Of Particular Significance

Advance Thoughts on LIGO

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 02/11/2016

Scarcely a hundred years after Einstein revealed the equations for his theory of gravity (“General Relativity”) on November 25th, 1915, the world today awaits an announcement from the LIGO experiment, where the G in LIGO stands for Gravity. (The full acronym stands for “Laser Interferometer Gravitational Wave Observatory.”) As you’ve surely heard, the widely reported rumors are that at some point in the last few months, LIGO, recently upgraded to its “Advanced” version, finally observed gravitational waves — ripples in the fabric of space (more accurately, of space-time). These waves, which can make the length of LIGO shorter and longer by an incredibly tiny amount, seem to have come from the violent merger of two black holes, each with a mass [rest-mass!] dozens of times larger than the Sun. Their coalescence occurred long long ago (billions of years) in a galaxy far far away (a good fraction of the distance across the visible part of the universe), but the ripples from the event arrived at Earth just weeks ago. For a brief moment, it is rumored, they shook LIGO hard enough to be convincingly observed.

For today’s purposes, let me assume the rumors are true, and let me assume also that the result to be announced is actually correct. We’ll learn today whether the first assumption is right, but the second assumption may not be certain for some months (remember OPERA’s [NOT] faster-than-light neutrinos  and BICEP2’s [PROBABLY NOT] gravitational waves from inflation). We must always keep in mind that any extraordinary scientific result has to be scrutinized and confirmed by experts before scientists will believe it! Discovery is difficult, and a large fraction of such claims — large — fail the test of time.

What the Big News Isn’t

There will be so much press and so many blog articles about this subject that I’m just going to point out a few things that I suspect most articles will miss, especially those in the press.

Most importantly, if LIGO has indeed directly discovered gravitational waves, that’s exciting of course. But it’s by no means the most important story here.

That’s because gravitational waves were already observed indirectly, quite some time ago, in a system of two neutron stars orbiting each other. This pair of neutron stars, discovered by Joe Taylor and his graduate student Russell Hulse, is interesting because one of the neutron stars is a pulsar, an object whose rotation and strong magnetic field combine to make it a natural lighthouse, or more accurately a radiohouse, sending out pulses of radio waves that can be detected at great distances. The time between pulses shifts very slightly as the pulsar moves toward and away from Earth, so the pulsar’s motion around its companion can be carefully monitored. Its orbital period has slowly changed over the decades, and the changes are perfectly consistent with what one would expect if the system were losing energy, emitting it in the form of unseen gravitational waves at just the rate predicted by Einstein’s theory (as shown in this graph.) For their discovery, Hulse and Taylor received the 1993 Nobel Prize. By now, there are other examples of similar pairs of neutron stars, also showing the same type of energy loss in detailed accord with Einstein’s equations.

A bit more subtle (so you can skip this paragraph if you want), but also more general, is that some kind of gravitational waves are inevitable… inevitable, after you accept Einstein’s earlier (1905) equations of special relativity, in which he suggested that the speed of light is a sort of universal speed limit on everything, imposed by the structure of space-time.  Sound waves, for instance, exist because the speed of sound is finite; if it were infinite, a vibrating guitar string would make the whole atmosphere wiggle back and forth in sync with the guitar string.  Similarly, since effects of gravity must travel at a finite speed, the gravitational effects of orbiting objects must create waves. The only question is the specific properties those waves might have.

No one, therefore, should be surprised that gravitational waves exist, or that they travel at the universal speed limit, just like electromagnetic waves (including visible light, radio waves, etc.) No one should even be surprised that the waves LIGO is (perhaps) detecting have properties predicted by Einstein’s specific equations for gravity; if they were different in a dramatic way, the Hulse-Taylor neutron stars would have behaved differently than expected.

Furthermore, no one should be surprised if waves from a black hole merger have been observed by the Advanced LIGO experiment. This experiment was designed from the beginning, decades ago, so that it could hardly fail to discover gravitational waves from the coalescence of two black holes, two neutron stars, or one of each. We know these mergers happen, and the experts were very confident that Advanced LIGO could find them. The really serious questions were: (a) would Advanced LIGO work as advertised? (b) if it worked, how soon would it make its first discovery? and (c) would the discovery agree in detail with expectations from Einstein’s equations?

Big News In Scientific Technology

So the first big story is that Advanced LIGO WORKS! This experiment represents one of the greatest technological achievements in human history. Congratulations are due to the designers, builders, and operators of this experiment — and to the National Science Foundation of the United States, which is LIGO’s largest funding source. U.S. taxpayers, who on average each contributed a few cents per year over the past two-plus decades, can be proud. And because of the new engineering and technology that were required to make Advanced LIGO functional, I suspect that, over the long run, taxpayers will get a positive financial return on their investment. That’s in addition of course to a vast scientific return.

Advanced LIGO is not even in its final form; further improvements are in the works. Currently, Advanced LIGO consists of two detectors located 2000 miles (3000 kilometers) apart. Each detector consists of two “arms” a few miles (kilometers) long, oriented at right angles, and the lengths of the arms are continuously compared.  This is done using exceptionally stable lasers reflecting off exceptionally perfect mirrors, and requiring use of sophisticated tricks for mitigating all sorts of normal vibrations and even effects of quantum “jitter” from the Heisenberg uncertainty principle. With these tools, Advanced LIGO can detect when passing gravitational waves change the lengths of LIGO’s arms by … incredibly … less than one part in a billion trillion (1,000,000,000,000,000,000,000). That’s an astoundingly tiny distance: a thousand times smaller than the radius of a proton. (A proton itself is a hundred thousand times smaller, in radius, than an atom. Indeed, LIGO is measuring a distance as small as can be probed by the Large Hadron Collider — albeit with a very very tiny energy, in contrast to the collider.) By any measure, the gravitational experimenters have done something absolutely extraordinary.

Big News In Gravity

The second big story: from the gravitational waves that LIGO has perhaps seen, we would learn that the merger of two black holes occurs, to a large extent, as Einstein’s theory predicts. The success of this prediction for what the pattern of gravitational waves should be is a far more powerful test of Einstein’s equations than the mere existence of the gravitational waves!

Imagine, if you can… Two city-sized black holes, each with a mass [rest-mass!] tens of times greater than the Sun, and separated by a few tens of miles (tens of kilometers), orbit each other. They circle faster and faster, as often, in their last few seconds, as 100 times per second. They move at a speed that approaches the universal speed limit. This extreme motion creates an ever larger and increasingly rapid vibration in space-time, generating large space-time waves that rush outward into space. Finally the two black holes spiral toward each other, meet, and join together to make a single black hole, larger than the first two and spinning at an incredible rate.  It takes a short moment to settle down to its final form, emitting still more gravitational waves.

During this whole process, the total amount of energy emitted in the vibrations of space-time is a few times larger than you’d get if you could take the entire Sun and (magically) extract all of the energy stored in its rest-mass (E=mc²). This is an immense amount of energy, significantly more than emitted in a typical supernova. Indeed, LIGO’s black hole merger may perhaps be the most titanic event ever detected by humans!

This violent dance of darkness involves very strong and complicated warping of space and time. In fact, it wasn’t until 2005 or so that the full calculation of the process, including the actual moment of coalescence, was possible, using highly advanced mathematical techniques and powerful supercomputers!

By contrast, the resulting ripples we get to observe, billions of years later, are much more tame. Traveling far across the cosmos, they have spread out and weakened. Today they create extremely small and rather simple wiggles in space and time. You can learn how to calculate their properties in an advanced university textbook on Einstein’s gravity equations. Not for the faint of heart, but certainly no supercomputers required.

So gravitational waves are the (relatively) easy part. It’s the prediction of the merger’s properties that was the really big challenge, and its success would represent a remarkable achievement by gravitational theorists. And it would provide powerful new tests of whether Einstein’s equations are in any way incomplete in their description of gravity, black holes, space and time.

