By artificial force, I mean a physical force applied by us onto an object which sets it in an accelerated motion (& not a natural force like gravity). eg: hitting a ball. Excuse me if the terminology is wrong. I didn't know what to call it otherwise. On the contrary, we have well established names for the 4 natural forces.

Consider I am on a spaceship orbiting the earth. My fist punches a ball initially at rest wrt me. Next, I will see the ball accelerating & moving away wrt me. Could this acceleration be explained in terms of curving of spacetime?

For instance, this is what think. Both my fist ( & ofcourse me) & the ball have mass. Mass is energy. Mass or energy curves spacetime which appears to us as gravity. Both my fist (& obviously me) & ball have different patterns of curved spacetime around them. Punching the ball by my fist transfers some energy onto the ball. Because now the ball has an excess energy, the curved pattern around it in the spacetime changes. And this curving of spacetime makes us to think as if the ball is accelerating through it. Is this thought or explanation correct?

Another implication of this thought if its correct is that it would mean the artificial force as described in the above context produced curving of spacetime (which is gravity) or in other words, that there is no distinction between artificial force and gravity.


2 Answers 2


The evolution of "energy" is coupled with the evolution of curvature of spacetime, so when you transfer energy from your fist on the ball the curvature of spacetime changes. But then again, so it does if the ball is moving away from you with constant speed due to the fact that distribution of "energy" in universe as a whole is changing, you don't need to hit the ball.

The changes are continuous though, there is no instantly in relativity. No information can move faster than light so the curvature at distant point will change only when the information about the balls movement will reach the point.

And this curving of spacetime makes us to think as if the ball is accelerating through it.

That is not so. If the acceleration was due to spacetime curvature, everything in the vicinity of the ball at the moment of you hitting it will be influenced by this curvature. So for example if the ball would be wet and you would punch it, the droplets on the ball should move (more or less, depends on the exact spacetime curvature around the ball) with the ball. That does not happen. When you hit the ball, the droplets inertia will force them to "remain" at the original movement and they will separate from the ball.

  • $\begingroup$ @ Umaxo - edited by removing 'instantly' from 'Because now the ball has an excess energy, the curved pattern around it in the spacetime changes instantly'. $\endgroup$ Oct 22, 2019 at 12:26

When you are talking about pushing the ball, you are indirectly talking about electromagnetic force since electromagnetic force "pushes" things apart from each other. But electromagnetic force doesn't require space time curvature.

  • $\begingroup$ -Are you saying when I hit the ball, the reason why the ball accelerates away is only due to the electromagnetic force causing repulsion between the electrons of the ball & my fist? And that there is no need for an artificial force? $\endgroup$ Oct 22, 2019 at 12:41
  • $\begingroup$ -Lets say an example where there isn't any collision of masses. For instance, I move my hands in the air randomly. Clearly it will be an accelerated+decelerated motion. There is a force involved in it. This force allows us to move our hands against force of gravity. But what force? It kind of feels we are producing this force from the energy of biological processes taking place in our body. Life is an open system. Work can be done by a living system. How do you see this? $\endgroup$ Oct 22, 2019 at 12:42
  • $\begingroup$ Again, while moving our hand, we use contraction/expansion of muscle fibers. Loosely speaking, this occurs due to a change in distance between neighboring atoms resulting in a deviation from the equilibrium state. This deviation results in an imbalance of electromagnetic forces and the atoms try to settle back into the equilibrium state, giving rise to force like in case of a stretched rubber band. $\endgroup$
    – NiRVANA
    Oct 22, 2019 at 13:06
  • $\begingroup$ PS: Muscles work in a much more complicated way which involves reconfigurations of proteins and stuff, but the analogy is same. $\endgroup$
    – NiRVANA
    Oct 22, 2019 at 13:07
  • $\begingroup$ @Astik- So basically there are only natural forces, right? $\endgroup$ Oct 22, 2019 at 13:13

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