Video Infoblog: The Weird Experiment that Changes When Observed

The double-slit experiment is the strangest phenomenon in physics.

Transcript

0:00

The double-slit experiment  defies our understanding.

0:03

It involves a laser shooting beams  of light particles, called photons,  

0:07

at a screen with two parallel slits.

0:10

If we mark all the spots where  the photons hit the second wall,  

0:13

we should expect to see two strips  to correspond with the two slits.

0:17

But that’s not what we see.

0:19

Instead, we see an alternating  pattern of light and dark bands.

0:23

This is characteristic of the behavior of waves  that pass through both slits simultaneously,  

0:29

interfering with each other to create  the pattern on the second screen.

0:32

The bright lines are where the top of waves  meet, resulting in a more intense or brighter  

0:38

area on the screen. The darker bands are where  the top of one wave meets the bottom of another,  

0:43

so they cancel each other out, resulting in  a less intense or darker area on the screen.

0:48

Here is a photo of a real interference pattern.

0:52

The original double-slit experiment was conducted  in 1801 by British polymath Thomas Young.

0:57

Since then, it’s been performed  with various types of particles,  

1:01

including electrons, and they were  found to behave in the same way.

1:05

This phenomenon occurs even if the  photons are fired one at a time,  

1:10

which suggests that the photon is interfering with  itself as if it passes through both slits at once.

1:16

Physicists were so stumped that they decided to  observe which slit the particle went through.

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And that’s when things get really weird.

1:25

When scientists used a measuring device to  observe the slit that each photon passed through,  

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the interference pattern disappeared, and  the photons started behaving like particles.

1:36

Instead of the spectrum of light and  dark bands, we see two bright bands,  

1:40

indicating that the photons  chose one slit or the other.

1:44

So light can display characteristics of both  

1:47

particles and waves, known  as wave-particle duality.

1:50

It appears that light decides  to behave as a wave when it’s  

1:54

not being watched and acts like a  particle when it is being measured.

1:58

The mere act of observing which slit it went  through changed the behavior of the photons!

2:04

Almost as if they were aware  they were being watched!

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It’s kind of like how in a video game,  

2:10

the environment and objects only load when  the player focuses on or interacts with them.

2:15

The entire world isn't rendered all at once,  

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allowing the game to save processing  power and optimize resources.

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Likewise, light seems to behave  like waves, but when we observe it,  

2:27

it's as if we're "loading" its properties,  causing it to change and act like particles.

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Physicist Richard Feynman famously said,  

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"the double-slit experiment is absolutely  impossible to explain in any classical way  

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and has in it the heart of quantum mechanics.  In reality, it contains the only mystery."

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The most widely accepted theory to explain this  phenomenon comes from physicist Niels Bohr,  

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who proposed that particles like photons don’t  have definite properties until they’re observed.

2:57

So in the case of light, photons exist  in a wave-like form representing a range  

3:02

of possible positions until they are observed.

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When we measure or observe the particle,  

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we force it to “choose” a definite  state, behaving like particles.

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This is called the Copenhagen interpretation,  named in honor of Bohr’s home city.

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However, to this day, no one knows why observing  a particle causes its behavior to change.

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Not everyone was a fan of Bohr’s theory.

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Einstein disagreed that particles could be  in multiple states at once until observed.

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He was uncomfortable with the  inherent randomness of such an  

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interpretation and famously said, “God  does not play dice with the universe.”

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Einstein believed that the universe  followed a consistent set of rules,  

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like how his general relativity theory  predicted the motion of planets around the sun.

3:49

He spent his later years trying to develop a  

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unified theory that explained everything  in the universe, but he didn’t succeed.

3:57

Physicists recently conducted a  variation on the double-slit experiment.

4:01

In April 2023, researchers at Imperial  College London swapped out the screen  

4:07

with two slits for a transparent material  used in smartphone screens called ITO.

4:12

They first fired a laser beam at the material.

4:16

When a second laser was directed at  the material and pulsed super quickly,  

4:20

in quadrillionths of a second, it caused the  material to become temporarily reflective.

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The interaction of Laser 2 with the  material changed the frequency - and  

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therefore the color - of the light from  Laser 1 that was reflected off the material.

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This experiment shows that by manipulating time  rather than space, they were able to create  

4:41

interference patterns similar to the interference  observed in the original double-slit experiment.

4:47

There are certain things in  life we take for granted.

4:50

If you don’t get enough sleep,  you’ll feel tired and groggy.

4:53

If you drive recklessly, you are more  likely to get into a car accident.

4:56

But sometimes, there are principles that  govern our lives that we just can’t explain.

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The perplexing result of the double-slit  experiment is up for interpretation,  

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with some suggestions that this is  evidence our world could be “programmed”.

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So far, not even the best scientific minds  

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can explain the bizarre behavior  of particles…at least, not yet.

5:19

There are many puzzles that are perplexing.

5:22

For example, can you tell what’s  wrong with the gears Leonardo da  

5:25

Vinci wanted to use in his military  tank design from the 15th century?

5:29

This is one of the thousands of  problems you’ll find on Brilliant,  

5:32

a website and app where you  can learn STEM interactively.

5:35

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5:44

causing the front and back wheels  to rotate in opposite directions.

5:47

So the tank will never be able to move  at all. This was possibly an oversight  

5:53

or an intentional mistake to protect his  design from being built without his support.

5:58

For more puzzling questions like these, you can  try out Brilliant for free for 30 days by heading  

6:04

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6:10

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6:14

which gives you access to all the offerings. Thanks for watching. I’m Cindy Pom.