During the recent merger of two Neutron stars the lead up to the merger was detected as gravitation waves. This was the merger of two spinning bodies that had very strong magnetic fields and they were orbiting each other. How sure is the community of physicists that the signals detected were not ULF radio waves? Radio waves would have had the same frequency and arrived within the same time frame.
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4$\begingroup$ LIGO detects the way a passing gravitational wave changes the geometry of spacetime. Radio waves cannot do this, so we can be absolutely sure that what LIGO detected was gravitational waves and not electromagnetic waves. $\endgroup$– John RennieCommented Nov 26, 2023 at 11:53
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$\begingroup$ It was my understanding that the detection was the lead up to the merger which then ceased when the merger took place. I perhaps wrongly thought that this would be the way radio waves produced by orbiting magnetic bodies would show. i assumed that the greatest gravitation waves would be after the merger. $\endgroup$– Michael McgarryCommented Nov 26, 2023 at 12:45
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$\begingroup$ No, the gravitational waves are produced as the two bodies are spiralling inwards towards each other. $\endgroup$– John RennieCommented Nov 26, 2023 at 13:42
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2 Answers
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LIGO itself is designed to be insensitive to ULF waves. If it wasn't, the strong terrestrial ULF background would interfere with its operation.
ULF waves cannot propagate through interstellar space, where the plasma frequency is ~1 kHz. Below the plasma frequency, the plasma acts as a conductor: it's an effective Faraday shield.
The interplanetary plasma frequency is ~10 kHz, and the ionospheric plasma frequency is a few MHz, so the Earth's surface is even better shielded. Terrestrial ULF comes from things like lighning discharges below the ionosphere, inside all these layers of shielding.
We'd love to be able to detect extraterrestrial ULF: we'd get a better look at cosmic particle accelerators (pulsars and accretion disks) that way. We believe that extremely powerful ULF is involved in the acceleration process, but we can't study it directly.
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LIGO's mirrors are charged as calculated in this answer. The LIGO team has a technique to measure and keep this charge from getting too large over time. However, the calculation shows that a ULF (~50 Hz) radio wave of 10 pT B-field amplitude would be in the noise of their magnetic field veto and cause a strain of $3 \times 10^{-21}$.
The LIGO team has applied a slowly varying B field to the mirror support + magnetometer, and demonstrated that it takes a large easily seen B field to make the event strains they see. Unfortunately for the same E, $B_{Wave}=(1/c)E$ for an electromagnetic wave is much smaller than $B_{Loop}=(\frac{2}{r\omega})E$. Thus for a radius $r=1m$ loop driven at a frequency $\omega = 2\pi 50$, $\frac{B_{Wave}}{B_{Loop}}=5\times 10^{-7}$. So if a ULF wave came in with the same E as generated by their calibration loop, the $B_{Wave}$ would not be seen by the magnetometer.
However, as John Doty explains, any extraterrestrial ULF radio wave would be attenuated away by the plasmas in space and in the earth's ionosphere. This leaves terrestrial ULF signals like Q-bursts propagating in the earth-ionosphere cavity as a possible source. It would be interesting to record the V(t) output of a short dipole antenna and see if chirp like signals exist in coincidence with LIGO events ...just to be sure they are not there!
Despite the above arguments, LIGO has most probably observed gravitational waves. LIGO monitors and vetoes on many phenomena such as ground motion, lightning, storms, ....It is hard to imagine a source of terrestrial ULF chirps which escape the vetoes. Also, after years of observing, LIGO seems able to explain the shapes and magnitudes of their many events as gravitational waves from physically reasonable sources. Perhaps there are also other glitches in their data which ULF waves might explain?
answered Nov 26, 2023 at 23:23
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$\begingroup$ I suppose if they were really ULF radio waves they would have been picked up by other ULF radio wave receivers. I am sure the team at LIGO would have checked for that before releasing their results. $\endgroup$ Commented Dec 3, 2023 at 13:10