[MUSIC PLAYING] There is one interpretation
of the meaning of quantum mechanics that somehow manages
to skip a lot of the wildly extravagant or near-mystical
ideas of the mainstream interpretations. It's the de Broglie-Bohm
pilot-wave theory. And despite its alluringly
intuitive nature, for some reason it
remains a fringe theory. [MUSIC PLAYING] Misinterpretation of the
ideas of quantum mechanics has spawned some of the worst
quackery pseudoscience hoo-ha and unfounded
mystical storytelling of any scientific theory. It's easy to see why. There are some pretty
out there explanations for the processes at work
behind the incredibly successful mathematics
of quantum mechanics. These explanations
claim stuff like things are both waves and
particles at the same time, the act of observation
defines reality, cats are both alive and dead,
or even that the universe is constantly splitting into
infinite alternate realities. The weird results of
quantum experiments seem to demand
weird explanations of the nature of reality. But there is one interpretation
of quantum mechanics that remains comfortably,
almost stodgily, physical. That's de Broglie-Bohm
pilot-wave theory. Pilot-wave theory, also
known as Bohmian mechanics, stands in striking contrast to
the much more mainstream ideas of, for example,
the Copenhagen and many-worlds interpretations. Now we've covered
both of those before. And those episodes are
really worth a look. Pilot-wave theory is perhaps
the most solidly physical, even mundane, of the complete and
self-consistent interpretations of quantum mechanics. But at the same
time, it's considered one of the least orthodox. Why so? Because orthodoxy equals
radicalism plus time. And the founding fathers of
the Copenhagen interpretation of quantum mechanics-- Niels
Bohr and Werner Heisenberg-- were radicals. When quantum theory was
coming together in the '20s, they were fervent about
the need to reject all classical thinking
in interpreting the strange results of
early quantum experiments. One aspect of that
radical thinking was that the wave function is
not a wave in anything physical but an abstract distribution
of probabilities. Bohr and Heisenberg
insisted that in the absence of measurement, the
unobserved universe is only a suite of possibilities of the
various states it could take were a measurement to be made,
and that upon measurement fundamental
randomness determines the properties of, say, the
particle that would emerge from its wave function. This required an almost mystical
duality between the wave and particle-like
nature of matter. Not everyone was so sure. Einstein famously hated the
idea of fundamental randomness. But to counter Bohr
and Heisenberg there needed to be a full theory
that described how a quantum object could show both wave
and particle-like behavior at the same time without being
fundamentally probabilistic. That theory came from Louis de
Broglie, the guy who originally proposed the idea
that matter could be described as waves right at
the beginning of the quantum revolution. De Broglie's theory
reasoned that there was no need for quantum
objects to transition in a mystical way
between non-real waves and real particles. Why not just have
real waves that push around real particles? This is pilot-wave theory. In it, the wave
function describes a real wave of some stuff. This wave guides the motion
of a real point-like particle that has a definite
location at all times. Importantly, the wave
function in pilot-wave theory evolves exactly according
to the Schrodinger equation. That's the equation at the
heart of all quantum mechanics that tells the wave
function how to change across space and time. This means that
pilot-wave theory makes the same basic predictions
as any other breed of quantum mechanics. For example, this guiding wave
does all the usual wavy stuff, like form an interference
pattern when it passes through a pair of slits. Because particles follow the
paths etched out by the wave, it'll end up landing
according to that pattern. The wave defines a set
of possible trajectories and the particle takes
one of those trajectories. But the choice of
path isn't random-- if you know the exact
particle position and velocity at any point, you
could figure out its entire future trajectory. Apparent randomness arises
because we can't ever have a perfect measurement of
initial position, velocity, or other properties. But this hypothetical
predictability means that a pilot-wave universe
is completely deterministic. When de Broglie presented
his still-incomplete theory at the famous Solvay
Conference of 1927, it didn't go down so well. Technical objections were
raised and Niels Bohr doubled down on the
probabilistic interpretation. De Broglie was convinced and he
dropped pilot-waves altogether. The idea was
