Gravitational waves
Physicists make rapid progress in bounding the speed of gravity
“The speed of gravity, like the speed of light, is one of the fundamental constants in the Universe,” Neil Cornish, a physicist at Montana State University, told Phys.org. “Until the advent of gravitational wave astronomy, we had no way to directly measure the speed of gravity.”
Over the past few months, physicists have made very rapid progress in bounding the speed of gravity using gravitational wave observations.
Initially, the first LIGO detections of gravitational waves constrained the speed of gravity to within 50% of the speed of light.
In a paper published last week in Physical Review Letters, Cornish and his coauthors Diego Blas at CERN and Germano Nardini at the University of Bern have combined the first three gravitational wave events reported by the LIGO and Virgo collaborations, allowing them to improve the original bounds to within roughly 45% of the speed of light.
Just two days later (and after the physicists mentioned above wrote their paper), another paper was published in The Astrophysical Journal Letters by the LIGO and Virgo collaborations, whose authors are affiliated with nearly 200 institutions around the world. By using data from the gravitational waves emitted by a binary neutron star merger detected in August, they were able to constrain the difference between the speed of gravity and the speed of light to between -3 x 10-15 and 7 x 10-16 times the speed of light.
The reason for the huge leap in precision is that the neutron star event did not emit only gravitational waves, but also electromagnetic radiation in the form of gamma rays. The simultaneous emission of both gravitational waves and light from the same source allowed the scientists to set bounds on the speed of gravity that is many orders of magnitude more stringent that what could be set using gravitational wave signals alone.
Depending on whether an astrophysical source emits both gravitational waves and light or only the former, scientists take different approaches to constraining the speed of gravity. When a source emits both gravitational waves and light, scientists can measure the difference (if any) in the arrival times of the two different types of signals at a single detector. In the AJL paper, the scientists measured an arrival delay of just a few seconds between signals that traveled a distance of more than one hundred million light years. Such a small delay across this distance is considered virtually nothing.
On the other hand, when a source emits only gravitational waves, scientists must detect the same signal in multiple Earth-based detectors and measure the (very slight) difference in arrival times. The scientists of the PRL paper did this by comparing signals detected by two LIGO detectors located 1800 miles apart: one in Hanford, Washington, and the other in Livingston, Louisiana.
As the physicists explain, it’s possible to greatly improve the bounds on the speed of gravity using sources that emit only gravitational waves. For example, using four detectors located at different places on Earth, with five gravitational wave events for comparison, the constraints could improve to within 1% of the speed of light. But they could still not reach the degree of precision of experiments that have access to both gravity and light.
Overall, bounding the speed of light has many significant implications for fundamental physics and cosmology. One of the biggest implications is that the tight bounds provide a more precise test of general relativity and rule out proposed alternatives to general relativity.
“Many alternative theories of gravity, including some that have been invoked to explain the accelerated expansion of the Universe, predict that the speed of gravity is different from the speed of light,” Cornish said. “Several of those theories have now been ruled out, thereby restricting the ways in which Einstein’s theory can sensibly be modified, and making dark energy a more likely explanation for the accelerated expansion.”
Read more at: https://phys.org/news/2017-11-physicists-rapid-bounding-gravity.html#jCp
Are gravitational waves kicking this black hole out of its galaxy?
Astronomers have just spied a black hole with a mass 1 billion times the sun’s hurtling toward our galaxy. But scientists aren’t worried about it making contact: It’s some 8 billion light-years away from Earth and traveling at less than 1% the speed of light. Instead, they’re wondering how it got the boot from its parent galaxy, 3C186 (fuzzy mass in the Hubble telescope image, above). Most black holes lie quietly—if voraciously—at the center of their galaxies, slurping up the occasional passing star.
But every once in a while, two galaxies merge, and the black holes in their centers begin to swirl around each other in a pas des deux that eventually leads to a devastating merger. The wandering black hole (bright spot above), may be the result of one such merger. Based on the wavelengths of spectral lines emitted by the luminous gas surrounding the black hole, the object is traveling at a speed of about 7.5 million kilometers per hour—a rate that would carry it from Earth to the moon in about 3 minutes. If the most likely scenario is true, then a massive kick from the merger of two black holes some 1.2 billion years ago would have created a ripple of gravitational waves, the researchers suggest in a forthcoming issue of Astronomy & Astrophysics. And if the precollision black holes didn’t have the same mass and rotation rate as each other, the waves would have been stronger in some directions than others, giving the resulting object a jolt equivalent to the energy of 100 million supernovae exploding simultaneously, the researchers estimate. Other runaway black holes have been proposed, but none of them has yet been confirmed.
This Mind-Bending Theory Joins Black Holes, Gravitational Waves & Axions to Find New Physics
We haven’t seen physicists this excited for a while.
Scientists have proposed a new theory that combines some of the most mysterious phenomena in the Universe – black holes, gravitational waves, and axions – to solve one of the most confounding problems in modern physics. And it’s got experts in the field very excited.
The theory, which imagines a Universe filled with colossal ‘gravitational atoms’ that are capable of producing vast clouds of dark matter, predicts that it could be possible to detect entirely new kinds of particles using a giant gravitational wave detector called LIGO.
“This is probably the most promising paper I’ve seen so far on the new physics we might probe with gravitational waves,” MIT particle physicist Benjamin Safdi, who wasn’t involved in the research, told Nature.
“It’s an awesome idea,” adds particle astrophysicist Tracy Slatyer, also from MIT. “The [LIGO] data is going to be there, and it would be amazing if we saw something.”
