I think it would be helpful for laypersons like myself to understand how, in practice, a "real" force differs from a pseudo-force. Virtually all explanations (eg, on this stack, Wikipedia, Quora, great Veritasium video) of the difference between a pseudo-force and a real force that use the freefalling/accelerating reference frame thought experiment, focus on how the falling/accelerating object cannot tell whether it's falling/accelerating in space or on Earth, but fail to explain how this differs from a real force.
As I see it, an argument that gravity is not a real force should proceed as follows:
- Here is how a falling/accelerating object would behave were gravity a real force.
- A falling/accelerating object does not behave this way;
- Hence, gravity is not a real force.
Based on this great answer, I believe that the difference between a real force such as say, the electromagnetic force, and gravity, is that the object's mass does not affect its acceleration under gravity, while the object's mass (or rather, the ratio of mass to the strength of the field) does affect its acceleration under the electromagnetic force. I interpret this to mean that if I drop iron objects of various masses from equal height, then they will accelerate toward the Earth at the same rate (reaching the ground at the same time), whereas if I place them an equal distance from a strong magnet, then they will accelerate toward the magnet at rates proportional to their masses (reaching the magnet at different times).
Additionally, I believe that the implication behind the thought experiment that one cannot tell the difference between weightlessness and freefall, or acceleration and gravity, is that with respect to a real force, one would be able to tell the difference. This begs the question ... how?
With focus on how gravity is not a real force, rather than on how gravity is a pseudo-force, I believe that the below questions are essentially equivalent, so answering any one of them should be sufficient, but I may be wrong about that:
- If gravity were a "real" force, then how would I be able to tell if I'm weightless in space or free-falling to Earth?
- If gravity were a "real" force, then how would I be able to tell if my ship is accelerating "up" in space or I'm on Earth being accelerated "down" by gravity?
- If I'm wearing a ferromagnetic (chainmail) suit while my non-magnetic (plastic) ship flies nearby a magnetar, then how would I be able to tell whether my ship is accelerating away from the magnetar, or I'm being pulled toward it?
- If I wake up in a hospital bed in an unknown spaceship, and I feel that the bed is pushing up against me / I'm pressing down on the bed, then how would I be able to tell whether I'm now Wolverine and have a metal skeleton while the ship is made of plastic (magnet below the ship), or if the ship is made of metal (magnet above the ship)?
Note: Force gradients (tidal forces) are ignored in the original thought experiment, and should be ignored here too.