Second Gravitational Wave Detected From Ancient Black Hole Collision (theguardian.com)

An anonymous reader quotes a report from The Guardian: Physicists have detected ripples in the fabric of spacetime that were set in motion by the collision of two black holes far across the universe more than a billion years ago. The event marks only the second time that scientists have spotted gravitational waves, the tenuous stretching and squeezing of spacetime predicted by Einstein more than a century ago. The faint signal received by the twin instruments of the Laser Interferometer Gravitational Wave Observatory (LIGO) in the US revealed two black holes circling one another 27 times before finally smashing together at half the speed of light. The cataclysmic event saw the black holes, one eight times more massive than the sun, the other 14 times more massive, merge into one about 21 times heavier than the sun. In the process, energy equivalent to the mass of the sun radiated into space as gravitational waves. Writing in the journal Physical Review Letters on Wednesday, the LIGO team describes how a second rush of gravitational waves showed up in their instrument a few months after the first, at 3.38am UK time on Boxing Day morning 2015. An automatic search detected the signals and emailed the LIGO scientists within minutes to alert them. The latest signals arrived at the Livingston detector 1.1milliseconds before they hit the Hanford detector, allowing scientists on the team to roughly work out the position of the collision in the sky. In February, LIGO scientists officially announced the first-ever observation of gravity waves.

Comments

Why?

[phantomfive]

Why did it take so long to detect these? I know that there have been plenty of experiments attempting to measure them before. Are the waves smaller than expect, thus harder to detect? What was the thing preventing discovery?

Re:Why?

[Edis Krad]

Are the waves smaller than expect, thus harder to detect?

Indeed. They're very small. We're talking about a shift in space the size of a very small fraction of a proton. So yes, with the current detectors they're pretty hard to detect.

Re:Why?

[CRCulver]

I would like to know this as well. When I was a teenager and first read Larry Niven's story "The Borderlands of Sol" (namely when it was republished in the collection Crashlander), where a gravity detector plays a role in the story, I assumed this was a real technology. After all, ways to detect electromagnetic waves or particle radiation had long been around. I thought nothing of it for a couple of decades, and then was surprised to read the news a while back of the "first ever detection of gravitational waves".

Re:Why?

[The Evil Atheist]

It took so long because the signal is mind-bogglingly weak. No detector was sensitive enough or well designed enough to rule out false positives. The LIGO experiment is much more sensitive and a lot of effort put in to detect false positives (including some social engineering). The detectors also underwent a very extensive testing phase before they were considered ready. We also weren't sure how frequent these events were, but now we are expecting a few more events.

But, it must be said indirect evidence of gravitational waves already were detected through the observation of two pulsars orbiting and closing in on each other at a rate predicted by the theory.

Re:Why?

[phantomfive]

The LIGO experiment is much more sensitive and a lot of effort put in to detect false positives (including some social engineering).

How can social engineering be useful at all in this case?

Re:Why?

[bjorniac]

Noise. All kinds of noise.

The system is an interferometer - basically two lasers set up in a large L shape with mirrors (massive simplification). When the lengths of the arms are the same, the beams cancel, when they differ a signal is recorded.

Now, the differences in length due to a gravitational wave is tiny, and the problem that kept LIGO from their detection is that there are huge numbers of sources of vibrations around the same frequencies as expected from gravitational waves that have far larger amplitudes. Thermal vibrations, for example, are a killer for experiments like this.

The waves themselves have almost exactly the waveforms that were predicted - the template fits from simulations match amazingly well in terms of amplitudes, frequencies and their evolution. What stopped experiments like this from making the observation was simply a lack of technical skill to make a precise enough instrument. Following the development of LIGO over the last decade, this is precisely what everyone working on the project said - once the noise curve is reduced to form Advanced LIOG (recent upgrade) the noise would be sufficiently small than an integrated signal against a template would be clearly visible, and now it is.

Re:Why?

[MightyMartian]

The current detectors are the most sensitive instruments ever developed by humanity, and in and of themselves mark a major leap forward in technical ability.

Re:Why?

[MightyMartian]

If I had to hazard a guess, with incredibly fine instruments, there's a risk of experimenter effect.

Re:Why?

[quenda]

But gravity waves are like elephants in your fridge compared to the problem of detecting gravity particles.

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, ...

https://en.wikipedia.org/wiki/...

Re:Why?

[MightyMartian]

You have no idea what it will or will not address. When the first scientists were mucking around with electricity in the 18th century they were giving people shocks and making frogs legs jump. Within a hundred years they were rolling out the world's first global high speed communication's system.

So take your contrarianism and stick it up your ass. Your type would have us still beating each other with sticks, because, you know, what good does that shiny shit in the ground do us?

Re: Why?

[Anonymous Coward]

They had a system to inject test data into the final stage analysis. The people doing it did it blind to publication ready point. Partly it was to ensure nobody leaked before the 'envelope was opened'.

Re:Why?

[Megol]

Strangely enough humanity as a whole can multitask. Individuals can too.

For your complaint to be valid humanity could only do one thing at a time, that specialization doesn't help when developing something (humans are replaceable cogs) and that somehow your idea what is important is the key to future advancements. You also disregard the fact that basic research often helps progress in unexpected ways - including the area of microscopy.

However you seem to think that just throwing monkeys (read: humans) on typewriters (research) is the best way to develop solutions. That in itself should be a huge warning sign that you shouldn't be taken serious.

Re: Why?

[master_p]

And why isn't it detecting waves on a daily basis? The universe is supposed to contain billions of black holes.

Re: Why?

[bruce_the_loon]

LIGO is detecting the gravitational storm that happens when two black holes, each 10-30 times the mass of the sun, actually collide and merge. Standalone black holes shouldn't generate gravity waves unless disturbed by something massive close by.

Orbital binary systems should generate gravity waves, but those would be a couple of orders of magnitude less powerful than two colliding black holes and LIGO isn't sensitive enough to detect those out of the noise.

Re:Why?

[Maritz]

That's not the kind of "technical ability" nor the kind of instrument we need.

You think you know what we need. You're wrong. You don't.

Re:Why?

[ctrl-alt-canc]

Actually, given the level of some comments here, better microscopes are really needed just to detect the presence of a brain in some people.

Re: Why?

[Anonymous Coward]

What bruce said, and also remember that the strength of waves decays according to the square of our distance from them. This means that a black hole collision has to be "relatively" close to us to be detectable. The first collision was a bit over 1 billion light years away and was just detectable. The observable universe is a bit over 90 billion light years. This means that we miss a something like 99.9999% of similar events that happen in the universe simply because our detectors aren't sensitive enough. As we improve them we should start to see lots more events. You should expect several a week some time soon according to one of the scientists involved in the project who was on the radio.

Re:Why?

[peragrin]

When scientists first started mucky around with electricity it had an affect that could be understood and felt. same goes for lasers, and everything else you use.

