First I want to point out that the authors of the original 1998 paper who were the first to
conducted the Delayed Choice Quantum Eraser experiment implied retrocausality, or the
future affecting the past.
That’s why in my original video, I presented it that way.
And several other youtubers have done the same.
The authors would not be wrong if one interprets the results of their experiment in a nonstandard
way.
But I now admit that my buying into that interpretation was incorrect.
This experiment was clarified to me by one of my very astute viewers, who
has a PhD in quantum optics.
He understands the details of this experiment a lot better than I do.
And as he pointed out to me, the devil is in the details.
So how and why did the original authors conclude that this experiment implies retrocausality?
And why is it that I am now saying this is wrong?
I know that there are a couple of videos on YouTube now that have debunked the retro causality
claim.
But I found those videos to still be a bit confusing and lacking some details.
So stay tuned because I am going to simplify this notoriously confusing experiment so that
hopefully anyone with a little science background can understand it.
That’s coming up right now...
Before I explain how the delayed choice experiment actually works, what it means, and what I
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And now back to the show.
Basically, the retro causal conclusion of the original paper requires a non-standard
interpretation of quantum mechanics.
It implies retro causality if you assume a classical way of thinking.
What I mean by this is that you have to interpret that the photon is either a particle or a
wave from the moment it left the light source.
So when you decide how to measure it later, for example if you measure it to be a wave,
it must retro causally become a wave when it was created before you decided to measure
it.
And the same thing applies if you measure it to be a particle.
However, this retro causal interpretation falls apart if you assume the more standard
interpretation of quantum mechanics where a photon in flight is in a superposition of
states.
In other words it is a wave, a wave of potentiality.
Once it is measured, that is, once its path is known, its wave collapses to become a localized
particle and shows up as a particle distribution on a screen.
But if it is not measured, that is, its path information is not known, it remains a wave
and will show up as an interference pattern like a wave that interferes with another wave.
If you consider this latter interpretation, there is no retro causality whatsoever.
To understand what’s really going on, let me first start out by explaining how the simpler
double slit experiment works.
If you send photons, electrons or any quantum object one at a time through the slits, at
first you will see what looks like a random distribution of dots.
But after a while, you will see that those dots create an interference pattern.
It’s as if little bits of waves were going through the slits and interfering with each other.
But the weirdness doesn’t stop there.
If you then put detectors on the slits to measure which slit the photon passes through,
then you don’t see an interference pattern.
You see a pattern like you would if you were sending individual particles through the slits.
The act of measuring seems to affect the results.
This phenomenon has resulted in all kinds of crack pot new age quantum woo, theories
about universal awareness, consciousness affecting reality, and a whole lot more.
But you being quantum physics enthusiasts probably know that the change in behavior
of quantum objects does NOT occur because we are watching them or are conscious of them.
The change is due to the nature of quantum mechanics.
All quantum objects like photons and electrons are really waves.
But if they interact with anything, that is, if an irreversible energy exchange takes place,
such as what happens when these quantum objects interact with the measuring device, their
waves become localized.
The localized waves of electrons and photons then appear to be like classical particles.
This is sometimes referred to as, “wave collapse.”
This same wave collapse occurs WITHOUT the detector present, when the photon interacts
with the screen in the back.
The photon is like a wave until it hits the screen.
After hitting or interacting with the screen, the wave becomes localized, so it appears
as a discrete particle.
And we this as a dot on the screen.
But prior to hitting the screen, the photon “wave” interacts with itself similar to
the way any classical wave would.
This is why we see an interference pattern in the aggregate of many dots.
This wave to particle collapse seems incredible, but that’s the way quantum mechanics seems
to work.
How this so called wave collapse occurs, is not well understood.
If you want to learn more, I have a video on this phenomenon, which is now ascribed
a more modern terminology called decoherence, up here.
The Delayed Choice Quantum Eraser is like the double slit experiment on steroids, and
takes this to another level of confusion.
But before I describe that, let me debunk a misleading illustration commonly shown about
the double slit experiment.
If you have no detectors checking the path of the particle, then you get interference
as just like I said earlier.
If however, you have a detector that measures the path, you don’t really get two clean
lines of photons like I illustrated.
You actually get a single spread out distribution of photons.
What you get depends a lot on the physical dimensions of the setup.
If your slit is very wide, you get a relatively concentrated dot on your screen, because the
photon just goes straight through, almost as if it didn’t notice the slit.
And this is the scenario you see in almost all illustrations, a double line pattern on
the screen.
But if you narrow the width of the slits to the size of nanometers, which you generally
have to in this experiment because we are dealing with tiny quantum things, you enhance
diffraction.
In other words, the photons interact more with the slits, which causes the photons to
spread out as a fuzzy collection of dots.
They do not show up as clean narrow lines of dots.
For the typical double slit illustration, it is ok to ignore this.
But to understand the delayed choice quantum eraser, it is helpful to realize that what
you would REALLY see is a spread out pattern of dots, or an interference pattern if you
don’t measure which slit the photon passes through.
So how does the delayed choice experiment work?
Let me show you step by step.
I have simplified the illustration below so that you can better follow what’s going
on.
