[MUSIC PLAYING] This episode is
sponsored by Audible. 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. This episode of "Space Time"
is supported by Audible.com. Right now, Audible is
offering "Space Time" viewers a 30-day trial period. Check out audible.com/spacetime
to access their audio programs and titles. I recently listened to "The
Big Picture" by Sean Carroll. It's rare to find a brilliant
theoretical physicist who's willing and able to discuss
the philosophical and human implications of the science
without the nonsense mysticism. Go to audible.com/spacetime. And make sure you use that
link so they know we sent you, and to get a membership trial. 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? [MUSIC PLAYING]
I really liked this
your link is wierd
r/pbsspacetime/
What the fuuuck, that blew my mind.