The Japanese Space Agency, JAXA, has launched its heaviest space mission to date – a huge X-ray probe that will study galaxy clusters, active galactic nuclei and supernova remnants. Launched from the Tanegashima Space Center by a H-IIA rocket today at 17:45 local time, ASTRO-H – weighing 2700 kg and measuring 14 m long – will now be placed in a low-Earth orbit at an altitude of 575 km, where it will operate for three years.
Many objects in deep space – including black holes, neutron stars, and galaxy clusters – emit X-rays as well as visible light. As the Earth’s atmosphere blocks X-rays from reaching land-based telescopes, the best way to study X-rays from deep space is to use an orbiting telescope.
ASTRO-H is a multipurpose X-ray observatory that will aim to explore the structure and evolution of the universe, including the distribution of dark matter in galaxy clusters. “ASTRO-H will extend the frontier of high-resolution X-ray spectroscopy,” ASTRO-H project manager Tadayuki Takahashi from the University of Tokyo told physicsworld.com. “ASTRO-H will determine the velocity field of the gas in clusters of galaxies and allow sensitive and precise measurements of how clusters grow and evolve, as well as measure the chemical composition of the gas in active galaxies that should allow measurements of the strength of the winds in these sources.”
Unique instrument
ASTRO-H contains four instruments including a gamma-ray detector, a soft X-ray spectrometer (SXS) and imager, as well as a hard X-ray imager. The craft will cover a wide energy range from 0.3 keV (soft X-rays) to 600 keV (gamma-rays), providing the highest energy resolution ever between 3–10 keV.
Indeed, the SXS will have an energy resolution of 7 eV – much better than previous missions such as NASA’s Chandra and Swift missions, as well as the European Space Agency’s XMM-Newton probe. “The SXS will be the first instrument capable of high spectral resolution for extended sources such as supernova remnants, normal galaxies and clusters of galaxies,” says Takahashi, “while the Hard X-ray Imager is only the second instrument that can provide very sensitive images and spectra in the 5–80 keV band.”
Astronomer David Burrows from Penn State University also thinks the SXS is a special instrument. “The most important capability is the extremely high energy resolution of the SXS,” he adds. “This will allow much better characterization of hot gas in clusters and supernova remnants – including measurements of both temperature and abundance – than has been possible until now.”
The ASTRO-H launch comes shortly after India sent its own X-ray mission into space in September. Dubbed Astrosat, it surveys the skies in the hard X-ray and ultraviolet bands, as well as monitoring the sky for new transients and studying X-ray binaries, active galactic nuclei and clusters of galaxies. “ASTRO-H is a very exciting X-ray astronomy mission that the community of X-ray astronomers had been looking forward to for quite some time,” says K P Singh from the Tata Institute of Fundamental Research, who was a lead scientist for one of Astrosat’s instruments.
“ASTRO-H is a very exciting project for high-energy astrophysics and in some ways it will start a new era in X-ray astronomy,” says Kirpal Nandra, a director at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who is also lead scientist on the European Space Agency’s planned Athena X-ray observatory. “ASTRO-H is an extremely ambitious and complex mission. It continues a great tradition in Japanese X-ray astronomy.”
ASTRO-H has been built by an international collaboration of more than 70 contributing institutions in Canada, Europe, Japan and the US. Takahashi told physicsworld.com that he is “very proud” that such a large international team has come together to build the craft. “The power of ASTRO-H will certainly discover unexpected phenomena,” he adds.
Ground-penetrating radar can improve the automated harvesting of asparagus spears. That is the conclusion of a new study by researchers from Germany, which demonstrates how radar can detect the best height at which to cut asparagus plants to maximize crop yield. Asparagus harvesting is a labour-intensive process and the researchers believe that the new technique could maximize production while minimizing damage to the part of the plant that is left in the ground to produce future crops.
Asparagus plants consist of two parts – the root network and the edible spears. The root network comprises both downward-growing roots and the sideways-growing rhizome from which multiple spears grow upwards. White asparagus is popular in much of Europe and is usually grown buried in ridges of soil. This forces the spears to grow underground, where the absence of light stops them from turning green.
During harvesting, spears are cut from the root network. Manually harvesting is labour intensive because it involves cutting the spears individually. Automatic harvesting sees the whole top layer of the soil sliced off – from which severed spears are sieved out. The challenge with automation lies in selecting the appropriate depth of soil to remove. If too little is removed, then valuable crop remains wasted in the ground. If too much is removed, then the cutting machinery may damage the rhizomes or even kill the plants entirely. Individual plants normally provide a crop for 10 years, so not damaging the plants is desirable. While manual inspection of selected plants can help guide automatic harvesting, the variation between plants makes selecting the optimum height challenging.