To detect TWO gravity waves we had to wait for TWO massive blackholes to combine. And space itself moved by a proton fraction.

Do you have any idea how tiny of fraction that is to be useful? Right about now earth is about the best space ship we can hope for. As it is the only way to get one gravity.

Re:Why?

[Lumpy]

And the only reason we achieved so much in such a little amount of time is because the INFORMATION WAS SHARED FREELY. each scientist was able to use the work of the previous.

Now the stupidity of copyright and patents are slowing down progress at an alarming rate.

Re: Why?

[SharpFang]

Wouldn't it be able to detect weaker events if they happen closer to us?

Re:Why?

[toonces33]

For a while I was starting to wonder if we would need space-based interferometers to solve the noise/vibration problem. But fortunately that wasn't needed.

Re:Why?

[michelcolman]

But I'm detecting gravitons all the time! I wouldn't be sitting in this seat if it wasn't for gravitons. I think someone misunderstood something here. Possibly me ;-)

Re: Why?

[michelcolman]

So the graviton was both there and not there as long as you didn't open the envelope?

Re:Why?

[Bengie]

Money invested into a specific scientific discipline has greatly diminishing returns. Couple that with no one knowing which scientific discipline or combinations of will cause the next breakthrough, and spreading our money around is the best investment. Many domains already have huge amounts of private funding, little point in the government throwing money at better optics for microscopes.

Re: Why?

[Bengie]

Yes, but it's how much weaker. For a brief moment, those two blackholes released more energy that the rest of the entire observable Universe, that includes all of those quasars. Even gamma ray bursts only outshine their local galaxy.

Re:Why?

[K. S. Kyosuke]

We also weren't sure how frequent these events were, but now we are expecting a few more events.

I'm no statistician, but assuming that the intervals between detectable events are exponentially distributed (seems reasonable to me), the fact that we detected the first event pretty quickly within a certain time period would seem to suggest that too high or too low values of lambda (not fitting the first successful observation) are unlikely.

Re:Why?

[jfdavis668]

Back then, information was not shared freely. Scientists hid their findings until they could publish or present, often years later. That is why there they fought over who discovered what, since 5 people might discover it between the first person and when he got around to publishing the result. Hard to prove you did it first.

Re:Why?

[tomxor]

When scientists first started mucky around with electricity it had an affect that could be understood and felt. same goes for lasers, and everything else you use.

To detect TWO gravity waves we had to wait for TWO massive blackholes to combine. And space itself moved by a proton fraction.

Do you have any idea how tiny of fraction that is to be useful? Right about now earth is about the best space ship we can hope for. As it is the only way to get one gravity.

Your argument is selective at best, some of the most profound invention in history fell out of some obscure research that no one including the people researching them had ever imagined would be of practical use. Turing accidentally invented computer by investigating the nature of information, a fundamental property of the universe.

Put yourself in the position of those people, you are focusing on applications that you already know about - you can't think about the ones you don't know about unless you open your mind.

Re:Why?

[drinkypoo]

Do you have any idea how tiny of fraction that is to be useful? Right about now earth is about the best space ship we can hope for. As it is the only way to get one gravity.

Earth is not a space ship. It has no propulsion. At most it's a space probe, and even then, it's not just Earth. You'd need the whole solar system. Since it's not going anywhere (the idea that we're not part of the milky way has been debunked) it's not that, either.

Unless, maybe, the whole galaxy is a space ship. I suppose that's an idea which merits further investigation.

Re: Why?

[Lord Crc]

And why isn't it detecting waves on a daily basis? The universe is supposed to contain billions of black holes.

The black hole merger that was first detected had a peak power output that was 50 times greater than the total power output of all the stars in the observable universe.

The waves from that merger caused the arms of the LIGO detectors to differ in length by 0.000000000000000000001 meters, which is roughly like the earth getting wider by 10 protons.

This latest merger involved less massive black holes which should mean it had a lot less peak power.

Re:Why?

[angel'o'sphere]

INFORMATION WAS SHARED FREELY. each scientist was able to use the work of the previous.

That was most certainly not the case.

Or Newton and Leibnitz had not accused each other for stealing their ideas about differential equations and integrals.

Re:Why?

[Dcnjoe60]

Scientists that where mucking around with electricity in the 18th century was not government funded...

If society, today, practiced the same amount of philanthropy and charitable given as in the 18th (and 19th) century, the government wouldn't need to fund research today.

Re:Why?

[kimvette]

> Doesn't matter. That's not the kind of "technical ability" nor the kind of instrument we need. It won't help us to address a single pressing or practical problem here on Earth in the foreseeable future.

It most certainly does matter. If the alcubierre drive (aka "warp drive") is to ever become reality, we need to more fully understand how gravity works, and then we need to develop a way to create and manipulate gravity waves and development of the sort of power sources required will likely take centuries. Otherwise, we're doomed to exinction if we keep thinking "well, we can't do anything practical with it now, so let's not bother." Had past generations held this mentality, we would not have computers, the automobile, central heating and HVAC, or even the electric light today.

Given your lack of appreciation for the whole point of pure science, I'd have to guess you're a Trump-supporting Republican. ;)

Re:Why?

[Dcnjoe60]

And the only reason we achieved so much in such a little amount of time is because the INFORMATION WAS SHARED FREELY. each scientist was able to use the work of the previous.

Now the stupidity of copyright and patents are slowing down progress at an alarming rate.

And who is stopping the free sharing of information? It's not the government. It's not the public. It's the universities. Why? so they can capitalize on the monetization of their findings. This was less of a problem before there was large scale public funding (government grants).

One way to fix it is by having any royalties go back to the public coffers in proportion to the public spending on the research. If 10% of the funding is from the government, then 10% of the resulting royalties go back to the government. If the funding is 80%, then 80% goes. Government funded research should benefit the public, not the private company the university sells the rights to.

Re: Why?

[Dcnjoe60]

Fuck you -- how dare you try to dictate the good of humanity

You are a vile and clueless piece of shit that would hold humanity back in your asshole attempt to tell us how to think and live our lives

Let me guess -- you're a progressive liberal. Probably want to tell me what to eat as well

Isn't your condemnation of whomever you were responding to, just another example of of somebody trying to tell others how to think and live their lives?

Re:Why?

[Dcnjoe60]

You have no idea what it will or will not address. When the first scientists were mucking around with electricity in the 18th century they were giving people shocks and making frogs legs jump. Within a hundred years they were rolling out the world's first global high speed communication's system.

So take your contrarianism and stick it up your ass. Your type would have us still beating each other with sticks, because, you know, what good does that shiny shit in the ground do us?

The usefulness of gravitational waves to society will probably not be on the equivalent of electricity. Electricity always existed. However, prior to being able to generate it for ourselves, it was random in occurrence (lightning), much like gravitational waves. So, unless you think we are on the verge of being able to generate our own gravitational waves, it is unlikely that the confirmation of Einstein's theory of their existence will have a practical application any time soon and almost certainly, not in the next 100 years.

