Hacker News new | past | comments | ask | show | jobs | submit login
Second Gravitational Wave Detected at LIGO (aps.org)
621 points by specialp on June 15, 2016 | hide | past | web | favorite | 174 comments

For those curious about the future of LIGO...

At present there are two LIGO facilities - one in Hanford, Washington and another in Livingston, Louisiana. This is necessary for both denoising (it's unlikely that a seismic event or random perturbation would effect both simultaneously) and triangulation via parallax.

Right now having just two facilities (that are relatively close together) limits localization to broad regions of the sky. Additional facilities are under way/in discussion for Europe, Japan, and India. This would significantly improve both the sensitivity of the array and its ability to localize events in a smaller region of the sky. Hopefully these projects get funded. LIGO stands to resolve some of the biggest open questions we have in cosmology.

There is also an effort to get these detectors in space with eLISA: http://physics.aps.org/articles/v9/63 This will be able to detect wave frequencies that would normally be drowned out by terrestrial sources.

I recommend the two animations on the wiki page of eLISA, they are worth a thousands words to understand the (crazy!) orbit of this contraption:


Wow, zero-drag satellites are a really elegant idea!


You must see umbrellas and parasols as elegant as well.

A precursor concept of a zero-drag satellite can be seen in Stargate SG1: "The Serpent's Venom", where an armed mine from an orbital minefield is brought aboard a shuttle, and the pilot must match the mine's trajectory changes.

"The Drag-free Satellite", the paper referenced in the Wikpedia article is from 1964, slightly older than SG1.

Also, your comment would be much nicer without the first sentence. And if you think about it, umbrellas are pretty neat. The folding mechanism is nifty, if you ask me.

look at this idiot, he can find beauty in elegant mechanisms!

It's so beautifully elegant to hold it aloft with your outstretched arm.

Wow, I didn't think it was possible to have three orbits that maintain an equilateral triangle like that. (I'm assuming the satellites only need to thrust against smaller influences like solar wind.)

If you like that orbit, look into the hopf fibration!

There is a current space experiment to measure the behavior of two elisa nodes. They are only a foot apart instead of a planned million kilometers. The purpose is a low cost in situ test of instruments. And also to survey the types of in situ instrumental and evironmental noise. It took decades to reduce LIGO noise to acceptable levels.

They should build one on the Moon.

One of the opening scenes from the transhuman adventure novel Diaspora by Greg Egan involves a robot on the moon tending a large laser interferometer which observes two inspiraling neutron stars. Perhaps my favourite book.

Egan's Quarantine had me checking out dozens of books about Quantum Mechanics while I was in High School. Love Egan's work, even if I had quite a bit of trouble with Diaspora (I probably just need to give the audiobook a second listen).

Love Diaspora, but it is really more of a philosophical thought experiment that drops you into two worlds rather than a novel with a plot. Half the book is geometry!

One of my favorite books as well. Every time I've gotten to the ending (especially the part where they discover the massive sculpture through trillions of universes) my mind is blown and I end up sitting and thinking about it for hours.

I have a similar reaction to Egan's Permutation City when the couple living in the simulation adjust the clock rate so the computer running them only executes one clock tick every second, every day, every year, every million years, every billion years, every trillion years, etc. Puts me in a contemplative mood.

But bad things happen on Earth after they coalesce. But that was in our galaxy. I guess it's just that these detected events are much farther away. Right?

Moonquakes exist too... I'd naively expect a lot less noise in space, and you don't have to worry about moon dust gumming up the works. And you can just lob three satellites up one at a time and arrange them more or less as you like, rather than having to deal with intervening geography.

The Moon also has seismic waves, albeit less than Earth. seismic waves have similar waveforms and frquencies as gravitational events. However the delays between two detectors is much larger for seismic since they propagate 50,000 times slower than gravitational waves.

terrestrial sources of gravitational waves?

Terrestrial sources of run-of-the-mill vibrations. The detector can pick up both.

One thing I don't understand in your post: given how ridiculously far away these black holes are, how could you possibly even dream of triangulating them with two facilities on Earth via parallax...?

You don't use parallax, rather you calculate the direction of the source by using the arrival times of the signals. The nearer detector will receive the signal slightly earlier by an amount that will typically be on the order of a few hundredths of a second. This is essentially the same method that geologists use to determine the epicenter of an earthquake.

This seems horribly prone to manipulation by cosmic radiation, gravity, and a million slower alterations like the rotation of the earth which when combined produce a noticeable effect.

The assumption here is that no force has acted on the energy arriving enough to distort two arrays on earth.

> seems horribly

No physicists here but you don't seem to be either. Irc from what had been said with the first event gravitational waves pass through matter and aren't disturbed like em waves, that's why they supposedly open new doors. Secondly, just take it at face value, if they say so it is so, it might seem improbable and "horribly prone to manipulation" but they probably thought of that and have machines precise enough to compensate for whatever effects you or me imagined e.g. "LIGO is designed to detect a change in distance between its mirrors 1/10,000th the width of a proton"[1]. It's like your grandma, never having touched computers, expressing opinions on the next release of some developer tool.


I think the assumption is that the paths taken are almost identical because the distance to the source is vastly longer than the distance between the detectors.

You do realise that gravitational waves are incredibly weakly interacting?

Also they travel at the speed of light, the rotation of the earth is pretty much constant (if not negligible).

The localization isn't performed through parallax, it's done through triangulation and time-of-arrival. For a pair of observatories, a difference in the arrival time of a signal (moving at the speed of light) defines a circle on the sky of possible source locations. With three observatories, there are three circles on the sky, which intersect at a single point.

The Virgo detector, outside of Pisa, is in the final stages of installing its advanced instrumentation. With LIGO+Virgo a source like the first detection, from September (a very loud event), could be localized to a patch of sky about 10 square degrees in area.