Big News in Astronomy

The third big story: If today’s rumor is indeed of a real discovery, we are witnessing the birth of an entirely new field of science: gravitational-wave astronomy. This type of astronomy is complementary to the many other methods we have of “looking” at the universe. What’s great about gravitational wave astronomy is that although dramatic events can occur in the universe without leaving a signal visible to the eye, and even without creating any electromagnetic waves at all, nothing violent can happen in the universe without making waves in space-time. Every object creates gravity, through the curvature of space-time, and every object feels gravity too. You can try to hide in the shadows, but there’s no hiding from gravity.

Advanced LIGO may have been rather lucky to observe a two-black-hole merger so early in its life. But we can be optimistic that the early discovery means that black hole mergers will be observed as often as several times a year even with the current version of Advanced LIGO, which will be further improved over the next few years. This in turn would imply that gravitational wave astronomy will soon be a very rich subject, with lots and lots of interesting data to come, even within 2016. We will look back on today as just the beginning.

Although the rumored discovery is of something expected — experts were pretty certain that mergers of black holes of this size happen on a fairly regular basis — gravitational wave astronomy might soon show us something completely unanticipated. Perhaps it will teach us surprising facts about the numbers or properties of black holes, neutron stars, or other massive objects. Perhaps it will help us solve some existing mysteries, such as those of gamma-ray bursts. Or perhaps it will reveal currently unsuspected cataclysmic events that may have occurred somewhere in our universe’s past.

Prizes On Order?

So it’s really not the gravitational waves themselves that we should celebrate, although I suspect that’s what the press will focus on. Scientists already knew that these waves exist, just as they were aware of the existence of atoms, neutrinos, and top quarks long before these objects were directly observed. The historic aspects of today’s announcement would be in the successful operation of Advanced LIGO, in its new way of “seeing” the universe that allows us to observe two black holes becoming one, and in the ability of Einstein’s gravitational equations to predict the complexities of such an astronomical convulsion.

Of course all of this is under the assumptions that the rumors are true, and also that LIGO’s results are confirmed by further observations. Let’s hope that any claims of discovery survive the careful and proper scrutiny to which they will now be subjected. If so, then prizes of the highest level are clearly in store, and will be doled out to quite a few people, experimenters for designing and building LIGO and theorists for predicting what black-hole mergers would look like. As always, though, the only prize that really matters is given by Nature… and the many scientists and engineers who have contributed to Advanced LIGO may have already won.

Enjoy the press conference this morning. I, ironically, will be in the most inaccessible of places: over the Atlantic Ocean.  I was invited to speak at a workshop on Large Hadron Collider physics this week, and I’ll just be flying home. I suppose I can wait 12 hours to find out the news… it’s been 44 years since LIGO was proposed…

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166 Responses

  1. A new physical theory with STW ( Static Time Waves)
    In the beginning was infinite geometric space. This space became filled with Static Time at absolute rest and absolute cold. Static time is strictly quantitative, and does not differentiate between past, present and future. Being quantifiable and measurable, static time is a scientific concept. Static time of 0.0033 microseconds per meter will be measured regardless of the direction chosen.
    Stars move through static time, which does not disrupt their motion. Static time exists but is imperceptible. Static time is the deepest secret of the universe.
    Static time fills all infinite space, eliminating the possibility of a vacuum. Static Time Waves (STW) travel through static time. The speed of STW is 300,000 km/second.
    Aetzbar in amazon
    The Newtonian universe is based on matter and force.
    The Einsteinian universe is based on matter and energy.
    The Aetzbarian universe is based on static time and energy.
    There is no gravity, and there is no gravity waves.
    There is Static Time , and there is Static Time Waves. (STW)
    There is a particles of Static Time.
    Static Time is real and measured.
    Everyone knows the Dynamic Time.
    It is time to recognize the Static Time.

  2. Please, Explain me how to know where comes that gravitational waves that they are about 360 angles that may have crossed such laser line, that still no any telescope or radioT or any directional detector to knows where it came from. Please explain me how do they knew about that black holes without such detectors. thanks

    1. LIGO consists of two detectors, one in Washington and one in Louisiana. The signal will reach both detectors at different times. You can get an idea of where the signal came from int he same way your brain uses your two ears to detect the direction a sound came from. This doesn’t let you pin down exactly where the signal came from but gives you a pretty good idea where to start.

  3. One last time … and then, like giuliohome, I am leaving this discussion. Gravitational waves are not the same as static gravitational gradients. In order for gravitational waves to produce the phenomena that you are discussing, their wavelength would have to be larger than our entire solar system (which they of course are not).

  4. Reply to Vincent:

    There is one salient point you have made with which I am in agreement, though not completely.

    There seems to be a common perception that Einstein worked in a vacuum, that he was so original in his insights that he needn’t have had much regard for the work of his predecessors. Nothing could be further from the truth. Einstein read and digested all theoretical material available at the time. He mastered Newtonian physics. He stood on the shoulders of giants, and he never pretended otherwise.

    Einstein also made mistakes. He referred to the cosmological constant as the biggest blunder of his career, and he was patently opposed to QM despite the fact that his work on the photoelectric effect paved the way for it.

    Still, we should not diminish his accomplishments. GR was a revolutionary breakthrough in physics, and without Einstein we cannot say where we would be today.

    It was Newton (undeniably a “serious scientific thinker”) whose mathematics predicted that gravity traveled at infinite speed. Einstein set the record straight on that issue.

    Finally, I’m not sure how to interpret your meaning when you suggest that Einstein should not be given credit for the concept of gravitational waves because “[gravitational waves] existed well before any human.” All physical laws existed well before any human, so what exactly is the logical implication here?

    1. I’m often seeing people write things that are implicated to exist because of some person or because of mathematics and it bugs me. A person could be the first to build an instrument to discover something new or think up a new model that seems to be a better understanding of a phenomenon, but they aren’t creators of the observation. They can be a discoverer. If they make a novel instrument to discover something they are creators of the instrument only, they are not creators of the phenomenon. Nature is here to be discovered. The fact is gravitational waves are an easy consequence to see of any moving massive body that only temporarily occupied a space. Do you know that the gravitational wave that comes from the moon raises the surface that you stand on about 30 centimeters every time the moon passes overhead? This is a gravitational wave that is very slow so we don’t readily perceive it, but it is a gravitational wave nevertheless and doesn’t require an Einstein to exist or for such a person to predict it. That is why I wrote the tongue-in-cheek slightly humorous comment.

    2. Imagination is the only thing imaginary.

      If I tell you that a black hole is a pink elephant sucking matter with is trump, everyone will make his own image of it according to his own elementary concepts like the elephant and its parts, the pink color etc…

      These concepts are passed from generations to generations, teachers to students, scientists to scientists. Everyone is the sum of a past and everyone is entangled with everyone in a way. So alone we are not much…

  5. QCD is non-Abelian, QED is Abelian. The electrodynamic is linear and general relativity is non-linear.
    The source of Gravitational waves (black holes) is billions of light years far.
    To travel in speed “c”, it must have massless quantization or very low frequency.
    Non-linear exibits itself the gravity – and it must also escape other gravitational waves inteference.
    More frequency more space, less frequency less space and more gravity by itself.
    Like electromagnetic, all waves has momentum – only at “c” has no mass.
    If GW has gravity, it has mass – how it can travel at “c” ?

    1. You also seem to write comments without reading this website too.
      According to GR, GW are caused by boosts of energy and momentum, or, if you let me use an slightly improper translation, by (very) massive objects. That doesn’t imply at all that the GW can’t travel at c, as already written above… besides that gravitons are supposed to be massless and are not the main topic of paper discussed here, though they actually wrote also about a bound constraint on the graviton’s lambda, etc… etc…

        1. Thanks guliohome, Kudzu,
          If we construct a Penrose stairs, if we got at one end as good working and beautiful Abelian and linear – if we construct a Model (reverse mirror image) – to connect the other end as non-abelian and non-linear – it also work in QCD and in observable spacetime geometry ?

