Download The historic detection of gravitational waves, announced in

Feature
9 May 2016
Gravitational waves: Your cheat sheet on
the find of the decade
The historic detection of gravitational waves, announced in February, left New Scientist with
an unprecedented postbag of questions from readers. We didn't have space to answer them all,
but we put a selection of them to gravitational wave expert Martin Hendry of the University
of Glasgow, UK, who is a member of the LIGO collaboration that made the discovery
A tiny blip was huge news
Caltech/MIT/LIGO lab
I feel slightly let down by the tiny “chirp” that signalled the gravitational wave – it seemed
rather insignificant for an event as momentous as the merger of two black holes. Why was
it so brief?
The gravitational waveform produced by the black holes as they spiralled towards each other
and finally merged would have lasted for many millions, perhaps even billions of years. Over
the past decade or so our theoretical understanding of general relativity has improved a great
deal, allowing us to calculate the precise pattern of gravitational waves it predicts for such a
merger.
This waveform is very distinctive, and features a pattern in which both frequency and
amplitude increase as the black holes approach each other, orbiting ever faster. Remarkably,
however, it was only within the final second that the signal reached high enough frequencies
and high enough amplitudes for LIGO to detect it, above the general background noise from
other non-cosmic sources.
I mean noise in a quite literal sense: the gravitational wave frequencies to which LIGO is
sensitive are frequencies that our ears can hear. So we can take the signal we detected and
turn it into sound waves – as indeed our collaboration spokesperson Gaby Gonzalez showed
when she broke the news of the discovery. This sound file first translates the frequencies of
the gravitational wave signal to the identical audio frequency, and then shifts the audio
frequencies upwards by a few hundred hertz. The second representation is analogous to how,
for example, astronomers working with the Hubble Space Telescope might use false colour on
their images to better bring out contrast. It is somewhat easier to hear the chirp in the shifted
representation, but with good quality speakers or headphones it should be audible in the first,
un-shifted representation too.
..
Visually, you can see the chirp pattern on our website, and it even features on a dress design
you can buy online!
Catch up: Einstein’s last theory confirmed – A guide to gravitational waves
Surely the universe must contain many other sources of gravitational waves. If so, then
how much information can actually be gathered from the chirp, when it might just be the
end result of interference from multiple other as yet unknown sources?
We believe that the universe does indeed contain very many sources of gravitational waves. In
fact, we estimate that an event like the one we detected last September occurs somewhere in
the universe every 15 minutes. But most of these sources are very far away and the waves
from them spread out in all directions, meaning they are far too weak to be detected by the
time they reach Earth. That is why it took 100 years from Einstein’s predictions for us to have
the incredible instrument sensitivity necessary to make the first detections.
The other aspect is that space-time is incredibly stiff: that’s why you need a cataclysmic event
like the merger of two black holes to produce a distortion that we can measure. In turn it
means that gravitational waves interact only very, very weakly with matter. That is another
factor that makes them difficult to detect, but it also means that they will pass virtually
unimpeded through everything they encounter. This includes our LIGO interferometers: we
were able to detect the presence of GW150914, as we call last September’s signal, but we
extracted virtually none of its energy, and it just carried on its merry way almost completely
unaffected.
The upshot is that we do not generally expect gravitational waves to be attenuated or absorbed
by intervening matter, or to exhibit interference effects, or to be otherwise affected in ways
that would render them difficult to interpret. Although the signal we detected was incredibly
weak, it was a pristine representation of the waves emitted by the source, unaffected by its 1.3
billion year journey to us. This is one of the main reasons why gravitational wave astronomy
offers such exciting prospects: we will be able to use gravitational waves to probe regions of
the universe that would be completely inaccessible using light signals alone.
How does the gravity of large bodies like the sun, Earth or black holes warp space? If space
is a vacuum, what is there for gravity get hold of?
This is a very interesting question. There is a phrase, originally attributed to the physicist John
Wheeler, that is often used when describing general relativity: “space-time tells matter how to
move and matter tells space-time how to curve”. This phrase captures the essence of
Einstein’s big idea: that we should not think of gravity as a force between massive bodies, but
as a curving or bending of space-time itself.