Success with tree roots
“A few centimetres of additional cutting depth, and hence average [spear] length, can make a few thousand euros difference on a typical asparagus field,” explains Jörg Schöbel of the Technische Universität Braunschweig – an institution in a region famous for its asparagus production. Schöbel and his colleagues’ previous research had focused on more established applications for ground-penetrating radar, such as the detection of buried pipes and cables for civil-engineering purposes. The technique uses reflected radio pulses to image beneath the ground and had previously been applied successfully to monitor tree-root growth. When presented with the asparagus problem, Schöbel’s team immediately recognized the technique as a potential solution.
The researchers created a detection system consisting of a radar transmitter and receiver mounted on an adjustable framework, attached to a rail-guided trolley (see image). The system scans the top surface of the asparagus bed using radio-wave pulses in the 0.2–2 GHz band as well as continuous-wave radar. To test their set-up, the researchers planted a 9 m long test bed of asparagus, on which was heaped a ridge of soil about 0.5 m in height. Plants in the first part of the bed were spaced at a distance of 1 m apart, while the rest were planted 0.3 m apart, the latter being typical of a large-scale asparagus plantation.
As long as the asparagus are planted close together, where radar signals overlap, the top of the plant’s root networks can be detected as a horizontal reflection pattern in the radar data. To calculate depth from the low-contrast reflections recorded, the researchers used a digital processing algorithm called “phase congruency” – which provides edge detection based on the frequency, rather than time, domain. Knowing the rough depth of the asparagus, reflections from the plants can be distinguished from those from the soil surface above and the solid ground beneath.
Margin of safety
From this, a small safety margin can be added to produce a single cutting depth for the entire field or, to better maximize crop yield, the cutting depth could be dynamically adjusted as the harvester moves across each ridge. Alternatively, the radar apparatus could be attached to the harvester itself, allowing scanning and cutting to be undertaken in tandem.
With their initial study complete, the researchers are now looking to further develop and simplify their detection technique, with the long-term aim of working towards a commercially viable application. One particular challenge to be overcome is how to refine the signal-processing technique to handle different soil conditions. While the researchers tested their system with the dry, sandy soil found in the Braunschweig region, detection becomes more difficult with heavier and more humid soils, which more strongly absorb high-frequency radar signals.
Researchers from the LIGO collaboration who last week announced they had detected the first ever gravitational waves – spewed out from two merging black holes – have also picked up a second possible gravitational-wave event. Although the signal from “LVT151012″ is much weaker than the confirmed “GW150914” event, the LIGO team says it most likely has an astrophysical source and arose from two coalescing black holes. The researchers have in addition spotted “several even less significant events in the data, most likely just due to some disturbance at the detectors”, which they are now analysing to see if any are from gravitational waves. Their conclusions, expected over the course of this year, will see the new era of gravitational-wave astronomy finally start.
While LVT151012 is the next most interesting candidate event, the data for it are currently not statistically significant enough (about 2σ) for it to be declared a “detection” and the team will need to analyse it further to say if it is a true event or noise. Despite this, LIGO scientist Amber Stuver, who is based at the LIGO Livingston Observatory in Louisiana, US, told physicsworld.com that the signal from the candidate event, which was detected last October, was similar to that from GW150914 and was “clean and clear”. These events suggest that the rate of binary-black-hole mergers is higher than expected, between six and 400 per cubic gigaparsec per year.
Indeed, Stuver points out that the stellar-mass black holes that merged in the GW150914 event are themselves surprising. Astronomers previously thought that such stellar-mass binaries would either not form at all or, if they did, they would be too far apart to merge within the age of the universe. LIGO’s detection has showed that this is untrue, prompting what Stuver hopes will be a revolution in astronomy.
First black-hole signals
James Hough from the University of Glasgow in the UK agrees with Stuver, pointing out that LIGO’s discovery is also the only direct evidence we have for the existence of any black holes. Astronomers had previously obtained only indirect evidence in the form of X-rays from matter falling into other black holes and the distortion of the orbits of stars at galactic centres that host supermassive black holes.
Hough says the team can be sure that the waves in the GW150914 event came from two merging stellar-mass black holes because the waves are directly related to the size of the system. The radius of the objects is such that they must be black holes for the mass that they have, he explains. “I think that there is no doubt about that at all.”