          1. Relativity equations were localized to keep the mirror image. But mass and thus the gravity violates the relativity.
            How the gravitational wave is a general relativity phenomenon ?

            If the universe were reflected in a mirror, most of the laws of physics would be identical—things would behave the same way regardless of what we call “left” and what we call “right”. This concept of mirror reflection is called parity (P). Gravity, the electromagnetic force, and the strong interaction all behave in the same way regardless of whether or not the universe is reflected in a mirror, and thus are said to conserve parity (P-symmetry). However, the weak interaction does distinguish “left” from “right”, a phenomenon called parity violation (P-violation).

          2. Sorry, but your sentences range from wrong “gravity violates the relativity” to off-topic/unrelated, like “QCD”, “non-abelian”, so – at best – you should post about parity violation in a thread concerning primordial G waves and inflation, but surely not here. Sorry, I’m not going to reply more…

          3. Here (GW) energy and momentum at work at light speed, so newton physics does not work – only relativistic physics.
            But in macro level everything is almost empty – only at quantum level we could fix 99% mass as color charge dance.
            The Higgs mechanism which try to explain mass, was originally intended for QCD.
            At the absence of Lorentz invariance and relativity, Goldstone massless mode became the longitudinal polarization of a massive spin-1 “photon”.
            So a gauge invariance to become massive, it must violate Lorentz invariance and relativity.

            So gravitational wave’s energy-momentum to curve spacetime, it must violate relativity ?

          4. While there is disagreement between QM and relativity gravitational waves have not yet been found to be a part of this. According to relativity GW are not particle in nature, they are waves in space itself. So far all of the (little) we have measured agrees with relativity.

            It is possible that we may discover disagreement with relativity. We do not know that the waves have traveled exactly at light speed for example, if they did not it would indicate a number of things (Massive gravitons, non-infinite range of gravity.) Likewise the finer details of these sorts of interactions haven’t been mapped out, these may disagree with relativity also. But so far relativity remains intact.

          5. To fit the common sense of mass Higgs mechanism was introduced. To fit the common sense of gravity Geneeal relativity was introduced.
            But it was localized (euler-lagragian) by point “particle” or light quanta. ?

            Einstein’s equations as we have derived them are indeed the correct field equations for the metric, let’s see how they can be derived from a more modern viewpoint, starting from an action principle. (In fact the equations were first derived by Hilbert, not Einstein, and Hilbert did it using the action principle. But he had been inspired by Einstein’s previous papers on the subject, and Einstein himself derived the equations independently, so they are rightly named after Einstein. The action, however, is rightly called the Hilbert action.) The action should be the integral over spacetime of a Lagrange density (“Lagrangian” for short, although strictly speaking the Lagrangian is the integral over space of the Lagrange density.

          6. By localizing the relativity (as point parricle), like in quantum decoherence and in ultraviolet catastrophe, – the superposition Energy (Cosmological cobstant) is not accounted – because the dark eneegy has no Momentum like the light quanta (mass energy or zero mass momentum) ?

          7. /Physicists like Rayleigh, Jeans, and Lorentz set Planck’s constant to zero in order to align with classical physics, but Planck knew well that this constant had a precise nonzero value. “I am unable to understand Jeans’ stubbornness – he is an example of a theoretician as should never be existing, the same as Hegel was for Realism philosophy (localization)/.

            A wasp does not drown on water. We evolved with knocking of molecules around us – and gravity – what we call momentum.
            A wave in our decoherence must have a flip – above momentum is the flip – or consciousness ?

          8. GW is not energy and momentum, it is highenergy photons (+ cosmological constant) entangled with photons traveling at c (discreteness) – could be quantized.
            If it is more than “c” (consciousness) – could not be quantized – and causes energy&momentum along Geodesic – we observe as lagrangian.

            If the Gravitational waves from the Big Bang travels slightly more than the light speed – it could reach us unlike big bang light – which could not overcome the space expansion (by dark energy) – which causes the darkness of the sky – and discreteness in light, called the photons.
            We now measure the slight tremours of upcoming bigbang waves – before that we see a blue light (cherenkov effect).

          9. Gravitational interaction of antimatter ? : Neutrino oscillations induced by a flavor-dependent violation of the Einstein Equivalence Principle have been considered as a suitable explanation of the solar electron-neutrino deficiency.
            Neutrinos can undergo flavor–oscillations if they possess flavor–dependent couplings to the surrounding gravitational field. ?

  6. Any body can explain it me? If we are wrapped by gravity waves, by all sides, why we cant detect earth waves but only a billion lightyear waves? Are there not enough earth mass to do it? thanks

    1. Gravitational waves are different from gravity in the same way ocean waves are different from the ocean. The Earth’s gravity affects us very much, like fish in the ocean we are adapted to it, live in it. Gravitational waves however are different. They do not attracts, they stretch and squash. And they are very weak. The whole Earth emits in a year enough gravitational wave energy to power a lightbulb for an hour or so. The event we have seen is the single most violent even ever witnessed by man and even then its echoes were so weak we needed incredible precision to detect them.

      1. I knews from other sources that the gravitational waves coming from Earth, are relevant and detectable, as well as the earth-moon system and earth-sun too. What happens is that as seismic disturbances and other external factors, are all annulled, should be canceled, due to the laser system stability. Just when constructed the detector system, that stability is not taken into account or to detects, that impossible to detect because after assembled all this, there are no further disturbance / imbalance to detect. So would remain to be another new disorder to detect, and then, even vanishingly lower effects than from earth systems. And it will be a “new” or disturbance-imbalance that lasers detects. At least that’s what I knows from another source.

        1. Continuing about what knew: then,thats only detects momentary events like explosions, large masses passages, etc. and nothing stable, that for example, that if black holes merges remain in progress for a long time, the disturbance of these laser would start, but the disturbance that would start, will be as stable system at same time pursuing the merger of these black holes, then, a new fact perturb, but if it continues, it adds to the Earth-Moon-Sun system and should be abandoned again as disturbance to be incorporated into the balance. This does not help at all to accept the existence of particles such as gravitons, but only to be accept the existence of masses that create gravitational fields and that are creating it by that waves. That is, all facys of relativity (deformation of space and nothing else) and no facts by particle physics. Is It? That is my question. thanks

  7. In reply to Doc:

    Doc on February 12, 2016 at 11:51 PM
    “I think there is some confusion here. Einstein’s GR predicted the existence of gravitational waves, which is what all the LIGO fuss is about This is a concept very distinct from gravity waves. This is not a semantic difference; gravitational waves and gravity waves are both scientifically accepted terms. (The latter, for example, is responsible for the tidal force.)”

    … When I speak here about gravity waves I’m referring to gravitational waves because our context here is spacetime ripples from sources external to only Earth related gravity. So when I mentioned the moon’s effect on the Earth tides as an example of a gravity wave effect this should be understood as a gravitational wave effect. I know that earth scientists use the term gravity wave in a different context and in that context they don’t mean any effect that goes beyond the bounds of the Earth.

    I don’t agree that the tidal effect fits in the category that earth scientists use for gravity wave. The tidal effect comes from another massive body in motion outside of the environ of the Earth so it falls within the category of gravitational wave effect.

    Acknowledgement of this does steal some of the excitement of what the LIGO scientists are doing, yet if they are seeing a signal from a real black hole merging event there is still plenty to be excited about.

      1. You are confusing a tidal bulge which can be illustrated well in a static picture with tides which is a dynamic phenomenon.

