Physicists Detect Gravitational Waves, Proving Einstein Right - <AUTHOR> http://www.nytimes.com/2016/02/12/science/ligo-gravitational-waves-black-holes-einstein.html <comment><USER4> 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](https://en.wikipedia.org/wiki/Graviton)

<comment><USER5> 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.

<comment><USER6> 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).

<comment><USER7> > 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.

<comment><USER8> Photons are massless particles that have energy. All massless particles travel at the speed of light.

<comment><USER7> 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

<comment><USER9> What? Protons most definitely have mass.

<comment><USER10> They obviously meant photons

<comment><USER7> Yes, fingers kept typing wrong word.

</comment></comment></comment></comment></comment></comment></comment></comment></comment><comment><USER11> 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.

<comment><USER12> 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.")

<comment><USER13> 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?

<comment><USER12> 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).

<comment><USER14> 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?

<comment><USER15> 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.

[https://www.wolframalpha.com/input/?i=mass+of+the+solar+syst...](https://www.wolframalpha.com/input/?i=mass+of+the+solar+system+*+5.6+%2F+%283%C3%9710^25+kg%29)

<comment><USER16> 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.

<comment><USER12> 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.

</comment></comment></comment></comment></comment></comment></comment></comment><comment><USER17> 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](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.

<comment><USER18> They also injected fake signals into the detector now and then, partly to keep the analysts on their toes.

[http://www.ligo.org/news/blind-injection.php](http://www.ligo.org/news/blind- injection.php)

<comment><USER19> 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".

<comment><USER20> That's a rather loaded philosophical question you are asking there - <USER2> you are serious about the double blind experiment.

</comment></comment></comment></comment><comment><USER21> > _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?

<comment><USER22> 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.

<comment><USER23> 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.

<comment><USER24> This is about detection.

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

<comment><USER25> 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!

</comment></comment></comment></comment></comment><comment><USER26> 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?

<comment><USER27> 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.

<comment><USER28> 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.

<comment><USER27> 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...

<comment><USER29> 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?

<comment><USER27> 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...

<comment><USER28> 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.

<comment><USER29> 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.

<comment><USER28> 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.

</comment></comment></comment></comment></comment></comment></comment></comment></comment><comment> hun-nemethpeter According to this paper ( [https://dcc.ligo.org/LIGO-P150914/public](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](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?

<comment><USER30> 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.

        H-----L</comment></comment><comment>-s

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

           s
          /|\
         / | \
        L-----H

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

<comment><USER31> Weird, that's exactly what I was thinking.

</comment></comment><comment><USER32> Paper here: [https://dcc.ligo.org/LIGO-P150914/public](https://dcc.ligo.org/LIGO-P150914/public)

</comment><comment><USER33> 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?

<comment><USER34> 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)

<comment><USER33> 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?

<comment><USER35> 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).

<comment><USER33> 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.

<comment><USER36> 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.

</comment></comment></comment></comment></comment></comment><comment><USER37> 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.

<comment><USER38> 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.

</comment></comment><comment><USER39> Weird nobody mentioned here the excellent threads on this topic here [https://www.reddit.com/r/science/comments/458ppo/ligo_makes_...](https://www.reddit.com/r/science/comments/458ppo/ligo_makes_gravitational_wave_announcement_today/) and here [https://www.reddit.com/r/askscience/comments/458vhd/gravitat...](https://www.reddit.com/r/askscience/comments/458vhd/gravitational_wave_megathread/)

</comment><comment><USER40> Are there any potential competing theories this detection could also support? I'm wondering how much room there is here for confirmation bias, but I suppose that's a pretty hard thing to measure without the benefit of hindsight.

<comment><USER41> Even before this discovery, it's been pretty solidly established that any alternative theory to General Relativity would need to behave essentially identically to GR in the limits where we've been able to test it. So, for example, the "low energy limit" of string theory is general relativity (plus other content, in most cases). I'm not sure whether the loop quantum gravity folks have a working low-curvature limit yet (I'm out of touch), but that would be a requirement for them, too.

At first glance, I'd guess that this discovery only strengthens that conclusion: even a small deviation from GR might well change the detailed behavior of an immensely high curvature situation like a black hole merger, and what we saw seems to have been a spot on match for the GR-based models.

<comment><USER42> Well, they extracted a lot from the waveform: Distance, the two masses, the resulting mass. I could imagine that a competing theory gives the same waveform maybe with different values for these parameters.

<comment><USER43> A "competing theory" would first have to match the GR predictions in all the other regimes where it's already been tested. But doing that is an extremely strong constraint on a theory, to the point where the only theory that can meet it is GR itself. Physicists know this because alternative theories to GR have been constructed and tested, and they have all failed. See, for example, here:

[https://en.wikipedia.org/wiki/Alternatives_to_general_relati...](https://en.wikipedia.org/wiki/Alternatives_to_general_relativity#Results_of_testing_theories)

<comment><USER44> That's a very common sentiment, but a mistaken one. Theories do not get accepted only if they match the predictions of the previous theories. People value other features besides accuracy of predictions, like simplicity and explanatory power. Just recall how Kopernik's theory of solar system got accepted. It had worse predictions than Ptolemy's scheme at the time it was introduced; Ptolemy's scheme was way better in accuracy, but utterly complex and explained little.

<comment><USER43> You're missing the point. I agree that matching the predictions of _experiments_ (not previous theories--I'm talking about experimental results that match the predictions of GR, not just those predictions themselves) is not a _sufficient_ condition for a theory to be accepted (which is what you are saying); but it is certainly a _necessary_ condition (which is what I was saying).

 _> Just recall how Kopernik's theory of solar system got accepted. It had worse predictions than Ptolemy's scheme at the time it was introduced_

Yes, and it wasn't accepted at the time it was introduced. Actually, Copernicus' theory in its original form was never really "accepted"; what was accepted was Kepler's reformulation using elliptical orbits, based on Brahe's more accurate observations. Kepler's model was _more_ accurate than Ptolemy's, and that was a key factor in its acceptance.

<comment><USER44> I agree with you that if a new theory was to replace the old one for making specific set of predictions, it should give predictions of similar or better accuracy. But I do not think _that replacement_ is necessary for the new theory to compete or be accepted; it is the new benefit it brings, whatever its nature may be, that is crucial. The two can temporarily both be accepted to coexist, if both have their strengths. For example, quantum theory does not make the same predictions as classical theory when it comes to classical experiments (mechanics, basic EM phenomena) and is largely useless in that domain. It only gives probabilities of results of specified experiments of certain kinds; it does not reproduce the old predictions (like definite trajectories, Moon phases or solar eclipses), but provides new results (like resonance frequencies of atoms and molecules and their bond energies). Similar thing can happen with a new theory of gravity; it may not give the same prediction for Mercury perihelion precession, but it may be able to explain other things, like why the inverse square law, why no repulsive gravity or why the mutual gravity force between electrons is so much lower than the mutual EM force. Explanation for oddities in Mercury motion could then wait for further data and repetition of calculations. It is natural to expect of any new theory to bring new results, but demanding that it reproduces all the old ones along is too much. That happens rarely and such expectation only prevents any new ideas from being considered.

<comment><USER43> _> quantum theory does not make the same predictions as classical theory when it comes to classical experiments (mechanics, basic EM phenomena)_

Yes, it does. Do you know how the classical limit of quantum theory works? That limit is what allows us to use classical physics in the domain where it works. If that limit didn't work, we would have a serious problem with consistency.