He adds that the signal from GW150914 was so perfect and clear that it almost needed no sophisticated data analysis to tease it out of LIGO’s data. The signal lasted in the detector for nearly 0.2 s, sweeping from about 30 Hz to 150 Hz, almost exactly as was expected for such a wave.
Matching the templates
According to Stuver, LIGO has a large “template bank” containing detailed simulations and predictions for every possible type of merger – be it binary black holes or neutron stars – with many different permutations and combinations of possible masses. Each template produces a unique gravitational-wave signal and the researchers’ computer system actively looks for a high correlation between it and an incoming signal. If the two are close, a detection is flagged. For GW150914, this correlation was extremely clear and immediately noticeable.
LIGO uses another method of detecting “burst” gravitational waves from an unknown source for which we have no models (such as supernovae) that involves looking for a “statistically significant anomaly” in the data. Both computational methods easily picked up the GW150914 signal, boosting the chances of it being a detection right from the start. Still, the team spent the next four months confirming its find. “We tried everything to show that it was not an actual signal, but it passed every hurdle,” she says.
As the detection was so clear, the researchers were able to tease out information such as the final black hole’s spin. LIGO spokesperson Gabriela González, from Louisiana State University in the US, explains that this spin distorts the gravitational waveform, leaving a stamp on it that LIGO was able to detect. “We produce all kinds of waveforms with all kinds of spins, all kinds of masses,” she says. These are then matched to the data to see which fit best.
González adds that the spin information from GW150914 is also interesting as the spin is not large. The “spin parameter” for the final black hole was found to be just 0.67, which is quite low as high-mass black holes are expected to have a spin near the maximum value of 1. González says that finding out why from LIGO’s data will be fertile ground for theorists and astrophysicists to dig into. “We’ll just keep detecting waveforms and gathering data,” she adds.
Gravitational waves may also contain key information about the nature of dark matter. Although it is too soon to say for sure if the current detection will reveal any information about dark matter, Hough thinks there is a good possibility that “we may see something in the future, about the way the signals are distorted when they reach us”.
A sample of asphalt to which steel-wool fibres have been added, making the material magnetic.
By Margaret Harris at the AAAS Meeting in Washington, DC
Although Thursday’s LIGO result was extremely exciting, I’m afraid I can only spend so much time pondering ripples in the fabric of space–time before I start yearning for something a little more…concrete. Like, well, concrete. And asphalt. And cement. These decidedly ordinary materials were the stars of two of the most fascinating talks I’ve seen at the AAAS meeting here in Washington DC over the past two days.
First up was Erik Schlangen, a civil engineer at the Delft University of Technology in the Netherlands who develops “self-healing” materials. One of his projects (which you can watch him demonstrate in a TED talk) involves mixing porous asphalt with fibres of steel wool. The resulting conglomerate is magnetic (that’s a magnet sticking to it in the photo), which means that microscopic cracks in it can be repaired using induction heating. The heat melts the bitumen in the asphalt, allowing it to re-fuse, but the surrounding aggregate remains relatively cool – meaning that cars can be driven over asphalt road surfaces almost as soon as the repair is complete.
For years, physicists have debated how to quantify the entanglement of identical particles. Now, two theorists in Italy have shown that this can be done using the formalism usually applied to non-identical particles, so long as the particles are considered together as an indivisible whole. They say their work could improve quantum-information processing, where the entanglement of identical particles is essential.
Entanglement is a purely quantum-mechanical process that allows two or more particles to have a much closer relationship than is allowed by classical physics, such that measuring the quantum state of one of them will instantaneously fix that of the other, no matter how far apart they are.
To the max
For example, if one particle is revealed to have its intrinsic angular momentum (spin) pointing up, then the other will automatically have its spin pointing down, and vice versa. The two particles are said to be "maximally entangled" when, over the course of repeated measurements, the states spin-up/spin-down and spin-down/spin-up along any axis are observed with the same frequency. If either combination appears more often than the other, then the entanglement is less than one.
To work out the amount of entanglement in any particular quantum system, physicists calculate the quantum-mechanical analogue of classical entropy known as Von Neumann entropy. To date, however, this approach has been limited to what are known as non-identical or distinguishable particles. Any two particles are non-identical if they are of two different types, such as an electron and a proton, or they are of the same type but are far enough apart in space that their quantum wavefunctions do not overlap.
