Folks, we’re going to call it a day now. But don’t forget that if you are awake at 01.00 AEDT (14.00 GMT), then CERN physicist Jon Butterworth will be here, trying to answer all your questions again!
What a day! Katie Mack did an amazing job explaining the ins and outs of what she described as a bigger discovery than the Higgs boson. She reassured us that gravitational waves weren’t going to hurt anyone and that – coincidence or not – there was definitely no conspiracy behind the discovery.
Katie explained that they’re a whole new window into the universe, allowing us to study the very fabric of reality, and that although it turned out Einstein was right – and they do travel at the speed of light – it wasn’t necessarily so.
Thanks again to everyone for all the fabulous questions and, of course, to Katie Mack for spending so much time lending us her expertise.
You can read all about the discovery below, and be sure to check back soon as we publish more about the exciting news.
Assuming conservation of energy, what is 'lost' when a body emits gravitational waves? Is a body losing mass to emit the gravitational wave energy? Or does this phenonimina only apply to bodies orbiting one another which lose orbital angular momentum as they emit gravitational waves and so spiral in to another in a form of orbital decay?
The short answer is “yes”! Mass was lost when the two black holes collapsed. One of the black holes was 36 times the mass of our sun and the other was 29 times its mass. When they collapsed, the resulting black hole was only 62 solar masses. That means that a mass three times the mass of the sun was lost!
That mass was turned into energy, which caused the ripples in spacetime – gravitational waves – that LIGO detected 1.3 billion years later.
A massive thanks to Dr Katie Mack from the University of Melbourne for answering all our questions. We’ve had to let her get back to her life (which I’m guessing means talking about this stuff with other people!)
In about 11 hours time (14:00GMT), CERN physicist Jon Butterworth will be doing something very similar!
But before we wrap up here, there are a couple of questions from the comments and Twitter that I’ll be able to help you out with. So stay with me a little longer!
There’s been lots of amazing questions. Some of them have been very complex!
One such complex question, which quite a few people have asked, is whether this discovery could help physicists get closer to squaring our two best theories in physics, which currently conflict with each other.
Quantum mechanics (which is roughly about very small things) and general relativity (roughly about very big things!) are both superbly accurate.
But they conflict with each other. They can’t both be right. So physicists have been trying to break them both for decades but they always come out of tests unscathed. You can read all about that here:
Katie says studying gravitational waves could potentially help unify the two theories.
“I think that’s definitely a possibility,” she says. The trick to solving the problem could be to observe how gravity works in very extreme environments – which is exactly what LIGO just did when it observed the gravity wave:
That’s difficult to study otherwise, and so it gives us a lot more information about how gravity works outside of our everyday experience of it.
We are already pretty sure that general relativity has to break down somewhere because it doesn’t play nicely with quantum mechanics... So far, everything we’ve seen is completely consistent with general relativity, but the more we learn about it from experiments like LIGO the better idea we’ll have of where the edges of the theory might be.
Katie says she’s not quite sure how a machine running on gravitational work would work. But then again, it’s not impossible, she says:
So far all we’ve been able to get a gravitational wave to do for us is move a mirror suspended in an extremely well seismically isolated vacuum by a tiny fraction of the diameter of a proton. And it took decades of planning and construction to be able to do that.
But, in a sense, that little mirror wiggle was a machine running on gravitational waves! It’s conceivable that we could someday find a way to extract the energy from gravitational waves to do something useful, but it’s hard to imagine a scenario in which some other energy source wouldn’t be more efficient.
Thanks so much for your thoughtful (and in some cases very informed!) questions in comments – we’ve done our best to put as many as we can to Katie, and apologies to those of you who didn’t get a response.
Obviously, a text Q&A isn’t the best way of digging deep into “spacetime”, but the enormity of this discovery means we’ll be publishing more on it in the days and weeks to come.
So we’ll be wrapping up soon. But I think there’s time for maybe two more questions before we let Katie get back to her life!
