## 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 In…

“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!

## Black Hole Mergers Might Actually Make Gamma-Ray Bursts, After All

“If there is a gamma-ray signal associated with black hole-black hole mergers, it heralds a revolution in physics. Black holes may have accretion disks and may often have infalling matter surrounding them, being drawn in from the interstellar medium. In the case of binary black holes, there may also be the remnants of planets and the progenitor stars floating around, as well as the potential to be housed in a messy, star-forming region. But the central black holes themselves cannot emit any radiation. If something’s emitted from their location, it must be due to the accelerated matter surrounding them. In the absence of magnetic fields anywhere near the strength of neutron stars, it’s unclear how such an energetic burst could be generated.”

In 2015, the very first black hole-black hole merger was seen by the LIGO detectors. Interestingly, the NASA Fermi team claimed the detection of a transient event well above their noise floor, beginning just 0.4 seconds after the arrival of the gravitational wave signal. On the other hand, the other gamma-ray detector in space, ESA’s Integral, not only saw nothing, but claimed the Fermi analysis was flawed. Subsequent black hole-black hole mergers showed no such signal, but they were all of far lower masses than that very first signal from September 14, 2015. Now, however, a reanalysis of the data is available from the Fermi team themselves, validating their method and indicating that, indeed, a 3-sigma result was seen during that time. It doesn’t necessarily mean that there was something real there, but it’s suggestive enough that it’s mandatory we continue to look for electromagnetic counterparts to black hole-black hole mergers.

The Universe continues to be full of surprises, and the idea that black hole mergers may make gamma-rays, after all, would be a revolutionary one! Come get the full story today.

## Ask Ethan: Why Did Light Arrive 1.7 Seconds After Gravitational Waves In The Neutron Star Merger?

“Please discuss significance of the 1.7 sec. difference in arrival time between GW and Gamma Ray burst for the recent Neutron star event.”

Every massless particle and wave travels at the speed of light when it moves through a vacuum. Over a distance of 130 million light years, the gamma rays and gravitational waves emitted by merging neutron stars arrived offset by a mere 1.7 seconds, an incredible result! Yet if the light was emitted at the same time as the merger, that 1.7 second delay shouldn’t be there, unless something funny is afoot. While your instinct might be to attribute an exotic cause to this, it’s important to take a look at “mundane” astrophysics first, such as the environment surrounding the neutron star merger, the mechanism that produces the gamma rays, and the thickness of the matter shell that the gamma rays need to travel through. After all, matter is transparent to gravitational waves, but it interacts with light all the time! 30 years ago, neutrinos arrived four hours before the light did in a supernova; could this 1.7 second difference be an ultra-sped-up version of the same effect?

There’s no doubt that the first gamma rays from this neutron star-neutron star merger arrived after the gravitational waves did. But why? Find out on this week’s Ask Ethan!

## Seeing One Example Of Merging Neutron Stars Raises Five Incredible Questions

“1.) What is the rate at which neutron star-neutron star mergers occur? Before this event was observed, we had two ways of estimating how frequently two neutron stars would merge: from measurements of binary neutron stars in our galaxy (such as from pulsars), and from our theoretical models of star formation, supernovae, and their remnants. That gave us a mean estimate of around 100 such mergers every year within a cubic gigaparsec of space.

Thanks to the observation of this event, we now have our first observational rate estimate, and it’s about ten times larger than we expected. We thought we would need LIGO to reach its design sensitivity (it’s only halfway there) before seeing anything, and then on top of that we thought that pinpointing the location in at least 3 detectors would be unlikely. Yet we not only got it early, we localized it on the first try. So now the question is, did we just get lucky by seeing this one event, or is the true event rate really so much higher? And if it is, then what is it about our theoretical models that are so wrong?”

Now that we’ve observed merging neutron stars for the first time, in many different wavelengths of light as well as in gravitational waves, we’ve got a whole new world of data to work with. We’ve independently confirmed that gravitational waves are real and that we can, in fact, pinpoint their locations on the sky. We’ve demonstrated that merging neutron stars create short gamma ray bursts, and shown that the origin of the majority of elements heavier than the first row of transition metals comes primarily from neutron star-neutron star mergers. But the new discovery raises a ton of questions, too. Seeing this event has presented theorists with a number of new challenges, ranging from the event rate being some ten times as great as expected to much more matter being ejected than we’d thought. And what was it that was left behind? Was it a neutron star? A black hole? Or an exotic object that’s in its own class?

There are some great advances that the future will hold for gravitational wave and neutron star astronomy, but it’s up to theorists to explain why these objects behave as they do. Here are five burning questions we now have.

## Astronomy’s ‘Rosetta Stone’: Merging Neutron Stars Seen With Both Gravitational Waves And Light

“For the first time in history, gravitational wave astronomy isn’t a pipe dream, nor is it a way of looking for esoteric objects we can’t see via any other means. Instead, it’s truly a part of our night sky, and the first signpost of an astronomical cataclysm. In the future, as gravitational wave astronomy improves, it may even serve as an early warning system, enabling us to locate sources about to merge before they ever do so. It may grow to include not only black holes and neutron stars, but white dwarfs and supermassive black holes swallowing objects as well. Gravitational wave astronomy is only two years old, and we haven’t even taken it to space yet. The next step in understanding the Universe is before us. Sit back and enjoy the ride!”