That's not a bad thing. Historically, knowledge for the sake of knowledge was a good thing. It is only a recent trend in human history that it must be able to be monetized to be useful.

Re:Why?

[amRadioHed]

The waves aren't smaller than expected, this recent upgrade is actually the first time they had an instrument they thought would be sensitive enough to detect a gravity wave.

Re:Why?

[chihowa]

You also disregard the fact that basic research often helps progress in unexpected ways - including the area of microscopy.

Indeed. My research is in making better microscopes (to cure cancer and whatnot...) and I've been personally looking at some of the advances in interferometry that allowed LIGO to be built (and the analytical techniques that allow useful data to be recovered).

So Midas' criteria has been fulfilled and he can quit his bitching now.

Re:Why?

[MightyMartian]

In the short term perhaps not. But no one can really say what the value of basic research is. Even if takes centuries for an application to be found, well, we know they exist, and further we now have an instrument capable of measuring some of the smallest known perturbations in space-time.

Re:Why?

[MightyMartian]

Governments have always invested huge amounts in technical advancement, although historically much of it has been military advancement. But if you look at Rome, it made huge investments in public works, and out of that investment came numerous technical advancements, some still incredibly stunning, The Pantheon in Rome, built by state funds, is still the largest freestanding unreinforced concrete dome ever built.

But the Romans were hardly the first. The Greeks, Egyptians, Akkadians, Chinese, Sumerians, Inca, Aztecs and numerous other civilizations made great technological and engineering advancements, all largely financed by their governments. Even in the US, many of the major technical innovations made by private enterprise have either rested in part on taxpayer-funded research, or in some cases, like the development of the space and maritime technology, and even Internet itself, via direct funding by government.

Re:Why?

[Dcnjoe60]

Yes, all of those civilizations you mention, did many public works projects that had technologies come from them. But, they did not fund research for the sake of research, but instead to solve a problem. In most cases, it was not the government, per se, funding things, but the ruler. When one is organized in a monarchy or pseudo-monarchy, there is no real distinction between public funds and personal funds so the funding of projects is not truly by the government, any more than when the Renaissance popes commissioned great works of arts, it was the Catholic Church doing it. No, it was the ruler using those funds for what he believe to be a worthy endeavor. By default, since such rulers were the government it was government spending. But the reality was that scientists and artists, throughout history had wealthy benefactors, whether they be the king, the pope or some wealthy aristocrat. They weren't funded by the government in the sense we use the word today.

Re:Why?

[Dcnjoe60]

In the short term perhaps not. But no one can really say what the value of basic research is. Even if takes centuries for an application to be found, well, we know they exist, and further we now have an instrument capable of measuring some of the smallest known perturbations in space-time.

I am not saying the research isn't valuable, it is. It just isn't the same thing as the early research into electricity. Maybe some day it will bear fruit, but that isn't likely as the theories that predict gravitational waves also preclude them being very useful (kind of like Einstein's theories also posit that time travel is possible. Of course, it would require virtually all of the energy known to exist in the universe, but it is possible, if not very practical.).

Re: Why?

[TechyImmigrant]

And why isn't it detecting waves on a daily basis? The universe is supposed to contain billions of black holes.

I went to a talk by one of the LIGO scientists where pretty much this questions was asked.

It's a simple answer. Two black holes colliding is fantastically rare, but very the universe is fantastically big, so it happens a hell of a lot if it happens at all.
The LIGO experiment is limited by noise and can 'see' out to some distance. So all the events happening within the sphere of that radius get detected. The rate of detection is a function of the radius of detection. The rate of detection tells us something about the actual density of black holes in the local part of the universe.

The nice thing is as they increase the sensitivity, the volume goes up as a the third power of the radius, so the rate of detection should go up by the same amount. The rate of detection at the increased ranges tell you lots of things about the uniformity of distribution and the tightens the statistics on the detection rate. If they improve the sensitivity by a few db, they might well be detecting on a daily basis. Before the upgrade they were getting nothing. After the upgrade they have got 2 in a year.

I think they should build a couple more of these things around the globe and so get a higher resolution of direction determination. If you have a few billion, you should fund that.
 

Re: Why?

[TechyImmigrant]

It detects weaker events all the time. They can detect the gardeners mowing the lawn outside. But the gardeners aren't interesting to astronomers, black holes are.
 

Re: Why?

[TechyImmigrant]

>remember that the strength of waves decays according to the square of our distance from them.

But the detector's detection rates increase with the cube of the sensitivity distance of the detector. That's why it works.

Re: Why?

[Dcnjoe60]

At least gravitational waves are real. That alone makes it more useful to study than things like global warming. Humans aren't causing the Earth to get hotter, no matter how much you insist that your pseudoscience is real. Let's stop spending money to study fiction like global warming and then we'll have enough money to fund real research.

How would one know whether or not humans are causing the earth to get hotter, unless one were to research 1) if indeed the earth is getting hotter and 2) if so, the cause? Isn't that what research is all about -- forming a hypothesis and then conducting research to prove it true or false?

With global warming, there seem to be two possibilities -- either thousands of scientists with different political, spiritual and ethnic backgrounds have come to the conclusion that the planet is getting warmer or they all have conspired together against the big bad US for some ulterior motive. Even most skeptics to human caused global warming agree the planet is warming. The dispute is about the cause, man made or natural phenomenon.

Once again, we are faced with all of these researches involved in some massive conspiracy or not. However, it doesn't matter. If the planet is warming, regardless of whether it is from natural phenomenon or man-made, the research shows it will have devastating effects: crop producing areas will no longer be sustainable, coastal areas and many islands will flood, more violent weather patterns will occur and millions of people will be displaced because of these things. At least, that's what the research shows.

But that is the beauty of scientific endeavors. Others are free to research it and come up with different conclusions and then the scientific community gets together and works for consensus. But, let's say for the moment that climate change is pseudo-science and not occurring. By cutting off the research funds to study it, one actually plays into the hands of all of those conspiring scientists because their won't be anybody to refute their claims.

In the end, people are free to believe what they want about many things, whether right or wrong. But government leaders have a social contract to act on the best information they have available to protect their people. And obtaining that information requires research.

Re:Why?

[EndlessNameless]

And space itself moved by a proton fraction.

You must have a sense of scale.

Space moved a tiny amount, yes, but at a distance of 1.3 billion light-years. It is so small because we are so far away.

Imagine saying light waves are useless because a distant galaxy is so dim that it is barely detectable. But when you account for the distance, you realize that there was some serious energy in that light, and the light traveled a vast distance without being concentrated or guided in any fashion.

Where would telecommunications be now, if we looked disdainfully at a dim galaxy and decided not to research lasers?

Gravity is the weakest of the fundamental forces, but it is the one to study if you want to manipulate space.

If gravity wave intensity follows the inverse square law, that would be a ripple in space of roughly 1500 meters at a distance of 1 light-year from the collision (for a local deflection of 1 photon's circumference).

