There’s a crucial rule out relativity that — as far as we all know — all objects must obey. If you’ve got no mass as you travel through the vacuum of space, you absolutely are compelled to travel exactly at the speed of light .This is often exactly true for all massless particles, like photons & gluons, approx. true for particles whose mass is small compared to their kinetic energy. , like neutrinos, and will even be exactly true for gravitational waves. If gravity isn’t inherently quantum in nature, the speed of gravity should be exactly adequate to the speed of light if our current laws of physics are correct. And yet, once they saw the 1st neutron star-neutron star merger in both gravitational waves and with light, the gravitational waves came first by almost 2 seconds. What’s the explanation? That’s what Mario Blanco wants to understand , asking:
“I read your articles and located the one on gravitational waves very interesting. What would account for the 2s delay of gravitational waves over light waves?”
If everything traveled at an equivalent speed, and both are generated at an equivalent time, then why would one arrive before the other? It’s an excellent question.
On Aug 17, 2017, the signal from an occasion that occurred 130 million light-years away finally arrived here on Earth. From somewhere within the distant galaxy NGC 4993, 2 neutron stars had been locked during a gravitational dance where they orbited each other at speeds that reached a big fraction of the speed of light . As they orbited, they distorted the material of space due to both their mass and their motion relative to the curved space through which they traveled.
Whenever masses accelerate through curved space, they emit tiny amounts of invisible radiation that’s invisible to all or any telescopes: gravitational, instead of electromagnetic, radiation. These gravitational waves behave as ripples within the fabric of spacetime, carrying energy faraway from the system and causing their mutual orbit to decay. At a critical moment in time, these 2 stellar remnants spiraled so on the brink of each other that they touched, and what followed was one among the foremost spectacular scientific discoveries of all-time.
As soon as these two stars collided, the gravitational wave signal came to an abrupt end. Everything that the LIGO & Virgo detectors saw was from the inspiral phase up until that moment, followed by total gravitational wave silence. consistent with our greatest theoretical models, this was two neutron stars inspiraling and merging together, likely leading to an interesting end result: the formation of a black hole region. .
But then it happened. 1.7 seconds later, after the gravitational wave signal ceased, the 1st electromagnetic (light) signal arrived: gamma rays, which came in one enormous burst. From the mixture of gravitational wave and electromagnetic data, they were ready to pin down the location of this event better than any gravitational wave event ever: to the precise host galaxy during which it occurred, NGC 4993.
Over the approaching weeks, light began to arrive in other wavelengths also , as on the brink of 100 professional observatories monitored the spectacular afterglow of this neutron star merger.
On the one hand, this is often remarkable. We had an occasion occur some 130 million light-years away: far enough away that light took 130 million years to travel from the galaxy where it occurred to our eyes. Back when the merger happened , planet Earth was a vastly different place. Feathered birds had been around for less than 20 million years; placental mammals for 10 million. the 1st flowering plants were just starting to emerge, and therefore the largest dinosaurs were still 30 million years in Earth future.
For all that point , from then until this , both the light and therefore the gravitational waves from this event were journeying through the Universe, traveling at the only speed they might — the speed of light and therefore the speed of gravity, respectively — until they received Earth after a journey of 130 million years. First the gravitational waves from the inspiral phase arrived, moving the mirrors on our gravitational wave detectors by an incredibly small amount: but a 10,000 of the dimensions of a individual proton. And then, just 1.7 seconds after the gravitational wave signal ended, the 1st light from the event arrived also .
Immediately, this gave us the foremost impressive physical measurement of the speed of gravity ever: it had been adequate to the speed of light to raised than 1 part during a quadrillion (1015), because it takes around 4 quadrillion seconds to form up 130 million years, and that they arrived but 2 seconds aside from each other. before that, we had excellent theoretical reasons for knowing that the speed of gravity need to equal the speed of light , but only had indirect constraints that the 2 were like within 0.2% or so.
Does this mean that the speed of gravity and therefore the speed of light aren’t quite equal, then? That perhaps either gravity moves slightly faster than c, the speed of light during a vacuum, or that light itself might actually move a small bit slower than c, as if it had a small but non-zero mass to it? that might be a unprecedented revelation, but one that’s highly unlikely. If that were true, light of various energies (and wavelengths) would travel at different speeds, and therefore the level at which that might got to be true is far overlarge to be according to observations.
