIRGO and CLIO in addition to the two LIGO detectors), we can better constrain the position of these sources, and perhaps find out once and for all what kinds of electromagnetic radiation these black hole mergers produce.

Illustration of a fast gamma-ray burst, previously only thought to occur from the merger of neutron . [+] stars. Image credit: ESO.

We're right on the frontiers of opening up the Universe in a brand new way. The event of September 14th detected by LIGO was only the first of what's sure to be a massive influx of new data that will teach us about the Universe in a form of energy we've never directly probed before. It's time to embrace this new form of astronomy, and to open our window on the Universe as never before. It's an incredible time for any curious minds to be alive.

## Gravitational waves discovery: I was right, says Stephen Hawking

The discovery of gravitational waves could “revolutionise astronomy”, Prof Stephen Hawking said as he congratulated scientists on their groundbreaking work.

The top cosmologist said the breakthrough tallied with predictions he made more than 40 years ago at Cambridge University.

He told the BBC: “Gravitational waves provide a completely new way of looking at the universe. The ability to detect them has the potential to revolutionise astronomy. This discovery is the first detection of a black hole binary system and the first observation of black holes merging.”

Hawking, research director at Cambridge University’s Department of Applied Mathematics and Theoretical Physics, said: “The observed properties of this system is consistent with predictions about black holes that I made in 1970 here in Cambridge. The area of the final black hole is greater than the sum of the areas of the initial black holes as predicted by my black hole area theorem.”

Asked what more could be discovered if scientists scanned for gravitational waves, he said: “Apart from testing general relativity, we could hope to see black holes throughout the history of the universe. We may even see relics of the very early universe during the big bang at the most extreme energies possible.”

Prof John Womersley, chief executive of the Science and Technology Facilities Council, paid tribute to the UK’s contribution to the discovery. He said: “It has taken 100 years and the combined work of many hundreds of the cleverest scientists, engineers and mathematicians on Earth to prove that this key prediction of Albert Einstein is correct and show that gravitational waves exist.

“Of course, Einstein was always the smartest guy in the room. Today’s results also remind us just how important the UK’s contribution to world-leading science is. I’d certainly like to think that some of the smartest people on earth today are living and working in the UK.”

## How does LIGO look for gravitational waves?

Think of it as the most sensitive ruler in the galaxy.

Update: On 12 February 2016 scientists announced they detected gravitational waves using LIGO after its refurbishment in September 2015.

Some cosmic events are so cataclysmic they shake the very fabric of space-time itself. Two black holes smashing into one another at close to the speed of light, two neutron stars colliding – these are the earthquakes of the Universe.

And LIGO listens for their shockwaves – ripples in space-time, which Albert Einstein predicted in 1916, coining the term “gravitational waves”.

But, while we have indirect evidence of their existence, until now nobody has directly caught one, making them the last, great unproved prediction of general relativity, Einstein’s masterpiece theory of gravity.

That’s where LIGO (the Laser Interferometer Gravitational-Wave Observatory) comes in.

This \$600 million instrument is designed by physicists at MIT and Caltech (including Kip Thorne, the astrophysicist advisor on the movie Interstellar) specifically to catch gravitational waves.

You can think of LIGO as the most sensitive ruler ever made. As gravitational waves ripple over us, they cause tiny changes in the dimensions of whatever they pass through.

Everything on Earth, including your own body, expands and contracts in concert with the waves. These expansions and contractions are unbelievably tiny, far smaller than a single atom – which is why they’ve never been detected.

LIGO is designed to detect changes in its length of about one thousandth the diameter of a proton – 10 −18 metres – maybe just enough to catch a gravitational wave sent out by a cosmic cataclysm in a nearby galaxy.

LIGO has two detectors, one at the Hanford nuclear complex in Washington and one in Louisiana, 3,000 kilometres away.

Each detector looks like a giant “L” stamped on the landscape – two four-kilometre arms at right angles to one another.

Inside each arm is a tunnel carrying a laser beam.

The laser bounces up and down each arm of the detector before recombining at a detector.

Any expansion of one of the arms relative to the other will shift a mirror slightly, changing the pattern revealed on the detector.

