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# Article: Gravitational waves: talk to a scientist behind the discovery watch

1. @Fullofsurprises

Gravitational waves are ripples in space-time - the fabric of the universe, if you will. You have seen the animation of a mass distorting a sheet, and the mass distorts the sheet?

What if we dropped the mass? The sheet would be distorted more than if the mass was just placed there. The sheet would then re-form until it reached equilibrium, producing waves.
2. (Original post by Fullofsurprises)
What exactly is 'waving'?

I mean, what are gravity waves waving in?
There's been some further discussion of this below, so let me build on that.

When we describe gravitational waves as ripples in spacetime itself, we often use the analogy of a stretched sheet of rubber - like a trampoline - onto which a heavy object is dropped - like a bowling ball (say). The effect of dropping the ball will be to make the sheet wobble, and the ball will also bounce a little, before both settle down to a stable state with the ball sitting on the trampoline.

One aspect of this analogy that isn't maybe so good is that we see the trampoline moving in a 3rd dimension - i.e. we measure the change in the shape of our "spacetime" from outside of that surface. This often leads people to ask the (quite reasonable!) question "what are the gravitational waves waving in?" This is a very similar train of thought to the idea of the expansion of the universe, that was discovered by Edwin Hubble in the 1920s and is *also* predicted by Einstein's theory of general relativity, which leads to a question along the lines of "if the universe is expanding, what is it expanding into?..." At first glance this seems like an eminently reasonable question, and certainly left me scratching my head when I first began studying cosmology as a student.

The short answer, however, is that it doesn't have to be expanding into anything! We often use a 2-D analogy (like the trampoline) for the expansion of space, thinking about how for example dots on the surface of a balloon appear to move apart from each other as the balloon inflates. Here it's the 2-D surface of the balloon that is the counterpart of our 3 dimensions of space, but of course we can see - from outside, in the 3 dimensions that we inhabit - the balloon expanding into that 3rd dimension. Nevertheless, by looking at the properties of those dots on the surface, and how curved lines and angles behave as the balloon expands, the crucial point is that we could tell something about the curvature of the balloon's surface (and in particular distinguish its shape from the surface of a flat piece of paper, or the surface of a saddle) while still being completely embedded *within* the surface itself. It's what we call the intrinsic curvature of the surface.

In a similar way, then, the changes in the curvature of spacetime that produce gravitational waves can be characterised and measured in terms of the intrinsic curvature of the surface - without requiring us to think about a higher dimensional "surface" in which those changes might betaking place.

So the gravitational waves - the changes in the curvature of spacetime - don't need to be waving in a higher dimensional space. Even more mind-blowing, they don't need to be waving in *anything*: they can propagate through empty space! In fact so too can electromagnetic waves: neither needs any kind of medium to travel through. (A few people have mentioned the ether elsewhere in the chain of comments).

But just how empty is empty space? That's where one has to go *beyond* Einstein's theory of gravity (which is what we call a classical theory - which means in essence that we can treat spacetime as a smooth surface) and consider the weird physics rules of the quantum world: in other words we need to think of a quantum theory of gravity - which means that we no longer think of the vacuum of empty space as empty at all; instead it's a seething cauldron of virtual particles popping in and out of existence but not hanging around long enough for a "classical" observer to really notice.

So what is really going on in the fabric of spacetime? There is a phrase (originally attributed tothe physicist John Wheeler) often used in popular descriptions of generalrelativity that says "spacetime tells matter how to move and matter tells spacetime how to curve". This phrase captures the essence of Einstein's big idea: that we should not think ofgravity as a force between massive bodies, but as a curving or bending of spacetime itself.

However, John Wheeler's phrase does rather side-step aneven deeper question: *how* is it that matter "tells" matter how to curve? To fully answer this we need to understand something about the fundamental nature of spacetime itself, and we expect that this will require a fundamental linking together of the general relativity description and the rules of quantum physics as they apply to the vacuum of empty space. Future observations of gravitational wave source may lead to further insights about how such a quantum gravity theory would work - although in truth for most of the sources that LIGO will see, the waveforms that we will measure should be perfectly consistent with the general relativity description - i.e. our data won't have the detailor "resolution" necessary to allow us to investigate many aspects of quantum gravity.
3. (Original post by Kyx)
How did you make sure it was gravitational waves, and not something else?
This is, of course, an absolutely crucial question.

