Uncertainty Principle
You can't know position and momentum exactly at once.
About this topic
Heisenberg's uncertainty principle says that position and momentum can't both be known precisely at the same time: Δx · Δp ≥ ħ/2. It's not a limit of measuring equipment but a fundamental property of nature, arising from wave-like behaviour.
A wave squeezed into a small region must contain many wavelengths, so pinning down position makes momentum spread out. Confining an electron to an atom, about 10⁻¹⁰ m, gives a momentum uncertainty big enough to produce speeds of millions of metres per second.
Energy and time have a similar relation. The principle explains why electrons don't spiral into the nucleus and allows quantum tunnelling, which makes nuclear fusion in stars and some electronics possible. It marks the end of strict classical determinism, as we can predict only probabilities.
Students often say that measuring disturbs the particle, but the principle is deeper than clumsy measurement. It also doesn't matter for large objects, because Planck's constant is so small.
Key ideas
- Δx Δp ≥ ħ/2
- Not about measurement clumsiness
- Limits classical thinking
Quick check
Use cases: where Uncertainty Principle helps
- Homework: review the key idea behind Uncertainty Principle before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Uncertainty Principle without notes.
- Visual learning: play with the interactive lab to see how Uncertainty Principle changes when you adjust its inputs.
- Teaching a friend: explain Uncertainty Principle in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Uncertainty Principle?
You can't know position and momentum exactly at once.
What will I learn in Uncertainty Principle?
Δx Δp ≥ ħ/2. Not about measurement clumsiness. Limits classical thinking.
Is Uncertainty Principle part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Uncertainty Principle lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Uncertainty Principle?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.