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Hey Crazies.
Figuring out what’s going on with quantum particles can be a real hassle sometimes,
mainly because their behavior works against us.
You’ve probably heard of the infamous double-slit experiment
because everyone and their mother has done a video on it.
But most of those videos are designed to mystify you.
Let’s bring quantum mechanics back to reality.
Whenever you’re trying to learn something about quantum particles,
never forget the following maxim:
Quantum mechanics is not magic!
There are real physical things happening here that are independent of human existence.
We just need to figure out what they are.
The double-slit experiment is a good place to start.
It’s an example of something we call a “which way” experiment
where all we want to do is determine which way something went.
If we know it started here and ended here,
did it take this path? Or that one?
Or some other weird path?
Honestly, it’s a pretty common question to ask about anything physical.
Being able to make predictions is kind of a big deal.
Adding in barriers is a perfect way to narrow down the options and focus our attention.
As we move forward in this video,
we’re going to ask progressively deeper questions about this experiment,
about what its results actually mean.
We’re going to add pieces to it as we go until it looks like this:
the delayed-choice quantum eraser.
That’s when we’ll find the deeper answers, but we have to work up to it slowly.
So settle in!
In a previous video, we did the double-slit experiment with a full beam of light.
If you’re not familiar with that experiment or how wave interference works,
you should go watch this video first and come back after.
Ok, let’s keep going.
We want to take the double-slit experiment into the quantum realm
and no, it doesn’t look like this at all.
In fact, it doesn’t really look like anything
because light doesn’t work the same way on that level.
Down there, light is made of individual photons.
What’s a photon?
The smallest piece of energy in a beam of light.
We’ve spent a lot of time modeling light as a wave in the electromagnetic field.
But, that’s not always the best model, especially for a laser like this.
If we make it really dim, the beam stops being continuous.
We start to see individual packets of energy called photons.
If we make that laser dim enough, it’ll release one photon at a time.
Then we can send each one into the double-slit experiment.
Every time a photon hits the screen, it records the interaction as a dot.
At first, everything looks fine.
Individual photons are making individual dots.
After a while, you’ll notice a pattern build up on the screen.
It’s an interference pattern.
The first thing this result tells us is that the photon is a wave.
Wait wait wait! Isn’t the photon a particle?
Yes, but not in the way you’re probably thinking.
When you hear the word “particle” you probably think of something like dust.
It gives you the impression that quantum particles like photons are little balls,
but they’re not.
Quantum particles are just localized waves.
At all times, their behavior is described by a wave function,
which is a solution to a wave equation.
They are always waves!
A pattern like this in the double-slit experiment tells us there is interference
and a wave is the only thing that can interfere like that.
Just to be clear though.
While the interference pattern tells you the thing is a wave.
The non-interference pattern does not tell you the opposite.
This pattern does not mean “particle.”
It just means we’re inconclusive about waviness.
So, photons are waves, but their tininess can give them some semblance of location
and that’s going to be very important as we try to understand their behavior.
In general, this isn’t a waviness test or a particle test.
It’s a “which way” test.
The point is to test “which way” something went.
Did it go through slit A, slit B, or both slits simultaneously?
That’s the second thing our photon result from earlier can tell us.
The only way this interference pattern is possible
is if each individual photon went through both slits simultaneously.
Seriously! Both slits!
How is that possible?
Well, if each photon is actually a wave, then this isn’t all that surprising.
But what if we try to detect which slit each photon passed through?
Sure, we could do that, but then we get a different result.
Hmm...
Polarizers do a pretty good job of separating photons.
Just after slit A, we’ll put in a horizontal polarizer
and just after slit B, we’ll put in a vertical polarizer.
That way we’ll be able to know which photon came from which slit.
Unfortunately, now the pattern looks like this instead.
No interference.
Doesn’t that mean light is a particle?
Dude, keep up! We just went over this.
While the interference pattern tells you the thing is a wave.
The non-interference pattern does not tell you the opposite.
This pattern does not mean “particle.”
It just means we’re inconclusive about waviness.
But it doesn’t matter if we’re inclusive this time.
The previous experiment told us photons were waves already.
This new result tells us something else entirely.
It tells us the detection of each photon changed their behavior.
Now each photon only goes through one slit or the other, not both.
The photons knew we were watching?!
No, that’s not how this works.
That’s not how any of this works!
Remember, quantum mechanics is not magic!
I never said any person knew which slit each photon went through.
I only said they were able to know.
Each photon interacted with the polarizers.
That interaction is what changed the behavior of the photons, not the person.
It made the slit information available.
Whether or not a person actually took advantage of that availability is completely irrelevant.
The humans are not important!
So, then, what’s actually happening?
Probability is happening.
All the photons come out of the same hole in the laser.
