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Physicists Detect Gravitational Waves, Proving Einstein Right (nytimes.com)
2011 points by intull on Feb 11, 2016 | hide | past | web | favorite | 483 comments

This made me wonder how far we are from being able to create and detect gravitons. The Wikipedia page on gravitons [0] addresses this question:

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. [...]

However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are underway (such as LIGO and VIRGO). Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass [...].

Fascinating! I take it that the question of whether the graviton could have mass is now considered to be well answered in the negative.

[0] https://en.wikipedia.org/wiki/Graviton

Here's a quote from John Baez's G+:

"Gravity is a weak force, in the sense that the gravitational force between two protons is about 10^33 times weaker than the electric force between them. And I'm using protons rather than electrons here to make the gravity stronger - with electrons gravity would be almost 10^40 times weaker.

This has various consequences, but one is that gravitational waves are absorbed by matter much less than electromagnetic waves. It would be fun to estimate the amount of energy absorbed by the Earth as this particular gravitational wave came through, but it would be absurdly small. Gravitational waves make neutrinos look like rampaging gorillas."

Rephrasing, and assuming waving commutes with rampaging, gravitons make neutrinos look like WAVES of rampaging gorillas. I'm no physicist but to answer the original question I'd hazard a guess: quite far!

If gravitons have mass, then the universe is too strange to exist. Gravity is an interaction that defines the presence of matter (see dark matter). For the object that transmits that force between masses to itself have mass ... how can a black hole then project gravity?

Imho whatever is carrying gravity between masses cannot itself have a mass.

Force carrying particles in general don't have mass. Except that some of them seem to do, which was rather puzzling for some time, but was solved using the Higgs mechanism.

I can't think of an obvious reason the Higgs mechanism wouldn't work for gravitons, but I could be mistaken, it's not exactly the most intuitive area of physics.

Also, keep in mind that the strong force transmits the force between colour charges while also having a colour charge itself, so it isn't entirely inconceivable for the force transmitting the attraction between masses to have a mass.

No clue if a massive graviton would allow for black holes, but it's not entirely sure what black holes even are (especially quantum mechanically). At the very least it's presumably possible for some particles to escape it (e.g. as Hawking radiation).

If the massive gravitron was leaving a black hole it would be slowed by the black hole's gravity.

(1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons, pulling more in.

(2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves?

(3) If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron? This would require a new set of particles be created by non-gravitron massive objects (ie black holes) alongside the gravitrons. Like I said, too strange to exist.

(None of my points below say the graviton is massless, just that it's not crazy. As another post says, this new observation probably confines the graviton mass to be less than 10^-55 grams)

> If the massive gravitron was leaving a black hole it would be slowed by the black hole's gravity.

A graviton wouldn't be able to escape a black hole. A photon can't, and it's massless. The gravity of a black hole is actually a self-sustaining effect of the curvature of the spacetime around the black hole.

> (1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons.

We don't know details of the gravitational field around black holes and the mass that created it, because none have been observed close up. To an extent, the mass of a black hole is defined by its gravity.

> (2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves?

Again, photons are massless and subject to the doppler effect. Gravitons, massless or not, will be too.

> (3) If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron? This would require a new set of particles be created by non-gravitron massive objects (ie black holes) alongside the gravitrons. Like I said, too strange to exist.

Force carying particles can interact with themselves, c.f. gluons in QCD. In fact, GR is a non-linear theory so there will be non-linear interactions (as far as you can describe them in the weak limit).

> A graviton wouldn't be able to escape a black hole ... the curvature of the spacetime around the black hole

But isn't the curvature of spacetime around the black hole supposed to be the effect of its interaction with the graviton??

Is this where the translation from GR -> QM breaks down?

> Is this where the translation from GR -> QM breaks down?

Sort of, to get a graviton you introduce perturbations on a background metric. (Basically small wiggles of spacetime around an 'average.') You don't do anything like that when you solve the Einstein equations. Consequently, the background spacetime ( that is, the black hole) is not really made of gravitons. (At least in some sense.)

So in GR the metric itself is warping, which is a problem if you need a background reference/stable metric to define your graviton field? Is that close? Anyway thanks.

Yes, pretty close.

> We should see this as some inconsistency in how gravity scales with the mass of a black hole.

This inconsistency is a part of general relativity (even though gravitons themselves aren't). A black hole does not follow the GM/r^2 gravity law.

> If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves?

Yes, there is a doppler effect.

> If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron? This would require a new set of particles be created by non-gravitron massive objects (ie black holes) alongside the gravitrons. Like I said, too strange to exist.

Gravitons. Every now and then a pair of gravitons may exchange more gravitons (these are also virtual particles, so it's fine). This is why Feynman diagrams exist, so that you can not only calculate the interaction between two particles through a graviton, but also calculate the contribution of the lower-probability situations where more gravitons magically appear to transmit gravity between gravitons.

This is nothing new. Gluons (the carrier for the color/strong force) do basically the same thing. They are also bound by the force they carry, and thus gluons can interact with each other with more gluons.

Because these are virtual particles and only exist as a probability, this doesn't lead to infinite recursion. The secondary gravitons are improbable, and the tertiary gravitons more so, and so on, and the final series converges.

> and the final series converges.

Or not, and we yell "look behind you!" and then dump the infinities out the window.


Hah, that too :p

When I first learned about renormalization formally (i.e., with equations) I realized it wasn't that bad. But still, pretty sneaky :P

>(1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons.

Perhaps, no clue how a quantum mechanical gravity would interact with a black hole.

>(2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves?

The Doppler effect happens even for light, which isn't massive at all (that we know of).

>(3) If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron?

There's no reason they couldn't interact with themselves, in fact I guess that's probably the most likely case.

If they interact with each other, then wouldn't any "wave" collapse?

The particles leave the event in a smooth wave. Then they run into other waves, or each other, or just the background gravity fields. This perturbation should cause them to clump together. So in short order the smooth wave would become large blobs of gravitrons more akin to raindrops than waves. And without anything holding them apart, might not some of these clumps condense into some sort of ... I don't have the words for such an object. I wouldn't want to get in its way.

I think what you're trying to describe is something like 'confinement'. I'm not confident that a massive graviton will necessarily lead to confinement, neither gravitons nor confinement are that well understood.

It's a converging series; instead of getting a clumping you may get a slight mass increase. These new gravitons that modulate gravity between existing gravitons are not 100% there, so they do not contribute that much.

I'm going with the theory that black holes ARE gravitons themselves.

There always seem to be pairs (as in yin and yang), so could something like anti-gravitons exist too?

Sure, and in several theories of gravitation where there are gravitons as an uncharged massless spin-2 gauge boson (General Relativity isn't one of these; it doesn't have any gravitons at all, although the non-quantized classical gravitational waves have spin-2 symmetry) then gravitons are their own anti-particles, just as photons (uncharged massless spin-1 gauge bosons) are their own anti-particles in the Standard Model.

(i.e., anti-gravitons and gravitons are the same thing, just as anti-photons and photons are the same thing).

There are a variety of other theories of gravitation with gravitons, but as far as I know, there are none in which gravitons are not their own antiparticles. (There may be such theories available in universes with a very different cosmological constant or with different numbers of dimensions than the one we are in).

I love groups who, while considering something as crazy as time turning into a physical direction as it does in a black hole, can still manage voting down someone down thinking outside the box.

I like the idea of anti-gravitons being the inside of a graviton, or inside of a black hole. Given the inside of a black hole is essentially the end of time, coming out of a black hole or coming out of an anti-graviton, could equal going back to the beginning of time.

> Force carrying particles in general don't have mass.

Massless particles don't have energy. Massless and energyless particles have no speed. I have no interest in massless and energyless particles that stand still.

Photons are massless particles that have energy. All massless particles travel at the speed of light.

Protons are said to be massless, but a proton may have mass that is so small that we cannot measure it easily. We can't currently say with 100% certainty that it is massless- only that it is at most very, very small: < 1×10−18 eV/c2

What? Protons most definitely have mass.

They obviously meant photons

Yes, fingers kept typing wrong word.

First, I'll assume you meant 'photon'. With that in mind, is it your assertion that if photons have no mass, they necessarily possess no energy?

Because if so, and assuming you have some reason for believing this i.e. you can prove it, I would urge you to forward these findings to a physics journal of your choice posthaste, as this basically represents a total refutation of much of physics of the last century or so. You will easily win a Nobel Prize.

