Of Particular Significance

A Scientific Breakthrough! Combining Gravitational and Electromagnetic Waves

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 10/16/2017

Gravitational waves are now the most important new tool in the astronomer’s toolbox.  Already they’ve been used to confirm that large black holes — with masses ten or more times that of the Sun — and mergers of these large black holes to form even larger ones, are not uncommon in the universe.   Today it goes a big step further.

It’s long been known that neutron stars, remnants of collapsed stars that have exploded as supernovas, are common in the universe.  And it’s been known almost as long that sometimes neutron stars travel in pairs.  (In fact that’s how gravitational waves were first discovered, indirectly, back in the 1970s.)  Stars often form in pairs, and sometimes both stars explode as supernovas, leaving their neutron star relics in orbit around one another.  Neutron stars are small — just ten or so kilometers (miles) across.  According to Einstein’s theory of gravity, a pair of stars should gradually lose energy by emitting gravitational waves into space, and slowly but surely the two objects should spiral in on one another.   Eventually, after many millions or even billions of years, they collide and merge into a larger neutron star, or into a black hole.  This collision does two things.

  1. It makes some kind of brilliant flash of light — electromagnetic waves — whose details are only guessed at.  Some of those electromagnetic waves will be in the form of visible light, while much of it will be in invisible forms, such as gamma rays.
  2. It makes gravitational waves, whose details are easier to calculate and which are therefore distinctive, but couldn’t have been detected until LIGO and VIRGO started taking data, LIGO over the last couple of years, VIRGO over the last couple of months.

It’s possible that we’ve seen the light from neutron star mergers before, but no one could be sure.  Wouldn’t it be great, then, if we could see gravitational waves AND electromagnetic waves from a neutron star merger?  It would be a little like seeing the flash and hearing the sound from fireworks — seeing and hearing is better than either one separately, with each one clarifying the other.  (Caution: scientists are often speaking as if detecting gravitational waves is like “hearing”.  This is only an analogy, and a vague one!  It’s not at all the same as acoustic waves that we can hear with our ears, for many reasons… so please don’t take it too literally.)  If we could do both, we could learn about neutron stars and their properties in an entirely new way.

Today, we learned that this has happened.  LIGO , with the world’s first two gravitational observatories, detected the waves from two merging neutron stars, 130 million light years from Earth, on August 17th.  (Neutron star mergers last much longer than black hole mergers, so the two are easy to distinguish; and this one was so close, relatively speaking, that it was seen for a long while.)  VIRGO, with the third detector, allows scientists to triangulate and determine roughly where mergers have occurred.  They saw only a very weak signal, but that was extremely important, because it told the scientists that the merger must have occurred in a small region of the sky where VIRGO has a relative blind spot.  That told scientists where to look.

The merger was detected for more than a full minute… to be compared with black holes whose mergers can be detected for less than a second.  It’s not exactly clear yet what happened at the end, however!  Did the merged neutron stars form a black hole or a neutron star?  The jury is out.

At almost exactly the moment at which the gravitational waves reached their peak, a blast of gamma rays — electromagnetic waves of very high frequencies — were detected by a different scientific team, the one from FERMI. FERMI detects gamma rays from the distant universe every day, and a two-second gamma-ray-burst is not unusual.  And INTEGRAL, another gamma ray experiment, also detected it.   The teams communicated within minutes.   The FERMI and INTEGRAL gamma ray detectors can only indicate the rough region of the sky from which their gamma rays originate, and LIGO/VIRGO together also only give a rough region.  But the scientists saw those regions overlapped.  The evidence was clear.  And with that, astronomy entered a new, highly anticipated phase.

Already this was a huge discovery.  Brief gamma-ray bursts have been a mystery for years.  One of the best guesses as to their origin has been neutron star mergers.  Now the mystery is solved; that guess is apparently correct. (Or is it?  Probably, but the gamma ray discovery is surprisingly dim, given how close it is.  So there are still questions to ask.)

Also confirmed by the fact that these signals arrived within a couple of seconds of one another, after traveling for over 100 million years from the same source, is that, indeed, the speed of light and the speed of gravitational waves are exactly the same — both of them equal to the cosmic speed limit, just as Einstein’s theory of gravity predicts.

Next, these teams quickly told their astronomer friends to train their telescopes in the general area of the source. Dozens of telescopes, from every continent and from space, and looking for electromagnetic waves at a huge range of frequencies, pointed in that rough direction and scanned for anything unusual.  (A big challenge: the object was near the Sun in the sky, so it could be viewed in darkness only for an hour each night!) Light was detected!  At all frequencies!  The object was very bright, making it easy to find the galaxy in which the merger took place.  The brilliant glow was seen in gamma rays, ultraviolet light, infrared light, X-rays, and radio.  (Neutrinos, particles that can serve as another way to observe distant explosions, were not detected this time.)

