The Narrower You Look: Seeing Heisenberg's Uncertainty Principle with a Slit
Project Overview:
Heisenberg's uncertainty principle says that the more precisely you know a particle's position, the less precisely you can know its momentum, and this is not a limitation of our instruments, it is a fundamental fact about nature itself. In this project, you will see a visible version of this trade-off using nothing but a laser pointer and an adjustable slit, watching a beam of light spread out more and more as you squeeze it through a narrower and narrower gap, the same underlying math that governs Heisenberg's principle for particles like electrons.
Materials Required:
A laser pointer, red or green both work
Two straight edged objects to form an adjustable slit, such as two index cards, two razor blades, or two rulers
A dark room
A flat wall or screen at least a couple meters away
Optional, a ruler for measuring the spread
Background: Why This Actually Demonstrates Uncertainty
When you shine light through a narrow slit, you are forcing the light to have a well defined position, since you now know it passed through a small, specific gap. But narrowing that gap also forces the light to spread out more after passing through, meaning its direction, which corresponds to its momentum, becomes less well defined. Squeeze the position down, and the momentum spreads out. Widen the slit, and the light travels in a tighter, more predictable beam, but now you know less precisely where exactly it passed through.
This same trade-off applies to particles like electrons, just described with different math, called the Heisenberg uncertainty relation. You are not seeing electrons here, but you are seeing the exact same underlying wave behavior responsible for the effect, since both light and matter follow wave mechanics at a fundamental level.
Step by Step Instructions
Hold your two straight edged objects close together to form a narrow vertical gap between them, adjustable by how close you push them together.
Darken the room and shine the laser pointer through the gap toward the wall, keeping the beam steady and roughly centered on the slit.
Start with the slit fairly wide, a few millimeters, and observe the shape of the spot on the wall. It should look like a fairly tight, narrow beam.
Slowly squeeze the two edges closer together, narrowing the slit, while keeping the laser aimed steadily through the gap.
Watch the spot on the wall as you narrow the slit. It should visibly widen and spread out, and you may start to see faint secondary bands appear on either side of the main spot.
Continue narrowing the slit as much as you can while still letting light through, and note how much wider the final spread becomes compared to where you started.
Reverse the process, slowly widening the slit back out, and watch the spread shrink back down to a tight beam again.
Optional, measure the width of the spot on the wall at your widest slit setting and your narrowest slit setting, and compare the two numbers.
Congratulations, you just visually demonstrated the same position-momentum trade-off that Heisenberg's uncertainty principle describes mathematically, using nothing but light and a slit.
Fun Fact:
Heisenberg originally derived his uncertainty principle from thought experiments about trying to observe electrons with light, realizing that any attempt to pin down a particle's position with high precision necessarily disturbs its momentum, a limit built into nature itself rather than a flaw in any particular measuring device, no matter how advanced.
