Tag Archives: black holes

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

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.

BASIC QUESTIONS ABOUT NEUTRON STARS, BLACK HOLES, AND MERGERS

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.)

THE SIGNIFICANCE(S) OF YESTERDAY’S ANNOUNCEMENT OF A NEUTRON STAR MERGER

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.

MISCELLANEOUS QUESTIONS

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.

 

 

 

LIGO and VIRGO Announce a Joint Observation of a Black Hole Merger

Welcome, VIRGO!  Another merger of two big black holes has been detected, this time by both LIGO’s two detectors and by VIRGO as well.

Aside from the fact that this means that the VIRGO instrument actually works, which is great news, why is this a big deal?  By adding a third gravitational wave detector, built by the VIRGO collaboration, to LIGO’s Washington and Louisiana detectors, the scientists involved in the search for gravitational waves now can determine fairly accurately the direction from which a detected gravitational wave signal is coming.  And this allows them to do something new: to tell their astronomer colleagues roughly where to look in the sky, using ordinary telescopes, for some form of electromagnetic waves (perhaps visible light, gamma rays, or radio waves) that might have been produced by whatever created the gravitational waves. Continue reading

LIGO detects a second merger of black holes

There’s additional news from LIGO (the Laser Interferometry Gravitational Observatory) about gravitational waves today. What was a giant discovery just a few months ago will soon become almost routine… but for now it is still very exciting…

LIGO got a Christmas (US) present: Dec 25th/26th 2015, two more black holes were detected coalescing 1.4 billion light years away — changing the length of LIGO’s arms by 300 parts in a trillion trillion, even less than the first merger observed in September. The black holes had 14 solar masses and 8 solar masses, and merged into a black hole with 21 solar masses, emitting 1 solar mass of energy in gravitational waves. In contrast to the September event, which was short and showed just a few orbits before the merger, in this event nearly 30 orbits over a full second are observed, making more information available to scientists about the black holes, the merger, and general relativity.  (Apparently one of the incoming black holes was spinning with at least 20% of the maximum possible rotation rate for a black hole.)

The signal is not so “bright” as the first one, so it cannot be seen by eye if you just look at the data; to find it, some clever mathematical techniques are needed. But the signal, after signal processing, is very clear. (Signal-to-noise ratio is 13; it was 24 for the September detection.) For such a clear signal to occur due to random noise is 5 standard deviations — officially a detection. The corresponding “chirp” is nowhere near so obvious, but there is a faint trace.

This gives two detections of black hole mergers over about 48 days of 2015 quality data. There’s also a third “candidate”, not so clear — signal-to-noise of just under 10. If it is really due to gravitational waves, it would be merging black holes again… midway in size between the September and December events… but it is borderline, and might just be a statistical fluke.

It is interesting that we already have two, maybe three, mergers of large black holes… and no mergers of neutron stars with black holes or with each other, which are harder to observe. It seems there really are a lot of big black holes in binary pairs out there in the universe. Incidentally, the question of whether they might form the dark matter of the universe has been raised; it’s still a long-shot idea, since there are arguments against it for black holes of this size, but seeing these merger rates one has to reconsider those arguments carefully and keep an open mind about the evidence.

Let’s remember also that advanced-LIGO is still not running at full capacity. When LIGO starts its next run, six months long starting in September, the improvements over last year’s run will probably give a 50% to 100% increase in the rate for observed mergers.   In the longer run, the possibility of one merger per week is possible.

Meanwhile, VIRGO in Italy will come on line soon too, early in 2017. Japan and India are getting into the game too over the coming years. More detectors will allow scientists to know where on the sky the merger took place, which then can allow normal telescopes to look for flashes of light (or other forms of electromagnetic radiation) that might occur simultaneously with the merger… as is expected for neutron star mergers but not widely expected for black hole mergers.  The era of gravitational wave astronomy is underway.

Giving two free lectures 6/20,27 about gravitational waves

For those of you who live in or around Berkshire County, Massachusetts, or know people who do…

Starting next week I’ll be giving two free lectures about the LIGO experiment’s discovery of gravitational waves.  The lectures will be at 1:30 pm on Mondays June 20 and 27, at Berkshire Community College in Pittsfield, MA.  The first lecture will focus on why gravitational waves were expected by scientists, and the second will be on how gravitational waves were discovered, indirectly and then directly.  No math or science background will be assumed.  (These lectures will be similar in style to the ones I gave a couple of years ago concerning the Higgs boson discovery.)

