How a chirp won the Nobel prize in physics


Gravitational waves won the Nobel prize in physics today! Well, actually, it was the people who led the way in getting the instruments built that detected gravitational waves for the first time, they won the prize. It was such a great breakthrough!

A fun cosmology book I read a few years ago (yes, cosmology is fun!), called Black Holes & Time Warps: Einstein's Outrageous Legacy, is by Kip Thorne, one of the three winners announced today. Reading that book, watching him on YouTube a couple of times, and hearing him on the radio a couple of times, inspired me to write this brief appreciation of why he deserves the big prize, along with the two others who share the honor with him

At LIGO in Hanford. (The Washington Post)
Back in the year 2000, after the world did not crash, as some had feared, due to a software artifact (those who lived through this will remember what I mean), we took a group of science students from Omak to the LIGO detector at Hanford. They gave us a tour, the students asked a lot of questions, and we were very impressed.

I left thinking it was a long shot, a shot in the dark if you will, for that instrument to ever be able to detect gravitational waves. At that time, they were still not past working out how to eliminate all the noise the instrument picked up, including waves crashing on the ocean coast over a hundred miles away during storms, trucks passing by on the nearby highway, and so on, signals orders of magnitude larger than the distortions caused by gravitational waves which they were looking for, waves which warp spacetime on spatial scales the size of an atom - at least, that is what Thorne and the others had calculated that the big gravitational waves would do. Good luck with that!

At LIGO in Hanford. (The Washington Post)
But about 15 years later, they worked out all the problems in the instruments, mainly by engineering and installing better and better hardware and electronics in the LIGO instrument, partly by writing better and better software, and then, wouldn't you know it, they really detected gravitational waves, waves created when, in a galaxy far away, two extra-big black holes fell into each other and merged. Massively. I was so excited when I read this news and saw the signal of the black holes merging, and I kicked myself for being so skeptical of the prospects after we toured LIGO.

Kip Thorne. (LiveScience)
That is why this Nobel prize is so deserved. Kip Thorne began working on this back in the 1970s and 80s. He worked the equations, performed the calculations, and wrote the logical arguments, and published papers, showing how gravitational waves could be detected by this method, and the secrets of the universe they would reveal. He did talks, interviews, meetings, and lobbying. He co-authored big grant proposals, proposing how this method would detect gravitational waves.

Thorne was a lead strategist in getting the whole multi-LIGO system up and keeping it running (the second LIGO installation was in the state of Louisiana, and that second instrument was necessary to be sure of detecting gravitational waves), and he kept with it, year after year, for decades. All while doing other research and teaching classes at CalTech, and, who knows, maybe even having a life.

Graphic art visualization of merging pair of black holes. To say it the old Newtonian way, due to acceleration by gravity, the black holes revolve around each other faster and faster until they collide and become one. In relativistic terms, this produces a distinctive set of gravitational waves that move out in all directions at the speed of light. (Quanta Magazine)

So while there is no doubt that Thorne is one of the leading physicists in the study of gravity and relativity in recent decades, it wasn't just his prowess in being able to solve equations in theoretical physics that earned him the Nobel prize. It was also his insistence that the detection of gravitational waves was technologically feasible, and his leadership in making it happen.


The two other co-winners of the Nobel prize in physics this year, for the detection of gravitational waves, along with Kip Thorne, are Rainer Weiss and Barry Barish.

To give credit to the many other people who made LIGO able to detect gravitational waves, see this article in Popular Science, "Three men just won a Nobel Prize for the work of more than a thousand people." Along with listing all the names, it points out some examples of what selected individuals have done to make LIGO work.

For example, "Sheila Dwyer, Hanford Observatory: Ripples in spacetime make LIGO’s 2.5-mile-long arms quiver so its lasers hit detector mirrors inside. Dwyer kept the mirrors aligned."


It is a fun exercise to study the graphs and illustrations of how the black hole merger worked, and how spot-on the signal detected by LIGO is, so I'll end with this run-through of the visuals.

The signal of a pair of black holes falling into each other (above), detected by LIGO Hanford (in Washington) and LIGO Livingston (in Louisiana). The theoretical form of the signal in the middle graphs, calculated from relativity theory, is subtracted from the signal measured by the data to produce the residual curves in the bottom graphs. The residuals are the "noise," from other sources besides the black hole merger, that the instruments also picked up. Even with the noise in the signal, analysis of the two curves (one from each instrument) along with analyzing the curve made by combining the two together, finds they match the theoretical signal to a high level of certainty, ruling out random coincidence. In other words, it's a beautiful thing!
(Physical Review Letters)









This graph (above), which includes all the noise, shows the frequency of the same gravitational waves shown in the previous graph, on the same time scale. The frequency corresponds to how close together the waves in the previous graph are: the closer together, the higher the frequency. Imagine these black holes, each of them many times the mass of our sun, circling in closer and closer, faster and faster...until they merge into a single black hole.

This Physical Review Letters web page has the full scientific paper that was published to announce the first verified detection of gravitational waves, the waves from this black hole merger.


This diagram with graphs (above) illustrates the black holes merging (top) and the predicted pattern of gravitational waves (middle, in terms of strain), red line. 

The red line, from relativity theory, is superimposed on the signal detected by the LIGO instrument at Hanford, in gray, which has the noise (residual) stripped out. What a match! 

That one graph, in red (theory) compared to gray (measurement), is compelling evidence of two things:
  1. Einstein was correct in his prediction of gravitational waves.

  2. Black holes exist.
The bottom graph shows how the two black holes went faster and faster (green) as their distance apart got smaller and smaller (black), leading up to the merger.



"The Chirp from Space"
OK, I said this would be run-through of the "visuals," but these gravitational waves can be simulated as sound waves, enabling us to hear them. The frequencies shown in the graphs, 30-500 waves per second, are at the low end of human hearing range. No problem - we can adjust the sound simulation to a higher setting, making it easier to hear the pattern the sound makes, which is the key to hearing the famous chirp.

To hear a sound simulation of the chirp made in spacetime by the black hole merger, open this video on the Space.com web site.

In this short video, I like how the sound of black holes merging is introduced and explained by Dr. Gabriela Gonzalez, spokesperson for LIGO.

(The Washington Post)

Comments

Post a Comment