This year marks the centenary of quantum mechanics. The Post asked local
science writer and mathematician Robyn Arianrhod to “please explain”
a field of study that perplexed even Einstein.
science writer and mathematician Robyn Arianrhod to “please explain”
a field of study that perplexed even Einstein.
Shooting the breeze: Niels Bohr and Albert Einstein had long debates about quantum mechanics. Photo: Paul Ehrenfest, December 11, 1925 Public domain, via Wikimedia Commons Post: A very personal question to start: do you actually understand quantum mechanics?
Ha! I’ll let the experts answer this one. Richard Feynman told his students at Caltech that the subatomic quantum realm is so different from the world we see around us that “Even the experts do not understand it the way they would like to.” And just recently, a survey by Nature showed that physicists are still divided on what quantum mechanics really means.
If this is the centenary of quantum mechanics, what happened in 1925?
The first inklings that the subatomic world plays by different rules from the everyday world of ordinary physics came in 1900 and 1905, with the work of Max Planck and Albert Einstein. In our everyday experience, things such as temperature increase continuously, but Planck and Einstein discovered that in certain circumstances, heat and light behave like a series of discrete bundles or “particles” – i.e. quanta – of energy.
Ha! I’ll let the experts answer this one. Richard Feynman told his students at Caltech that the subatomic quantum realm is so different from the world we see around us that “Even the experts do not understand it the way they would like to.” And just recently, a survey by Nature showed that physicists are still divided on what quantum mechanics really means.
If this is the centenary of quantum mechanics, what happened in 1925?
The first inklings that the subatomic world plays by different rules from the everyday world of ordinary physics came in 1900 and 1905, with the work of Max Planck and Albert Einstein. In our everyday experience, things such as temperature increase continuously, but Planck and Einstein discovered that in certain circumstances, heat and light behave like a series of discrete bundles or “particles” – i.e. quanta – of energy.
Then, in 1911, Ernest Rutherford discovered that atoms are made up of a nucleus of neutrons and protons, which is surrounded by electrons. Various theories were propounded about this newly revealed atomic structure, but the year 1925 is when quantum mechanics (QM) was born.
QM gives the mathematical equations that show how electrons and other subatomic particles behave – analogously to Newton’s laws of motion, which describe the way ordinary objects move under various forces.
The stuff about particles being in two places at once and the thing with the cat is pretty weird. Does quantum mechanics belong in the realm of physics or metaphysics, or something else?
It’s pretty weird indeed – Einstein and Erwin Schrödinger certainly thought so! Schrödinger made up his cat problem to highlight this weirdness. The cat being both dead and alive, until you open the box and take a look, illustrated the fact that solutions of the equations of QM are probabilistic – they give the likelihood of something happening at a given time and place. This means that until you actually make an observation, the equations just say there is a certain probability that the cat is dead and another probability that it is alive. It is a bit like saying that until you toss a coin, all you can say is that there is a 50% chance that it will land heads up, and a 50% chance that it will be tails.
In ordinary (“classical”) physics, by contrast, if you have the right initial information, the equations let you predict in advance where individual objects – cricket balls, spaceships, planets, and so on – will be at any given time. (That’s why we have such things as GPS and communications satellites, to take just two examples.) Such physics is “deterministic” – you can pre-determine what is going to happen – but quantum behaviour is intrinsically probabilistic.
Yet these quantum mechanical equations do work – they do give accurate information about how electrons and photons behave in general, as repeated experiments have shown.
So, this is definitely the realm of physics. Trying to understand what’s going on, however, can take you into the realm of metaphysics.
Einstein famously hated quantum mechanics – what was his complaint?
Einstein was a quantum pioneer, because he showed that light seems to behave not just as a wave but also as a particle (the photon). Because quantum mechanics gives probabilities rather than certainties, however, Einstein felt that “the Old One” – God, or Nature – would have to be playing dice with the world if QM were the last word. This didn’t make sense to him, so he kept trying to find loopholes.
In 1935, he played his trump card: along with two colleagues (Podolsky and Rosen), he discovered a truly weird theoretical consequence of QM: the concept now known as “entanglement.”