Here’s a flyer with the details:  http://berkshireolli.org/ProfessorMattStrasslerOLLILecturesFlyer.pdf

Advance Thoughts on LIGO

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…

The Black Hole’s Tale

[Inspiration strikes in odd ways and at strange times.  Don’t ask me why I wrote this, because I’ve no idea.  In any case I hope some of you enjoy it; and the science behind it is described here.]

Quantum Theory claims: “All tales are told!”
But gravity demurs; for Einstein’s bold
Equations show that black holes tell no tales
And keep their secrets hidden deep within.

So it remained til nineteen seventy-four
When Hawking’s striking calculation showed
That black holes aren’t exactly black: they glow!
They shrink, wither, and in a flash they die
And take their hidden secrets to their graves,
Killing Quantum Theory as they go.

And if you disagreed, and did believe
that black holes’ tales are written in their glow,
No matter; this kills Quantum Theory too,
For once inside, a story can’t come out,
And copying puts a quantum world in doubt.

Thus Hawking argued that he’d made it clear
That Quantum Theory had to be revised.
“But not so fast” cried Susskind and ‘t Hooft,
For Quantum Theory’s cleverer than you think;
T’was twenty years ago the claim was made
That black holes may be complementary:
While those who venture in do find the tales
Are written clearly in the black hole’s deep,
Those outside have a very different view.
They think the stories rest upon the edge
And later end up written in the sky.

So strange this sounds! And yet, it has been shown
That in a quantum world of certain type
The information stored within a space
Can also seem to be upon its face!

Consensus grew that Quantum Theory’s safe
And even Hawking painfully agreed
The argument was strong; nine years ago
He publicly announced his change of heart.

But still it wasn’t yet precisely clear
Just how it is that black holes disappear
Without undoing Quantum Theory’s base;
And then the AMPS collaboration found,
While trying to ensure the case was sound,
The complementary black hole in fact
Could not exist! At least not as we thought,
For when the tale’s half written in the sky
The black hole’s inside could no longer be,
And anyone who reached the edge would die.

“Firewall”, the cry rose from the crowd;
And troubling it was; such walls would flout
The principles that Einstein had set out
To underpin his theory of space and time
And gravity — the very one we used
To show black holes exist, and find them too.

So something’s wrong! But what? What must we change?
Which principle is it that we must revise?
Which equation fails, and in what guise?
Confusion spreads across the blackened skies…

Proposals have been made, but none is firm.
Among them Hawking’s recent; he suggests
A black hole’s even less black than he thought:
Not only does it faintly glow, it leaks
Like politicians, whispering its tales
In code; and thus whatever is inside
Gets out! Though in a highly scrambled form.
(So do not try to enter and return!)
These holes aren’t complementary; instead
Their inner stories are somehow released
Before the holes that store them are deceased.

But be not sure; for Hawking’s story’s vague
And many others have suggested ways
That current controversy may be stemmed.
Yet none of them seem likely soon to lift
The murky darkness that still makes us blind
And hides the truth from all of humankind.

© Matt Strassler February 5, 2014

How Black is a Black Hole? An Introduction to the Paradoxes

Following on Thursday’s post and yesterday’s about black holes, specifically about Hawking’s recent vague proposal that was so widely (but rather misleadingly) reported in the media, and about the back-story which explains why there’s so much confusion about black holes among scientists interested in quantum gravity, and why Hawking made his suggestion in the first place, I’ve been motivated to write up a new introduction to the black hole information paradox.  This should provide the basic knowledge and the context that I’m sure many of you are looking for.  Please take a look and send comments!

Learning Lessons From Black Holes

My post about what Hawking is and isn’t saying about black holes got a lot of readers, but also some criticism for having come across as too harsh on what Hawking has and hasn’t done. Looking back, I think there’s some merit in the criticism, so let me try to address it and flesh out one of the important issues.

Before I do, let me mention that I’ve almost completed a brief introduction to the “black hole information paradox”; it should be posted within the next day, so stay tuned for that IT’S DONE!  It involves a very brief explanation of how, after having learned from Hawking’s 1974 work that black holes aren’t quite black (in that they slowly radiate particles), physicists are now considering whether black holes might even be less black than that (in that they might slowly leak what’s gone inside them, in scrambled form.)

Ok. One of the points I made on Thursday is that there’s a big difference between what Hawking has written in his latest paper and a something a physicist would call a theory, like the Theory of Special Relativity or Quantum Field Theory or String Theory. A theory may or may not apply to nature; it may  or may not be validated by experiments; but it’s not a theory without some precise equations. Hawking’s paper is two pages long and contains no equations. I made a big deal about this, because I was trying to make a more general point (having nothing to do with Hawking or his proposal) about what qualifies as a theory in physics, and what doesn’t. We have very high standards in this field, higher than the public sometimes realizes.

