Nuclear, Weak & the Limits of Classical Physics
Two short-range forces rule the nucleus, and classical physics has a border.
Lesson at a glanceShow the key points
- Nuclear (strong) forces act between protons and neutrons; they are attractive and very strong at about 10⁻¹⁵ m but negligible beyond a few times that.
- Nuclear forces overcome the Coulomb repulsion of protons and so keep nuclei stable; they power fission and fusion.
- Weak forces drive beta decay (a neutron becoming a proton, electron and antineutrino); their range is smaller than a proton.
- Classical physics works for bodies larger than about 10⁻⁹ m moving much slower than light (3 × 10⁸ m/s).
- Smaller scales need quantum physics; speeds near c need Einstein's relativity.
Inside every atom sits a nucleus that, by the rules of electricity alone, should fly apart. Something stronger holds it together.
A puzzle in the nucleus
An atom’s nucleus is astonishingly small: about 10⁻⁴⁵ m³ against roughly 10⁻³⁰ m³ for the whole atom, a mere 10⁻¹⁵ of its volume. Yet it holds about 99.98% of the atom’s mass. And the nuclei of ordinary (non-radioactive) elements are stable.
Take the nucleus of helium, called an alpha particle: two protons and two neutrons. The protons repel each other through the Coulomb force, which at such tiny separations is fierce. The neutrons carry no charge, so they are mere spectators in the electrical tug-of-war. Gravity between the four particles is far, far too weak to help. So why doesn’t the alpha particle burst apart?
The nuclear (strong) force
A third kind of force is at work, separate from gravity and electromagnetism: the nuclear force. It acts between protons and neutrons (in any combination), is mostly attractive, and has a peculiar character:
- At separations of about 10⁻¹⁵ m it is much stronger than the Coulomb repulsion, which is why nuclei hold together.
- Beyond a few times 10⁻¹⁵ m it fades to almost nothing: it is short-ranged.
Because of that short range, nuclear forces only matter when something changes inside a nucleus. Bare nuclei are rare in daily life, so you never feel them directly. But radioactivity, nuclear power (fission) and the energy of the Sun (fusion) all come from them.
The weak force
A fourth interaction shows up when particles transform into one another. A free neutron can turn into a proton while emitting an electron and an almost massless particle called an antineutrino. This is beta decay. A proton inside a nucleus can likewise become a neutron, emitting a positron and a neutrino (beta-plus decay).
Where classical physics stops
A remarkably small set of laws (Newton’s laws of motion, his law of gravitation, Maxwell’s electromagnetism, the Lorentz force and the laws of thermodynamics) describes the behaviour of everything from raindrops to planets. Physics built on them is called classical physics. It has two borders:
For most of this course we work inside the classical region: bodies larger than about 10⁻⁹ m moving well below the speed of light. There, nuclear and weak forces have no noticeable effect, and only gravity and electromagnetism matter. Occasionally we will apply classical ideas to subatomic particles; the answers are then approximations, but often illuminating ones. We also assume Newton’s third law holds for every force we use. Near the end of the course we take a first look at quantum physics.
- 1Nuclear forces act between protons and neutrons: strong, attractive and short-ranged (≈10⁻¹⁵ m). They keep nuclei stable.
- 2Weak forces drive beta decay; their range is smaller than a proton.
- 3Classical physics applies to objects larger than about m moving much slower than light.
- 4Smaller scales need quantum physics; speeds near c need relativity.
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