Imagine that two quantum particles are prepared together so that their properties are correlated in some sense – analogously to having two jellybeans, each in its own sealed box, correlated in terms of their colour so that one is red and the other is green. Now, without looking inside the boxes, transport the second box to the other side of the world. If you then look inside the first box and find a green jellybean, then you automatically know that the other jellybean is red. But with quantum jellybeans, all you know beforehand is that the particle inside each box has a 50% chance of being green and a 50% chance of being red. The jellybeans will randomly manifest one or other of these colours when the boxes are opened, and because of the 50-50 probability, it is quite possible that they could both turn out to be green, or both red. But in this analogy, the original correlation required the jellybeans to have different colours, and here’s where it gets “spooky,” as Einstein put: this initial correlation will remain, and even though the particles are separated by thousands of kilometres, if the first jellybean’s randomly manifested colour is observed to be green, the second jellybean instantly “knows” it will have to manifest as red.
This seems to mean that the second jellybean’s colour is determined by the first observer – it is not pre-existing, fixed at the outset, the way the ordinary jellybeans’ colours are. This seemed too bizarre a result for a complete theory of quantum behaviour, so Einstein assumed there must be more information about the second particle’s quantum properties that QM had not yet included. (Schrödinger agreed, and so he created his cat example the same year.)
Decades later, it turned out that entanglement really does happen as QM suggests, and that QM is not incomplete in the way Einstein had hoped.
Do physicists still argue about quantum mechanics or is it all settled?
The basic theory has been applied for a century, and so far, none of its predictions has been wrong. But physicists are still developing new ways to apply it – such as quantum computing – and they’re still debating what it all means. For instance, when that analogical “quantum jellybean” randomly manifests a colour (or more realistically, when an electron manifests a spin direction or a photon manifests a polarization direction), the probability wave describing it is said to have “collapsed” during the observation process. This is a way of trying to explain why you do get a definite observation or measurement, even though you can’t predict exactly what will happen beforehand. But there are competing explanations, such as the “many worlds” idea, in which all possibilities are manifested in different universes (each inaccessible to the other), and some quasi-deterministic “hidden variables” theories reminiscent of Einstein’s attempt to “complete” QM.
Is there anything left to discover about quantum mechanics?
The big one is a theory of quantum gravity – how to blend Einstein’s general theory of relativity with quantum theory. No one has cracked it so far.
What’s quantum mechanics ever done for you and me?
Love the Monty Python riff! Well, thanks to QM we have the essential components for computers, mobile phones, TV, LED lights, and all the other microelectronic tech we use every day. QM also gave us lasers, used in bar codes, high-speed fibre optical communications, and surgery, for example. And QM is used in other medical devices, such MRI machines.
Does knowing this quantum stuff change the way you see the world?
I think going deeper into modern science has changed the way I see the world, in the sense that it is awe-inspiring in a way that goes way beyond our everyday imaginations. Thinking of how the universe evolved, and all the chance events and mutations along the way, makes me feel so lucky to be here!
Robyn Arianrhod is a science writer, and a mathematician and historian affiliated with Monash University’s School of Mathematics. Her books include "Einstein’s Heroes: Imagining the world through the language of mathematics". Her latest book, "Vector", has just been shortlisted for the NSW History Awards. She lives in Wonthaggi.
QM gives the mathematical equations that show how electrons and other subatomic particles behave – analogously to Newton’s laws of motion, which describe the way ordinary objects move under various forces.
The stuff about particles being in two places at once and the thing with the cat is pretty weird. Does quantum mechanics belong in the realm of physics or metaphysics, or something else?
It’s pretty weird indeed – Einstein and Erwin Schrödinger certainly thought so! Schrödinger made up his cat problem to highlight this weirdness. The cat being both dead and alive, until you open the box and take a look, illustrated the fact that solutions of the equations of QM are probabilistic – they give the likelihood of something happening at a given time and place. This means that until you actually make an observation, the equations just say there is a certain probability that the cat is dead and another probability that it is alive. It is a bit like saying that until you toss a coin, all you can say is that there is a 50% chance that it will land heads up, and a 50% chance that it will be tails.
In ordinary (“classical”) physics, by contrast, if you have the right initial information, the equations let you predict in advance where individual objects – cricket balls, spaceships, planets, and so on – will be at any given time. (That’s why we have such things as GPS and communications satellites, to take just two examples.) Such physics is “deterministic” – you can pre-determine what is going to happen – but quantum behaviour is intrinsically probabilistic.