But with such sensitive equipment, local effects – even traffic rumbling 10 kilometres away – can create vibrations that might be confused with gravitational waves.

That’s why LIGO needs two detectors set up far apart from one another. Any local noise will only affect one of the detectors, while a real ripple in spacetime will show the same signal for both.

Meanwhile, astrophysicists have used general relativity to work out what signals of extreme events would look like.

They believe a collision of two black holes, for example, would make a characteristic “chirp” as the black holes spiral into one another, a “burst” as they merge, then a “ringing” recoil before a final decay. A neutron star collision would sound similar, but at a higher pitch.

LIGO performed its first demonstration run from 2002 to 2010, although it was not then sensitive enough to detect anything.

It was then shut down for a five-year upgrade, while engineers improved the isolation from vibrations, work that should improve its sensitivity 10-fold.

The rechristened Advanced LIGO was switched on again in September 2015, now capable of detecting gravitational waves from as far away as 225 million light years. Its reach encompasses the giant Virgo supercluster of more than a million galaxies.

With so many galaxies to choose from, LIGO is much more likely to catch the rare extreme events such as black hole or neutron star collisions.

The only shortcoming of the current LIGO system is that we only have two detectors and will be unable to triangulate any signal. That means we won’t know where it’s coming from.

But if we had more detectors, we’d be able to localise the signal (as the wave would wash over detectors on one side of the world before reaching the others).

The LIGO team are already planning that sort of system, with the first likely set for India in the early 2020s.

But first we need to see if we have the proof from the current instruments.

Within weeks of the restart, the physics community was abuzz with that LIGO had detected a spectacular event, although the researchers themselves have remained coy while poring over the data.

If LIGO has detected gravitational waves it would be the culmination of a 100-year quest, and mean a certain Nobel prize for its leaders.

More significantly, it would pave the way for a new era in astronomy, in the same way that the telescope did in the 17th century or the radio telescopes did in the 20th.

The Royal Institution of Australia has an Education resource based on this article. You can access it here.

### Cathal O’Connell

Cathal O'Connell is a science writer based in Melbourne.

### Read science facts, not fiction.

There’s never been a more important time to explain the facts, cherish evidence-based knowledge and to showcase the latest scientific, technological and engineering breakthroughs. Cosmos is published by The Royal Institution of Australia, a charity dedicated to connecting people with the world of science. Financial contributions, however big or small, help us provide access to trusted science information at a time when the world needs it most. Please support us by making a donation or purchasing a subscription today.

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## NASA Blueshift

In space, no one can hear you scream. Any sci-fi buff worth their dilithium crystals knows why: sound requires a medium such as air or water in which to propagate and empty space is well, empty. But what if I told you that there are waves that can travel through space itself and that physicists and astronomers are developing machines that will allow us to listen to them for the first time?

They are called gravitational waves and they are a prediction of general relativity, Albert Einstein’s famous theory for understanding gravity which turns 100 years old this month. You may be familiar with explanations of the theory that describe spacetime as a “rubber” sheet that deforms when a massive object is placed on it. Well imagine that you were to press a finger down on such a sheet and release it. You’d get a wave that would travel outwards, somewhat like the waves in a pond when a stone is dropped in it.

In this artist’s conception, a binary black hole produces gravitational waves that travel outwards and carry energy away from the system. Image Credit: NASA

With considerably more effort, it’s possible to crank through Einstein’s equations and show that there is indeed a mathematical solution that describes waves in spacetime and that these gravitational waves travel at the speed of light, come in two polarizations, and can carry energy, momentum, and information. You can also determine what it takes to generate these waves and where in the universe you might expect to find such systems.

It turns out that the best sources typically involve exotic and extreme astrophysical objects such as white dwarfs, neutron stars, and black holes. One of the best sources is a tightly-bound binary system with two such objects orbiting around one another. The gravitational waves generated by such systems carry away energy and cause the objects to fall towards one another, which increases the amplitude of the gravitational waves and the rate at which they sap energy from the system. The result is a runaway collapse of the system that produces a cataclysmic merger of the objects and a burst of gravitational waves.