> 10 square degrees in area

Wow, that's much bigger than I expected. So basically we can only determine the general direction?

> So basically we can only determine the general direction?

With current-generation detectors installed at three stations which is all we can hope to achieve at the current level of funding in the very near future.

More money, more detections at lower energies, better idea where they come from. There are plenty of instruments in cosmology, astronomy, and physics that have already been designed and planned out, but which do not have the funding to be built; Wait 20 years and five percent of them might come to fruition. You could pour trillions of dollars on these problems without running out of novel questions that we've already proposed ways of answering, and novel results from exploratory instruments we've already proposed building. We spend about 30 billion a year on basic science research according to the NSF, spread over all fields. For comparison, the military gets upwards of 600 billion.

We have good ideas about how to make gravitational wave detectors much, much better, but not how to make them much, much cheaper.

Not so bad as that, but it is a large area, and it's a challenge for optical astronomers to detect a faint source in such a large patch of sky. It's about the size of your two palms held at arms length. (Or, 40 times the size of the full moon, but that sounds less optimistic.)

> > 10 square degrees in area

> So basically we can only determine the general direction?

10 square degrees is about 0.3% of the full spherical sky.

It looks 45 times as big as the sun.

Well, the goal of LIGO is to detect if gravity is a wave. So we had to build sensitive sensors, over large distances to be able to pull off this feet. It was (to my knowledge) only build to do this. The fact you can use it for triangulation is a bonus. If they wanted triangulation, I suspect they would have made it with 3 sensors instead of 2.

Correction to my post above:

I used the word "parallax" poorly to refer to the offset between the detectors - localization is most definitely not done via parallax. It's arrival time + some signal processing, so with two detectors, resolution is limited.

Additional details and an image depicting the improvements made by adding a third, properly positioned detector are available at: https://www.ligo.caltech.edu/news/ligo20160404

I believe you use timing differences, not parallax (it takes ~40 ms for gravitational waves to travel through Earth).

You don't definitely don't, you only get angular information for exactly the reason you say. The distance information comes (I believe) by first determining the masses, and then computing how fast the energy falls off with distance.

What are some of those big open questions in cosmology that this could answer?

Got one answer to my own question. Waves from the Big Bang would be a big deal in cosmology.

Ditto. We cant see past the photon soup of the cosmic microwave background of the first 380,000 years of creation using EM detectors.

This is the EU Virgo collaboration based in Tuscany, Italy. It is in the process of being upgraded:


how much does each facility cost (ballpark)?

>At this point, NSF has invested approximately $1.1 billion in construction and upgrades, in operational costs, and in research awards to individual scientists, who study LIGO data to learn more about our universe.


But it's important to point out that

1) That $1.1 billion was for two facilities

2) LIGO underwent a huge upgrade over the last decade, making the instruments tremendously more sensitive, and the cost of this upgrade is included in there as well.

So I suspect that building a third facility with the sensitivity of the two existing facilities would be a much more affordable endeavor. The first time around is always the most expensive.

"“It is very significant that these black holes were much less massive than those observed in the first detection,” Gabriela Gonzalez, LIGO's spokesperson, said in a statement. “Because of their lighter masses compared to the first detection, they spent more time—about one second—in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our Universe.”"

from http://arstechnica.com/science/2016/06/ligo-data-includes-at...

My wife has this really stupid (or brilliant?) question - if during the collision one solar mass turned into gravitational waves is it possible to create mass from gravitational waves?

In principle, yes. The Einstein field equations are time symmetric, so you could reverse the situation by pumping in gravitational waves from a great distance in a spherically symmetric way, and have them converge in some central region in such a way as to increase the mass of a black hole at the center, or have it split up into two more massive black holes.

Isn't it in principle equally difficult as generating concentrial waves on the surface of a swimming pool in such way as to eject a swimmer back on the podium where he jumped in from?

Flow Wave's tribute to LIGO: https://www.youtube.com/watch?v=qvU5ytghMdQ

That's an impressive swimming pool ;-)

So you could theoretically build a black whole generator by building a sphere that could emit gravitational waves with precision?

You'd need the sphere to be more than massive enough to collapse under its own gravity, into a black hole; such a structure wouldn't be hypothetically possible. Building black holes with anything other than stars or giant gas clouds (in the early universe) turns out to be hard; your black hole generators inevitably keep collapsing into black holes or at least neutron stars.

> You'd need the sphere to be more than massive enough to collapse under its own gravity, into a black hole; such a structure wouldn't be hypothetically possible.

I don't think that's technically true. You could build a bunch of catapults on the edge of the sphere, and when they all launch rocks at the center of the sphere, they would eventually form a black hole. The catapults could be arbitrarily far from each other as the radius of the sphere increases, such that they would not really do much to each other gravitationally. You'd just have to wait a real long time for the rocks to hit the middle.

> Building black holes with anything other than stars or giant gas clouds (in the early universe) turns out to be hard;

Well yes, galaxy-sized intelligently designed structures don't really happen.

I think by "Black hole generator" the other person meant a device which could create black holes remotely through some kind of process, not just a mass that collapses under its own gravity. In that sense, if you could find an old, spun-down neutron star (and man wouldn't that be a fun search! Massive, but tiny and dim...) then as you say, you could just keep adding mass until crush.

Any black hole generator that doesn't itself become a black hole is essentially throwing part of itself in the black hole (Yay conservation laws!). You can replace the catapults with lasers or plasma guns or whatever.

Sure, but I think the original commentator was imaging a massive sphere that could emit such powerful and precise gravitational waves, that you could create more black holes as a result. My point was just that any mass capable of achieving that, even hypothetically, would have long since collapses into a black hole.

I take your point however, that you could coordinate in some way separated masses, but at some point you'd probably run into issues with the aforementioned galactic-scale of engineering.

Some A+ structural integrity fields I suppose.

Oh yeah, with daily baryon sweeps to keep the tribbles out!

