Controversy over gravitational wave discovery

Scientists first detected the shudders in space-time last year. However, since releasing their data other researchers say they have found 'strange correlations that shouldn't be there' in the signals.

A scientific argument has broken out over claims that gravitational waves, ripples through the fabric of space-time predicted by Albert Einstein a century ago, have been detected for a third time.

Scientists first detected the shudders in space-time last year and the discovery was hailed the 'biggest scientific breakthrough of the century' - and researchers have since done it twice more. 

However, since releasing their data in February, the results have come under scrutiny from other researchers - who say they have found 'strange correlations that shouldn't be there' in the form of unexpected noise in the signals.

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Scientists in the Ligo Scientific Collaboration have directly observed the ripples of gravitational waves for a third time. Pictured is an artist's impressions of the waves

Scientists in the Ligo Scientific Collaboration have directly observed the ripples of gravitational waves for a third time. Pictured is an artist's impressions of the waves

HOW LIGO WORKS 

LIGO's two observatories detect gravitational waves by splitting a laser beam and sending it down several kilometer-long tunnels before merging the light waves together again. 

A passing gravitational wave changes the shape of space by a tiny amount, and the LIGO was built with the ability to measure a change in distance just one-ten-thousandth the width of a proton.

However, this sensitivity means any amount of noise, even people running at the site, or raindrops, can be detected.

The team of independent physicists, led by Andrew Jackson, a physicist at the Niels Bohr Institute in Copenhagen, published their findings earlier this month.

'In our opinion, a discovery of this importance merited a genuinely independent analysis of the data,' the team wrote

LIGO scientists measure the interference in the two beams of light when they come back to meet, which reveals information on the space-time disturbance.

Gravitational waves are exceedingly faint, so LIGO was built with the ability to measure a change in distance just one-ten-thousandth the width of a proton. 

The ensure the results are accurate, LIGO uses two observatories, 3,000 kilometers apart, which operate synchronously, each double-checking the other's observations.

The noise at each detector should be completely uncorrelated, meaning a noise like a storm nearby one detector doesn't show up as noise in the other.

Some of the sources of 'noise' the team say they contend with include: 'a constant 'hiss' from photons arriving like raindrops at our light detectors; rumbles from seismic noise like earthquakes and the oceans pounding on the Earth's crust; strong winds shaking the buildings enough to affect our detectors.'

However, if a gravitational wave is found, it should create a similar signal in both instruments nearly simultaneously.

The main claim of Jackson's team is that there appears to be correlated noise in the detectors at the time of the gravitational-wave signal.    

WHAT'S THE PROBLEM? LIGO'S NOISE ISSUES

Teasing out a gravitational wave signal from LIGO data requires weeding through a lot of instrument and environmental noise. The image on top shows the signature of our latest gravitational wave detection

Teasing out a gravitational wave signal from LIGO data requires weeding through a lot of instrument and environmental noise. The image on top shows the signature of our latest gravitational wave detection

Gravitational waves are exceedingly faint, so to catch them LIGO was built with the ability to measure a change in distance just one-ten-thousandth the width of a proton.

The LIGO detectors are interferometers that shine a laser through a vacuum down two arms in the shape of an L that are each 4 kilometers in length.

The light from the laser bounces back and forth between mirrors on each end of the L, and scientists measure the length of both arms using the light.

If there's a disturbance in space-time, such as a gravitational wave, the time the light takes to travel 4 kilometers will be slightly different in each arm making one arm look longer than the other.

LIGO scientists measure the interference in the two beams of light when they come back to meet, which reveals information on the space-time disturbance.

The ensure the results are accurate, LIGO uses two observatories, 3,000 kilometers apart, which operate synchronously, each double-checking the other's observations.

The noise at each detector should be completely uncorrelated, meaning a noise like a stormnearby one detector doesn't show up as noise in the other.

An aerial view of the Laser Interferometer Gravitational-wave Observatory (LIGO) Hanford lab detector site near Hanford, Washington

An aerial view of the Laser Interferometer Gravitational-wave Observatory (LIGO) Hanford lab detector site near Hanford, Washington

Some of the sources of 'noise' the team say they contend with include: 'a constant 'hiss' from photons arriving like raindrops at our light detectors; rumbles from seismic noise like earthquakes and the oceans pounding on the Earth's crust; strong winds shaking the buildings enough to affect our detectors.'

However, if a gravitational wave is found, it should create a similar signal in both instruments nearly simultaneously.

The main claim of Jackson's team is that there appears to be correlated noise in the detectors at the time of the gravitational-wave signal.   

 

'We focussed our attention mainly on the first event, GW150914, with special attention to the time lag between the arrival times of the signal at the Hanford and Livingston detectors, the team say. 

'In our view, if we are to conclude reliably that this signal is due to a genuine astrophysical event, apart from chance-correlations, there should be no correlation between the 'residual' time records from LIGO's two detectors in Hanford and Livingston,' they team say. 

'Our investigation revealed that these residuals are, in fact, strongly correlated.