NASA scientists used supercomputers to solve Einstein’s equations for a merging black hole binary to predict the precise gravitational wave output. Image Credit: NASA

Now if the objects in question happen to be made of matter, for example if they are neutron stars, the merger will produce a burst of electromagnetic energy that can be detected with today’s telescopes. In fact, this is a favored model for one type of gamma-ray burst. If on the other hand, the two objects are black holes, there may be no electromagnetic signal at all. In terms of energy released per unit time, what astronomers call luminosity, the final inspiral and merger of a binary black hole system is the most luminous event in the universe since the big bang and yet it is completely invisible to any instrument that observes in the electromagnetic spectrum.

Physicists and astronomers have recognized the potential for building gravitational wave detectors for decades. They offer an opportunity to both understand the nature of gravity in its most extreme forms as well as an entirely new tool for doing astronomy. In many ways, a gravitational wave observatory is more like a listening device than a telescope. They tend to be sensitive to sources over a wide area of the sky, much like your ears can sense sounds coming from different directions. It’s also possible to observe multiple sources simultaneously, as the human ears (and brain) do when you talk on the phone with the TV on, the refrigerator humming, and a siren blaring in the distance.

The problem is that detecting gravitational waves is hard. What you want to look for is the stretching of spacetime itself. This is done by placing two or more objects in near-perfect free-fall so that their motion will only be affected by gravity. You then closely monitor the distance between these objects and look for distortions caused by the passing gravitational waves. The size of these waves is measured with a dimensionless number called strain, which tells you the total displacement caused by the waves divided by the initial distance between the objects. For a typical astrophysical gravitational wave source, the strain at Earth is about one sextillionth. Don’t know your prefixes that far out? Neither did I. It’s one part in a billion trillion or 10 -21 . This incredibly small number is sometimes wrongly interpreted as evidence that gravitational waves are weak. A better description is that spacetime is extremely stiff and it takes a tremendous amount of energy to make even a small distortion. That rubber sheet in the model should be more like a titanium sheet.

The LIGO observatory in Hanford, WA is one of several kilometer-scale interferometric gravitational wave detectors currently in operation. LIGO is sensitive to “high frequency” gravitational waves with with periods of seconds to milliseconds. Image credit: LIGO Laboratory

You might think that measuring a number as small as 10 -21 would be in the realm of fantasy but it is in fact within the grasp of modern precision measurement techniques. Right now, several large collaborations of scientists are operating kilometer-scale detectors that can measure distance fluctuations on the order of 10 -19 m, more than a thousand times smaller than a proton. The most sensitive of these, known as LIGO, has just begun operating after a major upgrade and will likely make the historic first detection of a gravitational wave in the next few years. In fact there is a scandalous rumor circulating that they may have already detected one but are keeping it secret until they’ve checked and double-checked everything.

The LISA mission is a concept for building a space-based gravitational wave observatory that spans millions of kilometers. Such an instrument would be sensitive to “mid-frequency” gravitational waves with with periods of hours to seconds. Image credit: NASA

At the same time, others like myself are developing concepts for million-kilometer scale detectors in space. In fact, I’m part of an international effort led by the European Space Agency that will launch a satellite called LISA Pathfinder in December of this year. The purpose of this satellite is to demonstrate some of the novel technologies and measurement strategies that will be needed to realize a space-based gravitational wave observatory.

The LISA Pathfinder mission is designed to demonstrate several of the key technologies that are needed to implement the LISA concept. Led by the European Space Agency with contributions from NASA and several European institutions, LISA Pathfinder will launch from French Guiana on Dec. 2nd, 2015. This image shows the spacecraft being fueled prior to being mated with the launch vehicle. Image credit: ESA-CNES-Arianespace / Optique Vidéo du CSG – P. Baudon

In the coming series of blog entries, I plan to introduce you to the LISA Pathfinder mission as well as talk more about gravitational waves and the technologies being developed to detect them. If you can’t wait for those entries, I encourage you to watch this Google Hangout hosted by the American Astronomical and Astronautical Societies on November 20 th , featuring me, Dr. Joey Shapiro-Key of the University of Texas Rio Grande Valley, and Dr. Shane Larson of the Adler Planetarium.

## A new door opens

Until now, astronomers have only been able to view the cosmos either directly using space telescopes, or by analysing objects in deep space using instruments that measure ultraviolet light.

