Of Particular Significance

The Significance of Yesterday’s Gravitational Wave Announcement: an FAQ

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 10/17/2017

Yesterday’s post on the results from the LIGO/VIRGO network of gravitational wave detectors was aimed at getting information out, rather than providing the pedagogical backdrop.  Today I’m following up with a post that attempts to answer some of the questions that my readers and my personal friends asked me.  Some wanted to understand better how to visualize what had happened, while others wanted more clarity on why the discovery was so important.  So I’ve put together a post which  (1) explains what neutron stars and black holes are and what their mergers are like, (2) clarifies why yesterday’s announcement was important — and there were many reasons, which is why it’s hard to reduce it all to a single soundbite.  And (3) there are some miscellaneous questions at the end.

First, a disclaimer: I am *not* an expert in the very complex subject of neutron star mergers and the resulting explosions, called kilonovas.  These are much more complicated than black hole mergers.  I am still learning some of the details.  Hopefully I’ve avoided errors, but you’ll notice a few places where I don’t know the answers … yet.  Perhaps my more expert colleagues will help me fill in the gaps over time.

Please, if you spot any errors, don’t hesitate to comment!!  And feel free to ask additional questions whose answers I can add to the list.


What are neutron stars and black holes, and how are they related?

Every atom is made from a tiny atomic nucleus, made of neutrons and protons (which are very similar), and loosely surrounded by electrons. Most of an atom is empty space, so it can, under extreme circumstances, be crushed — but only if every electron and proton convert to a neutron (which remains behind) and a neutrino (which heads off into outer space.) When a giant star runs out of fuel, the pressure from its furnace turns off, and it collapses inward under its own weight, creating just those extraordinary conditions in which the matter can be crushed. Thus: a star’s interior, with a mass one to several times the Sun’s mass, is all turned into a several-mile(kilometer)-wide ball of neutrons — the number of neutrons approaching a 1 with 57 zeroes after it.

If the star is big but not too big, the neutron ball stiffens and holds its shape, and the star explodes outward, blowing itself to pieces in a what is called a core-collapse supernova. The ball of neutrons remains behind; this is what we call a neutron star. It’s a ball of the densest material that we know can exist in the universe — a pure atomic nucleus many miles(kilometers) across. It has a very hard surface; if you tried to go inside a neutron star, your experience would be a lot worse than running into a closed door at a hundred miles per hour.

If the star is very big indeed, the neutron ball that forms may immediately (or soon) collapse under its own weight, forming a black hole. A supernova may or may not result in this case; the star might just disappear. A black hole is very, very different from a neutron star. Black holes are what’s left when matter collapses irretrievably upon itself under the pull of gravity, shrinking down endlessly. While a neutron star has a surface that you could smash your head on, a black hole has no surface — it has an edge that is simply a point of no return, called a horizon. In Einstein’s theory, you can just go right through, as if passing through an open door. You won’t even notice the moment you go in. [Note: this is true in Einstein’s theory. But there is a big controversy as to whether the combination of Einstein’s theory with quantum physics changes the horizon into something novel and dangerous to those who enter; this is known as the firewall controversy, and would take us too far afield into speculation.]  But once you pass through that door, you can never return.

Black holes can form in other ways too, but not those that we’re observing with the LIGO/VIRGO detectors.

Why are their mergers the best sources for gravitational waves?

One of the easiest and most obvious ways to make gravitational waves is to have two objects orbiting each other.  If you put your two fists in a pool of water and move them around each other, you’ll get a pattern of water waves spiraling outward; this is in rough (very rough!) analogy to what happens with two orbiting objects, although, since the objects are moving in space, the waves aren’t in a material like water.  They are waves in space itself.

To get powerful gravitational waves, you want objects each with a very big mass that are orbiting around each other at very high speed. To get the fast motion, you need the force of gravity between the two objects to be strong; and to get gravity to be as strong as possible, you need the two objects to be as close as possible (since, as Isaac Newton already knew, gravity between two objects grows stronger when the distance between them shrinks.) But if the objects are large, they can’t get too close; they will bump into each other and merge long before their orbit can become fast enough. So to get a really fast orbit, you need two relatively small objects, each with a relatively big mass — what scientists refer to as compact objects. Neutron stars and black holes are the most compact objects we know about. Fortunately, they do indeed often travel in orbiting pairs, and do sometimes, for a very brief period before they merge, orbit rapidly enough to produce gravitational waves that LIGO and VIRGO can observe.

Why do we find these objects in pairs in the first place?

Stars very often travel in pairs… they are called binary stars. They can start their lives in pairs, forming together in large gas clouds, or even if they begin solitary, they can end up pairing up if they live in large densely packed communities of stars where it is common for multiple stars to pass nearby. Perhaps surprisingly, their pairing can survive the collapse and explosion of either star, leaving two black holes, two neutron stars, or one of each in orbit around one another.