 _> It only gives probabilities of results of specified experiments of certain kinds; it does not reproduce the old predictions (like definite trajectories, Moon phases or solar eclipses)_

Are you aware that all of those "old predictions" can indeed be derived from quantum theory, using the classical limit I described above? The reason that works is that, in the classical limit, quantum theory predicts a probability of 1 for one result--the classical result.

 _> It is natural to expect of any new theory to bring new results, but demanding that it reproduces all the old ones along is too much._

You appear to have a mistaken understanding of how new theories get accepted. New theories that don't reproduce all of the predictions of the theory they replace, in the domains where the old theory is verified by experiment, are not accepted. If general relativity had not reproduced all of the predictions of Newtonian gravity in the weak field, slow motion limit, it would not have been accepted. And if quantum theory had not reproduced all of the predictions of classical physics in the classical limit, it would not have been accepted.

<comment><USER44> As I wrote above, I agree with you on the requirements for replacement theory. My point is that a new theory of a phenomenon does not need to replace and reproduce all the results of the old theory to be considered worthwile, competing, acceptable.

<comment><USER43> Can you give an example of a new theory that was considered worthwhile even though it didn't replace and reproduce all the results of the old theory? I'm not aware of any. (The Copernicus example given upthread is not a valid example, as I said in response to that post.)

<comment><USER44> Schroedinger's theory of hydrogen atom and his wave mechanics (1926). It explained positions of emission lines of excited hydrogen, but it didn't explain how the atoms lose excitation energy as there is no _c_ and no spontaneous emission in that theory. Larmor's older theory (1897) explained how the energy is lost - by EM radiation - and gave formula connecting acceleration and losses that is used to this day.

Joseph Larmor, LXIII, _On the theory of the Magnetic Influence on Spectra ; and on the Radiation from moving Ions_ , Philosophical Magazine Series 5 Vol. 44, Iss. 271, 1897

Erwin Schrodinger, _Quantisierung als Eigenwertproblem_. Annalen der Phys. 384 (4) (1926)

<comment><USER43> _> Schroedinger's theory of hydrogen atom and his wave mechanics (1926)._

This was not a "new theory" that was competing with any "old theories". It was a tentative model in a regime where _no_ previous theory existed, and it was never claimed to cover anything outside that limited regime. It wasn't competing with any other theories, because there were no other theories to compete with. The question of whether or not Schrodinger's model reproduced the predictions of the "old" theory never arose, because there was no "old" theory. (Technically, there was a sort of "old" theory of the hydrogen atom-- Bohr's model--but Schrodinger's model did reproduce all of its correct predictions, plus it added more correct predictions of things that the Bohr model got wrong.)

The position with regard to gravitational waves is very different; we already have a comprehensive, fundamental theory--General Relativity--that explains them. Any alternative theory that only explained GWs, and didn't also explain all the other experimental results that GR explains, would be a nonstarter.

 _> Larmor's older theory (1897)_

This wasn't a separate "theory" at all; it was just a derivation of a particular formula using an already known theory, Maxwell's Equations.

<comment><USER44> Schroedinger theory certainly was a new theory of the atom and later of molecules at that time, successfully competing and largely replacing classical EM models of atoms and molecules such as Larmor's theory of molecules, although it didn't cover the EM radiation aspect and EM theory needs to be used in parallel with Schroedinger's to get, say, intensities of emission lines. I think this is a good example of what I was saying in the first post. It is the new benefit that the theory brings, not reproduction of every single result of the previous theories, that makes the new theory interesting and helps its adoption. Cases where the new theory completely replaces the old theory and reproduces all of its positive results happen too, but are not the only way how new knowledge is adopted.

<comment><USER43> _> I think this is a good example of what I was saying in the first post. It is the new benefit that the theory brings, not reproduction of every single result of the previous theories_

Of course Schrodinger's model didn't reproduce the results of classical EM with regard to the atom. It wasn't supposed to, because those results of classical EM were _wrong_. In other words, there wasn't a correct "old theory" that covered the regime the Schrodinger model covered (the atom)--there was only a wrong "old theory".

As far as using Schrodinger's model plus classical EM theory to get results like emission line intensities, there also there was no correct "old theory"; there was only a wrong "old theory" (classical EM by itself, which did not predict emission lines at all, let alone their intensities--it predicted a continuous emission spectrum). Also, this hybrid classical-quantum model was known to be incomplete at the time; it was only used because nobody had yet figured out how to quantize the EM field.

 _> It is the new benefit that the theory brings, not reproduction of every single result of the previous theories_

Once again, this is _not_ the situation under discussion in this thread (gravitational waves). In the case you describe, the results of the previous theories were _wrong_ in the regime the new model covered, so there was nothing to reproduce; there was no correct "old theory" for the new theory to compete with.

In the case of gravitational waves, we have a _correct_ "old theory"\--General Relativity--so any new theory that did not match that _correct_ old theory would be a nonstarter. I am not aware of any case where a new theory was accepted as interesting when there was a correct old theory covering the same regime and the new theory did not reproduce its results.

<comment><USER44> > _It wasn 't supposed to, because those results of classical EM were wrong._

You're badly mistaken. Although nobody succeeded in obtaining the emission line frequencies of gases out of the classical EM theory, the theory did correctly give other results consistent with observations. One of them is the formula for emission intensity that connects energy radiated with second derivative of electric moment; it goes back to Larmor's work. This _was_ the result the new theory would preferably reproduce or at least be consistent with. Wave mechanics wasn't consistent with it - the hydrogen atom oscillates indefinitely in wave mechanics. Schroedinger himself viewed this as a deficiency and planned to get back to it - check the ending part of his seminal papers on wave mechanics. The classical formula is taught to this day both in macroscopic EM theory and quantum optics courses, although there are some deficiencies and problems about the formula that Larmor did not know.

> _In the case of gravitational waves, we have a correct "old > theory"\--General Relativity--so any new theory that did not match that > correct old theory would be a nonstarter._

I do not think any physics theory could even be "correct" in the sense of Platonic ideals, but I do not know what you mean by "correct". I do not claim a new theory could completely replace the old one before it could deliver the same or better results. I claim theory has value and is accepted based on its new benefits, not its superiority in every aspect the old theory was superior before. Calling incomplete theory non-starter makes no sense to me, as all theories, including General Relativity, are incomplete.

<comment><USER43> _> You're badly mistaken._

No, I'm not; you're just mistaken about which classical results I was referring to. I meant the results of classical EM that predicted that atoms could not exist--because the electrons would radiate until they fell into the nucleus. And what classical formula tells you how much the electrons will radiate because of their acceleration due to responding to the electric field of the nucleus? Larmor's formula.

In other words, Larmor's formula was not a "theory"\--it was a particular result derived within a theory. The particular _result_ happened to be correct, within a particular limited domain; but the underlying _theory_ that was used to derive it could not explain _why_ it was correct--because the same theory, and indeed the same particular result--the same formula--made other predictions that were obviously egregiously wrong (like predicting that atoms would collapse).

 _> nobody succeeded in obtaining the emission line frequencies of gases out of the classical EM theory_

You're drastically understating the failure of classical EM here. It's not that classical EM couldn't predict the particular frequencies of emission lines. It's that classical EM couldn't predict the _existence_ of emission lines at all. Classical EM predicted that atoms would emit a continuous spectrum of radiation--not radiation sharply peaked at particular frequencies.