And with so much information, so much can be learned!

Most important, perhaps, is this: from the pattern of the spectrum of light, the conjecture seems to be confirmed that the mergers of neutron stars are important sources, perhaps the dominant one, for many of the heavy chemical elements — iodine, iridium, cesium, gold, platinum, and so on — that are forged in the intense heat of these collisions.  It used to be thought that the same supernovas that form neutron stars in the first place were the most likely source.  But now it seems that this second stage of neutron star life — merger, rather than birth — is just as important.  That’s fascinating, because neutron star mergers are much more rare than the supernovas that form them.  There’s a supernova in our Milky Way galaxy every century or so, but it’s tens of millenia or more between these “kilonovas”, created in neutron star mergers.

If there’s anything disappointing about this news, it’s this: almost everything that was observed by all these different experiments was predicted in advance.  Sometimes it’s more important and useful when some of your predictions fail completely, because then you realize how much you have to learn.  Apparently our understanding of gravity, of neutron stars, and of their mergers, and of all sorts of sources of electromagnetic radiation that are produced in those merges, is even better than we might have thought. But fortunately there are a few new puzzles.  The X-rays were late; the gamma rays were dim… we’ll hear more about this shortly, as NASA is holding a second news conference.

Some highlights from the second news conference:

  • New information about neutron star interiors, which affects how large they are and therefore how exactly they merge, has been obtained
  • The first ever visual-light image of a gravitational wave source, from the Swope telescope, at the outskirts of a distant galaxy; the galaxy’s center is the blob of light, and the arrow points to the explosion.

  • The theoretical calculations for a kilonova explosion suggest that debris from the blast should rather quickly block the visual light, so the explosion dims quickly in visible light — but infrared light lasts much longer.  The observations by the visible and infrared light telescopes confirm this aspect of the theory; and you can see evidence for that in the picture above, where four days later the bright spot is both much dimmer and much redder than when it was discovered.
  • Estimate: the total mass of the gold and platinum produced in this explosion is vastly larger than the mass of the Earth.
  • Estimate: these neutron stars were formed about 10 or so billion years ago.  They’ve been orbiting each other for most of the universe’s history, and ended their lives just 130 million years ago, creating the blast we’ve so recently detected.
  • Big Puzzle: all of the previous gamma-ray bursts seen up to now have always had shone in ultraviolet light and X-rays as well as gamma rays.   But X-rays didn’t show up this time, at least not initially.  This was a big surprise.  It took 9 days for the Chandra telescope to observe X-rays, too faint for any other X-ray telescope.  Does this mean that the two neutron stars created a black hole, which then created a jet of matter that points not quite directly at us but off-axis, and shines by illuminating the matter in interstellar space?  This had been suggested as a possibility twenty years ago, but this is the first time there’s been any evidence for it.
  • One more surprise: it took 16 days for radio waves from the source to be discovered, with the Very Large Array, the most powerful existing radio telescope.  The radio emission has been growing brighter since then!  As with the X-rays, this seems also to support the idea of an off-axis jet.
  • Nothing quite like this gamma-ray burst has been seen — or rather, recognized — before.  When a gamma ray burst doesn’t have an X-ray component showing up right away, it simply looks odd and a bit mysterious.  Its harder to observe than most bursts, because without a jet pointing right at us, its afterglow fades quickly.  Moreover, a jet pointing at us is bright, so it blinds us to the more detailed and subtle features of the kilonova.  But this time, LIGO/VIRGO told scientists that “Yes, this is a neutron star merger”, leading to detailed study from all electromagnetic frequencies, including patient study over many days of the X-rays and radio.  In other cases those observations would have stopped after just a short time, and the whole story couldn’t have been properly interpreted.



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46 Responses

  1. Thanks Michael Kelsey for explanation. I had a cursory glance at some of the papers. But I am still not very clear. I would appreciate it very much if Matt has some time and writes a blog about it. I think this is the most unusual case where you know amplitude at the emission point and absorption point, except perhaps supernovas where we have theory how the supernovas came about.

    1. There are a few other cases where we can (we think) calculate the absolute magnitude and use that to get a direct distance estimate. Astronomers (which I am not!) call these cases “standard candles.”

      The two most well known are the Cepheid variable stars, which have a relationship between their period of oscillation and their absolute luminosity, as discovered by Henrietta Leavitt and used by Hubble to infer the distances to other galaxies; and the Type Ia supernovae, which you mentioned yourself.

