So Quantum Mechanics, is the subject of the discussion here tonight.
And look it's the year 2014
And the first glimmers of the Quantum Mechanics
really came on the scene on the early part of the 20th Century.
Certainly by 1930 or so the basic framework of Quantum Mechanics was place and yet,
today right some 80 years later there are still questions at
our right for discussion at the foundations of the subject
questions that are still controversial, and we're going to discuss one of those
controversial questions here tonight
so you'll see various perspectives on that.
At the same time, quantum mechanics of all the subjects
of modern physics
has really been embraced by the culture of at least the language of QM
has been embraced by the culture I mean, just to give a couple examples right
it's there from hollywood, right we have quantum thrillers right
we also have, you know, quantum sporting goods right, golf clubs, quantum bowling balls, quantum baseball
we've got quantum cartoons right
"Oh Alice, you're the one for me"
"But Bob, in a Quantum world how can we be sure"
and the we have physicist talking dirty right
you fiinsh I'm not going, I, I...
It's a family program here right
so we also have quantum healthcare right
so this one, Quantum herbal canker sore gel, nothing to do with the previous cartoon, completetely seprarate
We have the Quantum Pendant, which protects the body against any harmful rays
I like this one, we got the QuantumVet right
Use your cellphone to instantly diagnose and medically treat your pet at home
and I love this tagline that you see at the bottom, Don't take me to the Vet, use QuantumVet
which makes use of the famous of all adages which is a sure fire way to get words to rhyme, is to use the same word
 
[laughter]
now this, this one is absolutely my favourite
the Quantum Sleeper
protection from bio-chemical terrorist attack, kidnappers, stalkers, bullet-proof
and look how... look at the loving couple over there
about to enter the sarcophagus over there
right so, so QM in this sense is certanly out there in the world
but of course, this is just a word
quantum is this really quantum bullshit right
so, um,  we're gonna talk about here tonight is
the real Quantum Mechanics
and what I'm going to do here for just about 10-12 minutes
is run through the basic ideas of Quantum Mechanics
again I think that it's probably familiar to many of you
but just to make sure we're all on the same page
and that'll actually lead us into the issue for tonight's discussion
which is this quantum measurement problem
okay so for the basic premier on Quantum Mechanics
the key experiment that really gets the ideas of Quantum Mechanics across is
the Double Slit Experiment
and I don't know if any of you go to Spooky Action last year
just by any chance. Yeah, so a handful of you
so you may recall that we actually did the Double Slit Experiment
on the stage last year
we're not going to do it again this year
we don't have enough time
rather these animations will give you the essential ideas
and if anyone doesn't believe the animations, just ask someone whose hand was up
cause they saw the real experiment done last year
OK. So, what is a Double Slit Experiment
the idea is this
imagine that I have a gun
that's firing ordinary pellets
like BBs at a barrier that has two openings
now you'd expect that those BBs that'd go thru the left slit
will land aligned in a band on the left
those pellets that go thru the right opening will land in a band aligned with it on the right
and indeed we're to do this experiment that is the result you'd find
now, what we're going to do is
imagine dialling down the size of those pellets, those BBs
making them smaller and smaller until they are the size of little tiny particles, let's just call them, electrons
imagine we're doing this with electrons
you would expect that if you ran the same experiment
again those electrons that pass through the left opening
will land in a band and aligned with the opening on the left
and similarly, for those that pass through the right
that is what you would expect
should
happen
and the thing is, many of you are familar, this is
not what happens
when you do this experiment
instead if you do this experiment you get results that look rather different
they look like this
you get a bright band next to a dark region
next to a bright region
next to a dark region
next to a bright region
and so forth
and that data, that strange data
is really what impels us towards
now that data is strange because it's not what we'd think should happen
we expect to get two bands and we get more
but it turns out that that pattern
bright, dark
bright, dark
is a familiar pattern to most physicist
and we're trained from birth to recognise that pattern
right if you're to go
to any physicist, 3'o clock in the moring
shake that physicist,
wake them up, and tell them "I got this data, bright band, dark band, bright band, dark band "
The very first thing they would say to you is, "What the hell are you doing in my room at 3 o'clock in the morning"
[laughter]
But after that, they would say, "I know what that is, that's an interference pattern"
that is the hallmark signature that there is some kind of wave phenomenon
Some kind of wave phenomenon taking place
right to understand that, let me just give you
a little visual here, where we are looking at water waves
Imagine I throw 2 pebbles
The first one over here it makes these circular ripples, and now I throw in a second one...
