Ripples in spacetime - from 1.3 billion light years away

by Mandeep S.S. Gill

By now most of you who are astro-enthusiasts have already heard the news originally announced in February 2016 of the gravitational wave event observed by Advanced LIGO in September 2015, and perhaps also heard a bit about how excited astrophysicists were about it.

LIGO Livingston site, showing the two 4km arms joined at a right angle. (Credit: Caltech/MIT/LIGO Laboratory.)

As for why we were all so enthused, maybe the simplest explanation is this: we have grown a new sense and have for the first time heard the ripples in spacetime emanating from two colliding black holes, spreading out throughout the Universe, and gently jiggling the Earth as they pass us by, in a way humans have never been able to before.

Above: Gravitational waves rippling out into the fabric of spacetime from a cataclysmic merger of two massive objects (such as black holes). (Courtesy Caltech/MIT/LIGO Laboratory.)

Gaining a new sense to detect the Universe in is a pretty big deal in anyone's book, at any point.

And in fact, gaining a new sensory modality is a decent analogy; e.g., we might compare gaining the ability to detect gravitational waves to when adults who have been deaf since birth first get cochlear implants—if you have ever seen their whole-body emotional reactions when the implants are turned on (e.g., see here), you will get some sense of what the hundreds who have worked so hard, and the thousands who have waited so long—25, 40, 100 years, depending on how you count it—felt after this announcement.

Above: The mechanics of a cochlear implant, showing the new sensory modality inserting information through sensors directly into the human brain (from this source).

There are any number of easy-to-find links about what it means, what we can learn from it in the future, how many more we may expect to see in the coming years. But as far as the reactions that the scientists felt that first day of the official announcement, here’s one link directly from the press conference the morning of February 11, 2016, in Washington, D.C.

It's definitely long, over an hour, and gets technical in places later—but if you want a sense of how excited people were about this when it was first announced, skip right to 20 minutes in to watch the well-done and stirring two-minute clip about what it means, then watch LIGO Lab Executive Director David Reitze announce, in no uncertain terms, straight up—that "WE DID IT."

(Reitze actually gave a nice talk at Stanford in May of 2015 and while it left many of us eagerly anticipating Advanced LIGO turn-on, he didn’t promise anything about how quickly the first events would be seen, and no one had any guarantees that anything would be seen so soon.)

As for my personal response, here is an edited version of an email I sent to some former students for whom I was a teaching assistant (in a beginning astronomy class the summer of 2015):

-------------------------------------------------------------------- Date: Thu, 11 Feb 2016:

heya Peter + Sean-

Glad you're psyched—I posted on FB [Facebook] a couple of days ago when the announcement date was publicized—but it's very cool that it's confirmed. I'd known of the rumors for a couple of months, like all of my colleagues, but I but was stunned when it was actually confirmed by the LIGO team in the last few days that it was really true. I remember the exact moment I saw the email, just as we remember those critical moments in our lives, clearly.

Above: Reenactment by J.Meyers of the reaction upon realizing the rumors about the discovery were true. (Credit: M.Gill.)

I did hear it though astronomy collaboration channels a few days before the official public announcement (though much after all the rumors had been already swirling, and months after the actual observation date in September 2015, when the 1000+ LIGO team members knew it right away)—and it was an amazing, fantabulous, jaw-drop-inducing moment when i actually realized it was true. But we all had to stay totally mum for several days, not even talking to colleagues who weren't in the know—but those who were, were totally buzzing about this.

Above: Small subset of the LIGO-Livingston team in cleanroom attire (Credit: LIGO Laboratory.)

Here's the deal—we knew it would happen someday, almost for sure, we just had no idea if that would be several years from now, because it had always seemed just around the corner for many years as they steadily improved the machinery, and no one had any hard estimate for the rates of neutron star or stellar mass black hole coalescences in an average galaxy.

But then out of the blue—they turned on the amped up version of the original gravitational wave detector (now called Advanced LIGO), and boom, there it was—two big black holes, each about 30 times the mass of the Sun, spinning around each other 1.3 billion light years away, then inspiraling, colliding and coalescing into one larger one, and for about a fifth of a second, sending power outward and sending out 50 times more energy in that instant in gravitational waves than is sent out from all the stars shining in the entire OBSERVABLE UNIVERSE(!!!).

Above: The three stages of the merger event that define different parts of the gravitational waveform signal that LIGO sees. (Credit: LIGO/A. Simonnet.)

That's what the conversion of three Solar masses to pure gravitational energy will do (through E = mc2—that c is one dang big number that we take for granted far too oten).

Above: Classic picture of Herr Einstein writing up his most famous equation on a chalkboard (yes, from an era where calcium carbonate was popularly used to good effect for these sorts of efforts).

So yeah, I'm rather psyched. These moments come only rarely in a scientist's life—and there are many scientists who have been working or waiting for this longer than you both have been alive, my young friends. :-)

-M

 

---------------------------------- And now a little more detailed info given in FAQ form (questions were from friends of mine on FB), in an Appendix:

Q: There has been a fair amount of discussion about a coincident GRB (gamma-ray burst) detected by the Fermi satellite at about the same time as the GW (gravitational wave) event. They are inconclusive about it and say it would be unexpected to have a GRB (gamma-ray burst) with a black hole binary collision. Is it likely to be connected? (By Tom M.)

