Uncertainty Principle
| Physics: | Quantum Mechanics, Heisenberg Uncertainty Principle |
| Grade Range: | Elementary School, Middle School, High School |
| Format: | Stage |
This demonstration is a part of the Quantum Mechanics Show. This demonstration is not typically performed by itself, since it does require background knowledge in the topic to be understood. In the Quantum Mechanics Show, this demonstration is scripted, so although this write-up does not provide a script, one should be aware of the differences that happen between scripted and non-scripted demonstrations.
Materials
- Ring Stand with Clamp
- Ping-Pong Ball on a String
- Modified Hairdryer
Safety Precautions
Please read the General Safety Precautions section of the Demonstration Safety page before performing this demonstration.
Demonstration
- Preparation
- Prior to starting this demonstration, ask the contact to show you where the controls for the lights are in the location. If you have a volunteer with you, assign them control of the lights for this demonstration.
- Set out the ring stand and hang the ping-pong ball from the clamp. Explain to the audience that we can track the location and movement of the ball quite easily.
- Explain that tracking objects on the atomic scale is much harder. Ask for the lights to be turned off, and explain that you can use your "Quantum Hairdryer" to try and find the atomic particle.
- Turn on the light and the hairdryer, and try to find the particle. Every time you find the particle with the light, the air blows it away! Chase the ball with the blower for a few seconds before turning it off, and having the lights come back on.
Why This Works
The Heisenberg Uncertainty Principle states that you can measure the momentum or the position of an atomic particle, but you cannot measure both at the same time. On the atomic scale, the act of measuring or observing a particle's state adds energy into the system, which affects the particle. If you try to measure the particle's momentum, the energy you add by measuring it moves the particle, affecting the position. Likewise, if you observe the particle's position, the energy you add by observing it changes the momentum it had. The Heisenberg Uncertainty Principle can be written as follows:
| σx: Standard Deviation of Position |
| σp: Standard Deviation of Momentum |
| ħ: Planck Constant, where ħ = h/(2π) |
Without getting too deep into the math, one can see that the values for momentum and position affect each other. If one value becomes smaller, or more precise, then the other value has to become bigger, or less precise. Therefore, if one knows a lot about the momentum, they cannot know as much about the position. Likewise, if one knows about the position, then they cannot know much about the momentum.
When we use the "Quantum Hairdryer" to try and find the particle, we are using the light on the hairdryer to try and observe it, or find the position. While we are doing that, however, the air blowing out of the dryer moves the ball around. The blowing air represents the energy we add into the system by trying to observe the particle, so although we are able to find the particle, there is no way we can measure the momentum that it will have, since the energy we add in is causing the momentum to constantly change. Likewise, we could try to find the momentum of the particle by having it blow the ball in a circle, making it swing around at the speed of the moving air. While the ball is doing this, however, we cannot pinpoint the exact location of the ball at all times with our blower, and only know that it will be in a general area swinging around. If we tried to pinpoint the location, the momentum will be lost, and vice-versa.
Additional Information
- This demonstration is a part of the Quantum Mechanics Show.