However, Wheeler’s phrase does rather side-step an even deeper question: how is it that
matter “tells” space-time how to curve? As the question hints, to fully answer this we need to
understand something about the fundamental nature of space-time. We expect that this will
require linking general relativity and the weird rules of quantum physics as they apply to the
vacuum of empty space.
We don’t yet have such a complete description, but the quantum notion of “empty” space
involves a bubbling mass of “virtual” particles that pop in and out of existence.
Future observations of gravitational waves may lead to further insights about how a quantum
gravity theory would work. In truth, though, for most of the sources that LIGO will see, our
measurements should be perfectly consistent with the general relativity description. Our data
won’t have the detail or “resolution” necessary to allow us to investigate many aspects of
quantum gravity.
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That said, our results can already be used to constrain aspects of those quantum gravity
theories by putting limits on the mass of the graviton, the hypothetical quantum particle of
gravity.
Read more: How we found them – Go inside a giant gravitational wave detector
When people talk about distorting space-time, what does it distort into? If I distort a (more
or less) 2D piece of paper, it moves into a third dimension. So do gravitational waves
require more than three dimensions of space and one of time? Is this a dumb question?
This isn’t a dumb question at all. It relates closely to a broader question that cosmologists
have wrestled with over the past century, often phrased as “if space is expanding, what’s it
expanding into?”. The short answer is that it doesn’t have to be expanding into anything!
We often use a 2D metaphor for the expansion of space, thinking about how, for example,
dots on the surface of a balloon appear to move apart as the balloon inflates. Here, the 2D
surface of the balloon is the counterpart of our three dimensions of space. Of course from the
three dimensions that we inhabit we can see the balloon is expanding into that third
dimension. But the crucial point is that we can still tell something about the 2D surface while
being completely embedded within it.
To elaborate, by looking at the properties of those dots on the surface and how curved lines
and angles behave as the balloon expands, we can distinguish its surface from that of a flat
piece of paper, without needing to think about any higher dimensions.
This is what we call the intrinsic curvature of the balloon’s surface. In a similar way, the
changes in the curvature of space-time that produce gravitational waves can be characterised
in terms of the intrinsic curvature of space-time – without requiring us to think about a higher
dimensional “surface”.
Discover: What will gravitational waves tell us about the universe?
What’s in these waves that are coming across 1.3 billion light years of space at the speed of
light? Is it like electromagnetism – where we talk about waves, but really it’s particles
called photons? Does the discovery tell us that particles of gravity must exist – and that they
must be massless like photons?
The analogy with light and electromagnetism is very interesting. About 150 years ago, James
Clerk Maxwell devised a set of equations that predicted the existence of electromagnetic
waves propagating at the speed of light. That was what physicists today call a classical field
theory, and it works very well for longer-wavelength electromagnetic waves such as radio
waves. It was only through considering shorter wavelength, higher frequency light – such as
visible light, ultraviolet and X-rays – that a quantum description emerged in the early 20th
century, leading to the notion of photons.
General relativity, Einstein’s theory that predicts gravitational waves, is a classical field
theory like Maxwell’s. And just as we can regard radio emissions as waves and not as photons
because of their long wavelength, the gravitational waves that we detected were of
sufficiently long wavelength that we could indeed regard them as waves. In the future, we
hope to be able to detect gravitational radiation with shorter wavelengths – where the wavelike description starts to break down and we would need to consider it in terms of particles of
gravity, gravitons.
The idea that gravitational waves travel at the speed of light is hard-wired into general
relativity. Our observations of GW150914 did not allow us to put tight constraints on the
speed of the gravitational waves, but the time delay between the arrival of the signal at the
two LIGO detectors is consistent with them travelling at the speed of light.
If so, and if the waves are at some level made of particles, then those particles would have to
be massless. That’s all speculation with the data we have for the moment, but our results can
already be used to put an upper limit on the graviton’s mass, because a very massive graviton
would affect the shape of the waves predicted by general relativity for the merger of two
black holes. You can read more about this in one of our papers