ing attosecond (1018 s) timescale, the time it takes an electron to orbit an atom, to the timescale of ≈14 billion years, the age of our universe. Fascination with the passage of time is a fundamental aspect of human endeavor, driving us to attempt to understand our world and indeed our universe.

The relevant timescales for the atomic and molecular processes of interest (see Figure 5–1) cover 19 orders of magnitude—a factor of 10 billion billion. The complex folding of a protein molecule can take milliseconds (ms) (103 s) or longer. On the other hand, a millisecond is a very long time for an atomic collision, unless the atoms are cooled to billionths of a degree above absolute zero, as discussed in Chapter 2. The small molecules in the air we breathe undergo collisions with each other approximately every 100 picoseconds (ps) (1010 s), and they tumble or rotate in space approximately every 1 to 10 ps (10−11 to 10−12 s). The atoms within

FIGURE 5–1 Characteristic timescales of atomic and molecular motions.

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