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
It's the de Broglie-Bohm 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 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
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.
Because orthodoxy equals radicalism plus time.
And the founding fathers of the Copenhagen interpretation
of quantum mechanics-- Niels Bohr and Werner Heisenberg--
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
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
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
Bohm took up where de Broglie left off and completed
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
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.
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,
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.