This is an awfully confident-sounding post for being so off base.

I want some of what you're smoking.

E = h*nu

> Imho whatever is carrying gravity between masses cannot itself have a mass.

Forget gravitons, this already exists in pure general relativity. Spacetime is curved around a massive object, and that curvature contains energy. That's just another word for mass, so the curvature itself exerts gravity. This creates more curvature (...etc etc). This is one of the reasons as to why Einstein's equations are nonlinear.

Note that the concept of having mass is separate from the gravity force. Interaction with the Higgs field gives rise to mass, whereas gravitons are the force carrying particle for gravity.

Only electrons get mass from the Higgs mechanism. Most of your mass comes from your protons and neutrons (or rather the energy "stored" in the bonds between the quarks that make them up).

You're talking about binding energy, and that's not where the majority of mass comes from.

All massive particles get their mass from the Higgs field.

All _fundamental_ particles gets their mass from the Higgs (up to some issues with the neutrinos). Composite particles, say the Proton, is a strongly coupled system (that is, can not be described by perturbation theory) and it does not have the mass being the sum of its constituents (not even most of it). Hence, it is not known what gives most of the mass of particles such as the Proton.

Er, sorry, implicitly was talking about fundamental particles.

IIRC we do know where protons get their mass. The internal color field has some energy, thus some mass, which comes from that field interacting with Higgs.

E=mc^2 doesn't come from the Higgs mechanism.

This is true for fundamental particles. However, more than 99% of the protons mass doesn't come from its constituent parts. I couldn't find any papers on this question in the 10 minutes I spent googling (and I don't have my QCD textbook handy), but here's a few links:

* http://physics.stackexchange.com/questions/64232/your-mass-i... * https://en.wikipedia.org/wiki/Proton#Quarks_and_the_mass_of_...

What do you think of the strong nuclear force? Gluons are the force carriers between color-charged particles. And gluons themselves have a color charge. So gluons transmit force between each other. Does that mean the universe is already too strange to exist?

By the way, this is why the strong nuclear force has such a short range. Gravity has infinite range as far as we can tell, so that makes it unlikely that the graviton, if it exists, has mass.

Photon's and gravitons have to be masses because Gravity and Light and interact with matter at finite distances.

If your 1 billion light years from earth, you are still effected by the Earth's gravity (well its likely smaller then experimental error but never mind that it still exist).

So there needs to be gravitons from Earth flooding the entire sphere of space for 1 billion light years around the Earth. All these gravitons have mass, and are emitting their own gravitons. Which all have mass and energy! Where is this mass and energy coming from? It really can't, it violates the laws of thermodynamics. But so does Dark Energy so who knows.

> Photon's and gravitons have to be masses because Gravity and Light and interact with matter at finite distances.

Photons don't have mass. If they had mass they couldn't travel at the speed of light.

The speed of light is the limit of time conduction in this universe.. It is the clock speed of the great animation

How does the mediating particle being affected by the force translate to a shorter range?

There are two aspects to it. And I'll be very hand-wavey because QCD is not my field.

Suppose you have two quarks and you start to pull them apart. The gluons that transmit the force between the two quarks tend to "bunch" together because they have their own charge. You can think of it roughly like a rope of gluons trying to pull the quarks back together.

If you keep pulling on the quarks, you might expect the gluons to eventually "break". But this doesn't happen, because the gluons act on each other. If there were ever a break, more gluons would join in, tugging the break back together. Eventually, you end up with so much energy density in all these gluons that they start forming new quarks and other particles. These new particles will bind with your quarks and each other to form color-neutral particles.

So I spoke a little imprecisely. Gluons, being massless, have infinite range. But you won't ever see the strong force acting over any large distance because anytime you try to get color-charged particles far enough apart, you'll end up making more particles.

Wikipedia has a brief write-up that provides an illustration. https://en.wikipedia.org/wiki/Color_confinement

No--the mediating particle having mass translates to a finite range.

I'm assuming it's because they interact with each other.

Gravitation does not technically interact with light either, but rather bends the spacetime the light travels through. So question is, what makes gravitons different?

I'm a physicist, but in a different field, but my (possibly incorrect) impression is what a quantized particle is is a bit mysterious when the field is strong, for example, with lensing.

Best example: the Hydrogen atom is supposedly quantum, but if it is quantum, where are the photons? the q^2/r potential is a mean field that one finds from classical electrodynamics, it isn't formed by the summation of photons. [Another mental poker, photons are momentum eigenstates, so how can potential be described in position space? You'd need to sum up an infinite number of them! (For EM students, recall how to represent 1/r in spherical harmonics or in terms of sines and cosines)]

What happens, as I understand it, is with strong fields, one tends to use a semi-classical description because in the strong field limit, one deals with many photons, which should approach the classical limit.

Basically, quanta are like "pertubations" of the fields from their "free" solutions, as they are in GR (linearization of the GR field eqns) and as they are in EM. Free essentially means in the absence of sources, like charges, or masses for GR. So trying to explain general phenomena in terms of "pertubations", which are basically the solutions for "free" fields, is not always fair.

One doesn't always face this in high energy physics because in HEP, most of the incoming and outgoing states in a problem are these "free" solutions. For example when doing scattering off a hydrogen atom, the incoming states are "free" (a free nuclei, a free electron), so one can use photons for that phenomena, and one finds that the scattering is like scattering against a (mean) 1/r potential.

But in the case where the strong fields don't turn off, like when you are bound to a Hydrogen atom, or when considering nucleons in nuclei in the low energy limit, one turns away from the pertubative, photon/gluon model and either solving the problem numerically or treats the fields as semi-classical, as with the Hydrogen atom. For my field of laser-plasma physics, this shows up in the so-called "Volker-state", rather than treating the strong laser field as a sum of innumerable (ie., not-simulatable) photons, one treats the Laser field as a semi-classical background for the quantum guys (electrons, ions).

I think lensing is like strong static fields in EM. One wouldn't really think of them in terms of quanta of the field.

maybe a black hole projecting gravity is how it sends information back out

>how far we are from being able to create and detect gravitons //

Surely we create gravitons whenever we move a mass just like we create photons whenever we move a body that interacts electromagnetically? Isn't the point that we're constantly exchanging gravitons with all matter as they mediate gravitational attraction.

I think you must be right -- but maybe there's some other way to create them, without needing to move a mass. The ability to manipulate gravity itself might be the key to space travel.

Flying cars man! Think of the flying cars!

And maybe real hover boards

First we need to find out how to create repulsion. Right now I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs.

Maybe we could make orbital graviton beam generator that could literally suck an object off the face of the Earth.

I'm not terribly knowledgeable about relativity, but I don't think that gravitational repulsion is a very meaningful concept in GR. I would appreciate being corrected on this matter if that is not true.

Well then prepare to stand corrected.

Inside a charged black hole there is a second horizon. Beyond this point the black hole is gravitationally repulsive. http://casa.colorado.edu/~ajsh/rn.html

So it's not explicitly ruled out by relativity. However, that link says:

> The Universe at large appears to be electrically neutral, or close to it. Thus real black holes are unlikely to be charged. If a black hole did somehow become charged, it would quickly neutralize itself by accreting charge of the opposite sign.

> It is not clear how a gravitationally repulsive, negative-mass singularity could form.

So it falls under the same sort of category as negative- or imaginary-mass 'exotic matter': not ruled out, but there's nothing suggesting that it actually exists.

I wasn't ever arguing that it occurs in nature, or even that we will one day engineer it. I was just answering your question that GR does indeed allow for such a thing.

I vaguely recall other weird edge cases where gravity is repulsive, but I can't find any links right now.

Of course, and I do appreciate your reply. I was purposefully vague in my original wording because there is that niggling difference between "unphysical" and "not proved impossible" that I didn't want to get on the wrong side of. You have enabled me to speak more precisely about the subject in the future -- many thanks. My last reply was merely trying to place this information in context; that is to say not unphysical, but in the same category as stable wormholes, Alcubierre drives, FTL/time travel, and other such theories. And ultimately it seems like either the guy talking about graviton beams was either talking about something extremely far-fetched, or a great way to destroy large parts of the planet, or both.

> a novelty device that weighs more than what it's mass would lead you to think it weighs

This is an interesting concept. As far as I'm aware, we have ways of measuring weight, but no way of measuring mass. How would you know whether something weighed more than it "should", based on its "mass"?

