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.
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.
“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.
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!
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!
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?
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?
Why Neutron Stars, Not Black Holes, Show The Future Of Gravitational Wave Astronomy
“3.) Gravitational waves move at exactly the speed of light! Before this detection, we never had a gravitational wave and a light signal simultaneously identifiable to compare with one another. After a journey of 130 million light years, the first electromagnetic signal from this detection arrived just 1.7 seconds after the peak of the gravitational wave signal. That means, at most, the difference between the speed of gravity and the speed of light is about 0.12 microns-per-second, or 0.00000000000004%. It’s anticipated that these two speeds are exactly equal, and the delay of the light signal comes from the fact that the light-producing reactions in the neutron star take a second or two to reach the surface.”
Detecting black holes and the gravitational wave signals from them was an incredible feat, but doing the same thing for neutron star mergers is a true game-changer. Instead of fractions of a second, neutron star mergers show up for up to half a minute. Unlike black holes, there’s an electromagnetic counterpart. Because of that, we can verify that the speed of gravity really is identical to the speed of light: to better than 1 part in 1,000,000,000,000,000. And perhaps most spectacularly, we can bring the electromagnetic and gravitational-wave skies together for the first time. Even though LIGO has seen more merging black holes, the fact is that there are more merging neutron stars. The key, now, is finding them. We live at a moment where gravitational wave astronomy is just in its infancy, giving us a whole new way to look at the Universe.
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.
All this is most certainly easily said than done and requires meticulous and extensive research, not to mention highly sensitive instruments.
Had they not have measured this time difference,
we might have had to wait for the merger for more massive black holes
to collide and maybe even build more sensitive instruments to detect these waves.
And Einstein predicted this a 100 years back!
Note: Hope you are able to understand and appreciate the profundity of the discovery done by mankind.
** All animations used here are merely for Educational purposes. If you have any issues, please write to us at : email@example.com
Why is this discovery a Big Deal ?
Gravitational waves gives
us another way to observe celestial phenomenon. These waves also form
when supernovae explode, when black holes collide and during many other
Detecting them might give us a new
perspective into the cosmic events. There is hell of a lot of space that
is left unexplored or lies beyond human exuberance and this discovery
might shed some light on it. ( like the big bang per se )
The ultimate goal is to
understand the fundamental laws of the universe. It is a quest through
the oblivion towards a theory of everything.
Although it is
unknown how many years/decades it might take to get us there, but these discoveries
are markers to getting there.
What is this Image that i see everywhere?
This is not the photograph of the actual event but a simulation run by NASA of two black holes merging.
How does the actual experimental setup look like ?
The actual experimental setup is a bit complex in its entirety. But the guardian has an elegant image that seems to cover its essence:
Have a great day!
Nobel Prize in Physics 2017
The Nobel Prize
in Physics 2017 was divided, one half awarded to Rainer Weiss, the other
half jointly to Barry C. Barish and Kip S. Thorne “for decisive contributions to the LIGO detector and the observation of gravitational waves”.