Detecting gravitational waves could be as important as first use of telescope

Mackeyser wrote:

Black hole colliding will sound like an extended Yanni and Kenny G jam. With Japanese drums like Drum Tao playing in the background…

But…we already knew that.

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Everything You Need To Know About Gravitational Waves

Answers by Peter Graham, Stanford Physics Professor and Gravitational Wave Researcher, on Quora. http://theramshuddle.com/topic/detection-of-gravitational-waves-marks-new-era-in-astronomy/

Q: How important is the discovery of Gravitational Waves?

A: Extremely important! This is surely one of the most important discoveries in physics in the past several decades. It is not even so much the confirmation of the gravitational waves themselves, we were very confident that they existed, it is that we now have the ability to observe the universe using this entirely new spectrum. Everything we currently know about astrophysics and cosmology arose from observations of electromagnetic waves. Gravitational waves give us a new and entirely different source of information. We will learn a great deal about the universe that we could never have learned any other way.

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Q: How did LIGO validate the signal they detected?

A: This is a very important question, and also very complicated. In fact, for the real answer I recommend reading the papers that LIGO put out. Here I can just give some of the basic ideas of how it is done. When looking for such a small signal, it is absolutely crucial not to fool yourself because there is always a lot of noise in such a precise measurement. The LIGO collaboration worked very hard for a long time to make sure that they could claim a detection with confidence when it occurred. There are many possible problems with making such a detection and so LIGO applies many different strategies for dealing with these problems. A few of these are as follows. First they built two detectors, one in Washington and one in Louisiana, and required that both detectors see the same signal at (almost) exactly the same time. This greatly cuts down on the chance that the signal is just coming from random noise. There are very few things besides a gravitational wave that are going to hit both detectors at basically the same time with the same signal. They also work very hard to make sure that each detector has as little noise as possible, and that the level of the noise is well understood. For example, the earth vibrates all the time at a small level and this is a noise source for LIGO so they use impressive mechanical systems, a bit like shock absorbers, to keep their mirrors from feeling these vibrations as much as possible. Even the best laser is not perfect, and will have some noise in it. So they build two baselines in each detector to allow them to subtract off the laser noise that will be comment to both baselines. This is just a small fraction of the number of different noise sources they have considered, calculated carefully, and worked hard to understand and reduce as much as possible. They also use blind analysis techniques to help them make sure that they are not fooling themselves and are estimating their uncertainties correctly. And they spend a long time understanding their detector and the noise in it after they turn it on.

The real answer is much longer than this, and is also crucial for truly being certain that gravitational waves were detected and not just noise. The LIGO collaboration has done a very impressive job on a very difficult measurement. It is their impressive work that has most physicists convinced that we have in fact detected gravitational waves. It is truly an amazing experiment.

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Q: How do I explain to students in the fourth grade what a gravitational wave is, and why its discovery is important?

A: Good question! A gravitational wave is a ripple in space-time. By this we mean that when a gravitational wave passes by us, all the distances appear to oscillate. If it passes between me and you, the distance between us would grow, then shrink again, and so on, oscillating until the wave had passed. We never see this because the gravitational waves that reach earth are so tiny. But if we did have a strong gravitational wave pass through us we would really see this oscillating distance, and it would look really weird!

Their discovery is so important partly because it confirms a key prediction of Einstein’s theory of gravity, called general relativity, which was formulated 100 years ago. But perhaps the main reason the discovery is important is that it opens a new window onto the stars. Gravitational wave detectors are a new kind of telescope that will allow us to learn a great deal more about the universe than we ever could otherwise.

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Q: What impact does the discovery of gravitational waves have on mankind’s future?

A:This is the opening of a new era in our ability to learn about the universe. People have always wondered what else is out there beyond our planet. We have learned a great deal from observing electromagnetic waves. Gravitational waves have the potential to reveal even more. Possibly, in the future, this could even include looking back to some of the earliest moments after the birth of the universe. I believe our understanding of the universe is one of humanity’s greatest accomplishments.

