08-29-2016, 09:59 PM
In 2015, a team of physicists published a paper in the journal Physical Review Letters with more than 1,000 contributing authors. As you might expect, this paper was of monumental historical importance: After a century of waiting, physicists finally confirmed the existence of gravitational waves. In so doing, they’ve confirmed the last major prediction of Einstein’s theory of general relativity and, they believe, offered a way forward for the study of black holes. This observation, catchily dubbed GW150914, looks to astronomers like the start of a whole new day for astrophysics.
But there were always other implications of the theory that were more difficult to test. In particular, if Einstein’s theory is accurate then we should observe that gravity “moves” at the speed of light. To illustrate this, there’s an old thought experiment: What would happen if the Sun popped out of existence right now, in an instant? Would the Earth fly off into space immediately, or would there be some delay as the planet continued to orbit an empty point while the information about the change in gravity propagated outward at the speed of light? The answer is the latter: The sharp, expanding interface between Sun-exists gravity and Sun-doesn’t-exit gravity is a gravitational wave (not a gravity wave).
But of course, stars don’t really pop in and out of existence like that. In principle, everything with mass causes gravitational waves — my fingers waving over my keyboard is technically making them — but unless we have unimaginably massive objects doing unimaginably violent things, they’re too small to have the slightest hope of detection. Physicists came up with a number of astronomical events which they hoped would satisfy both of the following conditions:
a) Must feature large and chaotic enough movements of mass to create detectable gravitational waves
b) Must actually exist
The best candidate was the at-that-point-unobserved phenomenon of a binary black hole, in which two black holes approach each other and eventually merge. Though it’s possible they could simply hit head-on, smash directly into one another and merge that way, it’s far more likely that they will miss and begin to orbit one another and circle inward to eventually meet in the middle. As they spiral inward, their velocity will increase until, for the last portion of the process, they are moving as quickly as half the speed of light.
Let’s take a moment just to appreciate what it is we’re talking about here. According to the LIGO team, the black holes involved in this collision were 29 times and 36 times heavier than our Sun, respectively. Each one is a singularity, a body so massive that it has actually torn space-time and bent the laws of physics until they no longer truly apply. By the end, these sucking, invisible monsters can orbit one another in a matter of milliseconds.
As they wrench around one another at such speeds, the black holes violently compress and distort spacetime. If it were possible to get close to them without being physically torn apart by the gravitational forces, we would go through some truly unimaginable loops and squiggles in time. To an outside observer, we would seem to move jerkily through time, moving irrationally from ultra-fast-forward to ultra-slow-mo and back again as we quite literally rode the gravitational waves. From our own perspective, however, we would be cruising forward with a uniform rate of time.
When singularities dance at relativistic speeds, the resulting gravitational waves could have enough energy for detection in our own solar system. That was the inspiration for a number of projects, including the Laser Interferometer Gravitational-wave Observatory (LIGO). LIGO’s two L-shaped antennae are some 1,900 miles apart, one held in in Hanford, Washington and the other in Livingston, Louisiana. Both are enormous in their own right, with each arm stretching about 2.5 miles outward through protective concrete tunnels.
At the end of each L-arm lies a highly sensitive mirror that is affected by changes in gravitational potential. Their tiny adjustments can change the length of the tunnel by less than a thousandth the width of a proton, as observed by the laser in their laser interferometer. They can even feed the antennae data to an audio visualizer so they can easily hear spikes in the data as changes in pitch. As heard below, by the time it reaches the Earth, the incredible destructive power of a gravitational wave sounds like a diminutive little ping.
But the detection didn’t play out as quickly as it was supposed to. GW150914 was actually observed while the only two operational LIGO detectors were still in “engineering mode,” getting tested for the beginning of real work later in the month. It’s a good thing they were, since on September 14, 2015, before the first official day of operations, LIGO detected a gravitational wave. It took five months to verify and publish the results — which means you should be wondering what else the LIGO detectors might already have observed that the team has yet to get ready for publication.