You don't need to emit gravitational waves, you can emit light that converges to a point.

This is amazing. How did you find this?

> How did you find this?

I found it extremely amazing as well! :p

Yes, it's almost an exactly analogous situation. The difficulty is that you're going against the second law of thermodynamics in both cases.

...Which is at least one going notion as to why the arrow of time exists at all.

That analogy was great. It does an awesome job at explaining the disproportionate amount of energy that needs to be focused at a point in space.

No. It's in principle as easy as using array of antennas designed to reinforce each others signals.

It's used in normal FM radio.

I think the practical limitation here is generating and concentrating/focusing the gravitational waves.

EDIT: And there would probably have to be some control of the relative phase of each component wave as well.

Hm, that's interesting. That means that a black hole could spontaneously disappear (or split into two) depending on its internal state. Observing that a black hole does not do this would then give you information about its internal state. This would be a counter-example to the no-hair conjecture, which would be a major breakthrough. So something must be wrong here. It can't be that simple.

No-hair is about stable black holes in a steady state. When (e.g.) two black holes merge to form a third, its transient state may be more complicated; I guess the "hair" decays exponentially.

(Disclaimer: Not an actual physicist.)

Yes, that's right. When two black holes first merge, the resulting black hole has a sort of peanut shape which contains lots of information (i.e., hair). Such a black hole is not stable, however, and emits lots of gravitational waves that carry all this information away (a process known as balding).

Thanks! But why then is that not a general solution to the black hole information paradox?

The paradox is an almost wholly thoretical problem. Our civilization will never observe even a stellar black hole evaporating, and UMBHs might never evaporate even in the enormously far future limit where their Hubble volumes are effectively de Sitter vacuum.

If a black hole is always colder than the relic and cosmological horizon radiation even as we approach de Sitter vacuum, then it won't evaporate. No evaporation, no missing information -- it's just somewhere else in spacetime.

If a black hole only mostly evaporates, the remnant holds information, and so no information loss paradox. Just a problem about how to assess the entropy.

If a black hole generically isn't no hair (i.e., if no hair doesn't hold up at all, or only holds up in cases like exactly spherical, stationary black holes in true vacuum) then the hair holds information, and so no information loss paradox. Just a problem about the nature of the hair. Hawking, Perry and Strominger are leaning towards "soft hair" as extremely long wavelength particles (wavelengths on the order of the Hubble diameter), and while there are lots and lots of as-yet-unanswered questions in that approach, it is not ridiculous.

There are other possibilities too. This is an issue which has had forty years of study.

The problem is that when you take a _model_ black hole deliberately arranged so that it has the greatest chance of being wholly determined by the eleven free parameters of no hair, and put it in dS electrovac and run that to the far future, so it also has the greatest chance of evaporating, there are a couple of problems.

Firstly, fully classically, we cannot know (because of no hair) if we grew the black hole by throwing in one shell of matter or two shells of matter of half the mass each, or ten shells of matter whose differing masses sum up to the same as the previous two cases. If we don't evaporate, we don't care; if we have a remnant, we don't care; if we fully evaporate, we have lost the information about the number of shells and their individual masses. Worse, when we throw in the shells we have the Hawking radiation temperature rise from T_start to T_end. If we arrange it so that an observer can measure the Hawking radiation as we throw in each shell, that observer will see T_start, T_start+1, ... T_end-1, T_end, so that observer has information that is otherwise lost. So, hair?

Secondly, we can make this worse by turning classical matter shells into individual quantum particles. I need to simplify here. If we throw in two alphas and a handful of electrons, and have no hair, an outside observer will not be able to know if you threw those in, or threw in two 4-He atoms, or 8 2-H atoms, and so forth. The no hair black hole evaporates into photons, and thus you have a version of the Knapsack Problem, with the unfortunate side effect that by the AMPS argument, you must have violated at least one of unitarity for at least some observers, the equivalence principle (in particular the "no drama" condition at the horizon of a sufficiently massive black hole that spacetime curvature is effectively flat at the horizon), the holographic principle (in particular, AdS/CFT's view of it), or semiclassical gravity as an effective (in the Wilson sense) theory of gravitation outside the horizon. People argue about which the worst thing to give up would be, and many qc-gr papers have been written in the past couple of years where one or more is gleefully abandoned, with the consequences followed to logical extremes.

The Hawking-Perry-Strominger approach only deals with the classical problem, hoping to extend that into the quantum realm.

There are several other ideas theirs competes with, many of which start with some quantum view hoping to extend that to realistically massive black holes.

In any event, resolution has proven to be non-trivial.

However, the tl;dr version is that yes, adding in some hair MAY solve the information loss paradox, but there are other approaches, and no good way to choose among them.

Real black holes might indeed be very hairy at all times. They certainly don't live in de Sitter vacuum (or even dS electrovac) today or any time soon, and they even less sit in AdS (and our universe's quantum fields are not conformally invariant). However, these still seem like acceptable conditions for _models_ of black holes where one is probing the nature of the mathematical theories that describe them in detail, and in particular General Relativity.

I've never bothered to really look into the BH information paradox (ie I might be missing something that's obvious to all those legions of theoreticians that have), but just from a cursory examination, it never made sense to me. The no-hair conjecture really only applies to stationary BHs, so once you introduce Hawking radiation, the conjecture evaporates right besides its subject.

The analogy I've come up with to illustrate my point is the 'thermodynamic information paradox':

An isolated system will tend towards a stationary equilibrium state, uniquely described by just a few macroscopic parameters. This is our version of the no-hair conjecture.

Now, instead of a completely isolated system, we allow interaction via absorption and emission of radiation. We assume that no matter the incoming radiation, outgoing radiation will obey totally probabilistic thermal laws as there are no hairs. Now, if we were to radiate away all energy (eventually reaching zero temperature), information will have been irretrievably lost.