 _> The classical formula is taught to this day both in macroscopic EM theory and quantum optics courses_

Sure, because _within its domain of validity_ , it works fine as an approximation. But that's all it is--an approximation. And we explain why the approximation works, and why it works only within a particular domain of validity, by reference to the more complete underlying theory--quantum electrodynamics.

 _> I do not know what you mean by "correct"._

I mean "makes predictions that match the results of experiments".

</comment></comment></comment></comment></comment></comment></comment></comment></comment></comment></comment></comment></comment></comment></comment></comment><comment><USER45> I sometimes wonder why tech people like space-related stuff so much. It is a major news indeed and a feat of science and technology, but why is space so popular? Because it's otherworldly, large-scale and kind of making you feel empowered or united? I'm probably more interested in mundane, obscure and humble stuff, so this disproportionate popularity of space-related news is always baffling to me.

<comment><USER46> In grappling with this question myself -- it's a good question -- I've concluded the interest is in understanding and predicting the behavior of a system based on a few laws.

The system is quite complex and full of exotic objects, so ordinary real world intuition is a poor guide. And the laws are couched in a mathematical language that is also foreign to our everyday world.

Yet, predictions can be made and tested. It's an intellectual puzzle like "what does this very tight loop do?", or "how does the Y combinator work?" \-- but in a different arena.

</comment></comment><comment><USER47> The mechanical and software engineering underlying these research endeavors is breathtaking. The laser apparatus, LISA pathfinder, ELISA - how on earth do they calibrate/debug/test such complex systems?

... and I shudder to think that more often than not, anything I code in C/C++ will segfault on first run.

<comment><USER48> Don't worry, their code segfaulted umpteen times, too :-)

</comment></comment><comment><USER49> If they build a third observatory, can they triangulate where in the universe the events are occurring?

<comment><USER50> My intuition is that this is unlikely, but I'd love to see someone do the math. Given the scale of interstellar distances, any locations on our planet (and even in our solar system) are going to effectively function as a single point. Given arbitrarily-accurate measurement, it could work, but I'd bet physical limitations will prevent that from being a possibility.

To my mind, it'd be roughly like trying to triangulate an earthquake in France with three sensors in a 1mm^3 cube in NYC (scale is probably way off, I definitely didn't do the math).

<comment><USER51> It would work. The delta-t is a few miliseconds. This gives enough precision for a decent estimate of the direction of the signal.

Now estimating the distance is a different matter.

<comment><USER52> The gravity waves redshift just like any other wave. So, they _have_ estimated a distance: 1.3B light-years.

</comment></comment></comment></comment><comment><USER53> I wonder of this means the space version of these antennas, eLISA, will get more funding. Using space seems like a much better way to access long distance laser conduits in a vacuum needed to detect gravitational waves.

<comment><USER54> Since the LISA has longer baselines, it measures gravitational waves of different frequencies, from different phenomena--supermassive black hole coalescence, not these (mere!) stellar mass black holes. So the experiments are complimentary

</comment></comment><comment><USER55> I generally dislike idolatry and pinning mayor scientific advancement on one single person, but honestly, Einstein really was something else.

<comment><USER56> In all honesty I do think Einstein is getting too much credit today. I'd paste the list of co-authors here to congratulate them but HN doesn't allow comments that large. The list is available here for reference, and I think every one of them deserves credit for this.

      http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102

On another note, I feel like the importance of this finding is less in proving Einstein's theory; having taken a formal relativity class and an degree in Physics, I think GR itself is an astounding mathematical framework for describing spacetime, to which Einstein deserves credit, but the existence of gravitational waves is completely natural consequence of the equations within. It's not very different from the existence of light being a natural consequence of Maxwell's equations.

I'd say the true importance of this discovery is in successfully creating an _experimental apparatus_ to detect what was previously almost universally agreed to probably exist but thought to be nearly impossible to detect. What's truly exciting isn't proving Einstein right, but the possibilities of what we'll be able to to detect with this apparatus in the future. So it's the team that built the apparatus which truly deserves the credit today.

</comment></comment><comment><USER57> I hope Kip Thorne gets a Nobel Prize for this, ideally while he's still alive.

<comment><USER58> Pedanticly, it'd have to be while he was alive since the Nobel Committee won't nominate anyone who's deceased. Rosalind Franklin would've surely received one for her work on DNA (among others who passed before their work was recognized).

</comment></comment><comment><USER59> It was mentioned that during this event, three sun's mass equivalents were turned into gravity waves, I guess that means that matter particles were turned into gravitons.

But what happens to them? Is there any way to turn them back into matter? If not, then at some point, will all matter in the universe end up as gravitons?

Also, if an object moving through space creates gravitational waves, doesn't that violate the law that states that a non-accelerating object will not lose/gain any energy? Because if you have to emit gravitons as you move in space, and emitting them requires energy or matter expenditure, then an object moving through space will slowly lose all it's mass?

</comment><comment> nappy-doo Does anyone know if G-Waves are effected by velocity, like EM-Waves are?

In other words, if two bodies are moving relative to one another, one emits G-Waves, and one detects them. Are the waves at the detector doppler shifted in frequency by the relative velocities?

<comment><USER60> I don't see how they could not be. I think that shift arises from transformations between different coordinate frames, it doesn't matter what you're observing.

</comment></comment><comment><USER61> _Lost in the transformation was three solar masses' worth of energy, vaporized into gravitational waves in an unseen and barely felt apocalypse. As visible light, that energy would be equivalent to the brightness of a billion trillion suns._

Beautiful.

</comment><comment><USER62> Note this is a stellar black hole merger of several tens of solar masses. Imagine the disturbance of a galactic core black hole mergers of millions of stellar masses. These are probably much rarer, but do occur when galaxies merge.

<comment><USER28> Oooh, I can do another back of the envelope calculation here! (cf [https://news.ycombinator.com/item?id=11081838](https://news.ycombinator.com/item?id=11081838) )

If this happened in the centre of the Milky Way, we're about 25k light years away.

Let's say 2 1 million solar masses black holes merged there... and they also gave off about 3/60 of their mass as radiation, that's about 100'000 solar masses being radiated 25k light years away.

Using my calculation in the other post, we're talking 10^52 Joules. Across a distance of 25'000 light years, or about 10^20 metres, that's then decreased by 10^40 (inverse square) so we're left with about 10^12 Joules...

Which is good news! If that happened in the Milky Way, we would probably survive it - though we'd definitely notice some strange atmospheric effects...

</comment></comment><comment><USER63> Does anyone else get a bit depressed when you consider how insignificant we are?

<comment><USER64> No. But with so much interesting things left to find I start to really really hate the concept of mortality :(

<comment> grif-fin I could not agree more. Everyday I wake up and I think why is it that we gave up on massive wave of research on immortality/life extension.

</comment></comment></comment><comment><USER65> I have a question: what does this mean for theoretical physics? (except for Einstein was right) Does it settle any major debates? Does it make any competing theory more or less likely?

Sorry I am not vary knowledgeable on the topic.

<comment><USER41> It's by far the most explicit verification we've ever had that black holes exist in pretty nearly the exact form predicted by Einstein's equations of general relativity, which is pretty cool. It provides the tightest limits on any possible mass for the graviton (the presumed particle carrying the gravitational force, which is generally believed to be massless but you always have to wonder about more exotic possibilities). It gives a stunningly clear confirmation that modern numerical simulations of relativistic dynamics are an accurate reflection of nature. (And by the same token, it presumably puts limits on the strength of any potential deviation in the laws of physics from the equations used in designing those simulations.) And it probably does something to give preference to models of _astrophysics_ in which binary systems with these characteristics are common.