Before we dive headfirst into the crazy physics of this new theory, let’s run through the major players.
Black holes are an easy one – vast, matter-annihilating objects that are so remarkably strange, when Albert Einstein’s equations first predicted their existence, he didn’t believe they could actually be real.
Black holes maintain such powerful gravitational fields, when two of them collide with each other, they produce gravitational waves.
Confirmed for the first time last year, but predicted by Einstein more than a century ago, gravitational waves are ripples in the fabric of space-time that emanate from the most violent and explosive events in the Universe.
And axions? Well, they’re a bit more tricky, because unlike black holes and gravitational waves, we’re not even sure if axions exist – and we’ve been searching for them for the past four decades.
Axions are one of the many candidates that have been proposed for dark matter – a mysterious, invisible substance whose gravity appears to hold our galaxies together, and is predicted to make up 85 percent of all matter in the Universe.
Axions are predicted to weigh around 1 quintillion (a billion billion) times less than an electron, and if we can prove their existence, these super-light particles could solve some major theoretical problems with the standard model of physics.
Okay, now that we have all the pieces in place, let’s get to this mind-bending new theory. (And yes, we’re calling it a theory, not a hypothesis, because it’s based on a mathematical framework. More on that here).
A team of physicists led by Asimina Arvanitaki and Masha Baryakhtar from the Perimeter Institute for Theoretical Physics in Canada have proposed that if axions exist and have the right mass, they could be produced in the form of vast clouds of particles by a spinning black hole.
This process would be enough to produce gravitational waves like the ones that were detected last year, and if so, we can use gravitational wave detectors to finally observe the signature of dark matter, and close the gaps in the standard model.
“The basic idea is that we’re trying to use black holes… the densest, most compact objects in the Universe, to search for new kinds of particles,” Baryakhtar told Ryan F. Mandelbaum at Gizmodo.
You can think of this scenario like this: a black hole is like the nucleus at the centre of a giant, hypothetical gravitational atom. Axions get stuck in orbit around this nucleus, whizzing around like electrons do in regular atoms.
“[E]lectrons interact via electromagnetism, so they let out electromagnetic waves, or light waves. Axions interact via gravity, so they let out gravitational waves,” Mandelbaum explains.
If an axion strays too close to the black hole’s event horizon, the spin of the black hole will ‘supercharge’ it, and due to a process called superradiance that has been shown to multiply photons (light particles) in many experiments in the past, this will cause the axions to multiply within a black hole.
These multiplying axions would interact with the black hole in the same way as the original axion near the event horizon, resulting in 1080 axions – “the same number of atoms in the entire Universe, around a single black hole”, says Mandelbaum.
“It’s so cool, and I haven’t read a paper that talked about [superradiance] in years,” Chanda Prescod-Weinstein, a University of Washington axion expert who wasn’t involved in the research, told him.
“[I]it was really fun to see superradiance and axions in one paper.”
These multiplying axions wouldn’t just pop into existence randomly – they’d group together in huge quantum waves like the electron clouds you see in an atom.
Within this cloud, any axions that collide with each other would produce gravitons – another hypothetical particle thought to mediate the force of gravitation.
Gravitons are to gravitational waves as photons are to light, and Baryakhtar and her team propose that they would set off continuous waves into the Universe at a frequency set by the axion’s mass.
With improved sensitivity, LIGO should be able to spot thousands of these axion signals in a single year, the researchers predict, finally giving them a way to observe the signature of dark matter – something that has eluded scientists for decades.
Of course, grand theories like these always come with some caveats – in order to work, the axions must have a very specific mass, and that mass doesn’t necessarily gel well with current predictions on dark matter.
But physicists are still excited by the idea, and with LIGO expected to increase greatly in sensitivity in the next couple of years, it might not be too long before we can test it out for real.
Source:http://www.sciencealert.com
Gravitational waves can’t solve our black hole problems, physicists warn.
When it comes to black holes, the past couple of years have seen a firestorm of disagreements about event horizons, firewalls, and the very nature of black hole life and death erupting between cosmologists. It’s admittedly been a quiet firestorm, with papers here and there carefully arguing their positions while being respectful of opposing views, but that still counts as a firestorm in science.
Some people thought that the gravitational waves observed earlier this year could put an end to the dispute, but a group of physicists now warns that we shouldn’t be so quick to jump to conclusions.
The arguments centre around two related disagreements over what we’re actually talking about when we call something a ‘black hole’.
The first is over what happens when something falls into a black hole. Traditionally, black holes are thought to be objects with a gravitational force so strong, light isn’t even going fast enough to escape their clutches. And if light – the fastest thing in the Universe – can’t escape, then neither can anything else.
A black hole is usually defined by its event horizon – the outline of the region in space where gravity is strong enough to hold light down. You wouldn’t even necessarily notice as you passed over the event horizon of a black hole, since it’s just a place in space like any other. You’d only notice when you tried to escape.
But a few years ago, a couple of papers suggested that this very simplified view leads to some problems that can’t be resolved with our current understanding of the laws of physics.
Instead, they said, there must be something special about the event horizon: just after something passed over the event horizon, it would be scrambled and burnt up beyond recognition by something called a firewall. These firewalls seemed to eliminate the theoretical problems, but they were a pretty weird solution – and not everyone was on board.
One of those not on-board was Stephen Hawking, who thought it was ridiculous. Hawking and those who agreed with him maintained that there was nothing special about the edge of a black hole.