Let’s take a bit of a step back. There are lots of advanced questions in the comments, which we’ll try to get to. But someone just asked: “Remind me again what spacetime is?”
That seems pretty important since remember, gravity waves are ripples in it.
So here’s what Katie said:
Spacetime is what we call the combination of the three dimensions of space and one dimension of time. It’s not a “stuff” exactly, but it sometimes behaves that way, stretching and curving and bending in response to the way mass is distributed in the Universe.
Relativity says that we need to treat time like it’s a dimension, similar to space, because of how we travel through it. And it turns out that when you have a big mass somewhere, it not only warps space, but also changes how things around it experience time. Time moves more slowly when you’re close to a massive object, for example.
So mathematically it makes sense to link up space and time together as one thing – spacetime – that gets warped by mass and energy. It’s a bit hard to conceptualize, I’m afraid!
My colleague Elle Hunt is getting into this. She wants to know if gravitational waves might help the search for extraterrestrial life.
Katie’s response was roughly “maybe”:
I don’t see a way for gravitational waves to directly help us find extraterrestrial life, but it could tell us more about how black holes and stars form and die, which could in turn tell us about the kinds of conditions some alien planet might on average expect.
For example, it can tell us more about the rate of gamma-ray bursts in the Universe, and gamma-ray bursts are the sorts of things that an alien planet probably should stay away from if it wants to harbour life.
And then Katie had another thought:
If you want to get REALLY sci-fi about it, you can imagine some super-advanced civilisation manipulating massive objects in a way that could let them modulate gravitational waves to carry a message.
But in general it would be a lot easier to just send radio waves.
A few people answered saying “the speed of light”. And they’re right. But Katie explains that it wasn’t necessarily so:
According to Einstein’s theory, gravitational waves travel at exactly the speed of light. And, in fact, in this observation, one of the things that the scientists were testing was exactly that. In some alternatives to general relativity, gravity can travel at different speeds for different frequencies of gravitational waves. LIGO didn’t see that at all – as far as we can tell so far, it looks like Einstein was right about the speed of gravity too!
(By the way, make sure you hit the refresh button every now-and-again. Our auto-refresh function isn’t working properly.)
We’ve got a great question from a reader in the comments:
How do they know that this particular gravitational wave they detected is from a specific event that happened a billion years ago, billions of light years away?
Katie says there’s lots of information you can get from the “chirp” signal of the black hole colliding.
Oh– and if you haven’t heard the sound, here it is. Physicists took the wave they detected and interpreted it as a sound:
Katie says this sound lets physicists figure out where the wave came from:
We can compare the waveform to simulated ones we get from numerical relativity calculations (solving Einstein’s equations in a supercomputer) and figure out how the distance to the black hole changes the shape. If we know the distance, we also know how long ago it happened, because we know the speed of light, so we can figure out how long the signal has been travelling toward us.
My colleague here in the Guardian Australia office, Elle Hunt, is wondering how this will help her. (She’s mostly joking.)
Katie got a little upset with this: “IT GIVES YOU A NEW WINDOW ON THE UNIVERSE, ELLE!”
(We’re doing this over instant messaging, so the capitals are all hers.)
She collected herself to explain:
Okay so in all seriousness your day to day life will not change now that astronomers can WATCH BLACK HOLES COLLIDING and study the details of the VERY FABRIC OF REALITY.
You can safely ignore it if you want to, but omg really it’s going to be amazing. We are going to learn so much, we are going to have a MUCH deeper understanding of the Universe and how it changes over time, and we’re going to be able to directly observe incredible violent events millions of light years away. It’s mind boggling. So maybe that’s it.
Maybe your mind will be boggled, and you’ll experience the cosmic vertigo that I feel every time I think about what’s out there, and how immense it is. Maybe you’ll be humbled, or inspired, or just awed. Gravitational waves will not cure cancer, but studying them can help us better understand areas of physics that might someday connect to new treatments, just like studying antimatter has led to PET scans, and observational methods used in astronomy are often applied to improvements in medical imaging.
