When Einstein indicated that a single force is fictitious,what did he means?

Question

Is the concept of acceleration fictitious? when some one experience a free fall isnt he accelerating?

Answer 1

It would have been helpful if you had cited where Einstein said this.

But here's the deal...all motion is relative (which is why the theory is called the theory of "relativity")...relative to something or somethings. Even things that seem to be perfectly still, like the tree in your front yard, are moving relative to something...relative to the sun, for example. In other words, there is no such thing as an absolute motion relative to absolute zero velocity.

With all these motions (velocities) we have accelerations (v = at; a = v/t) that are, like their velocities, not absolute. They, too, are relative. So, as F = ma or F = mv^2/r = (m(v/t)^2)/r, the associated forces are relative.

As a consequence, there are forces acting on everything, even when none is readily apparent. That tree, for example, has weight, which is one force. But it also has a kinetic force (centripetal) due to its spin around the Earth and its revolution around the sun while on the Earth. Then there's the motion of the solar system around the center of the galaxy, the motion of the galaxy through the universe, and the expansion of the universe itself.

So a single measured force is really the vector sum of all those other forces created by all those other motions. So the notion that there is but a single force at work is "fictitious." The good news is that the single force usually works in our observable world because we can ignore the effects of all those other forces. But, alas, that is not the case in the relativistic world of near light speeds. That is probably why Einstein would indicate what you claim he did.