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Assuming a human-like entity has the ability to visit a planet that is like (or is, if that makes this easier) Earth during two time periods:

  1. A time period where the planet's magnetic field is stable and Magnetic North is at Geographic North, and Magnetic South is at Geographic South;
  2. A time period where the planet's magnetic field is stable and Magnetic North is at Geographic South, and Magnetic South is at Geographic North.

Assuming also that

  • there aren't any digital or written physical records from a civilization detailing the magnetic field shift as it was happening;
  • the human-like entity lacks a magnetic compass to measure the change, and they don't have the ability to single-handedly check the sea floor for magnetic field stripes, at least until they have evidence that the magnetic field has already flipped.

What would be the easiest discernible visible change on a continent for said entity to notice that the planet's magnetic field must have flipped since the last time they visited the planet?

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  • $\begingroup$ Magnetic compasses are trivial to make for any spacefaring race. To not be able to measure a magnetic field you'd need to have a spaceship without any inductors. Which given how essential they are to do anything with alternating current is Highly unlikely. $\endgroup$
    – sphennings
    Feb 23, 2022 at 5:25
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    $\begingroup$ @Sphennings: My main concern with a magnetic compass is it would be hard for a species to be certain that just because Magnetic North appears to be Geographic South, that feels like it would run into the issue of being sure that they know where Geographic North would normally be, given a long period between visits. Sort of hoping for a not needing to specify a means between spacefaring or time travelling human-like entity being able to tell upon appearing on the land of the planet itself post time period-change - before checking a magnetic compass. $\endgroup$
    – Alexander The 1st
    Feb 23, 2022 at 6:48
  • $\begingroup$ (1) If they don't have magnetic compasses and they are not interested in geology, why do they even care? (2) Why are you shouting? $\endgroup$
    – AlexP
    Feb 23, 2022 at 14:53
  • $\begingroup$ Magnetometers exist? If the visitor recorded magnetic field previous visit the change in polarity would be obvious. Just as visible as any other EM that is outside of 'visible light' $\endgroup$
    – Gault Drakkor
    Feb 23, 2022 at 21:15

8 Answers 8

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Zero evidence to the naked eye

It seems like there wouldn't be any evidence that is readily apparent.

Humans are not sensitive to magnetic fields, so we couldn't directly perceive the field either before or after. We are dependent upon tools such as compasses to even detect the field as it is today.

The only traces that remain of past states of the magnetic field are things we can't see without looking very hard for them, such as the magnetic orientation of rocks. And we need to date the rocks using an independent method in order to see how they relate to the present.

And even if periods of reversal can trigger (or be triggered by) increased volcanism, collecting that data would be a non-trivial task that would take months or years of dedicated research.

This is all stuff you have to go looking for. None of it will just fall into your lap. Nobody will notice this simply by being generally alert to their surroundings.

Finally, it seems that reversals typically take a long time, too. From wikipedia:

Most estimates for the duration of a polarity transition are between 1,000 and 10,000 years, but some estimates are as quick as a human lifetime.

Given the average human lifespan, it seems overwhelmingly likely that it would not be obvious to a human visitor even if the planet were undergoing geomagnetic reversal at the present moment, because it would not be complete for very many lifetimes, and the transition would be very gradual. They would likely need to take precise compass readings over a long time to detect any change.

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First get an ocean floor map. A well equipped interstellar survey ship could probably achieve this from orbit. What you are looking for is an active spreading zone. (You did say Earth or Earth like.) The mid-Atlantic ridge or equivalent is what you want.

Next you need a few ocean-worthy boats, each dragging a high quality submersible magnetometer.

Your goal is to map the magnetic stripes in the former magma that cooled into oceanic crust. Other than some variance due to polar drift, the youngest ones (closest to where new magma is coming up) should line up very needly either withthe current field or be in (nearly) direct opposition to it. Grab a few core samples to measure sediment depth, radioactivity, and other signs of age, and you'll be on your way to building a timeline of magnetic reversals dating all the way back to that ocean first forming.

There's a great video on magnetic stripes in the Atlantic here:

https://www.geolsoc.org.uk/Plate-Tectonics/Chap3-Plate-Margins/Divergent/Mid-Atlantic-Ridge

When you come back many years later, check your compass and the stripes close to the ridge and you'll see what's happened magnetically (if anything) in your absence.

On dry land, you can get some info about magnetic history from volcanic rock, but it will be a huge project to get details approaching a magnetic survey in a spreading ocean.

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  • $\begingroup$ From the question itself "Assume... they didn't have the ability to single-handedly check the sea floor for magnetic field stripes, at least until they had evidence that the magnetic field had already flipped" $\endgroup$
    – Starfish Prime
    Feb 23, 2022 at 12:42
  • $\begingroup$ @StarfishPrime - I admit I missed that detail, but my answer would require more than one person. In terms of total labit needed, a few boats checking a small strip perpendicular to the spread would be massively less labor intensive than running around to dozens of recently (geologically speaking) volcanic sites and hoping to get lucky enough to gather enough data. $\endgroup$
    – Escaped Lunatic
    Feb 23, 2022 at 13:23
  • $\begingroup$ Measure the spreading rate and look for the closest stripe you can map with your magnetometers being reversed from the existing field -- that will tell you a) the last flip was "recent" and b) let you estimate how recent. $\endgroup$
    – Zeiss Ikon
    Feb 24, 2022 at 18:53
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No detectable biological effects

As stated here:

  • Earth's magnetic field varies between 25 and 65 μT.