The actual experiment looks different, but the practical results are the same.
We first have our double slit, but instead of putting up a screen to see an interference
pattern, we put a special optical device called a Barium Borate crystal, which is referred
to simply as a BBO crystal.
It splits a single photon into a pair of entangled photons with half the energy each of the original.
And this is done for every photon coming from either the left, or the right slit.
There is a critical important point to note here which distinguishes this from the standard
double slit experiment.
The process of creating entangled photons effectively results in a measurement.
In other words, the wave function of the photon collapses so that it is now a particle.
And since the path from the top slit to detector 1 is slightly different than the path from
the bottom slit, the which way information of the photon is known.
This means that the pattern that will show up at detector 1 will always be a spread out
pattern, not an interference pattern.
Note that I said it will ALWAYS show up this way, and never as an interference pattern.
It doesn’t matter what happens at any of the other detectors.
So why is it illustrated as changing depending on what happens at the other detectors?
This is the center of the confusion.
And to understand this, you first have to first understand what happens to the entangled
pairs of the photons in the rest of the apparatus.
So if we follow the entangled pair from the top slit, we see that it hits a set of beam
splitters which can randomly direct the photon to either detector 2, detector 4 or detector
5.
If the photon goes to detector 2, it will show a spread out pattern because its path
information is known, since the photon could only have come from the top slit.
Similarly, the entangled pair from the bottom slit, hits a set of beam splitters which randomly
direct the photon to either detector 3, detector 4 or detector 5.
If the photon goes to detector 3, it will also show a spread out pattern because its
path information is known.
It could only have come from the bottom slit.
What makes things interesting is what happens to the photons that don’t end up at 2 or
3, but end up at detector 4 or 5.
These photons are fed into a set of optical instruments that effectively merge the beam
from the right and left slit into a single recombined beam.
This is the “eraser” part of the experiment.
It’s called “eraser” because the photons path, or which-way information in these two
cases is erased.
Hence, the patterns that you will see at 4 and 5 are always going to be interference
patterns.
This is similar to the pattern made in the double slit experiment without a measuring
device.
The two interference patterns from 4 and 5 look somewhat like this:
Now you might ask, where does the confusion of retro causality come in?
Well the confusion is from the way this experiment is presented.
It is presented as D1 changing its pattern to match the interference pattern at D4 or
D5 when the photons end up there, but showing a different pattern, a spread out pattern,
if the photons end up at D2 or D3.
So this implies that what happens at D2, D3, D4 or D5 influences what happens at D1.
But since the path to D1 is shorter than the path to any of the other detectors, the photons
reach D1 BEFORE they reach D2, D3, D4 or D5.
So the implication is that the pattern at D1 which would be in the past, is being affected
by what happens in the future at D2, D3, D4 or D5.
So people have naturally been led to think that the interference pattern of the eraser,
will change the result on the original D1 screen.
This is wrong.
If we were to turn on the experiment and show the content of all five screens, that is the
screen from D1, the two screens showing photons from the individual slits D2 and D3, and the
two screens D4 and D5, each showing half of the recombined photons beam, then we would
see that the patterns look something like this:
The quantum eraser has no effect on the original
screen at D1.
What’s really happening is that the changing patterns are due to the scientists who are conducting
this experiment selecting subsets of the photons in D1 to show the same patterns as at each
of the other detectors.
How is this done?
Recall, the first things that happens after the double slit is the creation of entangled
photon pairs.
This means that there is a connection between the particles hitting the screen at D1 and
the particles going down to the other detectors where we either measure them coming from separate
slits, causing no interference, or as the recombined beam where we see an interference
pattern.
Since the path information of all the photons hitting D1 is known, It is possible to single
out or manually identify all individual photons ending up at any of the other detectors.
In other words, using entanglement, we can recover which entangled partner photons from
any of the other 4 detectors hit the screen at D1.
So for example, if we manually sort out the entangled partner photons of the photons hitting
D4 or D5, we can see the exact same interference pattern hidden within the spread out pattern at D1.
But we see this only because we have now chosen a subsample from all the photons at D1, which
of course is going to be interfering because they are the entangled partners of the corresponding
photons at D4 and D5.
The same thing applies to D2 and D3.
So in the presentations that you see, including the one I originally made, the interference
pattern you see get at D1 is nothing but a hand-selected subset of the actual original
spread out pattern at D1, corresponding to photons that ended up at D4 or D5.
In fact, if you combine the interference patterns at D4 and D5, you get a spread out patter
with no interference.
So, what the quantum eraser is doing is creating two subsamples of the full non-interference
picture, which individually show interference, but no interference when combined.
Here is the clincher.
If you combine all the patterns from D2, D3, D4 and D5, you get precisely the same pattern
that you have at D1.
This should make sense because every entangled pair that ends D1, has a corresponding pair
that ends up at one of the other 4 detectors.
So really you a are not changing the past.
What you are really doing is, in the future, you are choosing a subsample of the data you
made in the past.
I hope you find some comfort in knowing that no one in the future, can mess up your present.
They can only mess up your future.
What do you think?
Leave your comments below. And I'll see you in the next video my friend!