# Is motion really relative?

I had a question that had been bothering me for some time. I tried to come up with an explanation but I wanted to know whether it is right.

Imagine two different observers say A & B floating in space in their corresponding frames say P&Q with nothing else surrounding them. A observes that B is accelerating away from him and B observes that A is accelerating away from him. I feel that this situation is completely symmetrical for both A & B and thus if the motion is actually relative, then the observations of both A & B must be similar. However, then how is it possible that only one of them say, A, happens to observe a ball in his hand following Newton's laws of motion showing that he is in an inertial frame of reference whereas a ball in the hands of B seems to be acted upon by pseudoforces? Basically, my question is what is the reason for the seemingly asymmetry between them ?

• If both are in accelerating frames, then none of the observers is in an inertial frame. – AtmosphericPrisonEscape Jan 29 at 14:01
• A & B can't be floating (read: inertial) and accelerating (non-internal) at the same time. – JEB Jan 29 at 14:17
• @AtmosphericPrisonEscape I did not say that both are in accelerating frames of references. What I said was it seems to them that the other is accelerating in the frame in which they themselves is stationary. – Deepak M S Jan 29 at 14:32
• It would appear that the Higgs field, which opposes the acceleration of a mass, also determines whether a frame is inertial or not. – R.W. Bird Jan 29 at 16:24
• A related question of mine: physics.stackexchange.com/q/290851/20427. – Dvij D.C. May 24 at 0:43

## 3 Answers

Motion is relative but acceleration is absolute. You can know if you are being accelerated without any reference to the outside world. You will feel it in your internal organs and if you carry out an experiment you will observe pseudo forces, even if you don't look outside your space ship. If a body A feels none of these effects and sees that another body B is being accelerated ´relative´ to it, then A is in an inertial frame (but it is unknown whether it is at rest, since this is relative) and B must be accelerating. This is not symmetrical, since B will feel an acceleration.

...what is the reason for the seemingly asymmetry between them ?

People and balls and most other things don't just accelerate. If you are accelerating, it's because something is pushing you. If only one of "A" or "B" is feeling the push, then that obviously is not a "completely symmetrical" situation.

how is it possible that...a ball in the hands of B seems to be acted upon by pseudoforces?

If B thinks that a pseudoforce is acting on the ball, that's because a real force is acting on B, but not acting on the ball. B chooses to ignore the force that they feel, and pretends that their own frame of reference is unaccelerated.

• Well then, I think I need to rephrase the question. What/who decides whether or not a force is acting upon them? My idea was that a force is something that causes an acceleration and if motion is actually relative, I feel that B is perfectly justified in considering himself stationary and thinking that A is accelerating instead (because in his frame, it is the velocity of A that is changing, and not his own.). – Deepak M S Jan 29 at 14:30
• @DeepakMS, There is no physics police force who will stop B from declaring himself to be stationary, and declaring that that A and everything else in the universe except himself are acted upon by some pseudo force. But if B will choose to take the opposite viewpoint and recognize his own accelerated motion, then he will find that the mathematical laws that he uses to describe everything else suddenly become simpler. – Solomon Slow Jan 29 at 14:35
• Thanks, I think your comment then answers my question. It is equally valid to consider both frames but the frame of reference of A happens to be the easiest one to describe the laws of motion. There is nothing special about inertial frames other than the fact that it is the frame in which the laws of motion are simplest. Am I right? – Deepak M S Jan 30 at 10:26
• Also, if you don't mind could you add your comment to the answer so that I could accept the answer? – Deepak M S Jan 30 at 10:27

There are really 3 cases possible in this situation:
1) Only A accelerates with respect to an inertial frame,
2) Only B accelerates with respect to an inertial frame,
3) Both of them accelerate

In all of the cases mentioned above we can get the same relative acceleration, and this is why we will not be able to decide which case is actually happening if the only information that we have is about their relative acceleration.

In a sense, the situation is not symmetric at all. But, it becomes symmetric when we look at it from a reference frame P or Q. This is similar to how we cannot differentiate between us accelerating towards the Earth and a lift in space accelerating towards us with an acceleration equal to $$g$$.