and look what happens
when the waves overlap, in some regions they work together, but
in other regions, the peak of one wave is crossing the trough of another
causing the wave to cancel out, and those are those dark regions that you see
on the screen
So this is exactly the kind of data that we are encountering, and to make that point
a little more clearly. Let me show you an animation, same idea
Imagine you got water waves going toward the barrier with the two openings
and again regions where the waves work together the water is very agitated
regions in between, where the peak of one wave crosses the trough of the other
they cancel each other out, and if on the back screen I put bright regions
associated with very agitated water, Dark regions with not agitated water
Look what I get, Bright Dark Bright Dark Bright Dark
and again, just to remember that is the data that we find in this experiment
So this suggests, that there is some deep connection
between these particles, these electrons
and this other idea, a completely separate idea, of waves
Now, at first sight, that might not seem all that surprising
Right? Because water has water waves,  but we all know
that water is itself made of little particles, molecules, H2O molecules
So certainly we know, if a large collection
of molecules, behave in a choreographed manner
those particles can yield a wave.
So maybe what's going on in the experiment
is just that, you've got a lot of electrons
and maybe they are behaving in a choreographed manner
and in that way, yielding a wave just like H2O molecules
yield a water wave. Now how would you test that idea?
Well you could run this very experiment
But only firing particle by single particle at this barrier with the 2 openings.
and by recording dot by single dot where each of those single particles lands on the detector screen
So let's run that version of the experiment
and see what happens.
So again,
Particle by single particle,
we're recording on the back screen
the history of all the landing locations.
And this is what happens in this experiment.
Dot by single dot,
we build up the same pattern,
the same interference pattern,
the same data.
That suggests to us
That there must be some kind of
wave like phenomenon involved to yield this interference pattern.
So this is where things get really strange.
Particles, electrons little tiny dot.
That is the image that we always have in mind.
Waves are these spread out entities.
How could a dot particle and a spread out wave
somehow be connected
And this was the puzzle that physicist faced
In the early decades of the 20th century
And many tried to figure out
what could the connection between
a particle and a wave be.
The natural suggestion that you might throw out as you know
maybe particles somehow are kind of ...kind of smeared out in some way
there are kind of spread out the way we didn't realize
that deal out it doesn't really work. Because whenever you measure an electron you find all of it
if it was truly smeared out. You find a piece of it here and a piece of it there
And the idea ultimately was settled out the part  came from the mind of the physicist--Max Born.
without the way they think about the wave associated with the particle is this .
This is a new idea. The wave is a wave of probability.
The wave is telling us the likelihood of the probability that the particles at one location or another.
They show you a little visual of that. Imagine this is the wave associated with a particle like an electron.
Where the wave is big, that is a likely location to find the particle.
Where the wave is small, an unlikely location.
Where the wave is absolutely zero,
that is the place where you simply will not find electron at all.
That's the picture that comes forward.
Now how would you test this idea to see if it's actually describing reality how the world works.
For the first thing is you need some kind of mathematics equation
that allows you to understand how this wave evolved how it changes over time.
And that equation came to us from over Schrodinger.
Whether you understand the mathematics symbol or not, it doesn't matter.But it is good to see that there is
a bona fide rigorous mathematical equation behind all of the imagery that you will be see here tonight.
That is the equation that describes how this wave this probability wave evolves over time.
That's step one. So now you understand how things change in time.