What happens when these objects merge?

Not surprisingly, there are three classes of mergers which can be detected: two black holes merging, two neutron stars merging, and a neutron star merging with a black hole. The first class was observed in 2015 (and announced in 2016), the second was announced yesterday, and it’s a matter of time before the third class is observed. The two objects may orbit each other for billions of years, very slowly radiating gravitational waves (an effect observed in the 70’s, leading to a Nobel Prize) and gradually coming closer and closer together. Only in the last day of their lives do their orbits really start to speed up. And just before these objects merge, they begin to orbit each other once per second, then ten times per second, then a hundred times per second. Visualize that if you can: objects a few dozen miles (kilometers) across, a few miles (kilometers) apart, each with the mass of the Sun or greater, orbiting each other 100 times each second. It’s truly mind-boggling — a spinning dumbbell beyond the imagination of even the greatest minds of the 19th century. I don’t know any scientist who isn’t awed by this vision. It all sounds like science fiction. But it’s not.

How do we know this isn’t science fiction?

We know, if we believe Einstein’s theory of gravity (and I’ll give you a very good reason to believe in it in just a moment.) Einstein’s theory predicts that such a rapidly spinning, large-mass dumbbell formed by two orbiting compact objects will produce a telltale pattern of ripples in space itself — gravitational waves. That pattern is both complicated and precisely predicted. In the case of black holes, the predictions go right up to and past the moment of merger, to the ringing of the larger black hole that forms in the merger. In the case of neutron stars, the instants just before, during and after the merger are more complex and we can’t yet be confident we understand them, but during tens of seconds before the merger Einstein’s theory is very precise about what to expect. The theory further predicts how those ripples will cross the vast distances from where they were created to the location of the Earth, and how they will appear in the LIGO/VIRGO network of three gravitational wave detectors. The prediction of what to expect at LIGO/VIRGO thus involves not just one prediction but many: the theory is used to predict the existence and properties of black holes and of neutron stars, the detailed features of their mergers, the precise patterns of the resulting gravitational waves, and how those gravitational waves cross space. That LIGO/VIRGO have detected the telltale patterns of these gravitational waves. That these wave patterns agree with Einstein’s theory in every detail is the strongest evidence ever obtained that there is nothing wrong with Einstein’s theory when used in these combined contexts.  That then in turn gives us confidence that our interpretation of the LIGO/VIRGO results is correct, confirming that black holes and neutron stars really exist and really merge. (Notice the reasoning is slightly circular… but that’s how scientific knowledge proceeds, as a set of detailed consistency checks that gradually and eventually become so tightly interconnected as to be almost impossible to unwind.  Scientific reasoning is not deductive; it is inductive.  We do it not because it is logically ironclad but because it works so incredibly well — as witnessed by the computer, and its screen, that I’m using to write this, and the wired and wireless internet and computer disk that will be used to transmit and store it.)


What makes it difficult to explain the significance of yesterday’s announcement is that it consists of many important results piled up together, rather than a simple takeaway that can be reduced to a single soundbite. (That was also true of the black hole mergers announcement back in 2016, which is why I wrote a long post about it.)

So here is a list of important things we learned.  No one of them, by itself, is earth-shattering, but each one is profound, and taken together they form a major event in scientific history.

First confirmed observation of a merger of two neutron stars: We’ve known these mergers must occur, but there’s nothing like being sure. And since these things are too far away and too small to see in a telescope, the only way to be sure these mergers occur, and to learn more details about them, is with gravitational waves.  We expect to see many more of these mergers in coming years as gravitational wave astronomy increases in its sensitivity, and we will learn more and more about them.

New information about the properties of neutron stars: Neutron stars were proposed almost a hundred years ago and were confirmed to exist in the 60’s and 70’s.  But their precise details aren’t known; we believe they are like a giant atomic nucleus, but they’re so vastly larger than ordinary atomic nuclei that can’t be sure we understand all of their internal properties, and there are debates in the scientific community that can’t be easily answered… until, perhaps, now.

From the detailed pattern of the gravitational waves of this one neutron star merger, scientists already learn two things. First, we confirm that Einstein’s theory correctly predicts the basic pattern of gravitational waves from orbiting neutron stars, as it does for orbiting and merging black holes. Unlike black holes, however, there are more questions about what happens to neutron stars when they merge. The question of what happened to this pair after they merged is still out — did the form a neutron star, an unstable neutron star that, slowing its spin, eventually collapsed into a black hole, or a black hole straightaway?