However, this is clearly nonsense as it tries to apply conclusions drawn under idealized conditions to non-ideal situations: Real thermodynamic systems fluctuate and have hairs. In fact, outgoing thermal radiation necessarily disrupts equilibrium, so the whole question is ill-posed.

As I said, isolated black hole solutions are there to probe the UV end of General Relativity, rather than to act as practical astrophysical models. (We have those too, fwiw).

Hair or not there is _enormous_ entropy in black holes that have not yet evaporated. Since we are fairly sure that no physical black holes in our universe have evaporated or are likely to evaporate in the next 10^65 years (for the smallest extremely isolated unmerged stellar black holes), and black holes deep inside galaxies will be around much much longer, it is perfectly reasonable to ignore the outgoing radiation and say that black holes are _effectively_ bald until our eventually much bigger Hubble volume is much closer to de Sitter vacuum. Indeed, mergers of black holes in nature are expected to undergo rapid "balding" as their complicated dynamical modes decay. So I can't agree with your last paragraph.

It's relevant to consider the origin of the Hawking radiation. Differently accelerated observers looking at a region of a QM field that's in thermal equilibrium will disagree on particle count. The formation of a black hole horizon creates a dynamical spacetime in which there is an acceleration between past observers (before the horizon forms) and future observers; the future observer sees particles within a few Schwarzschild radiuses of the black hole that the past observer does not. We usually consider a photon field for simplicity, but the particle count difference applies to all matter fields generically.

These particles are effectively created by fossil curvature which classically puts them on complex geodesics that go to infinity. In departing the region near the black hole their contribution to the local energy-density drops, which dynamically shrinks the horizon area. That in turn reveals previously hidden fossil curvature (which runs all the way to the singularity), which again is equivalent to an acceleration between past observers (before the "shrink") and future ones (after the "shrink), with the latter seeing particles that the previous ones do not. Voilà: evaporation, provided that nothing replaces at least the energy density of the escaping Hawking radiation particles (since that increase the area of the horizon, hiding behind it some of the previously observable-outside-the-horizon fossil curvature and thus some of the dynamic production of particles).

The quantum picture is slightly different because the particles are extremely low energy, with wavelengths proportional to the curvature radius at the horizon. "Soft hair" makes sense when one realizes that the curvature at the horizon of supermassive black holes can be arbitrarily flat (i.e., you can in principle have particles with wavelengths as long as or longer than the Hubble length). That's a very different picture from fully classical pointlike particles. However it's not clear that that's sufficient to dent the enormous difference between the number of microstates inside the horizon and the eleven variables of the macrostate of a no-hair black hole.

Additionally, even if it did wipe out much of the black hole's entropy, there does not yet seem to be a way to unitarily evolve from pre-horizon state to fully evaporated state; the details of what exactly was in the black hole as the horizon formed and what drifted in later are not obviously encoded in "soft hair", and the assumption is that the encoding would involve strong gravity somehow (and it must as the horizon area gets small).

So even taking your idea of adding a bunch of additional macroscopic parameters to no hair's eleven (at a particular time coordinate), you still have a non-negligible fraction of a galaxy worth of mass inside the horizon of an SMBH that fits comfortably within Jupiter's orbit, and absent a quantum gravity theory you'll have little idea about how to count all the compatible microstates. The information loss you are blithely accepting in your second last paragraph is difficult to conceive: for a spherically symmetric black hole of merely one solar mass the horizon area is about 10^77 in Planck areas, which gives us the holographic entropy bound. 10^77 is greater than the value of S for all the gas, dust and noncompact stars in the Milky Way.

(Finally, I want to note that there are of course many approaches to the post-AMPS black hole information problem very different from Hawking et al.'s "soft hair"; however super- and ultramassive Schwarzschild black holes in dS vacuum are good model probes of each of these possible theories).

Wow, thank you for that awesome explanation! I wish I could give you 100 upvotes for that.

P.S. How do you know all this? Are you a physicist?

So can "specially crafted" gravity waves converge into a point of negative mass?

It won't be difficult once we understand how quantum gravity works. :)

Doesn't seem like a dumb question to me! AFAIK, our models of physics work forwards, as well as backwards. So in theory: yes

IANA physicist, and have 0 training in physics, so someone please tell me if I'm wrong

The answer is actually yes, at least in principle. In practice this would be extremely difficult (read impossible) due to the weakness of gravitational wave interactions.

i.e. probabilities.

The energy density of gravitational waves is only large enough to create matter in the vicinity of the collision. The energy density dissipates by spreading out quickly.

Astronomers are looking for such photon signatures simultaneous with collision. With two LIGO detectors we can determine the collision time, but the sky location to within an arc. The third detector will give more location precision. Several more are under construction.

The article linked from that page is more useful to the physics-challenged such as myself: http://link.aps.org/doi/10.1103/Physics.9.68

How come: 14.2 + 7.5 == 21.7 != 20.8 ?

Does the gravitational wave contain 0.9 solar masses of energy?

Yes (and good question!). From the article: "The initial binary was composed of two stellar-mass black holes with a source-frame primary mass m1 14.2 M⊙, secondary mass m2 7.5 M⊙, and a total mass of 21.8 M⊙. The binary merged into a black hole of mass 20.8 M⊙, radiating 1.0M⊙ in gravitational waves."

For reference, one solar mass of radiation is about the same as the output from a gamma ray burst, or about 2000 times the output from a supernova.

Or in less sensical units, the energy from detonating 1% of our galaxy's mass worth of TNT.

Gamma-ray bursts emission is strongly beamed into a small solid angle. So, although their equivalent isotropic energies (that is the energy that would be needed to power gamma-ray bursts if the emission was isotropic) are huge, the actual energies released in gamma-ray bursts are more or less compatible with those of supernovae (~0.1% of a solar mass).

