Is there really such a thing as an inertial frame? How would we know for sure if we were in one? Looking at space, things are moving and accelerating all over the place, it would seem impossible to tell whether a frame is really inertial or not. Imagine you are in space and everything around you is accelerating past you. How do you know if you are in a non-inertial frame or in an inertial frame where everything around you is experiencing a force?