But something important was already learned about the internal properties of neutron stars. The stresses of being whipped around at such incredible speeds would tear you and I apart, and would even tear the Earth apart. We know neutron stars are much tougher than ordinary rock, but how much more? If they were too flimsy, they’d have broken apart at some point during LIGO/VIRGO’s observations, and the simple pattern of gravitational waves that was expected would have suddenly become much more complicated. That didn’t happen until perhaps just before the merger.   So scientists can use the simplicity of the pattern of gravitational waves to infer some new things about how stiff and strong neutron stars are.  More mergers will improve our understanding.  Again, there is no other simple way to obtain this information.

First visual observation of an event that produces both immense gravitational waves and bright electromagnetic waves: Black hole mergers aren’t expected to create a brilliant light display, because, as I mentioned above, they’re more like open doors to an invisible playground than they are like rocks, so they merge rather quietly, without a big bright and hot smash-up.  But neutron stars are big balls of stuff, and so the smash-up can indeed create lots of heat and light of all sorts, just as you might naively expect.  By “light” I mean not just visible light but all forms of electromagnetic waves, at all wavelengths (and therefore at all frequencies.)  Scientists divide up the range of electromagnetic waves into categories. These categories are radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays, listed from lowest frequency and largest wavelength to highest frequency and smallest wavelength.  (Note that these categories and the dividing lines between them are completely arbitrary, but the divisions are useful for various scientific purposes.  The only fundamental difference between yellow light, a radio wave, and a gamma ray is the wavelength and frequency; otherwise they’re exactly the same type of thing, a wave in the electric and magnetic fields.)

So if and when two neutron stars merge, we expect both gravitational waves and electromagnetic waves, the latter of many different frequencies created by many different effects that can arise when two huge balls of neutrons collide.  But just because we expect them doesn’t mean they’re easy to see.  These mergers are pretty rare — perhaps one every hundred thousand years in each big galaxy like our own — so the ones we find using LIGO/VIRGO will generally be very far away.  If the light show is too dim, none of our telescopes will be able to see it.

But this light show was plenty bright.  Gamma ray detectors out in space detected it instantly, confirming that the gravitational waves from the two neutron stars led to a collision and merger that produced very high frequency light.  Already, that’s a first.  It’s as though one had seen lightning for years but never heard thunder; or as though one had observed the waves from hurricanes for years but never observed one in the sky.  Seeing both allows us a whole new set of perspectives; one plus one is often much more than two.

Over time — hours and days — effects were seen in visible light, ultraviolet light, infrared light, X-rays and radio waves.  Some were seen earlier than others, which itself is a story, but each one contributes to our understanding of what these mergers are actually like.

Confirmation of the best guess concerning the origin of “short” gamma ray bursts:  For many years, bursts of gamma rays have been observed in the sky.  Among them, there seems to be a class of bursts that are shorter than most, typically lasting just a couple of seconds.  They come from all across the sky, indicating that they come from distant intergalactic space, presumably from distant galaxies.  Among other explanations, the most popular hypothesis concerning these short gamma-ray bursts has been that they come from merging neutron stars.  The only way to confirm this hypothesis is with the observation of the gravitational waves from such a merger.  That test has now been passed; it appears that the hypothesis is correct.  That in turn means that we have, for the first time, both a good explanation of these short gamma ray bursts and, because we know how often we observe these bursts, a good estimate as to how often neutron stars merge in the universe.

First distance measurement to a source using both a gravitational wave measure and a redshift in electromagnetic waves, allowing a new calibration of the distance scale of the universe and of its expansion rate:  The pattern over time of the gravitational waves from a merger of two black holes or neutron stars is complex enough to reveal many things about the merging objects, including a rough estimate of their masses and the orientation of the spinning pair relative to the Earth.  The overall strength of the waves, combined with the knowledge of the masses, reveals how far the pair is from the Earth.  That by itself is nice, but the real win comes when the discovery of the object using visible light, or in fact any light with frequency below gamma-rays, can be made.  In this case, the galaxy that contains the neutron stars can be determined.

Once we know the host galaxy, we can do something really important.  We can, by looking at the starlight, determine how rapidly the galaxy is moving away from us.  For distant galaxies, the speed at which the galaxy recedes should be related to its distance because the universe is expanding.

How rapidly the universe is expanding has been recently measured with remarkable precision, but the problem is that there are two different methods for making the measurement, and they disagree.   This disagreement is one of the most important problems for our understanding of the universe.  Maybe one of the measurement methods is flawed, or maybe — and this would be much more interesting — the universe simply doesn’t behave the way we think it does.

What gravitational waves do is give us a third method: the gravitational waves directly provide the distance to the galaxy, and the electromagnetic waves directly provide the speed of recession.  There is no other way to make this type of joint measurement directly for distant galaxies.  The method is not accurate enough to be useful in just one merger, but once dozens of mergers have been observed, the average result will provide important new information about the universe’s expansion.  When combined with the other methods, it may help resolve this all-important puzzle.

