In my last video, I talked about how the double slit experiment was mind blowing. How the particle
passed through the slit, depended crucially on whether someone was detecting the particle or not.
In THIS video, I’m going to show you how scientists can not only undo that effect,
but how it’s related to time. Let’s take a look, shall we?
So, we’re about to tackle a very tricky concept in physics,
which involves the nature of space and time and whether the future can affect the past. But this
is so tricky, I think it would be best if you viewed my earlier double slit experiment video.
A link to the video is given in this video’s description. Go ahead and watch it. I’ll wait.
We saw in the previous video that a single photon, traveling through both slits,
leaves an interference pattern, which means the photon is a wave. If we looked at which slit the
photon goes through, we see a pattern that is consistent with the photon being a particle.
We call the second experiment a “which way” study.
This result is pretty freaky. Back in the 1930s, physicists like Neils Bohr speculated that human
consciousness changed the result. We no longer believe that. Any detection of the slit the photon
gives the same result, including detection by electronic circuits.
Of course, the earlier speculation led to a lot of quantum woo, ranging from the books
The Dancing Wu-Li masters and the Tao of physics, to more recent films, like John Hagelin’s portion
of the movie “The Secret,” to “What the Bleep Do We Know,” to much of what Deepak Chopra writes.
None of any of these things are in agreement with modern science.
Sorry if they resonated with you, but you should discard all of them.
But let’s make some changes to the single photon hitting two slits experiment.
They’re going to blow your mind. I promise.
Let’s add a crystal to the experiment which turns the incoming photon into two photons
each with half the energy of the first one. These two photons are what we call entangled,
which means that if you measure one, you know everything about the other. The fact
that one is yellow and one is red isn’t real. They’re actually the same color,
but I use the two colors to clarify what I’m talking about.
We then add two detectors, called A and B which measure not the photon hitting the screen,
but rather which slit the photon goes through, by measuring the cousin photon,
which never hits the screen.
If A and B are turned off, you see a wave pattern. That kind of makes sense. But if
A and B are on and you look to see which slit the photon went through, you get a particle pattern.
Okay, here’s the first mind-blower. Suppose you make the cables going to A and B
very long, so the photon hits the screen before you can measure its cousin in A
and B. If you do this, the result is unchanged.
So what does that mean? It means that if you turn A and B on,
detecting the cousin photon affects what the photon hitting the screen does in the past.
This experiment seems to completely screw up causality. The particle or wave pattern is
created by whether a detector in the future sees which slit the photon goes through.
Full But there’s
yet another step in weirdness. What if you can erase your knowledge of which
slit the photon goes through? What happens then? And how do you do it?
You do it by adding three half-silvered mirrors.
Half-silvered mirror will reflect half of photons and half will be reflected.
You put two of them, one in front of detector A and one in front of detector B.
This means that the beams of cousin photons can go
into the detectors or get passed along to additional instrumentation.
You also add two new detectors, C and D. These are farther away from the screen than the A and
B guys. You place mirrors in front of the beams of cousin photons into a single half-silvered mirror.
That means that the photons from A can either be reflected into detector C,
or pass through the mirror to detector D. Conversely,
the cousin photons from B can either be reflected into D, or pass through the mirror
to detector C. You can see it in the red and yellow paths. C and D can see yellow or red.
The bottom line is that whether you see a cousin photon from detector C or D,
you don’t know what slit the photon went through.
This addition of C and D completely erases the knowledge that A and B give you.
So,what happens if you turn off detectors A and B and look only at C and D? Since the detectors
don’t tell you which slit the photon went through, it once again acts like a wave. But C&D are a
little offset, so the you see the interference pattern only when you look a C & D separately.
Furthermore, if the cables to C and D are long enough,
the photons hitting the screen hit long before the cousin particles hit C and/or D.
So what I’m saying is the following. If you use A and B in the original experiment,
you get a particle pattern. However, if you use C and D in this new version, you get a
wave pattern. And detection in A, B, C and D, all happen after the interference pattern is created.
This bizarreness implies that in the quantum world, both location and time don’t act like
they do in our world. Quantum mechanics can be considered non-local (because of the cousin
photons) and non-temporal, because a detection somehow changes the past.
Now the most important thing to state is that these are experimental results.
You can discuss what they mean, but you can’t deny that this is how the quantum world works.
Because, without any chance of being wrong, this is how the quantum world works?
So how can we explain the delayed choice and quantum eraser experiment? The simple
and sadly unsatisfying answer is we don’t know. However, I have given a ton of links to
other descriptions of the delayed quantum eraser experiment, that you can find in the description.
Okay, so what’s the answer? Like I said, we don’t know. Could it be that the universe at
the quantum level doesn’t understand time in the way we do? Is the universe super
deterministic, which means that everything we do, past and future,
is predetermined? Or, getting even sillier, maybe the delayed quantum eraser experiment,
is just is a bug fix in our simulated universe, which his kind of like the Matrix.
Or it could be none of the above. Indeed, it almost certainly is none of the above.
The real answer is almost certainly going to make all of our heads explode.
Okay, so that was cool. I’m sorry that there’s no answer, but there isn’t, even for people like me.
And for those inquiring about the weight loss,
you wouldn’t believe how much a mustache weights. It was a time for a change.
The last time I was mustache-less was not quite 40 years ago. I haven’t decided if the
mustache will come back or not. Feel free to comment if you have a strong opinion.
In any event, if you enjoyed the video, please like and subscribe. And tell all of your friends
who enjoy having their mind blown. Those are also probably my kind of people, those who
know one of the key truths of science, which is, of course, that physics is everything.