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The Question

(Submitted January 08, 1997)

What holds the Universe together? What holds us together?

The Answer

Your very good questions are related to two different forces. The first, what holds the Universe together, is one that astronomers think about often. On large scales like the Universe, the most important force is gravity. Between any two objects the gravitational attraction is proportional to the product of the masses divided by the square of the distance between them. Gravity is the force responsible for keeping the Earth and other planets in our solar system in orbit around the Sun. Gravity also governs the motions of the Sun and nearly all the stars you can see in the sky, which are orbiting about the center of the Milky Way Galaxy. The Milky Way is part of a gravitationally bound collection of galaxies which includes Andromeda, and is called the Local Group. Apart from observing that objects large and small are gravitationally attracted to each other, astronomers also observe that the Universe is expanding: an after-effect of the birth of the Universe in the Big Bang.

Your second question, what holds us together, is closer to biochemistry than astrophysics. Human beings are composed of different types of large molecules: proteins, nucleic acids, lipids, carbohydrates, etc. These molecules are held together by intermolecular forces. An example is the peptide bond that links amino acids together. This bond is formed when atoms of Hydrogen, Oxygen, Carbon and Nitrogen share electrons. Molecular bonding is governed by the electrostatic force, which on small scales is much stronger than the gravitational force for charged particles. We human beings still feel the effects of gravity though: it keeps us from floating off the Earth.

Padi Boyd
for Imagine the Universe!

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