But the test is  while you said yourself this
look.Yep. At a given moment in time,  I’ve got this probability profile for where a particle should be located.
where I tested it, I run experiments of searching for the particles over and over again in identically prepared situations.
And I count the number of the times that I found it in one of the location or another.
And if the theory is correct,
you should find the particles more often where the wave is big, and the less often where the wave is small.
Qubism. So we've heard this word a few times mentioned.
Ruger tell us what this approach is about.
OK. I brought a Quantum System to experiment on
So I just flip this Quantum System
Ok, what is the probability of "heads"?
100 % by the "many worlds" approach.
Sorry. I will play along. 50%.
Stick with a 100%
So, I have 0 %.
My point being the probability, you have a different one from me.
So this is actually about you, it is not about me.
So the probability actually is your belief or my belief about what you will see if I show it to you. I can show it to you, you see. He's right.
So, probability in this is used in ??? ??? probability theory
which actually is a model on full account probability which is used in widely
in Statistic Economics in some parts of physics
So, this is a respected and very useful and I believe is the only really fully consistent approach of probability.
Probability is a believe of what you will experience of what you will see.
Now Qubism says that, you do not need to modify the probability of Quantum Mechanics.
So, even Quantum Mechanics Probabilites are personal degrees of belief
So the probability ??? a measurement outcome is my expectation of what I will experience from the experiment
The point is the Probability is not a property of the coin in my hand, the Probability is actually my belief.
Now, Quantum States,
are equivalent to probabilites.
Quantum States determine probabilities of outcomes
Quantum States are fully mathematical equivalant to probability distributions.
So in that sense, Probabilities are an experiment on agent's expectations
Quantum States encode an experiment on Agent's expectations for experimental outcome.
Now, what this says about the measurement problem is it just dissolves.
Exactly, I experience an outcome and I update my own expectations for the future.
This forth approach is the dart is going and the idea is
if we are not looking
if we are not actually yet doing the observation there is nothing really we can say
until the observer looks, so if we bring our observer
and then you look, you take in the observation, you update your understanding of the world
based on your observation.
In science, what are we doing? We are making predictions and
we test this predictions in experiments
So this is really what Qubism talks about
Quantum Mechanics, this must be the reason that most physicists are not interested in the potential of Quantum Mechanics.
because Quantum Mechanics is a tool they can use for predictions, to design experiments
to build machines,  Quantum Mechanics has transformed the world in a way that is unprecedented
In all these experiments, experimental designs, predictions, including cosmological predictions
You never actually need to talk about hidden variables or spontaneous collapse
You just apply Quantum Mechanics as a tool to make predictions
And that works, and this must be one of the reasons physicists are not interested in the interpretation of Quantum Mechanics.
When you look at Cosmology, what do you base Cosmology on
You base it on what you observe or experience now
What are the Cosmological predictions or your conclusions you draw, well they are about your future experience
You will never stop at just saying "Oh well, I have explained all things I know because
So, even Cosmology is about taking your present and the few past experiences and
Using the Theory, using Quantum Mechanics as you know it
to make predictions about future experiences.
In your approach the spookiness evaporates if I understand it, can you describe to us
where it goes.
In the Qubist approach these correlations,, this situation is very much like ???'s socks.
He used to wear different color socks and when his colleague so him coming in, when he saw one foot, one sock, he knew immediately
what the color of the other sock would be, namely different.
Nobody would call that Spooky action at a distance
What Qubism says here is, if I have a belief about
this is a belief about what I will see experience when I am making the experiment
on that particle here.
What happens is, if I make a measurement on this particle her I see it spins down and then I update my expectation
on the other particle here.
No, it is always about me and the world because Qubism emphatically says that it is not all about the subject
nor is it all about the world. Any measurement is an action, subject and observer, physicist, system
take on the world.
So measurements are active processes they are actions
Any measurement is an action on the world and it needs subject and the object,  it needs... is where the experience and the world meet.
But that experience is something  that the individual
,the observer has,that's the point ?Yeah good !
So,what do you think ? David ?