'We hope that interested people will repeat our calculations and will make up their own minds regarding the significance of the results. 

WHAT ARE GRAVITATIONAL WAVES? 

Gravitational waves are considered ripples in space time fabric. They can be produced, for instance, when black holes orbit each other or by the merging of galaxies.

Gravitational waves are considered ripples in space time fabric. They can be produced, for instance, when black holes orbit each other or by the merging of galaxies.

Scientists view the the universe as being made up of a 'fabric of space-time'.

This corresponds to Einstein's General Theory of Relativity, published in 1916.

Objects in the universe bend this fabric, and more massive objects bend it more.

Gravitational waves are considered ripples in this fabric.

They can be produced, for instance, when black holes orbit each other or by the merging of galaxies.

Gravitational waves are also thought to have been produced during the Big Bang.

If found, they would not only confirm the Big Bang theory but also offer insights into fundamental physics.

For instance, they could shed light on the idea that, at one point, most or all of the forces of nature were combined into a single force. 

In March 2014, a team operating the Bicep2 telescope, based near the South Pole, believed they had found gravitational waves, but their results were proven to be inaccurate.

'It is obvious that 'belief' is never an alternative to 'understanding' in physics.  

The potential effects of the unexplained correlations 'could range from a minor modification of the extracted wave form to a total rejection of LIGO's claimed [gravitational wave] discovery,' wrote Jackson in an email to Quanta

LIGO representatives say there may well be some unexplained correlations, but that they should not affect the team's conclusions. 

Ian Harry, a researcher at the Max Planck Institute for Gravitational Physics in Potsdam-Golm and a member of the LIGO Scientific Collaboration, initially published a public rebuttal five days later saying the team couldn't reproduce the claimed correlations - something the new research claims was down to buggy code.

However, neither team has admitted they are wrong, and according to Forbes, the LIGO group said it 'respectfully responds that we have talked at some length with the group in the past and do not agree on the methods being used and thus with the conclusions.'  

The third detection was made by the Ligo (Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration (LSC) on January 4, 2017.

As was the case with the first two detections, the waves were generated when two black holes collided to form a larger black hole. 

The newfound black hole, formed by the merger, has a mass about 49 times that of our sun.

This fills in a gap between the masses of the two merged black holes detected previously by Ligo, with solar masses of 62 (first detection) and 21 (second detection).

'We have further confirmation of the existence of stellar-mass black holes that are larger than 20 solar masses -these are objects we didn't know existed before Ligo detected them,' said Dr David Shoemaker, a scientist at the Massachusetts Institute of Technology and the spokesperson for the Ligo scientific Collaboration.

An artist's impression of two black holes merging while spinning in a non-aligned fashion. The new Ligo data implies that at least one of the black holes may have been non-aligned compared to the overall orbital motion

An artist's impression of two black holes merging while spinning in a non-aligned fashion. The new Ligo data implies that at least one of the black holes may have been non-aligned compared to the overall orbital motion

WHAT IS THE THEORY OF RELATIVITY? 

In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum was independent of the motion of all observers - known as the theory of special relativity.

This groundbreaking work introduced a new framework for all of physics, and proposed new concepts of space and time.

He then spent 10 years trying to include acceleration in the theory, finally publishing his theory of general relativity in 1915.

This determined that massive objects cause a distortion in space-time, which is felt as gravity.

At its simplest, it can be thought of as a giant rubber sheet with a bowling ball in the centre.

As the ball warps the sheet, a planet bends the fabric of space-time, creating the force that we feel as gravity.

Any object that comes near to the body falls towards it because of the effect.

Einstein predicted that if two massive bodies came together it would create such a huge ripple in space time that it should be detectable on Earth.

It was most recently demonstrated in the hit film film Interstellar.

In a segment that saw the crew visit a planet which fell within the gravitational grasp of a huge black hole, the event caused time to slow down massively.

Crew members on the planet barely aged while those on the ship were decades older on their return.

'It is remarkable that humans can put together a story, and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us.

'The entire Ligo and Virgo scientific collaborations worked to put all these pieces together.' 

In all three cases, each of the twin detectors of LIGO detected gravitational waves from the tremendously energetic mergers of black hole pairs.

These are collisions that produce more power than is radiated as light by all the stars and galaxies in the universe at any given time.

The recent detection appears to be the farthest yet, with the black holes located about 3 billion light-years away.

Scientists said gravitational waves open a 'new door' for observing the universe and gaining knowledge about enigmatic objects like black holes and neutron stars.

Understanding such astronomical phenomena could be useful in helping us decipher how the universe first came to be. 

'This [discovery is taking us deeper into time and space in ways we couldn't do before the detection of gravitational waves,' said France Córdova, director of the National Science Foundation.

'In this case, we're exploring approximately three billion light-years away. 

'Ligo continues to make remarkable discoveries, transitioning from experiment to gravitational wave observatory. 

'More importantly each detection has offered much more than just a sighting.

'Slowly, we are collecting data that unveil the origin and characteristics of these objects, further informing our understanding of the universe.' 