Beyond that, I guess I'd say that this particular signal doesn't feel like that much of a _surprise_ : we were already pretty confident that if a black hole binary were to merge, a signal more or less like this would be an expected result. The scientists were evidently surprised that their very first signal was so strong (this one was even borderline detectable by the previous version of LIGO), which may teach us something, but it's not revolutionary.

</comment></comment><comment><USER66> One thing I don't quite understand - how can the "chirp" from LIGO be unambiguously categorized as extraterrestrial in origin? The waveform shown onscreen in the NYtimes video looks like an extremely noisy signal - not sure if that's the actual sampled data or just an artistic rendition. Couldn't there be a variety of physical disturbances that explain a sine-tone sweep like that, given how sensitive the instrument is to physical vibrations?

<comment><USER67> There are two independent sites, 2000 miles apart. Additionally, each site has two perpendicular experiments: one shows contraction when the perpendicular one shows expansion. Orientations at both sites are aligned I presume.

<comment><USER68> And there is a third which was offline at the time, and 2 more being worked on. This is no the last experiment.

</comment></comment></comment><comment><USER69> So what does this mean about the future, now? What new capability do we have? What is possible now that wasn't possible before?

</comment><comment><USER70> How could this result be reproduced in an independent repetition, then?

<comment><USER71> In a very real way it was. There were two completely geographically separate instruments which recorded the same signal with a delay that is consistent with the light travel time between them.

In the future this will get better when VIRGO in Italy and KAGRA [2] in Japan come online. Then we will have 4 independent detectors which will be able to verify that same signal is observed at the same time.

Obviously of course given the transient nature of what is being observed once the merger has occurred it will very rapidly stop producing gravitational waves so we will not be able to measure the same event again.

[1]: [https://en.wikipedia.org/wiki/Virgo_interferometer](https://en.wikipedia.org/wiki/Virgo_interferometer)

[2]: [https://en.wikipedia.org/wiki/KAGRA](https://en.wikipedia.org/wiki/KAGRA)

</comment></comment><comment><USER72> This is live now: [https://www.theguardian.com/science/across-the- universe/live...](https://www.theguardian.com/science/across-the- universe/live/2016/feb/11/gravitational-wave-announcement-latest-physics- einstein-ligo-black-holes-live)

</comment><comment><USER73> Wow, the list of authors to the paper is three pages long:

[https://dcc.ligo.org/public/0122/P150914/014/LIGO-P150914_De...](https://dcc.ligo.org/public/0122/P150914/014/LIGO-P150914_Detection_of_GW150914.pdf)

</comment><comment><USER73> On November 25, 1915 (at the time of WWI) Einstein presented the actual Einstein field equations to the Prussian Academy of Sciences. Almost exactly 100 years later on September 14, 2015 LIGO observed the first gravitational- wave signal. Is that a coincidence?

<comment><USER74> Yes

</comment></comment><comment><USER75> The original science paper is here [0]

[0] [http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116...](http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102)

</comment><comment><USER76> ok, so some of the best minds on Earth can build a machine to detect gravitational waves from an event 1.3B light years away. This is an incredible motivation for those of us on what is possible with technology in simple terrestrial projects.

</comment><comment><USER77> Honest question: there is any example of Einstein being proved _wrong_?

Was he indeed always right on his theories for phenomenons before they could be proved by experiments; or is that the case that we only hear about when he is proved right?

<comment><USER78> He was a proponent of hidden variable theory, which tried to reconcile quantum mechanics with determinism, famously saying "God does not play dice". People often say that hidden variable theories were proven impossible, and thus Einstein was proven wrong. That's not quite true, and only _local_ hidden variables have been ruled out.

<comment><USER79> It also disparages his contribution to the scientific discussion to just state that he was "proven wrong".

Bohr's argument in the discussion was a bit of a mess and I couldn't pull anything out of his rebuttal to EPR other than an assertion that QM behaves the way it does and not to pay any attention to the man behind the curtain. Its a very philosophical argument with very little scientific content and he just proposes that the QM math is correct because its correct, as far as I can tell.

EPR made a logical cogent argument. It was based on the philosophical principle of the locality of physics. They translated that into the mathematics of Quantum Mechanics and proposed a simple experimental test. Later that was refined by Bell and tested experimentally by Aspect and others. It was the Einstein-Podolsky-Rosen paper that laid the groundwork of how to test the non-locality/hidden-variables of QM though.

EPR moved the scientific discussion forwards much more than Bohr did, but it turns out the test they proposed showed that the position they favored was incorrect.

Also Einstein was arguing first and foremost that physics must be _local_. That's in opposition to the "spooky action at a distance" bit that he didn't like. Since local hidden variables are ruled out then he really was proven "wrong".

TL;DR I think Bohr's argument is rubbish, and Einstein's is solid, but the Universe is a bitch and doesn't care...

</comment></comment></comment><comment><USER80> No they haven't proven Einstein right, they've failed to prove him wrong; that distinction is the essence of science. Theories are never proven right, they can only be proven wrong.

</comment><comment><USER81> Is the speed of light affected by the gravitational fluctuations mentioned in the article?

Or, put another way, is the speed of light only a constant because we measure it in constant gravity?

<comment><USER82> Any observer will observe the same speed of light in their location.

Any effect of gravitational fluctuations in spacetime on the speed of light is a bit like a car driving on a race track that has treadmills scattered around it pointing in various directions and speeds. The car's speedometer will always read the same value because it's measuring the speed of its tires on whatever it's driving on.

</comment></comment><comment><USER83> I am not sure why but I am really hung up on the quote "Finally, astronomy grew ears. We never had ears before." They are detecting gravitational waves not sound waves.

<comment><USER84> Any vibration is just a signal. We don't actually have a direct experience with a sound, hearing is our brains interpreting the nerve signals generated by tiny organelles jiggling in our ears. What is concrete then, is only the shape of the signal, but not the medium through which it propagates.

This device just acts like a gigantic hearing device. Except it's not pressure waves, but the fabric of the universe which reverbates.

Note that the frequency of the signal is indeed in the audible range.

Anyway, I was a bit irritated of this same phrase, but because I tought radio astronomers had been listening to skies for quite some time now.

<comment><USER83> I think it bothers me because it is focusing on the sound that some small piece of this operation creates when the amazing thing is to be able to measure gravitational waves. Who cares if you hook the output of some piece of LIGO up to a speaker or not and make a noise? Unplug that speaker and these results are just as amazing.

</comment></comment></comment><comment><USER85> But there is no "fabric of space-time". Time is a mental concept, it cannot be detected.

Whatever they have detected or calculated is something else.

</comment><comment><USER86> Better video directly from LIGO/Caltech: [https://www.youtube.com/watch?v=wrqbfT8qcBc](https://www.youtube.com/watch?v=wrqbfT8qcBc)

And here's more detailed from PBS Space: [https://www.youtube.com/watch?v=gw- i_VKd6Wo](https://www.youtube.com/watch?v=gw-i_VKd6Wo)

</comment><comment><USER87> I really wonder what the researcher sitting bored at their computer looking at random data thought when they noticed the interesting event!

