Magnetic field
A field of force created in the surrounding space by a magnet or an electric current, exerting force on magnetic materials and moving charges. It is described as a vector quantity with both magnitude and direction.
Earth's magnetic field has flipped hundreds of times; the last reversal was about 780,000 years ago. During reversals the field can weaken to under 10% of today’s strength, letting auroras appear near the equator. Since 1840, Earth’s field has already weakened by about 9%.
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Migratory birds may ‘see’ Earth’s magnetic field. In European robins, the eye protein cryptochrome 4 changes its electron-spin chemistry under magnetic fields when lit. A 2021 Nature study showed it is magnetically sensitive, linking bird navigation to quantum mechanics.
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