'We know this is just the beginning. This 'window on the universe' will continue to expand.

'We will watch eagerly as hundreds of researchers from around the world enhance this observatory to illuminate the physics of merging black holes, neutron stars and other astronomical phenomena.'

The newest observation also provides clues about the directions in which the black holes are spinning.

As pairs of black holes spiral around each other, they also spin on their own axes, much like a pair of ice skaters spinning individually while also circling around each other.

How our sun and Earth warp space and time, or spacetime, is represented here with a green grid. As Albert Einstein demonstrated in his theory of general relativity, the gravity of massive bodies warps the fabric of space and time, and those bodies move along paths determined by this geometry

How our sun and Earth warp space and time, or spacetime, is represented here with a green grid. As Albert Einstein demonstrated in his theory of general relativity, the gravity of massive bodies warps the fabric of space and time, and those bodies move along paths determined by this geometry

Sometimes black holes spin in the same overall orbital direction as the pair is moving, this is known as aligned spins, and sometimes they spin in the opposite direction of the orbital motion.

And black holes can also be tilted away from the orbital plane. 

The new Ligo data implies that at least one of the black holes may have been non-aligned compared to the overall orbital motion.

More observations with LIGO are needed to say anything definitive about the spins of binary black holes, but these early data offer clues about how these pairs may form.

'This is the first time that we have evidence that the black holes may not be aligned, giving us just a tiny hint that binary black holes may form in dense stellar clusters,' said Bangalore Sathyaprakash of Cardiff University.

There are two primary theories to explain how binary pairs of black holes can be formed.

The first model proposes that the black holes are born together: they form when each star in a pair of stars explodes, and then, because the original stars were spinning in alignment, the black holes likely remain aligned.

In the other model, the black holes come together later in life within crowded stellar clusters.

The black holes pair up after they sink to the center of a star cluster.

In this scenario, the black holes can spin in any direction relative to their orbital motion.

Because Ligo sees some evidence that the newly discovered black holes are non-aligned, the data favours this dense stellar cluster theory.

'We're starting to gather real statistics on binary black hole systems,' said Keita Kawabe, who is based at the Ligo Hanford Observatory.

'That's interesting because some models of black hole binary formation are somewhat favoured over the others even now and, in the future, we can further narrow this down.'

The study also once again puts Albert Einstein's theories to the test.

For example, the researchers looked for an effect called dispersion, which occurs when light waves in a physical medium such as glass travel at different speeds depending on their wavelength; this is how a prism creates a rainbow.

This computer simulation shows the warping of space and time around two colliding black holes, pictured are the spheres at the top of this image. The coloured surface is space represented as a two-dimensional sheet. The funnel-shaped warping is produced by black hole's mass. Colors near the black holes depict the rate which time flows: green, normal; yellow, slowed by 20 or 30 per cent; red, hugely slowed

This computer simulation shows the warping of space and time around two colliding black holes, pictured are the spheres at the top of this image. The coloured surface is space represented as a two-dimensional sheet. The funnel-shaped warping is produced by black hole's mass. Colors near the black holes depict the rate which time flows: green, normal; yellow, slowed by 20 or 30 per cent; red, hugely slowed

Einstein's general theory of relativity forbids dispersion from happening in gravitational waves as they propagate from their source to Earth.

And Ligo did not find evidence for this effect.

'It looks like Einstein was right - even for this new event, which is about two times farther away than our first detection,' said Laura Cadonati of Georgia Tech University.

'We can see no deviation from the predictions of general relativity, and this greater distance helps us to make that statement with more confidence.'

The team is continuing to search the latest Ligo data for signs of space-time ripples from the far reaches of the cosmos.

A bird's eye view of Laser Interferometer Gravitational-wave Observatory (LIGO) Hanford laboratory's laser and vacuum equipment area (LVEA) which houses the pre-stabilized laser, beam splitter, input test masses, and other equipment

A bird's eye view of Laser Interferometer Gravitational-wave Observatory (LIGO) Hanford laboratory's laser and vacuum equipment area (LVEA) which houses the pre-stabilized laser, beam splitter, input test masses, and other equipment

They are also working on technical upgrades for Ligo's next run, scheduled to begin in late 2018, during which the detectors' sensitivity will be improved.

At a press conference, the researchers spoke of their ambition to use Ligo to detect neutron stars. 

Neutron stars are created when giant stars die in supernovas and their cores collapse, with the protons and electrons essentially melting into each other to form neutrons.

Researchers hope that improving the sensitivity of Ligo's instruments will help them to finally pinpoint signs of a neutron star, which could come as soon as 'this run or the next'. 

They added a lack of understanding about how often neutron stars form could be holding them back from making a momentous discovery. 

'With the third confirmed detection of gravitational waves from the collision of two black holes, Ligo is establishing itself as a powerful observatory for revealing the dark side of the universe,' said David Reitze of Caltech, executive director of the Ligo Laboratory.

'While Ligo is uniquely suited to observing these types of events, we hope to see other types of astrophysical events soon, such as the violent collision of two neutron stars.'