Catching gravitational waves

OK, seems like I am on a track of rebuilding my daily routine under new circumstances of the offline life. So it’s a good time to incorporate some blogging activity in it.

While I was away, some fascinating things happened. First of all I mean the direct observation of gravitational waves, which were theoretically predicted by Einstein in 1915 (or Poincare in 1905, if you stretch your definition of “prediction”). As with all fundamental physics experiments, the measurement was not a trivial one. It required construction of two interferometers each having two orthogonal 4 km-long tubes to detect distortion of the spacetime by a fraction of a proton diameter.

The concept of spacetime ripples is so mind-boggling and escaping the common sense logic that the discovery faced a lot of skepticism (see for example commentaries for that Nature summary). Frankly, it took my full mental power and a significant share of pure faith to digest the discovery. The most concerns that popped out in my head after reading press-releases were:

  • I didn’t see an estimation of how far apart are those waves generated by collapsing black holes. It actually seems to be a pretty damn good timing to build LIGO these days and detect gravitational waves from an event that happened 0.6 to 1.8 billion years ago.
  • Is it really possible to back-calculate the direction of the gravitational wave front? At billion light-years scale, two interferometers are essentially located in one point. And 1.2 billion light-years-deep spherical volume element seems to be quite a large chunk of space, at least for a non-astronomer.
  • The detection itself seems to be quite unlikely event as spacetime stretching can occur in any direction in three-dimensional space, while detection is most effective along the axes of the interferometer.

So I took a look into the paper published by LIGO collaboration (open access) and found the answers to all my concerns.

gw-fig1
Raw signals, theoretically fitted wave, and frequency-time plots for detected gravitational waves (source)

First, I was fooled by a number of nice animations (including one from LIGO) of what exactly was detected. I thought it was one of those waves produced by spiraling black holes, so we could expect more of them coming periodically. But in fact that sinusoidal line reflects the last 200 milliseconds of two black holes merger. So now it makes sense because gravitational waves are transmitting, well, gravity. Unlike electromagnetic waves, once gravitational force is established, it’s there forever (while the matter itself exists), and the fabric of spacetime stays still until the next powerful event happens somewhere in the Universe.

gw-fig2a
Phases of black holes merger corresponding to the phases of the detected wave (source)

So what about the timing and location of the detected event? As expected, the location of the source of gravitational waves was impossible to calculate precisely but quite possible to narrow down to 600 deg2 (which is ~1.5% of the sphere surface). The observation was followed by the burst of gamma-ray radiation 0.4 second later, in approximately the same part of the sky, which was detected by Fermi telescope. However, the spatial and temporal coincidence of these two events doesn’t absolutely mean the common origin (actually, it contradicts to what is known about black hole binaries). So there are actually no ways to independently validate LIGO’s finding.

sky-map
Approximate area, where gravitation waves came from (source)

 

In fact, the gravitational wave detected on September 14, 2015 (dubbed GW150914) was not the only but certainly the strongest signal detected by LIGO. The other one, “candidate” signal LVT151012 (October 12, 2015) was much weaker and didn’t make it into press-releases.

For the detection part of the story again my lay understanding was wrong. The gravitational wave actually has to pass orthogonally through the detector “plane” with polarization vector parallel to the plane, for the most effective stretching of the detector. The time difference between signal observations by two detectors was 6.9 ms, which means that the direction of wave propagation was sub-optimal but still with significant z-component (if it was going straight from detector H1 to L1, it would make 10 ms delay).

ligoDetector
Location and relative orientation of two LIGO detectors; schematic of each detector; and the instrument noise (source)

So I was left with no doubts about the discovery. Seems like those who oppose the results are simply opponents of the general theory of relativity. Since I have no problem with the theory, I welcome gravitational waves as the great discovery of 2016, which has all chances of being considered as the discovery of the century!

P.S. The last couple of months are actually extremely rich for fundamental discoveries: completion of 7th row of periodic table, new planet in the Solar system and now gravitational waves. It is a really exciting time to live in!

P.P.S. You can even listen how gravitational waves would ‘sound’ if they were able to actually move molecules in the air.

 

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Author: Slava Bernat

I did my PhD in medicinal chemistry/chemical biology of G protein-coupled receptors and then explored some chemical biology of non-coding RNA as a postdoc. Currently I'm working in a small biotech company in San-Francisco Bay area as a research chemist. I'm writing about science, which catches my attention in rss feed reader and some random thoughts or tutorials.

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