Since the article indicates the detector is sensitive down to 1/1000 of a photon's circumference, it would indicate at least a 1.5-meter ripple at one light-year for the smallest detectable ripple. This would be at least a 10cm change on Earth if it happened in Alpha Centuari. Obviously, this number should be multiplied by the actual detected change, but unfortunately the article does not report it.

This translates to a ripple of ~196 km at a distance of one light-day, or 1.6 million km at 8 light-minutes (the distance from the Earth to the Sun).

I don't know what a 1.6 million km ripple in space would do to something within it, but I doubt I would like to experience it.

We've done amazing things with optics in the century or two after we figured out how light worked. Clever ways of generating it, filtering it, focusing it, etc.

Now that we know with certainty that gravitational waves exist, we can work on better observations and a better understanding of this force.

We won't have interstellar warp travel in the next decade---but looking back, we didn't build the first laser until 50 years after the first detection of photons. This is just one step on a long road.

Re:Why?

[budgenator]

Most taxes were a lot lower back then as well. Most Astronomers had to make their living casting horoscopes for the idle rich back in the day.

Re:Why?

[RespekMyAthorati]

That's because your mom just walked by.

Re:Why?

[gweihir]

To give you an idea, while there is a full sun-mass equivalent of energy in the wave, it gets spread over a sphere-surface with a radius of 1 billion light-years. There is almost nothing left when the wave arrives and these cataclysmic events are rare enough that we will likely not be getting a nearer one anytime soon. The current observations only work because the detectors have been refined and refined when they failed to find anything before. They are incredibly sensitive now.

Re: Why?

[nickersonm]

From the papers, the first detection was a merger which radiated three times the mass-energy of the sun in gravitational waves. The second was 'only' ~1 M*.

Re:Why?

[david_thornley]

When were the early electricity experiments done? When did electricity become seriously useful? Depending on the dates you assign, there's likely to be a century's worth of difference there, and Benjamin Franklin would have completely failed to expect most of the uses we put it to. When were the early gravity wave experiments done? What new stuff will we find out that works with gravity wave research by 2100 or 2150? I don't know.

Re:Why?

[david_thornley]

Quite the contrary, but you'd need a background in physics and technology, and experience, to understand that observing gravity waves and conducting space exploration in general cannot help us to address a single pressing or practical problem here on Earth in the foreseeable future.

You have just described all forms of basic research, not just gravity waves and space exploration. There was no particular use for lasers when they were discovered, but last night I watched episodes of the 1960s Batman TV show by using one. There was no particular use for electricity when the Leyden jar was invented.

Now, as far as space exploration goes, we have solved some practical problems already, most obviously with communications and GPS satellites. I believe the idea of communication satellites is older than actual space exploration, but it was space exploration that pushed development of rockets that could put something in geosynchronous orbit. GPS satellites are a combination of space exploration and development in other areas. By the time we could realistically plan them, we already had proven ways of putting things into low earth orbit.

The problem with "for the foreseeable future" is that in many ways that's a pretty darn short time. I'm nearing retirement age, but "the foreseeable future" is still considerably shorter than my expected remaining lifespan.

Re:Why?

[david_thornley]

No, you're detecting virtual photons. You're constantly pushed from your normal path through spacetime by electromagnetic repulsion between you and things like floors, the ground, chair seats, etc. What you know is that your chair is accelerating at about 9.8 m/s^2 relative to your unaccelerated path. What you can't detect by sitting on your ass is whether you're on a planet big enough to produce a significant acceleration field (involving gravitons, if there is such a thing), or in an accelerating spaceship (not involving gravitons), or maybe sitting in a very large centrifuge. Now, I'm fairly sure I'm on a planet of one Earth mass, and therefore I can infer that I'm under the influence of gravity and get a decent estimate of how much by sitting on my ass

Re:Why?

[RockDoctor]

Larry Niven was a friend of Robert Forward, a physicist and aerospace engineer who made various proposals for gravity wave detectors and other over-the-horizon technologies throughout the 1960s to 1990s. He (Forward) is considered one of the doyens of "hard" SF. (As is Niven himself.)

Re:Why?

[hr raattgift]

Ohhh, close!

In practically all theories with gravitons that take GR as the unremovable background, gravitons are the quantizations of the classical spin-2 gravitational wave.

Sure, there are lots of gravitational waves passing through you, but they're very very very low amplitude; also, the weak interaction of these waves are such that even the smallest wave must be made up of enormous numbers of gravitons. So the GWs being shed by the Earth-Moon orbital system results in many many many more gravitons passing through you than solar neutrinos. Good luck unambiguously measuring _just one_, which is pretty much what one will almost always mean by "detecting a graviton".

Additionally, gravitational waves aren't what's keeping you in your chair on the surface of the Earth -- that's mostly static curvature sourced by the Earth's mass-energy. There are theories which quantize that, but they're not gravitons (in some forms of quantized gravitoelectromagnetism they can be virtual gravitons, though, along the lines of virtual photons in normal electromagnetism.)

The Earth *does* shed very low amplitude gravitational waves though, because it is not exactly spherically or rotationally symmetric. GWs are never shed by bodies exactly symmetrical about their rotational axes, or spherically symmetrical. Consider that there are non-binary bodies which are very very close to axisymmetric in the sky: many of the suspected compact massive objects like regular millisecond pulsars and the X- and gamma- black hole candidates must be much closer axisymmetric than the Earth or the sun or the pretty damn spherical slowly-rotating shells inside ordinary stars. By the shell theorem, most ordinary stars have better rotational symmetry at their surfaces than the Earth does. Yet they each source a lot more curvature than the Earth does, even though they shed a lot fewer gravitational waves (after subtracting Earth-Moon and star-large_planet GWs).

However, ordinary stars, or neutron stars, or black holes that are in binary systems shed (a) more gravitational waves, and depending on the distance between the bodies (b) gravitational waves at much higher amplitudes. So binary systems throw off more gravitons. The two LIGO events from merging black hole binaries put out more (spin-2 massless gauge boson) gravitons than the solar system produces on its own over the course of billions of years. Yet these LIGO gravitons have little effect on you sitting in your chair, right?

Re: Why?

[hr raattgift]

Yes, but they'd have to be either only a little weaker or a lot lot closer.

LIGO is sensitive to the amplitude of gravitational waves. Wave amplitude falls off linearly with distance. It's wave energy-density that falls off with the square of distance, and LIGO can detect the range of amplitudes to which it's sensitive at very tiny energy-densities.

Re:Why?