You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. Or you throw it at something of known mass and measure the speed it imparts onto the known object. Or you hang the known mass and the unknown mass on strings and measure the force of gravity between them, which may seem hard but can be done with stuff from Home Depo.


> You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight.

I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one.

> Or you throw it at something of known mass and measure the speed it imparts onto the known object.

Blind application of the principle of conservation of momentum does indeed tell us that we can measure the mass of one object by colliding it at known velocity with another object of known mass and measuring the resulting velocities. But I tend to worry that the mechanism for transferring velocity from one object to another object in a collision is the force it exerts during the collision, and that that force might be determined by the object's weight (also a force) rather than mass (a platonic concept). But, I'm not sure here either.

This ties in to the "fun factoid" that physics has no explanation for inertial mass and gravitational mass being the same quantity. If they in fact aren't necessarily the same thing, momentum transfer, measuring inertial mass, would solve this problem. If there is a reason they coincide, this approach will be confounded by that reason.

> Or you hang the a known mass and the unknown mass on strings and measure the force of gravity between them

I'm absolutely certain this wouldn't work to distinguish the mass and weight of an object that has extra weight because it's emitting extra gravity. The measured force of gravity is going to include the extra gravity you're trying to ignore.

>>As far as I'm aware, we have ways of measuring weight, but no way of measuring mass.

Then we are speaking of different things. I understand 'weight' as how heavy something is within particular gravity field (ie on a bathroom scale on earth) whereas mass is independent of local gravity. The schemes I suggest measure mass without resort to weight.

>>I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one.

The motion of the more massive pair will describe a smaller circle than the lighter one. The ratios of the two circles/motions allows you to calculate the unknown mass from the known.

We should be speaking of "a novelty device that weighs more than its mass would lead you to predict", that is to say, an object the measured weight of which does not correspond to what a different object of the same mass would weigh in the same location.

Measuring the force of gravitation between two objects definitely doesn't measure the mass of those objects without resorting to weight; the weight is the quantity you're measuring. Similarly, the fact that the center of gravity for a two-objects-attached-by-a-string system will lie closer to the massier object relies on the massier object also being heavier. If the massier object weighs less, why do you believe the center of gravity would still be closer to it?

> I understand 'weight' as how heavy something is within particular gravity field (ie on a bathroom scale on earth) whereas mass is independent of local gravity. The schemes I suggest measure mass without resort to weight.

Yes, those are the definitions of weight and mass. We can measure weight directly, because it's a force and we have tools to measure those. All of our methods of determining the mass of something, as far as I know, rely on the assumption that if you know the gravitational field at a point, all objects with the same mass would, if located at that point, have the same weight. The most common method of determining an object's mass involves measuring the gravitational attraction between the object and the earth (colloquially known as the object's "weight"), and then imputing a mass to it based on that weight.

In the spinning example, I could say that two objects attached by a string and set spinning around each other will spin around a point that balances the torque from each object (this might not be, strictly speaking, correct, but it's close enough that I think it's suggestive). But torque is defined by force, not mass -- if one of the objects gets heavier without becoming massier, that should draw the center of rotation closer to that object, shouldn't it?

Or phrased yet another way: if one of the two attached objects is heavier than it "should be" according to its "true mass", then the two-objects-and-a-string system will have the center of mass and the center of gravity in different places. Those terms are currently synonymous, but if we had a novelty object such as undersuit described they would be distinct. Is there any reason to believe that the two-objects-and-a-string system would, if spinning, rotate around the center of mass rather than the center of gravity?

All you need is a switch to turn it on and off.

Tell someone to hold it. Turn it off. Watch them struggle with the sudden weight. Turn it back on.

With today's technology, for some compounds... you can count atoms by physical measurements and known structure, and figure mass by atomic weight.


That sounds like a tractor beam to me.

> I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs

Which would make space travel a lot easier - no more worrying about bone density loss!

Could such a device be used to increase the reaction mass of your fuel when it exits your engine? A sort of way to cheat F=MA by artificially boosting M, but only after you are in orbit?

No, you're confusing mass and weight. Mass is the amount of matter in a thing. Weight measures gravity's pull on the thing.

This theoretical device could make things weight more than with just Earth's gravity... but it wouldn't help your spaceship. Your engine is still pushing out the same amount of matter, so thrust remains unchanged.

toy model:

I, the spaceship, would like to accelerate through space. I take up some fuel and hurl it in the opposite direction, which requires me to apply force to the fuel I'm ejecting. It goes off into space at some rate determined by the impulse I applied and the mass of fuel I applied it to.

Newton's third law means that when I hurl the fuel, it applies a symmetrical impulse to me, accelerating me in the opposite direction.

In this model, the acceleration I get from the fuel doesn't depend in any way on the mass of the fuel I eject, only on the force I apply to it. What's wrong with the model?

You could definitely boost an orbit by increasing the force of gravity while approaching periapsis.

No more so than any other means of adding energy to the fuel.

"Here's a board with wheels we call a hover board!"

Keyword here is detect. As the post says, even if we created them we would have no way of knowing because they are virtually impossible to detect.

Related to this... According to Kip Thorne, this recent discovery alone places an upper bound of maximum mass for a graviton, if it exists and has mass, at no more than 10^-55 grams. Since otherwise the observed waves would have been distorted. It's cool that we are already seeing conclusions drawn from this observation. This single obsrrvation. https://starguyspeaks.wordpress.com/2016/02/11/on-gravitatio...

Gravitational waves are a prediction of classical linearlized gravity - gravitational waves are to (classical) gravitons what light waves are to photons.

But we have direct experimental confirmation of light behaving light both a wave and as a particle. Do we have any such evidence to show gravity exhibiting particle like behavior?

No, but we predict it to be essentially impossible to observe a graviton. http://arxiv.org/abs/gr-qc/0601043

I remember learning about the LIGO experiment back when it was being built, a decade ago, and at the time it seemed so amazing: a giant tube of vacuum, sealed underground and so sensitive that it could detect animals walking nearby, listening to the moving and twisting of space itself… I guess we're finally seeing that with immense human ingenuity and the most careful of engineering, the universe will offer its secrets up to us.

This also means that between LIGO and ATLAS/CMS, the last few years have screwed in the final screws on two of the big physics advances of the 20th century: quantum field theory and general relativity are now both experimentally complete, and both look nearly unassailed in their correctness. The next steps for physics look increasingly abstruse: understanding the exceptional cases, like black holes, holography, and the fundamentally computational form of the universe. It's an exciting time, and it looks more and more like we're close to the very bottom, since we have to look so far now to find anything outside our models.

Well we've accounted for about 5% of the universe--the stuff we know about.

Dark matter (about 25%) seems to only interact gravitationally, which means that we've just, today, proven that we have an instrument that could possibly observe it directly. To date, all our evidence for dark matter is indirect--observing the otherwise unexplained behavior of normal matter. Today is the gravitational equivalent to Galileo pointing his first telescope at the night sky.

Dark energy (about 70%) still seems to be a total mystery.

And of course there is our inability to reconcile quantum mechanics with gravity. With each further proof of the correctness of each of those theories, the mystery of their apparent incompatibility deepens.

All of these factors lead me to believe that we may still have a long way to go in our understanding of the physical universe. I hope I'm right.

This is also why I believe it is so important to pursue nuclear energy. If we do invent further theories and experiments, it's likely that they will require even greater energy levels than we can create now, and potentially imply even greater dangers. If we can't learn to manage nuclear physics in a practical, routine way, we'll never have a hope of going beyond it (if indeed there is a "beyond.")

How do we know dark matter is some mysterious form of matter and not just small distributed particles (gas or solid) that are beyond our ability to detect? Do we have proof of a specific, exotic, non-atomic matter?

Scientists are pretty sure that dark matter is not just regular gas and dust because the amount required to create the gravity we see, would be visible. It would block or reflect a lot of the nearby starlight.

Just on the back of an envelope: If we assume the percentages in my post above apply to an individual galaxy, then there has to be 5x as much dark matter mass as lit mass. There's no way you could have 5x as much gas and dust in a galaxy as stars, and not see it.

For comparison, the sun makes up about 99.8% of the Solar System mass (500x as much mass as all the planets, dust, etc. combined).