When the Advanced LIGO detectors turned on in 2015, it shook up the world when they detected their first event: the merger of two quite massive black holes. Since that time, they’ve observed black hole-black hole mergers multiple times, with the VIRGO detector in Italy joining them for the fourth event. But this wasn’t what LIGO/VIRGO expected to see; rather, they were built to hunt for merging neutron stars that were much closer by. Neutron star mergers would be superior to black hole mergers in an extraordinary way: it would enable other astronomers to get in on the action. Unlike black holes, merging neutron stars should emit radiation across the electromagnetic spectrum, from gamma-rays to UV/optical afterglows. On August 17th, LIGO and VIRGO saw their very first neutron star merger, pinpointing its location to galaxy NGC 4993, just 120 million light years away.

For the first time, we’ve joined the gravitational wave and light-based skies together with an incredible event. It’s a glorious step forward. And it’s just the beginning.

## The Nobel Doesn’t Mean Gravitational Wave Astronomy Is Over; It’s Just Getting Good

“We haven’t just detected gravitational waves directly, we’ve begun exploring in the era of gravitational wave astronomy. We aren’t just seeing the sky in a whole new way; we’re getting better and better at seeing it, and learning what we’re looking at. Because these events are transient, existing only for a short amount of time, we right now only get one opportunity to view these black hole-black hole mergers. But as time goes on and our detectors continue to improve, we’re going to continue to see the Universe as we never have before. The Nobel Prize may have been for already completed research, but the true fruits of gravitational wave astronomy are still out there amidst the great cosmic forest. Thanks to the groundwork laid by 100+ years of scientists, for the first time, it’s picking season.”

Yes, we detected gravitational waves, directly, for the first time! Just days after Advanced LIGO first turned on, a signal of a 36 solar mass black hole merging with a 29 solar mass black hole gave us our first robust, direct detection of these long-sought waves, changing astronomy forever. Einstein’s General Relativity was validated in a whole new way, and over 40 years of work on developing and building LIGO was vindicated at last. Now, it’s two years later, and yes, some of the most important team members have been awarded physics’ highest honor: the Nobel Prize. But gravitational wave astronomy isn’t over now; on the contrary, it’s only just beginning in earnest. With a third detector now online and two more coming along in the next few years, we’re not only poised to enter a new era in astronomy, we’re about to open up a whole new set of discoveries that would otherwise be impossible.

Here’s where we are, and here’s how we do it! Find out what advances are already underway since this Nobel-winning discovery was made!

## Gravitational Waves Win 2017 Nobel Prize In Physics, The Ultimate Fusion Of Theory And Experiment

“The 2017 Nobel Prize in Physics may have gone to three individuals who made an outstanding contribution to the scientific enterprise, but it’s a story about so much more than that. It’s about all the men and women over more than 100 years who’ve contributed, theoretically and experimentally and observationally, to our understanding of the precise workings of the Universe. Science is much more than a method; it’s the accumulated knowledge of the entire human enterprise, gathered and synthesized together for the betterment of everyone. While the most prestigious award has now gone to gravitational waves, the science of this phenomenon is only in its earliest stages. The best is yet to come.”

It’s official at long last: the 2017 Nobel Prize in Physics has been awarded to three individuals most responsible for the development and eventual direct detection of gravitational waves. Congratulations to Rainer Weiss, Kip Thorne, and Barry Barish, whose respective contributions to the experimental setup of gravitational wave detectors, theoretical predictions about which astrophysical events produce which signals, and the design-and-building of the modern LIGO interferometers helped make it all possible. The story of directly detecting gravitational waves is so much more, however, than the story of just these three individuals, or even than the story of their collaborators. Instead, it’s the ultimate culmination of a century of theoretical, experimental, and instrumentational work, dating back to Einstein himself. It’s a story that includes physics titans Howard Robertson, Richard Feynman, and Joseph Weber. It includes Russell Hulse and Joseph Taylor, who won a Nobel decades earlier for the indirect detection of gravitational waves. And it’s the story of over 1,000 men and women who contributed to LIGO and VIRGO, bringing us into the era of gravitational wave astronomy.

## LIGO-VIRGO Detects The First Three-Detector Gravitational Wave

“When you have a signal appearing in one detector, you can gain a rough estimate of its distance from you (with uncertainties), but with no information about its direction. A second detector not only gives another distance estimate, but the time difference between the two signals gives you some information about distance, allowing you to restrict yourself to an “arc” on the sky. But a third detector, with a third time difference, allows you to pinpoint a single point, albeit with significant uncertainties. This is where the word “triangulation” comes from, since you need three detectors to pinpoint a location-of-origin. That’s exactly what VIRGO was able to give.”

For over a century after the publication of General Relativity, it was uncertain whether gravitational waves were real or not. It wasn’t until their first direct detection less than two years ago, by the LIGO scientific collaboration, that their existence was spectacularly confirmed. With the VIRGO detector in Italy coming online this year to complement the twin LIGO detectors, however, so much more became possible. An actual position in space could be identified for the first time, enabling a possible correlation between the gravitational wave sky and the electromagnetic one. The three-dimensional polarization of a gravitational wave could be measured, and compared with the predictions of Einstein’s theory. And gravitational wave signals can be teased out earlier and measured to smaller amplitudes than ever before. Not only have we just seen our fourth gravitational wave event, we’ve seen it in all three detectors.

This discovery is, indeed, something big, but there’s even bigger science to come in the future! Come see what this first three-detector gravitational wave event has given us!