<comment><USER88> [http://www.sciencemag.org/news/2016/02/here-s-first- person-s...](http://www.sciencemag.org/news/2016/02/here-s-first-person-spot- those-gravitational-waves)

" On 14 September 2015, while Drago was on the phone with a LIGO colleague in Italy, his pipeline sent him an email alert—of which he receives about one each day—telling him that both LIGO detectors had registered an "event" (a nonroutine reading) 3 minutes earlier, at 11:50:45 a.m. local time. It was a big one. "The signal-to-noise ratio was quite high—24 as opposed to [the more typical] 10," he says."

</comment></comment><comment><USER89> A hole that is less than the sum of its parts. Three suns' worth of mass has been turned into energy, in the form of gravitational waves;

The coalescing holes pumped 50 times more energy into space this way than the whole of the rest of the universe emitted in light, radio waves, X-rays and gamma rays combined.

</comment><comment><USER90> Einstein wasn't sure at first. It was Feynman who introduced the thought experiment that settled the debate in the physics community:

[https://en.wikipedia.org/wiki/Sticky_bead_argument](https://en.wikipedia.org/wiki/Sticky_bead_argument)

<comment><USER91> Not sure why this received downvotes. There was significant debate until the 1950s about whether gravitational waves were real phenomena or just coordinate artifacts. The sticky bead argument played a big role in settling the issue.

</comment></comment><comment><USER92> Tangential question. With blackholes merging, can more and more merge some time in future, creating a net gravitational pull to slow down the expansion of the universe and then may be eventually cause the universe to collapse into that continually merging mega black hole.

<comment><USER93> The gravity pull of the new black hole is essentially equal to the sum of the gravity pull of the two isolated black holes, and it's essentially equal to the gravity pull of the two stars before they transformed into black holes. So merging black holes don't increase the pull.

Actually, in each transformation there is an explosion and part of the mass goes away (try to not be very close). So the total mass in the final black hole is smaller than the mass in the initial stars, the rest are debris forming a nebula or something around the black hole.

(And, as the other commenter said, gravity is too weak.)

</comment></comment><comment> oldo-nicho I'm interested to know how they can be so sure that the change in distance between the two arms of LIGO is attributed to gravitational waves? I would of thought that miniscule movements in the Earth's crust would be a more likely culprit.

<comment> oldo-nicho Ahh, just realised that there are two instruments located on either side of the states, so if both instruments register the same event then it is unlikely to be tectonic movement...

</comment></comment><comment><USER94> _once shot up the outside of one of the antenna arms in Louisiana, and a truck crashed into one of the arms in Hanford_

Does this mean an actual truck, a vehicle? Did they accidentally hurt someone?

I liked this quote: _The future for the dark side looks bright._

</comment><comment><USER95> I understood how LIGO works but one question I have is, how did scientists conclude the gravitational waves they detected are from a collision of 2 black holes that happened before 1.3 Billion/Million years ago.

Could someone explain ?

</comment><comment><USER96> Technical details and hints on future results: [https://news.ycombinator.com/item?id=11092982](https://news.ycombinator.com/item?id=11092982)

</comment><comment><USER97> Would such a gravity wave cause a tsunami on a world circling the two black holes?

</comment><comment><USER98> Live stream from the NSF: [https://www.youtube.com/user/VideosatNSF/live](https://www.youtube.com/user/VideosatNSF/live)

</comment><comment><USER99> For those who missed the live announcement by the team [https://youtu.be/aEPIwEJmZyE?t=27m13s](https://youtu.be/aEPIwEJmZyE?t=27m13s)

</comment><comment><USER100> Where can I find out more about how thermal effects in the LIGO optics are controlled for?

Basically, I want to understand how it's possible to measure a distance change on the femtometer scale.

<comment><USER101> You probably want to read about their active isolation measures. Then you should find anything by Vladimir Braginsky you can get your hands on. This is a good start: [http://www.amazon.com/Quantum-Measurement-Vladimir-B- Bragins...](http://www.amazon.com/Quantum-Measurement-Vladimir-B- Braginsky/dp/0521484138)

<comment><USER100> More accessible / concise info on the vibration isolation, here: [https://news.ycombinator.com/item?id=11084282](https://news.ycombinator.com/item?id=11084282)

</comment></comment></comment><comment><USER102> Are gravitational waves supposed to be that weak or is it because of the distance between us and those black holes? Do they lose power as they travel through space?

<comment><USER20> Think of waves in a pond when you drop a rock in. The energy spreads out as it travels into the farthest reaches of the pond.

<comment><USER42> Only that in space, it drops even faster: The wavefront carries the same energy, but spread out over ever increasing length/area. For gravitational waves (or radio waves) they form spheres, not circles, and the surface size scales with r^2, not r. (Also water waves are /complicated/)

<comment><USER27> Right, but LIGO detects wave amplitude, not wave energy, which goes down as 1/r.

<comment><USER42> Oh, interesting point! I now have to wrap my head around that information transfer normally means energy transfer...

</comment></comment></comment></comment></comment><comment><USER103> So does this verification of gravitational waves help with Physics theory- building? Have people really doubted Einstein : the existence of gravitational waves?

</comment><comment><USER104> Has anyone converted the spectrograms given in the paper, or better still the raw data, into sound yet ? Each location in each ear please!

<comment><USER71> The data release contains the data converted to sound if you scroll to the bottom [1].

I don't think anyone has combined them as you suggest though!

[1] [https://losc.ligo.org/events/GW150914/](https://losc.ligo.org/events/GW150914/)

</comment></comment><comment><USER105> Would it be possible to listen for information transmitted via gravitational waves? Would there be any benefit over radio?

<comment><USER106> This event was the equivalent of three of our suns turned into pure energy. Pretty expensive to send a message.

<comment><USER107> Burning a few suns per message would significantly reduce spam.

<comment><USER108> Your post advocates a

( ) technical ( ) legislative ( ) market-based ( ) vigilante (X) Physics-based

approach to fighting spam. Your idea will not work. Here is why it won't work. (One or more of the following may apply to your particular idea, and it may have other flaws which used to vary from state to state before a bad federal law was passed.)

(X) The amount of energy involved would likely destroy the planet.

(X) Many email users cannot afford to lose business or alienate potential employers

Specifically, your plan fails to account for

(X) The relative sparseness of non-dark energy in our vicinity

(X) Huge existing software investment in SMTP

and the following philosophical objections may also apply:

(X) Incompatiblity with open source or open source licenses

(X) I don't want the government reading my email

Furthermore, this is what I think about you:

(X) Sorry dude, but I don't think it would work.

(I'm sorry, but I couldn't resist)

</comment></comment></comment></comment><comment><USER109> Can't wait until this discovery can be utilized to creating some kind of a new technology! Such as time travel!

</comment><comment><USER110> Obligatory......

[http://xkcd.com/1642/](http://xkcd.com/1642/)

</comment><comment><USER111> Does gravity move at the speed of light?

<comment><USER112> The videos say the waves do move at exactly the speed of light. They are massless.

</comment></comment><comment><USER113> As with most physics experiments for the last 40 years, nothing new was discovered that we didn't already predict. Confirming something widely believed to be true isn't nearly as valuable as finding out we don't understand something. This is actually one of the reasons I dropped out of my physics phd program.

<comment><USER54> Because this is astrophysics and not particle physics, this discovery is just the beginning! We don't know the rates of these mergers, the distribution of the masses of the binary components, what electromagnetic signature accompanies the events (if any)...

