# Coupling only to the center of mass degrees of freedom?

by JK423
Tags: coupling, degrees, freedom, mass
 P: 381 I am interested in the following scenario. Suppose that you have a composite system, e.g. ion, atom or whatever you want. This system will have internal degrees of freedom (d.o.f.), i.e. relative positions and momenta, and also center of mass d.o.f..What i am looking for is to find an interaction (using e.g. E/M fields or something else(?)) that couples only to the center of mass d.o.f. and not to the internal ones. I'm not sure if that's even possible. So basically i want to push the system around in space without giving energy to each constituents. Do you know any way of doing that? Thanks
 Mentor P: 10,846 Homogeneous gravitational fields. This is the best, and the only general method.
 P: 381 Thanks mfb! I'm interested in non-zero proper acceleration, and for a free falling particle that would be zero. So i don't think that gravitation is suitable. You think it's not possible without gravity?
Mentor
P: 10,846

## Coupling only to the center of mass degrees of freedom?

You need some force which is proportional to mass, for every particle in the setup.

The weak and strong interaction do not have a sufficient range to provide such an interaction as soon as your system is larger than a nucleus. The classic electromagnetic interaction cannot do that either, as electrons and protons (or nuclei) have opposite charges. The Mössbauer effect provides momentum to the whole crystal without exciting any internal degrees of freedom, and scattering of light is similar, but I don't know if that counts.

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