But then Hawking went a step further, adding fuel to the second part of the debate. In his quest to disprove the firewall, he ended up with a black hole without an event horizon.
This turned the definition of a black hole upside-down: without the event horizon – the place beyond which nothing can ever escape the black hole – what even defines a black hole? Physicists weren’t exactly scrambling to the table with answers.
At the same time, there were some alternatives to black holes being developed that might still have extreme gravity but wouldn’t have a point of no return. These strange objects have been dubbed ‘black hole mimickers’.
And then there were those who kept black holes with event horizons but still refused to believe the firewall.
All of the different parties converged on the gravitational waves observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO). At first glance, the gravitational waves seemed like a clear victory for the black-holes-with-event-horizons camp. The pattern of the waves seemed to exactly match their predictions of what it should look like when two black holes with event horizons collide to form another black hole with an event horizon.
They were particularly excited about the ‘ringdown’ – when the final black hole sheds some energy and settles down after all the excitement of the collision. The ringdown, they said, precisely matched what they expected and didn’t match other contradictory predictions.
But the authors of a new paper say we can’t be quite so sure. They showed that the gravitational waves LIGO detected could have been made by any of those black hole mimickers – the objects that have gravity like a black hole without having an event horizon. So it seems like we’re back to square one.
But there is hope for distinguishing between the different hypotheses. They still disagree when it comes to the ringdown, but the disagreements are going to take better measurements to resolve. With better measurements of gravitational waves and of the ringdown, we should be able to answer these fundamental questions about the nature of black holes.
Watch the video. URL:https://youtu.be/XE5PNbsUERE
Physicists think gravitational waves might permanently alter spacetime.
Back in February this year, the world celebrated when physicists finally detected gravitational waves – the tiny ripples in spacetime first predicted by Albert Einstein a century ago.
We’ve since gone on to spot a second gravitational wave event – and now a team of physicists has suggested that these ripples might not just be short-lived occurrences. They think they might permanently alter the fabric of space.
Even more impressive – the researchers think they might have actually found a wayto detect these permanent shifts in spacetime, also known as gravitational-wave memory.
“For so many years, people were simply concentrating on making that first detection of gravitational waves,” lead researcher on the new project Paul Lasky, from Monash University in Australia, told Charles Q. Choi from PBS.
“Once that first detection happened, our minds have become focused on the vast potential of this new field.”
Let’s step back for a second though, and have a quick refresher. Gravitational waves are tiny fluctuations in spacetime that occur whenever an object with mass moves, just like ripples moving out after a pebble’s been dropped in a lake.
They were first predicted by Einstein’s theory of general relativity, but they’re so minuscule that we’d never been able to detect them.
Until this year, when we were able to measure gravitational waves that had originated from one of the most violent events in the Universe: two black holes merging (you can see them orbiting each other before merging in the gif above).
And when we say minuscule, we mean ridiculously tiny. The ripples that the Laser Interferometer Gravitational-Wave Observatory (LIGO) picked up in February this year were about a billionth of the diameter of an atom.
So how could these tiny shifts make permanent changes in spacetime? And what would that mean for the Universe?
The idea of gravitational-wave memory was first predicted by Russian scientists back in 1974, but seeing as no one had even confirmed the existence of gravitational waves back then, it went largely unnoticed.
But after the LIGO detection in February and again in June, Lasky and his team revisited the idea.
To explain gravitational-wave memory, Lasky uses the example of two black holes orbiting each other before they eventually merge, and two astronauts drifting side by side in orbit around this black hole binary system.
The astronauts are initially separated from one another by say, 10 metres. And as the black holes spiral towards each other, they’ll release gravitational waves that ripple spacetime and cause the distance between the two astronauts to fluctuate ever so slightly.
After the black holes collide and merge, the gravitational waves will stop, and the astronauts’ distance will once again be constant – but not the same as the original distance.
And that’s what gravitational-wave memory is – a permanent stretching or shrinking of spacetime as a result of gravitational waves.
This effect would hypothetically be detected as an additional flare of gravitational waves near the end of the initial event. Which sounds straightforward enough, but as with most theoretical physics, there’s a problem. If gravitational waves were hard to detect, gravitational-wave memory will be even harder, because its ripple in spacetime will be even smaller.
“In general, we expect the size of the memory effect to be between about one-tenth and one-hundredth of that of the gravitational waves,” Lasky told PBS. “For almost all events other than the most catastrophic collisions in spacetime, the effect cannot be measured.”
In fact, in general it’s been assumed that LIGO would never be able to detect these memory flashes, no matter how catastrophic the event they originated from.
But Lasky and his team have now come up with a way that it could work – and it all comes down to volume.
Basically, with LIGO now expected to detect an increasing amount of gravitational waves, the researchers suggest that, over time, they’d be able to see a pattern of these memory events emerge.
“Our work has shown that the combination of all these mergers will enable us to measure the memory effect over time,” he explained. “The key is being able to stack the signals from all of the events in a clever way.”
The researchers estimate that LIGO would be able to detect the memory effect after observing 35 to 90 mergers as dramatic as the one back in February, but if the observatory becomes more sensitive, it might happen even sooner.
No one can confirm that this technique will work until then, but the physics community is pretty impressed.
“This is a very clever way of measuring gravitational-wave memory and exploring it observationally,” LIGO co-founder Kip Thorne from the California Institute of Technology, who wasn’t involved in the study, told Choi. “I never thought it’d be possible with LIGO.”
