My coworker told me gravity isn’t a force it’s an effect mass has on space time, like falling into a hole or something. We’re not physicists, I don’t understand.
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Let’s look at Newton’s first law
> A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a force.
But we look up in the sky and see that the planets and the moon aren’t moving in straight lines and there aren’t any obvious forces acting on them. So Newton explained that with gravity as a force.
Have you ever seen the [flight path of plane on a map](https://imageio.forbes.com/specials-images/imageserve/5e546df1d378190007f43538/LONGEST-FLIGHT-IN-THE-WORLD/960×0.jpg?format=jpg&width=1440)? Why do they take such roundabout routes instead of just flying in a straight line? Well, they *are* flying in a straight line. But the surface of the Earth itself is curved, so any straight lines on the surface also become curved. Wait a minute…
So Einstein proposes that the planets and the Moon *are* moving in straight lines. And gravity is not a force. It’s just the stuff that they’re moving through, space and time, are curved, so their straight lines also end up curved. And that curvature of spacetime is called gravity.
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