Did LIGO Just Discover Two Fundamentally Different Types Of Neutron Star Mergers?
“The first neutron star-neutron star merger ever directly observed was seen in both gravitational waves and in various forms of light, giving us a window into the nature of short gamma ray bursts, kilonovae, and the origin of the heaviest elements of all. The second one, however, had no robustly confirmed electromagnetic counterpart at all. The only major physical differences were the combined mass (2.74 vs. 3.4 solar masses), the initial object formed (neutron star vs. black hole), and the distance to the event (130 vs. 518 million light-years).
It’s possible that there really was an electromagnetic counterpart, and we simply weren’t able to see it. However, it’s also possible that binary neutron star mergers that directly lead to a black hole don’t produce electromagnetic signatures or enriched, heavy elements at all. It’s possible that this binary neutron star system, the most massive one ever discovered to date, represents a fundamentally different class of objects than have ever been seen before. This incredible idea should get put to the test over the next few years, as gravitational wave detectors continue to find more and more of these mergers. If there are two different classes of neutron star mergers, LIGO and Virgo will lead us to that conclusion, but we have to wait for the scientific data to know for sure.”
Neutron stars, when they merge, can produce gamma ray bursts, ejecta, the heaviest elements of all, and electromagnetic afterglows that cover nearly the full spectrum of where light can exist. We saw this with a gravitational wave + gamma ray signal that occurred on August 17, 2017, leading to a revolutionary understanding of kilonovae. But our second merging binary neutron star system, which was seen in gravitational waves on April 25, 2019, displayed no such signal at all.
It might be a failure of detection, or it might be due to some fundamental differences between these different events. My bet’s on the latter; here’s what we think might be going on!
Advanced LIGO Just Got More Advanced Thanks To An All-New Quantum Enhancement
“The current observing run of LIGO has been going on since April of this year, and there are already more than double the number of candidate signals than the total number of signals from all previous runs combined. This isn’t due to using the same instruments for longer periods of time, but owes this newfound success to some very exciting upgrades, including this clever new technique of squeezed quantum states.
For decades, scientists have had the idea to leverage squeezed quantum states to reduce the quantum uncertainty in the most important quantities for gravitational wave detections. Thanks to hard work and remarkable advances made by the LIGO Scientific Collaboration, this new, third observing run is already seeing more success than any gravitational wave detector in history. By reducing the phase uncertainty in the quantum vacuum that LIGO’s photons experience, we’re in exactly the right position to make the next great breakthrough in astrophysics.”
Did you know that LIGO and Virgo have been engaged in a new observing run since April of this year? Have you heard that the new run is up to 50% more sensitive than prior runs? That’s true, and it’s due to a number of improvements in noise reduction, including one fascinating way to leverage and control how quantum uncertainty plays out. These squeezed quantum states enable you to put the uncertainty where you most want it, and measure the corresponding quantity even more precisely as a result.
Come find out how we’re bending the quantum rules of the Universe to our will for the benefit of science; it’s a remarkable story!
LIGO’s Lasers Can See Gravitational Waves, Even Though The Waves Stretch The Light Itself
“But this is where the puzzle comes in: if space itself is what’s expanding or compressing, then shouldn’t the light moving through the detectors be expanding or compressing too? And if that’s the case, shouldn’t the light travel the same number of wavelengths through the detector as it would have if the gravitational wave had never existed?
This seems like a real problem. Light is a wave, and what defines any individual photon is its frequency, which in turn defines both its wavelength (in a vacuum) and its energy. Light redshifts or blueshifts as the space it’s occupying stretches (for red) or contracts (for blue), but once the wave has finished passing through, the light returns to the same wavelength it was back when space was restored to its original state.
It seems as though light should produce the same interference pattern, regardless of gravitational waves.”
Have you ever thought about how gravitational wave detectors work? By passing light down two mutually perpendicular arms, reflecting them back and reconstructing an interference pattern, we can detect a passing wave by how it changes the arm-lengths of the light. But the light itself also gets compressed and expanded, and shouldn’t those effects cancel out?
Clearly, LIGO, Virgo and KAGRA all work, as many detected events bear out. But have you ever thought about how? Come get the answer today!
Is The Universe Filled With Black Holes That Shouldn’t Exist?
