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Patrick26 t1_j4yihgi wrote

Yes. The Earth and the Moon are tidally locked, so if the Moon was the size of the Earth we would have what you posit.

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Princeofcatpoop t1_j4ys19n wrote

The moon is tidally locked to the Earth. The Earth is not tidally locked to the moon. If it was, the moon would only ever be on one side of the earth.

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Enorats t1_j4yzg6z wrote

That's not necessarily possible though. The Moon is tidally locked to the Earth because its smaller by a large margin. The Earth would eventually tidally lock to the Moon as well, but our Sun will destroy both the Earth and Moon long before that ever happens.

Two similarly sized bodies would take longer to lock to one another than the Moon did to the Earth, but less time than it'd take the Earth to lock to the Moon. I've no idea how long that'd actually take though, or if it'd be within the lifespan of the average star similar to the Sun.

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Underhill42 t1_j53hdho wrote

Keep in mind the moon is only ~1% the mass of the Earth, it's not doing much.

I would imagine the time to tidal locking decreases at *least* linearly with increasing gravity from the "lock-er" (e.g. twice the force = half the time, maybe less. That's normal for most systems), in which case if the moon were Earth-mass, 100x larger, Earth would lock to it 100x faster, and the 50 billion years until tidal locking (~55 with time served) would be closer to 550 million - almost before liquid water appeared on our surface.

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PoppersOfCorn t1_j4yoba7 wrote

It wouldn't be that simple. You have to take the the inverse square law Into consideration

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aspheric_cow t1_j4yr0ib wrote

I don't get it. How exactly does the inverse square law tie into this specific case?

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PoppersOfCorn t1_j4yvmu3 wrote

The inverse square law proposed by Newton suggests that the force of gravity acting between any two objects is inversely proportional to the square of the separation distance between the object's centers.

So it is not as simple as the moon is tidally locked to earth, so, therefore, two earth sized planet could be tidally locked

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