I am currently reading The Science of Insterstellar, which explains most things very well, but some things leave me confuzzled, which I hope to get answers to here. I am no physicist, but highly interested and eager to learn.

I get tidal forces in principle, but one thing never clicked for me, and that is why tidal forces are the reason that the opposite side to the other object (moon, black hole, whatever) point away from said object.

Take this picture from wiki;

Tidal forces sum


I understand all vectors up to the middle of the circle, where they point inwards and ever so slightly to the right.

Everything left from that is a mystery to me. How can the sum of all the forces pointing towards the object on the right result to vectors pointing away from it?

Even when I remembered that the object itself also has gravity in itself, all those vectors should also point inwards to the object.

How can the vectors (which I understand to be the sum of all gravitational forces ... ?) point away from the mass?

  • 1
    $\begingroup$ physics.stackexchange.com/q/121830 $\endgroup$ Jul 20, 2015 at 8:29
  • $\begingroup$ @dmckee Interesting... But that only means that our tidal waves of the sea level don't exist, right? The forces themselves (as depicted here) are still valid? $\endgroup$
    – F.P
    Jul 20, 2015 at 8:49
  • $\begingroup$ No, it means that the picture the book offered you is highly idealized. $\endgroup$ Jul 20, 2015 at 9:07

1 Answer 1


This link explains it:

The Earth experiences two high tides per day because of the difference in the Moon's gravitational field at the Earth's surface and at its center. You could say that there is a high tide on the side nearest the Moon because the Moon pulls the water away from the Earth, and a high tide on the opposite side because the Moon pulls the Earth away from the water on the far side. The tidal effects are greatly exaggerated in the sketches.

enter image description here

  • $\begingroup$ Okay, that makes much more sense now. So all the vectors actually do point towards the other object, it's just that the "moving" of the first object is calculated out so it remains seemingly stationary, resulting in the "pointing away" vectors. Basically just a different frame of refence (outside the system vs. locked on the first object) $\endgroup$
    – F.P
    Jul 20, 2015 at 7:39
  • $\begingroup$ thats right, it is the relative motions we see as tides. $\endgroup$
    – anna v
    Jul 20, 2015 at 8:11

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