As usual, XKCD provides some help to understand how much energy a supernova is:

"Which of the following would be brighter, in terms of the amount of energy delivered to your retina:

1. A supernova, seen from as far away as the Sun is from the Earth, or

2. The detonation of a hydrogen bomb pressed against your eyeball?

Answer: the supernova, by nine orders of magnitude."


Because the event is extremely violent.

Just as a nuclear bomb converts a small amount of mass to radiant energy (grams?), a merger can convert mass into other forms of energy, too.

Gravitational waves, of which there are probably many passing through us all the time from many sources, carry a lot of power. The only reason we don't see them every day is that spacetime itself is extremely stiff.

Isn't it also that power decreases over the cube of the distance to the event? A truly gargantuan amount of power (1 solar mass equivalent) distributed across a 3-dimensional shell dozens or hundreds of light years across should be minuscule at any Earth-sized point along that shell.

They put the distance at 440−190+180  Mpc. 440 megaparsecs is is around 1.2 billion light years away(!).

Wouldn't it be inverse square, not inverse cubed? Or do gravity waves behave different than other things we're used to, like light?

Actually the nice thing about gravitational waves is that we are sensitive to their amplitude, which scales as 1/r, and not to their energy, which scales as 1/r^2.

When you say that spacetime itself is extremely stiff, I can't help myself but to visualize a lattice of discrete points through which the wave propagates through. I understand this would be the wrong way about thinking about it, but is there another way?

This isn't really a wrong picture to imagine. In a way, this is what the experiments are trying to measure. You have a mirror and a laser at two points of your lattice (the space itself), and the "region" between them compresses and expands, which makes the laser take a slightly different time to bounce back.

Far away in linear approximation (which is what we saw here), the lattice picture is quite appropriate as a first order analogy.

That's an open problem right now. Lots of proposals, insufficient evidence to resolve the question.

Stoset is right. The reason why we do not notice them is due to the inverse square law. This event was 1.3 billion light years away. 1 Solar mass of energy being turned into a gravitational wave nearly instantly creates a very strong gravitational wave. But by the time it comes here, it deforms things by distances that we normally reserve for subatomic particles. What this finding shows is that this happens somewhat regularly in our Universe, it was just that we were previously unable to achieve the precision to measure such small deformations.

Not inverse square law, but simply inverse law.

As energy radiates outwards, it becomes diluted on a sphere that increases its surface area as r^2. Thus the local energy density goes with inverse square law in the distance to the observer. But we don't measure the local energy density of gravitational waves, but the local amplitude. And in wave mechanics energy is the square of the amplitude. So the amplitude goes down with 1/r.

Not really. You could have been within a few thousand miles of this collision and survived. In fact, it would be well below your ability to notice if it were 1 AU away.


Needless to say, you would not survive if 1 solar mass of energy were released in any other form (even neutrinos). The key feature is that that tremendous amount of energy results in only a minuscule deformation of spacetime, i.e., spacetime is extremely stiff.

By the way, the distances are way smaller than merely subatomic. The strain sensitivity is 10^-21, which for the ~ 1 km arms of LIGO is a shift of just 10^-8 the width of an atom.

> you would not survive if 1 solar mass of energy were released in any other form (even neutrinos)

But it is close!

A supernova releases "few times 10^45 J of neutrino energy" [1], so let's say 5. 5e45 J is about 6e28 kg, while solar mass is 2e30 kg. And neutron radiation from a supernova would get fatal when closer than about 2.3 AU [2]. So we have a factor of 30 from the masses and a factor of 5 (1^2 AU vs. 2.3^2 AU) from the distance.

So about 1/150 of solar mass released in neutron radiation would be survivable at the distance of 1 AU.

[1] http://wayback.archive.org/web/20120313045458/http://www.and...

[2] https://what-if.xkcd.com/73/

Yea, I mentioned this because I just read it a couple of days ago :)

Hey, let's do some math!

The formula for gravitational potential energy is just mgh (mass times gravitational acceleration times height), and g is just GM/r^2, so the potential energy of one black hole in the other's gravitational field would be GMm/r, which would be the same for the other, so the total gravitational potential energy would be twice that.

Also, the schwarzschild radius of a black hole is about 3km per solar mass (2e30 kg).

Which means that before their event horizons touch the two black holes should be separated by a distance of at least 65.1 km.

So, 2 * G * 14.2 * 7.5 * (2e30kg)^2 / 65.1 km is...

8.73e47 joules

divide by c^2:

9.7e30 kg or 4.86 solar mass

So the system actually had nearly 5 solar masses of gravitational potential energy in it, some of which was radiated away as gravitational waves.

You're just calculating how much kinetic energy the black holes had when they collided. This energy goes into the spin of the final black hole. The energy for the gravitational waves comes from loss of mass of the final black hole.

Yes. Gravitational waves transmit both energy (i.e., mass) and angular momentum.

So... build more detectors and pinpoint the direction the gravity wave is coming from?

Yes! There are 2 now, so they can give a very coarse estimate of the direction with a precision of 'somewhere in this swath of the sky' (using the relative location, the wave propagation speed [light speed], and the arrival times relative to each detector).

As more detectors come online they will be able to triangulate more accurately. The next one is supposed to come online in 2018 iirc.

That's the idea! I seem to remember that there are plans to build another detector in India. I'm not sure what its status is.


Note that the VIRGO detector, located near Pisa (Italy), has already taken a set of measurements a few years ago, and it would have been sensitive enough to detect waves like the one from last September. Unfortunately, Virgo was down for upgrades when GW150914 came. I recall that they plan to restart Virgo by early 2017, so it should be operational well before Ligo India. (And I like to underline that Ligo and Virgo joined together in the Ligo/Virgo collaboration and thus work together: as a matter of fact, the first guy who saw GW150914 in the raw data was from Virgo, and the papers presenting the detection were signed by people in both collaborations.)