That always confused me. We have an Oort cloud, whose members we cannot resolve very well/at all. Why do we assume only our star has such a thing? If all stars did, that isn't enough mass to explain dark matter?

The total mass of the Oort cloud is guessed at (3×10^25 kg), or about five Earth masses. With dark matter, we are talking about roughly 5.6x the amount of the total solar system mass. The Oort could would need to be about 371,691x more massive than it is.


Could be an anthropic explanation for that. In a solar system with a hypothetically-"normal" Oort cloud, comets and debris from the cloud might wipe out life on the habitable inner planets every few hundred million years, never allowing it to advance to human-like levels.

So we might be here only because our solar system is surrounded by an unusual amount of nothing.

But we also look at a lot of other stars in the sky. If every single one (or almost every single one) had a massive 5x mass Oort cloud around it, it would affect the light we see from that star.

Consider that we can currently detect differences in luminosity small enough to tell whether an Earth-size planet is passing between us and the star. A 5x mass Oort cloud would be thousands of times more mass than that. It would have noticeable effect on luminosity.

And, while our sun has an Oort cloud, there are a lot of stars out there that probably don't--too small, too big, too hot, too young, too old, etc.

Amazingly small! It extends halfway to the next star (a lightyear or so) which seems like it would add up to a lot.

Well for that explanation to scale up, the Oort Cloud would have to total about 5x the mass of the sun. That would have a pretty good chance of perturbing the orbits of all the planets, and vice versa.

A bit of Googling tells me that the current estimate of its mass is in the order of 5-10 Earth masses--not nearly enough to explain dark matter.

> For comparison, the sun makes up about 99.8% of the Solar System mass (500x as much mass as all the planets, dust, etc. combined).

That leaves just 0.2% for all the planets, dust, Oort cloud, Kuiper belt, etc. So ... no.

Google "sun percentage mass solar system" and the highlighted answer is "By far most of the solar system's mass is in the Sun itself: somewhere between 99.8 and 99.9 percent."

Please don't just disagree when you don't know what you are talking about.

I think you've misinterpreted my post. The "No" was in response to this:

> That always confused me. We have an Oort cloud, whose members we cannot resolve very well/at all. Why do we assume only our star has such a thing? If all stars did, that isn't enough mass to explain dark matter?

No, that isn't enough mass to explain dark matter, since it's only 0.1% to 0.2% of the mass of the solar system.

The text I quoted was in complete agreement with what you and others have posted. I was pointing out that the questioner's point had already been answered.

Ok, that's just a really confusing way of communicating, nobody is going to puzzle that out when the obvious way of looking at your response is disagreement with the grandparent.

Scientists are not prone to falling back on explaining observations via postulating a new kind of matter we can scarcely observe. Ever since the first indications of "dark matter" scientists have been attempting to explain it as something more familiar to us, some kind of atomic matter or some-such, maybe gas or dust or lots of planets or dark stars or something. At every single turn they've been stymied, and instead of eliminating the idea of dark matter as an ethereal particle they've instead eliminated other possibilities.

Don't look at the current theory of dark matter (weakly interacting massive particles) as some hare-brained scheme that scientists thought up, instead look at it as the hard-fought victor of numerous observational challenges. Dark matter is the theory that survived. We tried explaining things a zillion other ways (gas clouds, compact objects, neutrinos) and those theories just didn't match the observations. There are also a few exceptional circumstances (such as the bullet cluster) that indicate very strongly that dark matter is something different than either gas clouds or stuff like stars and planets, because in the bullet cluster we can observe the gas and the stars and planets and the mass, and each of them are in different places because each of them follow different rules when it comes to interacting during a galactic cluster collision.

> Scientists are not prone to falling back on explaining observations via postulating a new kind of matter we can scarcely observe.

You mean like ether?

Dark matter was "invented" because there wasn't enough observable mass in galactic-scale objects to account for their behavior. In other words, they acted like they had more mass than we could observe. Dark matter is basically characterized by not responding on the electromagnetic spectrum, which is what we use to do these observations. Since all the matter we know of generally does respond on this spectrum, that's why dark matter is considered to be "exotic".

https://en.wikipedia.org/wiki/Bullet_Cluster is fairly good evidence that dark matter isn't just unobserved regular matter. In these massive cluster wide collisions the dark matter seems to have "kept going" (you see very strong gravitational lensing where there appears to be nothing) while regular matter that we know is subject to forces besides gravity collided together, slowed down, and became very hot.

"Dark Matter" is an unfortunate name, since it sounds like regular matter that is not adequately lit.

In reality, it's something we have no idea what it is, except that it's not visible and a big source of gravity.

Completely agree. We have a long way to go.

>It's an exciting time, and it looks more and more like we're close to the very bottom, since we have to look so far now to find anything outside our models.

For what it's worth we thought the same thing a little over 100 years ago. We just had to figure out a few pesky things like blackbody radiation and physics would be all wrapped up.

This is a popular thinking, but actually there were people like Kelvin, Jeans, Rayleigh, Planck and many others who did not get famous who knew there were problems with the theory. In no point in time of modern science there was widespread opinion that "it's mostly done".

You're way too optimist.

You forgot about dark matter.

And the devices required to probe Plank length/mass/energy are way beyond even our imagination.

"fundamentally computational form of the universe", you must be a seth lloyd guy :)

But yes, it's the fringes that we'll find new physics. It's not unlike the late 19th century when newtonian + E&M seemed to account for all there was to know.

There hardest thing in fundamental physics right now is to know what questions to ask. We've got answers that work for a lot of the biggest ones that the last 100 years have been spent developing and exploring.

> I guess we're finally seeing that with immense human ingenuity and the most careful of engineering, the universe will offer its secrets up to us.

That's been going on for a few hundred years now.

> quantum field theory and general relativity are now both experimentally complete, and both look nearly unassailed in their correctness

Well, we know that both theories are "wrong" in the sense that they give nonsense answers if you ask them the wrong questions. It's just that all of those questions are well beyond our ability to test experimentally.

It's not underground - it's raised up about a meter off the ground.

An underground device sensitive enough to detect animals walking around could be useful for other things... (from ecology research to large-scale surveillance)

A conceptual issue that some of the commenters may have missed is that part of the detection is done by matched filtering (https://en.wikipedia.org/wiki/Matched_filter), in which it is necessary to have a good idea of the signal you're looking for. This detection has built upon analytical and numerical advances in relativity. While people may not know about the prevalence of e.g. binary black hole collisions, they have a pretty good idea of the signal that would result if such a collision were to occur. Similarly with other potential sources like binary neutron star collisions.

They also injected fake signals into the detector now and then, partly to keep the analysts on their toes.


I don't think they are expected to tell its fake though? It's hard to do a double blind experiment without a "placebo universe".

That's a rather loaded philosophical question you are asking there - assuming you are serious about the double blind experiment.

Yeah, too many LHC reports have primed people to expect counting experiments where the scientists struggle to get to 5 sigma. The waveforms we're talking about here have a signal to noise ratio over 20.

Sorry, what does that ratio imply?

I'm assuming that the same rules apply as do in straight RF detection. A signal becomes a decent signal at 6db above noise and gets exponentially better every 6db above that. Something 20db above noise is rock solid reliable.

Uhh, 20db isn't the same as 20s/n ratio though?

SNR = 10log(Ps/Pw), solving

Ps/Pw = 10^(SNR/10) = 10^(20/10) == 100

A signal 20db above the noise, you could put your eye out with it.

db is confusing, when you're talking voltage it's 20log(Vs/Vw) And in absolute terms engineers talk about the power over 1mW.

Myself I get miffed a bit because people have been conditioned to think in terms of trying to pull facts out of crappy data sets using poorly thought out statistics. However in a lot of engineering and physics fields the data is often really good. Often good enough that you can work off a single measurement.

That one can be very confident the effect observed was real.

Are you saying there is some kind of confirmation bias at play, or does this increase confidence in the result?

The latter.

> And then the ringing stopped as the two holes coalesced into a single black hole, a trapdoor in space with the equivalent mass of 62 suns. All in a fifth of a second, Earth time.

Am I reading this correctly, that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided?

Was that extremely coincidental? Or do these events happen all the time, and so if it wasn't those two black holes it would be two others?