While you, me and everyone you know creates ripples in space-time simply by, say, dancing around with another person, this rippling is practically insignificant and undetectable.

However, with major cosmic events &ndash like the collision of two black holes, or the birth of a supernova &ndash huge ripples expand into the universe over vast distances that, eventually, strike our planet, but very faintly.

LIGO, the L-shaped telescope, consists of two 4km, highly-sensitive laser beams that can detect even the faintest of these changes.

LIGO&rsquos Sensing and Control (ISC) system. Image via Caltech/MIT/LIGO Lab

When a gravitational wave hits Earth, LIGO and advanced monitoring systems can detect the smallest stretching or squeezing of space-time between two points of each laser.

And so it was in September last year that such a warping of space-time was detected at a frequency of 100Hz.

This discovery allows us to throw open the door and view the first one trillionth of a second of the Big Bang.

&ldquoIn short, this is a really, really difficult experiment.&rdquo Those are the words of Dr David Reitze, the man who announced to the world last September: &ldquoLadies and gentlemen, we have detected gravitational waves. We did it.&rdquo

## Definitioner

In a general sense: any travelling pattern, whether or not it involves matter being transported as well. Simple examples are water-waves - wave crests and troughs travelling over a water surface, and a Mexican wave in a football stadium, with fans alternately standing up and sitting down - the pattern moves throught the stadium, not the fans themselves.

An especially simple form for a wave is a sinus wave, a regular pattern of wave crests and troughs.

In a general sense: any travelling pattern, whether or not it involves matter being transported as well. Simple examples are water-waves - wave crests and troughs travelling over a water surface, and a Mexican wave in a football stadium, with fans alternately standing up and sitting down - the pattern moves throught the stadium, not the fans themselves.

An especially simple form for a wave is a sinus wave, a regular pattern of wave crests and troughs.

In a general sense: Collective name for all phenomena in which energy is transported through space in the form of waves or particles. In a more restricted sense, the word is often used synonymously with electromagnetic radiation.

Measure for the rapidity of an oscillation, defined as the inverse of the period of oscillation: A process that, in oscillating, repeats itself after 0.1 seconds has the frequency 1/(0.1 seconds)= 10 Hz. (The unit Hertz, abbreviated as Hz, is defined as 1 Hz = 1/second.)

For a simple wave, the frequency is given by the number of maxima going by a stationary observer in a second. Ten maxima going by per second correspond to a frequency of 10 Hz.

free In the context of relativity theory, a particle (object, observer. ) that is not acted upon by any force except gravity is said to be free or, a bit more specific, to be in free fall. Free test particles play an important role in understanding the structure of general relativity. field A field describes how a physical quantity is distributed in space and time. For instance, the area where electric forces act on a test particle is subject to an electric field. Or the gravitational forces which act on the mass of a test body define a gravitational field. In general a field contains energy, occupies space and can change over time. energy Physical quantity with the special property that, in physical processes, energy is neither destroyed nor created, simply transformed from one form of energy to another. Some of the different forms of energy that are defined separately are kinetic energy, thermal energy and the energy carried by electromagnetic radiation. Processes that transform one form of energy into another take place in all machines we use in everyday life, from the engine of a subway train (electrical energy into kinetic energy of the train) to an electric blanket (electrical energy into thermal energy). One important consequence of special relativity is that energy and mass are completely equivalent - two different ways to define what is, on closer inspection, one and the same physical quantity. See the keyword equivalence between mass and energy. [email protected] Project that uses private computers to search for gravitational waves in the data of current gravitational wave detectors. More information can be found on the project webpages. > [email protected] project page Earth Our very own planet in the solar system - the third planet from the sun. The earth has a mass of about 6 trillion trillion (in exponential notation, 6·10 24 ) kilograms. Doppler shift (Doppler effect) Effect named after the Austrian scientist Christian Doppler concerning the emission of waves by moving sources. Consider a wave-source (for instance, a device that sends out sound-waves or light-waves). Also consider two observers A and B, with observer A moving relative to the source, while observer B is at rest relative to it. When a source that moves relative to an observer emits a wave, the frequency measured by this observer is different from what a measuring instrument would record that is at rest relative to the source: If source and observer approach each other, the observer measures a higher frequency, if they move away from each other, a lower frequency. In everyday life, the Doppler effect is readily apparent when we listen to sound waves from moving sources. If a police car or fire truck with blaring horns first races towards us, then passes us and races away, the characteristic horn sounds change dramatically in pitch, the moment the car passes us. This is because, at first, the car is moving towards us, and there is a Doppler shift towards higher pitch compared with a listener in the car. From the moment the car passes us, it becomes a source that moves away from us, with all sounds being shifted to lower pitch. In the context of relativity, the most important Doppler effect is that for light waves. In this context, a shift towards higher frequencies is called blueshift, one to lower frequencies redshift. current