        The root cause of a tidal bulge is the gradient of the static gravitational field, and this can be seen if there were no continents and the ocean levels were all the same and no Earth rotation and no motion of the moon around the Earth. In such a situation though you would have a bulge but no tides.

        If you made an elevation marker on most places of an Earth with equal ocean depths and now introduce a moon in motion around the Earth you can then see tides relative to the marker.

        I conclude that I am still correct to say Earth tides are a demonstration of gravitational waves.

        See: https://www.lhup.edu/~dsimanek/scenario/tides.htm

        1. I think we both agree on the effect of the static gradient. Then of course you have a sort of movie composed of a sequence of slightly different static pictures, but I’m not confusing that with gravitational waves: they are due to the relativistic principle of locality, I.e. the fact that the action of gravity is not instantaneous, something that you can’t see from the tidal effects you’re referring to

        2. Moreover, your link is very detailed about tides and can be interesting in confuting possible misrepresentation of some textbooks, but there cannot be any effect due to GR there and it is not mentioned in fact.

          1. Gravitational waves are not due to Einstein’s relativity theory. Gravity and gravitational waves existed well before any human.

            I don’t know why so many people are giving so much credit to Einstein for something that is just a part of nature and should have been easily foreseen.

            The talk of whether the action of gravity is finite or infinite is interesting for historical reasons, but as far as I’m concerned something moving at infinite speed is just bonkers and should have never been seriously considered by any serious thinker.

            1. I’m tired to read this nonsense. The phenomena you’re mentioned are described by Newtonian physics and they don’t require nor prove GR. If you don’t understand this after all this debate, I won’t spend further time for your insisting comments.

        3. In a sense, you are right Vincent. Both, the tidal force and the gravitational waves have the same origin: gravity. But the tidal force is explained by Newton, not the gravitational waves. GWs are relative to an outside observer and the fact that Einstein’s GR predicted them to this precision is simply amazing. It is the instability of the orbital motion of the two bodies which produces GWs. In the case of the earth and the moon coupling, the obit is pretty stable, though not totally because of entropy…

        4. By the principle of action equal reaction, which is true in General Relativity too, it is understandable that the acceleration of two bodies toward each other asks for and equal and opposite release of energy. But what is the nature of this energy? General Relativity gives a general solution without even knowing the specific mechanism at the particle level. There is no real description of matter in GR. So how could Einstein have predicted it? Einstein has been struck by a lightning having more than 1.21 gigawatts… Einstein was a master at synthesis thinking, he liked to see the big picture without forgetting about the small things…

          1. Well, your answer appears to be unnecessary complex for this context.
            These are clearly examples newtonian physics, that’s it.
            Anyway, one could simply answers your question about the nature of this release of energy, saying that it is mass lost and transformed into gravitational waves. In fact it is the standard answer here: https://www.theguardian.com/science/live/2016/feb/12/gravitational-waves-an-astrophysicist-answers-your-questions-live?page=with:block-56bd4fa7e4b0bef6d53d3a87#block-56bd4fa7e4b0bef6d53d3a87
            But if you invoke the relativistic conservation of momentum, then it looks like you’re going to introduce mathematical complexity, pseudo-tensors and advanced staff like jet bundles… and finally one needs to introduce parallel transport so that you can get an “energy conservation law” in integral form. Look at http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

          2. In GR, energy is conserved in a local interaction (conserved locally). The exchange of gravitational energy in a two rotating black holes system occurs at the speed of light and it is local by definition. There is no fuzz here. At large scales, it is not the case, you may have to consider the entire universe…

          3. I should have written the momentum is conserved.
            Momentum is certainly related to energy.
            Conservation of momentum is included in both, Newton and Einstein theories, but only Einstein gave us a global theory including momentum, energy, mass and gravity, with the help of relativistic principles. Even though you could infer a kind of gravitational waves in Newton’s theory, Newton didn’t.

  8. Thanks for sharing this report. To add a waveklet of infoirmaiton, Kip Thorne compared the energy emtting when the two black holes merge to 50 times the total power radiated by all stars in the visible universe. I investigate this value and found that it is approx. a power of about 10^-3c^5/G or about 3×10^49 Watts. It is a bit less than 10^23 times the luminosity of the Sun ! It is 100 million higher to the energy emitted by the most powerful supernova ASASSN-15lh that exploded in 2015 and released 2×10^38 W, or 10 times more energy than the prior record.

  9. Question:
    What is the probability that a chirped wave train 20ms long,after travelling for 1.3 billion years through outer space,will encounter the LIGO detector
    at exactly the time when LIGO was just ready to detect this signal???

        1. The specific answer is that it is not the timing of the event that restricts the probabilities, but the volume in which the event could be located, given the uncertainty about the direction of the source. Numerically it is one in “400 million galaxies in that 600 sq deg patc”.

        1. Are you serious? Have you even read the above article? Prof Strassler says several avents per year, I can’t remember now where I read 3 or 4 however.

  10. Does this mean that the universe can’t expand faster than c? If the answer is yes, there is a limit to the expansion could it give more credibility of a cyclic universe? And that we should stop asking where did all this come from because there is no beginning and no end. Indeed, there is no time, only the “now”.

    1. Matt Strassler explained the inflation here: http://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/inflation/
      There you can read:
      “Doesn’t that incredible expansion mean that things moved apart faster than the speed of light … the universal speed limit?
      Yes it does.
      And doesn’t that violate Einstein’s theory of relativity?
      No it doesn’t.” etc…

      I don’t see a strong relation between infation and G waves, anyway…
      prof Strassler answered a (FWIU similar) question about gravitational waves in his comment there: http://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/inflation/#comment-183958

  11. … gravitational waves would be OK if there exists gravitational field. But as I understood there is no such thing as gravitational field and what we call gravitation is just twist of spacetime. So “then they propagate in empty space” is like spacetime distrortions propagate in empty space? In other words space propagates in space?

    1. Please, it would be better if you use the “Reply” link next to my answer when you want to refer to it! Gravitational field does exist, its relativistic description is a twist of spacetime, in particular that twist follows the relativistic principle of locality, in other words gravitational waves propagate with finite speed c (minor technical note, speed c in case of planar waves, speed less than c for signal… but ignore the technicality in the parenthesis).
      Using your wording, the spacetime twists along the axis perpendicular to the wave direction. And yes, that wave is originated from supermassive binary objects and propagates through empty space

      1. “Please, it would be better if you use the “Reply” link next to my answer when you want to refer to it!” – my bad
        and thank you for explanation, as well as for sharing your comments on most hot science topics

  12. nope, BHs distortion of spacetime is there in place circumfering the blackholes. But so called “gravitational waves” propagate without mass propagation. Did you get the question?

  13. from my laypesrson perspective: gravity is spacetime distortion due to presence of a mass. You can’t separate mass and its gravity effect on spacetime. Gravitational waves is wave-like spacetime distortion, but without wave-like moving mass? I can’t comprehend logic behind claims of existance of spacetime deformations without presence of mass. My guess LIGO did detected, but something not gravitational. And please leave away silly analogies with waves in pond.

    1. Without presence of mass? The source is claimed to be 2 black holes: they’re in fact supermassive objects moving towards each other. So the “mass moving” is originating the waves, according to GR, and then they propagate in empty space. Of course there is much more complexity in the mathematical equations and in the implementation of the observatory… devil hides in details…

    2. That’s a nice opinion but not one that’s scientifically supported. Gravity is an infinite range force, the sun’s gravity extends out billions of miles, out to neighboring stars and beyond. If that is not separate from its mass, a small blip of plasma at the center, then how are gravitational waves separate?

      And we have seen neutron stars losing energy, spiraling in towards each other, not just once but dozens of times now. Te first was PSR B1913+16, known as the Hulse–Taylor binary. If there are no gravitational waves then something is causing them to lose orbital energy and head towards merger, something behaving exactly as Einstein predicted. that’s one heck of a coincidence.