      The latter are white dwarves which accrete matter from a companion until they explode; it appears that Type Ia’s (to first order) all generate more or less the same total energy output (absolute luminosity), which allows us to compute their distances directly.

      What’s new about gravitational waves is that the systems which generate them (at least, the systems which we can observe) are simple enough (two compact objects in close orbit) that we can compute the amplitude very precisely! Most of the uncertainty in the distance estimate (for GW170817 it was 40 +/- 8 Mpc, a 20% error bar) comes from the uncertainty in estimating the masses of the system from the signal, not from the calculation itself.

  2. It is too bad that very few people have Heard of “OSM,” the Oscillators in a Substance Mode which does a quite good job of explaining everything except the fact of Essence itself. The following vornado rtr based on that Model.
    1. 1. Electromagnetic and “gravitational “ waves are the sase type of phenomenon. If ultra low frequency , and, probably caused in part, by effects of the “One Force” in its guise as Gravity, are called gravitational waves,
    2.:Neutron” stars moodle best as grossly oversized atoms, which probably contain no neutros as all, and always spew some heavy atoms . A collision would spill a lot!
    3 “black Hole collisions” the merging of the centers of mass of galaxies, would be expected to give a quick click, that merger would be the end of a long process but very fast. (Black holes re a fanciful misinterpretation of the ions surrounding the centers of mass of galaxies.)
    3.Sorry, Matt, butt Einstein’s gr3at break through was gross mistake which has led scientists astray of rovere a century.
    4. The people who did the Experiment of 1890 did not realize the true significance of the speed of light as an average velocity of information criers and got us started on the “Photons- leaking-through- nothing nonsense..
    5, Space-Time mathematics is actually a kind of “Substance model The “Substance” in which there are distortions is called “Space-Time.”
    7. Sound and light are both compression Qcw phenomena =,They simply operate in different portions of reakutt,,
    That should be enough heresy…..

  3. Matt,
    How do “: the gravitational waves directly provide the distance to the galaxy”. The only way I can think of is if you know the amplitude exactly at the point of emmision. We know the amplitude here on earth. Thanks for resuming your blog.

    1. That’s exactly how it works. The orbital information (the period of the waves) gives you information about the masses involved. Once you have the masses, it is possible* to calculate the absolute amplitude of the generated gravitational waves. That amplitude falls off like 1/R (not 1/R^2); thus, the amplitude at detection gives you a direct measurement (with some uncertainty, about 20% for GW170817) of the distance.

  4. I am lsuspicious of result that fulfill prophecies. There is too much of current theory that seems to have been based on self fulfilling prophecies, Black Hokes, most of the Standard Model, (Including the Higgs) even neutrons in atomic nuclei are some examples.. o
    Signed, Dean LeRoy, Sinclair , PhD Author of
    The Physicists’ Grail, Harvard Book Store Press, 2017

    1. Dean,
      As I recall, many of the creators/discovers of QM and GR/SR hated some of the results that fell out from research, including their own. Was nature forcing itself on them, not the other way around? I think you are being too critical.(Although it is true that data starved eras do result in many fanciful models) But that’s “life in the big city”!

  5. Huge breakthrough! Almost worth a second Nobel…

    About ”hearing” gravitational waves – it isn’t such a bad analogy really as the frequencies are audible. Which raises the question, If you happened to be close enough to a merger, would the gravitational waves deposit enough energy in your ears that you could actually hear it?

  6. Thanks for your post, Professor.
    It’s really impressive and clarifying.
    I wonder, though, does the fact ( now confirmed ) of coincidence of GW and EM-waves somehow follow from Einstein’s General Theory of Relativity ?
    Yours, bob

    1. The coincidence between GR and EW waves refers to their propagation speed (“c”) and it is indeed a prediction of Einstein’s theory. The topic is covered in many textbooks on GR and gravitational waves.

  7. Fascinated by all this. But I am still struggling with the concept of gravitational waves. First of all, LIGO describes it as ” ‘ripples’ in the fabric of space-time”, but what exactly is rippling here physically still eludes my understanding. Is that space-time itself ripples or something physical within this space-time is rippling? And more basically, what is meant by ‘rippling’?

    1. It’s a bit like a sound wave. When sound emanates from a source, a train of alternating high-pressure and low-pressure regions travels through air. If you stand in one spot, your ear vibrates as it is pushed by the high and low pressure as the sound wave passes by.

