How does an isolated observer know if they're accelerating?

Question

I'm aware of this question and read its answers, but I'm not sure if it specifically addresses what I want to ask, and also it'd be nice to have a more accessible answer since I don't know GR.

I'm reading a book on principles of relativity, and am going through the definitions of reference frames and free particles. From what I've understood from there and other answers on SE, a non-inertial reference frame is one in which a free particle is measured to be undergoing acceleration. This requires an observer to identify a free particle in the first place.

If I'm an isolated observer, there can be two scenarios:

I see a particle at rest w.r.t. me. How do I tell if that's a free particle without communicating with someone I know is in an inertial frame (and hence can confirm whether or not the particle is free)?
I see a particle that's accelerating: how do I know whether I'm in an accelerating frame, or if I'm in an inertial frame but that particle is accelerating?

As someone who started with the subject, it's very confusing for me. I'm not sure if in either scenario it'd be possible for me to confirm whether the particle is free or not.

Does this answer your question? How does an isolated body in deep space 'know' it's rotating? — sammy gerbil, Mar 23 '20 at 21:08
Can’t you feel something when your car, bicycle, train, etc. accelerates or decelerates? — G. Smith, Mar 23 '20 at 21:10
@G.Smith: Hmm I'm considering the situation where I'm in empty space, with only 1 more particle to look at as per the 2 scenarios I described in the question. — Shirish Kulhari, Mar 24 '20 at 07:06
@sammygerbil: Thanks for the link! But I already had looked at that and it concerns rotation. For now I'm just wondering about straight-line acceleration. — Shirish Kulhari, Mar 24 '20 at 07:22
@ShirishKulhari Rotation is also acceleration. The same principles apply. If you see another particle rotating around you, how do you know whether it is rotating or you are spinning about your own axis? — sammy gerbil, Mar 24 '20 at 08:33

score 1 · Answer 1 · answered Mar 23 '20 at 21:29

In my comments to your previous question that I answered I gave the example of driving a car quickly around a corner. In one direction you feel pressed against the driver side door. In the other you feel pushed toward the other side the other side. Things on the seat next to you slide outward as if there were some force pushing them.

Similarly, when you accelerate or brake you feel pressed back against your seat or thrust forward, respectively. In each of these cases you feel like there are forces at play. In actuality you are experiencing your body's inertia. That is, your body's resistance to a change in motion per Newton's 1st Law. That's how you know you are in an accelerating frame.

Hope this helps.

score -1 · Answer 2 · answered Apr 11 '22 at 05:16

-1

By having a standard to which they measure their displacement vs time on an ongoing basis.

answered Apr 11 '22 at 05:16

MEONH.

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If you want to determine which is moving, you or the object, then establish a second standard whose position from both is known at the start of measurement. Then ask, did I move away from that standard or did the object. Pretty easy. Frame of reference. Look at something else nearby like a tree. Am I moving past the stars or is the stars moving past me? Is the tree moving away from you?No. So you are not moving relative to the object in question. The object in question is moving relative to you. – MEONH. Apr 11 '22 at 05:19
I think you may be slightly confused on the nature of Relativity. – shai horowitz May 14 '22 at 14:31

How does an isolated observer know if they're accelerating?

2 Answers2