Matter in coordinated, flowing motion - think of water flowing in a pipe. An important example is the electric current associated with moving electric charges. Electric currents are the sources of magnetic fields.

continuous Space as we are used to thinking about it is a continuum or, equivalently, continuous space: Between every two points, there always exists an infinity of other points, and every volume can be divided into smaller and smaller parts without ever reaching a limit. binary (binary star) A system consisting of two stars in orbit around each other. From a relativistic point of view, there are binaries that are of special interest, namely those in which at least one of the partners is a neutron star or a black hole. Potentially, such systems are effective sources of gravitational waves. Albert Einstein Institute One of the research institutes of the Max Planck Society an international centre for research on Einstein's theory of gravity - from the mathematical fundamental, astrophysics and gravitational waves to quantum gravity. Founded in 1995, the institute is situated in Golm near Potsdam in Germany. In 2002, the experimental branch of the institute was opened in Hannover. It is dedicated to research with the gravitational wave detector GEO 600 . AEI webpages Website of AEI-Hannover acceleration

Every change of velocity with time is an acceleration.

This definition is slightly different from our everyday usage of the word. Ordinarily, we talk of an object accelerating when it becomes faster and faster. The physics definition covers two more situations. An object that decelerates, becomes slower, thus changes its velocity and, in the physics sense, undergoes a (negative) acceleration. Also, in physics, velocity is not the same as speed. A constant velocity implies not only constant speed, but also a constant direction of movement. Once the direction changes, so does the velocity - the change in velocity is associated with the change in the direction of movement. Thus, in the physics sense, even a car going around a curve of the road at constant speed undergoes acceleration.

## LISA: Detecting Exoplanets Using Gravitational Waves

Humanity is experiencing a revolution in astronomy. Until recently, we've depended on the electromagnetic spectrum (i.e. light) to make discoveries from our solar system's backyard to the furthest-most reaches of the cosmos by using telescopes. Now, with the first historic detection of gravitational waves on Sept. 14, 2015, a whole new universe awaits us, one in which we can analyze the spacetime ripples washing over us from black hole collisions and, possibly, alien worlds as they orbit their distant stars.

In a study published July 8, 2019, in Nature Astronomy, a group of researchers have explored the latter possibility to reveal extrasolar planets, or exoplanets, that would otherwise remain invisible to traditional astronomical techniques.

"We propose a method which uses gravitational waves to find exoplanets that orbit binary white dwarf stars," Nicola Tamanini, of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Potsdam, Germany, said in a statement.

So far, the gravitational waves generated by massive collisions in the deep cosmos have been detected by two observatories, the U.S.-based Laser Interferometer Gravitational-wave Observatory (LIGO) that uses two detectors in Washington and Louisiana, and the Virgo interferometer near Pisa, Italy. Both projects use L-shaped buildings that house advanced laser interferometers that can detect the minute fluctuations in distance as gravitational waves wash through our planet. LIGO was the first to detect the gravitational waves that were theorized by Einstein more than a century ago and now both LIGO and Virgo work in concert to make regular detections of black hole and neutron star collisions.

In 2017, another historic milestone was reached when both the gravitational waves and gamma-ray radiation were detected at the same time when two neutron stars collided in a galaxy 130 light-years away. This event launched a new era of "multimessenger astronomy" that enabled astronomers to pinpoint the location of the event, understand the physical mechanisms behind short gamma-ray bursts, confirm that colliding neutron stars are the culprit, and provide an intimate look at the nuclear processes that manufacture heavy elements (such as gold and platinum) in the cosmos.