In contrast, physicists have been unable to agree on a way of quantifying entanglement between identical particles. In this case, the particles are close enough that their wavefunctions overlap, and it is impossible to say whether the outcomes of two successive measurements relate to a specific particle or not. In other words, the inherent quantum correlations between the two particles muddy the waters when it comes to establishing the amount of entanglement between them. According to Rosario Lo Franco of the University of Palermo, attempts to quantify entanglement between identical particles "remain technically awkward and not intuitive" and, he says, do not always generate the same result.
Two become one
In the latest work, Lo Franco and Palermo-colleague Giuseppe Compagno have shown that it is possible to use the Von Neumann formalism, even in the case of identical particles. To do so they avoid, as Compagno puts it, "artificially assigning unphysical labels", such as "1" and "2" or "A" and "B", to two identical particles. Instead, they consider the two particles as a single entity described by a wavefunction expressed in terms of physical quantities of a single particle.
By doing this, the researchers were able to quantify the effect of particle type and particle separation on entanglement. They found that two particles with opposite spin and partially overlapping wavefunctions are more entangled when they are closer together, and they also found that the amount of entanglement depends on whether the particles are bosons (having integer spin) or fermions (which have half-integer spin). But they found that two particles with opposite spin will be fully entangled when they are located at the same point in space (within the limits of Heisenberg's uncertainty principle), regardless of particle type.
Efficient entanglement
These characteristics, say the researchers, allow the creation of what they call "entangling gates", in which particles with opposite spin become more entangled as they are brought closer together and become fully entangled when they occupy the same site. Indeed, the researchers point out that such a device was demonstrated last year by physicists at the University of Colorado in the US, who showed that two rubidium atoms placed in opposite spin states became fully entangled when brought together using optical tweezers.
Lo Franco and Compagno also found that identical particles will always be at least as entangled as non-identical ones placed in the same quantum state. "This suggests that identical particles may be more efficient than distinguishable ones for entanglement-based quantum-information tasks," says Lo Franco.
Nathan Killoran of the University of Ulm in Germany believes that the new research helps to support the idea that entanglement between identical particles is not merely a mathematical artefact, as some physicists have argued. He also thinks it could help scientists to "tap the large stores of entanglement" contained within identical particles for use in applications such as state teleportation, quantum metrology and quantum cryptography. "Entanglement can be thought of as a 'fuel' for many quantum-information technologies," he says.
Alice Prochaska, the principal of Somerville College, Oxford, told me yesterday that she is “absolutely thrilled” that Mary Somerville (1780–1872) will appear on a new £10 Scottish banknote. Prochaska believes the decision will help to give the Scottish polymath, whose work led to the discovery of Neptune, the wide recognition she has not yet received. Somerville will be the first woman other than a royal to appear on a Scottish banknote.
The decision had been announced earlier this week by the Royal Bank of Scotland (RBS), following a somewhat bungled public vote. On 1 February, RBS launched a week-long Facebook poll to determine whether Somerville, the engineer Thomas Telford or the physicist James Clerk Maxwell should adorn the new note, which will be issued in the second half of 2017. Having led comfortably throughout, Somerville was overtaken at the eleventh hour by Telford, following a suspicious flurry of votes mainly from outside of the UK. This triggered a three-day stewards’ inquiry before the bank declared Somerville the winner on Wednesday.
LIGO leaders at the press conference (from left): executive director David Reitze, spokesperson Gabriela González, Rai Weiss and Kip Thorne.
By Margaret Harris at the AAAS meeting in Washington DC
Not with a bang, but a chirp.
That's how the 2016 meeting of the American Association for the Advancement of Science (AAAS) kicked off on Thursday, thanks to the spectacular news that the Laser Interferometer Gravitational-wave Observatory (LIGO) has, for the first time ever, directly observed the ripples in space–time known as gravitational waves. As our news story explains, LIGO’s twin interferometers picked up the waveform produced as two black holes spiralled into each other, emitting gravitational waves at frequencies and amplitudes that rose sharply with time, like the chirp of a cricket.
The LIGO researchers announced their discovery at a packed press conference in downtown Washington, DC. The excitement in the room was palpable, even though, as it turned out, most of the journalists present already knew what they were about to hear. This actually isn’t unusual. It’s common practice for scientific journals to send new research papers to journalists a few days ahead of publication; the idea behind this so-called embargo system is that it gives journalists time to report accurately on complex science stories.
What was unusual was that this time, there was no embargoed paper. Instead, there was a vigorous rumour mill casting out information in a messy, somewhat underhand and highly anisotropic way. This is rather interesting, and I wish that LIGO’s Gabriela González hadn’t dismissed the journalist who asked about it with an incredulous “The facts are so beautiful – why do you talk about rumours?”