As I mentioned in answer to another question, in the final few weeks before the announcement, as we put the finishing touches on our scientific papers, one of my particular tasks was to lead the writing of a "Science Summary" of the main detection paper: an article whose aim was to summarise all of our main conclusions and findings but without the technical language and jargon found in the main scientific paper. You can find a copy of it on ourwww.ligo.org website at: http://www.ligo.org/science/Publicat...0914/flyer.pdf

You'll see from the science summary that we devoted most of the 4th page to this question. Let me quote from the science summary:

"Firstly, the time delay between the observations made at each LIGO detector was consistent with the light travel timebetween the two sites. Also, the Hanford and Livingston signals showed a similar pattern, as would be expected given thenear alignment of the two interferometers, and were strong enough to ‘stand out’ against the background noise around the time of the event– like a burst of laughter heard above the background chatter of a crowded room."

"Understanding this background noise is an essential part of ouranalysis and involves monitoring a vast array of environmental datarecorded at both sites: ground motions, temperature variations andpower grid fluctuations to name just a few. In parallel, many datachannels monitor in real time the status of the interferometers –checking, for example, that the various laser beams are properlycentred. If any of these environmental or instrumental channelsindicated a problem, then the detector data would be discarded.However, despite exhaustive studies, no such data quality problemswere found at the time of the event."

"But perhaps GW150914 was a rare noise fluctuation, whichhappened to occur simply by chance with similar characteristics atboth sites? To reject this possibility we need to work out just howrare such a fluctuation would be: the less often it could occur bychance, the more confidently we can rule out this scenario in favourof the alternative – that GW150914 was indeed a real gravitationalwave event."

"To carry out this statistical analysis we used 16 days’ worth ofstable, high quality detector strain data from the month followingthe event. GW150914 was indeed by far the strongest signalobserved in either detector during that period. We then introduceda series of artificial time shifts between the H1 and L1 data,effectively creating a much longer data set in which we could searchfor apparent signals that were as strong (or stronger) than GW150914."

"By using only time shifts greater than 10 milliseconds (the light travel time between the detectors) we ensured that theseartificial data sets contained no real signals, but only coincidences in noise. We can then see, in the very long artificial data set, how often acoincidence mimicking GW150914 would appear. This analysis gives us the false alarm rate: how often we could expect to measure such aseemingly loud event that was really just a noise fluctuation (i.e. a ‘false alarm’)."

"Our analysis showed that a noise event mimicking GW150914 would be exceedingly rare – indeed we expect an event as strong as GW150914 toappear by chance only once in about 200,000 years of such data!"

On that basis, we are pretty sure that the event was a real gravitational wave.
4. (Original post by Joinedup)
How dangerous are events like the one LIGO detected?

How near to earth could black holes / neutron stars collide without exterminating life on earth

edit - came over a bit daily mail for a minute there... congratulations on the detecttion
The gravitational waves themselves would stretch and squash spacetime - indeed that's the effect we measured here at the Earth. The closer you were to the merger, the greater the stretching and squashing. However, *even if the merger were only as far away as the Sun is from the Earth* the stretching and squashing would still be small - roughly about a billionth of a metre. That's a *huge* effect compared with the stretching and squashing we actually measured from GW150914, which was less than a million millionth the width of a human hair, but it wouldn't tear the Earth apart or anything as dramatic as that.

The electromagnetic radiation given off by the merger of two compact objects like neutron stars could do a lot more damage. Such an event we believe could produce a burst of gamma rays - high-energy light radiation that, if we were caught in the firing line of a gamma-ray burst right on our cosmic doorstep, could wipe out all life on Earth!
5. (Original post by Kyx)
@Fullofsurprises

Gravitational waves are ripples in space-time - the fabric of the universe, if you will. You have seen the animation of a mass distorting a sheet, and the mass distorts the sheet?

What if we dropped the mass? The sheet would be distorted more than if the mass was just placed there. The sheet would then re-form until it reached equilibrium, producing waves.
I suppose maybe my question is 'what is gravity' then. They don't sound like gravity waves to me. They sound like universe waves or bumps in reality or something.
6. (Original post by Martin Hendry)
There's been some further discussion of this below, so let me build on that.

.................

