Earlier today I was reading through this entire thread: https://www.physicsforums.com/threa...tatement-acceleration-is-not-relative.670653/ And I remained confused about one thing in particular. The original poster made a statement (bolded below) on page 1 that seemed intuitive to me, and it seemed like the other posters never did sufficiently engage with it and explain why it was mistaken: ---------------- >"An accelerometer is a device that measures proper acceleration." >>"This is an inference." >"Conceptually, an accelerometer behaves as a damped mass on a spring. When the accelerometer experiences an acceleration, the mass is displaced to the point that the spring is able to accelerate the mass at the same rate as the casing. The displacement is then measured to give the acceleration." >>"Again, the bold text is an inference. Einstein interprets the behavior of the instrument in two ways. Observed from the inertial reference frame, it is indeed acceleration that causes the displacement and counteracting force. Observed from the non-inertial frame, it is a gravitational field and the forcible restraint from acceleration that displaces the mechanism. There is no acceleration in the non-inertial frame, according to Einstein's interpretation." -------------------- Basically, my question is this: how do we know that an accelerometer is measuring proper acceleration? How do we know that fictitious forces are not real forces? This is not meant to try to debunk relativity. I am just having a hard time grasping how we can be so confident about detecting absolute acceleration. Imagine that you have lived in a windowless space station your entire life (with internal power sources to provide lighting to grow plants, etc.). For all of your life, you have freely floated with the space station. Any relative motion with the walls of the space station had to come from the work of your limbs against the walls of the space station, in your experience. That's all you know so far. Imagine that, one day, you notice the entire space station shift in relative motion with respect to you after you were just freely floating in mid-air, and you feel yourself pushed against one side of the interior of the space station for one minute. Then, after that minute, the apparent "force" stops, and you feel yourself freely floating again. You are just a lowly space station janitor, so you know nothing about any sort of rocket propulsion with which the space station might be equipped. All you know is, one minute there was this change in relative motion and, even after this relative motion stopped once you made contact with the side of the space station, this apparent "force" that continued to push you against one of the sides of the space station with considerable force, and then the next minute it stopped. In this scenario, should such a person be able to deduce with the information on hand that the apparent force was a fictitious force and that, in reality, it was the space station that was undergoing proper acceleration? Could such a person be able to rule out the idea that they themselves were the ones that initiated the relative motion, and the space station was merely resisting their progress? Would the person need any more information in order to figure this out? If the latter, then what additional information would that person need? Does the person need to know about the rocket engine? Does the person need to be able to look at the fixed stars and measure a change in relative velocity with the fixed stars in order to deduce that it was the space station undergoing proper acceleration, and not the person themselves? To be sure, an accelerometer attached to the space station would show tension in the springs. However, how do we know that it is not the springs themselves that are flexing and moving the internal bits of the accelerometer (or however an accelerometer works), rather than a force acting on the accelerometer?