As you’ve no doubt already heard, it was announced earlier today that the Laser Interferometer Gravitational-wave Observatory (LIGO) has detected gravitational waves1 — i.e., ripples in space-time, which in this case were produced by a merger of two black holes that took place about 1.3 billion years ago (and, accordingly, 1.3 billion light years away) over the course of about 20 milliseconds.
Yes, ripples in space-time are a real thing; they are a predication of the theory of general relativity (which last year celebrated its centennial) and now they have also been measured. Serendipitously, the measurement itself also took place last year, on September 14, 2015 at 09:50:45 UTC (i.e., in the middle of the night at the detectors themselves, which are located in the U.S.), when those ripples finally reached Earth2.
This is a really, really impressive achievement. It involved decades of work on the part of thousands of people1, detectors that are miles in length, and the measuring of distances a tiny fraction of the size of a proton.
It’s thrilling to see how many quality write-ups there are out there about the news (e.g., BBC, NYT, NPR). You can get an explanation of the physics in comic form or in more detail, read about how the detection event went down (i.e., not exactly as planned, in that the machine hadn’t quite started its official “experiment run”), read an eloquent retrospective from a fellow Caltech professor, or even get a tour inside the facility.
Given all that great material, I don’t have much more to add in terms of explaining the physics . . . but watching the National Science Foundation’s live stream of the press conference produced a few highlights (besides the science itself, of course), which I didn’t want to let go unremarked.
- At one point, close to 84,000 people were watching the stream on that YouTube page alone. That’s not quite TV ratings numbers, but it’s not bad for a handful of physicists showing videos of their research!
- The paper was published not in Nature or Science (which certainly publish a lot of good stuff, but also have issues with hype and access), but in Physical Review Letters — the physicist’s physics journal — under the terms of a Creative Commons license.
- “That’s the chirp we’ve been looking for.” — Gabriela González, after having just played the sound of two black holes merging (Despite the very large and very small scales involve, the frequency of the signal happens to be in the range that humans can hear.)
- “He was smart enough, and he knew enough physics. He wasn’t just a theorist.” — Ray Weiss, about Einstein, just as Weiss is about to introduce his colleague, the eminent theorist Kip Thorne (whom a clearly uncomfortable Weiss quickly assured us is “both a theorist and really an experimenter”)
- “I know it in kilometers also.” — David Reitze, about the limit that the paper places on the mass of the graviton (Neither he nor Thorne knew off the top of their head what the number is in such conventional units as grams or kilograms.)
Furthermore, here are some fun images from the event, put together into a gif:
The events each frame depicts are:
- Applause right after Reitze announced, “We did it.”
- Two black holes, about to merge together.
- Gravitational waves produced by the merger, propagating out from the single black hole that remains.
- The signal detected in Livingston, Louisiana . . .
- . . . alongside the signal detected in Hanford, Washington.
- Both signals on the same graph, with the Hanford signal shifted by the time it takes for the waves to travel between them (7 milliseconds). What beautiful data!
- “That’s the chirp we’ve been looking for.” (as the view faded from the video back to the speaker)
- How LIGO works. (Each “arm” of the interferometer is four kilometers long.)
- The “fabric” of space-time 40 milliseconds before the black holes merged.
To replay the individual videos shown during the press conference (which are much better than the gif-compressed images above) or relive the press conference in full, just head on over to ligo.org.
Finally, I’d like to share two of my favorite physicists’ reactions, both of whom have captured the spirit of the day better than anything I’ve come up with:
“My favorite part was the NSF director’s intro. It sounds the like the premise of a Star Trek movie — just the way she says ‘laser interferometer gravitational observatory’, like it is going to detect a space-time anomaly in the gamma quadrant that the Enterprise is going to have to investigate. I guess the point is that humans achieved the exact sort of thing that Star Trek would show as an aspirational goal.” — Brian Standley
and more simply:
“The human race has detected the collision of two black holes 1.2 billion years ago, by measuring a flutter of less than four thousandths the diameter of a proton. It’s a good day to be alive.” — Jessie Rosenberg
1. Well, more precisely, gravitational waves were detected by the team of collaborators who built and upgraded and calibrate the instrument, who analyze the data, who run simulations, who do theoretical calculations . . . The journal article — which was published online at the same time the announcement was made (after having already gone through the peer review process) — is 16 pages long, the last six of which are devoted a list of something like a thousand authors (I didn’t count exactly), of whom all but three are still alive, and their 133 affiliations.
2. Why did it take almost five months before it was announced, then? Because this is the kind of result that you want to be really sure about before telling the world — not just that there wasn’t an error of some kind (e.g., a “test” signal that was sent through the system by accident, or as a prank), but also that the analysis is done very thoroughly and very correctly.