We've known gravity waves existed since the Hulse-Taylor pulsar, so just observing them for the first time is not nearly as interesting as the science to come in the next decade. Advanced LIGO is a powerful new tool that will open up exciting new observations.

</comment></comment><comment><USER114> And we are one step closer Gravity Shockwave Generating Division Tool.

</comment><comment><USER115> Could someone explain what possible applications this might produce?

<comment><USER112> It's super early, and detection seems at the limits of our technology.

This is just understanding at present. That, in itself is worth it.

As that understanding develops, and our tech advances, engineering may be able to apply it in useful ways, maybe object detection above a specified mass? New ways to visualize things?

One "application" is to serve as a ruler to measure out tech with. The limits are there, putting these observations just within reach.

Now that we have some confirmation, we also have the metrics as well as the compelling new science that may arise from all of this as a strong motivation to advance.

It's like being able to detect color for the first time. At first we understand what color is, then we refine, and after iterations, engineering, experiments, we get to a place where we see it all in color.

Applications will follow.

These waves being confirmed are like a new sense. Crude, but real. We can now follow this new perception to its conclusion, just as we have many other things.

We don't always know what that conclusion will be, or the form an application may take, but we do know we won't actualize any of it if we don't do the basic, hard, expensive work needed first.

</comment></comment><comment><USER116> 1.1. billion and 40 years vs. pencil and paper and a few years. That should tell us something about education, mode of thinking and research. And a few other things.

</comment><comment><USER89> Did collision of 2 black holes prove E = MC2

</comment><comment><USER117> The "proving Einstein right" part would be more fitting if there was some independent evidence of the collision. As it is it seems to go in circles.

But that's the NYT I guess.

<comment><USER5> Maybe we will get lucky and literally see two black holes merge somewhere nearby. But I think these detectors will be the last of our concerns with all the praying and bunker building going on. Bruce Willis won't save us from that one.

</comment></comment><comment><USER118> Does this bring us any closer to warp drives?

<comment><USER119> Uh, I guess it might help with testing things that would?

Iirc the idea would be that inferometers would be used to test things like that.

I don't think we (humanity) will achieve warp drives, though, I do support the attempt.

</comment></comment><comment><USER120> Dicussed here: [https://news.ycombinator.com/item?id=11079462](https://news.ycombinator.com/item?id=11079462)

<comment><USER121> To be fair, that article is pre-announcement and doesn't have much detail on the actual discovery.

Actual paper here: [http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116...](http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102)

</comment></comment><comment><USER122> the wave can come from two slits, one beam comes later maybe

</comment><comment><USER122> the wave can from two different slits, so one beam travels longer

</comment><comment><USER123> > The future for the dark side looks bright.

Oh no you didn't.

</comment><comment><USER124> I wonder if gravitational waves are responsible for the video starting automatically without me clicking play.

<comment><USER125> Yes, in that any given event is ultimately the result of all previous events within its past light cone.

</comment></comment><comment><USER126> I swear a big bang theory rerun about this was on last night. Sheldon detected waves at the north pole, but they were actually a blender turned on by the rest of the gang. He's embarrassed and goes home. Leonard beds Penny. Decent episode.

</comment><comment><USER127> I am a bit skeptical of the conclusion given the methods. Here, there's no observable phenomena independent of the test apparatus that corresponds to the proposed cause. The conclusion is circular.

1\. Theory predicts gravitational waves when massive objects collide and that the gravitational waves would have an effect that could be measured by the experimental instruments.

2\. The experimental instruments measure something.

3\. This is considered proof that massive objects collided.

4\. Therefore gravitational waves exist.

To reframe my skepticism, the experiment measures something. The conclusion as to what it measures, however, is unsupported by statistical inference or direct experience of a causal phenomenon. That's not to say that what the phenomena measured -- the earth resonating -- is uninteresting or unimportant or even inconsistent with the theory of gravitational waves.

Yet, I don't find the possibility of a geophysical cause -- i.e. that the earth maintains consistent dimensions at a sub-atomic scale -- the many orders of magnitude less likely than gravitational waves necessary to reach a conclusion. In particular, I find natural fluctuation to be more likely because the experiment acknowledges its existence.

For a point of comparison, consider the Perihelion precession of Mercury that provides evidence in support of general relativity. The theory was used to predict the results of an observable event. The experimenters trained their telescopes at a particular location and particular time and observed phenomena consistent with a prediction based on the theory. The same is true of the Higgs. In both cases the experiment is of the form "when X, I will observe Y."

The reasoning here is:

      If X, then Y.
      Y, therefore X.

It treats an ordinary implication as mutual implication.

<comment><USER128> I think your argument fails for a few reasons:

(1) It simply fails to understand the scientific method, which is empiricism, not mathematical/logical proof. Scientific evidence is essentially failed disproof, not logical proof.

(2) It mischaracterizes the nature of the prediction, which includes not merely that _something_ will be measured, but that a particular pattern will be measured.

(3) It proposes unspecified "geophysical causes" as an alternate explanation, but there was no pre-existing geophysical model which predicted the pattern observed. (Any after-the-fact geophysical -- or other -- alternative explanation which explains the observed pattern would also need to perform differently on some other test to be verifiably different, and then we could do the test to distinguish the source.)

(4) It misstates the reasoning to contrast it with other experiments, this is exactly "when X, I will observe Y" (where X is "I construct detectors of a particular type in more than one location" and Y is "I will periodically detect particular patterns of signals on those detectors -- not just one of them alone -- which the model predicts will be produced by collisions of massive, distant objects.")

<comment><USER127> Under the theory of science as the study of what is falsifiable, there's nothing here to falsify because there is no way to disprove that a conjectured but unobserved collision of two massive bodies was something else or didn't occur. Which is to say that it is impossible to falsify that a conjecture is a conjecture.

That there is not a geophysical theory, doesn't have a bearing on the correctness of the gravitational wave theory one way or the other...anymore than the absence of a helio-centric model for the solar system made the geocentric model more correct or the absence of a theory of oxygen made the theory of pholgiston more correct. More importantly, both these incorrect theories had reasonable explanatory power to the point that they were useful.

The reason they were useful theories is because they were predictive, pholgiston allowed a person to calculate the weight of ashes after burning and the geocentric solar model made the prediction of the location of stars possible with reasonable precision. On the other hand, theories that offer conclusions about unfalsifiable propositions are what Carnap and the Vienna circle termed "metaphysics".

The conclusion that the experiment justifies is that the Earth resonates. There is no external event to which the measurements can be correlated to establish causality. There's no confidence interval. It's a case where the observations confirm a pre-existing world view under the same human cognitive structures by which seashells on mountain tops confirm a world-wide flood. It assumes that because we live on the Earth we know everything about it.

Anyway, it's a case of over-reaching with the conclusions. It's an argument from design.

<comment><USER128> > Under the theory of science as the study of what is falsifiable, there's > nothing here to falsify because there is no way to disprove that a > conjectured but unobserved collision of two massive bodies was something > else or didn't occur.

Yes, there is: if the predicted kind of observations did not occur, it would imply one of two things:

(1) the model of gravity waves and their generation and propagation on which the prediction was based was incorrect, or

(2) the expectation of large-object collisions on which the prediction was _also_ based is incorrect.

Now, were that the case, _distinguishing_ which of those assumptions was false would require coming up with a new set of experiments that would have different results if the first was correct and the second false than if those were flipped, and yet a different set of results if both were false.