    1. I would imagine that in least in theory it would be possible to make a laser out of gravitons, because (as bosons) gravitons do not obey the Pauli exclusion principle. The big “IF” is whether or not gravitons actually exist. Gravitational waves may shed a lot of light on this existence question.

  14. Matt,

    I sorry to add to the pile-on, but I have a question about how LIGO works, and you are the only one who might be able to answer.

    LIGO is an interferometer. It observes changes in the length of its arms (caused by gravitational waves) thru the interference of the two beams sent down the arms.

    I just looked up on Wikipedia that a uv laser has wavelenghts around 180 to 400 nanometers. X-ray lasers have a wavelength of the 10s of nm. Optical lasers have wavelengths even longer than uv lasers. (As a kid I saw a uv laser burn wood at the Ontario Science Center and they are my standard laser.)

    Media sources, citing, I think, experimenters, have said that LIGO detected a spatial change on the order of 1/1000th of the width of a proton.

    The radius of a proton is measured in femtometers.

    I am thinking that is a difference of like 9 orders of magnitude smaller than LIGO’s “measuring stick”, the wavelength of the laser.

    Can you accurately track changes caused by interference that are so much smaller than the wavelength of the laser? Are there other techniques that allow them to do such a mind-boggling measurement?

    Any light you could shed on this would be much appreciated.

    And again, as so many have said, very grateful for your blog and your willingness to engage your readers.


    1. The trick is that it uses an alignment of two waves. This is far more precise. For example you may know of “beat frequency”, where two tones of nearly identical frequency are played and the resulting interference crates a variation in volume whose period depends on the difference between those two frequencies.

      If you use one reference frequency you can take a sample frequency and from the resultant beat determine its frequency, accuracy improving the closer the two frequencies are. Classically such a system could measure an infinitely small difference between two frequencies.

      This works with light also, both with frequency and phase. If you’re very,v very careful and use a few tricks to get past (or rather avoid) the uncertainty principle and you can use interference to measure distances far smaller than the wavelengths of light used.

      1. Kudzu,

        Yes, your point gets right at what I am asking. If my naive calculation is correct, LIGO can detect that one of the waves is on the order of one billionth of a wavelength out of phase from the other?
        Just amazing.



        1. That’s correct but as you imagine it’s not easy. Background noise is a big issue and you need some physics and math trickery to help you out. This is why the project not only took years to design and build but has had two phases (so far.) When the original LIGO was built scientists were still working through the problems of detecting such small changes, and they still are. It may take us decades to reach the point where all the relevant issues are dealt with and out detectors are as good as they can be.

    1. Not really, dark energy is everywhere, exceeds matter energy and causes space to expand. Gravitational waves are discreet things that are always far less than the matter that makes them and they cause space to temporarily expand AND contract.

  15. From the post:

    “Imagine, if you can… Two city-sized black holes, each with a mass [rest-mass!] tens of times greater than the Sun, and separated by a few tens of miles (tens of kilometers), orbit each other. They circle faster and faster, as often, in their last few seconds, as 100 times per second. They move at a speed that approaches the universal speed limit. This extreme motion creates an ever larger and increasingly rapid vibration in space-time, generating large space-time waves that rush outward into space. Finally the two black holes spiral toward each other, meet, and join together to make a single black hole, larger than the first two and spinning at an incredible rate. It takes a short moment to settle down to its final form, emitting still more gravitational waves.

    During this whole process, the total amount of energy emitted in the vibrations of space-time is a few times larger than you’d get if you could take the entire Sun and (magically) extract all of the energy stored in its rest-mass (E=mc²). This is an immense amount of energy, significantly more than emitted in a typical supernova. Indeed, LIGO’s black hole merger may perhaps be the most titanic event ever detected by humans!”

    I have a question about this, related to Joe Darcy’s questions above that Mark addressed. I read through the LIGO paper and didn’t find an answer.

    I understand that the binary black hole system radiates energy in the form of “inspiral” gravitational waves before the merger. I presume that, at this stage, the “lost” energy comes not from the mass-energy of either of the black holes, but rather from other system energy (is it gravitational binding energy?).

    What I’m having trouble understanding is where the radiated energy comes from DURING the merger. I see that in the detected LIGO event, the initial [rest] masses of the black holes were 36 and 29 solar masses (36+29=65), but the mass of the final black hole is only 62 solar masses, which means (I think) that 3 solar masses of mass-energy from the initial black holes themselves were radiated away in the form of gravitational waves.

    My question is: where, specifically, do those 3 solar masses of energy come from, and how does it “escape”?

    I’ve read enough of this website to know that a black hole’s mass (like any system’s mass) is greater than the sum of the masses of the elementary particles within it; all other forms of energy inside must also contribute by m=E/c^2, and I’m sure there’s a hell of a lot of it!

    So, is there an accepted model for the mechanism of energy conversion during such a merger? What I mean is: what form(s) of energy within the black holes is converted and radiated away via gravitational waves? To put it in the language of particle physics, how are the gravitons created? Are massive particles converted into them? Are massless ones? Does it have something to do with internal motion-energy, or something else entirely?

    And then, does anything “escape” the event horizons during the merger?

    In short, I guess I’m asking where those 3 solar masses of energy come from and how they exit the confines of the merging black holes.

    1. The energy originates as gravitational potential energy, the energy all the mas sin the holes has because it is separated by distance instead of in one large, low energy lump. As the holes approach each other this is converted into kinetic energy, orbital and rotational and eventually (a lot but not all of) it becomes gravitational wave energy.

      The amount released is so extreme because the holes are so small and heavy and can get so close. Indeed it is the maximum amount of gravitational potential energy the system can lose.

      1. Thanks for the response, Kudzu, but that doesn’t quite answer my question.

        I understand that the binary system loses energy as the black holes spiral inward toward each other. What I don’t understand is how and why the final black hole is 62 solar masses when the individual masses of the source black holes added to 65.

        What happened to those 3 solar masses of MASS-ENERGY? How and why was that mass-energy carried away as gravitational radiation upon the merger?

        1. Umm interesting Mike, I don’t know the answer but wasn’t it hawking that showed that when 2 black holes merged the total entropy? is always greater then the sum of both holes?, is that not part of the generalised second law.. But maybe entropy and mass(energy) are different in this respect?

        2. The ‘mass energy’ isn’t strictly just relating to mass. The mass of any particle is a combination of many different kinds of energy, a proton in empty space is more massive than one on Earth’s surface since the latter loses gravitational potential energy as it enters Earth’s gravity well.

          Likewise any two bodies that merge will release energy, their combined particles will have less mass then they would if separated. Often this is released as electromagnetic radiation of various kinds, light, infrared and so on with gravitational waves being a small component.

          Black holes however cannot release energy in this way, their sole means of losing energy is gravitational waves. The holes simply cannot merge without losing energy and a large amount of it. (In fact there is an unresolved issue regarding energy loss at larger distances, ‘the final parsec problem’.) The amount can be roughly calculated using the equation for gravitational potential energy, u = -GMm/r. (Roughly since the holes are losing mass as they approach each other.) The difficulties are outlined in the wikipedia article: https://en.wikipedia.org/wiki/Binary_black_hole which itself lacks fine detail but provides links to papers that go further into the calculations.

          Take our two holes just before and just after merger. Before we can consider them as two point masses separated by their schwarzschild radii. For our two holes that’s 10.6 and 8.6km, total 19.2km . After merger the new, 65 Sol hole’s radius is 19.2km meaning the average distance of mass from the enter is half that. Back-of-the-envelope and noting the hole has increased in volume compared to its components tells us around 1.5% of the mass should be lost. The actual amount is grater though I am not sure how well it is known.

          1. @Kudzu:

            My genuine thanks for another informative reply!

            I still, however, don’t understand how the loss in mass-energy of the individual black holes arises.