If we really can detect gravitational-wave memory, it won’t just be a momentous day for our understanding of the Universe – it could also help solve a problem that physicist Stephen Hawking has been puzzling over for decades: the black hole information paradox.
Basically, the paradox stems from the fact that conventional physics states that nothing, not even light, can escape a black hole’s event horizon. But quantum physics tells us that information can never be destroyed.
Stephen Hawking has recently tried to solve the paradox by suggesting that information can be carried out of a black hole by something known as ‘soft hairs‘, which are essentially zero-energy forms of electromangetic and gravitational radiation that release information as black holes evaporate.
And gravitational-wave memory could actually measure those soft hairs and determine whether they exist once and for all.
We’re a long way off doing that, but at least now, we have a plan. And with a new space-based gravitational wave observatory set to go online by 2029, we might not have to wait another 100 years for results.
Gravitational waves: A monumnetal discovery, but questions on black holes remain
The maiden direct detection of gravitational waves (GW160914), announced on February 11, the culmination of intense worldwide scientific efforts spanning almost four decades, is one of the greatest scientific discoveries of all times.
In particular, kudos to the Laser Interferometer Gravitational-Wave Observatory (LIGO) team for being successful in one of the greatest experimental physics feats which was originally conceived way back in 2002. Now we have a new window to unravel the most violent phenomenon of the cosmos.
It is very heartening for Indian scientists, who have played a very significant role carrying forward frontline research on various aspects of gravitational wave astronomy.
In fact, two of the pioneers here are Prof. Bala Iyer at Raman Research Institute, Bengaluru, and Prof. (emeritus) Sanjeev Dhurandhar at the Inter-university Centre for Astronomy and Astrophysics (IUCAA) in Pune. The gravitational astronomy group at IUCAA and some other Indian centers have contributed to various relevant research as part of the global LIGO team.
One may be tempted to fancy that this discovery has suddenly elevated Einstein’s General Theory of Relativity (GTR) from the status of being a mere “electrostatic” effect to that of dynamic “electromagnetism” where the effect propagates like a wave with a finite speed (of light).
However, it should be borne in mind that unlike the electromagnetic case, one cannot obtain any realistic analytical derivation for such waves. The solution Einstein obtained 100 years ago is only a highly approximate solution involving only weak gravity.
This has to do with complex non-linear nature of GTR vis-Ã -vis simple linear nature of electromagnetism. And despite dramatic improvements in the prowess of numerical relativity, in order make prediction about the expected pattern of gravitational waves resulting from, say the merger of two compact objects orbiting one another.
One needs to make various tacit assumptions or simplifications. Such computations, in a sense, become easiest when the merging compact objects are black holes. This is so because they are the simplest objects in the Universe: a black hole is just a single point mass with an imaginary spherical boundary called the event horizon.
Thus, when one considers a black hole-black hole collision, one essentially considers gravitational interaction of two point masses dressed by their horizon.
In contrast, a compact object such as a neutron star has a dense extended body with complex structure about which we have little idea. Hence, the study of the merger of two neutron stars is far more complicated than the corresponding black hole case.
The present event – GW160914 – has been interpreted to have resulted from two compact objects having masses 36 (+5/-4) and 29 (+4/-4) solar masses respectively. These objects have been interpreted to be “black holes” because the original separation of them was only around 350 km.
Once the two black holes merge, a dynamic single hole black hole is supposed to be born. This dynamic black hole is expected to go through a stage called “ring down”, where any distortion in the shape is dissipated as more gravitational waves. The mass of the final black hole has been estimated to be about 62±4 solar masses. And the missing 3.0±0.5 solar masses of energy was presumably radiated away in the form of gravitational waves, in accordance with mass-energy equivalence.
But one may ponder and introspect whether the 0.02 second duration burst of gravitational waves has confirmed that those massive compact objects involved here indeed possessed event horizons from which nothing, not even light, can escape. If an energy equivalent to three solar masses have been radiated out, a significant portion of it must have come from the mass-energies of the two progenitor black holes.
The site of mass energy in a black hole is the central singularity. How can so much energy be extracted from a singularities and in a fraction of a second? Also during the “ring down” stage, can any energy come out of the trembling horizon of the dynamic black hole?
For a broader scientific perspective, one may note here that, in the past 15 years, many authors have pointed severe conceptual difficulties associated with the black hole paradigm, and have argued that so-called black holes are only quasi-black holes.
Some of the black hole alternatives which have invoked some quantum gravity ideas are ‘gravastars’, ‘dark energy stars’, ‘black stars’ and ‘fuzzball’. On the other hand, the maiden suggestion for a quasi-black hole is called ‘eternally collapsing object’ that is an ultra-hot ball of plasma.
All such black hole alternatives are practically as compact as true black holes and can very well be accommodated in a 350 km orbit. But there is an important difference: Unlike true black holes, these are dense extended objects without any event horizon.
Hence, the radiation of mass-energy of around three Suns may be better understood in scenarios where the compact objects involved for such mind boggling luminosities are quasi-black holes having no horizons. Of course, as of now, nobody has studied the expected pattern of gravitational waves resulting from the coalescence of quasi-black holes.
If these results are correct, they would imply that so-called massive black holes are only quasi-black holes. It would mean GW150914 resulted from the merger of two quasi- black holes and hence so much energy could be radiated out from the event.
Coalescence of two neutron stars are expected to generate not only bursts of gravitational waves but also bursts of gamma rays. Similarly, if the progenitors of powerful gravitational wave bursts are say balls of plasma, such events should be followed by bursts of gamma rays or x-rays.