[david_thornley]

Let's look at space exploration. The original space exploration rockets in the US were based on IRBMs and ICBMs, but they didn't stay that way. The military still has rockets of various sorts, and the space program still does also, but they're not the same. For a long time now, NASA rockets have been designed for space exploration purposes, not military purposes. Once we had relatively efficient ways to put objects into the orbit of our choice, we found more and more applications. We developed capabilities for scientific purposes, and these capabilities proved to be very useful in everyday life. If we hadn't indulged in space exploration, we wouldn't have had the ready-made ability to put up GPS satellites once we could build them. We've also learned a whole lot scientifically, but it's much harder to trace that into useful results. What you are saying is that past space exploration efforts were good because you can see good results from them, and the current ones are wasteful since you lack imagination.

There's no difference between gravitational waves and lasers as you explain them. Both were theorized and apparatus constructed. The big difference is that lasers were created long enough ago that we've found very large numbers of useful purposes.

There's no difference between gravitational wave detectors and Leyden jars as you describe. Neither was useful for any practical purpose. Both demonstrated properties of matter here on Earth. Scientists working on electricity early on didn't know that they could manipulate and exploit it for anything useful. The big difference is that the Leyden jar era of electricity was a long time ago, and we've had lots of time to develop uses.

Re:ALIENS.

[MobileTatsu-NJG]

I tried building a sensor that detects gravity but in all my bench-tests it just kept pointing at your mom.

Re:ALIENS.

[PopeRatzo]

I tried building a sensor that detects gravity but in all my bench-tests it just kept pointing at your mom.

FYI, your dick is not a gravity detecting sensor.

Re: radiated into space as gravitational waves?!

[Anonymous Coward]

How should the statement be written?

Small black holes, right?

[GoodNewsJimDotCom]

Is it just me or black holes just slightly bigger than the sun sound small?

Re:Small black holes, right?

[hcs_$reboot]

Provided that if the Sun were to become a black hole, its diameter would only be 6 km ... (ie 200,000 times less), the thing that gives a current-Sun size black hole is gigantic! (one two).

Re:Small black holes, right?

[burtosis]

Is it just me or black holes just slightly bigger than the sun sound small?

I think it's a reference to the overall idea there are two size ranges of black holes. Small (stellar) and super massive. For technical reasons there seem to be a lack of many holes in the 100 to 100k solar mass size range. Supermassive mergers are thought to happen sometime after galaxies merge, but this is far less common than thier smaller counterpart mergers. Intermediate holes seem to be quite rare.

Re:Small black holes, right?

[Maritz]

It's easy if you just think of it in terms of kilopounds per megainch.

Re:Small black holes, right?

[Maritz]

It could be that intermediates are just very hard to spot with conventional astronomy. Judging by the results of LIGO so far, we might be starting to "see" a more expected amount.

Re:Small black holes, right?

[SharpFang]

To be precise, it's size would be zero. It's a singularity, no actual size.

The diameter given when describing size of a black hole is the diameter of event horizon created by the black hole. It's still just "empty space", but that's a point-of-no-return, and whatever's inside, will not escape (minus Hawking Radiation, but that's nitpicking) and that's an important characteristic that describes a limit beyond which everything is "as good as inside the black hole", the 3km from the center not doing a squat of a difference when the outside world is concerned.

Re:Small black holes, right?

[Rob Riggs]

Unless you have a testable theory to show that its size is is anything less than the event horizon, you just have faith that its size is 0 and nothing more.

Re:Small black holes, right?

[amRadioHed]

It's unlikely there is a literal singularity at the center of a black hole, but we have no theories that can make sense of what's actually going on beyond the event horizon.

Re:Small black holes, right?

[TechyImmigrant]

It's easy if you just think of it in terms of kilopounds per megainch.

No km is kibibytes per millimeter.

Re:Small black holes, right?

[budgenator]

It's easy if you just think of it in terms of kilopounds per megainch.

How many stones per megainch is that for our British readers?

Re:Small black holes, right?

[hr raattgift]

Semiclassical gravity gives a perfectly consistent picture right to the limit of strong gravity -- that's where the radius of curvature is on the Planck scale. For stellar black holes, that's well inside the horizon. At the horizon of supermassive black holes, the curvature can be arbitrarily flat, and microscopic physics just inside the horizon is no different from well outside the horizon. (This is the basis for the "no drama" conjecture frequently discussed in the aftermath of the Polchinski et al. (AMPS firewalls) paper).

There is no reason at all to conclude that semiclassical gravity is incomplete in weak gravity. The problem with GR vs QFT making incompatible predictions is not at the horizon, but much nearer to the singularity (and the presence of the zero-radius singularity itself, and you're right that most gravitational physicists hope that the singularity vanishes in a quantum theory of gravity).

You'd be right if you said we aren't certain about what happens at and very near the singularity. But very near, for a stellar black hole, is on the order of light-picoseconds to light-nanoseconds, and that distance doesn't climb nearly as quickly with its mass as the Schwarzschild radius does. For instance, you can cram a large large large number of high energy particles obeying the spin statistics for bosons into a compact area; near that seething mass you can also convert a large number of particles obeying the Pauli exclusion principle into ones that do not; the problem is in the details about the mechanisms for shortening of particle wavelengths that are already very short versus degeneracy pressures, which nobody has a clear answer for at this time. (GR has no such mechanism, so the particle wavelengths go to zero. Ideally a quantum theory of gravity has a mechanism that keeps those wavelengths from becoming arbitrarily short.)

Re:Small black holes, right?

[amRadioHed]

Thank you for the detailed response. Yeah, my comment was poorly worded, I shouldn't have said beyond the event horizon.

Re:ALIENS.

[Khyber]

Correction: It must be very tiny to be sensitive enough to accurately detect such distant large masses like GP's mom.

Re:ALIENS.

[Lorens]

In the David Weber's "Honor Harrington" universe he uses gravity waves. His gravity waves are faster than light (cue interesting plot details, of course), but the real ones detected by LIGO seem to propagate at something more on the order of 0.01 c. Does someone have a more exact value?

Don't forget Australia

[EEPROMS]

there are two detectors not one, one is based in Australia the other in the USA. Also many of the parts used in both sensor arrays were designed and created in Australia.

https://en.wikipedia.org/wiki/AIGO

Re:Don't forget Australia

[dargaud]

And what about VIRGO in Italy which is also part of the collaboration ? It was offline during the 1st detection; did it catch it this time ?

Re:Don't forget Australia

[Anonymous Coward]

There are three main detectors; two in the USA (LIGO) and one in Italy (Virgo). Currently, a detector is under construction in Japan and a fifth one using LIGO components will be built in India. Additionally, there is a smaller detector in Germany that is only sensitive in higher frequencies. It is mainly used to test technology, but it is also used for certain types of sources.

Having multiple detectors is very useful, because coincidence is used to determine the sky position. A single detector can only determine the distance, but with three or more detectors, a spot on the sky can be identified by triangulation. Moreover, having multiple detectors increases sensitivity, so weaker signals can be detected and more accurate measurements can be done on the properties of the system that is observed (e.g., the masses of merging black holes).