A: The upshot is that the signal was not very strong at all, and the correlation with that GRB is quite weak, so most astrophysicists don't believe it's really connected. As GW astronomy really begins in earnest this year, and beyond, and we finally see our first real EM (electromagnetic) counterpart at some point, we'll know if these black hole-black hole collisions could cause GRBs of any form. The scenarios to do so seem far-fetched so It's not very likely, is what most would say.

Above: Two neutron stars colliding, giving off GW’s and shining in bright gamma rays as well. (Courtesy NASA.)

Q: But about that signal—wouldn't the EM signal be received at the same time as the GW signal? By the time the teams get telescope time to look for EM signals, wouldn't it be too late? Or would they be doing retrospective analysis of some recordings that were being done anyway? (Also Tom M.)

A: So this is quite right that if all the photons were coming from the same place at the same and having no interference in between, they'll get to us at the same time, but these are very messy and complex processes and that's generally not what happens.

E.g., when a supernova goes off, know what gets to us first? The neutrino burst from the core collapse (there were ~13 seen in SN 1987A that went off in the LMC). Because the opacity is just too high for photons to escape at that point. (And this is even though neutrinos have mass by the way, but their kinetic energy is much higher than their rest-mass energy in these supernova explosions, so they still beat the first photons by a big lead in getting to us.)

Above: artist’s conception of neutrinos being emitted from a supernova, and arriving earlier than the photons from the event. (Credit: Sandbox Studios, Chicago.)

So, coming back to this—we know now that the gamma-ray signal was seen, 0.4s after the GW burst (which remember was just ~0.2s long) in the same general area—which for both signals, is a fair fraction of the sky, by the way (like 500 sq deg, of the 41 thousand sq deg of the full sky). And several other gamma-ray events of similar size were seen in that general area over the two day window they looked in. So it's likely a coincidence, on this one. But future events will reveal much more to us (e.g. the GBM (Gamma-ray Burst Monitor) on Fermi sees on average 70% of the sky at any time, so it works as a near 'whole sky monitor' and will often overlap the entire GW localization area).

Above: The packing of the various instruments, including the GBM, into Fermi, prior to launch. (Credit: NASA.)

But here's the deal—if the BHs had no accretion disks at the point when they inspiralled and coalesced, which is what simulations generally show happening by the time a BH-BH collision takes place (i.e., that through powerful gravitational interactions, they clear out the common disk of material between them in the final phase of coalescence) there will be nothing visible in the EM spectrum in the end. That is—pure BHs with no matter outside the event horizon will show zero EM signal upon final coalescence.

(There are some exotic theories that predict some kind of EM counterpart in these BH collisions, but even if they’re right, most assuredly there will not be much of an EM signal, compared to, for example, a supernova.)

So... there probably is not a real EM signal on this one, is my guess.

Q: To sum it up, after the GW event, they went back to look at the data Fermi had recorded and found what is a coincident GRB, but it is not yet clear whether that is consistent with it being from the same BH (black hole) collision event, yes? I would assume that, since the LIGO folks asked the EM observatories for coincident observations, that there is reason to believe it is possible. (Tom M. and Dennis G.)

A: In the case of Fermi, yes, they looked at archival data from that time, and yes, you're right—we're not sure yet.

Above: tapes upon tapes of archival data (in this case, at NASA Ames research center, in Mountain View, CA, and the hexagonal shape in the foreground is the pattern of the separator fence in front). (Credit: M. Gill.)

For the other telescopes, they asked them to look as soon as they could because, e.g., a kilonova event expected from a neutron star collision should have a 10-day decay from its peak brightness (where normal classes of supernovae have 60-day (or sometimes more) lightcurve decays as they get dimmer) so there was just a hope that something might be seen, if there had indeed been some colliding accretion disks or whatever. Anyway, people definitely wanted to make the effort to look, for this first-ever event.

Above: the lightcurves (i.e. brightness vs. time) of two types of supernovae. (From Astronomy Today, 3rd Ed., by Eric Chaisson and SteveMcMillan, Prentice-Hall. Image © 2005 Pearson Prentice-Hall, Inc.)

Q: The articles say the merger generated a lot of brightness, and also that all the energy was released as GWs. Do GWs also look bright unto themselves, like light? (from Jacqueline R.)

A: No, that's the interesting thing—GWs are totally different from photons; as “ripples in spacetime” they stretch and squeeze matter by tiny fractions as they pass through, so when any article says this BH collision was “bright,” it's only in this kind of emitted energy that our other normal telescopes (of any form) can't see.

The best analogy probably is if you make some immense sound of crashing cymbals or an intense airhorn or something in an otherwise totally dark room, you know you could never see it with your eyes, or with a camera (without a flash)—it wouldn't even occur to you to detect it in this way, naturally enough. You know you would need to use sound detectors (like your ears).

And for the first time in human history—we just heard last year what it sounds like when two massive black holes whirl themselves into a crazy frenzy and then careen into each other in a final fraction of a second and merge, perturbing spacetime so intensely that we eventually hear the faint echoes of this cataclysm, even at 1.3 billion light years away...

----------- For further viewing:

For one other personal connection, for those who want to check it out, i'm going to pass on this cool photo album posted publicly by a friend who gave me a tour of the LIGO lab instrumentation at Caltech in 2007(these are his own pictures of it though, vs. my old ones, and this does require a FB acct).

Research on mirror coatings at SLAC that could make the LIGO detectors ultimately 10 times more sensitive than the currently are (from July 19, 2016).