### Launching Detectors Into Space

With these incredible advancements facilitated by our new ability to detect gravitational waves, what does the future hold? Well, why not launch a gravitational wave observatory into space! As discussed in the Nature Astronomy study, the planned Laser Interferometer Space Antenna (LISA) will do just that and its extreme sensitivity will give us a brand new look at cosmic targets that are currently hiding in the dark. One of these targets will be binary white dwarf star systems that may be accompanied by orbiting exoplanets (with masses of 50 Earth-masses and greater) that cannot be seen using current exoplanet-detection techniques. Theoretically, LISA will be sensitive to gravitational waves coming from white dwarf binaries throughout our galaxy.

"LISA will measure gravitational waves from thousands of white dwarf binaries," said Tamanini. "When a planet is orbiting such a pair of white dwarfs, the observed gravitational-wave pattern will look different compared to the one of a binary without a planet. This characteristic change in the gravitational waveforms will enable us to discover exoplanets."

White dwarfs are the stellar corpses of sun-like stars that have run out of fuel and died long ago. Our sun will run out of fuel in 5 billion years or so, which will cause it to swell up into a bloated red giant. After the red giant phase, the star will shed its layers of hot plasma, creating a so-called planetary nebula, leaving a tiny spinning object approximately the size of Earth in its wake. This dense object will then be crushed under its own immense gravity, creating a blob of degenerate matter.

White dwarfs are well studied and represent the final, dead phase of our sun's life, but they could also be invaluable objects in our pursuit to find new worlds far beyond the solar system.

If, for example, two white dwarfs orbit one another as a binary system, the gravitational perturbations they create will act like a spinning child's toy in a swimming pool — ripples in spacetime will propagate in all directions, carrying energy away from the orbiting stars at the speed of light. Current gravitational wave detectors can only measure the most powerful cosmic clashes, but with LISA, these more subtle events that produce a weaker gravitational wave signal will be within reach.

### Hidden Alien Worlds

Currently, astronomers use two primary methods to detect exoplanets orbiting other stars: the "radial velocity method," which uses very sensitive spectrometers attached to telescopes that can detect the Doppler shift caused by an orbiting exoplanet, and the "transit method," which NASA's Kepler space telescope (and others) use to detect the very slight dip in star brightness as a world orbits in front.

Although over 4,000 exoplanets have been discovered primarily by using these two methods, some exoplanets remain hidden and, in the case of binary white dwarfs, we know little about whether they can host exoplanets. But, if LISA can measure the space-time ripples emanating from these systems, it might also detect the slight tugging of exoplanets as they orbit, in a similar manner that the radial velocity method measures the Doppler shift of electromagnetic waves, only using gravitational waves instead.

LISA is a project led by the European Space Agency and is currently scheduled to launch in 2034. Consisting of three spacecraft flying in formation, they will beam ultra-precise lasers at one another to create a vast equilateral triangular laser interferometer with each spacecraft separated by 1.5 million miles (2.5 million kilometers). LISA will therefore be an interferometer a million times bigger than anything we currently have, or ever will have, on Earth.

"LISA is going to target an exoplanet population yet completely unprobed," added Tamanini. "From a theoretical perspective, nothing prevents the presence of exoplanets around compact binary white dwarfs."

If these binary white dwarf star systems are found to also host exoplanets, they will help us better understand how star systems like our own evolve and whether planets can survive after their binary star systems have run out of fuel and died. The researchers also point out that they could also reveal whether second-generation exoplanets (i.e. planets that form after the red giant phase) exist.

Beyond the gravitational wave detections of exoplanets, the possibilities are endless. If there's one thing that the current "new age" of gravitational wave astronomy has taught us, future space-based observatories like LISA could reveal phenomena that occur in the dark that we never thought we'd ever witness.

About 1,600 light-years away from Earth, in a binary star system known as J0806, two dense white dwarf stars orbit each other once every 321 seconds. Based on data from the Chandra X-Ray Observatory, astronomers believe that the stars' already super short orbital period is steadily becoming shorter, which will eventually cause the two stars to merge.