Re:Don't forget Australia

[Anonymous Coward]

No, Virgo was offline during the entire observation run. The collaboration decided that it would be better to perform upgrades at LIGO and Virgo at different times, so Virgo observed for another year when LIGO went offline and Virgo continues finishing upgrades for a year after LIGO resumed operations. That way, the period without any observation run was reduced and duplicate efforts could be reduced.

LIGO and Virgo will start a joint observation run at the end of this year.

Re: ALIENS.

[Anonymous Coward]

0.010000000c

Getting close to design sensitivity

[Laxator2]

Every time they get closer to the design sensitivity the detector can spot signals coming from farther away, as the wave amplitude follows the inverse square law.

This increase in range will result in a great increase in the _volume_ they can observe, and remember that these detectors do not need to be pointed they way telescopes do.

The project can clearly follow the Type 1a supernova project (which brought the Nobel Prize to Saul Perlmutter) and go from detecting one signal every few months to detecting a few signals per day.

Re:Getting close to design sensitivity

[TMB]

Counter-intuitively, the strain amplitude goes as the inverse of the distance, not the inverse square!

Re:Getting close to design sensitivity

[TMB]

Actually, let me rephrase that -- it's not counter-intuitive at all that amplitude goes as 1/r, but what's odd is that with gravitational waves, you directly detect the wave amplitude, so detectability falls off as 1/r. For most waves, you detect the wave intensity, which goes as amplitude squared and therefore 1/r^2.

Good that it was ancient

[drolli]

I guess I wouldn't want to observe something like that if i was only a light year away.

Re:ALIENS.

[Warma]

Can you elaborate what makes you think, that the detected gravity waves propagate at 0.01c?

Re:Why LIGO is a scam

[Maritz]

I just don't know who to believe. Albert Einstein and several generations of cosmologists, or a AC blowhard on Slashdot. TORN.

Re:Crazy scientists!

[Maritz]

Sorry, what?

Re: radiated into space as gravitational waves?!

[JaredOfEuropa]

No, instead we get that awful animated infographic (complete with obligatory nondescript upbeat music). To be honest I prefer a nice dry and poorly designed Powerpoint over that.

Re:ALIENS.

[Lumpy]

How do you know they are not just reflections off the edge of the glass jar the universe is in?

Re:ALIENS.

[michelcolman]

but the real ones detected by LIGO seem to propagate at something more on the order of 0.01 c. Does someone have a more exact value?

Sure, as far as we know, the exact value is c. Where did you find 0.01c?

The distance between Livingston and Hanford is 3002 km, and the signals were received 1.1 milliseconds apart. In a straight line that would be rougly 3 million km/sec, or 10c. But obviously the signal came in at an angle. If it had come in perpendicular to the line between the two detectors, they would have detected it simultaneously. So it must have come from somewhere in between, I would say around 6 degrees off the perpendicular plane between the two detectors.

Re:ALIENS.

[rgbatduke]

I think you slipped a decimal. The LIGO observatories are roughly 3000 km apart, so a straight line lag between them is around 10 milliseconds. A lag of 1 millisecond meansi that the (essentially plane) wave came in at a small angle relative to the perpendicular plane separating them. The triangle involved would (conveniently enough) have a short leg around 300 km long, and that's still a small angle so without a calculator roughly 0.1 radians on one or the other side of the perpendicular plane. I'm not certain how they manage to set the azimuthal angle and decide whether the source is on the northwest or southeast side of the plane (at the moment of detection, which then has a very particular orientation relative to universal coordinates as the earth orbits and rotates) -- maybe they use the shift in the lag AS the earth rotates to do azimuthal triangulation if the signal is long enough, maybe they use multiple detectors at right angles to each other to get an extra angle -- I suppose I could look, but detecting signal lags across meters is easy enough with modern electronics (nanosecond plus time resolution) so they could even do both. It would have been and still would be a lot more precise if they had three, or better, four detectors and could do honest to god 3D triangulation -- they aren't going to do parallax until they put a second detector on (say) the moon or at the lagrange points but they could get a very precise line to the event that way.

Any LIGO-ites on /.? Surely somebody who does this is around to comment with something other than references to the mass of their male parts? Which, on the scale we are discussing, is truly infinitesimal (reminding me of the flea and the elephant, but that's another story...:-)

rgb

Re: radiated into space as gravitational waves?!

[ultranova]

How should the statement be written?

"Massive ego, detecting an accomplishment that did not originate from itself, quickly moved into action to diminish it, causing the fabric of noosphere to ripple in a disgusted reaction."

Re:Crazy scientists!

[Bengie]

And a blackhole takes an infinite amount of time to form. Why are you complaining about what blackholes can or cannot do if you don't believe they exist?

Re:Why LIGO is a scam

[Deadstick]

Quit bogarting, dammit.

Re:ALIENS.

[Dunbal]

How does a hydroelectric dam grab you?

Re:ALIENS.

[LifesABeach]

Maybe you could just ask them how they did it?

Collision?

[DarthVain]

"Collision" might be a bit of a stretch. It implies immediacy. I didn't RTFA and I am no physicist, however I expect that the "Collision" took an extraordinary amount of time (galactic even as opposed to geologic time periods). Millions of years maybe? I have no idea. Seems if that is the case the summary is a bit sensational, in that it could more accurately be described as the waves of two black holes that slowly eventually merged into one... The end may have happened a lot quicker I suppose, but the lead time was probably enormous.

Re:Collision?

[amRadioHed]

Yes, the black holes had been circling each other for billions of years and slowly leaking energy in the form of gravity waves which resulted in them gradually moving towards each other. As their orbits tightened their orbital velocities increased and it was only in the very last fraction of a second before this long process ended that they were orbiting fast enough to create waves we could detect.

Re:ALIENS.

[rgbatduke]

If I had any ligoites in my addressbook, sure, but lacking that, posting on /. is a good way to proceed. OTOH, reading the wikipedia page would probably do it too. At the moment I'm making up a physics final and don't have time -- I was just dangling bait to see if I could get a lazy answer in the meantime...;-)

Re: radiated into space as gravitational waves?!

[dave420]

No, instead of reading a scientific report on these findings you went to the mass media and complained it was the mass media. The problem lies with you not knowing where to find what you're looking for, not that what you're looking for doesn't exist.

Re:ALIENS.

[Maritz]

That there's millisecond delays at all at the scale we're talking has me unconvinced that we've detected a gravity wave.

Hyperskeptical I see. I presume you've read up on all the measures that they take to find the real signal and subtract noise? They weren't sufficient for you?

Re:ALIENS.

[number6x]

If the gravity waves were travelling faster than the speed of light, we would not have detected them. We wouldn't be here to detect them because gravity waves propagating through spacetime faster than the speed of light would mean that the universe doesn't work and that would be the end of everything. I suggest Misner, Thorne and Wheeler, an appropriately weighty tome, for more information on the nature of space time.

whenever I have trouble sleeping, this book, saved from my graduate school days always does the trick. Much better than counting sheep.

Re:ALIENS.