The first ever direct detection of gravitational waves has been made by researchers working on the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) in the US. The breakthrough – announced today at a news conference in Washington, DC – ends a decades-long hunt for these ripples in space–time. This monumental observation marks the beginning of the era of gravitational-wave astronomy and provides evidence for one of the last unverified predictions of Einstein's general theory of relativity.
The waves were produced from the collision of two black holes of 36 and 29 solar masses, respectively, which merged to form a spinning, 62-solar-mass black hole, some 1.3 billion light-years (410 mpc) away in an event dubbed GW150914. The detection was made on 14 September last year and was measured while the newly upgraded aLIGO detectors – one in Hanford, Washington, and the other in Livingston, Louisiana – were being calibrated before the first observational run began four days later.
The gravitational-wave signal lasted in both of LIGO's interferometers for 0.2 seconds and has been measured to a statistical certainty above 5.1σ. In fact, the signal from the event was so strong that it could be visually "seen" in the data by eye. It was measured in both of LIGO's interferometers, arriving within seven milliseconds of each other. The observation is also the first time a stellar-mass binary black-hole system has been detected. The data also showed that gravitational waves travel at light speed and that gravity has no mass, as predicted by general relativity.
“The effect we are trying to measure is so tiny that it takes something like LIGO to measure it," says David Reitze, LIGO laboratory executive director. “It’s mindboggling." He goes on to say, “We have been deaf, but now we can hear them. We now expect to hear things we never expected as we open a new window of astronomy. This was a scientific Moon shot, and we did it, we landed on the Moon."
Ripples in the cosmos
Just as accelerating a charged particle produces electromagnetic radiation, so accelerating mass produces gravitational radiation – this energy is lost from the system in the form of "gravitational waves". But unlike electromagnetic waves that travel through space–time, gravitational waves actually ripple the fabric of space–time. Such waves travel away from their source in all directions at the speed of light, compressing and expanding intervening space–time as they flow.
Any accelerating mass will produce gravitational waves so long as it is not spherically or cylindrically symmetric, which means that a perfectly spherical spinning star will not create the ripples. Since Einstein published his general theory of relativity 100 years ago, scientists have predicted that binary-star or black-hole systems would be prolific sources of gravitational waves in our universe, but such waves had never been directly detected, until aLIGO's measurement last September.
Ringing chirp: the waveform of event GW150914 The vital waveform of gravitational-wave event GW150914 showing the inspiral, chirp and ringdown features. (Courtesy: LIGO/Phys. Rev. Lett.116 061102)
If two black holes are stably orbiting each other, they produce a continuous stream of gravitational waves at twice the orbital frequency, carrying away the system's rotational energy and angular momentum. Such ripples are thought to have wavelengths that are tens of light-years and are relatively weak. But if the initial separation between the two black holes is not too large then – at some point – the orbit will get smaller as the system loses rotational energy and the two holes will eventually "inspiral".
Chirp and ring
The closer a binary pair is initially, the more radiation is emitted as the two black holes plunge into one another, which accelerates the inspiral. This process produces a characteristic "chirp" waveform in which both the amplitude and frequency of the gravitational waves increases – sometimes for less than a second – until it peaks at the merger. Given off during the last few seconds of the merger, these gravitational waves are characteristic of the mass and spin of the final black hole.
The single black hole created by such a cataclysmic merger is initially highly distorted. However, the nascent hole loses its deformity almost instantly by ringing like a bell and producing further gravitational radiation. The system quickly loses energy and the strength of the waves decays exponentially to form a "ringdown" signal. For event GW150914, aLIGO detected the chirp and the ringdown note at the end.
As the final black hole was 62 solar masses, this means that 3 solar masses' worth of gravitational radiation was emitted during the event. The signal also revealed that the new-born black hole is a rotating Kerr black hole (with a spin parameter of 0.67). Cosmologists have modelled such a gravitational-wave signal as audible sounds, based on the frequencies of the waves as they would arrive at LIGO's detectors. (You can listen to a chirp and ringdown here.)
Across the universe: LIGO’s extended reach with the recent update to aLIGO Schematic showing the much greater volume of the universe to which the Advanced LIGO detectors will be sensitive. (Courtesy: Caltech/ MIT/ LIGO Lab)
The length of time that a signal remains in LIGO's interferometers – and hence the quality of a potential detection of gravitational waves – depends inversely on the frequency that LIGO is set up to measure and the masses of the binary objects involved. It is therefore easier to detect gravitational waves at lower frequencies and from lighter objects. Before its upgrade, LIGO was able to detect gravitational waves from 40 to 10,000 Hz, but since aLIGO came online, the interferometers have been able to detect waves down to a frequency of just 10 Hz, thereby greatly extending LIGO's reach.
