**Length**

Atomic nuclei range from about 10^{-4} to 10^{-5} of the size of an atom. If the atom were about the size of a medium-sized airport (say, 3 km) then the nucleus would be about 30 cm, about the size of a basketball. Now imagine the airport, 3 km, having a sphere encompassing it. If you change the basketball to a golf ball you have a rough scale of the hydrogen atom with its central proton. Inside the golf ball are the quarks. Change the scale from the proton being the size of a golf ball to the size of a marble, about 1 cm. The sphere is now the size of the earth’s orbit. The actual size of a proton is about 10^{-15}m. This is equivalent to one femtometer, or one Fermi (1 fm). The smallest distance probed is 10^{-18}m, which is one thousandth of a fermi. The fundamental particles such as quarks are smaller than this.

The radius of the Hubble volume, or known universe, is about fourteen billion light years, which is about 10^{26}m away. The size of your desk is about 10^{26} times smaller than the universe and only 10^{18} times larger than the smallest probed distance. The *mean* distance between the large distance of the universe and the smallest distance probed is 10^{4}m, or 10 km. This means that the mean distance of the universe is about six miles.

The Planck length is about 10^{-35}m. This is 10^{20} times smaller than a proton. This proton is a hundred billion billion times larger than the Planck length. This is the scale at which strings perform their gymnastics and the quantum foam bubbles.

**Speed**

A snail’s speed is about 0.01m per second. A person walks at about 1m/s and drives a car at about 30m/s. The typical speed of an airplane reaching the speed of sound is about 740 miles per hour. Light travels much faster, almost a million times faster at about 3×10^{8}m/s. The hypothetical particle, or a massless photon, which is suspected to travel faster than the speed of light is called a *tachyon*.

**Time**

For humans, a year is a long time and a second is a short time. For a particle, a hundredth of a second is an eternity. For the universe a few hundred million years is simply dinner. Particles that live long enough to leave tracks in a detector have lifetimes of 10^{-10} to 10^{-6} s. A neutron is abnormally long with a lifetime of 15 minutes. If the universe is 13.7 billion years old then it is roughly 10^{18} seconds old. The Planck time is 10^{-44} seconds, smallest meaningful unit of time.

Material from Ken Ford’s *The Quantum World* (Cambridge, MA: Harvard University Press, 2004), 10-16.

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