      For gravitational waves, it is compressed and expanded regions of space that travel away from the source. For a detector, imagine two rocks out in space at rest a small distance from each other. As a gravitational wave passes by, the space between the rocks alternately compresses and expands, causing the rocks to oscillate closer and farther from each other. This changing distance is what LIGO detects, only they use mirrors instead of rocks so they can use lasers to measure how far they move..

      1. Sure, that’s the part I understand. What I don’t get is that usually for waves to travel, you need a medium. For sound it is air (or a gas, more generally), so what is the equivalent medium for gravity waves, would it somehow require empty space not to be empty at all but filled with matter/energy/virtualparticles through which the waves travel, or is this completly wrong type of thinking here?

        1. The medium of gravitational waves is spacetime itself. In General Relativity, gravity is described as the effect of spacetime that is curved by nearby masses. Spacetime is a thing that is acted upon, not just a static background. As the configuration of masses changes, so does the curvature of space. If you have a configuration of masses that exhibits periodic motion (orbits, for example), then the changes in curvature will also be periodic (i.e., oscillating). These oscillations can then propagate outward at the speed of light (they travel at a finite speed since instant communication is impossible). These are gravitational waves.

          1. Just wanted to confirm for other readers that Mark H is correct. The great breakthrough of Einstein was to realize that no material medium is needed for either electromagnetic waves or for gravitational waves. It’s hugely counterintuitive, but there’s a reason why it took Einstein to figure it out… it’s not at all obvious that waves without an ordinary medium are possible. But it is true.

          2. As per quantum physics, instantaneous communication is possible, as in quantum entanglement.
            GR describes the evolution of the spacetime continuum, and the events embedded within it, and not an actual movement “through” a preexisting, static, distorted, space. The movement “through” a preexisting space is an illusion. (As per Einstein, “Reality is merely an illusion, albeit a very persistent one.”)
            As the continuum evolves forward, apparent curvature of motion is manifested due to time dilation.
            In a gravitational field, i.e., time dilation gradient, an outside observer perceives the next instant manifesting first in the faster time frames and then shifting down the dilation gradient. This evolves events within the continuum down gradient. This is the “force” of gravity.
            Einstein’s fundamental metric in GR describes the straight line forward evolution of events in a null gravitational field. This is the forward evolution of events when there is no time dilation (this state only exists for us in our personal sense of the invariant rate of time we each experience in our own inertial frames of reference as per SR, or worldlines in GR).
            The curvature of motion we perceive as per GR is the resultant of this straight line evolution and the down gradient evolution induced by time dilation.
            Gravity waves are merely a slight acceleration in the rate of time shifting through the spacetime continuum. This acceleration shortens the length of a meter (to maintain c) and this distorts the shape of LIGO’s antenna, which then sets off a signal.

          3. Thanks for taking the time to reply Mark and Matt. I could also try to understand it by comparing gravitational waves to light waves (or electromagnetic radiation, in general) instead of p-Waves in a medium,. These do not require a medium but they require the existence of a particle called photon. If gravitational waves do not require (a physical) medium, they still require a ‘force carrier’ like a graviton, right? Wouldn’t then the proof of existence of graviational waves be indirect proof of the existence of gravitons?
            PS: I am certainly no Einstein, so I have issues imagining a ‘curvature in nothing’, i.e. a curvature without a physical surface (or boundary) of a physical object. In fact that is the way curvature is normally illustrated – with the surface of a physical sphere or torus, etc.

  8. Thanks Doc. You are always way ahead of the news media and far more explanatory. It seems to me that since we cannot see the resultant after the stars merge, that they have become a black hole. This means most of their energy (information) is lost to us. The emissions would only be occurring prior to the merger. This could possibly explain the weakness of the gamma rays and different timing of the x-rays.

  9. Matt, how does the heavy material (like gold, etc.) get out into the Universe where it can be incorporated into planetary bodies that are being formed in new solar systems? I can understand a supernova blasting out material but it seems like in this case, 2 neutron stars smash together, create a lot of heavy elements, then turn into a black hole which sucks all those elements in and that’s the end of it.

  10. I read in The Guardian report that at the moment of merger LIGO briefly measured no gravitational wave disturbance, unlike the waves detected prior to and following the merger. Is this something expected in the models of this kind of merger, or something unusual? Is this additional evidence pointing to a black hole as the result of the merger?

  11. How do astronomers define X-rays vs. gamma rays? I’m coming from particle physics where these terms mostly overlap.

        1. Hi, Mark. I come from an HEP background and this confused me as well. An expert (from the Geant4 group) on electromagnetic interactions explained it to me as: X-rays come from electron behaving semi-classically (cyclotron emission, bremsstrahlung, close-in orbitals, like K- or L- shells), while gamma rays come from nulcear or particle interactions (nuclear excited states, e+/e- annihilation, etc.). The distinction you gave about imaging vs. counting makes a lot of sense, too.