[bondsbw]

gravity waves propagating through spacetime faster than the speed of light would mean that the universe doesn't work the way we thought

FTFY.

Science.

Re:ALIENS.

[T.E.D.]

I bet you are by far the best Dozens player in the laboratory.

So far away

[TheSync]

Both supposed gravitational wave detections were >400 megaparsecs (1.3 billion light years) distant. That is really, really far.

For example, the CfA2 Great Wall of galaxies is only 300 million light years from Earth.

Are there really no black hole collisions happening closer to us? Are these really so rare?

Re:So far away

[amRadioHed]

We are only detecting the last split second of a multi billion year process, so yeah pretty rare that the orbiting black holes are in a detectable state.

Re:ALIENS.

[micahraleigh]

Should be c.

Re:ALIENS.

[TheSync]

I'm not certain how they manage to set the azimuthal angle

See: Rapid Bayesian position reconstruction for gravitational-wave transients

"We introduce BAYESTAR, a rapid, Bayesian, non-Markov chain Monte Carlo sky localization algorithm that takes just seconds to produce probability sky maps that are comparable in accuracy to the full analysis. Prompt localizations from BAYESTAR will make it possible to search electromagnetic counterparts of compact binary mergers."

Re:Why LIGO is a scam

[micahraleigh]

Gravity is continuous, not instantaneous.It takes about 8 mins for the gravity from the sun to arrive on earth.

You seem to have bought into the A theory of time which has some unlikely implications (like no temporal differences between objects moving at high speeds and stationary objects, but this has been measured and is consistently applied in applications such as GPS).

Re:ALIENS.

[Dcnjoe60]

Doesn't all technology that we know of depend on gravity in some form or another?

Technology does not exist in nature, it is created. For people to exist to create the technology, they need a planet capable of sustaining life. To have a planet with an atmosphere or even to have a planet at all, requires gravity.

Ergo, all technology depends on gravity.

Not black holes

[zero_out]

These gravity waves were not from two black holes colliding. It was just Chuck Norris doing push-ups again.

Re:ALIENS.

[Lorens]

I was interested in the maximum possible speed, distance between the two points divided by 1.1 ms. Of course speed can be much less; if gravity wave propagation is orthogonal to the line between the two points then the wave should have been detected at the same instant. You might take into account the speed of the Earth and its rotation, but that should be insignificant and it gets complicated. That is why I asked if someone had a more exact (read: better) value :)

Re: radiated into space as gravitational waves?!

[JaredOfEuropa]

I'm not going to read a scientific report as it's most likely going to be waaay over my head. But I remember when mass media journalists were able to report stuff as if addressing adults instead of dropping to the level of Yo Gabba Gabba.

Re:ALIENS.

[budgenator]

If the gravity waves were travelling faster than the speed of light, we would not have detected them.

Not necessarily, C, the speed of light (In Vacuum) in is available to both sub-luminal and super-luminal events, so a super-luminal particle could cause a flash of light that could be seen by us in our sub-luminal universe.
The effect would be similar to but not the same as Cherenkov radiation where neutrons travelling faster than the speed of light in water cause a blue glow in reactor pools.

Re: radiated into space as gravitational waves?!

[TechyImmigrant]

A real physicist on the radio called it "swimming in jello" (That's jelly for the rest of the world).

That seems a pretty good analogy. Radiating gravity waves seems fine too. People are so picky.

Re:Why LIGO is a scam

[TechyImmigrant]

Stable Planetary Orbits or Newton Was Right

In spite of the incessant propaganda over the last century from the general relativity camp, gravity is an instantaneous or nonlocal phenomenon, just as Isaac Newton assumed. If changes in gravity traveled at the speed of light, as relativists claim, Newtonian gravity equations would not work at all and all planetary orbits would become unstable. There would be no planetary systems orbiting stars and there would be no galaxies. The reason is that it would take more than eight minutes for changes (caused by its motion around the Milky Way galaxy) in the sun's gravity to reach the earth and even longer for the more distant planets. So the earth's orbit around the sun would depend on where the sun was eight minutes ago, the time it takes changes in the gravitational field to reach the earth, and not on where it is now. This is not observed.

[emphasis added]

If true, this was the most interesting part of your article, Louis. Got an external link to back it up?

It's not true. I don't have a link to back it up, but I did read it somewhere plausible written by a real physicist. The motion of the planets fits the relatavistic model. One of the problems with Newton's laws is that the motions of the planets did not fit the predictions of Newton's laws. Relativity made up for the discrepancy. The motion of the planets is conditionally stable, just like any control system in the real world (as opposed to your simulator).

Re:ALIENS.

[budgenator]

If the detectors arms aren't parallel and perpendicular to each other and the source that would give them an additional data point and make it possible to triangulate; 4 arms in two locations, could give you 3 data points.

Re:Why LIGO is a scam

[budgenator]

Regardless of whether "gravity" is "an instantaneous or nonlocal phenomenon" or not, LIGO detects the changes in gravity and the propagation of that change isn't instantaneous. I didn't understand how LIGO worked, until I wrapped my head around the fact that gravity waves are non-instantanious.

Detection claimed in 1960s

[peter303]

By Maryland physicist Joesph Weber. He used pizeolectric strain sensors on large metal cylinders. No one could repeat his results nor prove his experiment was sensitive enough. The LIGO project is a direct reaction to this failure with a supposedly better approach. But it took over 30 years of fund raising and technology development to make LIGO work. During those 30 years Congress threatened termination several times. And skeptics like myself though there might be something incorrect with LIGO physics until they finally got a result.

Re:Why LIGO is a scam

[Lord Crc]

If true, this was the most interesting part of your article, Louis. Got an external link to back it up?

Here's a link to an article explaining of why it's false: http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

Any issue raised woth Hawking radiation?

[pmowry911]

From what I read the loss of mass in the system was converted into the gravitational waves and no gamma rays or anything were produced. If we assume this is true does it make the theory of Hawking radiation incorrect?

Forgive me if this is already covered. Couldn't find a similar post on my cell phone

Re:Any issue raised woth Hawking radiation?

[nickersonm]

No, Hawking radiation is related to the area of the event horizon (and therefore the mass), and is only relevant for very small black holes, as for anything above ~0.4 lunar masses (1.5e-8 solar mass) it will absorb more energy from cosmic microwave background than it emits in Hawking radiation.

Re:Why LIGO is a scam

[TechyImmigrant]

You don't know what the fuck you're talking about.

Oh Mr (or Mrs, or Ms) Coward. You seem to be scientifically illiterate.

Re:ALIENS.

[Warma]

Well, the speed of the gravity wave is exactly c, to the best of our knowledge. I still don't understand where you're getting that 0.01c from.

Re:ALIENS.

[Coren22]

The latest signals arrived at the Livingston detector 1.1milliseconds before they hit the Hanford detector, allowing scientists on the team to roughly work out the position of the collision in the sky.

https://www.google.com/search?...