forgotten for decades and Copenhagen
became the orthodoxy. It took until 1952 for
another physicist, David Bohm, to feel very
uncomfortable with some of the wackiness of Copenhagen
and to rediscover de Broglie's old idea. Bohm took up where de Broglie
left off and completed the theory. The result was Bohmian
mechanics, also known as de Broglie-Bohm
pilot-wave theory. These days, more
and more serious physicists are
favoring Bohm's ideas. However, it's far from
being broadly accepted. De Broglie himself remained
firmly in the Copenhagen camp even after Bohm's efforts. See, although
pilot-wave theory makes all of the usual predictions
of quantum mechanics, it has some really
fundamental differences. Those differences are
in the special thinking you need to do in order
to accept pilot-waves over other interpretations. In fact, most of the
arguments for or against it are about this special thinking. Are you more or less
comfortable with the oddness of pilot-waves versus
the oddness, say, of Copenhagen or many-worlds? So what uncomfortable thinking
does pilot-wave theory require? Well, for one thing, it needs
a teensy bit of extra math that mainstream
interpretations don't. As well as the
Schrodinger equation that tells the wave
function how to change, it also has a
guiding equation that tells the particle how to move
within that wave function. That "extra math" is considered
un-parsimonious to some, a needless added complexity. However, the guiding
equation is derived directly from the wave function,
so some would argue that it was there all along. A more troubling requirement
of Bohmian mechanics is that it does contain
real complexity that is not encoded in the wave function. That's something that Niels
Bohr was so fervently against. Bohmian mechanics has
so-called hidden variables, details about the state of
the particle that are not described by the wave function. According to pilot-wave
theory, the wave function just describes the
possible distribution of those variables given our
lack of perfect knowledge. But hidden variables have a
bad rap in quantum mechanics. Pretty soon after de Broglie
first proposed pilot-waves, the revered mathematician
John Von Neumann published a proof showing that
hidden variable explanations for the wave function
just couldn't work. That proclamation contributed
to the long shelving of pilot-wave theory. But in fact, Von Neumann didn't
quite get the full answer. It turns out that
the restriction against hidden
variables only applies to local hidden variables. So there can't be
extra information about a specific region
of the wave function that the rest of the wave
function doesn't know. This was figured out pretty
soon after Von Neumann's paper by German mathematician
Grete Hermann, although her refutation wasn't
noticed until it was re-derived by John Bell in the 1960s. This helped the resuscitation
of pilot-wave theory, because Bohmian
mechanics doesn't use local hidden variables--
its hidden variables are global. The entire wave function
knows the location, velocity, and spin of each particle. This non-locality is
another weird thing you have to believe in
order to accept pilot-waves. Not only does the
entire wave function know the properties
of the particle, but the entire wave function
can be effected instantaneously. So a measurement at one
point in the wave function will affect its shape elsewhere. This can therefore affect the
trajectories and properties of particles carried
by that wave, potentially very far away. But quantum
entanglement experiments show that this sort of
"spooky" action at a distance is a very real phenomenon. Again, we've gone
into the non-locality of entangled particles
in detail before. Also worth a look. It's a tough idea to
swallow, but experiments indicate that some
type of non-locality is real, whether or not
we accept pilot-waves. It would be remiss of me
to talk about pilot-waves without mentioning the
amazing analogy that was discovered in
bouncing droplets on a vibrating pool of oil. I'll let Veritasium
cover this one in detail. It's pretty amazing. But we see many of the
familiar quantum phenomena appear in this macroscopic
system of a suspended oil droplet following
its own pilot-wave. Now, we shouldn't take
a macroscopic analogy as proof of microscopic
reality, but it certainly demonstrates that
this sort of thing does happen in this universe,
at least on some scales. I should probably also
add that de Broglie-Bohm pilot-wave theory
is certainly wrong. And I don't think
anyone could deny that, because it doesn't
account for relativity, either special or general. That means it's at
best incomplete. While regular mechanics
has quantum field theory as its relativistic
version, pilot-wave theory hasn't quite got there yet. Quantum field theory
