rest frame principle

[somanystylez]

Hi guys, this is my first post here, and I've just recently started studying special relativity. I don't understand some thing so I hope to get an appropiate answer.

If we have an object that serves as an inertial reference frame, is this object simultaneous with itself from its own rest frame? And are all of its parts mutually simultaneous when viewed from the rest frame of that object? I hope you can answer me.

[ajb]

Really one should think of rest frames of massive particles, which is really just the same as picking an origin for your inertial coordinate system. For objects with some extent we need to be a bit careful. For most purposes a "small enough" object will pay the same role as a particle.

 

When we have larger object we need to take care as it takes light some finite time to cross the object.

[swansont]

"Simultaneous with itself" is an awkward phrase. Simultaneity refers to events. Events that are simultaneous in one frame generally are not in other frames.

[somanystylez]

But if an object has parts, surely from its rest frame it must be simultaneous with its parts at some time. Otherwise we couldn't even define the object because without its parts it doesn't exist.

[ajb]

What do you mean by this statment?

 

It reminds me of the fact that there are no true rigid bodies in special relativity.

[somanystylez]

For instance, take an intertial frame, let's say a cell phone which is at rest with respect to the earth. From its perspective it's parts are simultaneous with one another and simultaneous with the object itself. That's what I mean.

[ajb]

For instance, take an intertial frame, let's say a cell phone which is at rest with respect to the earth. From its perspective it's parts are simultaneous with one another and simultaneous with the object itself. That's what I mean.

 

 

As swansont has said, it is events that you should be talking about. I am not sure what simultaneous with one another means here.

[somanystylez]

As swansont has said, it is events that you should be talking about. I am not sure what simultaneous with one another means here.

 

I mean parts in the sense of temporal parts which are extended in space and time. So basically I'm refering to stages of the parts of that object. The stages make the object what it is at a particular time.

[ajb]

I think I see what you are asking, however I am not sure how to answer it.

[somanystylez]

My opinion is that everything else than 'yes, from its frame the events are simultanoues' would be illogical, since then we wouldn't be able to define the object which serves as a reference frame itself

[ajb]

But there are issues here with using extended objects to define inertial reference frames. In particular object can not be rigid and it takes time for light to propagate from one end of the object to the other.

 

Just thing about blinking lights on say both ends of a very long ruler. Can I set the blinks to be simultaneous?

[studiot]

An object that has spatial extent will appear to have a different shape depending upon the observer's reference system.

 

So any 'part' of an object may 'exist' in one frame but not another, depending upon how you chop it up.

[xyzt]

Hi guys, this is my first post here, and I've just recently started studying special relativity. I don't understand some thing so I hope to get an appropiate answer.

 

If we have an object that serves as an inertial reference frame, is this object simultaneous with itself from its own rest frame? And are all of its parts mutually simultaneous when viewed from the rest frame of that object? I hope you can answer me.

In SR objects cannot be reference frames, reference frames are geometric constructs. They have infinite extents in terms of space.

An object at rest in a certain frame has all its points at rest in that frame. I hope this answers your questions (and that it corrects some of your misconceptions).

[somanystylez]

In SR objects cannot be reference frames, reference frames are geometric constructs. They have infinite extents in terms of space.

An object at rest in a certain frame has all its points at rest in that frame. I hope this answers your questions (and that it corrects some of your misconceptions).

 

It does, thank you for your very well formed answer.

[studiot]

In SR objects cannot be reference frames, reference frames are geometric constructs. They have infinite extents in terms of space.

 

True but we often specify the reference frame by reference to an object.

 

For examples

the reference frame with respect to the person on the spaceship/train/vehicle.

the frame with respect to the observer waving from the platform

the frame with respect to the person in the jet flying by

 

etc.

 

Further there is one frame in which any object can be considered at rest we say that it is at rest with respect to itself.

[xyzt]

True but we often specify the reference frame by reference to an object.

true

[ajb]

True but we often specify the reference frame by reference to an object.

 

For examples

the reference frame with respect to the person on the spaceship/train/vehicle.

the frame with respect to the observer waving from the platform

the frame with respect to the person in the jet flying by

Right, but you then have to pick an origin for the coordinate system you employ. You can then either think of the object as being a point and place this at the origin or use the centre of the object as defining the origin.

 

As long as we have scales at which the object you use to define the origin of the coordinate system is small there is no problem; both the ways of thinking agree.