So, gravitational waves exist. That not only confirms Einstein’s relativity, but it has the potential to help advance black hole science as well. The black holes don’t just send out waves when spinning, but also after they’ve merged. The process of melding into a single spherical unit doesn’t happen instantly, and as the two singularities ripple and shift, mass moves around in the new super black hole violently enough to make a whole new set of detectable waves. It’s these waves that could be a boon to black hole scientists, who have all kinds of predictions about exactly what sorts of characteristics they ought to have.
This is an important point: Detecting a gravitational wave is a fundamentally different sort of astronomy than detecting a ray of visible light. Rather than looking at particles or large objects moving through space, gravity science looks at the medium of the universe itself. That means it could offer insight into previously impossible areas of study — like the interior of a black hole. Black holes refuse to offer any data in the form of light, but they of all objects can’t avoid causing ripples in spacetime. It’s a bit like deep-sea fish with exterior nodes for detecting shockwaves in the water; even when there’s no light available to collect, disturbances in the medium that surrounds us can still offer vital information about the universe.
So, what we have here is not just a confirmation of gravitational waves, not just a window into black hole collisions, but a proof that we have at our disposal a whole new form of astronomy. Any insight these readings eventually offer into the black holes will be minimal next to the simple fact that these readings offer any insight into a black hole. That’s a truly revolutionary thing, and it shows that we might one day be able to use gravity itself to study everything from galactic collisions to dark matter.
Going forward, we’ve got to remember that the LIGO experiment is really only part of the larger world of gravitational wave science. It detects so-called high-frequency gravitational waves — but there should also be low-frequency gravitational waves, and to capture those the ESA recently finished an incredibly ambitious project called the Evolved Laser Interferometer Space Antenna (eLISA). eLISA is to be a constellation of three satellites arranged in an equilateral triangle over 600,000 miles to a side, and put these satellites into perfect free fall around the Sun. If the free fall is indeed flawless, then any detectable differences in the movements of the satellites should be due to differences in the force of gravity acting upon them. If a low frequency gravitational wave passes over this area of space, eLISA should be able to reflect this.
What we really saw with the discovery of gravitational waves was the start of a new era in astronomy. With the current eLISA project online, entire careers have shifted to study the impending gathering of data.
But in another sense it’s incredibly hopeful, as it offers a way past seemingly insurmountable obstacles, and toward seemingly unattainable truths about our universe.
But there were always other implications of the theory that were more difficult to test. In particular, if Einstein’s theory is accurate then we should observe that gravity “moves” at the speed of light. To illustrate this, there’s an old thought experiment: What would happen if the Sun popped out of existence right now, in an instant? Would the Earth fly off into space immediately, or would there be some delay as the planet continued to orbit an empty point while the information about the change in gravity propagated outward at the speed of light? The answer is the latter: The sharp, expanding interface between Sun-exists gravity and Sun-doesn’t-exit gravity is a gravitational wave (not a gravity wave).
But of course, stars don’t really pop in and out of existence like that. In principle, everything with mass causes gravitational waves — my fingers waving over my keyboard is technically making them — but unless we have unimaginably massive objects doing unimaginably violent things, they’re too small to have the slightest hope of detection. Physicists came up with a number of astronomical events which they hoped would satisfy both of the following conditions:
a) Must feature large and chaotic enough movements of mass to create detectable gravitational waves
b) Must actually exist
The best candidate was the at-that-point-unobserved phenomenon of a binary black hole, in which two black holes approach each other and eventually merge. Though it’s possible they could simply hit head-on, smash directly into one another and merge that way, it’s far more likely that they will miss and begin to orbit one another and circle inward to eventually meet in the middle. As they spiral inward, their velocity will increase until, for the last portion of the process, they are moving as quickly as half the speed of light.
Let’s take a moment just to appreciate what it is we’re talking about here. According to the LIGO team, the black holes involved in this collision were 29 times and 36 times heavier than our Sun, respectively. Each one is a singularity, a body so massive that it has actually torn space-time and bent the laws of physics until they no longer truly apply. By the end, these sucking, invisible monsters can orbit one another in a matter of milliseconds.