“What about at the high end of the stellar mass range of black holes? It’s true that pair instability supernovae are real and are indeed a limiting factor, as they don’t produce black holes. However, there’s an entirely separate way to produce black holes that is not particularly well understood at this time: direct collapse.
Whenever you have a large enough collection of mass, whether it’s in the form of a cloud of gas or a star or anywhere in between, there’s a chance that it can form a black hole directly: collapse due to insufficient pressure to hold it up against gravitation. For many years, simulations predicted that black holes should spontaneously arise through this process, but observations failed to see a confirmation. Then, a few years ago, one came in an unlikely place, as the Hubble Space Telescope saw a 25 solar mass star simply “disappear” without a supernova or other cataclysm. The only explanation? Direct collapse.”
As far as our best theories are concerned, the Universe isn’t filled with black holes of all different masses. Instead, the black holes that the Universe forms are inextricably linked to the processes by which the Universe makes the objects that then become black holes. From stars, there’s a theoretical lower limit of about 5 solar masses, and yet we saw a black hole of about 3 solar masses get created. There should be an enormous drop in black hole frequency above about 50 solar masses, but LIGO may be about to challenge that. And even at the highest end, there should be an upper limit to the masses of supermassive black holes, but a few of the ones we’ve found challenge that limit, too.
Does this mean the Universe is filled with black holes that shouldn’t exist? Or does it simply mean that we need superior models? Get the full story today.
Has LIGO Just Detected The ‘Trifecta’ Signal That All Astronomers Have Been Hoping For?
“Of course, all of this is just preliminary at this point. The LIGO collaboration has yet to announce a definitive detection of any type, and the IceCube event may turn out to be either a foreground, unrelated neutrino or a spurious event entirely. No electromagnetic signal has been announced, and there might not be one at all. Science moves slowly and carefully, as it should, and all of what’s been written here is a best-case scenario for the optimistic hopefuls out there, not a slam-dunk by any means.
But if we keep watching the sky in these three fundamentally different ways, and keep increasing and improving the precision at which we do so, it’s only a matter of time before the right natural event gives us the signal every astronomer has been waiting for. Just a generation ago, multi-messenger astronomy was nothing but a dream. Today, it’s not just the future of astronomy, but the present as well. There’s no moment in science quite as exciting as being on the cusp of an unprecedented breakthrough.”
There haven’t been any official releases, announcements, or claimed discoveries, but many of you may be aware that back in April, LIGO turned on again and began searching the Universe for gravitational waves, this time with improved range and sensitivity. Over that time, some 24 candidate events have been seen, and the most recent one, from July 28, 2019, is perhaps something special. Located 2.9 billion light years away and likely to be a black hole-black hole merger, it just happens to coincide with the arrival of a cosmic neutrino, in both space and time, as seen by IceCube.
Electromagnetic follow-ups are currently underway, and this could mark the first threefold multi-messenger astronomy signal ever! Watch this one closely, as it could herald a new dawn for astronomy in human history!
Ask Ethan: Why Haven’t We Found Gravitational Waves In Our Own Galaxy?
“Why are all the known gravitational wave sources (coalescing binaries) in the distant universe? Why none has been detected in our neighborhood? […] My guess (which is most probably wrong) is that the detectors need to be precisely aligned for any detection. Hence all the detection until now are serendipitous.”
On September 14, 2015, our view of the Universe changed forever with the first direct detection of gravitational waves. Since then, we’ve detected a variety of black hole and neutron star binaries in the final, end-stages of coalescence, culminating in a spectacular merger. But they’re all hundreds of millions or even billions of light-years away!
Simultaneously, we know that we have neutron stars and black holes in binary systems here in our own galaxy. But of all the gravitational waves that LIGO and Virgo have detected, none of these objects are among them. This remains true, even though we can identify many of them from their electromagnetic signatures.
Why haven’t we found gravitational waves in our own galaxy? Give us a better observatory and we will! Here’s the full scientific story on that.
Ask Ethan: Why Don’t Gravitational Waves Get Weaker Like The Gravitational Force Does?
“You have stated:
1) The strength of gravity varies with the square of the distance.
2) The strength of gravity waves, as detected by LIGO, varies directly with the distance.
So the question is, how can those two be the same thing?”
