Tuesday, May 29, 2007

Go Nuclear on Me

Once I was attending a class taught by Dirk Walecka, a fairly famous nuclear physicist, renowned also for his pedagogical skill. At one point he was describing the difference between the nuclear force, which holds protons and neutrons together in a nucleus, and the electromagnetic force, which holds electrons in their orbits about the nucleus. He argued that the most profound difference is this:

  • When nuclei get bigger, they get bigger
  • When atoms get bigger, they get smaller

His point was that as you increase in atomic number, nuclei just get bigger while atoms (until a shell is completed) will get smaller as the electrons get pulled in more by the increased positive charge coming from the additional protons in the nucleus. From this fact one can conclude two important features of the nuclear force:

  1. Since the protons in the nucleus should strongly repel each other from the electromagnetic force (like charges repel) the nuclear force must be much stronger than the electromagnetic force.
  2. Since atoms get smaller when they get bigger, the electromagnetic force extends over large distances, while the nuclear force does not.

Another amazing fact about the nuclear force is that it is just strong enough that the deuteron (a nucleus consisting of one proton and one neutron (that makes it a hydrogen isotope) is barely bound, while the diproton—a two proton nucleus (a helium isotope) is not bound.

If the strong nuclear force were just 4 percent stronger, the diproton would form, which would cause stars to so rapidly exhaust their nuclear fuel as to make complex life impossible. On the other hand, if the strong nuclear force were just 10 percent weaker, carbon, oxygen, and nitrogen would be radioactive and again life would be impossible.

Pretty darn cool, the nuclear force.

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