By Abhas Mitra
On October 3, 2017, the Nobel Prize for Physics was awarded to three American physicists “for decisive contribution to Laser Interferometry Gravitational Observatory (LIGO) detector and observation of gravitational waves”, a sort of extremely faint ripples propagating through the very fabric of space-time resulting from most powerful cosmic catastrophes.
Some Nobel prizes in the past have kicked up controversies. The latest award may join the list if one seriously considers the questions raised by some physicists in the recent years in peer-reviewed journals and which remain unanswered.
While everybody would be in awe for the success of the mechanical and optical engineering behind LIGO, the claim for decisive discovery of gravitational waves did not appear convincing to all.
The maiden detection of gravitational waves occurred on September 14, 2015 and was announced few months later. This event was due to a catastrophic collision between a pair of unusually massive black holes, a kind of extremely compact objects whose gravity is so strong that nothing, not even light, can escape from them.
However, having examined the same LIGO data, a group of astrophysicists from The Niels Bohr Institute & Discovery Center, Denmark, concluded that, the claim was somewhat suspect as the LIGO team had failed to remove noise from the data. This group published this conclusion in August in the high-impact Journal of Cosmology & Astroparticle Physics.
They found similar presence of noise for two more events of gravitational waves detected by LIGO. Following the reanalysis of LIGO events, Sabine Hossenfelder, a German theoretical physicist and a famous science blogger, even wrote a blog titled: “Was It All Just Noise? Independent Analysis Casts Doubt On LIGO’s Detections.”
Gravitational waves are ripples so weak that even the proverbial “needle in the haystack” fails to adequately describe them. Actually, searching for gravitational waves is like searching for an atom in a haystack. Accordingly, a team of more than 1,000 physicists has been toiling for decades to improve the sensitivity of the gravitational wave detectors. By 2015, the sensitivity of an advanced version of LIGO seemed to be just adequate for this purpose.
However, a super-sensitive detector is like a double-edged sword: It picks up not just the feeble gravitational waves but even more, the faintest of noises too. Therefore, while the LIGO team made the utmost effort to indeed eliminate all known sources of noises, there could still be some poorly understood or even unknown noises contaminating the LIGO signal.
One such potential source of uncontrollable noise could be “Schumann resonances” — global electromagnetic resonances generated and excited by lightning discharges in the cavity formed by the Earth’s surface and the ionosphere. Since these are global disturbances created by strong (local) lightning, the associated electromagnetic disturbances (as reported in a paper published in Physical Review D) may affect the kilometer-long metal arms of advanced LIGO detectors spread across the globe and simulate fake gravitational wave signals.
More recently physicist Z. K. Silagadze of Russia’s Novosibirsk State University, in her blog, has suggested that the LIGO gravitational wave signal could be due to such global ionospheric disturbances.
The claim by LIGO team that the gravitational waves detected by it were due to collision of black holes has also been questioned — in a paper in Physical Review Letters — by a team of general relativists from Portugal who showed that the LIGO signal, even if true, need not be due to collision of “exact” black holes. On the other hand, stars almost as compact as black holes — but from which light may just be able to escape — could also generate similar signals.
A smoking gun leaves evidence to show it recently fired a bullet: light flashes, sound and the smell of gunpowder. Similarly, for cosmic catastrophes, one anticipates electromagnetic signals like a burst of gamma rays, X-rays and light or radio waves. Recently, 25 telescopes searched for signals of such smoking gun from the latest LIGO detection of gravitational waves.
Unfortunately, no evidence of emission of any attendant electromagnetic emission was found. And, only future detection of such circumstantial evidence may confirm the claims of direct detection of gravitational waves which are likely to be swamped by infinitely stronger noises.
The motion of compact celestial objects are indeed expected to emit bursts of gravitational waves and these have indeed been detected earlier — albeit in an indirect manner. The famous Hulse-Taylor binary comprising two neutron stars orbiting each other is known to shrink over years due to emission of such waves. In 1993, the Nobel for Physics was awarded for this discovery to Russel Hulse and Hooton Taylor. But this was after almost two decades of observation in a meticulous manner. And there was no controversy for the claims of such (indirect) detection of gravitational waves.
But this time, LIGO’s claim of direct detection of gravitational waves had triggered a controversy. No doubt the claim would eventually be confirmed after verification by independent groups of researchers. Pending that Physics Nobel for 2017 may be considered by some to be somewhat premature.
(Abhas Mitra is former head of the Theoretical Astrophysics Division of the Bhabha Atomic Research Centre, Mumbai and currently adjunct professor in Homi Bhabha National Science Institute, Mumbai. The views expressed are personal. He can be contacted at firstname.lastname@example.org)