[MUSIC PLAYING] Is there a hidden
physical reality that underlies the strange
behavior of the quantum world? Or is that reality an illusion
in the eye of the observer? The weird phenomenon
of quantum entanglement gives us quite startling
clues to the answer. [MUSIC PLAYING] Babies may suck at math. But they're actually
surprisingly good at quantum mechanics. Well, one fundamental
aspect of it anyway. The peekaboo game is thought
to be so hilarious to babies because they lack
object permanence. Hide your face with your hands,
and a baby won't automatically assume that you kept existing. Reveal your face again,
and it's as though you popped into existence
from nothing, which is, of course, a
hilarious thing to do. Kids get over this
pretty quickly as they learn that
things don't magically pop into and out of
existence for no reason. By the time they grow up and
go to college to study physics, the notion of object
permanence is so deeply embedded that we
don't even bother teaching it in physics 101. And yet the idea
that the universe keeps existing when
we're not looking at it is a pretty fundamental
implied assumption behind all of classical physics. Indeed, most of science
takes it for granted that the universe is
real, whether or not we're looking at it. This notion that
the universe exists independent of the
mind of the observer is called realism in physics. But quantum mechanics
is so bizarre that it still has
scientists wondering if we need to reject
even this basic premise. This was the source of one
of the most heated debates at the advent of
quantum mechanics. On the one hand,
Niels Bohr insisted that it was meaningless
to assign reality to the universe in the
absence of observation. In the intervals
between measurement, quantum systems truly
exist as a fuzzy mixture of all possible
properties-- what we call a superposition of states. In between observations,
the wave function describing this superposition
is a complete description of reality. And our experience of a
well-defined material universe only has meaning at the
moment of measurement. This peekaboo universe is the
heart of Bohr's Copenhagen interpretation. On the other hand,
Albert Einstein insisted on an objective
reality, a reality independent of our observation of it. He insisted that the
wave function, and by extension quantum
mechanics, is incomplete. There must exist what we
call hidden variables that reflect a more physical
underlying reality. In an effort to demonstrate
the silliness of Bohr's idea, Einstein along with Boris
Podolsky and Nathan Rosen proposed a quantum scenario
that showed that in order to abandon the
assumption of realism, you also had to abandon
a concept almost as sacred-- locality. Locality is the idea that
each bit of the universe only acts on its
immediate surroundings. This is fundamental to
Einstein's relativity, which tells us that the chain
of cause and effect can't propagate any faster
than the speed of light. The Einstein Podolsky
Rosen, or EPR, paradox introduces one of the
most mysterious ideas in quantum mechanics--
quantum entanglement. Here's the idea. Two particles interact briefly. They influence each other
so that their properties are somehow connected. And yet we refrain
from measuring these properties to preserve
quantum uncertainty. Quantum mechanics requires that
we describe the particle pair with a single combined
wave function that encompasses all possible
states of both particles. We call such particles
an entangled pair. Now, according to the
Copenhagen interpretation, any measurement of one particle
automatically collapses the entire entangled
wave function, and so affects the
results of measurements of the other particle. That's an influence
that could theoretically be transmitted instantly across
any distance, and even back in time, violating locality and
possibly violating causality. Einstein et al thought
this was very silly. They thought that every
special point in the universe must be real and
physical and defined by knowable quantities,
local hidden variables that could affect each other no
faster than the speed of light. The argument between
Bohr and Einstein felt a bit philosophical
at the time. But in 1964, Irish
physicist John Stewart Bell proposed an experiment
to resolve the debate. It involved entangled
electron and positron pairs. When spontaneously
created from a photon, these particles will always be
spinning in opposite directions to each other. However, until measured,
we can't know which direction either is spinning. We just know that they're
opposite each other. Their wave functions
are therefore entangled. Measurement of the spin
of one of these particles tells us the spin of the
other, no matter how large the distance between them. But in quantum mechanics,
measurement actually affects the thing
you are measuring. In the case of quantum spin,
that measurement effect is especially weird. We define spin direction
according to the spin axis. That axis can point
in any direction. But to measure
spin direction, we need to choose an axis to
align our measuring device. We always find that the observed
quantum spin aligns itself with our chosen
measurement axis. If we choose to
measure vertically, the spin will turn
out to be up or down. If we measure horizontally,
it will be left or right. Measurement forces the alignment
of the measured particle. But how does this affect the
spin of its entangled partner? The answer would settle
the Bohr-Einstein debate. Let me explain. Scenario one, if
Einstein was right, imagine the response of each
particle to all possible spin measurements is encoded
in each particle at the moment of their creation
as hidden variables local to each particle. Nothing we do later
to one particle will then affect the other. When we later measure the
spins of both particles, there will be a
correlation in the results because the particles
were once connected. But there'll be no
correlation due to our choice of measurement axis. Scenario two-- Bohr was right. What if between creation
and measurement, the electron and positron
only exist as a wave function of all possible states. In that case, measurement
of one particle spin should cause the entire
wave function to collapse, to take on defined values. Both particles should then
manifest opposite spins along whichever axis we choose
for one of the particles. That should lead to a
correlation between our choice of measurement axis for the
first particle and the spin direction then measured
for the second. This is exactly
the spooky action at a distance that made
Einstein so uncomfortable. So John Stewart Bell