            To recap, here are a few things that I DO understand:

            1) I do understand that the mass of a system (e.g., a single black hole, or a binary black hole [BBH]) consists of more than just the sum of the masses of its constituent particles. By m=E/c^2, any other “internal” energy contributes to the system mass, too: to use the language of our dear host, that would be interaction energy, motion-energy, and any other sources of field energy. I say this to make it clear that I’m not under the false impression that the loss of mass associated with the merger of black holes necessarily means that massive particles are escaping.

            2) I do understand that during the “inspiral” stage, the BBH system radiates gravity waves as the orbit gets smaller, but that the lost energy does NOT come from the mass-energy of either black hole.

            3) I do understand that during the merger itself, the radiation of gravity waves momentarily peaks and then quickly dies down. I’ve read that this has something to do with the newly merged black hole oscillating as it loses its deformed shape. This makes sense to me—just as an accelerated charge radiates electromagnetic waves, an accelerated mass radiates gravitational waves.

            4) I do understand that this final spike in gravitational radiation carries away A LOT of system energy. In the event detected by LIGO, it amounted to about 3 M☉ / c^2. But now, unlike in the “inspiral” phase, this loss of energy DOES come from the mass-energy of the black holes themselves: 39 + 26 = 65, and 65 ≠ 62.

            Now, what I don’t understand is the connection between points 3 and 4 above. The final spike and “ringdown” are caused by the oscillation of the newly merged black hole, but the energy carried away by the gravitational waves COMES FROM the mass-energy of the original black holes.

            What’s happening here? It sure seems like mass-energy within event horizons ends up radiating away in gravitational waves. How does this happen, and why? I accept that it happens, but there’s a piece of the puzzle that I’m missing conceptually.

            Which specific constituent(s) of the black holes’ mass-energy is converted to gravitational radiation? Is it mass-energy of particles? interaction energy? motion-energy? some other sources of field energy? And whichever it is, it starts out inside the event horizon and ends up outside it, right?

            My brain hurts!

      1. Thank you again, Kudzu.

        I understand at least some of what you’ve written, but it still seems like you’re bringing up energy (within the BBH system) OTHER THAN the mass-energy of the individual black holes, whereas its only the fate of that mass-energy that’s befuddling me.

        So despite your valiant efforts (for which I’m truly grateful), I can’t say that I’m any closer to grasping how and why that mass-energy becomes gravitational radiation.

  16. Great news, however you look at it. Small question, is the confirmation of gravitation waves consistent with quantum gravity?

    1. This is something currently being investigated. While the large scale structure of the merger seems (so far) to agree with relativity it is hoped that details will emerge that give us a clearer idea of how gravity works in such an extreme environment.

  17. So if detecting a ripple in the Higgs Field means we have discovered the Higgs boson (not “we have discovered Higgs waves”), does this new discovery, via wave/particle duality, mean we have discovered the graviton?

    1. No. There is a difference. The Higgs particle is the smallest possible ripple in the Higgs field, like seeing a single water molecule wave. Gravity waves MAY be large collections of gravitons, like ocean waves. Gravitational waves may tell us some properties of gravitons however.

  18. LIGO basically use the earth as the antenna for Gravity wave. With the mass as earth how can the deformation in the millisecond range? Even if the bang indeed generated a pulse of 0.25 second, when it went through a 13 billion light year thick “media” to finally arrive at the earth, it must have been greatly damped to much smoother shape and thus much wider signal. When you check the event against the astronomy observation database, you assumed that gravity wave travels at the same speed as that of light, which is still an assumption without theoretical base yet. Therefore, I would not be so confident on the claim yet.

    If LIGO detector has a sensitivity of 10 to 21st power, then it should be able to detect the deformation of the earth by the moon and thus should have a daily alternating signal even if the deformation may not be a quadruple one. I have checked from the Internet that there is no no such phenomenon reported from LIGO. Since the Michelson arms are on the surface of the earth, then filtering out noises would be critical; however, we do not know what kind of filters did you use and in which band you expected signal and noise respectively.

    1. I have the same concern about Earth Tide by moon and sun (not ocean tide) effect and how the LIGO team dealt with it. This can’t be found in the PRL paper. Search the online pdf, only found Earth once, no tide or tidal. Earth tide effect can be calculated for the LIGO location based on the position of moon, sun and other factors, and I would hope the background noises mentioned in PRL paper include these in some detail to convince people that Earth tide effects are eliminated in the data analysis.

      1. Tides are actually a minor issue. The signal looked for is a ‘wiggling’ of length as the waves pass, a shrinking then stretching. This is on the order of 100 times per second or so, anything lower frequency, even two supermassive black holes merging (10 times per second signal) just isn’t seen. Tides are a twice a day signal,thousands of times longer then what’s being looked for.

    2. 1.) LIGO is the antenna, not the Earth. LIGO’s arm lengths are altered and this is what is measured. This is why the deformation in the milisecond range can be detected. In fact-

      2.) -because of this LIGO CANNOT detect anything lower frequency than about a few Hz, the time it takes light to cross it. Not tides for example.

      3.) Gravitational waves are theoretically expected to travel at light speed, this is part of relativity. If they do not then they are not the gravitational waves Einstein predicted.

  19. We know there is ocean = Aether ?,
    Now we know there is waves = gravitational.
    LIGO = Michelson morely ?

    At the time we saw signals from black holes merger, the waves hit LIGO. This coincidence implies the speed “c” of the waves ?

    What about higgs field, dark matter, elephant swimming in the tar – does they also disturbed ?

    For a layman like me Einstein was great, mass, arrow of time, information (wave function) were well explained by relativity, time dilation and light quanta. That needs property of space like waves – but for, general relativity, rest mass, black holes and their geometry of spacetime… we need cultural change ??

      1. No momentum no mass. Under speed “c” is visible matter. No speed limit no matter including the dark one.
        In fractal inflation some patch stopped expansion at some speed limit (black body) – but more than “c”.
        But without momentum, no mass, no gravity.
        “Dark night sky paradox”, is the argument that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe.

        The “ant horizon” is a decent enough analog for the edge of the visible universe. The speed at which the ant runs is described with respect to the part of the balloon it’s presently standing on and the speed at which light travels is with respect to the space it travels through (technically with respect to objects that are “sitting still” in space). The oldest photons we see are those that have come from just barely on the near side of the distance at which light can’t close the gap. It’s not that things beyond that distance are moving away faster than light (almost all the galaxies and gas and whatnot are moving slowly with respect to “the balloon”), it’s that the light they emit just isn’t moving fast enough to overcome the expansion.

        So waves were from inflation or big bang ??

    1. Gravity waves are predicted to travel exactly at c, yes.

      LIGO is in fact related to the Michelson-Morely experiment, specifically it;s interfereometer.

      1. Thanks Mr Kudzu,
        I have a layman question. We have EM waves with electric and magnetic connected with a flip.
        So gravitational wave is a connection between visible matter and WIMP connected with entanglement of juxtapose information from unexpanded space (black hole ?). Information not destroyed but lost connection between rebirth and with decreased entropy – what we call the consciousness ?

        1. Gravitational waves are different from EM waves in a few respects. They are quadrupole waves while ME waves are transverse. As such gravitational waves do not have two different fields to ‘flip’ between, this relates to why gravity doesn’t have two opposite charges with only opposites attracting.

          1. There is no plus and minus charge, only together neutral. To consist speed “c” masslessness added in maths – To consist only plus (attraction), quadrupole monopole radiation ?
            EM allow superposition to travel at “c”. So gravitational wave also allow superposition to travel at c ?

            GW is an another phenomenon like entanglement (spooky action at a distance) – which violates relativity and anger Einstein ?

        2. No, the ‘charge’ of gravity is different from zero. It is like +-1, a charge that is its own opposite. Gravitational waves, IF they are made of gravitons would allow superpositions to travel as with photons, but we do not know if they are made of gravitons so as yet gravitational waves support Einstein.