pretty explicitly requires that all possible
particle trajectories be considered equally real. Pilot-wave theory postulates
that the particle really takes a single actual
trajectory, the Bohm trajectory. This is not consistent
with quantum field theory, and so there isn't a complete
relativistic formulation of Bohmian mechanics yet. But there is good effort
in that direction. Now let's not even start
talking about gravity-- no version of quantum
mechanics has that sorted out. Also, we can't ignore the fact
that the initial motivation behind pilot-wave
theory was to preserve the idea of real particles. And I think we need to be
dubious about that sort of classical bias. All that said,
pilot-wave theory does do something
remarkable-- it shows us that it's possible to have
a consistent interpretation of quantum mechanics
that is both physical and deterministic, no hoo-ha needed. Maybe something like
pilot-waves really do drive the microscopic
mechanics of spacetime. Hey, guys. So we recently launched
our Patreon page. And I want to thank those of
you who have contributed so far. This is really going
to be a huge help. And it's going to
be a lot of fun. I've loved the discussions
we've been having on the EM Drive and future
episode ideas, not so much the embarrassing quark
compilation-- damn you, [INAUDIBLE] Anyway, if you're interested
in supporting the show, heading over to the Patreon
page would be an amazing way to do that. Or, you know, just
keep watching. In our last episode, we talked
about the strangest of stars, the strange star-- aptly named. Your comments, by comparison,
were very reasonable. Burak asks why quark/strange
matter isn't found naturally in the universe given that
it's supposed to be so stable. OK, so the hypothesis
is that strange matter is the most stable
matter in the universe. But this only applies when
you have a large number of quarks mushed together. In that case, having one
strange for every one up and one down quark is a
very low energy state and so is very stable. However, this doesn't
work when there are only a small number of quarks, say
in the typical atomic nucleus. In that case, any
mass of strange quarks will decay into the
lighter up or down quarks. But during the quark
era, the universe was full of this
quark-gluon plasma. And the problem is that
the universe at this time wasn't dense enough
and was expanding too quickly for strange matter
to form in any great abundance. That said, some
strange matter made have formed during
the quark epoch. And the resulting particles
are called strangelets. They may be around today. Depending on the as-yet-unknown
physics of strange matter, these strangelets
may even be expected to be more stable the larger
they are, and so would grow over time. In fact, such
strangelets may even convert any regular matter
they come into contact to into strange matter. Sebastian Lopez asks how
are the magnetic fields of neutron stars created. Well, to create and
sustain a magnetic field, you need some charge that's
moving or spinning in some way. That might seem a problem
for an object made up of neutral particles
like a neutron star. However, a neutron star isn't
only made up of neutrons. You have an outer crust
of conductive iron that can support an enormous
current of electrons. And below that crust,
there's a region centimeters to meters deep in which you
have significant impurities of electrons and protons mixed
in with the neutrons, perhaps up to 10% electrons and
protons by mass of the star. With their extreme
rotation rates, neutron stars support
electric currents sufficient for magnetic fields
of up to 100 million tesla. And then you have
magnetars, which are believed to get to 10
to the power of 11 tesla. These fields are supported
by superconduction of protons beneath the surface. The757packerfan would like
to know where the neutronium compares to adamantium. I see where you've
gone with this. Neutronium pretty much sucks as
a skeleton graft for Wolverine. Firstly, it would weigh
as much as a mountain range, which isn't helpful. Also, it's dense but
not necessarily strong. In a neutron star, it's a
superfluid, so not ideal there. But at atmospheric
pressure-- or even inside Wolverine pressure--
it would expand into a gas cataclysmically and
the neutrons would decay to protons and electrons
and an awful lot of radiation. A five centimeter
tube of neutronium would explode with
the equivalent energy of around a
trillion hydrogen bombs. That's going to take
some serious regeneration to recover from. By the way, this is why
science is so important. [MUSIC PLAYING]
I cAnt stand the way he's talking in this video. He's good at what he does, but something doesn't seem right
Can be future be fully determined using pilot wave theory