A reasonable person could (and some did) point out that given Hawking’s extreme physical disability, a short equation-less paper is not to be judged harshly, since typing is a royal pain if you can’t even move. I accept the criticism that I was insensitive to this way of reading my post… and indeed I thereby obscured the point I was trying to make.  I should have been more deliberate in my writing, and emphasized that there are many levels of discussions about science, ranging across cocktail party conversation, wild speculation over a beer, a serious scientific proposal, and a concrete scientific theory. The way I phrased things obscured the fact that Hawking’s proposal, though short of a theory, still represents serious science.

But independent of Hawking’s necessarily terse style, it remains the case that his scientific proposal, though based on certain points that are precise and clear, is quite vague on other points… and there are no equations to back them up.  Of course that doesn’t mean the proposal is wrong!  And a vague proposal can have real scientific merit, since it can propel research in the right direction. Other vague proposals (such as Einstein’s idea that “space and time must be curved”) have sometimes led, after months or years, to concrete theories (Einstein’s equations of “General Relativity”, his theory of gravity.) But many sensible-sounding vague proposals (such as “maybe the cosmological is zero because of an unknown symmetry”) lead nowhere, or even lead us astray. And the reason we should be so sensitive to this point is that the weakness of a vague proposal has already been dramatically demonstrated in this very context.

The recent flurry of activity concerning the fundamental quantum properties of black holes (which unfortunately, unlike their astrophysical properties, are not currently measurable) arose from the so-called firewall problem. And that problem emerged, in a 2012 paper by Almheri, Marolf, Polchinski and Sully (AMPS, for short), from an attempt to put concrete equations behind a twenty-year-old proposal called “complementarity”, due mainly to Susskind, Thoracius and Uglom; see also Stephens, ‘t Hooft and Whiting.

As a black hole forms and grows, and then evaporates, where is the information about how it formed?  And is that information lost, copied, or retained? (Only if it is retained, and not lost or copied, can standard quantum theory describe a black hole.) Complementarity is the notion that the answer depends on the point of view of the observer who’s asking the question. Observers who fall into the black hole think (and measure!) that the information is deep inside. Observers who remain outside the black hole think (and measure!) that the information remains just outside, and is eventually carried off by the Hawking radiation by which the black hole evaporates.  And both are right!  Neither sees the information lost or copied, and thus quantum theory survives.

For this apparently contradictory situation to be possible, there are certain requirements that must be true. Remarkably, a number of these have been shown to be true (at least in special circumstances)! But as of 2012, some others still had not been shown. In short, the proposal, though fairly well-grounded, remained a bit vague about some details.

And that vagueness was the Achilles heel that, after 20 years, brought it down.

The firewall problem pointed out by AMPS shows that complementarity doesn’t quite work. It doesn’t work because one of its vague points turns out to have an inherent and subtle self-contradiction. [Their argument is far too complex for this post, so (at best) I’ll have to explain it another time, if I can think of a way to do so…]

By the way, if you look at the AMPS paper, you’ll see it too doesn’t contain many equations. But it contains more than zero… and they are pithy, crucial, and to the point. (Moreover, there are a lot more supporting equations than it first appears; these are relegated to the paper’s appendices, to keep the discussion from looking cluttered.)

So while I understand that Hawking isn’t going to write out long equations unless he’s working with collaborators (which he often does), even the simplest quantitative issues concerning his proposal are not yet discussed or worked out. For instance, what is (even roughly) the time scale over which information begins leaking out? How long does the apparent horizon last? It would be fine if Hawking, working this out in his head, stated the answers without proof, but we need to know the answers he has in mind if we’re to seriously judge the proposal. It’s very far from obvious that any proposal along the lines that Hawking is suggesting (and others that people with similar views have advanced) would actually solve the information paradox without creating other serious problems.

When regarding a puzzle so thorny and subtle as the black hole information paradox, which has resisted solution for forty years, physicists know they should not rely solely on words and logical reasoning, no matter how brilliant the person who originates them. Progress in this area of theoretical research has occurred, and consensus (even partial) has only emerged, when there was both a conceptual and a calculational advance. Hawking’s old papers on singularities (with Penrose) and on black hole evaporation are classic examples; so is the AMPS paper. If anyone, whether Hawking or someone else, can put equations behind Hawking’s proposal that there are no real event horizons and that information is redistributed via a process involving (non-quantum) chaos, then — great! — the proposal can be properly evaluated and its internal consistency can be checked. Until then, it’s far too early to say that Hawking’s proposal represents a scientific theory.