Best test so far of Einstein’s prediction that the speed of light and the speed of gravitational waves are identical: Since gamma rays from the merger and the peak of the gravitational waves arrived within two seconds of one another after traveling 130 million years — that is, about 5 thousand million million seconds — we can say that the speed of light and the speed of gravitational waves are both equal to the cosmic speed limit to within one part in 2 thousand million million.  Such a precise test requires the combination of gravitational wave and gamma ray observations.

Efficient production of heavy elements confirmed:  It’s long been said that we are star-stuff, or stardust, and it’s been clear for a long time that it’s true.  But there’s been a puzzle when one looks into the details.  While it’s known that all the chemical elements from hydrogen up to iron are formed inside of stars, and can be blasted into space in supernova explosions to drift around and eventually form planets, moons, and humans, it hasn’t been quite as clear how the other elements with heavier atoms — atoms such as iodine, cesium, gold, lead, bismuth, uranium and so on — predominantly formed.  Yes they can be formed in supernovas, but not so easily; and there seem to be more atoms of heavy elements around the universe than supernovas can explain.  There are many supernovas in the history of the universe, but the efficiency for producing heavy chemical elements is just too low.

It was proposed some time ago that the mergers of neutron stars might be a suitable place to produce these heavy elements.  Even those these mergers are rare, they might be much more efficient, because the nuclei of heavy elements contain lots of neutrons and, not surprisingly, a collision of two neutron stars would produce lots of neutrons in its debris, suitable perhaps for making these nuclei.   A key indication that this is going on would be the following: if a neutron star merger could be identified using gravitational waves, and if its location could be determined using telescopes, then one would observe a pattern of light that would be characteristic of what is now called a “kilonova” explosion.   Warning: I don’t yet know much about kilonovas and I may be leaving out important details. A kilonova is powered by the process of forming heavy elements; most of the nuclei produced are initially radioactive — i.e., unstable — and they break down by emitting high energy particles, including the particles of light (called photons) which are in the gamma ray and X-ray categories.  The resulting characteristic glow would be expected to have a pattern of a certain type: it would be initially bright but would dim rapidly in visible light, with a long afterglow in infrared light.  The reasons for this are complex, so let me set them aside for now.  The important point is that this pattern was observed, confirming that a kilonova of this type occurred, and thus that, in this neutron star merger, enormous amounts of heavy elements were indeed produced.  So we now have a lot of evidence, for the first time, that almost all the heavy chemical elements on and around our planet were formed in neutron star mergers.  Again, we could not know this if we did not know that this was a neutron star merger, and that information comes only from the gravitational wave observation.


Did the merger of these two neutron stars result in a new black hole, a larger neutron star, or an unstable rapidly spinning neutron star that later collapsed into a black hole?

We don’t yet know, and maybe we won’t know.  Some scientists involved appear to be leaning toward the possibility that a black hole was formed, but others seem to say the jury is out.  I’m not sure what additional information can be obtained over time about this.

If the two neutron stars formed a black hole, why was there a kilonova?  Why wasn’t everything sucked into the black hole?

Black holes aren’t vacuum cleaners; they pull things in via gravity just the same way that the Earth and Sun do, and don’t suck things in some unusual way.  The only crucial thing about a black hole is that once you go in you can’t come out.  But just as when trying to avoid hitting the Earth or Sun, you can avoid falling in if you orbit fast enough or if you’re flung outward before you reach the edge.

The point in a neutron star merger is that the forces at the moment of merger are so intense that one or both neutron stars are partially ripped apart.  The material that is thrown outward in all directions, at an immense speed, somehow creates the bright, hot flash of gamma rays and eventually the kilonova glow from the newly formed atomic nuclei.  Those details I don’t yet understand, but I know they have been carefully studied both with approximate equations and in computer simulations such as this one and this one.  However, the accuracy of the simulations can only be confirmed through the detailed studies of a merger, such as the one just announced.  It seems, from the data we’ve seen, that the simulations did a fairly good job.  I’m sure they will be improved once they are compared with the recent data.




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73 Responses

  1. How much closer to us would this event have to have been for the two Ligo detectors to have seen it all the way to merger and ringdown? (as a ratio of distances).

  2. /The physical reality of Time dilation and Rest mass cannot coexist/.
    Big crunch and big rip in Paradox.
    If you go to past tense, there might be more mass and gravity near big bang. But the time slowed – means, *zero rest mass and only Energy.

    .) The term classical theory means, a theory where reduced Planck’s constant h-bar = 0 is zero.
    If f =0, there is no photon, as there is no vibrating energy creating a wavelength.
    Angular frequency (gravity?) prevents space expansion ?