However, John Wheeler's phrase does rather side-step aneven deeper question: *how* is it that matter "tells" matter how to curve? To fully answer this we need to understand something about the fundamental nature of spacetime itself, and we expect that this will require a fundamental linking together of the general relativity description and the rules of quantum physics as they apply to the vacuum of empty space. Future observations of gravitational wave source may lead to further insights about how such a quantum gravity theory would work - although in truth for most of the sources that LIGO will see, the waveforms that we will measure should be perfectly consistent with the general relativity description - i.e. our data won't have the detailor "resolution" necessary to allow us to investigate many aspects of quantum gravity.
It's really nice of you to take so much time to reply to my (I'm sure dozy) question!

I still have problems really believing that part about the universe not expanding into something. I get that we can tell that it's expanding from seeing all the distant galaxies rushing away from us. That sounds like the universe is expanding into an infinity of emptiness. Maybe the problem is we just can't see far enough to tell what's 'outside'? The explanation you give about 2D/3D perspectives sounds like the kind of thing cosmologists make up to cover something that is unknowable? Just being facetious, forgive my ignorance.

The gravity thing sounds like the whole universe is wobbling, but obviously it must be cleverer than that.

Is gravity even a 'thing' really? Is it just our perception of something operating beyond our frame of reference?
7. (Original post by Martin Hendry)
This is, of course, an absolutely crucial question.

On that basis, we are pretty sure that the event was a real gravitational wave.
Or a London bus hitting a taxi.

Seriously, don't you need loads before you announce something as big as this? It seems like measuring a change the size of a proton (is it even that large?) is so exquisite, it's difficult to believe that you can ever detect enough about disturbances from other sources.

Shouldn't the detectors be in space?
8. (Original post by Laomedeia)
I have read contradicting information about black holes. It is often said that black holes are a point of singularity (as in infinitely small), on the other hand I heard some black holes are so massive that if you put one where our sun is, it would fill the orbit of Mercury. So do black holes vary in size or are they all the singularity thing?
What's going on here is the mixing up of two distinct properties of black holes. If you take the mass of the Sun (2 x 10^30 kg, or 2 million million million million million kilos) and could somehow squeeze it down to a sphere of radius 3km, then you'd turn the Sun into a black hole. What that means is: at a radius three kilometres from the centre of the Sun you'd encounter what we call the "event horizon" (also known as the Schwarzschild Radius) beyond which not even light could escape. It's meaningful, therefore, to think of this radius of the event horizon as an indication of the "size" of the black hole - as it does mark an important boundary between the "inside" and "outside" of the black hole.

So what happens to matter inside the event horizon? Well according to General Relativity there's basically nothing that can prevent that matter getting squashed all the way down to a point - producing the singularity that you mention in your question. So if (as I said at the start of my answer) we think of turning the Sun into a black hole by squashing down its mass into a smaller and smaller sphere, then you'd first make the event horizon - which is then the only surface anyone outside would see - but inside the event horizon the mass of the Sun would just keep shrinking down ever more densely until it reached the singularity.

So when people talk about a really massive black hole filling the orbit of Mercury, they are referring to supermassive black holes, such as those that we believe can be found at the centre of almost all galaxies. The radius of the event horizon is just proportional to the mass of the black hole; if you double the mass, you double the radius. So a black hole like the two that merged in GW150914 - each of which had a mass of about 30 times that of the Sun - would have an event horizon of radius about 90 km.

The biggest supermassive black holes, however, have masses about a billion times that of the Sun, which means a radius of their event horizon equal to about 3 billion kilometres. The radius of the Earth's orbit is only (only!) 150 million kilometres. So the event horizon of one of these monster supermassive black holes wouldn't just be bigger than Mercury's orbit, it'd go all the way out to the orbit of Uranus!

On the other hand many supermassive black holes have more modest masses of few million solar masses. That means their event horizons have a radius of perhaps about 10 million kilometres, whereas Mercury's average distance from the Sun is more like 60 million km.

But the important point is that even for these supermassive black holes, where the event horizon is big enough to swallow a planet's orbit, according to General Relativity, there's still a singularity at their centre - deep inside the event horizon.

Does the density really become infinite at a central singularity? That's another question. Most physicists think that the GR prediction of a singularity is really just a sign that GR is breaking down, and we need another - more complete - theory to describe such strong gravity conditions. Take a look at some of my other comments and answers where I talk about quantum gravity to see how such a theory might arise.
9. What can we, potentially, do with gravitational waves?
10. (Original post by Fullofsurprises)
Or a London bus hitting a taxi.

Seriously, don't you need loads before you announce something as big as this? It seems like measuring a change the size of a proton (is it even that large?) is so exquisite, it's difficult to believe that you can ever detect enough about disturbances from other sources.

