# University of California, San Diego Physics 11 - Survey of Physics

 H. E. Smith

 Physics 11 The Scale of the Universe

• The Atom -- the basic unit of every chemical element is the atom, consisting of the nucleus, composed of protons and neutrons held together by the Strong Force, with electrons "orbiting" the nucleus, attracted by the Electrical Force.

Atomic Particles
Particle Symbol Charge Mass
(g )
Mass
(amu )
Family
proton p+ +1 1.673 x 10-24 1.00727 baryon
neutron n0 0 1.675 x 10-24 1.00866 baryon
electron e- -1 9.109 x 10-28 5.485 x 10-4 lepton

• Atomic Nuclues -- ~10-15mm.
• Size of Atom -- the radius of the first electron "orbit" in the Hydrogen atom is 1/2Å = 0.5 x 10-10m.

The ratio of the size of the atom to the size of the nucleus is about 100,000; if the atomic nucleus were a pingpong ball (r ~ 1cm) its electrons would be in orbits about a kilometer away.

• Molecules - Atoms build themselves into molecules (e.g. H2O) with sizes ranging from a few Å to the largest molecules, such as DNA,at up to 10-5 m.

• Cells - 10-4 m.

• Humans - The largest humans who don't play basketball are about 2 meters in size; Professor Smith is 1.86 m in height.

• The Earth - The earth is about 25,000 miles around, implying a radius of about 4000 miles. More precisely, the equatorial radius, RE = 6378 km = 6.378 x 106 m. In other words it would take about 7 million people to span the distance from San Diego to China through the center of the Earth.

• The Sun - the Sun, our nearest star is 93 million miles or about 150 million km = 1.5 x 1011m away. It has a size, RS = 7 x 108m.

If the sun were a large grapefruit (r ~ 7cm) the earth would be a pinhead 15 meters away.

• The Stars - the nearest star, Alpha Centauri, a triple star system visible from the Southern Hemisphere in the constellation Centaurus, is 4.34 light years = 4 x 1016m away.

A light year is the distance light travels in a year = 3 x 108 m/second x 60 seconds/minute X 60 minutes/hr x 24 hrs/day x 365 days/yr ~ 1016 m. If the sun were a grapefruit in San Diego, the next grapefruit would be 2500 miles away.

• The Galaxy - The Sun and Alpha Centauri are among about half a trillion stars in the Milky Way Galaxy, which is about 100,000 light years = 1021m across.

• Other Galaxies - The Andromeda Galaxy, our nearest sizeable neighbor is about 2 million light years = 2 x 1022m away.

• The Universe - 1025m: there may not be a "size" to the Universe; it may be infinite, but a crude estimate of the scale of the Universe can be made from the fact that the Universe began with the Big Bang some 10 - 20 billion years ago. Thus the most distant objects that we can see are about 10 billion light years = 1025m away.

From the size of the atomic nucleus, important for the fusion energy of the stars, at 10-15m, to the scale of the Universe as a whole ~ 1025m, the distance scales of interest in Astronomy span a factor of about 1040 -- 40 orders of magnitude!

• Powers of ten a visual depiction of the scale of the Universe starting at 10 million l.y. from the Milky Way and zooming into the heart of the atom.

Conducted by Gene Smith, CASS/UCSD.
Comments? You may send email to hsmith@ucsd.edu

Prof. H. E. (Gene) Smith
CASS   0424   UCSD
9500 Gilman Drive
La Jolla, CA    92093-0424

Last updated: 30 September 1999