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Comment by darkmighty | Hacker News Reader

> no signal of terrestrial origin will travel at anywhere close to the speed of light through the Earth (earthquake waves travel orders of magnitude slower)

To be fair, electromagnetic (i.e. radio) waves do travel at the speed of light through the Earth, though almost none can significantly penetrate the Earth itself, except extremely low frequencies. Electromagnetic interference should be quite easy to measure and look for correlations though; if scientists did their homework (likely), it has been ruled out.

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> electromagnetic (i.e. radio) waves do travel at the speed of light through the Earth, though almost none can significantly penetrate the Earth itself, except extremely low frequencies

Yes, but they won't jiggle the test masses in LIGO, so they won't produce a signal.

LIGO does not record jiggling test masses. It records electrical signals that can be generated by jiggling test masses, and also by other means.

Every false alarm should be investigated to see what produces such frequent apparent events. They need to distinguish better real events from ersatz events, independently of simultaneous reports elsewhere or optical confirmations.

> LIGO does not record jiggling test masses. It records electrical signals that can be generated by jiggling test masses, and also by other means.

No, it records interference patterns at a detector which are produced by jiggling of the test masses. There are no other means of producing those interference patterns--certainly not by low frequency EM radiation happening to pass through the apparatus, which was what the post I was responding to was talking about.

The question is what makes the test masses jiggle. A gravitational wave is one possibility, but certainly not the only one. Seismic disturbances, a truck driving by the facility, someone banging on the wall of the vacuum chamber, etc.

The electrical signals you are talking about come from the interference detector. It is possible that there could be electrical noise elsewhere in the circuit, but that's something the experimenters already allow for and take great precautions to minimize, just as the test masses are isolated to minimize any source of jiggling of them other than gravitational waves.

> They need to distinguish better real events from ersatz events

What do you think they could do better?

They need to discover better ways to reject apparent events without relying on the other detectors. The frequency of apparent real events needs to approach that of actual events, for corroboration from other detectors to mean anything.

> They need to discover better ways to reject apparent events without relying on the other detectors.

They already have plenty of these. Comparison of signal arrival times at multiple detectors is not even close to being the only criterion they use. It's just an important one that is not mentioned in Hossenfelder's article.

Is the index of refraction of the Earth at low radio frequencies really 1? I'd expect it to be higher, and so there to be some slowdown for radio waves.

Depends on how low the frequency is :)

But indeed I'm not an expert by any means on magma properties, perhaps someone else can chime in. If magma is conductive then even less radiation goes through.

Keep in mind at extremely low frequencies the whole Earth is in mid-field or near-field of emissions, so it doesn't behave attenuated like light rays.

Magma is, for practical purposes, infinitely conductive. Only atmospheric EM events are plausible causes of spurious correlated events.

Even an infinitely conductive sphere does not screen the quasi-static near field very well. Although the delay is probably going to be weird, greater than direct visibility. Atmospheric EM events indeed are what comes to mind.

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