That’s about all we have time for today. Thanks to everyone who posted a question (or an answer) below the line, and to our three experts for taking the time to respond to your queries.
We’re off to eat some celebratory cake.
groove pounder asks:
If this had happened at 1 AU, what discernible effects, if any, would we have felt on earth?
polomosquito asks:
As gravitational waves travel at the speed of light is our light cone identical to our “gravitational wave” cone?
Yes it is. However, gravitational waves don't interact with matter, while light does. Therefore, in principle we can use gravitational waves to look further back in time than with light, although the the detectors required to do this look must look at different frequencies than LIGO.
SantosElHalper (excellent Simpsons reference there) asks:
Once we use our existing and future technology over the decades, for example putting the mirrors etc into space to detect “different” types of gravitational wave, what kind of other events might we be able to understand through this? So far two black holes collapsing into each other - what else?
This discovery begins a completely new way to view the universe. So far astronomers have only had access to the different wavelengths of the electromagnetic spectrum (visible light, radio, microwaves, X rays, gamma rays etc) to view the Universe. By using gravitational waves we can discover extreme astrophysical systems which are in regimes where gravity has a very strong influence on the evolution of the system -- black hole mergers of different mass ranges (all the way from BH of the mass of the sun to supermassive blackholes in the hearts of galaxies); mergers of neutron stars - imagine superdense rapidly-spinning spheres of nuclear matter roughly the size of London!). We might even discover gravitational waves from the first instants of the birth of the Universe!
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"From an astronomer's point of view, this discovery opens up a completely new way of looking at the Universe"
LucyInthesky asks:
As an observation, this is a brilliant combination of technology, engineering and science. But given that gravitational waves have been worked on/with theoretically for a century, and nothing in the theoretical work has ever contradicted Einstein’s assumptions, in what way does this observation tell us anything we didn’t already ‘know’, other than verifying it by observation? And how is this going to change future scientific work?
You are right that this spectacular verification of Einstein's prediction from general relativity could have been expected, given all the other ways General Relativity's predictions have been confirmed. The reason that thousands of scientists have been patiently looking for this for such a long time, steadily increasing the sensitivity of their detectors, is because they were fairly certain that a signal would eventually be detected. However, it is fundamental to the scientific method that theoretical predictions are verified by observation or experiment. Even for our most successful theories, we have to keep testing their predictions, because eventually the theory will break down, and we will discover new physics. We already know that Einstein's gravity is not the full picture, as it is incompatible with quantum mechanics. From an astronomer's point of view, this discovery opens up a completely new way of looking at the Universe, in gravitational waves rather than electromagnetic waves. It is very significant that this is not just the first discovery of gravitational waves, but also the first detection of the merger of a black hole-black hole system. We are going to make a lot of astronomical discoveries that no one has yet anticipated - this has happened every time a new part of the electromagnetic spectrum has been opened up to astronomers, and with gravitational waves we are in "here be dragons" territory.
Ab Wilson writes:
Firstly just wow. Now I’ve got lots of questions about what actually happens as the black holes merge and what we can learn from the signal.
Are the gravitational waves coming from inside the event horizon(s)? If so does this mean we’re getting information from inside the merged black hole?
Theoretically the black holes themselves are singularities with no extent. So what does it mean to say that they merge? They ought to be able to get infinitely close together without actually merging. As they do so their orbits should become infinitely fast. But the faster they orbit the more energy they radiate away as gravitational waves. At some point the orbital speed is going to become so high that relativistic effects will become significant. What happens then? Does this limit the orbital speed and therefore accelerate the merger? Is this effect measurable in the signal?
If the black holes are not points (as is kind of necessary to make quantum gravity work) would there be evidence for this in the signal? Could this possibly present experimental evidence to validate theories beyond the standard model?
Lastly I read that 3 solar masses worth of energy were radiated away as gravitational waves during the event. If the universe is open the radiation will spread out forever becoming fainter and fainter and eventually being lost in background noise. But if the universe is closed then the waves would eventually loop back on themselves - the waves would be contained. Would it be possible to determine the shape of the universe based on the nature of the gravitational background noise? I’d have thought it ought to be possible to put a limit on the minimum size of a closed universe based on the gravitational background noise although I expect we’d need much better detectors to make this possible.
Again wow. Probably the most significant scientific observation of my lifetime.
The gravitational waves that escaped the merger and eventually travelled to us were produced outside the event horizons of the original (and final) black holes, so that they are not coming from inside the event horizons. Since gravitational waves only propagate at the speed of light, they cannot escape from inside the event horizon of a black hole.
When we talk about two black holes merging, what we really mean is that the event horizons of the two black holes eventually overlap with each other, resulting in a single event horizon. The pre-merger black holes were already moving fast enough that relativistic effects were important. Further, since the two merging objects are black holes, the actual merger was governed by General Relativity and thus an example of an event where relativistic effects are extremely important.
