Friday Science: Hawking 5 (Particles)

Friday reviews of Stephen Hawking's A Brief History of Time so far.
Chapter 1: Heliocentric
Chapter 2: Spacetime
Chapter 3: Expansion of the Universe
Chapter 4: Uncertainty Principle

Chapter 5: Elementary Particles and the Forces of Nature

Particle Discoveries

• Aristotle--matter can be divided endlessly (300s BC)
• Democritus--matter is made up of atoms (400s BC)
• John Dalton--revived atomic theory (1800s)
• Albert Einstein--Brownian motion confirms atoms (1905)
• J. J. Thomson--demonstrated electrons (late 1800s)
• Ernest Rutherford--atoms have a nucleus (1911)
• James Chadwick--discovered the neutron (1932)
• Murray Gell-Mann--Discovered protons and neutrons made up of quarks (1964)
• Six flavors of quark: up, down, strange, charmed, bottom, and top.
• P. A. M. Dirac--antiparticles and combined with special relativity (1928)
• Four categories of force carrying particles:
• The graviton is proposed as the basis for gravity, and gravitational waves have been discovered since Hawking wrote this book. Personally not sure that gravity is based on particles. Einstein didn't look at it that way.
• The photon is the basis for the second force, the electromagnetic force. It is much stronger than the gravitational force but can be either attractive or repulsive and works over smaller distances.
• The particles known as the W and Z mediate what is called the weak nuclear force, which is the basis of radiation. Abdus Salam and Steven Weinburg worked out a way to unify the electromagnetic and weak force into an electroweak force, moving toward grand unification.
• The gluon is the basis for the final force, the strong nuclear force. I believe that the unification of this force with the electroweak force has also been achieved since Hawking wrote his book. Gravity is what is left to unify.

Other Nuclear Characteristics

• Particles have spin. The options are 1/2, 0, 1, and 2. Particles of matter are particles of spin 1/2. Particles of 0, 1, and 2 give rise to the forces between matter particles.
• Pauli's exclusion principle says that two particles cannot be in the same state at the same time.