Each photon could exit from anywhere along its diameter,
but they’re more likely to come out of the center than the edge,
so the photon’s quantum wave looks like this.
The height of that wave gives us some idea of where we might find the photon at any given moment.
Eventually, that wave encounters something like our slit wall.
Most of the photons localize on the barrier and are destroyed,
but some of them localize on the openings and pass through.
Each photon wave that passes through has both slits available to it.
They’re either detected by the wall and localize in one area or they pass through both openings.
That turns the slits into two independent sources of probability that can interfere with each other.
The new wave looks something like this on the opposite side of the slits.
It spreads out as it travels until the photon localizes on the screen.
It gets destroyed, its energy is transfer, and we see a dot.
Those dots have size because it’s impossible to localize a particle to a single point.
You know, the uncertainty principle and all that jazz.
Over time, those dots build up on the screen and eventually form a pattern.
Since each photon localizes randomly based on this interference wave.
That pattern on the screen is an interference pattern.
What about when we did all that detecting?
There’s an extra interaction.
The polarizers force each photon to take a particular polarization.
Since each slit has a different polarizer, each photon can only pass through one of them.
The new wave looks like this on the opposite side of the polarizers
and that remaining wave peak travels to the screen and deposits a dot.
But, that’s a different probability wave arriving at the screen,
which means we build up a different pattern over time.
The extra interaction with the polarizers prevented the interference.
Quantum mechanics is not magic!
But what about the delayed-choice quantum eraser?
Ok, fine. It’s time for that anyway.
But, just so you know, the same rules apply.
The best way to test a hypothesis is to try and prove it wrong.
Is there any way we can delay the detection until after the photon hits the screen?
Yes, but we need to be a little more creative.
The photon that hits the screen is immediately destroyed upon impact,
so the first thing we need to do is copy the photon.
If we put a special kind of crystal immediately after the slits,
every photon will be split into an entangled pair of photons,
each photon having half the energy of the original.
Conservation of energy shall not be violated!
After that, we need to split up the partners.
A special kind of prism should do the trick.
One partner goes off to the screen.
The other partner goes off to some detectors.
Detector A is for Slit A and Detector B is for Slit B.
By separating the two partners, we can make the detection happen whenever we want.
We could do it immediately
and it would be just like the previous experiment with the polarizers.
But, if we do it way later,
the upper photon will hit the screen and make a dot before we detect the slit information.
What kind of pattern do we get then?
Just like before, if we detect the photons, we don’t get interference.
For real!
It doesn’t matter when the detection is made.
All that seems to matter is that the information will become available.
Hmm, let’s take this up a notch.
If we add in a beam splitter before each of our detectors,
only half of the photons end up getting detected.
The other half pass through and we can send them somewhere else.
Why would we want to do that?
To erase the information.
We can send that half of the photons into a box called the Quantum Eraser.
If a photon is detected by that box, we have no idea which slit it came from.
That information has effectively been erased from existence.
This means we’re only collecting slit information from half of the photons.
In this case, the pattern on the screen looks like a big mess.
But don’t forget, we’re still only sending in photons one at a time,
which should let us keep a record of which hits are entangled pairs.
When we use that information to separate the dots on the screen,
we get interference for the erased half
and no interference for the detected half.
By performing all these detections and erasures much later,
it almost seems like a photon gets detected and is like
“Oh crap! I’m detected!”
Then it somehow communicates that to its entangled partner in the past,
so it’ll land without interference.
Get a hold of yourself Nick!
Quantum mechanics is not magic!
Ok, reality check.
This last experiment, is what we call the “delay-choice quantum eraser”
and it's rather elaborate and convoluted.
If we jump to too many conclusions about this kind of result
we’ll have to subscribe to a particular interpretation of quantum mechanics.
There’s a whole bunch of these interpretations.
Some have been ruled out, but some pretty crazy ones still remain.
Let’s not go down that rabbit hole today.
How about a more rational perspective?
We spent so much time focusing on the delayed detectors over here
that we didn’t stop to consider the screen is also a detection.
You could just as easily say the pattern on the wall
is what determines what happens at the other detectors.
Whichever detection happens first determines the other detection.
I mean, the pattern can’t even be seen until all the detections have been made anyway.
Quantum mechanics is a physical model that allows us to make predictions about really tiny things.
Maybe that’s all it is.
Maybe elaborate experiments like the delay-choice quantum eraser
just expose the limits of our physical models.
In quantum mechanics, maybe the rational perspective is the craziest.
So, do you feel any better about quantum mechanics?
Let us know in the comments.
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And until next time, remember, it’s ok to be a little crazy.
Doesn’t the photoelectric effect prove the photon is a particle?
Nope!
The photoelectric effect is a specific case of the energy conservation principle.
The photon is a discrete packet of energy.
That discrete packet just happens to be a wave.
Thanks for watching!