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Can reality be adjusted after events have occurred?
That's the unsettling implication
of the delayed choice quantum eraser experiment.
We recently talked about the weird results
of the single particle double slit experiment.
They imply some startling things about the nature of reality.
Today, I want to talk about an additional, possibly even
stranger, version of this experiment,
whose results force us to reconsider
the nature of causality itself.
Now this episode isn't going to make a lot of sense
unless you've seen the first one.
So go ahead and watch it, if you haven't already.
I'll wait right here.
OK.
So the single particle double slit experiment
suggests that things may not exist as well-defined, even
real particles, in that strange interim between creation
and detection.
There's a fuzzy space in which we don't know the particle's
location or path.
The Copenhagen interpretation would tell us
that in this space, a particle is only
its wave function, a distribution
of possible properties.
It's a probability wave that does
all the usual wave-like stuff like making interference
patterns, until something happens to collapse it.
At that point, the Copenhagen interpretation
tells us that a true transition happens
between wave and particle.
But is that right?
And if so, what really causes this transition?
Does observation of the particle's location
force the universe into settling down and deciding
which particular reality we happen to be in?
To head off any wild metaphysical giddiness,
I want to say right now that there's absolutely no reason
to believe that observation by a physicist
is better at collapsing wave functions then observation
by an electronic detector.
Or a houseplant, for that matter.
We'll talk about what observation really
means at a later point.
But it's still pretty interesting
to see what happens if we try to observe the wave
function at different points in the double slit experiment.
The great mystery of the experiment
is that very particle-like things
appear to traverse both slits simultaneously, like you
might expect of a wave.
Physicists love a good mystery, and so have
tried very, very hard to peek to see
which slit these particles actually travel through
before they produce the famous interference pattern.
Turns out the universe is on to us.
Any experiment that determines unambiguously
which slit the particle traverses
destroys the interference pattern.
Instead, particles land in simple clumps, one
for each slit, as though they were
traveling as simple particles the whole time.
This is even true if you place detectors
on the far side of the slits after the wave particle
thing should have already been interfering with itself, just
like the wave function is collapsing retroactively,
as if the universe is saying, OK, guys.
The human figured out which way you went.
No more funny stuff.
Better pretend like you are particles that whole time.
But there's a huge problem with this interpretation.
It's impossible to make these measurements without messing up
the wave.
The interference pattern happens because the waves emerging
from each slit are what we call coherent, which
is a fancy way of saying that the relationship
between the wave form is emerging from the two slits.
So the locations of peaks and valleys
is predictable and stays consistent as the waves
move forward.
This coherence is what allows the waves
to produce the interference pattern in the first place.
But when you place some device in the path of either wave,
you mess with this coherence, and so affect the pattern
that reaches the screen.
By the way, the double slit experiment
where you try to determine which slit is traversed
is called a "Which Way" experiment.
And if the test is done on the far side of the slits,
it's called a "Delayed Choice" experiment.
Physicists hate being outsmarted by the universe.
So they've come up with clever ways
to measure which way the particle traveled while still
preserving coherence.
I'm gonna talk about the most famous, performed in 1999.
This experiment made use of a very special type of crystal
that absorbs an incoming photon, and creates
two new photons, each with half the energy of the original.
These new photons are twins of each other.
In fact, they're an entangled pair.
So fundamentally connected in strange ways
that we'll come back to.
Place this crystal in front of the double slit
to make coherent entangled pairs of any photons passing through.
Send one of each pair off to the screen
to produce our interference pattern.
And use the other to figure out which
slit the original photon passed through.
Let's focus on detectors A and B here.
Detector A lights up if the original photon passed
through slit A. And detector B lights up for slit B.
If we run this for a bunch of photons,
we see that whenever detectors A or B light up,
we get a simple pile of photons here at the screen.
No interference pattern at all.
As though any knowledge of which way the original photon
traveled stops it from acting like a wave during its passage
through the slits.
And crazier, this experiment was set
up so that photons reach A or B after their twins reach
the screen.
So a photon lands on the screen according to the pattern
defined by its wave function.
And then later, its untangled partner
reaches detector A or B, and somehow retroactively
influences the previous landing position.
It's like the second photon is saying, whoa, whoa, whoa.
Someone figured out which slit I came through.
You better look like you came through that one, too.
This is amazing.
And given that we only interact with one of the entangled pair,
surely that means we aren't messing with the other.
So we aren't ruining the interference pattern
with decoherence.
Could it get any weirder?
This is quantum mechanics.
So, yeah.
This extra stuff is the quantum eraser.
Its job is to destroy any information
about the path of the photons.
These devices are beam splitters.
Just half-silvered mirrors.
They work by allowing 50% of the photons
through, while reflecting the other 50%.
Now you have a new possible outcome.
Instead of being reflected to detectors A or B,
half of the photons end up in detectors C or D.
But this clever arrangement ensures
that if C or D light up, we have no idea which
slit that photon came from.
If we only look at the photons whose twins end up
at detector C or D, we do see an interference pattern.
It looks like the simple act of scrambling
the "Which Way" information retroactively
sends the message, OK.
Chill.
The observer lost the info of which slit we went through.
It's safe to have gone through both again.