    If there is *zero rest mass, then, only – mass from motion (change in form of energy) – the “Kugelblitz” ?

  3. The design of these very large instruments (LIGO/VIRGO) is based on General Relativity (GR). GR is a classical theory, in the sense that there is only one single story, one single outcome.

    On the Contrary, the Standard Model is a mish-mash of Quantum Field Theories (QFTs) like Quantum Electro Dynamics and Quantum Chromo Dynamics, and it (SM) explicitly excludes an explanation of gravitation, even though SM in a way postulates that gravitation is a fundamental force.

    SM, being a collection of QFTs is clearly NOT a classical theory: there are multiple stories, multiple outcomes, each possible outcome has its own probability of ocurrence (this is exactly what we see and measure in particle accelerators like the LHC).

    For SM there are four fundamental forces: Electromagnetism, the Weak Nuclear Force, the Strong Nuclear Force and Gravitation.

    Kind regards, GEN

  4. So many of the observations stemming from this event seem to match predicted behavior very well. Since that seems to be the case, can this event be viewed as further affirmation of the Standard Model of particle physics?

    1. /The physical reality of Time dilation and Rest mass cannot coexist/.
      Extreme values ( like near infinity, near c) at one side of Numerical equation (equivalence principle) – will unbalance position or momentum.
      So the physical reality is only a balance.

  5. Almost all the planets, including our Earth are having curvature of space time. This curvature gives balance to the planet. Whether a planet is perpendicular or tilt is decided by this curvature only.

    Does the GW tilt this balance (disturb equivalence principle) ?

  6. Can a black hole “choke” on a star of much larger mass?
    By the way, my understanding is that gravitational waves do not bend space – they bend space-time.

    1. Black holes are very compact. This means that there is only so much matter that can fall into them at once. When this is exceeded (And it needn’t be by a much larger mass.) the matter starts to ‘pile up’ at the center. For supermassive black holes in the centers of galaxies this matter can be ejected at the poles as two highly energetic ‘jets’ that are a feature of quasars.

      Stellar mass black holes merely get an odd accretion disk, the matter lacks the energy to escape but is too energetic to fall in; it has to emit energy as EM radiation. In the extreme a black hole could collide with a star and devour its core with the energy released supporting its outer layers, much like how a traditional star’s core works. Such stars would be stable for hundreds of thousands of years at most and it is possible we may have detected one. (PSR J1740—3052 looks like it may BECOME one in future.)

  7. It is just the opposite, our matter was scrambled at the Big Bounce. There was an accretion limit, which I coined the Bojowald Limit, at the time of the Big Bounce. However, additional matter was still on its way in and I am theorizing that it’s momentum caused our matter’s Planck Length to shorten slightly, causing the interaction problem. If this is true, there are still multiples of wavelengths that are the same, perhaps 1 out of 100,000, that can be detected because they will interact. To prove this would require a detector with an extremely narrow bandwidth.

  8. I also need to add that a partial collapse of a preexisting universe gives a very simple answer to what is dark matter and dark energy. Dark matter is preexisting universe matter and dark energy is the “gravitational” force of that matter.

    1. How does that work? Does it involve additional dimensions, or is the dark matter ‘scrambled’ relative to our own and so unable to interact with light? Can it be disconnected from gravity also?

  9. Vincent Suave, you gave a good answer.
    If you dig into the archives of Scientific American, you will find an October 2008 article by Martin Bojowald “Follow the Bouncing Universe” He (and others) postulates that our universe bounced into being from the collapse of a preexisting universe. When the universe collapsed, there was a limit on how small it got (not a point), the physics of the matter was “scrambled”, and it bounced to form our universe (the Big Bounce). This negates the Big Bang theory in which our universe sprang into being from an infinitely small point.
    The point of this is that it means a black Hole does have size and I contend that the only singularities in our entire universe are the origins of coordinate systems.
    The only problem I have with the Big Bounce theory is why an entire universe had to collapse. Why could it not have been a partial collapse and our universe be inside the remains of the preexisting universe.

  10. A pity to mention firewalls. The consensus nowadays is that they don’t exist. The problem is just to explain why. Not in any way relevant to this topic.

  11. according to Sabine Hossenfelder , space only expands between galaxies and clusters . then we face a question ; there must be something that happens at the boundary of non expanding and expanding space …..what could that be since as you said that space IS an unknown physical medium with unknown properties , does GW change velocity ? …….thanks .

    1. Correct me if I’m wrong: Space expands uniformly, but it’s significant only on length scales of millions of light years and more?

    2. There are two ways of looking at this, both negate the need for a boundary of some kind.

      The first is to view all space as expanding, but with bound systems resisting this expansion. In this view the space you occupy right now is expanding, albeit very slightly, but the far more powerful forces binding your atoms together resist this, keeping their bound size. Because of this objects don’t move apart and so the expansion of space cannot be detected.