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How much can we learn about black holes from gravitational wave astronomy?

We will likely learn a great deal about black holes from observing gravitational waves. Of course it depends on what we see, how many black holes we can detect and with what significance. But it seems likely that we will learn much more about the number and masses of black holes that are formed, which in turn will tell us more about the formation of black holes and stars in general. We will learn important properties such as their spin, or angular momentum. We could learn about merger rates which may eventually teach us about how the supermassive black holes in centers of galaxies formed. We will also test the assumption that such astrophysical black holes follow the known laws of classical general relativity. By seeing black hole mergers we get a test of whether the physics near the event horizon (so-called strong field gravity) is indeed what we expect from our known laws. It is otherwise very difficult to test gravity in this regime and to know that black holes do indeed follow our predictions. Given how many black holes we expect to be able to observe, it also seems likely that we will learn much that is not yet anticipated about black holes. It is a common pattern that once we have the ability to observe a new system we learn many things that no one expected.

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Peter Graham: How do LIGO scientists know that gravitational wave signal came from two black holes?

Many people had previously calculated what the gravitational wave signals from a large variety of sources would look like. LIGO compares what they see against these “templates” to see which one it matches. Specifically, they look at things like the strength and frequency of the wave and how it changed with time during the event. This exact shape of the signal, or waveform, is then matched to expectations for expected sources such as neutron star binaries or black hole binaries. The expected signals are distinct enough that LIGO can conclude with confidence that they are seeing a black hole binary, and can even get a good measurement of the masses and other properties of the black holes all just from the waveform of the signal.

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Gravitational Waves: 6 Cosmic Questions They Can Tackle

The discovery of ripples in spacetime will vindicate Einstein—but it can also do so much more

By Davide Castelvecchi

http://www.scientificamerican.com/article/gravitational-waves-6-cosmic-questions-they-can-tackle/

When LIGO fought to get US government funding in the early 1990s, its major opponents at congressional hearings were astronomers. “The general view was that LIGO didn’t have much to do with astronomy,” says Clifford Will, a general-relativity theorist at the University of Florida in Gainesville and an early LIGO supporter. But things have changed now, he says.
Welcome to the field of gravitational-wave astronomy: we take a look at the questions and phenomena that it can explore.

DO BLACK HOLES ACTUALLY EXIST?

The signal that LIGO is expected to announce on Thursday is rumoured to have been produced by two merging black holes. Such events are the most energetic known; the power of the gravitational waves that they emit can briefly rival that of all the stars in the observable Universe combined.Black-hole mergers are also among the cleanest gravitational-wave signals to interpret.

A black-hole merger occurs when two black holes start to spiral towards each other, radiating energy as gravitational waves. These waves should have a characteristic sound called a chirp, which can be used to measure the masses of the two objects. Next, the black holes actually fuse. “It’s as if you get two soap bubbles so close that they form one bubble. Initially, the bigger bubble is deformed,” says Thibault Damour, a gravity theorist at the Institute of Advanced Scientific Studies near Paris. The resulting single black hole will settle into a perfectly spherical shape, but first it is predicted to radiate gravitational waves in a pattern called a ringdown.

One of the most important scientific consequences of detecting a black-hole merger would be confirmation that black holes really do exist—at least as the perfectly round objects made of pure, empty, warped space-time that are predicted by general relativity. Another would be that mergers proceed as predicted. Astronomers already have plenty of circumstantial evidence for these phenomena, but so far that has come from observations of the stars and super-heated gas that orbit black holes, not of black holes themselves.
“The scientific community, including myself, has become very blasé about black holes. We have taken them for granted,” says Frans Pretorius, a specialist in general-relativity simulations at Princeton University in New Jersey. “But if you think of what an astonishing prediction it is, we really need astonishing evidence.”

DO GRAVITATIONAL WAVES TRAVEL AT THE SPEED OF LIGHT?