Are we forced to double down on the interpretation
that observation of the path causes the collapse of the wave
function, and that the wave function can collapse
all the way back to wherever our new knowledge extends to
in the past?
Some sort of retroactive reality cascade?
This is a pretty wild story.
For that reason it makes sense to be cautious,
even of the Copenhagen interpretation.
Part of the appeal of the Copenhagen interpretation
is that it avoids any physical interaction that moves faster
than light.
See, when a spread out wave function
resolves itself into a set of known properties,
say, the location of a particle on the double slit
screen, somehow the entire wave function
knows to do this-- to collapse at the same instant.
But if these wave functions are physical,
as the Copenhagen Interpretation would tell us,
then there is no real instantaneous physical
interaction.
By contrast, a physical interpretation
of the wave function, like the De Broglie-Bohm Pilot Wave
Theory, requires an underlying physicality,
a set of defined properties that evolves with the wave function.
So-called hidden variables.
That's a bit uncomfortable, because to explain
experimental results, those physical properties
need to act and change instantly at any distance.
They need to have what we call non-locality.
Now the delayed choice quantum eraser double slit experiment
doesn't tell us whether the wave function is physical or not.
But it tells us that the Copenhagen,
or any other metaphysical interpretation of the wave
function, is no less well, crazy-sounding
than a physical interpretation.
In fact, the solution may lie in this fascinating phenomenon
of quantum entanglement.
As we'll see in the future, entangled particles
really are able to influence each other instantaneously.
And we'll see that their non-locality
doesn't violate causality.
So perhaps they can even affect coherence and decoherence
retroactively and physically without making a causal mess.
Perhaps this thing we call observation
is just entanglement between the observer and the experiment.
Perhaps the evolving tapestry of entanglement
in all its impossible complexity is
what really defines reality in this space time.
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I recently listened to "The Big Picture" by Sean Carroll.
It's rare to find a brilliant theoretical physicist who's
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Go to audible.com/spacetime.
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Last week, Neal Stephenson came on the show
to help us figure out how to save humanity
from the end of the world.
You guys had your own ideas.
Mircea Kitsune asks, what if a black hole
was headed towards the Earth?
Could we stop it?
This is a great "end of world" scenario.
And the short answer is, no.
Long answer-- depends on the type of black hole
and how close it gets.
The only black holes that we know
for sure are buzzing around our galaxy
are stellar remnant black holes.
And they're at least several times the mass of the sun.
Something like that passing through the solar system,
even if nowhere near the Earth, would probably
disrupt planetary orbits and either rearrange or scatter
our planetary system.
Even if it passed by the outer reaches of the solar system,
it would perturb the Oort cloud and send swarms
of comets plummeting inwards.
However, there's a theoretical type
of black hole, so-called primordial black holes,
which may have formed in the first instance
after the Big Bang.
These may have masses similar to planets rather than stars,
if they exist.
And one of these passing through the solar system
would only be dangerous if it was moving slowly enough,
and if it came very close to the Earth.
In fact, impact by a primordial black hole
was one of the hypotheses that Seveneves scientists
proposed for the moon's inexplicable destruction.
Tsjoencinema would like to know what
would exactly happen if a gamma ray burst hit the earth.
I'm glad you asked.
By happy coincidence, the wonderful channel, Kurzgesagt,
recently published a whole episode on gamma ray bursts.
I'll let them answer this one for you.
Check the link.
A few of you pointed out that I neglected
to mention certain known ends of the world that are
coming in the distant future.
And yeah.
Sure, Earth is certainly doomed.
The increasing temperature of the sun
will cause all of Earth's oceans to evaporate
in a billion years, plus or minus, depending on the model.
But that's the sort of predictable event
that hopefully a super advanced post-humanity
can deal with with a bit of basic geoengineering.
Increasing atmospheric reflectivity
for example, which is something we
could do in the very near future, if we wanted to.
As for the death of the sun and Andromeda's collision
with the Milky Way, OK.
Let's deal with one end of the world at a time, people.
I also asked you to submit your ideas
for extinction-proofing humanity.
The prize-- a place on the ark.
Here are the winners.
In the event of End of World, you guys
should show up at your local spaceport.
Your names will be on the door list.
Error 404, Hoder Not Found had a great idea
in which you collect asteroids and put them
in orbit around the Earth, ready to sling at any oncoming
impactors before they reach us.
This is actually a really nice one.
The rocks don't even have to be all that big,
as long as you spot the incoming rock on one
of its nearer passes before the impact pass,
you could play a little cosmic billiards to knock it just
off course.
TimacTR suggests we upload our minds
into deep underground computers.
Go full virtual?
I'm down with that.
Although who's to say we didn't already do that?
Caleb Tandberg wants to build underwater cities instead
of underground cities to protect against gamma ray bursts.
I really love this idea.
You don't even need much water at all
to fully protect us from the gamma ray
burst or the subsequent UV.
So just below the surface is great.
All surface life is laid waste.
But hey, The Deathless Face of the Unborn Mind
has an extinction proofing idea.
Reinvent our monetary economic systems
to produce efficient stable and healthy societies that actually
have the energy and collective intelligence to progress
technologically.
OK.
Well, if you're going to be sensible about it.
But can we please still have the underwater cities?
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