Oct 19 2017

More Gravitational Waves

Published by under Astronomy
Comments: 9

neutronStar_drupal_withNOcaptionThe winner of the Nobel prize for physics was the detection of gravitational waves. These are extremely subtle ripples in spacetime caused by massive cataclysmic events, such as black holes colliding. These ripples were predicted by Einstein, who thought we may never be able to detect them because they would be so unbelievably tiny.

How tiny?  LIGO (Laser Interferometer Gravitational-Wave Observatory), the device used to detect the waves, can detect changes as small at 10 -22 meters. Graviational waves detected so far have had an amplitude of 10 -18 meters, smaller than the radius of a proton.

How is that possible? That is where the interferometry comes in. LIGO uses a laser split into two beams that will travel for 8 Km and then reflect off mirrors and come back to the same detector. The two beams have traveled the exact same distance so that they are in phase, or at least they can be calibrated to be exactly in phase, meaning that the peaks of the waves line up. When a ripple in spacetime comes through, the length of the two arms (which are at 90 degree angles to each other) will change slightly, bringing the two beams out of phase. That slight phase shift can be detected and measured.

Prior to winning the Nobel prize LIGO had detected four gravitational waves wash over it. This was considered enough of a confirmation of the technology and science to award the prize.

I admit I was a little surprised. The discovery is certainly worthy, but usually the Nobel committee is very conservative and they will wait for a discovery to stand the test of time. The little doubt I had in the back of my mind was that, in order to detect the tiny gravitational waves they have to eliminate all sources of even the slightest interference. Animals walking on the grounds of LIGO will cause a jitter in the detector.

While I had no doubt the instrument worked as advertised I admit to being a little worried about how certain they could be that all sources of artifact have been eliminated. The signal to noise ratio is miniscule, requiring virtually all noise to be eliminated or accounted for, and that always makes me suspicious.

Well, I think my doubts have now been assuaged. LIGO has detected a fifth gravitational wave event. More importantly, this detection was then confirmed (for the first time) by observing the event itself in various parts of the electromagnetic spectrum.  As Science reports:

At 12:41 universal time on 17 August, physicists with three massive instruments—the twin 8-kilometer-long detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington, and Livingston, Louisiana, and the 6-kilometer Virgo detector near Pisa, Italy—spotted waves unlike any seen before. The four previous events lasted for, at most, a few seconds, with gravitational waves rippling at frequencies of tens of cycles per second. The new siren sang for 100 seconds at frequencies climbing to thousands of cycles per second. Whereas the earlier signal came from pairs of huge black holes quickly spiraling into each other, the new signal revealed lighter neutron stars, 1.1 and 1.6 times as massive as the sun, twirling inexorably together, researchers announced in parallel press conferences in Washington, D.C., and Garching, Germany.

Confirmation by three gravitational wave detectors is nice, but there’s more:

Because all three gravitational-wave detectors saw the signal, physicists could triangulate and locate the source to within a 30-square-degree patch of sky—about 60 times the size of the moon and much more precise than Fermi’s localization. Astronomers swiveled telescopes large and small to the spot in the constellation Hydra. The search got off to a slow start because that part of sky was in daylight for many observatories. But within hours, five groups had identified a new source of light in the periphery of galaxy NGC 4993, which they watched fade from bright blue to dim red in a matter of days. Nearly 2 weeks later, the source began to emit x-rays and radio waves.

The difference for this detection is that it was of neutron stars colliding, not black holes. Black holes are black, you can’t see them directly. But we can see neutron stars. They emit gamma rays, x-rays, and radio waves (what astronomers call an “optical counterpart”). This was no artifact – this was a real celestial event.

I appear not to be alone in my sentiments. Astronomer Andrew Howell is quoted as saying:

 “Sometimes I wonder whether we’re all just mucking around,” Howell says. “It’s moments like this that reassure me that science works.”

This event had some more scientific pay dirt as well. The colliding neutron stars formed what is called a kilonova – thousands of times more bright than an ordinary nova.

Further, our models predict less production of heavy elements like gold and platinum in supernova than we actually observe in the universe. Where are those extra heavy elements coming from? It was hypothesized that they were coming from neutron start collisions like this one. Indeed – astronomers observed clouds of heavy elements around the kilonova – planets worth of gold and platinum.

I find it ironic that the ultimate gravitational wave confirmation comes so soon after this new science was awarded the Nobel. But it’s actually not that much of a coincidence. Once the detectors were up and running, the gravitational waves started coming in.