Funding for its construction was approved by India. We are hoping that it might coming online in ~ 5 years. It takes a long time to build everything from the ground up.

This signal was reported on months ago. Can anyone explain what they did to move the GW151226 signal from 2-sigma to >5-sigma?

"The data is in fact completely open and you could analyse it yourself! In addition to the GW150914 event there are also two others that rise somewhat above the background ("GW151012" and "GW151226"). You can see them by eye in the above plot. They are clearly not statistically significant enough to announce a discovery alone, but still they are tantalising... with room for improvement to design sensitivity (by a factor of ~2 which increases the spatial reach by 2^3) and the construction of a third detector in India to triangulate the signal, the future of gravitational wave astronomy is exciting." http://syymmetries.blogspot.com/2016_03_01_archive.html

Their background analysis is data-driven; the more data they take the better they know their background noise. They are now at the point where they can definitely say that what looked like an unusual fluctuation above the background noise months ago was in fact very unusual. Enough for discovery significance.

Citation, as it says in the article:

"Two matched-filter searches used coincident observations between the two LIGO detectors from September 12, 2015 to January 19, 2016 to estimate the significance of GW151226. One of these searches was the off-line version of the online search discussed previously. The off-line searches benefit from improved calibration and refined data quality information not available to online searches."


The use of p-values with a research design like you are describing worries me:

'The garden of forking paths: Why multiple comparisons can be a problem, even when there is no \ shing expedition" or "p-hacking" and the research hypothesis was posited ahead of time' http://www.stat.columbia.edu/~gelman/research/unpublished/p_...

I think you might be confusing LVT151012 which was including in the announcement back in February and all the subsequent papers. That is a low significance trigger that may be another binary black hole merger. GW151226 is only being reported on now as it was still in the vetting process back in February.

Gosh, wake me up in 50 years, I can't wait to learn what is the nature of dark energy and what lays beyond standard model.

Wow 50 years, by then it might be that we find we were asking the wrong questions at this point. And even with the right questions, the answers were... unexpected.

Can someone who has expertise explain the significance of this in plain English please?

I'm definitely not a good person to explain or comment on this, but in a nutshell:

Black hole collisions were expected to be a fairly common occurrence, but we had no way of knowing for certain how frequent they are when only one had been detected. Now that another one has been detected a fairly short time afterwards, it supports the prediction that we should observe several of them every year.

If black hole collisions were very rare events, it would be harder to use them as a natural laboratory. It's much easier to demonstrate that some small deviation from theory is real when you have multiple observations of it.

Therefore, this second observation in a short time confirms that gravitational waves are likely to be a useful tool to explore fundamental physical principles.

Also, this event is much closer to what they would "Expect" e.g. a much more common predicted size of black holes; the first event was rather on the extreme end of things (which is why it was so much easier to see in the data).

> Therefore, this second observation in a short time confirms that gravitational waves are likely to be a useful tool to explore fundamental physical principles.

No, it just means there's a high probability that these events occur on a regular basis. Of course, "regular basis" really means we might be able to approach gaussian statistics within the lifetime of the experiment.

Agreed. The most important single fact (though there are many to be extracted here) is an estimate of the detectable merger rate.

They didn't expect these events to be very common, so it was suspicious that they got a signal almost as soon as they turned the interferometer on. Receiving additional events will help reassure people who were afraid that the first result was a glitch of some kind.

The more you read about how this particular instrument is constructed, the more incredulous you'll be that it could possibly work at all, or if it does, that it could pick up anything but noise. So it's very good news that these events are relatively common. (Well, good for the research teams at the LIGO installations, but bad for anyone within a few hundred light years of the events being detected.)

The end target is to measure gravitational waves of Big Bang and learn about the origin of universe.

PS: Watch this brilliant video [1] if you want to know what gravitational waves are.

[1] https://www.youtube.com/watch?v=4GbWfNHtHRg

The largest gravitational waves, emanating from black hole collisions, have now repeatedly been detected. Scientists can iterate on the design to build more sensitive detectors. This will give us a new way of studying the universe that doesn't rely on electromagnetic radiation (light, x-rays, etc). Perhaps it will lead to answers about the missing matter in the universe.

First time might be a fluke, second time might be a fluke, third time... The theory might be right.

Basic science.

Remember, this was imagined by some dude 100 years ago. That's nucking futs.

Some nice visuals to explain the concept http://www.npr.org/sections/thetwo-way/2016/06/15/481934630/...

When gravitational waves are released from the merger of two black holes, the combined mass of the resultant black hole is less than sum of its component black holes, because some of the mass was released in the form gravitational waves.


What are the implications of this on black hole entropy and temperature? Can black holes evaporate from gravitational radiation alone without Hawkins radiation?

What are the implications of this on the mass of any object in the universe, since all objects are related to each other gravitationally and the universe is expanding? Does it mean objects are constantly loosing mass and the universe is filled with energy from this release? Can dark energy be related to this process? Can universe be expanding because matter is constantly lost into the gravitational waves?

What are the implications of gravitational waves on the fabric of spacetime, if objects are constantly leaking gravitational waves in a nonstatic universe?

Cosmologically we turn to the conservation of energy-mass-momentum wherein the matter fields at one point in spacetime can donate energy-mass-momentum into the gravitational field, and can pick some of that up again at a distance from the gravitational field, typically changing local matter field content's (linear) momentum.

At cosmological scales the "objects" are the various matter fields (in the most general sense, everything that is neither the cosmological constant nor the gravitational field itself) and we concern ourselves with energy-density, which drops with the metric expansion of space.

Sean Carroll goes into some detail here:


Back to GWs: these change the peculiar motions of matter field content, at cosmological scales giving tiny nudges to galaxy clusters compared to what is imparted by Dark Energy. So even though we lose the conservation of energy, we get back the conservation of energy-momentum.