In simpler terms, if light had a non-zero mass , which mass were heavy enough to define why gravitational waves arrived 1.7 seconds before light after traveling 130 million light-years across the Universe, then they’d observe radio waves traveling significantly slower than the speed of light: too slow to be according to what we’ve already observed.
But that’s okay. In physics, we don’t have any problem considering all possible explanations for an observed puzzle. If they’re doing our jobs correctly, every explanation apart from one are going to be incorrect. The challenge is to seek out the right one.
And we think we have! The key’s to believe the objects that are merging together, the physics at play, and what signals they’re likely to produce . We’ve already done this for the gravitational waves, detailing how they’re produced during the inspiral phase and cease once the merger takes place. Now, it’s time to travel a little deeper & think about the light .
Up until these 2 neutron stars touched, there was no “extra” light produced. They simply shone as neutron stars do: faintly, at high temperatures but with tiny surface areas, and completely undetectable with our current technology from 130 million light-years away. Neutron stars aren’t like black holes; they aren’t point-like. Instead, they’re compact objects — typically somewhere between 20 & 40 km across — but denser than an atomic nucleus. They’re called neutron stars because they’re about 90% neutrons by composition, with other atomic nuclei and a couple of electrons at the outer edge .
When 2 neutron stars collide, there are three possibilities which will result. They are:
1. You can form another neutron star , which you’ll do if your total mass is a smaller amount than 2.5 times the mass of the Sun
2. You can form a replacement star briefly, which then collapses into a region in under a second, if your total mass is between 2.5 & 2.8 solar masses (dependent on the neutron star’s spin)
3. Otherwise you can form a black hole directly, with no intermediate neutron star , if your total mass is bigger than 2.8 solar masses.
From the gravitational wave signal that arose from this event, officially referred to as GW170817, we all know that this event falls into the 2nd category: the merger and post-merger signal existed for a few of 100 milliseconds before disappearing entirely all in a instant , which indicates that a neutron star formed for a quick time before an event horizon formed & engulfed the whole thing.
But nevertheless, light still got out. subsequent question was, simply, how?
How was the light that we observed generated? Again, there have been three possibilities that we could consider .
1. Immediately, as soon because the neutron stars touch, by processes that occur on their surfaces.
2. Only after material gets ejected, where it collides with any surrounding material and produces light from that.
3. Or from the inside of neutron stars, where reactions generate energy that only gets emitted once it propagates to the outside .
In each scenario, gravitational waves travel unperturbed once the signal is generated, but light takes an additional amount of your time to urge out.
If it’s the 1st option, and neutron star mergers generate light as soon as they touch, the light gets emitted immediately & thus must be delayed by passing through the environment surrounding the star . That environment must be rich in matter, as each fast-moving neutron stars, with charged particles on their surfaces and intense magnetic fields, is sure to strip and eject material from the opposite one.
If it’s the 2nd or 3rd option, merging neutron stars generate light from their mergers, but that light only gets emitted after a particular amount of your time has passed: either for ejected material to smash into the circumstellar material or for the light generated within the star interiors to succeed in the surface. It’s also possible, in either of those cases, that both “delayed emission” and “slowed arrival by surrounding material” are at play.
Any of those scenarios could easily explain the 1.7s delay of light’s arrival with reference to gravitational waves. But on April 25, 2019,they saw another neutron star-neutron star merger in gravitational waves, which was more massive than GW170817. No light was emitted of any type, disfavoring the primary scenario. it’s like neutron stars don’t generate light as soon as they touch. Instead, the emission of light comes after the emission of gravitational waves.
With only two direct detections of merging neutron stars through the emission of gravitational waves, it’s a testament to how incredibly precise the science of gravitational wave astronomy has become that we will reconstruct all we’ve . Once you add within the electromagnetic follow-up observations from the 2017 event that also produced light, we’ve definitively shown that an outsized fraction of the elements in our Universe — including gold, platinum, iodine & uranium — arise from these neutron star mergers. But not, perhaps, from neutron star mergers;
Perhaps it’s only those that don’t immediately form a black hole. Either ejected material or reactions within the neutron star’s interior is required to produce these elements, and hence, the light related to a kilonova explosion. That light is merely produced after the gravitational wave signal has ended, and should further be delayed by having to undergo the circumstellar material. this is often why, even light & gravity both travel exactly at the speed of light during a vacuum, the light we saw didn’t arrive until nearly 2 seconds after the gravitational wave signal ceased. As they collect and observe more of those events, they’ll be ready to confirm and refine this picture once and for all!