> > Acceleration is measurable with a device called an accelerometer. Acceleration is not relative (like velocity), it's absolute. The reason that you can't me

> > Acceleration is measurable with a device called an accelerometer. Acceleration is not relative (like velocity), it's absolute. The reason that you can't measure gravitation acceleration in free-fall in a uniform gravitational field[0] with an accelerometer is that all parts of the accelerometer are being accelerated together. [0] The "uniform gravitational field" part comes from Einstein's equivalence principle, and it's pretty obvious that if the field were sufficiently non-uniform then one could expect strain gauges to indicate that the free-falling object _is_ being accelerated. Imagine you have a 10km long space ship near a massive body like Earth, or the Sun, then the gravitational field will not be uniform across that space ship, and strain gauges will a) let you know, b) will even be able to tell you the strength of the field and the heading to the massive object. All these "gravity is not a force" arguments based on the equivalence principle are simply nonsense. The better argument is that gravity is both not a force because it only curves spacetime, and also yes equivalent to a force (especially when you do a Mercator-style projection of curved spacetime onto flat spacetime).*