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.