> That there is not a geophysical theory, doesn't have a bearing on the > correctness of the gravitational wave theory one way or the other

Science isn't about correctness, its about continuous refinement of models which better predict observations. The absence of a better alternative model doesn't "prove" that a given model is "correct", but science deals with neither proof (except in the negative sense) nor correctness. (Further, the model of gravity waves being tested here is an implication of broader models whose other implications have also withstood attempts to falsify them.)

</comment></comment></comment></comment><comment><USER89> Everyday we're getting sun rays [https://en.wikipedia.org/wiki/Crepuscular_rays](https://en.wikipedia.org/wiki/Crepuscular_rays)

Why are we surprised at gravitational waves when 2 black holes collided?

<comment><USER82> We actually aren't surprised. This confirms a theory we've been reasonably confident about for about 100 years.

This is less "wow, look at what an unexpected result we found!" and more that we finally managed to measure something we've been looking for.

<comment><USER129> Not only expected, but the predicted wave forms were virtually perfect. We knew what to look for, and we even knew what we saw as soon as we saw it.

</comment></comment></comment><comment><USER130> A detector of this sensitivity seems like a boon for spying. Rather difficult to relocate, fortunately, but it makes me wonder about the future of the technology. No one would have looked at the first computer and envisioned an iPhone.

<comment><USER131> _> No one would have looked at the first computer and envisioned an iPhone._

Not really. Portable computers were envisioned quite a long time ago.

<comment><USER130> Not before they were solid state. Computers had their own buildings and relied on glass tubes for operation. Screens weren't on the radar. Operators were highly educated. But technology did bring us to a point where the vision became more focused, and led to what we have today.

There are many possible paths to rebutting my statement, which to be clear is idle morning musing, but your objection doesn't hold water.

<comment><USER132> I agree that you do make a point. There is really no way for us to know what is to come. Sometimes we think we are looking at a square, but it really is a cube and we are just unable to look beyond are current perspective for whatever reason.

I would say one obstacle that stands in the way of "spying" on objects moving on the Earth's surface is that the gravitational wave energy emitted by accelerating objects on Earth would be "too small" for current detectors. Not to mention that there would be the issue of how to filter gravitational wave noise, and/or isolate frequencies. However, if it possible to build an amplifier or filter to resolve these issues, that remains to be seen - <USER3> maybe somebody else could chime in.

edit: typos, clarity

<comment><USER132> ok, here's an idea that someone might either build on or refute: would it be possible to build an amplifier of gravitational waves using some arrangement of microscopic and/or macroscopic objects having a "known" defined 3D physical relationship (say in a lattice), and under known interactional forces (including EM). You would have to take into account the uncertainty principle in system parameter measurement, though the propagating gravitational wave should have a deterministic effect on the potential well and thus the quantum wave(s) of the system(s). Thus, amplification through propagation in the space-time of the system might be feasible. This is of course all hand-waving, and very rough.

</comment></comment></comment></comment></comment><comment><USER133> I'm wondering no-one mentioned Electric Universe which is the greatest opponent of this gravitational hocus-pocus religion. It would be much more beneficial to the humanity to focus smart peoples' attention to Birkeland currents or plasma or to the recent experiments of the SAFIRE project. They have several series on their youtube channel:

[https://www.youtube.com/user/ThunderboltsProject/videos](https://www.youtube.com/user/ThunderboltsProject/videos)

<comment><USER134> You ... must be trolling.</comment></comment>


Six Pictures of Paradise - <AUTHOR> http://nautil.us/issue/101/in-our-nature/six-pictures-of-paradise-rp <comment><USER2> I have always been fascinated that people consider tropical rainforests "paradise". They are one of the most difficult places to live with the heat, humidity, disease, and dangerous flora and fauna. Fascinating places, but in my mind far from paradise.

<comment><USER3> visually they seem like paradise. full practical reality: no. the ideal we should seek would have the best of both worlds.

<comment><USER2> Yes. The closest I have ever experienced are the islands of the South Pacific. You get the same lush vegetation, but with pleasant temperatures and few of the nasty critters. Some of the Cook Islands are a very nice :)

</comment></comment></comment><comment><USER4> It's interesting to me that the author spent years looking at these photos and wrote a very long article about them and still doesn't seem to understand the photographs. I think I spent less than a minute before I at least I think I got something more out of it than what the author saw. I could be totally misunderstanding the author though.

All the reasons Nigel gives for what makes those photos interesting miss the point I think.

> "although any of us could have taken those pictures, only one person > actually did"

> "Just knowing that these scenes aren't the most beautiful places along the > trail system provokes a rush of tenderness for that forest. It's the same > thing I feel looking at pictures of my wife caught unaware she's being > photographed, when she looks ordinary instead of radiant."

> "reducing each of Struth's photographs to a list of the plant species it > contained and some observations on the successional state of the forest. "

I notice this miss the point on artsy/literary stuff happens a lot when talking to other engineers. It's like there's a blindness that some can't seem to overcome. To see the pictures in another way, the keyword here used in the article was "complexity". Look at one, and image this wasn't a photograph at all. Imagine it if were an abstract painting, like a pollock painting. Forget about the forest, and that there are plants in it, just focus a little bit less and look at the lines. The author is looking at all the wrong details. Struth here seems to care about the tangle of lines, the massive dark structures contrasting with the little bits of tiny complex jumbles. I'd guess the best I could do is to say to an engineer, try to write a program that generates patterns like this. What would that look like? These aren't photos of randomness, there is complex, sophisticated architecture in these plants trying to fight for scraps of light. It's humbling and in a way reminds me of other types of structures in nature like the superstructures in the cosmos, like the CfA2 Great Wall[1]. Try and focus out a little, like I said, try and see it as a painting. Turn off the forest, and then if you get it, turn it back on, because context is important.

A good analogy here might be the 3D Stereograms that were popular in the 90's. Some people looked at those things and couldn't see the 3D (I often couldn't) but would say they see something.

[1] [http://www.aanda.org/articles/aa/full/2004/31/aa0696/img12.g...](http://www.aanda.org/articles/aa/full/2004/31/aa0696/img12.gif)

<comment><USER5> Having art explained to you is like having a joke explained... now you "get" it, but that's beside the point.

<comment><USER6> In my experience, art invokes several ideas at the same time. Nigel Pitman explored several in these pictures, and VieElm pointed out another. Unlike most jokes, it is satisfying to spend time with each of the ideas.

I'm no Thomas Struth, nor had I heard of him before this article, but a couple years a ago I took several pictures in a similar spirit, of which I uploaded four: [http://thyrsus21.deviantart.com/gallery/](http://thyrsus21.deviantart.com/gallery/)

</comment></comment></comment><comment><USER7> "...all of us were burning to write up whatever we learned in some technical journal that, years later, an associate professor might glance at on his coffee break."

</comment><comment><USER8> That's a beautiful article and as apt an illustration of the proverb not seeing the wood for the trees as I've ever read.

</comment><comment><USER9> I only see four photos (3 inline and 1 in the header). I'm on mobile safari. Where are the other 2?

</comment><comment><USER10> What a beautiful article.</comment>


3D TV? Too Soon Now, but One Day You Will Want It - <AUTHOR> http://singularityhub.com/2010/01/14/3d-tv-too-soon-now-but-one-day-you-will-want-it/

<comment><USER2> 3D is probably inevitable if you think about it. Television has room to evolve until it reaches the limits of a pair of healthy human eyes. That includes factors like color, resolution, frame rate, and depth perception.