The predictions for the LIGO detection rate are very poor. They're based on a sample of just a handful of binary pulsars observed in our Galaxy, which would produce NS-NS mergers. The BH-BH merger rate is almost totally unconstrained, although it is generally thought to be less than the NS-NS merger rate. So the fact that a BH-BH merger was the first detection, and the fact that it was detected so soon after the sensitivity increases is evidence that the BH-BH merger rate is probably somewhat higher than expected. But we won't know for sure until LIGO detects more events and the rate can be better constrained. Sometimes you do just get lucky.

I should add that there are lots of selection biases and educated guesses in all of this, too. The signal from BH-BH mergers is louder and easier to detect from larger distances. At the same time, NSs are probably more common than BHs, but it's not really clear whether there are more NS-NS binaries than BH-BH binaries because NSs receive kicks from the supernova when they are born but BHs (probably) do not. This may have the effect of blowing apart many nascent NS-NS binaries but leaving the BH-BH binaries intact.

If this events are so rare (that we don't even know how rare they are), how is it possible that they achieved the required certainty (5 sigma)?

I guess you could count one looong wave as a series of one-time events/measurements, but it could as well be a loooong interference.

From the paper: "To account for the search background noise varying across the target signal space, candidate and background events are divided into three search classes based on template length. The right panel of Fig. 4 shows the background for the search class of GW150914. The GW150914 detection- statistic value of ρˆ_c = 23.6 is larger than any background event, so only an upper bound can be placed on its false alarm rate. Across the three search classes this bound is 1 in 203 000 years. This translates to a false alarm probability < 2 × 10^−7, corresponding to 5.1σ. A second, independent matched-filter analysis that uses a different method for estimating the significance of its events [85,86], also detected GW150914 with identical signal parameters and consistent significance" (https://dcc.ligo.org/LIGO-P150914/public). Take a look at Figure 4 as well.

In case you'd like to dig deeper, the 85 and 86 mentioned are:

[85] K. Cannon et al., Astrophys. J. 748, 136 (2012).

[86] S. Privitera, S. R. P. Mohapatra, P. Ajith, K. Cannon, N. Fotopoulos, M. A. Frei, C. Hanna, A. J. Weinstein, and J. T. Whelan, Phys. Rev. D 89, 024003 (2014),

It's not a counting experiment, which makes the calculation of a false positive rate somewhat harder. The key for LIGO is certainly that they saw the signal coincident at two stations, far apart.

This is about detection.

To put it another way, you need a single black swan to prove that black swans exists (to whatever sigma).

Isn't the point though that the gravitational wave observatories are looking specifically for "black swans" rather than just observing swans generally. So when a swan with a lower reflectivity is observed then it now fits the "black swan" profile. Could be just a swan covered in soot; you need more data to show that this swan is always black or that the lower reflectivity wasn't caused by a measuring anomaly, etc.

I may have pushed the analogy too far!


This comment is making the page formatting gross. Those special characters with the strike-throughs make the entire page over-wide, thus requiring horizontal scrolling to read comments.

The browser layout engine should break on the spaces (Chrome does). They are just normal spaces, the combining character should have no effect. You have a bug somewhere.

Also, I cannot edit nor delete it now, so tough luck!

        " "    SPACE	                        Basic Latin
        "̶"     COMBINING LONG STROKE OVERLAY	Combining Diacritical Marks

Gecko doesn't seem to break that line.

The Higgs detection was not a single event but resulted from the statistical analysis of many events.

Thank you for correcting me!

Because of the shape of the event, detection in two places, and more importantly, it matching the signature of the theoretical event extremely closely (especially the ringing at the end)

The detection uncertainty is a separate matter from the predicted rate. Sure, if you had a strong prior that GWs should be detected once in a billion years, then you would want a better detection. But as it is the priors on the detection are pretty weak and this is totally consistent with what is expected.

Both detectors detected the same event at slightly different times

About 7ms apart, which ties in well with the distance between the two detectors.

Others have answered other aspects of this, but as I understand it, it is not the case that we don't know how rare they (BH-BH events) are because they are so rare, we don't know how rare they are, because we don't have a really good model for them. So, we don't know how often we'd expect to detect them, once we had a detector.

I believe the issue is that we don't know of they are rare at all. Perhaps they occur quite frequently. That is the big question.

I recall reading some years ago that gravitational wave would be used to prove multiverse theory. How would that scale compared to bh-bh or ns-ns mergers?

Also, have read today that this discovery backs inflationary theory, how so?

It seems highly unlikely that they could say a specific bh-bh merger was the cause. It seems implied they are triangulating the source, with two detectors?

AFAIK no multiverse theory has yet been put forth that is experimentally testable (even in theory given infinite time, energy etc.) So it's not a proper (falsifiable) scientific theory at present, merely a (in my opinion wild) conjecture.

> no multiverse theory has yet been put forth that is experimentally testable

Just to be clear here; that's because there is no theory for a multiverse. Not yet, anyways. Nobody has put one forth yet. When you hear "multiverse" come out of physicist's mouth, it's because it's a concept indirectly related to other theories. The current popular theory which involves a multiverse is string theory. When string theorists do the math, there is some evidence that a multiverse is possible.

However, that doesn't mean much. Even if string theory was correct and little strings are really the fundamental component of everything in the universe, the multiverse part of string theory could still be wrong. The theory isn't reliant on it, it just doesn't forbid it.

I also wouldn't say that it's entirely untestable. There are a couple things that could be indicative of a multiverse that some physicists have looked for: http://phys.org/news/2010-12-scientists-evidence-universes.h...

The source isn't the greatest, but it shows that we can look at the CMB for indirect evidence. With higher resolution scanning years in the future, such a theory may be testable. I only mention this because the way your comment reads, it sounds like you're saying a multiverse would be inherently untestable.

These are very good points. I do agree that it's possible someone could come up with a testable multiverse-type theory.

How about a theory where spacetime touches upon another spacetime and interacts with it, perhaps through gravity?

Or if spacetime folds back onto itself?

You are suggesting possible future events that would provide evidence for a multiverse. That does not make something a falsifiable theory.

Analogy: it could happen that tomorrow Jesus Christ descends from the heavens and brings the day of reckoning. That would prove Christianity to be true, but the fact that this could happen does not make Christianity a falsifiable theory.

A falsifiable theory is a theory that predicts something that we can (in theory) measure today (possibly requiring infinite resources etc).

Your second example (which is not a multiverse theory at all) is actually a good example of a falsifiable theory. People have calculated [1] that if spacetime was folded back onto itself at even just a single point, it would leave a distinct signature in the cosmic microwave background. We do not observe this signature, so we are pretty sure spacetime does not fold back onto itself.

[1] http://arxiv.org/abs/1108.2842

Ok. What if spacetime folds back onto itself over a very long distance? Wouldn't that be (viewed locally, with our limited instruments) as if another version of spacetime touches upon our version of spacetime?

Physics will always be based on observations. Consciousness is fundamentally hinged TO observation. I'd argue that the way your brain works is more fundamental to reality than the physics causing a Mhz of conduction throughout your synapses. In summary, all bullshit theories are possible in spirit of deceit. For why should good senses be wasted on a cohesive system when the mind is simply a slave to its own devices?

I haven't heard of any credible connection between gravitational waves and multiverse theories.

Is there any idea how far away these black holes were? It would be interesting to know the volume of space it can potentially detect evens.

TFA says "they had heard and recorded the sound of two black holes colliding a billion light-years away" and "1.2 billion years ago".

And from the paper: "The source lies at a luminosity distance of 410+160-180 Mpcc corresponding to a redshift z=0.09+0.03-0.04.". (https://dcc.ligo.org/LIGO-P150914/public) Which corresponds to 1.337+0.522-0.587 billion ly (or between 750.2 million and 1.859 billion ly).

Oops, missed that. Thanks!

Looks like there are roughly three million galaxies within a billion light years. Seems like lots of space for black hole pairs to live in. I suppose over the coming years, these gravity wave observatories will nail down just how common they are.

So that's wayyyyyyy outside our galaxy? Any idea how many galaxies fit into a 1 billion ly sphere around the milky way? I'm guessing a shit ton, which makes the detection of a bh merger seem more realistic to me.

That's some serious range!

It was mentioned during the press conference today that gravitational waves are not affected by interstellar/intergalactic dust the same way light is. In theory, once our detectors are good enough we should be able to use gravitational wave astronomy to peer all the way back to the big bang!

So, we'll finally be able to hear what god said at the start of the universe?