My understanding is that gravitational waves are only radiated when there's a dipole moment: two (or more, I guess) objects in rotation around each other. A black hole just sitting on its own doesn't radiate any gravitational waves at all. According to http://www.ast.cam.ac.uk/public/ask/2519 , the Earth and Moon should also be radiating gravitational waves (and therefore losing a little bit of energy) but this is surely dwarfed by other effects and not measurable.

Important pedantic point: There are no gravitational dipoles, as there is no repulsive gravitational interaction.

A dumbbell-shaped mass is the sum of a gravitational monopole and a gravitational quadrupole. That gravitational radiation originates only from quadrupolar sources (which are far less-efficient radiators than an equal-sized dipole) is one reason that gravitational waves are hard to detect.

But in a nonstatic, expanding universe any two objects, no matter how far apart, are gravitationally related and have dipole moment. Isn't it true?

> What are the implications of this on black hole entropy and temperature? Can black holes evaporate from gravitational radiation alone without Hawkins radiation?

No, gravitational waves are only generated when you have large masses moving. When two black holes merge, they become a single spinning mass which then becomes uniform very quickly. Once the mass is uniform, it doesn't generate gravitational waves from rotation.

I have yet to study cosmology (I did some research in astroseismology), so I can't answer general cosmology questions.

Data and audio files relevant to this event are available to the public at https://losc.ligo.org/events/GW151226/

There are also Jupyter tutorials on processing GW signals at https://losc.ligo.org/tutorials/

How do we know that these waves they're detecting are from collisions of black holes? How do they locate these black holes and how do they make these conclusions that what they detect is coming from black hole Alpha & Beta colliding?

The waveforms are predicted by our knowledge of general relativity; specifically what the gravitational wave profile would be for two objects of certain masses spiraling into each other.

A semi-hand-wavy way we know that the objects in this particular inspired are black holes is that for them to be orbiting each other this fast, they must be very close. For them to be this close, they must be very small. Together, this implied the objects have a certain size which is smaller than their Schwarzschild radius, which means they are probably black holes.

It's mainly derived from the frequency and magnitude of the wave. The frequency derives from the period of the binary system, which in turn derives from the masses of the two black holes. Once the period has been determined it's possible to solve for the original masses. Given those we can estimate the energy produced in the collision, compare that to the magnitude of the wave we received, and then estimate the distance (similar to estimating how far away a sound is by how loud it was). LIGO can also roughly locate the direction the waves come from by comparing when the signals are received at the two different detectors (LA and WA). If we add a third observatory at some point we should be able to triangulate much more accurately.

We're pretty sure they come from black holes because the signals are uncannily similar to what theory says signals from black holes looks like. So either they're black holds or the theory is wrong or there's something else that acts very much like a black hole.

I was lucky enough to be in an astrophysics faculty (doing a research project) when the LIGO paper was published. Everyone was super excited and was printing off papers and discussing the experiment, results and future of astronomy. It was really something else to see that many clever minds so excited about a result that many scientists involved in the field of cosmology and relativity didn't live to see.

Interesting to note that the y-scale on figure 1 is 10^-21 (units of strain). Measurement at that scale is absolutely insane. The power of good engineering, incredible optics, and lots of averaging.

To give you an idea of how crazy this is. It is like measuring the distance from the earth to the sun to an atomic radius.

There's minimal averaging, as the signals are transient.

Depends on what you mean by averaging I suppose, but the signal is a measure of the average path length from several hundred reflections in the cavity. Furthermore, the signals are generated from the average interference of many photons. Even still, shot noise from the limited number of photons in the cavity is a major source of uncertainty.

All true.

To give perspective on where I was coming from: from the (human) perspective of looking at noise in 1/sqrt(Hz) units, the signals are actually less-than-averaged if they're less than a second in duration.

It's not like a continuous-wave source where one can integrate for months and hammer down the uncertainty to far lower strain values.

It's averaging in the sense of a long integration time. The signals of interest are glacial w/r/t speed of light, in the few Hz range, so you can integrate the optical signal over a longer period of time to improve SNR and beat shot noise.

I suppose one could rely on averaging the raw inputs of a large number of less reliable sensors, but I don't believe that's what the researchers have actually done here.

Something I've asked before but got no answer - at these extreme masses, velocities, and forces time dilation has got to be immense.

Yet the article makes no mention of this whatsoever.

Something to keep in mind is that gravitational waves are linearized solutions to Einstein equations. That is, they can only be meaningfully defined far away from those sources. What happens close to the black holes cannot be described in terms of waves. You should not think of the gravitational waves as being emitted directly by the black holes. Instead what happens is that the black holes distort spacetime in complex non-linear ways, which turn into gravitational waves at large distances (hundreds of Schwarzschild radii). As such, the time dilation imprinted in the waves depends on the motion of the "center of mass of the binary" and the cosmological redshift.

Does this work the same as it does with waves on the ocean?

The waves on the ocean are also turbulent distortions by the wind which pack together was waves (a group of 14 waves with the 7th as the highest and all waves after the 7th barely noticeable).

It seems that the 7th spike measured at LIGO is also the highest. Maybe the grouping of turbulent distortions as waves is something universal?

I keep trying to picture how this would work using a 'drain in a bathtub' image in my head where the water rushing down the plug is like a black hole pulling things in with gravity.

Is there any way thinking like that I can get an approximate idea of what the gravitational wave would be in that scenario?

It's not like a bathtub. (And let's ignore time dilation.)

Imagine you have a large mass (sun) moving in a straight line, and an object (sensor) 1 light minute from that mass. The gravity from the sun takes 1 light minute to reach the sensor - that would mean that the sensor is attracted to where the sun was 1 minute ago.

That can't work - it violates all sorts of conservation laws.

Instead what happens is that the gravitational force itself is ALSO moving in a straight line! So when the gravity from the sun reaches the sensor it attracts the sensor to where the sun is now because both the sun, and the gravitational force, are moving together.

