# Proper length increase/decrease

1. May 11, 2014

### analyst5

As I've understood so far, there are theoretically many types of acceleration.

In all cases except in the Born rigid acceleration the proper length of the object increases or decreases. If all parts of the object undergo the same acceleration simultaneously wrt to some IRF then the object will actually increase its length. If we move one part of the body, the other parts will accelerate one by one like in a domino effect. I'm still not sure what happens with the proper length in this scenario, but it looks like it's decreasing during the acceleration as the body gets compressed.
So my question is, in the case of these proper length changes, do the atoms that actually make up the body also shrink (or expand) or does only the space between atoms change, but atoms continue to have the same length as before acceleration. My guess is the first option, but I'd gladly listen to some replies and opinions.

Regards, analyst

2. May 11, 2014

### PAllen

It can also shear without change of proper length.
Understood you mean proper length, by context.
Depends whether you pull or push. If you pull, it will increase in proper length, if you push it will decrease. If the proper acceleration is sufficiently slow (how slow depends on how long the object's length) these changes can be made negligible. To approximate rigid acceleration, the effect of push or pull needs time to propagate to the far end at the speed of sound in the material. Thus the scale of quasi-rigid proper acceleration is <small velocity>/<time to propagate sound the proper length of the object>.
No, it is the second option. The intermolecular forces are so much weaker than forces within the atom, that any object is shattered or macroscopically deformed way before any changes to atomic shape occur.

3. May 11, 2014

### Bill_K

Yes, but they do eventually occur, don't they? Suppose you have one atom being accelerated, in a uniform electric field, say. What happens to its shape? For example, this would be expected to show up as level splitting.