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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.
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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?
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