page_title: A Brief History of Time
section_title: The Uncertainty Principle
hierarchical_section_title: A Brief History of Time / Summary / The Uncertainty Principle
context_page_description: A Brief History of Time is a book written by the scientist and mathematician Stephen Hawking. The subject of the book is cosmology, the story of the universe.
  There are two other versions of this book: The Illustrated A Brief History of Time and A Briefer History of Time. The Illustrated A Brief History of Time has pictures to help explain its ideas. It was also updated because new information was found. A Briefer History of Time is shorter than the first version and was also updated.
  This book is very popular and well-known. This book was on the London Sunday Times bestseller list for over 4 years.
context_section_description: The uncertainty principle says that the speed and the position of a particle cannot be found at the same time. To find where a particle is, scientists shine light at the particle. If a high frequency light is used, the light can find the position more accurately but the particle's speed will be unknown (because the light will change the speed of the particle). If a lower frequency light is used, the light can find the speed more accurately but the particle's position will be unknown. The uncertainty principle disproved the idea of a theory that was deterministic, or something that would predict everything in the future.
  How light behaves is also talked more about in this chapter. Some theories say that light acts like particles even though it really is made of waves; one theory that says this is Planck's quantum hypothesis. A different theory also says that light waves also act like particles; a theory that says this is Heisenberg's uncertainty principle.
  Light waves have crests and troughs. The highest point of a wave is the crest, and the lowest part of the wave is a trough. Sometimes more than one of these waves can interfere with each other - the crests and the troughs line up. This is called light interference. When light waves interfere with each other, this can make many colors. An example of this is the colors in soap bubbles.
caption: Light interference causes many colors to appear.
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