Here’s a puzzling fact for you: if you get ten times as far away from a source of gravitational waves, how much less would you expect the signal to be in your gravitational wave detector? For light, brightness falls off as the inverse of the distance squared: it would be 1/100th as bright. For the gravitational force, it also falls off as the inverse of distance squared: 1/100th the force. But for gravitational waves, the signal strength only drops as the inverse of the distance; the signal would be 1/10th the original strength.
Why is this? Believe it or not, it’s mandated by physics! Come find out the deep truth behind why on this special* edition of Ask Ethan!
(* – special because I had to derive this myself; nobody gives the full explanation anywhere I can find!)
Ask Ethan: Do Merging Black Holes Create An Information-Loss Paradox?
“When black holes merge they [lose] energy through gravitational waves. Does this pose the same problem as Hawking radiation does, with respect to loss of information? Or is the information on what has gone into the black hole somehow encoded into the gravitational wave? And if it is could we someday hope to decode what went into the black hole using gravitational waves?”
Black holes and entropy have long been a problem. If you calculate a black hole’s temperature and entropy in General Relativity alone, you get an absurd answer: 0. That implies that the pre-existing object that forms a black hole, which definitely has entropy, would see it disappear when it formed a black hole. But the entropy of a system can never decrease, implying that black holes must have an entropy after all: encoded on the surface of the event horizon. So let’s take two black holes with entropy on their event horizon and merge them together. What happens? You form a larger black hole with a larger event horizon, while gravitational waves carry away roughly 5% of that mass in the form of gravitational waves.
Who gets the entropy? Is it still conserved? And where does it go, in theory and in practice? Here’s what we know so far, for the final Ask Ethan of 2018!
Five Surprising Truths About Black Holes From LIGO
“1.) The largest merging black holes are the easiest to see, and they don’t appear to get larger than about 50 solar masses. One of the best things about looking for gravitational waves is that it’s easier to see them from farther away than it is for a light source. Stars appear dimmer in proportion to their distance squared: a star 10 times the distance is just one-hundredth as bright. But gravitational waves are dimmer in direct proportion to distance: merging black holes 10 times as far away produce 10% the signal.
As a result, we can see very massive objects to very great distances, and yet we don’t see black holes merging with 75, 100, 150, or 200+ solar masses. 20-to-50 solar masses are common, but we haven’t seen anything above that yet. Perhaps the black holes arising from ultra-massive stars truly are rare.”
Well, the LIGO and Virgo collaborations have put their head together to re-analyze the full suite of their scientific data, and guess what they found: a total of 11 merger events, including ten black hole-black hole mergers. Now that we’re in the double digits, we can actually start to say some incredibly meaningful things about what’s present in the Universe, what we’ve seen, and what that means for what comes next. Which black holes are the most common? How frequently do these mergers occur? What’s the highest-mass black hole binary that LIGO could detect? And how many black holes do we expect to find when run III starts up in 2019?
The answers are coming! Come see what we know so far, and learn five surprising truths about black holes, courtesy of our findings from LIGO!
Ask Ethan: If Light Contracts And Expands With Space, How Do We Detect Gravitational Waves?
“If the wavelength of light stretches and contracts with space-time, then how can LIGO detect gravitational waves. [Those waves] stretch and contract the two arms of the LIGO detector and so the the light waves within the the two arms [must] stretch and contract too. Wouldn’t the number of wavelengths of light in each arm remain the same hence cause no change in the interference pattern, rendering [gravitational waves] undetectable?”
Three years ago, we detected the very first gravitational wave ever seen, as the signal from two massive, merging black holes rippled through the Universe, carrying with it the energy of three solar masses turned into pure energy via Einstein’s E = mc^2. Since that time, we’ve discovered more gravitational waves, mostly from black hole-black hole mergers but also from a neutron star-neutron star merger.
But how did we do it? The LIGO detectors function by having two perpendicular laser beams bounce back-and-forth in a long vacuum chamber, only to recombine them at the end. As the gravitational waves pass through, the arm lengths extend and compress, changing the path length. But the wavelength of the light inside changes, too! Doesn’t this mean the effects should cancel out, and we shouldn’t see an interference pattern?
It’s what you might intuit, but it’s not right. The scientific truth is fascinating, and allows us to detect these waves anyway. Here’s how it all works!