  12. is possible measure the nêutron eletric dípole momentum,explaining the conservation of cp for strong interaction Given by hidden symmetry or news spacetime originated by the violation of PtT and does appear the antiparticles as asymmetries of the matter and energy

  13. the gravitational Waves solve not the trouble of the Gravity ,and its origins,and relations with the weakness of gravitational force what would lead to the naked singulaty.because appear as particles and antiparticles are originated of differents axions,that could implying the called dark matter and energy.the conservation of cp for strong interactions are measured by hidden symmetry,that is the violation of PT that does the connection of space and time into spacetime continuos and the “constant” speed of light,that implies symmetrically non linear electromagnetics fields.the gravity and eletrocmagnetic forces are dual,it is asymmetrics

  14. Yes. As I have read that gravitational waves are distortions in spacetime, so they are ofcourse not sound waves. But as you answered that sound waves do give us analogy to feel what is going on.

    So these sound waves do not give any additional information about the collision?
    In some simulations Earth is shown to be distorting (exaggerated) due to passing gravitational waves. And If I were very close to the source of the gravitational waves, I would also be distorted along with the distorting spacetime. Then I won’t have to listen to the converted sound waves to feel what is going on. 🙂

    1. The gravitational waves from the collision DO give us an enormous amount of new information about the collision. The electromagnetic radiation mostly comes from after the collision, and was’t even detected for many hours (because the telescopes weren’t informed instantly and then weren’t able to look for a while); only the gamma rays seen by FERMI and INTEGRAL came directly from the collision. By contrast, the gravitational waves are seen before, during, and slightly after the collision, and they provide lots of information about the two neutron stars that would otherwise be completely impossible to know. Even to believe that the gamma-ray burst was from a neutron star merger would be pure guesswork — as it has been for decades of gamma-ray burst studies. Only gravitational waves make it clear.

      And yes, you would be noticeably distorted by the gravitational waves if you were sufficiently close. You would be stretched one way and compressed the other, like gum — and then the reverse, back and forth. You’d have to be so close, though, that you’d be fried by all of the high-energy electromagnetic radiation!

  15. I was wondering whether the gravitational waves are converted into sound waves just to give no-specialists a feeling of how the compact stars collide or it really helps in determining few things about the collision.

    1. You mean, why scientists are speaking of them as though they are sound waves? There are a couple of reasons. I think you’re right that it is about conveying that it is a form of vibration generated by spinning objects and by their collision — on earth we’d expect to hear sound. So it somehow feels analogous. I think much of it is also about trying to convey that this, like sound, is a type of wave you can’t see, and its wavelength is large. The important thing is that is something that isn’t light (or more general electromagnetic waves) but which also carries information about its source. In any case, it is definitely confusing many non-experts, who have long been taught (correctly, of course) that there is no sound in outer space (because sound, unlike gravitational waves, must travel through an ordinary medium, while there’s ordinary about space-time.)

  16. Fascinating! That said – there was one sentence I did not understand. You wrote: But now it seems that this second stage of neutron star life — merger, rather than birth — is just as important.” Since the merger of binary neutron stars is comparatively rare, how could this source of heavy elements be “just as important”? Thanks Matt as always!

    1. The point is, apparently, that the mergers are much more efficient than supernovas in creating heavy elements… making up for the rarity. This is not an area that I know much about, unfortunately, so I can’t give any direct insights. Heavy element nuclei have lots of neutrons, so I’m sure it helps to start out with huge piles of neutrons in the first place.

  17. This is a great Monday morning! The discovery of gavitional waves was historic, but with the resources available to correlate the EM Wave to the gravitational wave is huge for astrophysics and QM. This discovery seems to provide evidence that gravity “Spacetime” is faster than light or that Gravity slows light. Either way it’s exciting! We’ve seen the creation of matter and the sequence of events that leading up to it! I’ve been watching and reading from LBL, Caltech, MIT, NASA, ESA, and others.

      1. Prof. Strassler,

        How would the LIGO discoveries impact the (too many, perhaps) so-called “alternative” theories of gravitation?

        1. Each LIGO measurement so far closely agrees with general relativity, so certain kinds of deviations are now more tightly excluded. It’s hard to say for a theory like MOND, because the details of that theory aren’t made sufficiently clear in the context of something as complex as black hole or neutron star mergers.

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