This signal that was detected was created well within the classical regime, so that it can't be used to directly tell us anything about quantum gravity. Further, the various sources of noise in the detector are much, much larger than any quantum gravitational effects we expect in the signal so that with only the one event we can't probe the quantum regime.
The current bounds on the size of the universe are already stronger than limits we would be able to obtain from observations of binary merger events such as those LIGO will see.
Peter Nilsson asks:
Would the LIGO benefit from a third detector, in the Z-direction?
Yes, another detector will help with determining the direction that any signals detected originated from. This information can then help astronomers to point a telescope in the direction the gravitational wave signal came from to look for electromagnetic counterparts such as gamma rays.
XanthanGumsSon asks:
Is the gravitational wave from those two black holes colliding still present? Will the wave always be there or does it or would it disappear after some amount of time has passed?
The wave will continue to propagate at the speed of light away from the original black hole collision. However, it will not pass by us again.
Christian Beaumont asks:
If gravity waves traveled at the speed of light away from the Big Bang, then surely they would be long gone, since the Earth is not travelling away from the Big Bang at the speed of light.
How would it therefore be possible to probe “..relics from the Big Bang”, given the wave-front already passed?
I suppose one could argue that measurements of the density structures of the cosmos itself are indirect measurements of the Big Bang... although that seems a little misleading.
The Big Bang wasn't an event that occurred at one point within our universe. It occurred everywhere. This means that any relics of the big ban would have been produced at all points in the universe. Therefore, wavefronts originating from more and more distant points away from us are constantly passing us, allowing us to look for such Big Bang relics.
Dr Jonathan Braden has also been busy below the line answering questions. One commenter asks:
How are they able to be so specific about the two merging black holes, distance, direction and mass?
The exact shape of the signal that they saw depends on various properties of the two black holes that collided, so by looking carefully at the shape of the signal they were able to understand something about the masses of the black holes. To get the direction, they needed to use the fact that they have two detectors. As more GW detectors come online, the ability to determine the direction of events will improve.
peterSpain asks:
How do they filter out waves from similar events producing similar waves? How do they know the start and end points of the wave with so many event data?
Guardian contributor this is a rather distinctive pattern of wavelengths, so there aren't many events like this. But the most impressive test is the near-identical traces seen in the two LIGO detectors, separated by a time consistent with a wave travelling across them at the speed of light. Local background "noise" would not do that.
Hi Jon - Graham Taylor once said “in football time and space are the same thing” a) is he right and b) if so, does this mean he’s as smart as Einstein (assuming he came up with his own independent thought experiment)?
Guardian contributor
UhOhSeven has a question, which was answered by an amateur physicist in the comments, to which Jon Butterworth has added his thoughts.
Can you explain why the passage of the gravitational wave through the detecting apparatus didn’t correspondingly alter the dimensions of the apparatus itself, cancelling out the interference effect?
I'm only an amateur physicist, but I think I can answer your question. It's the change in the dimensions of the detecting apparatus itself that's measured in the LIGO experiment. The basic set-up is two, long, high vacuum tunnels, at right angles to each other. Laser beams travel up and down the tunnels several times, reflected by mirrors at each end, multiplying the distance travelled, and the beams are finally brought together where the the arms meet, where they form an interference pattern.
When the passage of a gravitational wave causes one of the arms to lengthen by a tiny amount, the laser beam in that arm takes fractionally longer to travel the distance than the beam in the other arm, and the change shows up in the interference pattern.
What I don't know is whether time is affected by the gravity wave. Since we're often told that the universe is actually a spacetime continuum, I imagine it is. So I imagine there's a way of accounting for that. Or maybe in the mathematics of the Standard Model there's no need to distinguish between time and space. Professor?
Guardian contributor you're pretty much right, except that the waves propagate like this:
https://upload.wikimedia.org/wikipedia/commons/b/b8/GravitationalWave_PlusPolarization.gif
so they stretch one arm and shrink the other, and then back again. And the speed of light remains constant, so the time taken for the light to travel along the arms changes.
Just in case you need some visualisation...
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Jethrain asks:
So from the simulation that was shown during the press conference yesterday, there was a visible (quite large, or at least large-looking) spike in potential at about the point where the black holes merged. What order of magnitude of energy are we talking about here? Are these gravitational waves the only direct signal we have of such an event, or would it be possible for other particles to be produced and flung out by this? And (slightly different topic) how strongly attenuated would the waves be travelling through matter and/or free space?