B S Sathyaprakash – a physicist at Cardiff University in the UK and a member of the LIGO collaboration – says the facility is currently functioning at 30 Hz, which was still sufficient to pick up the signal at 410 mpc. Although he admits that a heavier object will last for a shorter time, the signal itself is really strong. "Big objects have a larger amplitude, so a [gravitational wave] signal from a binary black-hole system can be detected from a much greater distance than a similar signal from a neutron-star system," he explains.
Long arm of LIGO
LIGO's successful detection of gravitational waves is thanks to its simple but ingenious design. The two observatories are essentially Fabry–Pérot interferometers consisting of two 4 km-long arms at right angles to each other, with "test masses" in the form of pure silicon primary mirrors – each weighing 40 kg and suspended as a pendulum – at both ends of the arms. Both interferometer arms are housed in an ultrahigh vacuum.
During a run, laser light with a wavelength of 1064 nm and a power of 200 W is sent to a beamsplitter, which transmits one half of the light into one of the arms and reflects the rest down the other arm. As each arm itself is a Fabry–Pérot cavity, the light is allowed to bounce back and forth some 400 times in each arm before returning to the beamsplitter. This effectively increases the arm length to nearly 1600 km, boosting aLIGO's sensitivity.
After the bounces, light from each arm returns to the beamsplitter, where the two beams combine. Some of this light is again transmitted through the beamsplitter and is detected at the photodetector (see diagram below).
Follow the beam A schematic showing aLIGO's interferometer. (Courtesy: IOP Publishing)
Riding the wave
If the light travels exactly the same distance down both arms, the two combining light waves interfere destructively, cancelling each other so that no light is observed at the photodetector. But if a gravitational wave slightly stretches one arm and compresses the other, the two beams would no longer completely subtract each other, producing an interference pattern at the detector. This pattern contains information about how much the two arms have lengthened or shortened, which in turn tells us about what produced the gravitational waves.
The aLIGO facility does not, however, measure the change in path-length because the gravitational wave compresses or expands the light's wavelength too. Instead, what the device reveals are tiny shifts in the period of the two light beams. If the crests or troughs of the wave arrive out of synch, they produce an interference pattern, meaning that the light acts as a clock and not a ruler.
Apart from using a Fabry–Pérot cavity to increase their sensitivity, the interferometers also have a "power-recycling mirror" placed just behind the beamsplitter. This mirror, which is partly reflective, slowly boosts the laser power from 200 W to 750 kW by reflecting nearly all of the laser light back to the beamsplitter and into the arms. Despite all of these upgrades and modifications, even a strong gravitational wave from colliding black holes displaces the mirrors by barely 10–19 m, making LIGO's successful detection even more triumphant.
Testing Einstein
With the gravitational-wave data from the GW150914 event, the LIGO researchers were also able to check a key prediction made by general relativity, which is that gravitational waves travel at the speed of light and that the currently unknown carriers of the force – often dubbed "gravitions" – are massless. If gravitons did have mass, some physicists have reasoned, it could explain the accelerating expansion of the universe without resorting to the concept of "dark energy". However, the aLIGO data show no evidence that the gravitational waves were anomalously dispersed, as they would if gravity had some small mass.
Chirps from the LIGO team
LIGO spokesperson Gabriela González from Louisiana State University says, "It's been a very long road, but this is just the beginning and more is to come. We can now begin listening to the universe.” She continues, “It’s a gift of nature."
“LIGO has opened a new window on the universe – a gravitational-wave window,” says LIGO co-founder Kip Thorne from the California Institute of Technology. “Each time a new window has opened up there have been big surprises – LIGO is just the beginning.” He continues, “Until now, we as scientists have only seen warped space–time, when it’s very calm. It’s as though we’d only seen the surface of the ocean on a very calm day when it’s quite glassy. We had never seen the ocean in a storm, with crashing waves. All of that changed on 14 September 2015. The colliding black holes that produced these gravitational waves created a violent storm in the fabric of space and time. A storm in which time speeded up and slowed down, speeded up again."
“My reaction was ‘wow’, I couldn’t believe it,” says Reitze. “We should be seeing more in the coming year," says Thorne. "We are going to have a huge richness in gravitational-wave signals."
