Guest post By Robin Bisson
With Halloween and Bonfire Night out of the way, mince pies comfortably established on the supermarket shelves and town light displays ready to be switched on by a passing celebrity, Christmas is on the horizon. And so too is Science Brainwaves’ Christmas Lecture, a free event being held on the 17th on December at the University of Sheffield, which promises to be a real cracker *groan*. The lecture, titled ‘Weird Physics’, is being given by Dr Paul Stevenson of the University of Surrey.
That physics is weird might seem to be a given, after all physics fans have a reputation for getting all excited over obscure things that happen millions of light years away, having some outlandish tastes in music, and developing a tendency in later life to wear jackets with leather elbow patches. However, we’re not talking about any normal kind of weirdness here; we’re talking about the seriously bizarre world of quantum mechanics, a world in which even the most basic facts about the world get turned upside down. Now, if all that the word ‘quantum’ makes you think of is daytime repeats of Quantum Leap, you’re probably not alone. So to get swotted up before the lecture, and to give you a taste of some of the strangeness to come, here’s an introduction to the weird world of quantum mechanics…
Perhaps the most well known illustration of why the quantum world is at odds with the rest of the world is that of Erwin Schrödinger’s famous cat. Schrödinger proposed an experiment where a cat is placed in an opaque box, in which there would also be a phial containing poisonous gas, one radioactive atom and a mechanism to smash the phial if the atom decayed. If the atom were to decay then the cat would die, but if the atom does not decay then the cat survives. The weirdness comes in because quantum mechanics tells us that until we look at the atom, it is in the state of being decayed and non-decayed at the same time, and the consequence for our unhappy cat is that it is both alive and dead at the same time. “Ridiculous!” you might well shout, which is exactly what Schrödinger was pointing out, all be it in a slightly morbid way – the quantum world does not appear to fit in with the world of big things that we know, in which cats most definitely don’t wander about in limbo between life and death.
To make it clearer what exactly we’re dealing with here, a little explanation is needed. The quantum world is that of atoms and subatomic particles: the familiar electrons, protons and neutrons that we all learned about at school, as well as other more exotic particles. One of the things that quantum mechanics says about these little bits of matter, is that once you have observed them being in a particular place at a particular time, you cannot say for sure exactly where they will be at any point in the future. Instead you can only give a probability of them being in a particular place until you have another look and make sure. So far so normal. After all, we can’t expect to know everything.
What quantum mechanics sneaks in and confuses us with, however, is the assertion that these particles aren’t actually anywhere until we look to check – while our backs are turned they are in one place, and another place, and even another, and another and another place all at the same time, but when we look at them BANG, they are somewhere definite again. It’s a bit like playing “What’s The Time Mr. Wolf?” with subatomic particles, except that instead of you not knowing where your fellow players are, they don’t know where they are until you turn around to look (and also, subatomic particles don’t run away screaming when you growl at them).
If this all seems a bit stupid and it’s obvious that physicists just haven’t understood some basic stuff, like that things can’t be in two places at once, there are some awkward experimental results that show the quantum world simply can’t be like the world we all know and love. For instance, quantum particles get ‘entangled’ with each other, so that if you do something to one of the particles the other particle notices. It doesn’t matter if the other particle is on the other side of the room, the other side of the world or the other side of the universe, it will ‘respond’ when its partner has something done to it. Weirder still, these particles may not even only be particles – quantum mechanics regards them as having some properties of waves, and some properties of particles, something that Richard Feynman called “the only mystery in physics”.
Don’t worry if your brain is beginning to throb alarmingly, the physicists are confused too. Since quantum mechanics was first formulated there has been a raging debate about how it should be interpreted. One interpretation that has steadily gained support is that quantum mechanics only makes sense if we live in just one of many co-existing parallel universes. Every time we look in the box to see the mortal state of Schrödinger’s cat we set off down one leg of the trousers of time, let’s assume the one where the cat is alive, while in the universe of the other leg, the cat has met a sticky end.
While it may be mind-bending to try and understand what quantum mechanics implies about the universe and to imagine what the quantum world looks like up close, the more we understand about it, the more we can manipulate it to do some pretty nutty things. We all know that as long as watertight bowls are kept steady, any liquid inside them is going to stay there, right? Not if it’s a superfluid – liquids predicted by quantum mechanics that creep slowly out of any container holding them (click the link for a video). Even more amazingly, there is some early evidence that supersolids can exist – solids that literally move through each other like ghosts, without being affected. In any case, I think we can all agree that a world with quantum mechanics is a much weirder world indeed than a world without. To find out more, come to the Christmas Lecture, but don’t say you weren’t warned if you lose grip on reality…
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