          1. Thanks Kudzu,
            People like Roger Penrose believe that gravity is going to behave differently and he is with Einstein on this – I also.

  20. and what about a seismic tremor disrupting the laser measurement, even protected? Are that any chance to system error

      1. What is the probability for an event having these parameters(20ms length wave train,250 Hz,30 solar masses BH’s etc),first of all to happen,and then,after travelling for 1.3 billion years,to encounter the LIGO experiment on earth just at the time when LIGO was ready to detect it ?

  21. What a chance to have been a boson (or other subparticle) to disrupting the laser measurement? After all, bosons also have mass and make gravitational field

    1. Bosons have a pathetic mass. The detectos constantly pick up noise, form night and day temperature changes to traffic driving by. That is why there are two observatories, one in Washington and one in Louisiana. A signal is only considered if it appears in BOTH places at about the same time. The signal also isn’t one ‘blip’ it’s a special secondish-long wave, starting slow and rising before suddenly stopping.

  22. One cannot overestimate how amazing Einstein was as an old school natural philosopher, as well as a mathematical physicist. First came the empirically-driven intuitive insights about fundamental principles, and only then did he bring in the tools of mathematical physics (with lots of help from others in the case of General Relativity) to give rigor, details and predictive capacity to his conceptual discoveries.

    Maybe it is time to reassess the conventional scoring of the Einstein-Bohr debates and to reconsider the reservations Einstein had about the incompleteness and opaque quality of Quantum Mechanics. His intuition was legendary, and well deserved.

  23. I thought the gravitational wave affects not only the length of arms in the interferometer but also the wavelength of the laser beam. But it seems like only the length of arms changes and the wavelength of the laser doesn’t change to produce the path difference and so the interference. Am I missing something here? Please clarify this if anyone knows. Thanks!

    1. K.L., That is a great question. I think the laser beam should be affected in the same way as the rest of the equipment. I suspect that they resolve this by a comparison between the other tunnel which is at 90 degrees.

      Note, I am highly skeptical of their claimed find, especially so soon after their upgrade and calibrations. I think they are now at the level of instrumental accuracy where they can get away with pulling a fast one on the public who has been funding this stuff for some 40 years.

      1. General relativity is always mysterious to me. If one thinks the snapshot under gravitational wave then yes, the light also stretches/compresses as the space does. So at glance it looks like there is no path difference. But if the light “propagates” along the arms then the snapshot doesn’t propagate as it does in free space. Instead, dt^2=(1+h)dx^2 (c=1 and assume h is a constant and very small for simplicity). So there is a deviation in time proportional to h/2 from the time taken in free space as the light propagates along the deformed arms. Each arm has different h and the net deviation in time is non-zero and so there is phase difference at the end.

      2. I also wonder how the black hole merger “signal” could be so short; less than a second. Surely we should expect to see ripples in spacetime that rise above the noise that last longer than that.

        1. Not yet, the energy released increases exponentially recall. Taking the first nineteen powers of 2 gives a million or so, of that 90% or so is in the last three. During a merger pretty much all the energy is released in five seconds, starting when the holes are still thousands of miles apart. The rate has been doubling for millennia. We’re getting the final flash, perhaps the most dramatic example of a vicious cycle.

    2. The Reference Frame has an excellent answer/explanation (basically, the warp in the light recovers much more quickly than the warp in the solid arms)

    3. The experiment uses a prism to split the light so the beam goes down both tunnels. However, it is designed to under normal conditions the peak from one tunnel exactly matches the trough of the other. So if any light is seen, then the distance over at least one of the tunnels must have changed.
      Having a second observatory a long way away helps filter out false signals.

      One would have seen the exact same signal at both observatories separated by some amount of time.

  24. Question: If Einstein hadn’t come up with this when he did, how long would it have been before someone else would have?

    1. People give way too much credit to Einstein for the idea of gravity waves. He didn’t originate the idea. A recent post on Google+ claims Einstein was told of this, rejected the idea, then later accepted it and then many years later rejected again and then later accepted it.

      There were others before Einstein who believed that light would curve and fall near a massive body, in particular Johann Soldner in 1801 and John Mitchell in 1783 and even Newton.

      1. Yes and no. While everything you state here is accurate, Einstein was the first to develop a sound and cogent *mathematical* theory that predicted the existence of gravitational waves. Newton did not accomplish this in his mathematics. In the absence of mathematical theory we can only be talking speculation, regardless of how brilliant these past insights turned out to be in the end.

        1. It is obvious that if light is affected as it passes by a mass as Newton suggested and Soldner mathematically demonstrated and Einstein later calculated, which had almost the same calculated value, that massive bodies in relative motion would create gravity waves. I believe that the moon’s motion around the Earth and the Earth ocean tides are a good example of gravity waves in action. It was not until 1915 that Einstein changed his calculation and doubled the value for the deflection.

          1. I assume that the concept behind G waves is that the effect of gravity is not instantaneous along distances, as opposite to the classical examples of moon’s orbit and ocean’s tides that don’t prove a finite speed propagation, so they’re effect of gravity, not of G waves…

            Again, thanks for your comments, I’m also skeptical about the complete “calibration” of the advanced interferometer and about the tendency to justify funding with “quick” results… that, as you noted, could be a hidden/biased motivation to emphasize Einstein’s predictions.

          2. I think there is some confusion here. Einstein’s GR predicted the existence of gravitational waves, which is what all the LIGO fuss is about This is a concept very distinct from gravity waves. This is not a semantic difference; gravitational waves and gravity waves are both scientifically accepted terms. (The latter, for example, is responsible for the tidal force.)

  25. Is it a direct measurement? I’d consider the fact that the nature of the detector, an interferometer, is electromagnetic.
    Besides the certainty of having seen a signal, how can we be sure that the only source of that signal is gravity?
    Classically experiments should be reproducible. This is more an observation… when will we find gravitational waves experimentally?

    1. When? According to what Kip Thorne said, probably at least once more this year, and from now on averaging multiple times per year, beating on into the future.

      Did you watch the press conference? It explained how this worked and what’s coming up behind it.

      1. Sure, but I’ve read also someone else’s doubt about the need of multiple detections in order to localize the source more accurately.
        The setup works by capturing the interference of the light from two perpendicular arms, etc… so I would call this an indirect measure of the spacetime gravitational ripple…
        I’m wondering whether an experiment – completely based on mechanics and not electromagnetism – would be ever possible…

    2. You may have a good question here. Consider the last two paragraphs of The NYT’s article about this claimed result and interference:

      “Dr. Weiss, who is retired with emeritus status at M.I.T., said his life now was more like that of a graduate student — that is to say, tinkering and making things work. This tendency was almost the undoing of the LIGO discovery. Only three days before the black hole chirp came in, Dr. Weiss was at the Livingston site, he recalled, and was horrified to find that the antenna readings were plagued by radio interference.

      That needs to be fixed, he told his colleagues, imploring them to delay the engineering run. But they demurred, saying that everything was ready, that it was too late to stop the program. Lucky for them.

      “We would have missed that big event,” Dr. Weiss said.”

      Now, how do we know that there wasn’t a cosmic, or some other, radio source that affected the two distant instruments?

    3. It’s about as direct as you can get; an detector was there and was altered by the waves.

      It is important to note that LIGO is not one detector at once place but TWO detectors, one in Washington and one in Louisiana. This in itself is useful for eliminating interference since for anything to be misinterpreted as a signal it would have to be large enough to affect both detectors. (Simple radio interference won’t do it, the signal is too local.) It would also have to look like a gravity wave, a steadily and exponentially increasing frequency heading up to 100Hz or so before rapidly fading. We got a signal remarkably close to what was predicted, not just in general but in specific details. The chance of that being noise is similar to that of the static in your car radio calling out your name.

