Quantum physics has a mystique of being complicated and hard to understand, in fact Richard Feynmann
who won the Nobel prize for his work on quantum electrodynamics said: “If you think you
understand quantum physics, you don’t understand quantum physics”. Which is kind of disheartening
for us because if he didn’t understand it, what chance do the rest of us have?
Fortunately this quote is a little misleading. We do in fact understand quantum physics really
well, in fact it is arguably the most successful scientific theory out there, and has let us
invent technologies like computers, digital cameras, LED screens, lasers and nuclear power
plants. And you know, you don’t really want to build a nuclear power plant if you don’t
really understand how it works.
So quantum physics is the part of physics that describes the smallest things in our
Universe: molecules, atoms, subatomic particles thing like that. Things down there don’t
quite work the same way that we are used to up here. This is fascinating because you and
everything around you is made from quantum physics, and so this is really how the whole
universe is actually working.
I’ve drawn these protons, neutrons and electrons as particles, but in quantum mechanics we
really describe everything as waves. By the way I'm using quantum physics and quantum
mechanics interchangeably, they are the same thing. So instead of an electron looking like
this, it should look something like this. This is called a wave-function.
But this wave-function isn’t a real physical wave like wave on water or a sounds wave.
A quantum wave is an abstract mathematical description. To get the real world properties
like position or momentum of an electron we have to do mathematical operations on this
wave-function, so for the position we take the amplitude and square it, which for this
wave would look something like this. This gives us a thing called a probability distribution
which tells us that you are more likely to find the electron here than here, and when
we actually measure where the electron is, an electron particle pops up somewhere within
this area.
So with quantum physics we don’t know anything with infinite detail, we can only predict
probabilities that things will happen, and it looks like this is a fundamental feature
of the Universe which was quite a departure from the clockwork, deterministic universe
in classical physics, the kind of thing Newton derived.
This wave-function model predicts what subatomic particles will do incredibly well, but weirdly
we've got no idea if this wave-function is literally real or not. No one has ever seen
a quantum wave because whenever we measure an electron all we ever see is a point like
electron particle. So there is like this hidden quantum realm where the waves exist, and then
the world we can see, which is where all the waves have turned into particles. And the
barrier between these is a measurement. We say a measurement ‘collapses’ the wave
function, but we don’t actually have any physics to describe how the wave collapses.
This is a gap in our knowledge that we have dubbed the measurement problem, and this is
one of the things that Feynmann was referring to with his quote.
Another confusing thing is how exactly to picture an electron. It seems to be a wave
until you measure it, and then it is a particle, so what actually is it? This is known as particle-wave
duality, and here is an example of it in action: the famous double slit experiment.
Imagine spraying a paintball gun at a wall with two openings in it, you’d expect to
see two columns of paint go through and hit the wall behind. But if you shrink this all
down to the size of electrons you see something quite different. You can fire one electron
at a time at the slits and they appear on the back wall, but as they build up over time
you get a whole pattern of stripes, instead of just two bands, this pattern of stripes
is called an interference pattern, something you only see with waves. The idea is that
it is the electron-wave that goes through both slits at the same time, and then the
waves from each slit overlap with each other, and where the waves add together you have
a high probability of the electron popping up at the wall, but where the waves cancel
out the probability is very low. So actually on the back wall the highest probability of
finding the electron is in the middle of the slits, and then it goes down and up again,
and down and up again and this is the interference pattern. So when you fire one electron after
another they follow this probability distribution and this interference pattern starts building
up, and that's exactly what we see in experiments. So this shows that electrons behave like waves
in this experiment.
A question is what actually happens to this spread-out electron-wave when you do a measurement?
It seems like it goes from this spread out wave to this localised particle, but like
I said, there's nothing in quantum mechanics that tells us how the wave-function collapses.
And this is not only true for electrons, but for everything in the Universe, so this double
slit experiment has huge consequences for our model of the Universe, and it was very
surprising the first time it was done. Physicists are still grappling with this question today
and have come up with many interpretations of quantum mechanics to try an explain these
results, and explain how reality actually works.
Okay lets go back to the wave-function. Now we can use this picture to explain other features
of quantum physics that you may have heard about.
So this is just one possible wave-function for an electron, but there are many others.
Like this one for instance. This says that the electron has a probability of being over
here, and a probability of being over here, and very little probability of being in the
middle. This is perfectly allowable in quantum physics and this is where the phrase ‘things
can be in two places at once' comes from. This is known as superposition, which comes
from the fact that this wave can be made by adding, or superimposing these two waves.
The word superposition just means the adding together of waves and we already saw this
in the double slit experiment, and is not really a very special phenomenon. You can
even see superposition by dropping two pebbles into a pond where the ripples overlap.
