4 ms·

> 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

> 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.

tzs 7y ago
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.
- darkmighty 7y ago
  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.
  - ncmncm 7y ago
    Magma is, for practical purposes, infinitely conductive. Only atmospheric EM events are plausible causes of spurious correlated events.
    - darkmighty 7y ago
      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.
pdonis 7y ago
> 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.
- ncmncm 7y ago
  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.
  - pdonis 7y ago
    > 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?
    - ncmncm 7y ago
      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.

tzs 7y ago

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.

darkmighty 7y ago
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.
- ncmncm 7y ago
  Magma is, for practical purposes, infinitely conductive. Only atmospheric EM events are plausible causes of spurious correlated events.
  - darkmighty 7y ago
    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.

pdonis 7y ago

> 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.

ncmncm 7y ago
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.
- pdonis 7y ago
  > 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?
  - ncmncm 7y ago
    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.