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 is the de broy berm
pilot wave theory and despite its
alluringly intuitive nature
for some reason it remains a fringe
theory
misinterpretation of the ideas of
quantum mechanics has spawned some of
the worst
quackery pseudo-science 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 stodgely physical
that's de bruy bone pilot wave theory
pilot wave theory also known as bohmian
mechanics
it 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
boron 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
particle-like nature
of matter not everyone was so sure
einstein famously hated the idea of
fundamental randomness
but to counter borin 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 bruy
the guy who originally proposed the idea
that matic would be described as waves
right at the beginning of the quantum
revolution
de bruy'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 bruy 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 board doubled down on the
probabilistic interpretation
de bruy was convinced and he dropped
pilot waves all together
the idea was forgotten for decades and
copenhagen became the orthodoxy
it took until 1952 for another physicist
david boehm to feel very uncomfortable
with some of the wackiness of copenhagen
and to rediscover de bruy's old idea
berm took up where de broglie left off
and completed the theory
the result was bohmian mechanics
also known as de bruy boom pilot wave
theory
these days more and more serious
physicists are favoring boehm's ideas
however it's far from being broadly
accepted
de bruy himself remained firmly in the
copenhagen camp
even after berm'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 sort of 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 unpassimonious 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 ball was so
fervently against
birmingham 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 wrap in
quantum mechanics
pretty soon after de bruy 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 greta herman
although
her refutation wasn't noticed until it
was rederived 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
affected 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 bone 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 bone 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 space-time hey guys so we recently
launched our patreon page
and i want to thank those of you who've
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 compilation
damn you louis anyway if you're
interested in supporting the show
heading over to the patreon page would
be an amazing way to do that
well 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
buddha asks why strange mata
isn't found naturally in the universe
given that it's supposed to be so
stable okay 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 quite long 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 may 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 sort of 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
ion that can support
an enormous current of electrons and
below that crust there's a region
centimeters to metres 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
electrical 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
the 757 pack-a-fan would like to know
whether neutronium compares to
adamantium
i see where you're going 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 into protons and electrons
and an awful lot of radiation a five
centimeter
cube 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