      The bigger problem is just random noise. The detectors aren’t silent so that signals are rare, there is constant and continual background noise from traffic passing by to earthquakes of all magnitudes, temperature variations… a huge amount of analysis is needed to compensate for these sorts of things. In general all signals are recorded then anything that doesn’t look like a gravitational wave is discarded. This may cause us to miss a few things we don’t know are out there, but it is vital for picking the gold from the mud.

      1. “We got a signal remarkably close to what was predicted, not just in general but in specific details.”
        Fair enough.
        Thank you 🙂

        1. Thanks Kudzo, why does most everyone think these people are daft? Surely the first things they eliminate are the obvious….Seismic activity or the moon etc. Some of you guys are insulting and over suspicious imo. Some doubt is good, but common.

          1. I’m not insulting and doubts are legitimates, not meaning that they are daft but they have to eliminate much more that the obvious… A final comment, just read Washington Post about LIGO detection of gravitational waves:
            “To catch movement that small, scientists have to filter out ambient vibrations all the time. And sometimes even seemingly perfect results can end in disappointment: To prevent false positives, LIGO has an elaborate system in place to occasionally inject ersatz signals. Only three scientists on the team know the truth in such cases, and in at least one instance their colleagues were prepared to publish the results when they finally revealed the ruse.”.

          2. Sorry maybe that was insulting to the insulters 🙂 ….. Of coarse debate and doubt is good, I didn’t want you to leave the conversation with a bye, but good points on paragraph you quoted. 🙁

    4. The LIGO experimental setup was specifically designed to detect disturbances in the spacetime field within a certain frequency range (the range provided by GR for merging massive objects). The experimenters then proceeded to measure every source of noise, external and internal, and minimize those sources. Having multiple, widely separated sites detect a GW signature within a certain time allowed by ‘c’ further reduces the possibility of a false detection. Eventually we’ll have a space-based interferometer which would provide yet another ‘ear’ for localizing these GW point events. IMO the ultimate proof for GW would be for these GW interferometers detect a nearby SN event and localize it enough so that x-ray & optical telescopes can point at a region of space and study the star before shock ‘breakthrough.’ I think we’d get a few hours lead time. Neutrino detectors would ‘see’ the event as well a few seconds after the GW event. but they would be less precise in localizing the source.

      1. Interesting thoughts! As already pointed out, it is away from the noisy surface of the Earth, that researchers expect to see small chinks start to emerge…

      2. Mark
        I would inquire whether a neutrino signal had been detected at the same time,at some of the existing neutrino detectors

  26. Nobels? I think the usual 3 for a ‘project’ based discovery, either one human and two orgs or two humans and one org. Odds? Kip Thorne looks good here, a mortal lock if they go 2 humans deep, and they could of course go all human like with the accelerating expanding universe findings a few years back. The other? Gotta be MATHEW McCONNAUGHEY (“Alright alright alright”) … they WISH – but no. Maybe Gabby Gonzalez. And then, of course, there’s the movie:

    Every thing is massive,
    And every thing that’s massive
    Means there’s gravity
    Every thing’s attractive
    Including you and me
    Tho simultaneously repulsive
    When considered asymptotically
    Side by side Isaac
    Maxwell, Max Planck & of course Big Al
    Neils, Wheeler, Steve, Frank & Higgsy
    And don’t you dare leave out John Stewart Bell
    Every thing is massive
    Every thing is E equals MC squared
    Even gravity is massive
    Though it only works to ensure stuff is snared
    Every thing is massive…

  27. Thank you for reading this.

    The article mentioned that the gravity wave(s?) that was measured occurred in the last fraction of time just before the 2 black holes merged. It also mentioned that a long time ago these two orbiting black holes slowly lost orbit leading to the final merger.

    Question: Why did they lose orbit? Planets orbit the sun (elliptically) and do not lose orbit.

    Question: Were the masses of the individual black holes known prior to this merger? if so, does the difference in the final mass of the new (somewhat bigger) black whole account for the energy (frequency) of the strength of the wave(s?) being detected?

    Question: Were the orbits of the two black holes elliptical?

    Again, thanks for reviewing this.

    Joe Darcy


    1. Someone correct me if I’m wrong but all objects lose “orbit” as you put it (or gain). The moon is slowly moving away from the earth, just as some planets are moving closer to the sun.

      If gravity is more powerful then motion energy if Will win, if the motion energy is fast enough to win over gravity that will win. I don’t think anything is so finely balanced that they are in perfect orbit without moving in or out or circular orbit.

    2. The BH pair lost energy due to gravitational wave generation. The energy loss is very small at first, but exponentially increases with time. The loss is proportional to the masses involved and the acceleration they experience as the revolve around their center of mass. Google PSR B1913+16 which is the Taylor-Hulse binary pulsar system which gravitational wave generation was first indirectly detected and measured.

      For our solar system, the loss due to gravitational wave generation is extremely small (dominated by the Sun and Jupiter — a ‘tiny’ mass), and thus, changes to planetary orbits are extremely small.

      The “chirp” that LIGO detected provided the masses of the BH pair just before their merger. The energy of 3 sols of rest mass was dumped into gravitation wave generation, thus we were able to detect it despite being 1.3 Billion LY away. (From Fig 2, having two ~30 sol masses gain ~0.25c within 0.15 sec!!!!)

      The initial orbit had to be elliptical (unequal BH masses) for there to be gravitational wave generation.

      Earth-Moon orbit is increasing since Earth’s spin is slowing down and angular momentum of the Earth-Moon system is conserved so Moon is moving outward to compensate.

  28. Agree with the above posts about having you back Matt. Your blog is the first place I go when I hear about something like this.

  29. Thanks so much for the post Matt. Once again your precise and down to earth explanation has put to rest something that has been bugging me since high school.

  30. Came here to get a clear and informative explanation. Thanks for the paragraph about sound waves, made me logically understand why these things are finite…

    Hope you back full time Proff. Strassler.

  31. Very clear explanation of gravitational waves.
    Thank you, Professor.
    I am very happy you are back with us, common folks.

    Your fan,

  32. Assuming that LIGO works. Assuming that special relativity is accurate. Would LIGO, or one of it’s enhancements, be able to prove or disprove some of the various models for dark matter?

    1. That’s unlikely, the gravitational waves emitted by dark matter should be far weaker even than those emitted by orbiting planets and of the wrong frequency.

  33. I read the following in an article about the upcoming LIGO announcement about gravitational waves: “Crucially, because they travel straight through matter, nothing can obscure the source of these waves – there are no shadows. And they could offer an unparalleled ‘view’ of objects that don’t emit light, like black holes.” The comment about “no shadows” got me thinking about what happens when gravitational waves pass through a black hole on their way to being observed by us. Are they not perturbed? No shadow produced passing through the event horizon or the singularity?

    1. Richard, I would like to reply to your question. It is very unlikely that a black hole would hide or perturb a signal from a pair of distant massive fast orbiting objects. Black holes are small compared to the scale of a gravity wave from a very distant source. If a black hole was close enough to us to perturb (or act as an obscurer) the signal from a distant source we would have more important concerns to deal with. The article in which you read that the signal travels _straight_ through matter should not be interpreted as without any distortion.

      1. Thanks for your reply Vincent. The observations from Earth are not really what I was interested in (we can always construct a thought experiment where a third black hole very close to the colliding pair is occluding a relatively close observer.)
        I’m more interested in the theoretical implications of gravity waves somehow passing through the third black hole. What does the existence/absence of any perturbations tell us about the nature of space-time and the ability to probe within the event horizon?

          1. I also would be interested in Matt or one of the lesser mattlets tackling this, but until that happens, Imma thinking angular momentum from all that spinning, each black hole as influenced by the other and then both against each other but consider together as a related system.

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