  • MRI systems used for medical diagnosis expose patients to fields as high as 2.5 T.

  • MRI operators can be exposed to fields up to 5 mT while standing by the patient.

Studies have shown that slight biological efects appear only for exposure to a magnetic field of >2 mT.

No effects on materials

In this study, materials were exposed to static magnetic field of ~2 mT.

Therefore magnetic fields of 65 μT may not have any significant effect on biology or materials.

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  • $\begingroup$ I'm not sure this answer makes a whole lot of sense... there are species which can sense the planetary magnetic field, and there are records of magnetic field reversals in igneous rocks. It also doesn't really answer the question. $\endgroup$
    – Starfish Prime
    Feb 23, 2022 at 12:39
  • $\begingroup$ @Starfish Prime ! The question is about "the most easiest evidence to see that a planet's magnetic field has flipped" by a human-like entity which cannot sense magnetic field. To check the magnetic field reversal in igneous rocks is not "most easiest evidence". $\endgroup$
    – imtaar
    Feb 23, 2022 at 13:08
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Their are no signatures that would be visible without taking large scale, detailed, and quite sensitive, scientific observations. The examples that come to mind are:

  1. There will be an ozone layer depletion associated with the reversal but peak ozone concentrations are so low that you need good instruments just to detect the ozone layer exists. By the time the magnetic field has fully stablised the change will probably be small enough to disappear into the background variation.
  2. There will be radiological evidence burned into exposed rock and soil surfaces from the increased cosmic ray and UV flux caused by the weakened magnetosphere and associated ozone depletion but that evidence requires huge labs, contrasting rock units to measure against and fades quite rapidly unless buried relatively deeply quite soon after the radiation event.
  3. There will be fossil atmospheric evidence of the ozone depletion but that will be hard [impossible?] to differentiate from a nearby supernova or a glancing hit from a gamma ray burst and it won't be immediately apparent to a surface observer.
  4. There will be indications that something very strange happened in the dendrochronology, probably including the lose of some high altitude communities and bizarre growth behaviour in survivors but it would take a board ranging survey to even realise that this was not a local phenomenon let alone the implications and causes.

Arrive during a reversal and the changes would probably present a picture of some grand change underway but again knowing what would be very hard without very good instruments and a an extended study period.

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Confused birds

If the magnetic reversal occurred extremely quickly, for example "within a human lifetime" as is sometimes suggested as an extreme, it might be possible that this would mess up the migration patterns of long-lived birds. Certain migratory birds can live decades, and if the shift is occurring within 100 years, the movement of the magnetic poles might be significant enough that some birds might not have time to adapt as they would if the shift occurred on a 1000-10000 year timescale.

As to actually observing this, maybe the visitor would notice that there are less of a specific species of bird around than there are before? It probably wouldn't be obvious to anyone who hasn't specifically studied this, but an ornithologist might notice.

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Difference from prior observation.

The primary evidence would be a change from the previously recorded magnetic field.

The entity if they are the least bit scientific /methodical / profit oriented they would have observations from previous visit(S). Then a quick comparison would indicate the change, Likely causing further investigation if they are surprised or wanting confirmation.

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Recent meteor strike (I think)

An iron-rich meteorite struck the ground, leaving a fair sized Meteor Crater full of little novelty grains of iron. If these solidified in a magnetic field, and were compressed into surrounding sediment, there ought to be traces of magnetization if you measure very carefully. You compare those to the current magnetic field and see if they're backward despite the recent age of the crater. Caveat: I don't know if that really works.

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Extinction Events

The magnetic field of a planet helps stop background radiation of space, and it's believed that during a pole switch, the field weakens to allow radiation through, although that's just currently a theory and doesn't have much evidence for it.

What does is magnetoreception - the phenomenon of migrating animals using magnetic fields for direction. Essentially this means that every major migration pattern on the planet will flip and lead to the extinction of the majority of migrating species.

(I assume that this planet has animals, since you allowed Earth to sub in for the planet in the question, and Earth has animals.)

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    $\begingroup$ If flipping the poles killed all "magnetic migrators," that seems to imply that all such species currently on Earth evolved that behavior during the current "chron," i.e. within the last ~780,000 years. Is that right? $\endgroup$
    – Tom
    Feb 23, 2022 at 5:45
  • $\begingroup$ @Tom Just to clarify - most of such species, not all of them. $\endgroup$
    – Halfthawed
    Feb 23, 2022 at 6:01
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    $\begingroup$ I've been doing some reading (because this is fascinating), and wikipedia says: "Statistical analysis shows no evidence for a correlation between reversals and extinctions." $\endgroup$
    – Tom
    Feb 23, 2022 at 6:05
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    $\begingroup$ @Tom It says that at the end of a paragraph explaining how unreliable statistical analysis of the fossil record can be. Which makes sense, given that the modern fossil record is very, very incomplete. $\endgroup$
    – Halfthawed
    Feb 23, 2022 at 6:09
  • $\begingroup$ I did wonder about this, but the flips occur over many generations and as such they might be associated with dramatic changes in eg. migratory patterns, but not necessarily extinctions... the animals might reasonably change their behaviour in response to the changes in their environment, etc. $\endgroup$
    – Starfish Prime
    Feb 23, 2022 at 12:41

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