In contrast, the coalescence of true singular black holes need not give rise to strong electromagnetic counterparts. Thus, one wishes, in future one would plan for multi-spectral observations in the coincidence of gravitational burst observations. This may help unravel true physical nature of the progenitors of gravitational waves.
Such questions apart, it’s a moment for celebration for astronomy and in particular for Einstein’s theory of general relativity which just completed its centennial.
All of us are fortunate that this great development of natural sciences happened in our lifetime and many of us could witness the LIGO press conference on the night of February 11, 2016. If September 11, 2001, was one of the worst memories for modern history, February 11, 2016 turned out to be one of the best nights of human intellectual history.
How gravitational waves went from a whisper to a shout
On 11 February 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its sister collaboration, Virgo, announced their earthshaking observation of Albert Einstein’s ripples in spacetime. LIGO had seen the death dance of a pair of massive black holes. As the behemoths circled each other faster and faster, the frequency and amplitude of the spacetime waves they produced grew into a crescendo as the black holes became one. Then the new doubly massive black hole began to ring softer and softer like a quieting bell. The escalating chirp and ringdown is also a metaphor for public information flow about the discovery. It could have unfolded differently.
When scientists make a discovery, they must choose how to disseminate it. A big decision they must make is whether to reveal the results before or after peer review. Reveal before peer review—sometimes even before the paper is written—and the community can use the results right away, but there is an increased risk that problems will be found in a very public way. Reveal after peer review, and the chance of such problems decreases, but there is more time for a competitor to announce first or for rumors to leak. At Physical Review Letters (PRL), where I am an editor, we allow authors to choose when they want to reveal their results. The LIGO collaborators chose to wait.
Just before LIGO’s experimental run began in September 2015, the team held a vote on which journal they would pick if they made a discovery. They picked PRL. Five days after the vote, LIGO’s detectors seemed to hear the universe sing out for the first time.
Had LIGO just confirmed a 100-year-old prediction made by Einstein? Had they discovered the first black hole binary? Had they opened a new era of astrophysics? With the stakes so high, the collaborators wanted to keep their results secret while they determined if the results were real. It was unfortunate that some onlookers chose to publicize vague rumors when the internal vetting had just begun.
By early December the collaboration was convinced that the results were real, and LIGO spokesperson Gabriela “Gaby” González let me know that we would be receiving a paper from the group in mid- to late January. When she told me that they had convincingly observed gravitational waves, that it was not a test, and that the source was the merger of two huge black holes, my jaw dropped.
Gaby stressed LIGO’s desire for strict confidentiality, so for a month I told only one other person in the world: my fellow editor Abhishek Agarwal. By mid-January we had to bring others into the loop to prepare for the paper’s arrival, to review it, and eventually to publish it. To avoid information slipping out from a casual conversation or a glance at a screen, we used the code name “Big Paper.” (The code name for the second LIGO, announced in June, discovery was “Big Two.”) To the best of my knowledge no information leaked from us. Inside the LIGO team, for similar reasons, the discovery was referred to as “The Event.”
Big Paper on The Event arrived at PRL on the evening of 21 January 2016, and we immediately sent it out to experts for anonymous peer review. The referees, like everyone involved, were sworn to secrecy. Informed, unbiased advice is central to picking which papers are published and to improving those that are. In this case it was clear that the paper was important and interesting enough for PRL. As expected, the reviews were very favorable and conveyed the message that the paper would be an inspiration to physicists and astronomers alike.
As the time for the announcement drew closer, the rumors increased. In one case, a preprint was spotted on a printer, then a physicist emailed his whole department about the results, one tweet quoted the email, and a science reporter based an entire story on that tweet. The information was incomplete, though correct—except for the journal where the paper would be published. That reporter learned at the press conference that PRL would publish the paper and sheepishly congratulated me.
Meanwhile, we continued to protect the information from leaking. My son, who is a budding science reporter, texted me a few days before the announcement, asking if I’d seen the rumors. That led to an awkward phone call—I still couldn’t tell him about the discovery. When we ordered a celebratory cake for the editorial office, we avoided any mention of the result on the frosting, lest it lead to an information leak. It turned out that we were not being overly cautious: A tweet containing a picture of a cake at NASA’s Goddard Space Flight Center on the morning of the announcement did leak news of the discovery!1 Confidentiality requires vigilance.
Everyone at LIGO’s press conference was given access to the PRL paper hours beforehand, on condition that they not publish their stories until after the announcement was made and the news embargo lifted. Actually, it might have been better in some ways had the press had access to the paper a little earlier, but that also would have increased the risk of the paper leaking prior to the announcement.
We had an agreement with the LIGO team to publish the paper online on 11 February at 10:45am Eastern Time, 15 minutes after the press conference was to begin. But I learned that morning from the reporters around me that the embargo was being lifted at 10:30am, and they planned to publish their stories then, which would create 15 minutes of pent-up demand for the PRL paper. So I found the spokesperson minutes before she went to the microphones and asked her if we could publish at 10:30. Gaby smiled and simply said yes.
After a few frantic emails, all the plans were changed, and at 10:30 we published the LIGO paper.2 It didn’t help: The demand for the paper was still so great that our site crashed under a load of 10 000 hits per minute.3 After we added a slew of servers, our site came back up, and the paper was downloaded an unprecedented quarter of a million times on the first day.
The LIGO researchers had chosen to maintain confidentiality because they wanted their results carefully vetted before they went public. They also wanted the information to come from them, not from rumors. Although some of the information leaked before the announcement, they still did get the glory of presenting the full results to the world. And the ringdown phase has been impressive, as news of the result continues to spread far and wide.
