As a physicist, there are few things as exciting as experimental confirmation of our theoretical understanding of the universe. This week brought us groundbreaking news in the field of astronomy, and I didn’t want to pass the opportunity to learn more details about it.
On Monday morning, during a conference at the National Science Foundation, it was announced that the merger of two neutron stars had been observed for the first time, both with gravitational wave detection and in the entire electromagnetic spectrum. This is a tremendous achievement; it is the first time that the gravitational wave signal of the merger of such objects has been observed (until now it was only black holes), but it has also confirmed predictions about electromagnetic emissions in such event. Until now only a prediction, we have observed the burst of highly energetic gamma rays that are produced during these events as well as the production of over 40 heavy elements, including gold, platinum, and uranium.
These experimental results have been obtained by a combination of over 70 Earth-based observatories in all continents and seven space observatories. It is an incredible example of the power of scientific collaboration across many countries in high-risk high-rewards science, such as the LHC effort which confirmed the existence of the Higgs boson. For the first time, we have electromagnetic and gravitational data for the same astronomical event, which opens a new field of multi-messenger astronomy, which will help us understand the universe to a whole new level. Our movies of the stars just got sound.
What happened and how did we see it?
On August 17 at 12:41:04 UTC, the LIGO and Virgo telescopes detected a gravitational wave signal lasting 100 seconds. This signal was much longer than previous ones detected, indicating that the objects producing them were much lighter than black holes.
About 1.7 seconds after, Fermi, the gamma-ray orbiting telescope, detected a burst of gamma rays. It was the first time that these two signals had been detected for the same astronomical event. The gamma rays and gravitational waves traveled together for 130 million years and arrived within 2 seconds of one another, both moving at the same speed as predicted by Einstein.
The three-detector network of gravitational telescopes, together with Fermi, were able to pinpoint the region of the sky that had witnessed the event and the astronomical observatories that detect in the electromagnetic spectrum, turned their eyes. About 10 hours and 52 minutes after the gravitational wave detection, a new visible light source was found in the NGC 4993 galaxy in the Hydra constellation. Further observations across the entire electromagnetic spectrum were made, including infrared emission (+ 11 hours and 36 minutes), ultraviolet (+15 hours), X-rays (+9 days) and radio (+15 days).
Astronomers now have an understanding of what happened through the emissions detected by the telescopes. The pair of neutron stars were spinning around one another while orbiting in their galaxy. As they spun, they lost energy through the emission of gravitational waves and got closer and closer to each other. It was only when they were about 200 miles apart, with the merger imminent, that the gravitational waves they emitted got strong enough for our detectors to be able to detect them. About ten milliseconds before the merger, the neutron stars started feeling tidal forces and up to few seconds after the merger matter was violently ejected into the interstellar medium, this ejection was detected as a burst of gamma rays. The ejected matter was the perfect site for heavy elements to form. The decay of some of these elements powered the light observed in the blue, red and near-infrared spectrum. The features of the light spectrum observed throughout the following days to the gravitational wave detection are a consequence of the merging of spectrums emitted by different elements. We now have confirmation that over 40 elements of the periodic table, including gold, platinum, and uranium, are formed in merging neutron stars.
What are gravitational waves?
Gravitational waves are perturbations in spacetime caused by the movement of large massive objects, they are particularly strong during the collision of very large astronomical bodies, such as black holes and neutron stars. The gravitational waves travel through the fabric of spacetime like ripples in the water. However, unlike water, spacetime is incredibly stiff — a gravitational wave caused by the merger of two black holes would only change the distance from Earth to Alpha Century by the width of a human hair. To detect these incredibly weak signals, the LIGO and Virgo observatories are miles long to boost the signal. A laser beam is split in two, and the beams travel back and forth in the two tunnels for exactly the same distance before interfering again. If the distance traveled by the beams has changed in the slightest, due to space being stretched by a gravitational wave, the interference pattern of the light when is combined together will give it away. The accuracy of these telescopes is mind-boggling, and the science that makes them work is even more fascinating: they use quantum phenomena to be able to measure beyond the accuracy of our classical tools — it is the science governing the smallest pieces of the universe that helps us understand the physics of the largest bodies. But this deserves a closer look altogether, for now, we can truthfully say that these observatories are the biggest and most accurate rulers that exist in the universe.
What are gamma rays?
Gamma rays are the most energetic electromagnetic emissions. We have specialized telescopes to look for these rays, both Earth-based telescopes and Earth-orbiting telescopes such as Fermi, which observes the entire sky (except the part blocked by the Earth at any given point). Fermi detects flashes of gamma rays about twice a day; these are usually caused by thunderstorms, flares from our sun and exotic stars from our galaxy. A few hundred detections a year, however, are huge explosions coming from galaxies far away. These are conjectured to be caused by the expulsion of matter at high speed during the creation of black holes and the merger of neutron stars, but there was no experimental evidence to back that claim. Until now!
What are neutron stars?
Neutron stars are the remnants of massive stars, about ten to twenty times the size of the Sun. These stars die after they run out of fuel for the nuclear fusion reaction that makes them shine, their core collapsing due to its own gravity, while the outer layers are violently expulsed resulting in a supernova. The star shrinks to the size of a small city, and the pressure inside is so large that atoms disintegrate into their constituents, protons and electrons, which recombine into neutrons.
The event observed in August was the first direct observation of the merger of a pair of these stars. However, we have long had experimental evidence of their existence. They were first observed by Jocelyn Bell Burnell and Anthony Hewish in 1967, who identified them as pulsars — radio-wave pulsing stars. In fact, they don’t emit the radio waves in pulses; rather they do so continuously. However, as they rotate around their own axis (which often does not coincide with the axis of the emission), we can only observe their waves when the beam is pointed towards Earth. They are akin to lighthouses in deep space.
In 1974, Joseph Taylor and Russel Hulse discovered the first pair of orbiting neutron stars in a binary system, which were orbiting around one another. They were found in an unstable orbit, (from radio observation) that was spiraling in, the spiraling was happening at the speed that Einstein predicted. Their observation provided the first firm observation of a system that confirmed that gravitational waves must exist in nature, confirming Einstein’s predictions. These neutron stars were over a million miles apart and won’t merger from another 300 million years. In contrast, the binary system that starred the event in August, these neutron stars started emitting detectable gravitational waves at 200 miles apart, and 100 seconds later they merged.
The era of multimessenger astronomy has started, and we expect to be able to gain a much deeper understanding of the universe by combining gravitational wave signals with detections from other sources. If anything I hope that the announcement on Monday, will inspire many old and young minds to learn more about the universe that surrounds us, because it is still full of secrets.
I thoroughly recommend watching the live announcement and sharing in the excitement of the physics community. There are, first and foremost, scientific articles describing the discovery, press releases by all parties involved, numerous first-hand accounts from the scientists that took part in this experiment, as well as many popular-science articles on the topic; I hope you find the reading of these accounts as interesting as I have.
For now, I will leave you with this intriguing musical piece that my talented friend Cora Miron, composed while inspired by these astronomical discoveries.