# Is an atom's nucleus a single particle?

1. Jan 17, 2010

### Nasher

Is an atom's nucleus a single particle?

Take for example the nucleus of a lithium atom which contains three protons and three neutrons.
Do the wavefunctions of these three protons and three neutrons merge together to form a single particle, in the case of the lithium's nucleus, a tri-proton-tri-neutron particle, with a single wavefunction?
Or is it that the lithium's nucleus contains three protons and three neutrons close together,
but these particles do not merge into a single particle with a single wavefunction?

Last edited: Jan 17, 2010
2. Jan 17, 2010

Staff Emeritus
It's not an either-or thing. At large wavelengths, a nucleus acts as a single particle and it is well described by a single wavefunction. At smaller wavelengths, the behavior of the individual nucleons becomes important, and at smaller wavelengths still, the behavior of the individuals quarks and gluons becomes important.

3. Jan 17, 2010

### bcrowell

Staff Emeritus
Here's another way of describing it. Suppose you have a system composed of two identical particles in one dimension with coordinates x1 and x2. You can change to a new set of coordinates u1=(x1+x2)/2, u2=x2-u1, where u1 is the location of the center of mass, and u2 is the displacement of one particle from the center of mass. If the system is isolated from external interactions, then u1's behavior is fixed by conservation of momentum, so the internal dynamics depend only on u2. On the other hand, if you throw the system at a wall and measure how loud the splat is, it's only u1's conjugate momentum that matters.

In a frame of reference where a deuterium nucleus is seen as moving at .9999999999999999c, the wavelength corresponding to the degree of freedom u1 is actually much shorter than the scale of u2, but nevertheless you don't need any knowledge of the internal structure to predict the nucleus's motion.

An issue that I think is much deeper and more difficult is the idea of, e.g., treating a composite system made of an even number of fermions as if it were a boson. We know we can do the double-slit experiment with atoms, but it's not clear if we can do it with viruses or cars.

4. Jan 17, 2010

### Naty1

Why use three protons and three neutrons in your example? Are you aware neither is a "particle" in the sense you might mean; each is composed of other more fundamental constituents...say, quarks....as noted in post #2.

Furthermore, you can consider all of these to be composed of vibrations of energy...called strings... like particle theories, it is incomplete [and not a part of the standard model].

In general, as implied above it's best to consider a "particle" to have different characteristics which are apparent in different conditions. for example, if you observe a lithium atom from a distance of three feet, at low energies, perhaps viewing it as a single entity is adequate. If you want to observe the behavior as a gas you don't even need to worry about the nuclear structure. But when you begin to bombard it with other particles and wish to understand the scattering, picturing the composition of the nucleus as a single particle will usually not do.

It's analogous to relativity: things look one way at low velocities, but strange things begin to happen when velocities become a significant proportion of that of light approached.

5. Jan 17, 2010

### robert2734

Lithium 6 exists and is important for thermonuclear bombs. Most lithium is lithium-7.

6. Jan 17, 2010

### Naty1

Even more weird:
per Wikipedia....