Gravitational waves are not going to be powering cars any time soon, but general relativity is already part of GPS. And the development of the LIGO experiment has been a massive effort in developing new ways of using lasers and new materials science, which will have all sorts of applications and are probably already improving lives. We don’t know yet all the ways this kind of work will affect our lives, but it’s a good bet we’ll find some amazing ones.
A lot of people are saying that the discovery of gravitational waves opens a new window on the Universe. Katie confirms that that is completely true and not even a bit of an exaggeration. It could have implications for theories of gravity as well as our understanding of black holes, stars and entire galaxies, she says:
It’s an entirely new way of looking at the cosmos, using an entirely new way of seeing (or “hearing” if you prefer that analogy, keeping in mind that it’s not exact). We’ll be able to study things that we would never be able to see in any kind of detail or perhaps even at all. We’ll be able to watch as black holes (and galaxies) merge and grow, throughout an incredibly huge volume of the Universe. We can test theories of gravity, find out how black holes spin, learn about the formation of stars and their deaths.
It’s an immense treasure trove of information and I can’t wait to see what it will tell us.
Twitter user John Stetson asks Katie to put the gravitational waves discovery in the context of other recent scientific breakthroughs. How might it change how we think about physics in future?
@guardian@mikeyslezak how many doors does this discovery open to the future of physics? Can you compare it with recent science big-deals?
In Katie’s informed opinion, it’s a bigger deal than the detection of the Higgs boson – “because it gives us more stuff to do and more opportunities to discover new things”:
It’s probably on par with the discovery of dark energy, though in that case, we really weren’t expecting dark energy at all. In this case, we knew pretty well that gravitational waves existed, but we couldn’t detect them and thus use them to study the cosmos.
We had to build a system that could measure a change in length of a thousandth the diameter of a proton just to be able to detect this signal at all. And all gravitational waves do is stretch and squeeze things a tiny, tiny bit – so I don’t see a way for them to harm us.
If something was massive enough and close enough to produce an even slightly non-tiny gravitational wave effect, then it would probably obliterate us from whatever it was doing to make that wave.
Guardian Australia’s Walkley-winning marsupial correspondent First Dog On The Moon is wondering whether it is a conspiracy?! (Of course he is.)
It’s not a conspiracy, but people on Twitter have asked similar questions about whether it was a coincidence that as soon as the LIGO machine (a “tube with a laser shooting through it”... broadly) was turned on, it detected a massive gravitational wave in action.
@guardian@MikeySlezak Was it purely coincidence they captured the merger of these black holes? Seems so improbable.
Here’s First Dog’s question in full (caps original...):
As I understand it, these two black holes crashed into each other a billion years ago, a gazillion miles away in space – and the resulting gravity waves have been hurtling toward us ever since.
Is the arrival of these waves like a one off event? Or is it like the ocean – a constant rippling sort of thing?
Because if it is a one-off event it is a pretty amazing coincidence that all these scientists decided to flick the switch on the LIGO machine JUST IN TIME FOR THESE GRAVITY WAVES FROM A BILLION YEARS AGO TO ARRIVE ON EARTH.
THAT IS EVEN MORE OF AN AMAZING COINCIDENCE THAN PRETTY MUCH ANYTHING EVER!
Or have these waves just been pootling through here the whole time – and if they have, how can we then get the distinct “sound” of the two black holes colliding?
Katie says: They crashed into each other about a billion years ago and yeah, really far away (depending on how you measure it, somewhere around a billion light years away). And yeah, the waves have been coming toward us ever since (“gravitational waves”, not “gravity waves”, as it happens).
It’s a one-off for that PARTICULAR binary system — the two black holes have merged now and they’re done. But there are LOTS of systems like this around the Universe, and probably many more also happening right now. And if there’s a binary system that hasn’t yet merged, that’s sending us a kind of constant rippling.