As they wrench around one another at such speeds, the black holes violently compress and distort spacetime. If it were possible to get close to them without being physically torn apart by the gravitational forces, we would go through some truly unimaginable loops and squiggles in time. To an outside observer, we would seem to move jerkily through time, moving irrationally from ultra-fast-forward to ultra-slow-mo and back again as we quite literally rode the gravitational waves. From our own perspective, however, we would be cruising forward with a uniform rate of time.
When singularities dance at relativistic speeds, the resulting gravitational waves could have enough energy for detection in our own solar system. That was the inspiration for a number of projects, including the Laser Interferometer Gravitational-wave Observatory (LIGO). LIGO’s two L-shaped antennae are some 1,900 miles apart, one held in in Hanford, Washington and the other in Livingston, Louisiana. Both are enormous in their own right, with each arm stretching about 2.5 miles outward through protective concrete tunnels.
At the end of each L-arm lies a highly sensitive mirror that is affected by changes in gravitational potential. Their tiny adjustments can change the length of the tunnel by less than a thousandth the width of a proton, as observed by the laser in their laser interferometer. They can even feed the antennae data to an audio visualizer so they can easily hear spikes in the data as changes in pitch. As heard below, by the time it reaches the Earth, the incredible destructive power of a gravitational wave sounds like a diminutive little ping.
But the detection didn’t play out as quickly as it was supposed to. GW150914 was actually observed while the only two operational LIGO detectors were still in “engineering mode,” getting tested for the beginning of real work later in the month. It’s a good thing they were, since on September 14, 2015, before the first official day of operations, LIGO detected a gravitational wave. It took five months to verify and publish the results — which means you should be wondering what else the LIGO detectors might already have observed that the team has yet to get ready for publication.
So, gravitational waves exist. That not only confirms Einstein’s relativity, but it has the potential to help advance black hole science as well. The black holes don’t just send out waves when spinning, but also after they’ve merged. The process of melding into a single spherical unit doesn’t happen instantly, and as the two singularities ripple and shift, mass moves around in the new super black hole violently enough to make a whole new set of detectable waves. It’s these waves that could be a boon to black hole scientists, who have all kinds of predictions about exactly what sorts of characteristics they ought to have.
This is an important point: Detecting a gravitational wave is a fundamentally different sort of astronomy than detecting a ray of visible light. Rather than looking at particles or large objects moving through space, gravity science looks at the medium of the universe itself. That means it could offer insight into previously impossible areas of study — like the interior of a black hole. Black holes refuse to offer any data in the form of light, but they of all objects can’t avoid causing ripples in spacetime. It’s a bit like deep-sea fish with exterior nodes for detecting shockwaves in the water; even when there’s no light available to collect, disturbances in the medium that surrounds us can still offer vital information about the universe.
So, what we have here is not just a confirmation of gravitational waves, not just a window into black hole collisions, but a proof that we have at our disposal a whole new form of astronomy. Any insight these readings eventually offer into the black holes will be minimal next to the simple fact that these readings offer any insight into a black hole. That’s a truly revolutionary thing, and it shows that we might one day be able to use gravity itself to study everything from galactic collisions to dark matter.
Going forward, we’ve got to remember that the LIGO experiment is really only part of the larger world of gravitational wave science. It detects so-called high-frequency gravitational waves — but there should also be low-frequency gravitational waves, and to capture those the ESA recently finished an incredibly ambitious project called the Evolved Laser Interferometer Space Antenna (eLISA). eLISA is to be a constellation of three satellites arranged in an equilateral triangle over 600,000 miles to a side, and put these satellites into perfect free fall around the Sun. If the free fall is indeed flawless, then any detectable differences in the movements of the satellites should be due to differences in the force of gravity acting upon them. If a low frequency gravitational wave passes over this area of space, eLISA should be able to reflect this.
What we really saw with the discovery of gravitational waves was the start of a new era in astronomy. With the current eLISA project online, entire careers have shifted to study the impending gathering of data.
But in another sense it’s incredibly hopeful, as it offers a way past seemingly insurmountable obstacles, and toward seemingly unattainable truths about our universe.