Authors may have good reasons to announce their results prior to the completion of peer review—reasons that include competition from other groups, hope for informal community feedback, and desire to control the announcement and avoid weeks of rumors. But if authors choose that path, they should consider the possibility that peer review will turn up problems they did not think of, and they should tailor their announcement accordingly. Authors may instead choose to wait for the completion of peer review, especially when they have no concerns about competition. In such cases it is an even greater pity when rumors leak, because the leakers provide disincentive for such patience.
For LIGO, although much of the information leaked before the press conference, the researchers still had much to announce, probably in part because they had emphasized confidentiality. Announcing early makes sense in some cases, but the LIGO group made the right choice to wait.
Has giant LIGO experiment seen gravitational waves?
An improbable rumour has started that the observatory has already made a discovery — but even if true, the signal could be a drill.
On 25 September, a sensational rumour appeared on Twitter: Lawrence Krauss, a cosmologist, reported hearing that the world’s largest gravitational-wave observatory had seen a signal, barely a week after its official re-opening.
The rumour had been spreading around physics circles for at least a week. If it is true, and if the signal seen by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) genuinely represents the signature of a gravitational wave, it would confirm one of the most-elusive and spectacular predictions of the general theory of relativity almost exactly 100 years after Albert Einstein first proposed it.
But those are two big ifs. LIGO will not confirm or deny the rumours. “The official response is that we’re analysing the data,” says spokesperson Gabriela González, a physicist at Louisiana State University in Baton Rouge. Gonzalez was upset at the possibility that someone in the LIGO team might have initiated the rumour, although Krauss and other researchers told me that they did not hear it directly from members of the LIGO collaboration. “I give it a 10–15% likelihood of being right,” says Krauss, who works at Arizona State University in Tempe.
And even if LIGO has seen some promising data, it could be the result of an elaborate drill — a false signal deliberately injected into the detectors to train LIGO’s data-analysis team. At this stage, only three people would know the truth, and they would not reveal that until much later, when the collaboration is ready to publish a paper and to hold a press conference.
LIGO on the lookout
In principle, LIGO could have already spotted a signal. Its two detectors, in Hanford, Washington, and Livingston, Louisiana,operated from 2002-2010 without detecting any gravitational waves — ripples in the fabric of space–time that, according to Einstein’s theory, are produced by cataclysmic events such as the merging of two black holes. LIGO’s interferometers bounce laser beams between mirrors at the opposite ends of 4-kilometre-long vacuum pipes, aiming to detect passing gravitational waves that stretch and compress the length of the pipes — along with the rest of space.
But Advanced LIGO, which officially began taking data on 18 September, represents a US$200-million overhaul. The detectors are now three times more sensitive than their predecessors, González says, and they have already stayed simultaneously online for up to 24 hours at a time. (This is a big improvement from earlier runs, when operations could be interrupted by a truck hitting a pothole on a road kilometres away from either site).
The detectors are expected to run for three months before shutting down for a further upgrade. Some in the LIGO collaboration reckon that they have a one-in-three chance of recording an event during that time, making a detection over days or weeks of operation improbable.
But LIGO also conducted several ‘engineering runs’ beginning in June, during which the detectors were both running and recording data. Those data have been examined, though some in the collaboration have cautioned against this. At the time, the team was still calibrating the instruments and understanding noise — such as fluctuations in the laser beams or thermal vibrations — that would make it hard to pick out genuine signals. (Detecting gravitational waves is an extremely delicate task, as it involves measuring changes in the length of each interferometer’s arms by about one part in 1022).
Rapid analysis
Even if a signal has been seen, and if it is a genuine discovery, an official announcement probably will not happen until next year. The Advanced LIGO team plans to give itself about three months from the time of a signal detection to analyse it, write up a paper and vote to decide whether to announce it.
But an astrophysical event such as the collision of two black holes could produce an unequivocal detection of gravitational waves, if it happened in a galaxy close enough to the Milky Way to produce a loud signal, several LIGO members have told me.
This is especially true for what gravitational-wave scientists call a ‘chirp’: a clean-looking sinusoidal wave that becomes higher in pitch and louder as time goes by, resembling the sounds of certain birds. Chirps are the signature of two neutron stars or black holes as they spiral into each other, emitting gravitational waves in the process. In the final stages of the stars’ dance, the waves soar past the 10-Hertz pitch threshold needed for the Advanced LIGO to detect a signal.
False alert
But then there is the other possible scenario: that the researchers have seen a false signal planted deliberately as a drill. The LIGO is almost unique among physics experiments in practising ‘blind injection’. A team of three collaboration members has the ability to simulate a detection by using actuators to move the mirrors. “Only they know if, and when, a certain type of signal has been injected,” says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who leads the Advanced LIGO’s data-analysis team.
Two such exercises took place during earlier science runs of LIGO, one in 2007 and one in 2010. Harry Collins, a sociologist of science at Cardiff University, UK, was there to document them (and has written books about it1). He says that the exercises can be valuable for rehearsing the analysis techniques that will be needed when a real event occurs. But the practice can also be a drain on the team’s energies. “Analysing one of these events can be enormously time consuming,” he says. “At some point, it damages their home life.”
The original blind-injection exercises took 18 months and 6 months respectively. The first one was discarded, but in the second case, the collaboration wrote a paper and held a vote to decide whether they would make an announcement. Only then did the blind-injection team ‘open the envelope’ and reveal that the events had been staged.