      Conversely space in gravity dominated systems are best described using he Schwarzschild metric, which doesn’t expand. In this case we can consider the effect of gravity itself as swamping the effect. As you leave such a system the geometry of space changes as gravity weakens, slowly giving way to the expanding space of low density voids.

      1. That is very fair description of the standard view of expanding space. I just don’t believe that all the evidence favors that interpretation for the cosmological redshift. But my position is in the minority.

  12. One other reason this is important, at least in my mind, is that it eliminates once and for all any tiny lingering doubt about whether LIGO/VIRGO are detecting real events, rather than instrumental artefacts.

  13. Have we made enough spectroscopic observations of regular supernovas to compare how heavy element formation happens in them vs. in this kilonova? How are they different?

    1. There are a host of problems with spectroscopic observations. Chief among these is the swamping of rare elements by those like iron and carbon that can be millions of times more abundant.

      Models predict that the abundance of an element should fall off exponentially with an increase in atomic weight, drastically enough to cause the aforementioned abundance problems. What evidence we have shows a lot of elements too faint to get a real idea of their abundance; this matches our models for the most part but there’s enough wriggle room in the orders of magnitude for vastly more elements to be produced by some poorly understood mechanism while providing similar spectroscopic results.

      As such our standing (Until now) has been ‘theory predicts and abundance problem and observations are consistent with that.’ Now we have confirmation (And testing and refinement) of an additional angle that relaxes the demands put on supernovae.

      The main differences between the two novae are the neutron flux and the starting material. Supernovae start with iron and related elements, mostly lighter, and a neutron flux produced by breakdown of nuclei in situ (as it were.) A kilonova starts with iron (from the neutron star’s ‘crust’) that has already endured a supernova, enriching it in heavy elements and a neutron flux generated by tearing apart neutron-rich material. This should (in theory now looking increasingly proven) a much higher flux for a longer time allowing heavier nuclei to build up.

  14. Matt,
    Is there any indication as to the size of the black holes. Some believe they are infinitesimally small point objects or do they have a finite size.

    1. The big problem is that if black holes have a ‘size’ it is likely smaller than their event horizon. Due to the nature of the horizon we can’t say much about what happens below it; if black hole ‘cores’ really are smaller than their horizons these measurements won’t tell us that. Only if they were larger (Not impossible but not favored) would we see something amiss.

      As it is you can mostly treat the size of a black hole as the size of its horizon, the ‘no return’ point. It’s not a size in the way we usually think of it, but shares some properties.

  15. A day late with my question. How does neutron-star matter behave when not confined at degenerate pressures? Would the nuclear strong force would keep large clumps of neutronium (my word) stable? I visualize chunks of neutron-star crust being flung off during merger rather than fragmenting into individual neutrons. Is that accurate? Followed by furious beta-decay to synthesize the elements from heavest to lighter elements? Are we able to detect spectroscopically if any “stable” elements with A>260 (aka theoretical island of stability for very heavy elements)? Or are emission lines impossible to know in advance for such super-heavy elements?

  16. Mat,
    could you explain please ” there are two different methods for making the measurement, and they disagree” – what are they, and by how much they disagree? Also, how do “the gravitational waves directly provide the distance to the galaxy”?

    1. The various measured values of Hubble’s constant are listed here: https://en.wikipedia.org/wiki/Hubble%27s_law#Observed_values

      Of this the topmost three are the ones of note, 71.9, 67.6 and LIGO\VIRGO’s 70.0.

      The highest vaules come from the Hubble Space Telescope and relies on using gravitationally lensed objects; these can produce multiple images that have different path lengths so that light takes longer to reach us from one image than another. This time delay gives us the path difference and geometry gives us the linear distance away from us.

      The lower value comes from BOSS, a survey to map the distribution of galaxies and from this get an idea of the strength f primordial sound waves in the early universe. Both methods have considerable uncertainty, but do not really overlap, suggesting an error, bias or unknown factor in one of the measurements.

  17. “gravitational waves _directly_ provide the distance to the galaxy”

    Pls explain — how & why is that possible? Even with the 1st LIGO obseevation couple years ago, distance to the event was cited, but AFAIK was never explained in the reports..

    Also, do gravitational waves experience either doppler or cosmological “red shift” equivalent?

    1. The absolute amplitude of a GW signal (at the source) can be calculated directly from the masses, their separation, and their orbital period (the latter two are redundant, of course). That absolute amplitude can be directly compared to the amplitude measured by the detector. The amplitude falls off like 1/R for gravitational waves so you can extract R from the ratio.

