# Accelerating neutral atom

1. Dec 19, 2008

### spidey

If charge radiates on acceleration,what would happen if we accelerate a neutral atom..will this radiate or not? this neutral atom also has electrons and protons which are charged so they should radiate but as a whole this neutral atom should not radiate?both are contradictory..

is this problem only with neutral atom or also with neutrally charged particle neutron? neutron shouldn't radiate since it is neutral but what it is doing with the energy got from acceleration? charged particle radiates from the energy of acceleration so neutron will also get the same energy then why it is not radiating?

2. Dec 19, 2008

### turin

How do you propose to accelerate a neutral atom? I suppose that is somewhat of a distracting/irrelevant issue, because I suppose you could have just as well asked about two concentric insulators, one with charge +Q and the other with charge -Q, so that the total charge is zero. I must say that I don't know the answer to this, but I would guess that the radiation from the +Q would exactly cancel the radiation from the -Q, so that the result would be zero radiation. You should note that radiation is a far-field phenomenon.

Why do you say that? Do you believe that a dipole or loop antenna should not radiate?

Again, why do you say that?

OK. Now I can ask the fair question: how do you accelerate a neutron? And, BTW, acceleration does not always change a particle's energy. For example, a magnetic field accelerates a charged particle in a direction transverse to its direction of motion, so a magnetic field alone does not change the energy of a charged particle. Furthermore, if a particle does receive energy by virtue of acceleration, this is usually accounted for in the form of kinetic energy, radiation being a loss term.

3. Dec 20, 2008

### Redbelly98

Staff Emeritus
I agree, with both the "I don't know" part and also I suspect the radiated electric fields would simply cancel as turin says here.

FYI, a neutral atom could be accelerated with a non-uniform electric field. The induced dipole would be attracted toward regions of higher E-field magnitude.

4. Dec 20, 2008

Staff Emeritus
First, a neutral atom may have a magnetic dipole moment or a higher moment (such as an electric quadrupole moment). If I accelerate one of these atoms (by smacking it with a baseball bat - nothing esoteric required) it will certainly radiate.

Now, suppose we are moving something that doesn't have any of these moments - helium, for example. It will not radiate. One reason is because there is no change to the electric or magnetic fields as the atom is accelerated - so how can it radiate? The other is, as said before, that the radiation from the positive charges interferes destructively with the radiation from the negative charges.

If you look closely, you'll see that these are the same thing, just applied at a different order in the calculation. In one case you sum up the charges first and then see how much it radiates and in the other you see how much each charge radiates and then sum.

5. Jan 2, 2009

### Alex Barri

Last edited: Jan 2, 2009