Thick and fast
Researchers hope that, once Advanced LIGO completes further upgrades a year from now, it will detect mergers between black holes and between neutron stars on a regular basis, perhaps ten times a year. And if detections become routine, the researchers will learn to recognize them virtually in real time.
Advanced LIGO could detect the loudest merger signals for a minute or more, meaning that computers in the control rooms would make loud noises and send out automatic alerts even before the event is over. The idea of this is to flag possible events to astronomers, who could then search the sky for the type of ‘fireworks’ — such as supernova explosions — that might be associated with gravitational waves.
Chad Hanna, a physicist at Pennsylvania State University in University Park who leads the search for mergers, told me earlier this year that he has already got permission from his wife to keep his mobile phone on at night, waiting to receive alerts. “When the signal of a merger arrives, we’ll know 30 seconds later,” he says.
Feeling the pulse of the space-time continuum.
Humans have known about the force of gravity since ancient times. Yet, we are still exploring its true nature, how it works, and why it works the way it does.
Haaaaaaaaaaaave you met PSR B1913+16? The first three letters of its name indicate it’s a pulsating radio source, an object in the universe that gives off energy as radio waves at very specific periods. More commonly, such sources are known as pulsars, a portmanteau of pulsating stars.
When heavy stars run out of hydrogen to fuse into helium, they undergo a series of processes that sees them stripped off their once-splendid upper layers, leaving behind a core of matter called a neutron star. It is extremely dense, extremely hot, and spinning very fast. When it emits electromagnetic radiation in flashes, it is called a pulsar. PSR B1913+16 is one such pulsar, discovered in 1974, located in the constellation Aquila some 21,000 light-years from Earth.
Finding PSR B1913+16 earned its discoverers the Nobel Prize for physics in 1993 because this was no ordinary pulsar, and it was the first to be discovered of its kind: of binary stars. As the ‘B’ in its name indicates, it is locked in an epic pirouette with a nearby neutron star, the two spinning around each other with the orbit’s total diameter spanning one to five times that of our Sun.
Losing energy but how?
The discoverers were Americans Russell Alan Hulse and Joseph Hooton Taylor, Jr., of the University of Massachusetts Amherst, and their prize-winning discovery didn’t culminate with just spotting the binary pulsar that has come to be named after them. They found that the pulsar’s orbit was shrinking, meaning the system as a whole was losing energy. They found that they could also predict the rate at which the orbit was shrinking using the general theory of relativity.
In other words, PSR B1913+16 was losing energy as gravitational energy while proving a direct (natural) experiment to verify Albert Einstein’s monumental theory from a century ago. (That a human was able to intuit how two neutron stars orbiting each other trillions of miles away could lose energy is homage to the uniformity of the laws of physics. Through the vast darkness of space, we can strip away with our minds any strangeness of its farthest reaches because what is available on a speck of blue is what is available there, too.)
While gravitational energy, and gravitational waves with it, might seem like an esoteric concept, it is easily intuited as the gravitational analogue of electromagnetic energy (and electromagnetic waves). Electromagnetism and gravitation are the two most accessible of the four fundamental forces of nature. When a system of charged particles moves, it lets off electromagnetic energy and so becomes less energetic over time. Similarly, when a system of massive objects moves, it lets off gravitational energy… right?
“Yeah. Think of mass as charge,” says Tarun Souradeep, a professor at the Inter-University Centre for Astronomy and Astrophysics, Pune, India. “Electromagnetic waves come with two charges that can make up a dipole. But the conservation of momentum prevents gravitational radiation from having dipoles.”
According to Albert Einstein and his general theory of relativity, gravitation is a force born due to the curvature, or roundedness, of the space-time continuum: space-time bends around massive objects (an effect very noticeable during gravitational lensing). When massive objects accelerate through the continuum, they set off waves in it that travel at the speed of light. These are called gravitational waves.
“The efficiency of energy conversion – from the bodies into gravitational waves – is very high,” Prof. Souradeep clarifies. “But they’re difficult to detect because they don’t interact with matter.”
Albie’s still got it
In 2004, Joseph Taylor, Jr., and Joel Weisberg published a paper analysing 30 years of observations of PSR B1913+16, and found that general relativity was able to explain the rate of orbit contraction within an error of 0.2 per cent. Should you argue that the binary system could be losing its energy in many different ways, that the theory of general relativity is able to so accurately explain it means that the theory is involved, and in the form of gravitational waves.
Prof. Souradeep says, “According to Newtonian gravity, the gravitational pull of the Sun on Earth was instantaneous action at a distance. But now we know light takes eight minutes to come from the Sun to Earth, which means the star’s gravitational pull must also take eight minutes to affect Earth. This is why we have causality, with gravitational waves in a radiative mode.”
And this is proof that the waves exist, at least definitely in theory. They provide a simple, coherent explanation for a well-defined problem – like a hole in a giant jigsaw puzzle that we know only a certain kind of piece can fill. The fundamental particles called neutrinos were discovered through a similar process.
These particles, like gravitational waves, hardly interact with matter and are tenaciously elusive. Their discovery was predicted by the physicist Wolfgang Pauli in 1930. He needed such a particle to explain how the heavier neutron could decay into the lighter proton, the remaining mass (or energy) being carried away by an electron and a neutrino antiparticle. And the team that first observed neutrinos in an experiment, in 1942, did find it under these circumstances.