Most 3D tech seen today is in its infancy. There isn't much content for it. You still need to wear 3D glasses for most of it (those are probably going to get tossed before 3D takes off in the home - it's not natural enough to use).

HDTV took a while before becoming mainstream. We were hearing about it back in the mid-1990's. There was no content at first, TV sets were expensive, certain standards didn't solidify yet (Blu Ray, HDMI to name a few), etc... It didn't start taking off until recently.

Right now we're seeing the same for 3D. A bunch of different standards, all in their infancy. It might take 5-10 years before it starts becoming mainstream, similar to HDTV. The 3D standards you see today may end up being toys for rich people only to be replaced with something better shortly after, like Laserdisc's were before DVD's came to be.

Beyond 3D TV running at 240fps at 20K x 10K pixels, what else is there? Probably something that links directly into your brain, bypassing your eyes (and ears) entirely. A few decades away from that I'm afraid though. :)

</comment><comment><USER3> Although I'm interested in 3D vision I'm not very interested in this sort of technology, which is not far removed from 19th century anaglyphs or the cinema crazes of the 1950s. I expect that this has an initial novelty value, which will quickly wear thin.

This is yet another attempt to reinvent TV, which will probably not succeed. Having recently been involved with people in the TV industry it's clear that they're struggling to compete against internet based delivery of video content, and are desperately searching for ways to attract or at least retain conventional TV viewers, and the advertising revenues which go along with that. Ultimately, this is a battle which they're going to lose.

</comment><comment><USER4> I'm not sure how 3D can catch on when so many people seem to have an adverse reaction to it. Lots of people I know who saw Avatar in 3D complained of headaches or feelings of motion sickness. I don't have true stereo vision so the whole effect is lost on me anyway. It just ends up looking blurry and distorted to me. I'm much more interested in 2K displays personally. That's something everyone can appreciate and enjoy.

</comment><comment><USER5> I don't see grandma putting on the shutter glasses to watch Leno in 3D. Even with the ideal target market - kids watching 3D animated movies - do you really want your kids tethered with expensive glasses to an even more expensive television?

I could see 3D TV appealing to console gamers, but the hardware cycle isn't at a point right now where any of the console manufacturers is likely to add 3D support. Particularly since 3D glasses have been available on PCs for years with little apparent demand.</comment>


Ask HN: Which documentaries are must-watch? - <AUTHOR> <comment><USER2> In no particular order:

\- Inside Job - [https://www.imdb.com/title/tt1645089/](https://www.imdb.com/title/tt1645089/)

\- Planet Earth - [https://www.imdb.com/title/tt0795176/](https://www.imdb.com/title/tt0795176/)

\- Restrepo - [https://www.imdb.com/title/tt1559549/](https://www.imdb.com/title/tt1559549/)

\- The Tillman Story - [https://www.imdb.com/title/tt1568334/](https://www.imdb.com/title/tt1568334/)

\- Chernobyl (??) - [https://www.imdb.com/title/tt7366338/](https://www.imdb.com/title/tt7366338/)

\- Hoaxed - [https://www.imdb.com/title/tt8991264/](https://www.imdb.com/title/tt8991264/)

</comment><comment><USER3> The "Seven Up" series. [https://en.wikipedia.org/wiki/Up_(film_series)](https://en.wikipedia.org/wiki/Up_\(film_series\))

From WP: Roger Ebert said that it is "an inspired, even noble, use of the film medium", that the films "penetrate to the central mystery of life", and that the series is among his top ten films of all time.

</comment><comment><USER4> Unnatural Selection: [https://www.netflix.com/title/80208910](https://www.netflix.com/title/80208910)

</comment><comment><USER5> Science Fair[1]

[1]: [https://www.imdb.com/title/tt7153434/](https://www.imdb.com/title/tt7153434/)</comment>


Ask HN: Best blogging platform for a startup's blog - <AUTHOR>

I'm already running a tumblr blog for my startup but tumblr is not quite what we need. Both medium and svbtle look nice and pretty useful but is it a cheap thing to use those platforms? <comment><USER2> "Cheap" shouldn't be your first thought. "Effective" matters more. Can you get the layout you want? Will you have to deal with embedded ads? Are you vulnerable to the vagaries of someone else's failure?

Wordpress is very popular, of course. Pay for a hosted solution somewhere (Dreamhost is like ten bucks a month), and use the standard.

Personally, I'm moving over to WebPop ([http://www.webpop.com](http://www.webpop.com)). It's a little more expensive, but it breaks out of the Wordpress paradigm, it's based on Foundation so it has all the modern responsive stuff without looking like Bootstrap, it provides total control over your code, and it has a clean separation of structure and content. Also, they're hosting on Rackspace and boosting with Akamai, so it should be robust against traffic spikes.

</comment><comment><USER3> Anchor CMS and Fortrabbit. It's what I use for [http://blog.vpsstat.us/](http://blog.vpsstat.us/). Totally free.</comment>


LL vs. LR vs. GLR - <AUTHOR> http://panopticoncentral.net/archive/2009/03/13/24813.aspx

<comment><USER2> A great example of the power of simple explanations. I wish more of math and CS were taught this way. Right now those areas are overflowing with complex- sounding simple ideas.

<comment><USER3> The _what_ can be conceptually simple, but the _how_ is not. The justifications for how we know that you can generate a parser for any LR grammar, and what exactly that parser will act like, is more involved.

</comment></comment><comment><USER4> Wow. I've written LL (handwritten recursive descent) and LR, and worked with GLR generators, and apparently I didn't really understand why any of them worked that way. This guy is amazing.</comment>


A responsive image slider/carousel in AngularJS and CSS - <AUTHOR> http://andyshora.com/wallop-slider-angularjs-css.html

<comment><USER2> [http://shouldiuseacarousel.com/](http://shouldiuseacarousel.com/)

<comment><USER3> Thanks for this actually. I'm trying to talk people out of using this, but it's a touchy subject rife with politics. Hard statistics is useful.</comment></comment>


Ask HN: ratio of unit/functional tests to monitoring checks - <AUTHOR>

I'd writing a talk on web application monitoring and if people have a moment I'd love some data to use in that talk.
For your currently live project/team/organisation/whatever what is the ratio of unit, functional, integration tests to checks in your monitoring system?
For instance in a recent project (medium sized rails stack) we have ~2000 tests to about 240 checks in Nagios.
Any data points would be interesting. Anonymous data is fine. <comment><USER2> I'm surprised at that amount of active checks for a web app. What are you checking really - things like responsiveness, or more correctness? And do you include only the webapp checks, or also the underlaying system.

I'd expect somewhere around 20-40 checks for the system itself, so it looks like a reasonable number for a single app/deployment. Then again, I'm a passive monitoring fanboy.

<comment><AUTHOR> Actually I think active is probably the wrong word, so changed in the post, but to answer your question:

It's a system based on multiple web services and run across multiple hosts, and I've included all system checks in that number. So we have checks for:

* ping, ssh, disk, load, network io, etc. * error rates from request logs * request rates (warning on high values) * smoke tests for key functionality (i.e. does the search engine return results, can you complete certain forms, etc.) * connection tests from relevant hosts to relevant services or databases

</comment></comment><comment><USER3> ~300 unit tests across all site components.

145 nagios checks, including some smoke tests.

</comment><comment><USER4> I'm not really familiar with Nagios. What sort of active monitoring checks do you have that you use? I'd be interested :-)</comment>