Alan's Guide to Practical Quantum Theory for the uninitiated
There was no Winding Down on 1 April 2018 because it was a holiday. However, while cleaning the office we did find a coffee stained paper serviette scrawled on in green crayon. So we thought we would help readers to get over withdrawal symptoms by sending it out while the editor and the journalist droids are on holiday.
Signed: Rosie The Chief Cleaning Droid.
Today, I want to talk about quantum theory. As you probably know, scientists believe that only very tiny things exhibit quantum behaviour. Don't believe a word of it. Scientists are really unobservant (with the possible exception of Erwin Rudolf Josef Alexander Schroedinger), except when it comes to doing experiments. Let me tell you, there are plenty of macro-sized quantum objects in the world around us - they just don't happen in ivory towers.
Before I give some examples we need to look at something that quantum droids* call the object's 'Wave Function'. Written down, it is a maths formula full of Greek letters (hence the saying, "It's all Greek to me."). Quantum scientists use a lot of Greek letters to make sure we realise how clever they are... Anyway, this wave function basically tells you how likely the quantum object is to be found in a given place if you look for the object. Some places are more likely than others. What it comes down to is that the object is in multiple places, until you look at it, when it stops being everywhere and decides to be somewhere instead - usually where you are looking.
Scientists don't say that's what happens. If they explained it in ordinary language anyone could understand it, and we wouldn't think that scientists were clever. What they say is that "the wave function collapses", which sounds much more complicated! OK - now we know, let's have a look at some examples with common objects.
We can start with something simple. How about a cardboard packet of medical pills? You know the sort, the medicine name is printed on the side, and you can open it at either end. Right! Have you noticed that whichever end you open it at the patient information leaflet is always at that end, making it difficult to get at the medicine? The reason is simple, patient information leaflets are quantum objects. Inside the box the leaflet cannot be seen, so it has a wave function which smears it out onto both ends of the box. When you open the box you see the leaflet at the end you open, because the wave collapses to present the leaflet at the end you opened and are viewing!
Still not convinced? OK, let’s take a somewhat larger object - a bus. Yes - even something as large as a double decker bus can be 'quantum'. Have you ever stood in the rain or snow in a city waiting for the bus, only to see masses of buses going in the opposite direction, but none going in the direction you want to go? As a child I thought that this must be because there was a large hole in the road at the other end, into which they were all falling! But now I'm grown up (though some would claim otherwise) I know better. If fact, this is because quantum buses are going in both directions at once. That is to say the wave function of the bus gives the probability of it going in each direction. When someone waiting for a bus sees the bus, its wave function collapses to make it travel in the opposite direction to the one in which the viewer wants to go.
But if that's the case, I hear you ask, how is it that a bus will eventually come the way you want to go? Ah ha! That's very easy. First we note that when there are lots of buses going the other way, there is, of course, no one waiting at the bus stop for going the other way. Once there is someone going the other way and waiting for the bus, then, if they see it before you do, the wave function collapses so the bus is going your way.
There is one exception to this - school buses. As my younger readers will be well aware, the wave function for school buses in the morning -always- collapses in the direction of going to school, so there is no excuse for being late or skiving off!
And then of course there is the matter of cats. Have you ever noticed that cats can follow you but still manage to be in front of you, so you can trip over them? That's because cats are quantum creatures. Their wave function smears their probable position out so that they are potentially both ahead and also behind you. When you look down, they are in front, when you look away the wave function re-asserts itself and they go back to being quantum. Note that the wave function re-asserting itself is very unusual. As far as we know only cats can do this.
Ernie Schroedinger was the first to realise that cats are quantum creatures and which is why he used a cat in his famous experiment. You can read all about it in the Wikipedia (URL at the end), although you should be aware that there is school of thought that believes that it was Schroedinger in box, and the cat was doing the experiment...
Probably the most famous person to realise that cats are quantum was the poet TS Eliot in his collection of poems, 'Old Possum's Book of Practical Cats'. Let me give you an extract from his poem 'Macavity the Mystery Cat':
Macavity's a Mystery Cat: he's called the Hidden Paw -
For he's the master criminal who can defy the Law.
He's the bafflement of Scotland Yard, the Flying Squad's despair
For when they reach the scene of crime - Macavity's not there!
That is a classic description of quantum behaviour!
There's one other thing we need to talk about before we finish this discussion on quantumology. It's what Einstein called "Spooky Action at a Distance". The idea is that if two quantum objects are closely linked (also called entangled) and you measure one of their properties, no matter how close or how far away they are, then the same property in the other object will be the opposite. The instant you change a property in one object it's entangled sibling will also change. For instance, if the property on one of them is up, on the other it will be down. Scientists have done this with things like electrons to prove this is what happens. Einstein was definitely unhappy about this, because it meant that information was being transmitted faster than light. Science fiction writers, on the other hand, were very happy, claiming complete vindication of their space travel master works!
Of course, they needn't have used anything as fiddly to work with as electrons, there are plenty of examples of entanglement all around us, but you don't get oodles of government money for using everyday objects. There is an obvious, and ubiquitous, pair of everyday items that are always quantum entangled - USB connectors. Every USB stick is entangled with the connector you are trying to insert it into. As soon as you decide which way to insert it, the connector becomes, instantly, the wrong way round for the insertion!
Incidentally, in keeping with their quantum reputation, cats also practice quantum entanglement. Have you ever noticed that whenever you open a closed door in a house with a cat, the cat is -always- sitting neatly outside it waiting to walk in, tail in the air? It's because cats are entangled with closed doors in the houses. Even if you live in a five story 80 room sprawling mansion it happens. You open the door at one end knowing for sure the cat is asleep on a bed at the other the end of the mansion. Despite that, it will be instantly teleported to sit neatly outside the opening door.
Now if that's not "Spooky Action at a Distance", I don't know what is!
https://en.wikipedia.org/wiki/Erwin_Schr%C3%B6dinger
https://en.wikipedia.org/wiki/Schr%C3%B6dinger%27s_cat#/media/File:Schroedinger_cat.jpg
https://www.technologyreview.com/s/427174/einsteins-spooky-action-at-a-distance-paradox-older-than-thought/
http://monologues.co.uk/Childrens_Favourites/Macavity.htm
Happy April 1st everyone.
Alan Lenton
* My spelling checker suggested 'druids'. I was tempted, very tempted....