1, #129 - My Favorite Physics Experiment

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Have you ever heard of the Double-Slit Experiment?

I’m not exaggerating when I say it’s one of the most bizarre and fascinating things I’ve ever learned about.

Here’s the gist: Light is weird. Sometimes it acts like little particles, other times like waves. Scientists have tested this by shining light through two narrow slits onto a screen. If light were just particles, you’d expect two bright lines behind the slits. If it were waves, you’d see an interference pattern—like ripples overlapping in water.

And here’s the twist: Even when photons (individual particles of light) are sent through one at a time, they still form that wave-like interference pattern.

It’s like each photon is somehow interfering with itself.

But it gets weirder. The moment scientists try to measure which slit a photon goes through? The interference pattern vanishes. Now, the photons act like particles again—just two simple stripes.

So… just watching changes the outcome?

Yes. Somehow, the mere act of observation can change our physical reality.

And it doesn’t stop there. Follow-up experiments—like the (amazingly named) delayed-choice quantum eraser—suggest that choices made after a photon hits the screen can still influence what pattern appears. It’s like the future is rewriting the past.

Wild, right?

So, what’s the point?

First of all: How cool is that? It still stops me in my tracks every time I think about it.

And second: It reminds us there's still plenty of mystery out there. There's a lot we know—but it’s humbling to realize how much we still have to discover.

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Insight inspired by: Science! And Richard Feynman.