Shouldn't the detectors be in space?
We have more data that we are currently analysing - who's to say we won't have more detections to report soon....

I would say that you don't need loads of confirmed detections, provided you have a robust and reliable way to quantify the chances that what you *think* is a detection was instead just an unfortunate and coincidental example of a local "noise" disturbance. I believe we have exactly that, via the "false alarm rate" calculation that I described briefly in the earlier reply, and which is described more fully in our detection paper science summary. Have a read of that....

Putting a gravitational wave detector in space is a very good idea, and there are plans for doing exactly that - but it's expensive and challenging, and we have to take it a step at a time. In December 2015 a technology demonstrator mission called LISA Pathfinder was launched by the European Space Agency, and is designed to test some key methods and principles that will be used (we hope) in about 15-20 years to operate a planned space-borne GW detector called LISA.

But it isn't a case of either / or. Just like with light, there are cosmic sources of gravitational waves at a wide range of frequencies. The high frequency radiation we can "see" from ground-based detectors like LIGO; lower frequency waves will need a spaceborne detector like LISA. Just as we learn more about the universe by combining observations made with radio telescopes (low frequency EM radiation) through optical telescopes all the way to X-ray and gamma ray telescopes (high frequency EM radiation) similarly we want to be able to observe across the entire GW spectrum too.
11. (Original post by Kelmanator)
What can we, potentially, do with gravitational waves?
Surf them dude.

Seriously, they are made by like, black holes colliding and stuff. Not really anything much down on the level of flying or making stuff or building apps.
12. (Original post by Martin Hendry)
We have more data that we are currently analysing - who's to say we won't have more detections to report soon....

I would say that you don't need loads of confirmed detections, provided you have a robust and reliable way to quantify the chances that what you *think* is a detection was instead just an unfortunate and coincidental example of a local "noise" disturbance. I believe we have exactly that, via the "false alarm rate" calculation that I described briefly in the earlier reply, and which is described more fully in our detection paper science summary. Have a read of that....

Putting a gravitational wave detector in space is a very good idea, and there are plans for doing exactly that - but it's expensive and challenging, and we have to take it a step at a time. In December 2015 a technology demonstrator mission called LISA Pathfinder was launched by the European Space Agency, and is designed to test some key methods and principles that will be used (we hope) in about 15-20 years to operate a planned space-borne GW detector called LISA.

But it isn't a case of either / or. Just like with light, there are cosmic sources of gravitational waves at a wide range of frequencies. The high frequency radiation we can "see" from ground-based detectors like LIGO; lower frequency waves will need a spaceborne detector like LISA. Just as we learn more about the universe by combining observations made with radio telescopes (low frequency EM radiation) through optical telescopes all the way to X-ray and gamma ray telescopes (high frequency EM radiation) similarly we want to be able to observe across the entire GW spectrum too.
I hate to be a cynic, but we've been here before with other big science announcements like life on Mars and faster than light, only for them to be subsequently demolished.

Being a total cynic, is there a fresh grant application in the middle of all this somewhere?
13. (Original post by Kelmanator)
What can we, potentially, do with gravitational waves?
We can probe regions of the universe where violent cosmic events are happening, because those are the sorts of regions that can produce gravitational waves. This can give us insights into the nature of gravity itself and tell us more than we could learn from studying light along about the objects that emit GWs. For example we could learn about the interior structure of exploding stars called supernovae, or the remnants of dead stars called neutron stars. We could possibly also learn more about the earliest moments after the Big Bang, which we can't "see" in any form of light because of something called the cosmic bakcground radiation, which acts like a fog bank and stops us from seeing any further back than about 400,000 years after the Big Bang. Gravitational waves just pass through that fog bank, however, so if we could detect GWs from the very early universe (i.e. when it was younger that 400,000 years old) that could help us understand how the universe began.
14. Is it possible that dark energy is caused by gravitational waves? Like, from the big bang.
15. Does this discovery make any changes to our understanding of the universe?
16. (Original post by Fullofsurprises)
Brian! Sigh. .
Off topic, but I watched a movie called Red Eye last week, and it said it had a Brian Cox in it, and all throughout the movie I mistook Cillian Murphy for Brian Cox the scientist. I kept thinking that it's strange Brian is suddenly such a good actor...

I haven't got any questions for the Q&A but I'm enjoying reading the previous responses and would like to thank you for your time, Professor Hendry.

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