figured out a set of observable results, the
so-called Bell inequalities, that we'd expect to see in the
case that Einstein was right and quantum mechanics needs
local hidden variables. But if an entanglement
experiment violates the Bell inequalities,
then local realism is also violated. By the way, Veritasium
describes this experiment in much more detail
in this video. I recommend you check it out. It's a tricky experiment
because entangled quantum states are hard to produce, but
even harder to sustain. Any interaction can
destroy the entanglement. But in the early '80s,
French physicist Alain Aspect succeeded. Instead of looking at the
entangled spins of an electron positron pair, he
used photon pairs with entangled polarizations. Polarization is
just the alignment of a photon's electric
and magnetic fields. But the principle is the same. And Aspect found that
there was a correlation between the choice of
polarization measurement axis for one photon and the
final polarization direction of its entanglement partner. The Bell inequalities
were violated. The experiment
was even set up so that the influence had to travel
between the photons at faster than the speed of light. Since then, many
experiments have verified this result over
larger and larger distances. The instantaneous
influence has been observed over many
kilometers at this point. The delayed choice
quantum eraser, which we already covered, is yet
another example of this strange result. It's now been thoroughly
confirmed that the Bell inequalities are violated,
suggesting that the wave function cannot have
local hidden variables. So does this confirm the
Copenhagen interpretation and kill both
locality and realism? Do we live in a
peekaboo universe that vanishes into
quantum abstraction when we aren't looking at it? Are babies really better
at quantum mechanics than Einstein? Well, not so fast. The results of these
entanglement experiments do seem to violate
local realism. But that may mean a violation
of realism, or just of locality. In fact, it was
Doctor Bell's opinion that the violation
of his inequalities disproved only locality. Realism could be salvaged. Non-locality requires that
entangled particles affect each other instantaneously. That sounds
blasphemous to anyone who accepts Einstein's
theory of relativity. However, non-locality
and relativity can actually be
perfectly consistent. Relativity requires
that causality is preserved, so no faster
than light information flow. But none of these
entanglement experiments allow any real information to be
transmitted between particles. It's only possible
to see the influence between the entangled
partners after measurements have been made and those
measurements are compared, just as we saw with the
delayed choice quantum eraser. The universe seems
to conspire to avoid the paradox of information
traveling faster than light, or backwards in time. The Copenhagen
interpretation remains consistent with all
quantum observations. Niels Bohr's peekaboo universe
may be the universe we live in. However, realist and hidden
variable interpretations are also consistent as long
as they abandon locality. For example, entangled particles
may be dimensionally connected by Einstein-Rosen
bridges, wormholes that allows instantaneous
contact even between great
spatial separations. Also, the De Broglie-Bohm
Pilot Wave Theory works by assuming real and
non-local hidden variables. There is even a way around all
of this without sacrificing realism or locality. That's the many
worlds interpretation. We'll get back to
these in an upcoming episode of "Space Time." Last minute announcement--
tomorrow night, Thursday, the 22nd, is PBS
Nerd Night at the YouTube space in New York. Myself and several
PBS Digital hosts will be talking
about nerdy stuff. Link in the description. OK, last week we talked about
self-replicating spacecraft and why it may be
surprising that none have found their way to earth. There was a lively
discussion in the comments. Let's see what you had to say. Daniel Oberley and
others point out that a Von Neumann probe
may be in our solar system, but be well hidden. Would we ever even
see a single probe if it didn't want to be seen? OK, so the same
statistical arguments for Von Neumann probes tells
us that if we expect one, then we expect lots. OK, so given that, we can't
expect all probes to be equally cautious. Remember, these things may
be created by civilizations only slightly more advanced
than our own, by a few centuries perhaps. Humans are not on track to
extreme cultural enlightenment over time scale. But we are on track to
some unthinkably powerful technological advancements. There's no good reason to
insist that a species that can reach an interstellar
capable state will be highly culturally
evolved, let alone to insist that all will be. So even if there are
invisible probes hiding behind the moon put there by
some utopian prime directive of bane civilization,
there should also be probes blundering through
the solar system with no concern about being noticed. Strofi Kornego and
others wonder if life might be the ultimate
Von Neumann machine. You know what? That's not entirely crazy. We're on the verge of creating
synthetic life ourselves. This type of
militarization may indeed be the most sensible
for building self-replicating explorers. The question then becomes,
if Earth was seeded once, was it seeded only once? Again, if one Von Neumann
machine entering our system is likely, then lots are likely. All life on earth pretty
clearly descended from the one self-replicating molecule type. That said, other
later invaders may have failed to find
purchase on this planet once the first had
come to dominate. But if these artificial
panspermianic seeds are common, then we should
expect to find them in space and on other
planetary surfaces. That's hard to do. But there may be a way. Borne Stellar asks,
do you want reapers? Because this is how
you get reapers. Why do they have to be reapers? Why can't they be autobots? [MUSIC PLAYING]
Space Time is a great series, I would highly recommend it to everyone interested in physics and space!
These guys are doing a real service to the public. The videos are very well done.
Did Bohr really sayy both those things at around 2 min in? It's meaningless to talk about the reality of quantum systems between observation, according to Bohr. Yet that is exactly what the next statement seems to be doing.
This completely explains why my wallet is never in my backpack the first and second times I look there, but on the third time it is there. Thanks, I have always suspected it was something like this.