This is the birth of an entire new field of astronomy, a new way to image the universe. Yet again, Einstein is vindicated, and this time we actually exceeded his technological expectations. Sometimes we can be clever little apes.


9 responses so far

9 thoughts on “More Gravitational Waves”

  1. SteveA says:

    Despite being a hard SciFi fan, I’ve never had much interest in astronomy and the mechanics of the cosmos. However, the idea of neutron stars colliding gives me chills (nice ones). Quite a ‘Tears in Rain’ moment.

  2. RickK says:

    What I find mind-bendingly fascinating to contemplate is how fast these neutron stars and black holes must be moving as they spiral into each other. The gravitational wave spikes LIGO has detected are so fast – just quick little chirps when rendered as audio. Unless I’m visualizing this wrong, they represent super-massive objects spiraling around each other for thousands of revolutions per second before colliding and merging. The violence of that motion and collision are truly awesome to imagine.

  3. RickK says:

    Steve said: “The little doubt I had in the back of my mind was that, in order to detect the tiny gravitational waves they have to eliminate all sources of even the slightest interference.”

    Yes, particularly considering how quickly they detected the first hit once the new, more sensitive detectors were on line. However, the fact that the events were “heard” by separate detectors thousands of miles apart reduced a lot of that doubt. Also, the wave pattern detected matched exactly to the wave pattern predicted, which is also the sign of good, successful science.

  4. SFinkster says:

    Science is sofa king cool!!

  5. Daniel Hawkins says:

    Probably an unpopular opinion, but I don’t think the detection of gravitational waves deserved a Nobel prize. Or rather, I think the Nobel should have gone to Einstein for their prediction, and for General Relativity as a whole theory, but that wasn’t an option.

    The existence of gravitational waves is not surprising—it would have been far more surprising if we didn’t detect them despite calculating that our instruments were more than sensitive enough to do so. The detection of them was a tour-de-force in measurement science, and an impressive accomplishment to be sure, but we’re only now at the stage where we can barely detect the “loudest” signals in the universe.

    If in 15 or 20 years, we use gravitational wave detectors to make observations that lead to a refinement of the Standard Model (or a rejection of it) then I could understand award a prize to the people that got it. But now seems premature.

  6. JimV says:

    From Alfred Nobel’s will:

    … The whole of my remaining realizable estate shall be dealt with in the following way: the capital, invested in safe securities by my executors, shall constitute a fund, the interest on which shall be annually distributed in the form of prizes to those who, during the preceding year, shall have conferred the greatest benefit to mankind. The said interest shall be divided into five equal parts, which shall be apportioned as follows: one part to the person who shall have made the most important discovery or invention within the field of physics …

    The statement of the 2017 Nobel Committee:

    The Nobel Prize in Physics 2017 was divided, one half awarded to Rainer Weiss, the other half jointly to Barry C. Barish and Kip S. Thorne “for decisive contributions to the LIGO detector and the observation of gravitational waves”.

    So it seems they considered the invention of the LIGO detector to be an important contribution to mankind in the field of physics. (For what little it is worth, I concur. I can’t think of a more important one during the past year.)

  7. Charon says:

    I’m curious who Daniel Hawkins would allow to have the physics Nobel. I’m guessing nearly all awardees wouldn’t qualify under his criteria (essentially no experiment would, including the Higgs boson detection or the Hulse-Taylor pulsar…). Clearly on-Earth gravitational wave detection is a landmark moment in physics.

    Steve, note that VIRGO also detected one of the merger signals prior to the Nobel being awarded. As RickK said, the two detectors matching each other (with appropriate delays), and matching theory so well did make this a pretty sure thing. The VIRGO detection sealed the deal. (Also the GRB detection was prior to the award of the Nobel. Yes, it wasn’t publicly announced beforehand, but it was somewhat common knowledge – with 70 observatories in on the study, it could hardly be otherwise.)

  8. RickK says:

    Charon said: “…the two detectors matching each other (with appropriate delays)…”

    As the guy said in the movie “Contact”: “Whatever it is, it ain’t local!”

  9. Pete A says:

    Not meaning to nitpick for the sake of it, but it’s not “VIRGO”, it was the “Virgo interferometer” located near Pisa, Italy.

    Whereas “LIGO” is capitalized because it’s an acronym for: Laser Interferometer Gravitational-Wave Observatory.

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