This is a beautiful premise for a mind bending book.

I'm sure He either said "let there be light" or "gee, that's funny...."

I'd prefer either "what does this button do?" or "shit, don't press that!".

Those are what you get when you figure out how to construct a machine that looks to the future and can hear what God says at the end of the universe.

"I clicked print and nothing happened!"

If these waves travel at the speed of light shouldn't the distance and time match up...?

This would be true in a static universe, but, during the 1.2 billion years the waves have been traveling, the universe was experiencing accelerating expansion.

For example, the edge of the observable universe is about 46.5 lightyears away, while the universe is thought to be 13.8 billion years.


is about 46.5 lightyears away

I assume you mean 46.5 billion?

Yeah, thanks! I can't edit it any more, unfortunately.

Not if space time it'self is stretching while the light is in transit.

That's around 1/60th the diameter of the observable universe!

Well, the number of solar-mass black holes in our galaxy is about 10^8. Since black holes form from stars, you can assume the probablity of having binaries is probably related to the probablity of having binary systems in stars, which is high. And the distance to the event is several megaparsecs (much bigger than our galaxy). The fact that they detected two 30 solar mass black holes coalescing 2 days after their sensitivity upgrades says that they almost certainly have had other, less pretty, detections in the few months they've been running their detectors for. Or they should go buy some lottery tickets.

What does that mean in plain English?

Estimates about the frequency of observable events are not yet very good.

Do you know the current best estimate for BH-BH and NS-NS mergers? (Or at least, what they were before this experiment moved the needle).

Interested to know what they were shooting for when they spun this experiment up.

>> that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided?

Counterintuitive, but yes. Because it happened billions of years ago, it happened a long long way away. The sphere of objects billions of years away/ago is far larger than those closer to us. So such a detector should be detecting exponentially more very old objects than new ones. Given the rarity, I would expect nearly all detected events to have happened long long ago in galaxies far far away.

Also, models point to such events being more common in the distant past where there were more black holes (primordials) floating around than there are now.

*Polynomially more objects.

The volume of a constant-thickness spherical shell is O(r^2).

*Quadratically more objects.


less than quadratically because space is expanding in between

If one event happens 1B years ago 1B light years away and another event happens at .5B years ago .5B light years away... how would we know there are two events?

The wave may contain the information necessary to describe the event that created it. If you know, perhaps by frequency, that the wave came from two super-massive black holes spinning around each other at a given rate, then you know how massive each hole should be. From that you can predict the energy of the wave. And from that you know how far away the event must be for the energy behind the wave to have spread so thin. It's basically the same process as calculating distance via cepheid variable stars.

(not a physicist) The universe is expanding, which makes the wavelengths longer the farther away the source is.

I am not a physicist, and dont have a good mental model of gravitational waves (or general relativity at all), so maybe someone can answer my laymans question: do these waves behave like ripples in water, so a single event generates multiple repetitive, concentric waves permeating through space? That would make me think that a single massive event would be easier to detect because it would leave many repeated "echoes", ringing space for a long time after the actual event.

The article makes it sound like the detection of these waves is just a quick one-time blip though. I'd expect something as big as black holes merging to generate more longer lasting waves than just a quick blip. What is the period of these waves?

https://www.black-holes.org/gw150914 has some visualization of the event. There is the initial inspiral, and then there is a ringing afterwards. However, the entire event is over in a fraction of a second, which may be a "blip" to humans, but is very long when things happen at the speed of light.

Thanks, that's helpful. It's hard to get my head around the idea that an event so massive can be over so "quickly", without any residual longer-lasting effects.

>without any residual longer-lasting effects

A massive ripple in the very fabric of reality?

Well it sounds like a massive ripple in the fabric of reality which passes by us in a fraction of a second, never to be seen again. So from my non-physicist point of view, no it doesnt seem like a very long lasting effect, relative to us at least. Thanks for the snark though.

You're left with a big-ass black hole...

There's a theorem that black holes have "no hair": two black holes of the same mass, charge and angular momentum are indistinguishable. So the merge must happen instantaneously: if the combined black hole were "sloshing" afterwards that would violate that theorem.

Whether or not black holes have "no hair" is currently an unsolved problem.

Hawking, for instance, believes that the Hawking Radiation from a black hole encodes the information that went into creating the hole.


"do these waves behave like ripples in water, so a single event generates multiple repetitive, concentric waves permeating through space?"

No they don't. There is a law known as [Huygens' principle](https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_princi...) which says that when a disturbance at a particular point creates a wave, that wave only propagates on an outwards-expanding sphere that is centered at that point of disturbance, and does not produce any effect on the interior of that sphere. This was originally formulated for light waves, but it also holds for other kinds of waves, such as sound waves or, in this case, gravitational waves. What this means is that when you look at something that's far away you see a sharp image of exactly what happened there a short time ago (the time it had taken the light to reach you), whereas if the principle did not hold, each light source would have a small "echo" after it which would blur the image.

However, one of the reasons Huygens' principle holds is that the waves are propagating over three dimensions. In contrast, water waves only propagate over two dimensions, so Huygens' principle fails. That is why ripples continue to emanate from a spot even long after the disturbance there is over. More generally, Huygens' principle holds whenever the number of dimensions is odd and fails whenever the number of dimensions is even.

[Note: I may be wrong on why Huygens' principle fails for water waves. Water waves are actually pretty complicated compared to other kinds of waves and I am not knowledgeable in all the subtleties.]

An echo is a reflection. I don't know whether gravitational waves can be reflected even in principle, but even if they can, space is so empty that in practice there's nothing to reflect them. So even a single massive event, like the one described in the article, will just send out a single expanding spherical wavefront; if you're not listening at the right moment, you'll miss it.

Sorry, "echo" wasnt the right term (hence my quotes).

What I am trying to ask is if these behave like concentric water ripples, where from a single event you get first one peak of a wave, followed by many more repeated concentric peaks gradually getting smaller in amplitude? It sounds like there is just a single momentary wavefront without any residual secondary waves? Why is that?

So if you look at the waveform of the signal, there are in fact smaller ripples after the main event. However, how long these ripples take to settle afterwards to equilibrium is related to how quickly the waves propagate. In the case of ripples on a pond, those travel at about 1 m/s; these gravitational waves travel at the speed of light, roughly 300,000,000 m/s, so we should expect it to settle to equilibrium about 300,000,000 times faster. If it takes 60 seconds on a pond, we would expect the gravitational waves to settle in about 0.0000002 s, or 200 nanoseconds.

Note that this is a _very_ rough estimate, but it should give you an idea of the order of magnitude for the settling time.

They state in the article that four events were detected during the engineering run. In the future tens of events could be detected per year.

> Was that extremely coincidental? Or do these events happen all the time, and so if it wasn't those two black holes it would be two others?

From the article, no one knows: "Black holes, the even-more-extreme remains of dead stars, could be expected to do the same, but nobody knew if they existed in pairs or how often they might collide. If they did, however, the waves from the collision would be far louder and lower pitched than those from neutron stars."

The coalescing and ring-down takes a fraction of a second. The fraction of a second refers to the duration of the event. It does not mean they caught an event a fraction of a second after they turned it on.

Here's a better article:


In the press conference Kip Thorne mentioned that the estimates were for several events per year, before sensitivity upgrades.

But he neglected to mention the error bars on this, which AFAIK are huge at least for BH-BH mergers. Every time we built a new instrument, we saw something new, whether in astrophysics, or nuclear physics, or particle physics. Maybe the BH-BH rate is much higher than expected.

> Or do these events happen all the time, and so if it wasn't those two black holes it would be two others?

This is right. Soon we'll have a much more precise value for "all the time!"

Typically one would assume that it was not coincidental and then adjust the bounds for how often we expect the event occurs based on observations, or lack thereof.

Excellent catch. That's damn suspicious.

I may have suffered permanent eye damage from the rolling.

Whatever you do, don't visit http://deepinsidetherabbithole.com/Is_the_earth_a_ball_.html or you'll be chasing your eyes across the floor.

From the abstract of the paper, energy equivalent to three solar masses were radiated away in gravitational waves. That's a simply incredible amount!

Possibly stupid question: Given how far away it was, and that the inverse square law applies, would the effect of these waves be visible on the human scale if we were closer? We can see the effects of the compression of spacetime with LIGO after all, so presumably we could?

