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: Are Gravitational Waves Themselves Affected By Gravity?
“Are gravitational waves themselves subject to gravity? That is, if a gravitational wave were to pass by a galaxy cluster, would its form get distorted (even though the wave, itself, is a distortion of space-time)? One side of me says gravitational waves are a form of energy so therefore must be affected by gravity. The other side of me says “Nah – that just doesn’t make sense!"”
Think about the fabric of space itself. All the masses and forms of energy in the Universe cause space itself to curve, while the curved space itself alters the path along which any matter or form of energy will travel. Massless particles, like photons, are bent by the fabric of space itself. But what about gravitational waves? Are they also subject to this, or does gravitation lack a self-interaction that it would require for this to be possible?
For a very long time, this was a question that was theoretical only. But over the last three years, we’ve observed a slew of gravitational waves, allowing this idea to be tested for the first time.
What were the results? Gravitational waves are affected by gravity, in at least three different observable ways. Come find out how today!
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!
Ask Ethan: If Mass Curves Spacetime, How Does It Un-Curve Again?
“We are taught that mass warps spacetime, and the curvature of spacetime around mass explains gravity – so that an object in orbit around Earth, for example, is actually going in a straight line through curved spacetime. Ok, that makes sense, but when mass (like the Earth) moves through spacetime and bends it, why does spacetime not stay bent? What mechanism un-warps that area of spacetime as the mass moves on?”
You’ve very likely heard that according to Einstein, matter tells spacetime how to curve, and that curved spacetime tells matter how to move. This is true, but then why doesn’t spacetime remain curved when a mass that was once there is no longer present? Does something cause space to snap back to its prior, un-bent position? As it turns out, we need to think pretty hard about General Relativity to get this right in the first place at all. It isn’t just the locations and magnitudes of masses that determine how objects move through space, but a series of subtle effects that must all be added together to get it right. When we do, we find out that uncurving this space actually results in gravitational radiation: ripples in space that have been observed and confirmed.
The deciding results are actually decades old, and were indirect evidence for gravitational waves long before LIGO. Come get the answer today!
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.
LIGO’s Greatest Discovery Almost Didn’t Happen
“If all we had done was look at the automated signals, we would have gotten just one “single-detector alert,” in the Hanford detector, while the other two detectors would have registered no event. We would have thrown it away, all because the orientation was such that there was no significant signal in Virgo, and a glitch caused the Livingston signal to be vetoed. If we left the signal-finding solely to algorithms and theoretical decisions, a 1-in-10,000 coincidence would have stopped us from finding this first-of-its-kind event. But we had scientists on the job: real, live, human scientists, and now we’ve confidently seen a multi-messenger signal, in gravitational waves and electromagnetic light, for the very first time.”
Imagine the scene: it’s mid-August, 2017, and the Virgo detector has just joined the twin LIGO detectors barely two weeks ago. Amazingly, on August 14th, you’ve seen a gravitational wave signal in all three detectors; another black hole-black hole merger. Then, all of a sudden, even though the LIGO detectors are set to shut down later in the month, an extraordinarily significant signal goes off… but only in one detector. The LIGO Hanford detector sees a signal with a false-alarm probability of just one part in 300 billion; a slam dunk. Yet both LIGO Livingston and Virgo see nothing. A non-coincident signal should automatically be rejected, but somehow, one of the young researchers working on the project thought to check the Livingston data by hand… and that was where the secret lay.
LIGO’s greatest discovery, of two merging neutron stars, almost was overlooked. Thankfully, the hands-on nature of the scientists working on gravitational waves were able to turn this into the discovery of the century! (So far!)
The Largest Black Hole Merger Of All-Time Is Coming, And Soon
“Over in Andromeda, the nearest large galaxy to the Milky Way, a number of unusual systems have been found.
One of them, J0045+41, was originally thought to be two stars orbiting one another with a period of just 80 days.
When additional observations were taken in the X-ray, they revealed a surprise: J0045+41 weren’t stars at all.”
When you look at any narrow region of the sky, you don’t simply see what’s in front of you. Rather, you see everything along your line-of-sight, as far as your observing power can take you. In the case of the Panchromatic Hubble Andromeda Treasury, where hundreds of millions of stars were captured in impressive fashion, background objects thousands of times as distant can also be seen. One of them, J0045+41, was originally thought to be a binary star system that was quite tight: with just an 80 day orbital period. Follow-up observations in the X-ray, however, revealed that it wasn’t a binary star system after all, but an ultra-distant supermassive black hole pair, destined to merge in as little as 350 years. If we build the right observatory in space, we’ll be able to observe the entire inspiral-and-merger process for as long as we like!
Come get the full story, and some incredible pictures and visuals, on today’s Mostly Mute Monday!
Ask Ethan: Could Matter Escape The Event Horizon During A Black Hole Merger?
If two black holes merge, is it possible for matter that was within the event horizon of one black hole to escape? Could it escape and migrate to the other (more massive black hole)? What about escape to outside of both horizons?
Imagine that you’ve got two black holes about to merge in space. They’re radiating energy away, spacetime itself is at its most distorted, and perhaps you have particles just crossing over the event horizon for the first time. Is there any way you could configure it to have them escape, or to have a particle jump from one event horizon to another? The situation is incredible, and involves some of the strongest gravitational fields ever considered in the Universe. But numerical relativists are up to the challenge of simulating these spacetimes, and we can see what happens! Believe it or not, even as energy is radiated away and the total mass drops, the event horizons themselves never decrease in size, and the total volume encapsulating the “no-escape zone” only increases as time goes on!
Come learn the reason why matter can’t escape the event horizon, even during a black hole merger, on this week’s Ask Ethan!
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!