Okay this video is on the interpretations of quantum physics, which… I’ve been down
a little bit of a rabbit hole over the last couple of months if I’m honest, because
the interpretations of quantum physics aren’t easy to understand. And I thought, hey I’ll
do a video on those that’ll be fun! And then I did my research and looked into it,
and kind of, by brain melted. Anyway this is my best effort at the interpretations of
quantum physics I hope you enjoy. So what are they? Well first of all there is a hole
in the bottom of physics when you are down at the level of atoms or subatomic particles,
you are in the realm of quantum physics and there are some things in quantum physics that
simply don’t make sense. Physicists don’t like things not making sense and so the interpretations
of quantum physics are an attempt of many different physicists to come up with ways
of making quantum physics make sense. Now if you are not familiar with the terminology
of quantum physics I recommend you watch my last video because it is kind of an introduction
to quantum physics and this video is a sort of part two to that. So interpretation number
one is the Copenhagen interpretation and that’s kind of the standard way we are taught quantum
physics when we learn it in University and it is also kind of the place where I can illustrate
where some of the problems start happening. So the Copenhagen interpretation, the standard
description is that subatomic particles, like and electron, is described by a wave function
and this wave function obeys the Schrödinger equation, which tells us how that wave sort
of smoothly evolves over time. And features of this wave gives us all of the properties
of of quantum physics that people talk about like superposition, entanglement, energy quantisation,
decoherence and quantum tunnelling, all of those things are phenomena that appear because
of the wave-like nature of subatomic particles. So the conceptual problems begin when we do
a measurement on these waves. So the Copenhagen picture of quantum physics is that you have
a wave function moving along smoothly according to the Schrödinger equation until it hits
an object like a detector. And then what we see, what we measure as a human, is a single
particle in a specific position, which means that the wave function which is like spread
out over space, when it hits the detector, it suddenly collapses, it is called localisation.
So the wave localises, from a spread out wave, to a very sharply defined wave where we see
the particle. The trouble is, is there’s no actual physics in quantum mechanics that
describes how that collapse happens, and this is known as the measurement problem. So this
measurement is sort of a barrier, which shields the quantum realm. We can never see those
wave functions, all we can ever see are particles. And in the Copenhagen interpretation of quantum
physics what they say is like, us humans, we’ll never see those waves, there’s nothing
we can do to access that realm, so don’t worry wondering whether they are real or not
real, the important thing are the measurements. Reality is in the observations. Which is why
it has been termed the shut up and calculate interpretation of quantum physics. But people
have not stopped there they don’t like this discontinuous collapse of the wave function,
so they’ve come up with other ways of trying to interpret what is going on. And this brings
us to the second interpretation that I’m going to cover which is the many worlds interpretation
of quantum physics. So the many worlds interpretation of quantum physics takes the wave function
to be physically real and says if the Schrödinger equation is just a description of reality,
what does reality look like? So what it says is that when you do a measurement on particle
that is in a superposition of many different places at once, actually that particle turn
up at all those different places, just in different versions of reality. Another way
to think of it is if a particle is in a superposition of two places at once, and it hits a detector,
it now puts that detector in a superposition of measuring a particle in once place or another,
and if you look at the results from that detector then it puts you in a superposition of seeing
the particle in one place, or seeing the particle in another place, but because the results
of one place or another are mutually exclusive it means that now your universe between the
two you’s has decohered from each other and are now split into different branches.
So a superposition is described by a wave function, so really there’s just one giant
mega wave function that describes all of the possibilities that could ever have happened
in the entire history of Universe. So if there is an interpretation of quantum physics that
you have heard about it’ll be this one, the many worlds interpretation because it
seems to capture public consciousness, and I think it is because people like the idea
that if you’ve made like a really bad decision that you regret in the past, you can kind
of take comfort in the belief that there is another universe out there where you made
a better decision and so there is a version of you who’s happy right now. It also seems
to be a really useful storytelling technique in films that want to have different sort
of realities, like it was just in the Spider-Man into the spider verse for the different kinds
of, well I shouldn’t ruin the plot, anyway it was a good film. It also seems to be pretty
popular amongst physicists, although I’m not such a huge fan of it because one of the
problems is it breaks probability. So imagine if you’ve got a particle that’s in a thirty
percent probability of being here, and a seventy percent probability of being here, when it
hits the detector it splits into two universes where it definitely is, with a hundred percent
probability it is in one place and a hundred percent probability it’s in the other place
so if everything all happens what do those original probabilities actually mean? This
problem is addresses in an interpretation called the cosmological interpretation which
says that the many worlds theory is trivially true if the Universe is infinitely big, because
there is an infinite number of you’s doing that experiment, and that infinite number
just splits in proportion, so thirty percent of those infinite you’s will see one thing
and seventy percent of those infinite you’s will see the other thing. Unfortunately there’s
no experiments that we can run to see if this interpretation is more valid than the Copenhagen
interpretation, and that’s actually a general feature of all of these interpretations is
that we can’t pick them apart with any experiments and so until we have experiments it’s all
kind of like physics-y story telling. Lets now look at another aspect of quantum physics
that doesn’t quite make sense to us which is called non-locality, otherwise known as
spooky action at a distance. And this can be summarised by the EPR experiment which
is where you take two particles, say electrons, and you entangle them together to have equal
and opposite spins say, and then you separate them by a very very large distance because
they are now inextricably linked the wave function that describes them has to describe
both particles and what this means in reality is when you do a measurement on one of these
particles it instantaneously influences the other one, which can be you know billions
of miles away, and it does that instantaneously. So it sort of feels like faster than light
action. That’s the action at a distance which Einstein was really creeped out by.