I guess it’s a coincidence for this particular system, but we’ve missed all the other black hole mergers that have happened since the beginning of time – and there are lots more happening that hopefully we’ll now see.
In short: the Universe has been buzzing away with all this gravitational radiation forever and we’ve been completely unable to perceive it until now.
A lot of people on Twitter are asking whether the discovery helps in any way with time travel?
Katie says – sadly – no:
All we’re doing here is listening to the disturbances in spacetime caused by massive objects (like black holes) far away. We still don’t have any way to manipulate spacetime ourselves (except, you know, by curving it very very slightly by piling up rocks or something). Even if we could manipulate spacetime easily, it’s not at all clear that time travel wouldn’t be completely ruled out by some as-yet unknown physical law.
Basically, you’d have to loop spacetime around itself in some complicated way and then travel through that loop and it’s so far beyond what we can even conceive of ever doing, even theorists think it may not be possible in principle.
It’s important to remember that spacetime is the sort of background in which everything happens, and it can be curved and stretched and squeezed and stuff, but that stretching and squeezing never reverses time. It can slow it down or speed it up, but that’s about it.
You guys will have to excuse me while I get my nerd on here for a second. This is something I’ve been wondering...
So as Katie just explained, they’re detecting tiny changes in the size of spacetime itself. But how can you measure that?
I mean, let’s say I try to measure the size of something with a ruler. If spacetime itself shrinks, both the thing I’m measuring and the ruler itself will shrink – so according to my ruler, it wouldn’t have changed size.
Here’s what Katie said:
Great question! To measure it we rely on another of Einstein’s great insights – that the speed of light is constant NO MATTER WHAT. That means that when you shoot a laser through a 4km tube (which is what the LIGO machine does) and that tube gets a little bit longer because spacetime is stretching, light just keeps on at its usual speed and so takes a little longer to get to the end.
So with LIGO, it’s set up so that the laser light going through each arm comes together at the middle and matches up perfectly (to cancel out), but when a gravitational wave comes through, one arm gets a little longer and the other a little shorter.
So the light takes a little longer to get down one arm than the other, and that messes up the perfect alignment at the middle, so you can see that the perfect cancellation has been lost. Assuming nothing else moved the mirrors, it’s a sign spacetime itself changed shape!
Katie: Spacetime! The fabric of the Universe. Einstein’s big insight was that space and time are both tied in together in this sort of single concept called spacetime, and it can curve and stretch and squeeze... What we feel as gravity is just how we experience the curvature of spacetime around massive objects. Gravitational waves are fluctuations of curvature in spacetime.
Michael: Hang on. “Fluctuations of curvature in spacetime”?
Katie: Yeah, so when you’re close to something massive, spacetime curves in toward that mass (usual visual analogy: a bowling ball making a dent in a rubber sheet – though you have to imagine that in a higher dimension!).
As a result, the paths of other masses and even of light get diverted a bit toward that mass, because the spacetime they’re travelling through is bent.
A gravitational wave is when spacetime ripples happen – the curvature of empty space gets disturbed because of the mass accelerating in some way nearby sending ripples through the space around it.
It’s kind of like if you put a bowling ball on a trampoline and then bounce it up and down – it sends little wobbles of disturbance out.
So, physicists have announced the discovery of gravitational waves.
You’ve heard they’re “ripples in the fabric of spacetime”. You’ve heard they were a prediction of Albert Einstein’s general theory of relativity. You probably even know the waves that were detected were created when two massive black holes circled each other and violently collapsed. You can read all about the discovery here, and we’ve already answered a few questions here.
But we know you’re still brimming with questions! So here we have Dr Katie Mack, an astrophysicist from the University of Melbourne. She’s on hand to answer Everything You Wanted To Know About Gravitational Waves But Were Too Afraid To Ask.
No question is too basic! Hit us up in the comments and I’ll put them to Katie, who will do her best to bring you up to speed.
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