This thing was a billion light years away. Say it were closer; let's put it at a single light year away.

LIGO measures wave amplitude, as far as I can tell, which goes down linearly with distance (unlike wave energy, which goes down quadratically, since it's proportional to square of the amplitude). So we could expect to see an effect about a billion times bigger.

The detected effect was a change in metric of one part in 6e20 if I'm not mistaken: (4e-3 * (diameter of proton))/4km based on the article's claim of "four one-thousandths of the diameter of a proton". So at one light year distance we could expect an effect of one part in 6e11.

Not really visible on the human scale, seems to me. You could detect it easily with something like the Mössbauer effect, I expect. Your typical lab bench laser interferometer has errors on the order of 1 in 1e6 as far as I can tell, so probably wouldn't be able to pick this up.

Disclaimer: I could be totally off on what a lab bench laser interferometer can do. I'm pretty confident in the rest of the numbers above.

On the other hand, we may well detect the 3 solar masses radiated away as energy. That decreases as an inverse square law, so as one solar mass is about 10^30kg, and 1kg gives off about 10^17 J, we're talking about an explosion releasing something around 10^47 J. For comparison, a 1 kiloton nuclear bomb gives off about 10^15 J.

So, inverse square that explosion... 1 light year is about 10^16m, so we square that and get 10^32m, so we're now talking about ... 10^15 J.

So, unless my maths is all off (which is possible), if this happened about a light year away, whoever's on the side facing towards the blast wouldn't get to observe very much because they'd feel as if a 1kt nuke just went off above their head. Not a great way to start the day.

Chances are it would wipe out life on Earth too, through the ensuing side-effects like lighting the atmosphere on fire, sterilising half the planet, significantly heating up the oceans, possibly even stripping part of the atmosphere away, etc.

For a great novel based around a strikingly similar premise to what was just observed (and the main reason I even bothered to calculate this), Diaspora by Greg Egan is a fantastic book.

The big question is how much of the energy would get transferred in practice.

I agree that 3 solar masses worth of electromagnetic radiation at 1 light year distance would feel like a nuke going off. What I don't know is to what extent the energy of the equivalent gravitational waves (which _would_ have a lot of energy I agree) would actually get transferred to things we care about, like the atmosphere and us. If it's a few percent, say, we'd clearly be in trouble. If it's more like what neutrinos do, it would probably be detectable but probably not by unaided human senses.

I tried doing some quick looking around for estimates of gravitational wave coupling and energy transfer and didn't find anything so far...

I would like to understand why a gravitational wave distorts length in relation to normal gravity wells; specifically is this particular to waves? Why don't lengths get distorted in a normal gravity well, or do they? In essence, what is different between a gravity wave and a gravity well, which i understand both distort space, but only the wave distorts it in a way we can measure? Does the gravity well change lengths proportionally in all directions and thus isnt measurable?

A gravity well also distorts lengths, as best I understand (which is not very well, to be honest; take everything I'm saying here with a big grain of salt).

The difference in terms of detection is that the wave does this in a time-varying, periodic fashion.

For something like LIGO, we're trying to measure length changes on the order of 1e-18 meters. We're not actually measuring the lengths of LIGO's arms to that accuracy, though. What we're measuring is the difference between the times light takes to travel down those arms. And even that's hard to measure on an absolute scale, so what we really measure is how that difference changes in time.

Or put another way, the effect of Earth's gravitational well is not really distinguishable from inaccuracies in making the two legs of the interferometer equal length to start with, and is a much smaller effect than those inaccuracies. Again, if I understand this right...

Actually, we have ample proof of the distortion of spacetime in a gravity well - gravitational lensing. It's an observed effect around very massive objects and we have been able to see it at work very well. Also, arguably, the fact that we're not falling towards the sky is itself evidence of a spacetime gradient near the Earth, but that was also explained by Newton's Law of Gravitation.

But back in 1916, Einstein also theorised, as part of his general theory of gravitation, that there would be such things as gravity waves, caused by very massive objects moving through spacetime making 4-dimensional ripples appear in spacetime. Until today, that was just an unproven theory, though everyone believed it was likely to be true. There is now solid evidence to back it.

Agree... my question, though poorly worded, is less about proof of spacetime gradients (they do in the ways you describe).

It's more about understanding what the measurable effects of a gravitational well on earth has on the LIGO experimental setup (or a similar one with infinite precision), in the absence of gravitational waves.

Well, something like LIGO can only measure gravitational waves, because it looks for changes in the geometry of spacetime. If you were to move the LIGO in and out of Earth's gravitational well, I guess then it would record a shift.

That is a good point - perhaps it would all come out as neutrinos or gravitons... then we'd be fine. I doubt such an event would result in no electromagnetic radiation at all however... Why would it? The creation of a new black hole typically releases enormous amounts of all kinds of energy, electromagnetic as well as in the form of neutrinos.

I really don't have any good answers here; this just isn't something I know very much about....

If the gradient is small (as it should be 1 ly away) then the coupling ought to be very weak.

The energy was all dumped into the gravitational waves we detected, not into electromagnetic radiation: Gravity waves don’t interact with matter very much (the cross section of the graviton is believed to be extremely small) so the quantity of energy transferred to matter as the wave passes through is likewise extremely small. I haven’t run the numbers, but I’m not sure you’d notice this even from a light year away without fairly sensitive detectors.

What I didn't understand after reading the article is how do they separate out a set of waves for one specific thing vs. the many other objects sending out waves. Is it just that the set of blackholes are the strongest set of waves and thus the ones we can detect?

That, and the high frequency. For example, the Earth orbiting the Sun produces gravitational waves at a frequency that's about (factors of 2 and pi here and there) the orbital frequency; order of 1e-8 Hz. The black holes were producing 250Hz waves if I read the article right.

The longer the interferometer arms, the better you can do in sensitivity. The reason LIGO has 4000 m long arms is that it makes the experiment 4000x more sensitive than something you can do on a bench. (and their laser stabilization is excellent, improving things further)

Sure, but LIGO is sensitive at something like 1 part in 1e20, which is a lot more than 4000x better than 1 part in 1e6. I agree that their laser stabilization is likely much better, their vacuum is likely a lot better, etc. I was just surprised by how much better, I guess; 10 orders of magnitude is a lot.

Part of the reason for that is that LIGO isn't exactly a Michaelson interferometer in that it has an extra pair of mirrors in each arm. If you look at this schematic [1] then in a traditional Michaelson interferometer you would only have the mirrors that are at the end of both arms.

With LIGO there is an extra set of mirrors within the arms this allows the light from the laser to bounce between them ~100 times or so increasing the effective path length greatly.

[1] https://www.nsf.gov/news/speeches/colwell/rc03_ligo/img009.j...

Ah, neat. Did not know that!

I see, thanks.

Yeah,I got to the point mentioning the masses of the black holes before and after collision and said, "What, they didn't just lose three solar masses..." But, they did.

Which was the order of predictions I'd read, years back, but egads. Considering how much larger that is than a supernova, I'd be concerned to have such an event happen in this galaxy...

The energy is dumped into gravitational waves rather than electromagnetic radiation & they don’t interact with matter much. I’m not sure you’d notice it happening in the same galaxy unless you were looking for it.

Yes, mind-boggling. 3 solar masses converted to energy in just a few seconds.

0.5 seconds actually.

In this case it's not an inverse square law, the amplitude is simply inversely proportional to the distance.

Yup. Possibly one of the most energetic events in the Universe. Fascinating in many ways when you think about it. That mass was once matter, and somehow it got converted into gravitons.

According to this paper ( https://dcc.ligo.org/LIGO-P150914/public ) they detected the signal first at Livingston, Louisiana and 6.9ms later in Hanford, Washington. The distance between them according to wikipedia ( https://en.wikipedia.org/wiki/LIGO ) is 3002km (Ok, the 3002 km distance is on the Earth). If the gravity wave travel at the speed of light they should detect 10ms later (300 000/3002 sec = 1/100 sec = 10ms ). From these data the gravity travels at 434 000km/sec instead of 300 000km/sec. Almost 50% faster then light... Is there any error in my calc?

10ms is the absolute maximum difference in time, if the source was located on a line running through the two detectors. If the source was located on a perpendicular bisector of the line running through the two detectors, the difference in time between detections at the two detectors would be zero. Any value between the two is possible depending on the geometry.