When scientists start to compare observations from LIGO with those from other types of telescope, one of the first things that they will check is whether the signals arrive at the same time. Physicists hypothesize that gravity is transmitted by particles called gravitons, the gravitational analogue of photons. If, like photons, these particles have no mass, then gravitational waves would travel at the speed of light, matching the prediction of the speed of gravitational waves in classical general relativity. (Their speed can be affected by the accelerating expansion of the Universe, but that should manifest only over distances much greater than LIGO can probe).

But it is possible that gravitons have a slight mass, which would mean that gravitational waves would travel at less than the speed of light. So if, say, LIGO and Virgo were to detect gravitational waves from a cosmic event, and find that the waves took slightly longer to arrive at Earth than the associated burst of γ-rays detected by a more conventional telescope, that could have momentous consequences for fundamental physics.

IS SPACE-TIME MADE OF COSMIC STRINGS?

An even weirder discovery would occur if bursts of gravitational waves were detected coming from ‘cosmic strings’. These hypothetical defects in the curvature of space-time, which may or may not be related to string theory, would be infinitesimally thin but would stretch across cosmic distances. Researchers have predicted that cosmic strings, if they exist, might occasionally develop kinks; if a string snapped, it would suddenly release a burst of gravitational waves, which detectors such as LIGO and Virgo could measure.

ARE NEUTRON STARS RUGGED?

Neutron stars are the remnants of bigger stars that collapsed under their own weight, becoming so dense that they pushed their constituent electrons and protons to fuse into neutrons. Their extreme physics is poorly understood, but gravitational waves could provide unique insights. For example, the intense gravity at their surface tends to make neutron stars almost perfectly spherical. But some researchers have theorized that there could still be ‘mountains’—at most a few millimetres high—that make these dense objects, themselves about 10 kilometres in diameter, slightly asymmetrical. Neutron stars usually spin very rapidly, so the asymmetric distribution of mass would deform space-time and produce a continuous gravitational-wave signal in the shape of a sine wave, which would radiate energy and slow down the star’s spin.

Pairs of neutron stars that orbit each other would also produce a continuous signal. Just like black holes, the stars would spiral into each other and eventually merge, sometimes producing an audible chirp. But their final instants would differ dramatically from those of black holes. “You have a zoo of possibilities, depending on masses and how much pressure neutron-dense matter can exert,” says Pretorius. For example, the resulting merged star could be a huge neutron star, or it could immediately collapse and turn into a black hole.

WHAT MAKES STARS EXPLODE?

Black holes and neutron stars form when massive stars stop shining and collapse in on themselves. Astrophysicists think that this process is what powers a common type of supernova explosion, known as Type II. Simulations of such supernovae have not yet clearly explained what ignites them, but listening to the gravitational-wave bursts that real supernova are expected to produce could help to provide an answer. Depending on what the bursts’ waveforms look like, how loud the bursts are, how frequent they are and how they correlate with the supernovae as seen with electromagnetic telescopes, the data could help to validate or discard various, existing models.

HOW FAST IS THE UNIVERSE EXPANDING?

The expansion of the Universe means that distant objects that are receding from our Galaxy look redder than they really are, because the light that they emit stretches as it travels. Cosmologists estimate the rate of the Universe’s expansion by comparing this redshift of galaxies with how far the galaxies are from us. But that distance is usually gauged from the brightness of ‘Type Ia’ supernovae—a technique that leaves large uncertainties.

If several gravitational-wave detectors across the world detect signals from the same neutron-star merger, together they will be able to provide an estimate of the absolute loudness of the signal, which will reveal how far away the merger occurred. They will also be able to estimate the direction it came from; astronomers could then deduce which galaxy hosted the merger. Comparing that galaxy’s redshift with the distance of the merger as measured by the loudness of the gravitational waves could provide an independent estimate of the rate of cosmic expansion, possibly more accurate than current methods.