Now for entanglement. Let’s say two electron-waves meet. Their waves interfere with each other
and become mixed up. This means that mathematically we now have one wave-function that describes
everything about both electrons and they are inextricably linked, even if they move far
away from each other. A measurement on one of the particles, like measuring if it is
spin up or down is now correlated with a measurement on the other, even if they move billions of
miles away. Einstein was very uncomfortable with this idea because if you measure one
of the particles here you instantaneously know what the other will be even if it is
billions of miles away, and that's got a sort of whiff of faster than light communication,
which is not allowed by the theory of relativity. But it turns out you can’t actually use
this to communicate information, because the measurements give you random results, but
the fact that they are correlated means that somehow there is a link that stretches over
that distance. This is called non-locality.
Quantum tunnelling. Quantum tunnelling is where particles have a probability of moving
through barriers, essentially allowing things like electrons to pass through walls. When
a wave-function meets a barrier it decays exponentially in the barrier, but if the barrier
is narrow enough the wave-function will exist on the other side meaning there is a probability
of the particle being found there when a measurement is made.
In fact the only reason you are alive is because of quantum tunnelling in the Sun which make
the Sun shine. Protons normally repel each other, but they have a small probability of
quantum tunnelling into each other which is what turns hydrogen into helium and releases
fusion energy. All life on Earth exists because of energy from the Sun, except for life around
hydrothermal vents.
Now on to the Heisenberg Uncertainty principle. I said that the beginning that this wave-function
contains all of the information like position and momentum of the electron, we just have
to do some maths on it. The position is given by the amplitude, or height of the wave, and
the momentum is given by the wavelength of the wave.
But for this specific wave the position gives us a probability distribution, so we don't
know exactly where the electron is. Also there is an uncertainty in the momentum because
this wave is made of many different wavelengths.
But we can reduce that uncertainty, let’s have a wave that only has one wavelength,
so a sine wave. Now we know the momentum exactly because the wavelength has a single value,
but look at the position. There is an equal probability of the electron being found anywhere
in the universe. Okay let's do the opposite let’s make a wave that has only got one
position. Now we know exactly where the electron is, but what is the wavelength of the wave?
Now the wavelength is very uncertain. Basically only a sine wave gives you a precise momentum,
and any wave that isn't a perfect sine wave, you have to build out of multiple different
sine waves, and each of those multiple different sine waves has got a different wavelength,
and hence you have a range of possible different values of momentum for the particle.
This is Heisenberg’s Uncertainty principle, you can only know certain things precisely,
but not everything. Either you have got a definite value of momentum, and don't know
anything about position, or you know the position very well, but don't know anything about the
momentum, or you are in some intermediate state. And this isn't a limit of our measuring
apparatus, this is a fundamental property of the Universe!
And finally, where does the name ‘quantum’ come from. Well a quanta is a packet of something
like a chunk of something, and one of the first quantum effects people saw were atomic
spectra which is where atoms give off light with specific discrete energies. It works
like this. Imagine a string that is tied at both ends, like a guitar string. If you pluck
it, only certain waves can exist because the ends are tied down, in this situation we say
that the wavelengths are quantised to certain values.
The same thing happens if you ties the ends of the string together because the waves have
to match up, they can only vibrate in certain restricted ways. And this is what is happening
to an electron in an atom. The electron-wave is constrained by the atom and quantised to
certain wavelengths, short wavelength have high energy and long wavelengths have a lower
energy. This is why the light emitted by an atom looks like a barcode because each bar
of light corresponds to an electron jumping from a wave with a high energy to one with
a lower energy, and at the same time emitting a quantised photon of light when it does this.
So the light from an atom is quantised to discrete packets of energy.
Okay so that’s all the basics of quantum physics, here are some technical notes which
aren’t essential to know, but pause the screen now if you are interested in a little
more mathematical detail.
So to round up. In quantum physics objects are described with wave-functions, but when
we measure them, what we see are particles, so this leads to particle-wave duality, and
also the measurement problem. And the consequence of these wave-functions are the quantum phenomena
of superposition, entanglement, quantum tunnelling, the Heisenberg uncertainty principle and energy
quantisation. So if you understand these things you have got a good basic understanding of
quantum physics.
Despite its reputation I think quantum mechanics isn’t too difficult for most people to get
the basics of what is going on. In the past I have relied upon analogies to try an explain
it, but here I have just described what is actually going on which I think might be more
helpful. But if you have more questions I'll be on the comments below so ask away.
For me the weird thing about quantum physics is that on the one hand it is incredibly accurate
and predictive but also it has got giant holes in it like the measurement problem which we
just don’t understand. So we can wonder, will we ever actually understand quantum physics,
or is it just too abstract for our human brains to comprehend. Well I hope this video has
helped you understand a little more about how quantum physics works.
And thanks to the sponsor of this video brillaint.org, who have just launched their daily problems
which you can dip into if you have a spare 5 minutes each day. Each problem teaches you
some interesting facts that you can then use to solve the problem. And if you enjoy that
specific problem there are links to more on the same topic so you can develop your framework
of knowledge around that subject. And as ever if you are confused and need more guidance,
then you can join with the community discussions. So this is a simple, fun way to keep learning
more. If that sounds interesting go to brilliant dot org slash dos or click on the link in
the description below, and the first 200 to do so will get 20% off the annual subscription
which unlocks all of their premium content.
Well that's it from me, thanks for watching and I'll see you next time.