This article is an excerpt from the Shortform book guide to "A Brief History of Time" by Stephen Hawking. Shortform has the world's best summaries and analyses of books you should be reading.

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What is the uncertainty principle? Why is it impossible to measure both the position and the velocity of a subatomic particle simultaneously?

The uncertainty principle states that it is impossible to determine both particle velocity and position with certainty. In order to gauge the position and speed of a subatomic particle, we’d need to expose it to light, but doing that would alter both properties.

Keep reading to understand the uncertainty principle and its implications for the study of our universe.

## What Is the Uncertainty Principle?

**In order to measure the position and speed of any particle, one would need to shine a quantum of light on it—but doing so alters the position and speed of the particle one is attempting to measure.** This is the foundation of Werner Heisenberg’s uncertainty principle, which states that **it is impossible to accurately gauge both the position and speed of any given particle. **

This was the nail in the coffin for the principle of scientific determinism, which held that we should be able to predict everything about the universe, provided that we have perfect and complete information about it at any one time. The uncertainty principle, however, held that it was impossible to measure the precise state of the universe at any *one *point in time—therefore, it was impossible to make a 100 percent accurate prediction for any *future *point in time. This was the beginning of the new field of quantum mechanics.

**Quantum mechanics takes a probabilistic approach rather than an absolute approach**. Obeying the uncertainty principle, it does not seek to establish a single, definitive interpretation or result for quantum mechanics and the uncertainty principle tell us that it’s not as useful to think of the physical world as being composed of waves (like light and radio) and particles (like the atoms out of which all matter is made). It is better to think of our *observations* of the world in terms of waves and particles—**depending on the phenomenon, sometimes it’s more useful to think of particles as waves, and sometimes it’s more useful to think of waves as particles.**

These theories that argue for the varying behavior of particles and waves have had a profound impact on our understanding of the structure of atoms and molecules. While we can never predict the movement of electrons within atoms with 100 percent certainty, we *can *predict the *probabilities *of different events, within the constraints imposed by the uncertainty principle. any observation. It predicts a *range* of possible outcomes, and tries to work out *probabilities* for each.

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Here's what you'll find in our **full A Brief History of Time summary**:

- The search for a theory that explains the history and evolution of our universe
- Stephen Hawking's discussions about time, space, dimensions, and quantum theory
- How time travel would theoretically work