      Yes, gravitational waves do experience both Doppler and cosmological redshifts, since they’re waves like any other wave. However, all of the detections so far have been close enough that the difference is small (for example, the GW170817 signal was at 40 Mpc, corresponding to z ~< 0.1).

  18. Hi Matt,
    Thank you very much for your articles! It is always a pleasure to read them!

    I have a question about the delay of x-ray and radio waves. The explanation given at the press conference was that they were late because we are not viewing the jets head on but rather at an angle. Initially I thought what they were hinting at was that it took a while for the jets to disperse enough to widen the angle and include earth in its extended path. But then I am still not clear why would that cause a delay for the x-ray and radio only? Why it did not affect other frequencies? Or did I completely misunderstand what they were saying?

    1. The idea is that the main jets of the merging pair have missed Earth entirely; we see the gamma rays because they were emitted powerfully in most directions. But we’ll never see ‘direct’ X-ray and radio photons from the jets.

      Instead the light ‘sideways’ from the jets heats material nearby which then emits photons in all directions, some of which we see. It takes time for the jets to encounter enough material for the ‘secondary glow’ to be bright enough to see. We’d expect z-rays to have a small delay (Being the most energetic secondary emission.) with radio waves having a longer delay. (Being the result of more dispersed,lower-energy processes.)

  19. Oh, and another question: What sized black holes would make the biggest splash (highest frequency, largest energy output)? The current data is bit too slim to try extrapolate for tiny hypothetical primordial black holes with masses less than the Sun’s mass, and for supermassive black holes. Tiny black holes can orbit very close to one another, but have relatively little mass so maybe their orbital velocity and gravitational waves aren’t that great after all? Supermassive black holes on the other hand have huuuuuuge masses, but are also very large and can’t get too close before merging. So how would the maximum frequency and energy output scale as a function of the binary’s mass (for simplicity, let’s assume the two components have identical masses).
    A big thanks for your time!

  20. Would the current detectors be able to capture a merger between two white dwarfs under any circumstances? If so, from how far could such collisions be detected?
    Thanks a lot!

    1. @Petri, Based on this paper, https://arxiv.org/abs/1408.0740, It seems that white-dwarf mergers could only be detected in our galaxy and then only with a space-based interferometer like LISA. The GW frequencies from merging white dwarfs is too low for ground based observatories to detect.

  21. Dear Matt,
    I have a rather fast question. Why a binary system spiralyzes and eventually collapses? Does che system loose energy and decreases its angular momentum because it is emitting gravitational waves ( in analogy with a accelerated charged particle emitting EM radiation? ) . Or the reason of the collapse is induced by other mechanisms?

    1. Several things cause the system to collapse. When the stars are distant gravitational wave emission is very weak. In that case friction with interstellar gas and nudges by nearby stars will slowly push the two together. As they get closer gravitational radiation grows stronger and eventually takes over, being responsible for the bulk of their orbital decay once their speed hits a few percent that of light’s.

      For supermassive black holes this ‘swapping’ actually causes an issue, the “final parsec problem”.

  22. The idea that GW could in the future give an early warning of an impending NS merger is fascinating. Optical and radio instruments could then be oriented to pick up signals of the entire merger process. It’s not clear though what extra info could be gleaned from that. any ideas?

  23. The LIGO/VIRGO results paper had over 3,600 collaborative authors, think it’s safe to say it has been proofread. Given that we7ll soon be able to observe in greater detail hundreds of thousands to millions of galaxies, observations of these rare neutron star collisions should become more frequent. If we see the lightning and .0000000000000000002 seconds later we hear the thunder, it should tell us we’re close.

  24. At the beginning of your talk you state that a black hole is matter that collapses “endlessly”.Well,nothing in physics is endless(infinite),so please kindly be carefull because such statement is simply false.
    Yours sincerely

    1. You are absolutely correct. The problem that I faced when writing that statement is that making a statement which is both clear and correct is very difficult indeed. What happens to the matter as it collapses depends on the perspective of the observer, and also on whether the matter has angular momentum, charge, etc. Let me think about whether I can word this in a way that isn’t false. It’s very difficult to do it in a way that is strictly true, and I didn’t want to get involved in a complicated discussion of the subtle details of how black holes form.

      1. Hello Matt. There is another significant problem with supposing that black holes can crush down to an infinitesimal speck of zero volume. We know from considerations of the geometry of forces of massive bodies that the center gravitational forces are zero; the gravitational pull from the mass from one direction is countered by the pull from the opposite direction. The net result is zero. This is true for every object in inertial motion, a moon, planet, star, neutron star, and a black hole. When we realize the truth of this we face the problem of explaining how something could have the most massive gravity (spacetime curvature) at the same center point where gravity (spacetime curvature) should be, in fact, zero. This is a significant contradiction for those who think that a singularity means zero or near zero volume. In my understanding, a singularity is a mathematical issue only for those who do not understand how to think through the physics over a spacetime horizon. To wit, black holes must have a real physical size just like neutron stars.