Yet these quantum mechanical equations do work – they do give accurate information about how electrons and photons behave in general, as repeated experiments have shown.
So, this is definitely the realm of physics. Trying to understand what’s going on, however, can take you into the realm of metaphysics.
Einstein famously hated quantum mechanics – what was his complaint?
Einstein was a quantum pioneer, because he showed that light seems to behave not just as a wave but also as a particle (the photon). Because quantum mechanics gives probabilities rather than certainties, however, Einstein felt that “the Old One” – God, or Nature – would have to be playing dice with the world if QM were the last word. This didn’t make sense to him, so he kept trying to find loopholes.
In 1935, he played his trump card: along with two colleagues (Podolsky and Rosen), he discovered a truly weird theoretical consequence of QM: the concept now known as “entanglement.”
Imagine that two quantum particles are prepared together so that their properties are correlated in some sense – analogously to having two jellybeans, each in its own sealed box, correlated in terms of their colour so that one is red and the other is green. Now, without looking inside the boxes, transport the second box to the other side of the world. If you then look inside the first box and find a green jellybean, then you automatically know that the other jellybean is red. But with quantum jellybeans, all you know beforehand is that the particle inside each box has a 50% chance of being green and a 50% chance of being red. The jellybeans will randomly manifest one or other of these colours when the boxes are opened, and because of the 50-50 probability, it is quite possible that they could both turn out to be green, or both red. But in this analogy, the original correlation required the jellybeans to have different colours, and here’s where it gets “spooky,” as Einstein put: this initial correlation will remain, and even though the particles are separated by thousands of kilometres, if the first jellybean’s randomly manifested colour is observed to be green, the second jellybean instantly “knows” it will have to manifest as red.
This seems to mean that the second jellybean’s colour is determined by the first observer – it is not pre-existing, fixed at the outset, the way the ordinary jellybeans’ colours are. This seemed too bizarre a result for a complete theory of quantum behaviour, so Einstein assumed there must be more information about the second particle’s quantum properties that QM had not yet included. (Schrödinger agreed, and so he created his cat example the same year.)
Decades later, it turned out that entanglement really does happen as QM suggests, and that QM is not incomplete in the way Einstein had hoped.
Do physicists still argue about quantum mechanics or is it all settled?
The basic theory has been applied for a century, and so far, none of its predictions has been wrong. But physicists are still developing new ways to apply it – such as quantum computing – and they’re still debating what it all means. For instance, when that analogical “quantum jellybean” randomly manifests a colour (or more realistically, when an electron manifests a spin direction or a photon manifests a polarization direction), the probability wave describing it is said to have “collapsed” during the observation process. This is a way of trying to explain why you do get a definite observation or measurement, even though you can’t predict exactly what will happen beforehand. But there are competing explanations, such as the “many worlds” idea, in which all possibilities are manifested in different universes (each inaccessible to the other), and some quasi-deterministic “hidden variables” theories reminiscent of Einstein’s attempt to “complete” QM.
Is there anything left to discover about quantum mechanics?
The big one is a theory of quantum gravity – how to blend Einstein’s general theory of relativity with quantum theory. No one has cracked it so far.
What’s quantum mechanics ever done for you and me?
Love the Monty Python riff! Well, thanks to QM we have the essential components for computers, mobile phones, TV, LED lights, and all the other microelectronic tech we use every day. QM also gave us lasers, used in bar codes, high-speed fibre optical communications, and surgery, for example. And QM is used in other medical devices, such MRI machines.
Does knowing this quantum stuff change the way you see the world?
I think going deeper into modern science has changed the way I see the world, in the sense that it is awe-inspiring in a way that goes way beyond our everyday imaginations. Thinking of how the universe evolved, and all the chance events and mutations along the way, makes me feel so lucky to be here!
Robyn Arianrhod is a science writer, and a mathematician and historian affiliated with Monash University’s School of Mathematics. Her books include "Einstein’s Heroes: Imagining the world through the language of mathematics". Her latest book, "Vector", has just been shortlisted for the NSW History Awards. She lives in Wonthaggi.