The double-slit experiment defies our understanding.
It involves a laser shooting beams of light particles, called photons,
at a screen with two parallel slits.
If we mark all the spots where the photons hit the second wall,
we should expect to see two strips to correspond with the two slits.
But that’s not what we see.
Instead, we see an alternating pattern of light and dark bands.
This is characteristic of the behavior of waves that pass through both slits simultaneously,
interfering with each other to create the pattern on the second screen.
The bright lines are where the top of waves meet, resulting in a more intense or brighter
area on the screen. The darker bands are where the top of one wave meets the bottom of another,
so they cancel each other out, resulting in a less intense or darker area on the screen.
Here is a photo of a real interference pattern.
The original double-slit experiment was conducted in 1801 by British polymath Thomas Young.
Since then, it’s been performed with various types of particles,
including electrons, and they were found to behave in the same way.
This phenomenon occurs even if the photons are fired one at a time,
which suggests that the photon is interfering with itself as if it passes through both slits at once.
Physicists were so stumped that they decided to observe which slit the particle went through.
And that’s when things get really weird.
When scientists used a measuring device to observe the slit that each photon passed through,
the interference pattern disappeared, and the photons started behaving like particles.
Instead of the spectrum of light and dark bands, we see two bright bands,
indicating that the photons chose one slit or the other.
So light can display characteristics of both
particles and waves, known as wave-particle duality.
It appears that light decides to behave as a wave when it’s
not being watched and acts like a particle when it is being measured.
The mere act of observing which slit it went through changed the behavior of the photons!
Almost as if they were aware they were being watched!
It’s kind of like how in a video game,
the environment and objects only load when the player focuses on or interacts with them.
The entire world isn't rendered all at once,
allowing the game to save processing power and optimize resources.
Likewise, light seems to behave like waves, but when we observe it,
it's as if we're "loading" its properties, causing it to change and act like particles.
Physicist Richard Feynman famously said,
"the double-slit experiment is absolutely impossible to explain in any classical way
and has in it the heart of quantum mechanics. In reality, it contains the only mystery."
The most widely accepted theory to explain this phenomenon comes from physicist Niels Bohr,
who proposed that particles like photons don’t have definite properties until they’re observed.
So in the case of light, photons exist in a wave-like form representing a range
of possible positions until they are observed.
When we measure or observe the particle,
we force it to “choose” a definite state, behaving like particles.
This is called the Copenhagen interpretation, named in honor of Bohr’s home city.
However, to this day, no one knows why observing a particle causes its behavior to change.
Not everyone was a fan of Bohr’s theory.
Einstein disagreed that particles could be in multiple states at once until observed.
He was uncomfortable with the inherent randomness of such an
interpretation and famously said, “God does not play dice with the universe.”
Einstein believed that the universe followed a consistent set of rules,
like how his general relativity theory predicted the motion of planets around the sun.
He spent his later years trying to develop a
unified theory that explained everything in the universe, but he didn’t succeed.
Physicists recently conducted a variation on the double-slit experiment.
In April 2023, researchers at Imperial College London swapped out the screen
with two slits for a transparent material used in smartphone screens called ITO.
They first fired a laser beam at the material.
When a second laser was directed at the material and pulsed super quickly,
in quadrillionths of a second, it caused the material to become temporarily reflective.
The interaction of Laser 2 with the material changed the frequency - and
therefore the color - of the light from Laser 1 that was reflected off the material.
This experiment shows that by manipulating time rather than space, they were able to create
interference patterns similar to the interference observed in the original double-slit experiment.
There are certain things in life we take for granted.
If you don’t get enough sleep, you’ll feel tired and groggy.
If you drive recklessly, you are more likely to get into a car accident.
But sometimes, there are principles that govern our lives that we just can’t explain.
The perplexing result of the double-slit experiment is up for interpretation,
with some suggestions that this is evidence our world could be “programmed”.
So far, not even the best scientific minds
can explain the bizarre behavior of particles…at least, not yet.
There are many puzzles that are perplexing.
For example, can you tell what’s wrong with the gears Leonardo da
Vinci wanted to use in his military tank design from the 15th century?
This is one of the thousands of problems you’ll find on Brilliant,
a website and app where you can learn STEM interactively.
My viewers especially love Brilliant’s lessons in computer science, data science, and math.
causing the front and back wheels to rotate in opposite directions.
So the tank will never be able to move at all. This was possibly an oversight
or an intentional mistake to protect his design from being built without his support.
For more puzzling questions like these, you can try out Brilliant for free for 30 days by heading
to the custom link in my description: brilliant.org/newsthink. And the first
200 people who sign up with my link will get 20% off their Premium subscription,
which gives you access to all the offerings. Thanks for watching. I’m Cindy Pom.