The "spike" you saw in the plot of time vs strain is the "ringdown" as the blackholes merge. The gravitational wave signal is created by converting about three times the mass of the sun into energy in a fraction of a second. If it is a pure black hole - black hole merger, we don't necessarily expect a signature from this event other than in gravitational waves. However, because we don't know for sure about this type of system (its the first time one has been observed!) astronomers will be looking at various wavelengths on the electromagnetic spectrum (visible light, X rays, gamma rays etc) for a counterpart signal, so you'll have to wait and see. The waves are not attenuated by passing through matter but they are damped by the expansion of the universe itself as the waves travel from the source more than a billion light years to reach LIGO.
“New Scientist is saying they found two more in December,” points out one commenter.
Guardian contributor yes, rumours are they have more already. It will be fascinating to see how many, how often, what kind of events...
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thedavemc asks:
If you had an infinite budget, what would the next gravitational observatory look like?
Guardian contributor
hminhas asks:
Are these gravitational waves always present? As in, they keep reverberating throughout the space or are they a time bound event?
Guardian contributor The ones they saw were transient, from a short violent event. However, there will be weaker waves from other sources reverberating all the time.
Alexander Zaitlik asks:
If GWs have the character of waves then what does the spectral analysis look like?
Guardian contributor The analysis of the this signal is consistent with two black holes merging - different sources have different characteristics. Apparently they have a "library" of expected spectral characteristics from different sources.
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Professor Hiranya Peiris has joined the Q&A.
Chaminda Wijayasundara asks:
Does the discovery of gravitational waves prove the inflationary theory as well ?
While inflation predicts primordial gravitational waves, the LIGO discovery does not have any relation to the inflationary signal. LIGO has detected the gravitational waves coming from the merger of a pair of black holes - it's a brief "chirp" signal emitted as the black holes spiral in towards each other and merge. Inflation predicts a background "hum" rather than a short sharp "chirp". Cosmic microwave background measurements might detect the inflationary gravitational wave background in the future; future space-based successors to LIGO have sensitivity to detect this too. However, for now, we don't know if the inflationary prediction will be verified.
On location, I endorse this answer from Christian Beaumont:
The current LIGO detectors are planar setups, and a single LIGO detector can’t tell anything about the direction from where the event occurred. It was only because there are two detectors online that they were able to get a general idea of the location of the event. As more detectors come on line, it will be possible to pin point the location of the events with ever increasing accuracy.
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Here are some answers to get started... (I notice other commenters have already given some good answers to some of the questions - thanks!)
What are the chances that this finding will be retracted? A lot of effort went into verifying these results before they were announced, does this mean there's zero real possibility that we will find out that they were incorrect as some point in the future?
Thanks Jon!
- Chances of the result being retracted? Very small. The data are beautifully clear an unambiguous, in both detectors. They’ve had the data since Septamber and will have been checking it very carefully. Plus is has been reviewed by the journal. So while it is always possible some details of the interpretation may change, I don’t see anyway the fact that gravitational waves have been directly observed could change.
Will this discovery help scientists in actually observing dark matter? I've been doing some reading on it and the reports say that it has been unobservable. Could this lead to the possibility of actually observing dark matter?
- With improved sensitivity and more detectors coming online, we should get a lot of information about the number and distribution of the different sources of gravitational waves. It seems quite possible that this will affect our cosmological models. However, I don’t think it will have a very direct impact on understanding dark matter.
Hi Jon. Does this now make the eLISA project more likely to happen and what are the chances that NASA will now rejoin the project following today's announcement? Surely it strengthens the case for both?
- I doubt the eLISA space mission to measure gravitational waves would have gone ahead unless we’d detected some of them already, as LIGO just has. So this certainly increases the chances in that sense. The results from LISA pathfinder will also be an important factor. And it would be great if the US rejoined the mission with ESA, though I don’t know the chances of that.
Does this discovery mean anything in the search for a theory that combines quantum mechanics and relativity (e.g. a quantum theory of gravity)?
- These are “classical” gravitational waves, with long wavelengths, so they don’t tell us a great deal about quantum gravity. Though they do confirm that the mass of the graviton (which would be the quantised version of these waves) is practically zero, since otherwise waves of different frequency would travel at different speeds, for example.
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In just over ten minutes’ time, at 14:00 GMT, our live Q&A on the discovery of gravitational waves will get underway. Joining the previously mentioned Professors Jon Butterworth and Hiranya Peiris is Dr Jonathan Braden.
There’s still time to post your question below the line.
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On Thursday, physicists announced the discovery of gravitational waves, ripples in spacetime first anticipated by Albert Einstein a century ago. But what does it all really mean? Here’s your chance to find out: post your questions for Professors Jon Butterworth and Hiranya Peiris in the comments section below, and they will try to answer as many as they can between 14:00 and 15:00 GMT on Friday 12 February
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You would not be here to ask this question ;)