Relative movement in Special Relativity (not just Special Relativity , I guess)

[geordief]

Suppose we are an eye in the sky and we look down to the ground and see two objects which are moving relative to the other .

 

We can see that one is stationary (or more stationary ,perhaps?) wrt to the landscape and one is not.So clearly if the two objects are moving wrt each other it is the former that is moving. wrt to the latter and not vice versa.

 

Is there anything at all to this argument? Does the "all motion is relative" position rely upon the background being entirely free of sources of acceleration?

 

I am not espousing this position as it would break with my lifelong understanding but I would like to see it dismantled (if it is not too obvious to need dismantling)

Edited July 9, 2016 by geordief

[ajb]

So really you have yourself in sky and three objects - the two that you identify and then we can think of any marker on the Earth as the third object. To avoid curvature of the Earth we should take the marker to be close to the object in question - say a near by rock or something.

 

The point really is that there is no universal background to consider and all motion is thus relative to whatever you consider to be at rest.

[geordief]

So really you have yourself in sky and three objects - the two that you identify and then we can think of any marker on the Earth as the third object. To avoid curvature of the Earth we should take the marker to be close to the object in question - say a near by rock or something.

 

The point really is that there is no universal background to consider and all motion is thus relative to whatever you consider to be at rest.

Can we say ,generally that ,if we ascribe a locality(that can be as large as we like) to be our FOR then movement within that locality can be assigned non relatively?

 

If so does the universe need to be infinite or perfectly symmetrical (which it isn't) for relativity to hold ?

Edited July 9, 2016 by geordief

[ajb]

I don't really follow - but for sure we don't need the Unievrse to be infinite or symmetric in any way for relativity to hold.

 

Even before you think about Einsteinan relativity, we have a form of relativity behind standard classical mechanics. This form, known as Galilean relativity - which in part tells us that velocities need to be measured against something that is considered to be at rest.

[geordief]

I don't really follow - but for sure we don't need the Unievrse to be infinite or symmetric in any way for relativity to hold.

 

Even before you think about Einsteinan relativity, we have a form of relativity behind standard classical mechanics. This form, known as Galilean relativity - which in part tells us that velocities need to be measured against something that is considered to be at rest.

Yes but that "something that can be considered at rest" can be as large as you like can't it?

 

It can be ,for example the solar system . We could map the solar system and say A is more stationary wrt to its centre than B and so ,by that definition B is moving wrt A rather than vice versa.

 

And we can expand beyond the solar system provided that the "locality" is well defined (has an outer border) .

[swansont]

You get to choose what is at rest if there are no accelerations. The choices we make are ones of convenience, i.e. to make solving the problem easier. e.g. one of the objects, or the center-of-momentum. But the physics still works regardless of the choice of frame.

[ajb]

It can be ,for example the solar system . We could map the solar system and say A is more stationary wrt to its centre than B and so ,by that definition B is moving wrt A rather than vice versa.

Sure, so you want to think of the Sun as stationary and the planets moving arround it. That is fine, it is the natural choice - it respects the symmetry of the system. But it is a choice, and as swansont says we have to be careful with acceleration here. But still, we can pick other objects in the Solar System to be our origin of the coordinate system we set up.

Edited July 9, 2016 by ajb

[TakenItSeriously]

Also it is generally better to keep it down to two observers. Adding a third observer, especially if they are directly between the other two can create additional confusion since the middle observer combining the speeds of the other two wont be consistent with the speeds the other two are observing between each other.

 

For example if two people are approaching the center observer at 0.6c from opposite directions then the middle observer might incorrectly deduce that they are moving at 1.2c relative to each other while they would both experience the other approaching at a speed less than c

[J.C.MacSwell]

Yes but that "something that can be considered at rest" can be as large as you like can't it?

 

It can be ,for example the solar system . We could map the solar system and say A is more stationary wrt to its centre than B and so ,by that definition B is moving wrt A rather than vice versa.

 

And we can expand beyond the solar system provided that the "locality" is well defined (has an outer border) .

I don't think you can say A is not moving wrt B, as the "wrt" is defining a different reference frame, B's frame, and in B's frame that would not be true, A would be moving.

 

So B is moving wrt A, and vice versa, A is moving wrt B.

Edited July 20, 2016 by J.C.MacSwell

[geordief]

I don't think you can say A is not moving wrt B, as the "wrt" is defining a different reference frame, B's frame, and in B's frame that would not be true, A would be moving.

 

So B is moving wrt A, and vice versa, A is moving wrt B.

Yes, I see you are right.. was conflating frames of reference.

 

I was being devil's advocate but got taken in by my own brief

[Delta1212]

Yes, I see you are right.. was conflating frames of reference.

 

I was being devil's advocate but got taken in by my own brief

That's usually a good thing for the learning process. Figuring out something that seems to be a conflict within your understanding of something is an opportunity to figure out where your understanding needs firming up, whether because you haven't learned it yet or, especially if you are working on the bleeding edge of the body of human knowledge, it's something no one has figured out before. Either way, it's a good first step for advancing what you know about something.

 

It's only a problem for people who stop there and refuse to listen to the explanation when one exists.