I think your calculation assumes that the waves are traveling parallel to the line connecting Livingston/Hanford. In the diagram below, 's' is the source of the waves.

If instead the waves are traveling perpendicularly to the line between those two cities, they should be detected at the same time.

     / | \
Since the measured time difference is between 0ms and 10ms, the reality is probably somewhere in between these two extremes.

Weird, that's exactly what I was thinking.

Wild guess on my part. The wave is traveling through the earth, while the distance you measured is around the circumference and therefore larger.

Too bad, you had me excited for a moment at the thought of faster than light travel.

Even ignoring the curvature of the Earth, the signal source was not necessary located on the straight line between two LIGO locations, but rather at some angle to this line. For example if the signal origin was on the line that is exactly between the two LIGO detectors, the time delay would be zero.

I am just curious that is the speed of gravity wave travel at different speed in different medium?

Consider that the black holes merged about 1.3 billion years ago. If gravitational waves travelled 50% faster than the speed of light, they would've passed by earth long before our species came around, unless the effect of the collision went on for, say, a few million years after the event?

How do the detectors work? In my mind they don't make physical sense. They're saying the distance between the mirrors changes, but I don't understand how that's possible in this context.

Let's say a gravitational wave compresses space. To someone inside that compressed space, there should be no noticeable difference. Light will still flow the same way through the compressed space at the same speed relative to the compression. Matter will behave identically, because both light and matter are part of the fabric of that space. As I understand it, the only way the mirror lengths could change is if space is created or destroyed.

If that doesn't make sense, consider the 2d analogy of drawings living on paper. Assume also that light moves only along the surface of the paper. If you bend the paper, the light will bend with it. But when you bend the paper, the creatures living on the paper can't know it's bent. The fabric of the paper is still identical. Even if some of the paper gets compressed in one direction, it will still have the same amount of particles, so any light travelling through there will hit the same amount of resistance. And stretching the paper, even if you're a drawing on the part being stretched, would have no effect. A 2d creature looking at something 1 foot away, even if the paper is stretched to 10 feet, won't see any difference, because the fabric light travels through is also stretched.

The only way I can see this making sense is if light travels independent of the fabric of space, but it's my understanding that light travels through it, not independent of it?

I think the crucial detail you are missing from the article is this:

"According to the equations physicists have settled on, gravitational waves would compress space in one direction and stretch it in another as they traveled outward."

LIGO is two sets of 2 L-shaped antennas spread far apart on the globe, so that we can compare the compression of space in orthogonal directions and measure the very short delay between the gravitational wave hitting the first detector followed by the second. In this case, that difference was 7 milliseconds, which is also consistent with the speed of gravitational waves (also the speed of light)

I still don't understand. It doesn't matter where the compression happens, because it should be undetectable to any light/matter that's fundamentally a part of that space? If one of the arms gets compressed - the matter will be compressed too, so light still has the same density and amount of space to travel through?

Check the comic posted by AdrianN, it explains what you're missing. Basically light takes longer to travel stretched space (but matter does not, as you correctly said).

I see. If that's true, then light travels through a higher dimension, and this is definitive proof of at least a fourth spatial dimension. Otherwise there would be nothing for the 3d space to ripple through, or for light to travel through.

I'm surprised I haven't heard that light travels independent of 3d space compression before. That would also imply that if you enter a black hole with your feet at the bottom, you would see them visibly stretched far away from you (noticeably? I'm not sure) because light would take longer in the distortion to reach your eyes.

Doubts about higher dimensions and general relativity is common and a crucial point, so I dont think you should get downvoted.

Some points which might be helpful. We have a way, using the concept of a manifold, for ants on a surface like a sphere or a dougnut to figure this out without appealing to a third dimension. One could imagining say ant geographers making maps of portions of the surface, and noting how common regions covered in two different maps have different labels/coordinates. One can then figure out a definition of when two collections of maps(called an atlas) are equivalent and then show that an atlas for a plane, sphere, doughnut are mutually nonequivalent.

But all this is topology and involves global considerations. What is relevant here is local curvature. We can also do this appealing to an extra dimensions. Now, you used the example of a folded paper and you are correct that for an ant on the surface, the curvature is indetectable. The curvature of the paper is extrinsic and not intrinsic. We say that is isometric to flat space, and its curvature tensor is 0.

On the other hand, if the ant was on the surface of a ball, it could figure out this curvature intrinsically, for instance, by measuring sum of the angles of a triangle or the distance between parallel lines keeps shrinking. Not only is this intrinsic, but it is locally measurable. One cant have maps, even for a small area of the earth's surface, without some kind of distortion because of this intrinsic curvature.

An additional complexity - in GR, spacetime is curved rather that just space. Also, dont take 'curvature' too literally, it is just a way of measuring deviation from numbers that you would get in the flat scenario.

For more read up on manifolds, riemann curvature. John Baez had some essays on the geometric meaning of the curvature tensor in terms of the volume of a ball relative to the usual flat Euclidean case.

I don't really know how you jumped to 'higher dimensions' from that.

Thanks - still not sure I get it. The "fabric" is stretched in x, squeezed in y, sure. Is it that the wavelength of the light is -not- stretched? Guess I need to go back and study physics again :/

When one arm gets longer the laser takes a little longer to travel through it. That changes the interference pattern.


Still not explaining it. If spacetime is stretched why doesn't light "speed up" to accommodate for it is what he's asking. I'm assuming it's because the speed of light is invariant to that.

Exactly. The speed of light in a vacuum is constant no matter how quickly/slowly your frame of reference is (Special Relativity). You can think of it as the space "stretching/shrinking" because of the gravitational wave, as that is how it looks from the point of view outside the experiment.

Another way to look at it is to change your reference to be internal to the experiment. You can imagine the experiment moving through space-time at a constant rate of speed. When the gravitational wave hits the experiment its movement through space-time changes an infinitesimal amount (faster than slower as the wave passes, or vice-versa). Since the speed of the light passing through space-time has not changed (due to special relativity), the difference between its start and end points can be used to measure the amount of change caused by the gravitational wave, essentially in the same way you can use a laser to measure the speed of a moving object relative to a stationary object. Only you are basically "bouncing" the laser off of the experiment itself to determine the change.

Here's my re-statement of this confusion, isn't everything we can experience embedded in time-space, including the LIGO experiment itself? So how is there any relative shift allowed to be detected when everything we know is fundamentally intrinsic to time-space? That is, I too would appreciate having this mis-conceptualizing, of mine, cleared away.

Another re-phrase: how can we detect that space has stretched out if all of our rulers also get stretched by exactly the same amount?

The answer is that we have a ruler that doesn't get stretched in this way: light. The speed of light is a constant dictated by the laws of physics; stretching out our flashlight to twice its normal size wouldn't make the light it emits go twice as fast. So if you just measure the time it takes for a light ray to go from one point to another, you can compute the distance that it must have traveled, and if that distance changes, then you know that the space in between must have been stretched.

Thank you. i think that settles my confusion. (it's somewhat like we witnessed a length/Lorentz–FitzGerald contraction in a situation where there was apparently no reason to witness one). Now i can go back to being simply amazed by it all.

Thanks for the nice explanation! But another question: since the expansion rate of the universe has been different at different times, does this mean that the measured speed of light would be different at different times as well?

Because if the ruler you are using to measure is expanding at rate x, the measured speed of light would be different than when the ruler is expanding at rate y?

So, during the deflationary period of the universe, the speed of light would be significantly smaller, correct? In fact, would it be "negative" due to the universe expanding faster than light?

Does this mean that in earth's gravity well, there is an absolute difference in the time light takes to travel compared with light travelling in the void of space?

Can we compute the strength of a static gravity field we are inside, by measuring the time that light takes to propagate through it?

The light is constant, which means it moves at the same speed in both cases (assuming the light is in a vacuum). It won't move faster or slower based on the gravity field (other than in the case of black hole where it can't escape at all).

What happens instead is that the speed that an object moves through space-time changes dependent upon gravity. Using an atomic clock, we've actually measured the effect of gravity to show that time moves more slowly down on earth than it does in an orbiting satellite.

Thanks for this explanation. The universe is a pretty freaking amazing place.

For someone like me who knows next to nothing about this, that video was extremely well produced and it explained everything i was wondering about.

Jonathan Corum has had a fun career to watch; he's (I think?) a student of Tufte's. I discovered him with brunch.org; also, check out his style.org.

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