Waiting for a direct detection
On March 17, radio-astronomers from the Harvard-Smithsonian Centre for Astrophysics (CfA) announced a more recent finding that points to the existence of gravitational waves, albeit in a more powerful and ancient avatar. Using a telescope called BICEP2 located at the South Pole, they found the waves’ unique signature imprinted on the cosmic microwave background, a dim field of energy leftover from the Big Bang and visible to this day.
At the time, Chao-Lin Kuo, a co-leader of the BICEP2 collaboration, had said, “We have made the first direct image of gravitational waves, or ripples in space-time across the primordial sky, and verified a theory about the creation of the whole universe.”
Spotting the waves themselves, directly, in our human form is impossible. This is why the CfA discovery and the orbital characteristics of PSR B1913+16 are as direct detections as they get. In fact, finding one concise theory to explain actions and events in varied settings is a good way to surmise that such a theory could exist.
For instance, there is another experiment whose sole purpose has been to find gravitational waves, using laser. Its name is LIGO (Laser Interferometer Gravitational-wave Observatory). Its first phase operated from 2002 to 2010, and found no conclusive evidence of gravitational waves to report. Its second phase is due to start this year, in 2014, in an advanced form. On April 16, the LIGO collaboration put out a 20-minute documentary titled Passion for Understanding, about the “raw enthusiasm and excitement of those scientists and researchers who have dedicated their professional careers to this immense undertaking”.
The laser pendula
LIGO works like a pendulum to try and detect gravitational waves. With a pendulum, there is a suspended bob that goes back and forth between two points with a constant rhythm. Now, imagine there are two pendulums swinging parallel to each other but slightly out of phase, between two parallel lines 1 and 2. So when pendulum A reaches line 1, pendulum B hasn’t got there just yet, but it will soon enough.
When gravitational waves, comprising peaks and valleys of gravitational energy, surf through the space-time continuum, they induce corresponding crests and troughs that distort the metrics of space and passage of time in that area. When the two super-dense neutron stars that comprise PSR B1913+16 move around each other, they must be letting off gravitational waves in a similar manner, too.
When such a wave passes through the area where we are performing our pendulums experiment, they are likely to distort their arrival times to lines 1 and 2. Such a delay can be observed and recorded by sensitive instruments.
Analogously, LIGO uses beams of light generated by a laser at one point to bounce back and forth between mirrors for some time, and reconvene at a point. And instead of relying on the relatively clumsy mechanisms of swinging pendulums, scientists leverage the wave properties of light to make the measurement of a delay more precise.
At the beach, you’ll remember having seen waves forming in the distance, building up in height as they reach shallower depths, and then crashing in a spray of water on the shore. You might also have seen waves becoming bigger by combining. That is, when the crests of waves combine, they form a much bigger crest; when a crest and a trough combine, the effect is to cancel each other. (Of course this is an exaggeration. Matters are far less exact and pronounced on the beach.)
Similarly, the waves of laser light in LIGO are tuned such that, in the absence of a gravitational wave, what reaches the detector – an interferometer – is one crest and one trough, cancelling each other out and leaving no signal. In the presence of a gravitational wave, there is likely to be one crest and another crest, too, leaving behind a signal.
A blind spot
In an eight-year hunt for this signal, LIGO hasn’t found it. However, this isn’t the end because, like all waves, gravitational waves should also have a frequency, and it can be anywhere in a ginormous band if theoretical physicists are to be believed (and they are to be): between 10-7 and 1011 hertz. LIGO will help humankind figure out which frequency ranges can be ruled out.
In 2014, the observatory will also reawaken after four-years of being dormant and receiving upgrades to improve its sensitivity and accuracy. According to Prof. Souradeep, the latter now stands at 10-20 m. One more way in which LIGO is being equipped to find gravitational waves is by created a network of LIGO detectors around Earth. There are already two in the US, one in Europe, and one in Japan (although the Japanese LIGO uses a different technique).
But though the network improves our ability to detect gravitational waves, it presents another problem. “These detectors are on a single plane, making them blind to a few hundred degrees of the sky,” Prof. Souradeep says. This means the detectors will experience the effects of a gravitational wave but if it originated from a blind spot, they won’t be able to get a fix on its source: “It will be like trying to find MH370!” Fortunately, since 2010, there have been many ways proposed to solve this problem, and work on some of them is under way.
One of them is called eLISA, for Evolved Laser Interferometer Space Antenna. It will attempt to detect and measure gravitational waves by monitoring the locations of three spacecraft arranged in an equilateral triangle moving in a Sun-centric orbit. eLISA is expected to be launched only two decades from now, although a proof-of-concept mission has been planned by the European Space Agency for 2015.
Another solution is to install a LIGO detector on ground and outside the plane of the other three – such as in India. According to Prof. Souradeep, LIGO-India will reduce the size of the blind spot to a few tens of degrees – an order of magnitude improvement. The country’s Planning Commission has given its go-ahead for the project as a ‘mega-science project’ in the 12th Five Year Plan, and the Department of Atomic Energy, which is spearheading the project, has submitted a note to the Union Cabinet for approval. With the general elections going on in the country, physicists will have to wait until at least June or July to expect to get this final clearance.
Once cleared, of course, it will prove a big step forward not just for the Indian scientific community but also for the global one, marking the next big step – and possibly a more definitive one – in a journey that started with a strange pulsar 21,000 light-years away. As we get better at studying these waves, we have access to a universe visible not just in visible light, radio-waves, X-rays or neutrinos but also through its gravitational susurration – like feeling the pulse of the space-time continuum itself.
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