        When we look for where felt gravity is strongest we find this to be surfaces that are sufficiently strong to stop another object that has reached the surface from penetrating through to the center. Objects with distant surfaces are essential in my view for understanding how any object can cause gravity (spacetime curvature). Even when looking at this from the perspective of the equivalence principle we are always dealing with extended objects.

    2. Can we say this collapse as, “until the known physics works” (equivalence principle) .. or
      … collapse of, “numerical equality between the inertial and Gravitational mass” – by the disturbance in the so called spacetime ?

  25. Would the energy of a black hole be sufficient to change all that enters into energy of one dimension?

  26. It is not clear to me why the gamma rays, in the bursts, arrive at the same time as the gravitations waves. In Supernova SN1987, neutrinos arrived earlier than light waves, which interacted with charged particles, on their way to earth. I should have expected the same thing to happen to the gamma ray bursts, versus gravitational waves. Apparently this is wrong. Are the gamma rays so energetic that charged particles aren’t “seen” by them, accounting for their simultaneous arrival?

    1. Yes, the gamma rays are energetic enough that they aren’t affected. But the reason that the light from Supernova 1987a was seen later than the neutrinos is not because the light was affected by charged particles. It’s that nobody was looking for it. It was seen when sharp-eyed astronomers, outdoors, noticed something was funny-looking in the Magellenic Cloud. With modern techniques the glow of the supernova would have been picked up earlier.

  27. Colin: Kilonovas aren’t that common, and most observed are very distant.
    The distance of the gravity wave source can be read directly and had a pattern indicating neutron star masses; the optical component had a similar direction and the same distance. I THINK the directional sensitivity is about 5 degrees (but I don’t know how axis affects that). The GRB and LIGO triangulations overlapped; the optical component was found within that. The GRB and LIGO observations were essentially simultaneous and the following observations all fit the same model for intensity and luminosity curve.

    It’s a good question, but It seems unlikely. Of course LIGO is building up frequency data, which will help to refine those odds. It might be a while, though, before we see another within the 300m light year region for such observations. Roll on more sensitive equipment.

  28. The neutron capture process for heavy nucleii does seem a bit monkeys and shotguns compared to the moderation by the weak force for a few hundred million years. We can look forward to new exotic processes for their formation in kilonovas.

    1. I am cautious about answering this. If you are asking: is space a medium for waves in just the same way that water is a medium for waves, the answer is: no. If you are asking: is space a medium for waves, though in a way that does not, in key features, resemble water, then the answer is yes. Space is, in a sense, a medium, but it’s not like any one you’ve ever seen or felt. In particular, the speed of the waves as they pass you is the same — equal to the cosmic speed limit — no matter how you choose to move. That is not true of water waves.

  29. How likely would it be to see something similar to a kilonova somewhere in the regions they were looking if it was just any random night and not immediately following the gravitational wave detection. I’m trying to understand how strong the evidence is that the electromagnetic detection is the same event as the gravitational wave detection.

    1. Hi Colin, very unlikely. But the key is the combination of several things. First, a gamma ray burst signal at almost the same instant as the peak of the gravitational wave signal. Second, a bright transient in the rough region of the sky that behaves somewhat like other post-gamma-ray-burst afterglows. Third, the moderately good agreement of the afterglow (especially in visible and infrared light) with previous theory on how neutron star mergers ought to behave. Fourth, no other reasonable candidate gamma ray burst at around that time form that region of the sky.

        1. Short GRBs (as GRB170817A was) have a rate of roughly one per galaxy per million years (LIGO quotes the high end of their visible NS-NS merger rate at about 10k Gpc^-1 yr^-1). The combined LIGO/VIRGO/Fermi/et al. paper (https://arxiv.org/abs/1710.05834) estimated the probability of a chance coincidence at 5 x 10^-8.

      1. Perhaps the likeliness combination to look at is first the odds of an approximate coincidence between a GW signal and a GRB signal, this is fairly likely and has happened for instance to the first GW signal detected. Then the odds that the triangulation of the GW and the GRB overlap. And then the odds of observing an optical transient roughly in that area. The latter might be depending on how strong the previous evidence on short GRBs are associated with kilonovas. That would leave the overall likeliness basically to the chance of GRB and GW signal coinciding temporally within seconds and with overlapping rough source direction after enough interferometer observación time.

  30. Thank you. I shared this widely and even people who are not usually interested in this subject are engaged and amazed.
    Also, the links you provided, “in computer simulations such as this one and this one.” did not work. Thanks again.

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