The Earth’s Magnetosphere Could be Used as a Gravitational Wave Observatory
One of the challenges of gravitational wave astronomy is moving its abilities beyond observations of stellar mass mergers. The collision of two black holes or neutron stars releases a tremendous amount of gravitational energy, but even this is a challenge to detect. Gravitational waves do not couple strongly with most matter, so it takes a tremendous amount of sensitive observations to observe. But we are getting better at it, and there are a few proposals that hope to take our observations even further. One example of this is a recent study that looks at utilizing the magnetospheres of Earth and Jupiter.
The observation of high-frequency gravitational waves is one of the holy grails of gravitational wave astronomy. According to the standard model of cosmology, these high-frequency waves should have been produced during the early period of the big bang, particularly as a result of the inflationary period. As such, they would provide detailed evidence of both early inflation and cosmic evolution. But at the moment these gravitational waves are too faint and random to be distinguished from background noise.
So this new work looks at how they might be observed indirectly. Although high-frequency gravitational waves are faint, they do interact with the material they pass through, squeezing and flexing them ever so slightly. This is true not just for matter, but also for magnetic fields. When a gravitational wave passes through a magnetic field, the squeezing and flexing shift the magnetic field, which can produce photons. So the team wondered how this would occur when passing through fairly strong magnetic fields, such as the magnetospheres of Earth and Jupiter.
Artist impression of Juno at Jupiter. Credit: NASA
Both Earth and Jupiter have powerful magnetospheres. Earth’s magnetosphere is part of the reason we are protected from things such as solar flares. The team calculated the spectrum and intensity of photons that might be produced by Earth’s and Jupiter’s magnetospheres, and the results were encouraging. In fact, current Earth satellites could potentially capture some of these photons, as could the Juno spacecraft orbiting Jupiter. But since these spacecraft were never designed to observe such things, there’s no way for them to distinguish the signals from background radio noise.
But the study shows it could be possible to observe the effects of high-frequency gravitational waves. If we were to launch specially designed spacecraft into orbit around Earth and Jupiter, we should be able to detect gravitationally induced photons. What’s more, by combining observations from Earth and Jupiter, astronomers could also pinpoint the sources of the gravitational waves, since there would be a time delay from detection at Earth vs Jupiter.
This is an exciting time for gravitational wave astronomers. The field is still in its infancy, but it is quickly maturing. And studies such as this show we aren’t even close to the limits of what we may someday observe.
Reference: Liu, Tao, Jing Ren, and Chen Zhang. “Detecting High-Frequency Gravitational Waves in Planetary Magnetosphere.” arXiv preprint arXiv:2305.01832 (2023).
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Starship gave us quite a show during the first flight test of a fully integrated Starship (S24) and Super Heavy rocket (B7) from Starbase in Texas.
On April 20, 2023 at 8:33 a.m. CT, Starship successfully lifted off from the orbital launch pad for the first time. The vehicle cleared the pad and beach as Starship climbed to an apogee of ~39 km over the Gulf of Mexico – the highest of any Starship to-date.
With a test like this, success comes from what we learn, and we learned a tremendous amount about the vehicle and ground systems today that will help us improve on future flights of Starship.
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This is the vehicle trajectory and mission control audio without any additional commentary. There may be very long periods of silence. For our full hosted webcast, visit
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When two black holes collide, they don’t smash into each other the way two stars might. A black hole is an intensely curved region of space that can be described by only its mass, rotation, and electric charge, so two black holes release violent gravitational ripples as merge into a single black hole. The new black hole continues to emit gravitational waves until it settles down into a simple rotating black hole. That settling down period is known as the ring down, and its pattern holds clues to some of the deepest mysteries of gravitational physics.
Gravitational wave observatories such as the Laser Interferometry Gravitational-Wave Observatory (LIGO) have mostly focused on the inspiral period of black hole mergers. This is the period where the two black holes orbit ever closer to each other, creating a rhythmic stream of strong gravitational waves. From this astronomers can determine the mass and rotation of the original black holes, as well as the mass and rotation of the merged black hole. The pattern of gravitational waves we observe is governed by Einstein’s general relativity equations, and by matching observation to theory we learn about black holes.
General relativity describes gravity extremely well. Of all the gravitational tests we’ve done, they all agree with general relativity. But Einstein’s theory doesn’t play well with the other extremely accurate physical theory, quantum mechanics. Because of this, physicists have proposed modifications to general relativity that are more compatible with quantum theory. Under these modified theories, there are subtle differences in the way merged black holes ring down, but observing those differences hasn’t been possible. But a couple of new studies show how we might be able to observe them in the next LIGO run.
The modified Teukolsky equation. Credit: Li, Dongjun, et al
In the first work, the team focused on what is known as the Teukolsky Equation. First proposed by Saul Teukolsky, the equations are an efficient way of analyzing gravitational waves. The equations only apply to classical general relativity, so the team developed a way to modify the equations for modified general relativity models. Since the solutions to both the Teukolsky and modified Teukolsky equations don’t require a massive supercomputer to solve, the team can compare black hole ring downs in various gravitational models.
The second work looks at how this would be done with LIGO data. Rather than focusing on general differences, this work focuses on what is known as the no-hair theorem. General relativity predicts that no matter how two black holes merge, the final merged black hole must be described by only mass, rotation, and charge. It can’t have any “hair”, or remnant features of the collision. In some modified versions of general relativity, black holes can have certain features, which would violate the no-hair theorem. In this second work, the authors show how this could be used to test general relativity against certain modified theories.
LIGO has just begun its latest observation run, so it will be a while before there is enough data to test. But we may soon have a new observational test of Einstein’s old theory, and we might just prove it isn’t the final theory of gravity after all.
Reference: Li, Dongjun, et al. “Perturbations of spinning black holes beyond General Relativity: Modified Teukolsky equation.” Physical Review X 13.2 (2022): 021029.
Reference: Ma, Sizheng, Ling Sun, and Yanbei Chen. “Black hole spectroscopy by mode cleaning.” Physical Review Letters 130.2 (2023): 141401.
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