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We know that the current SETI efforts have not found evidence for the existence of intelligent exo-civilizations (XCIV). There also has not been any evidence found of ancient non-human civilizations in the fossil record. One possibility, among many, is that XCIVs only exist for a geologically short amount of time before collapsing/extinction. Then, it could be possible that, even though none exist now, one or more XCIV could have existed in the past, and it was capable of transmitting electromagnetic (EM) radiation signals.

So, consider the following hypothesis:

  1. In the last 100 million years, at least one XCIV capable of emitting directed EM radiation existed within 100 light-years of our solar system.
  2. The CIV sent directed EM beams towards all the planets in the star systems surrounding it.
  3. Based on what it could detect about each planet's composition, it designed the beams (frequency, signaling pattern, modulation, etc...) such that they would leave as large of a mark on the targeted planet's geologic record as possible (the mark).

Question: Is there any form of patterned, extra-solar EM radiation that could theoretically leave a mark in the geologic record that could be recognizable to us as such up to 100 million years later? And could a human-level civilization be capable of projecting such beams?

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    $\begingroup$ This is a really interesting premise. I'm surprised I haven't encountered it somewhere before in science fiction! $\endgroup$ – wogsland Feb 12 '17 at 23:38
  • $\begingroup$ This is somewhat related: arxiv.org/pdf/1804.03748.pdf $\endgroup$ – Justas Apr 18 '18 at 17:09
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No, it is not possible.

The only thing I could think about is re/de/magnetisation of magnetic minerals. Even then I'm not sure it's feasible, because you to form a magnetic field you need an electric current in motion, but that's a question for physics, not here.

So far, the major magnetic signals recorded in the geological history are well understood. There are many small scale signals that are less well understood, and are believed to be the result of internal processes within the Earth. You can look at my answer here for examples of these signals. Anyway, even if an extraterrestrial civilisation could somehow affect the geological record, it would have to be sustained for very long times (hundreds to thousands of years) for us to be able to see something, and even then we probably wouldn't be able to distinguish it from noise.

As an example, think of a river with rocks in it, constantly getting moved and tumbled. Then you go and throw a rock inside. There would be absolutely no way to know which rock was it (and even that someone threw a rock inside), even a day later, let alone 100 million years. The only way to distinguish something would be a huge amount of rocks, not just one. And then there's the question, of why would someone spend this enormous amount of energy of throwing rocks in the river or sending EM beams towards planets?

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  • $\begingroup$ It's like geophysics generally - there is always ambiguity, whilst any 'smoking gun' evidence is obscured by time and geological process. $\endgroup$ – Gordon Stanger Nov 8 '16 at 13:47
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You can just think of this in terms of energy. To leave a mark in the geologic record, you have to use something that is at least as strong as the forces that are shaping the world around you: erosion, melting, tectonic forces, wind and weather. These are all very energetic things that are happening around us all the time, and for long time spans.

If you want to compete with this, and leave a recognizable, permanent mark on Earth, you have to have something that is at least as energetic. That is difficult already for humans to achieve (though we do -- we have significantly accelerated erosion, and our moving of rocks and earth will clearly be distinguishable for a long time to come). But think of what it would take to do this from several light years away? Even a well focused laser beam would be far larger than Earth at these distances, meaning that you would have to send your laser beam with a far larger power and duration than the processes on Earth, just to leave a bit of a mark. This is clearly not possible.

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