This works out very nicely.

But what happens if the sun is moving in a circle? Otherwise known as accelerating?

The gravitational force can't know what the sun will do in the future (that it will move).

So as the sun moves it forces the gravitational force to change - before the force was moving in direction a, now it's moving in direction b. This change in the gravitational force is known as a gravitational wave.

This wave, because it is accelerating, has the ability to impart change in other objects, otherwise known as imparting energy. So gravitational waves can carry energy! Potentially huge amounts of it.

(And since they carry energy, they themself have mass, and therefor gravity, but these second-order effects as they are called, are too confusing, and weak, and everyone ignores them.)

Back to the black hole - as it orbits the other black hole (as they orbit each other), they change direction very very rapidly, causing huge gravitational waves - the waves steal energy from the orbit, causing the two black holes to fall into each other with smaller and smaller orbits, i.e. a spiral.

My problem is this: Near a black hole time dilation is enormous, huge gravity, plus huge velocity. So to an outside observer the black holes appear basically frozen and don't move. If they don't move they don't make gravitational waves, so we should detect nothing.

I have no answer to this question.

FWIW, for supermassive black holes, the curvature at the horizon can be arbitrarily flat.

It's referenced in the abstract in the phrase "source-frame". For the target audience, belaboring the time-dilation point would be a waste of time; it's basic knowledge to the intended readers of the abstract. We here on HN are by-and-large just sight-seeing.

For the most part, we concentrate on quantities far away from the sources as these are what we have a chance to measure and can more easily intuit. There are however, extremely precise simulations of GR that can numerically simulate the spacetime around close to mergers which of course includes all of these effects.

So, I remember hearing about LIGO when the first wave was detected, and was excited by it - but I had never heard of it before that.

What other experiments are running that would generate similar excitement from the science community that I probably haven't heard of yet?

>he signal persisted in the LIGO frequency band for approximately 1 s, increasing in frequency and amplitude over about 55 cycles from 35 to 450 Hz

Would love to hear an audio facsimile of what this might "sound" like.

For this particular event, see https://losc.ligo.org/events/GW151226/, near the bottom of the page

It's truly incredible that I can sit here, plug my headphones into this little electrical box and "hear" the gravitational sound of two black holes colliding. If only Mr. Einstein could be here to listen.

I love that the data-as-sound is so cute: https://losc.ligo.org/s/events/GW151226/GW151226_template_sh...

Truly an unbelievably jaunty little sound considering that it represents the entire mass of our solar system being converted into energy. Yow.

Indeed. And to put that into perspective:

"The amount of matter converted to energy in the atomic bomb dropped on Hiroshima was about 700 milligrams, less than one-third the mass of a U.S. dime."

(From http://discovermagazine.com/2010/jul-aug/24-numbers-nuclear-...)

One of my favourite things to do, when I'm back in California, is attend SLAC Public Lectures [1]. (There is a disturbing paucity of scientific cultural institutions in New York.) The most recent one was by Dr. Brian Lantz about LIGO [2].

[1] https://www6.slac.stanford.edu/community/public-lectures.asp...

[2] https://www.youtube.com/watch?v=EMzoQAmK8Dc

Is there any correlation between the fundamental vibrating of atoms in a vacuum at absolute zero, which I think is known as the zero-energy state, and gravitational waves? Meaning, I know quantum mechanics describes why they vibrate, but do gravity waves have any play in this, or effect? So exciting to have lived long enough to see some more theory become real with measurements!

I have a side question. If gravity (i.e. warping of space time) propagates at the speed of light. Then does that mean that the Alcubierre warp drive is fundamentally incapable of supra-light speeds?

What does the last line of the abstract refer to, about "deviations from general relativity"? Is it simply a statement that this (and other) observations of gravity waves are another tool for verifying GR's predictions?

Yes. These gravitational wave detections are the only direct measurements anyone's ever made in regions of such strong gravity. If strong-field gravity betrays any failing of the theory of GR, we'd expect to find it in gravitational-wave waveforms.

That these waveforms (the first one, in particular) appear to follow the basic GR plan at the time of the merger is actually kind of a bummer. As a physicist who makes precision tests of gravity for a living, I'd really hoped that we'd see something unexpected. Instead, the new thing that we learned is that GR continues to be an accurate model, something that was perhaps unexpected on its own.

I'm not so surprised that there are no GR deviations in these early days of detections that are just becoming significant over the instrument noise.

GW physicists are hard at work on the future generations of GW detectors... In 50 years, a signal like GW150914 might have an SNR of >1000, so we'll see all kinds of detail that may hold more information than what we've seen so far.

People have been testing GR really hard for a century, especially in the last fifty years. Nobody's convincingly found a single chink in the armor.

Clifford Will's "The Confrontation between General Relativity and Experiment" is an excellent introduction to the state of the art (this is the stock reference for everyone in the field).


> Is it simply a statement that this (and other) observations of gravity waves are another tool for verifying GR's predictions?

That is how I read it, yes.

How far are we from creating Gravity Shockwave Generating Division Tool? A.k.a Goldion Crusher.

Could black holes lose their mass through the propagation (generation?) of gravitational waves?

It seems that it happens when they merge, since:

> The cataclysmic event saw the black holes, one eight times more massive than the sun, the other 14 times more massive, merge into one about 21 times heavier than the sun. In the process, energy equivalent to the mass of the sun radiated into space as gravitational waves.

So the final black hole had rather less mass than the sum of the original two.


Yes, that's where the energy for gravitational waves comes from. In fact, if you check the original LIGO paper, that black hole merger lost something like 3 solar masses of energy.

I thought the shaking was Santa on the roof!

Applications are open for YC Winter 2020

Guidelines | FAQ | Support | API | Security | Lists | Bookmarklet | Legal | Apply to YC | Contact