So non-locality mean that one electron, its properties are not local to its physical place.
Its properties are dependent on something very very far away and this is an alien concept
in physics because if I throw a ball the balls position or velocity or colour or anything
physical to do with that ball they’re all localised to where that ball physically is,
whereas in quantum physics you can have things that are non-local. And that seems weird to
a physicist, because you are like ‘all physics is like this, except for this one bit here
in quantum physics’. So this has spawned other interpretations of quantum physics that
try and explain away this one, and one of these is called hidden variable theories.
So the idea in a hidden variable theory is that when you do this entanglement there’s
some secret label which captures the actual state of these two particles so it’s like
if I flip a coin and I hold it in my hands, even though I don’t know whether it’s
heads or tails, the coin actually is heads or tails when its in my hand, and I just reveal
it when I open up my hands. So that’s kind of what these hidden variable theories are
like, the particles are in a definite position we just don’t know what it is until we measure
it. Now this interpretation was actually killed by an experiment called Bell’s theorem.
I’m not going to go into the details of this because I don’t have time. But basically
the hidden variable theory and quantum physics predict different probabilities of you observing
certain things under Bell’s experiment. And when they did the experiment, they actually
found the results matched quantum physics and not hidden variable theories, so hidden
variables are out. Now it is not completely game over for the hidden variable theories
because Bell’s theorem is built on some axioms, some things that you assume to to
be true, and one of these is locality. And so you can have a successful non-local hidden
variable theory, also known as Bohmian mechanics also known as pilot wave theory. Now the idea
in this interpretation is that the particles are always real, but they are kind of surfing
about on an underlying wave, and so when you see the probability of particles appearing
in different places it’s because they have been nudged in different directions by this
underlying wave. But still we can’t see those waves. And the advantage of this is
that it kind of returns quantum physics to a sort of determinism, which none of the interpretations
of quantum physics do. But the trouble again is there’s no testable hypotheses so there
is no way of us doing an experiment to see if it is true. So another thread in the interpretations
of quantum physics is to instead focus on the collapse of the wave function, and try
and make that bit make sense. So there are interpretations called alternative collapse
theories that try and add some extra physics in to that collapse that either explain why
it happens or add some, describe the dynamics of how exactly the wave function localises.
One idea called spontaneous collapse theory is that the wave function has got a probability
of collapsing at any time a bit like radioactive decay. So for a small particle there is a
very low probability that its wave function collapses, maybe like once in a hundred million
years. Whereas for big groups of particles together in something like a football there
would be way more collapses because if any one those collapses, it has a knock on effect
to collapse the wave functions of all the others. And so there would be maybe a hundred
million collapses per second which kind of goes some way to describe why the world we
experience is all kind of solid and deterministic, and the quantum realm isn’t, so it describes
kind of the boundary between those two things. And the advantage of this interpretation is
it’s actually making a testable prediction. And so at some stage in the future we could
design an experiment that could maybe see if it is true or not. So, you know, it doesn’t
say that it is any more likely to be true than any of the others but at least it has
got a testable prediction which, as an experimental physicist, I like! So those are the main interpretations
of quantum physics but there’s many many more, I’m just going to cover a few more
here. But this isn’t an exhaustive list. Also, word of warning, this is where I started
to get really rather confused, so lets proceed with some amount of caution. First up is quantum
Bayesianism also known as Qbism, which takes the ideas of Bayesian probability and applies
it to quantum physics so it’s sort of like an informational theory of quantum physics.
And the idea as I understand it is that when you get new information about a state it updates
the probabilities of the things that you will measure. The consistent histories interpretation
of quantum physics apparently the mathematics is somewhat of a hybrid between hidden variable
theories and spontaneous collapse theories. It includes spontaneous collapses of the wave
function, but different aspects, not just just position can collapse. It also says that
those collapses are not physical events but just a way of you picking through a history
of that quantum object that makes sense, that’s consistent. The next one is quantum Darwinism,
which is where, when the quantum system interacts with the environment certain things are killed
off. So interactions between particles and the environment are kind of like natural selection
for the properties of that quantum object. Another interpretation is called the transactional
interpretation which, in the actual mathematics of quantum physics it allows solutions that
travel backwards in time as well as forwards in time, but we normally just throw away the
backwards in time ones, because you know you’re not going to travel backwards in time. But
like, I don’t know, maybe you can. So that’s what the transactional interpretation, it
keeps those and says that the properties that you have maybe are dependent on things that
happen to you in the future. And this is attractive because it can actually get around Bell’s
theorem. Another one is called the relational interpretation which doesn’t focus on the
properties of quantum objects but everything is defined by the relationships between them.
So those are the main interpretations of quantum physics there’s a few others but those are
the main ones. I sometimes get asked what is my favourite interpretation and I don’t
really have one. My thoughts about this are: because there’s so many different interpretations
of quantum physics it’s kind of a sign that we might be missing something kind of fundamental,
so maybe one way to attack this problem is to really go back to first principles and
back to the fundamental assumptions of quantum physics because there’s some interesting
assumptions that were made like when you have a wave function which predicts probabilities
of things happening that was just a guess, and it works really really well, but there
was no real reason for that other than nice guess work. So, yeah quantum physics is going
to… Well that’s the end. Thanks for sticking through all of that. I hope it was of interest.
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