(distance between the two, approximately 2300 miles)

Do the math, what speed do you get when you divide 2300 miles (or km equivalent) by 1.1 ms?

Re:ALIENS.

[Coren22]

Sorry, the LIGO wikipedia entry says 3002 km or 1865 miles.

https://en.wikipedia.org/wiki/...

Re:ALIENS.

[Coren22]

There must be some kind of error in 1.1 ms, as that is 9.1 x c, which doesn't seem possible...

https://www.wolframalpha.com/i...

Re:ALIENS.

[Coren22]

Electrons moving faster than the speed of light in the medium (water) you mean?

Re:ALIENS.

[ACE209]

Remember, the signal didn't travel between the detectors. Depending on the positions of detectors and sources, they could even have detected the signal at the same time.

Re:ALIENS.

[beastofburdon]

If they propagated at 0.01 c then the event would have been able to be seen on earth by the dinosaurs or before, and not by us, so that is impossible.

Re:ALIENS.

[beastofburdon]

Nope, it was dismissed out of hand. There was never any kind of test done to discredit it, the scientists simply said it couldn't possibly be and refused to even entertain the idea.

Re:ALIENS.

[beastofburdon]

I disagree with one thing. Cosmology is not at risk of becoming as dogmatic as religions, it has been at least that dogmatic for many decades.

Re:ALIENS.

[RockDoctor]

Anything - electron, proton, neutrino, banana - travelling faster than the speed of light in a particular medium will generate Cherenkov radiation.

Re:ALIENS.

[RockDoctor]

I'm not certain how they manage to set the azimuthal angle and decide whether the source is on the northwest or southeast side of the plane

Each of the two sites has two legs of their detector perpendicular to the other - a necessary part of the design. But if the legs at (say) LIGO-NW are oriented to bearing 000deg and 090deg, then the ones at LIGO-SE can be oriented at 045deg and 135deg.

Actually, it only requires that the legs not be parallel between the two sites, though you get a higher signal-to-noise ratio if the sites are at 45deg to each other, it's not much worse if hey're at 30deg or 60 deg. Further complicating matters is that you have to take the curvature of the Earth into account, because it's the bearing in three dimensions that you need, so again ideally you would want the sites at 45deg of latitude between each other, and 45 deg of longitude, but I'm not sure that can be accommodated within the USA, raising political pork barrel problems. But it's not critical, as long as there is a significant separation between the two stations.

They've been covering this for decades, since the proposal phase of the projects - including the international aspects, trying to combine data from LIGO with the other detectors in operation or under construction (VIRGO, MiniGRAIL, KAGRA ...).

The maths is actually congruent to analysing the birefringence figure of a mineral under crossed-polarisers, if you've ever had to do optical mineralogy. I couldn't do the spherical trigonometry these decades, but I passed the exam back in the 1980s.

Re: Trump will save us !

[StephenWilliamSchnar]

Sort of like how the left has been preaching that straight white males are the cancer that destroys the world? Don't think that either side is innocent of hatred. Trump is a symptom of a society being held ransom by busy-bodies that are trying to tell people how to live their lives, what they can and cannot say, and that have a singular hatred for one skin color and one gender.

Re:ALIENS.

[hr raattgift]

"c" (small letter) is the sole free parameter of the Poincaré group, which is the isometry group of Minkowski spacetime, which is the spacetime of Special Relativity. Locally, in our universe, spacetime far from black holes and the big bang is such an excellent approximation of Minkowski spacetime that Special Relativity passes all known tests (including those done by nature in pretty extreme conditions). The boundary of locally is determined by the radius of curvature, and for all practical purposes there is nowhere in the known universe except very near black holes (the singularity, not necessarily the horizon, where curvature radiuses can be arbitrarily large) and very near the big bang where that is less than light-microsecond scale (and typically curvature radiuses are many orders of magnitude larger).

The parameter corresponds physically to the speed of a particle whose mass is always zero under Poincaré group transformations. Those particles are said to be "massless" or to have zero rest mass or to have zero invariant mass.

The Poincaré group cannot be the isometry group of a spacetime in which objects travel on spacelike geodesics. Although one can _deform_ Special Relativity to incorporate the behaviours of objects moving faster than "c", the known deformations come at a cost, typically of making some physical observers see manifestly unphysical things, and we have substantial evidence against quite a few of those observations at any but the tiniest energy-densities in the known universe. (Deformed SR has been a topic of study for about sixty years.)

Finally, in Special Relativity, light is assumed to be massless. (There is ample experimental evidence which make the upper possible bound on light be extremely tiny, and all tests are compatible with light being actually massless).

However, In General Relativity we don't talk about light being "massless" at all, or having a particular velocity; it simply has the behaviour that anywhere in the manifold a particle of light's motions are constrained to the surface of a nonempty, open, convex cone of tangent bundles on that point as determined by the values of all the fields at that point. The cone we use in practice is called the "light cone" or the "causal cone", because as far as we can tell experimentally, light ALWAYS travels on the surface of that cone and nowhere else inside or outside it, and all other matter travels within the cone and never outside it. All known matter behaves this way. Additionally, (classical) gravitational waves travels on the cone (there is good evidence for that), and a quantization of them into gravitons would too (so in SR terms they would be "massless").

That said, in GR you can use a solution to the Einstein Field Equations of General Relativity that admit a hyperbolization AND also a causal cone structure whose slope is wider than that of light. General Relativity admits what Geroch calls "a democracy of causal cones". (He also will tell you that there is zero evidence for this in our universe). However, when you take that approach, you develop an initial values surface and evolve the solution to the EFE forward by doing the calculations. You fall into trouble if you try to set up an initial and a final values surface and try to match the two using your solution, as the results do not tend to match intuitions. Indeed, using just the standard light cone, Miguel Alcubierre found enough "bugs" in the wishful-thinking/surface-matching approach that he has been able to write a book about the topic in the setting of numerical relativity.

So, your intuition is that something whose causal cone is outside that of the standard one is that cherenkov radiation will result. There is a non-easy but robust way of checking that intuition: develop a solution to the EFEs that are a slight deformation of e.g. the Minkowski, de Sitter or Schwarzschild electrovacuum (with possibly more field content than just the electromagnetic one if you feel very ambitious) and solve the equations for your faster-than-light test particles.

Go for it, it has value as an exercise.

What is it?

[martinfb]

How fast do gravity waves travel?

Is gravity akin to a change in the (local) pressure of time-space, which increases on approach to mass. The more the mass, the more displaced time-space is, and the higher the pressure - the greater the gravity.

Re:ALIENS.

[Warma]

OK. It's not as if the other detector is sending the gravity wave and the other is receiving it, but that both are seeing an event that happens somewhere else. If the source of the event were equally far from both detectors, the time lag between them would be 0.

The arrival time between the detectors is used to calculate the direction, in the same way your ears determine the direction a sound is coming from. Of course, with only 2 detectors, this will not localize the direction perfectly, but it's a start.