Seeing Double
By Greg Spearritt
I failed Physics at high school. Now, in my (relative) dotage, I’ve become entranced by it, or at least by the science of the very small.
Quantum mechanics is fascinating. In it I’ve found unexpected resonances with my humanities-oriented worldview and a challenge to what I once considered a pretty straightforward and compelling materialism.
The Very Small
Although scientists tell us atoms are not actually the smallest particles out there, the term derives from the Greek atomos, meaning uncuttable. Atoms in the modern sense, however, have only been a thing since Scottish botanist Robert Brown – a one-time visitor to Australia – came upon an unusual phenomenon.
In 1827 Brown noticed pollen grains jiggling about while suspended in water. He found this movement in other matter also, living or not, when the particle size was small enough. Some 78 years later ‘Brownian motion’ was explained by Einstein: the pollen grains were being bombarded unevenly on all sides by tiny particles called molecules.
Why ‘quantum’?
Harold Camping could be justly accused of jumping the gun. He predicted the end of the world for 1994 (twice), then 1995 and finally for 2011.
Right up there with Camping (among many others) in the league of those with egg-stained faces would be Lord Kelvin, who declared in 1900 that our understanding of physics was virtually complete.
In that same year, Max Planck was studying the energy emitted by hot objects. The vibrations making up the wavelengths of energy he observed were puzzling until he hit on the idea that the energy levels increased in steps rather than transitioning smoothly.
Energy, it turns out, is parcelled up in a set of discrete values – quanta – rather than being fluid and continuous. In 1905 Einstein claimed (accurately as usual) that this applied to light energy as well.
The quantum nature of energy was evidence something fishy was afoot in the world of classical physics.
Seeing double
In the early 1900s things became not just confusing, but downright counter-intuitive. Danish physicist Niels Bohr (of whom, more later) declared:
Those who are not shocked when they first come across quantum mechanics cannot possibly have understood it.
Small things, it turns out, simply don’t behave like small versions of big things.
In 1801 Thomas Young conducted a very famous study of the nature of light, the double slit experiment. He wanted to test whether Newton was correct in thinking light energy was particle-like.
Young projected light through two side-by-side vertical slits onto a screen behind. If light was composed of particles you’d expect to see two vertical lit-up strips on the screen where the particles hit it. Instead, he found an interference pattern of light and dark strips. (See Figures 1 & 2.) It was as if water ripples had come out of each slit, crossed over each other with wave crests amplified wherever they met another crest, dips similarly amplified and no result where a dip and a crest cancelled each other out.
Figure 1 Figure 2
Light, apparently, can behave as if it were a wave. Yet we also now know light energy is bundled in discrete entities we call photons.
The double slit experiment has been performed ever since in all kinds of creative ways, for example using electrons or atoms rather than light, with the same result. But electrons, as we know (or thought we knew) are also particles.
Here’s an idea. Why don’t we send just one particle at a time through the slits? In this instance there couldn’t be a wave (interference) pattern on the screen behind, as a single electron or photon would have nothing to ‘interfere’ with. The result? As the particles are fired one by one at the slits and hit the screen, over time an interference pattern emerges.
What could be going on? Does the particle somehow go through both slits at once and interfere with itself?
I know! Let’s check to see exactly which slit the electrons or photons go through, since we don’t actually believe they could be going through both at once.
If we do that, we don’t get an interference pattern: we just see the two slit shapes reflected on the screen behind. Somehow the act of measuring has altered the result and ‘collapsed’ the wave. 1.
(To note, the whole business is – of course – more complex than can be conveyed here. In the case of electrons, for instance, scientists speak of an electron wave, not a wave of electrons: it’s a probability wave telling us the likelihood of where we might find the electron. True brain-pain territory, at least for those of us who failed Physics…)
So the quantum objects seem to be in some indeterminate state: if they’re measured, they take only one of the paths, through one of the slits and act like particles; if not, they seem to take both paths and behave as waves.
I know! Let’s set up an apparatus to measure them just before they hit a detection device, after they’ve come through the slits and ‘decided’ which state to be in. There’s no way for them to ‘know’, when they come through the slits, that we’re going to check up on them. This is the ‘delayed choice’ double slit experiment.
Too clever by half, apparently. Even if their path is detected after they’ve committed themselves, the result is the same: interacting with them by measuring collapses their wave-like behaviour. They act like particles. It’s as if they went back in time and changed their initial decision.
Light coming from a distant star, if it were lensed through a galaxy along its path, could be coming from two distinct points as it hits us on earth. In theory we could perform the delayed-choice experiment and discover that the light ‘knew’ billions of years ago whether it would be seen here on Earth in particle or wave form.
Two natures
In the fifth century, Nestorian, Cyril and others slugged it out concerning the true nature of Christ, resulting finally in the declaration at the Council of Chalcedon that he was “perfect in divinity and perfect in humanity”. Truly God and human, they averred.
All poppycock, in my (entirely humble, atheistic) opinion. Too much fevered imagination at work on what is in fact a category error.
Through repeatable experimentation – rather than the machinations of powerful churchmen – quantum objects can be predicted with astonishing accuracy to manifest in either wave or particle form: two apparently incompatible states. Until an experiment is performed, however, the actual state of these objects is indeterminate. Some suggest they are in a ‘superposition’ of both states, though according to Niels Bohr’s ‘Copenhagen Interpretation’ this too is a category error.
In fact, electrons and photons are not waves or particles: those are just our clumsy ways of visualising them. We are tempted to imagine a quantum particle in a confined space – an electron, say, trapped in an atom – as an extremely tiny entity zipping about very fast, or alternatively as a smeared-out wave that is everywhere within that space. Both images are wrong. On Bohr’s view, it makes no sense to ask where, or what, the electron is until we measure it. It’s like asking what’s north of the North Pole. In a sense, the electron doesn’t exist until we measure it.
Einstein couldn’t believe reality was like this. A colleague, Abraham Pais, reported:
I recall that during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it. 2.
There are many interpretations of the experimental facts, including the Copenhagen Interpretation (associated with Bohr), the Many Worlds Interpretation, the Ensemble Interpretation, the Pilot Wave theory and more. Thus far no experimental means of deciding between them has been devised. 3.
For some, it’s enough to say quantum objects are indeed real, but that they simply have indefinite properties. 4.
Weirder still
In just over a century we’ve learned a great deal more about quantum states that challenges common sense.
‘Entanglement’ is where two particles share a property. If you fire a quantum particle away from another entangled particle and change the state of the first, the second will also change instantly, even though there is no way the two could communicate. They are somehow fundamentally entangled. It makes as much sense as saying two balls don’t have their own colour, but rather share the property of colour. A connectivity exists where none should be possible.5.
Heisenberg (through his ‘uncertainty principle’) determined that we cannot know both the location and the momentum of quantum particles. That’s like saying we know a car is going 110 kph but haven’t a clue where it is. Or that it’s passing through Sydney but can’t tell if it’s going 10 kph or 110.
Further, there is true randomness at the base of reality. The results of tossing a coin seem random, but if we knew the exact state of each coin and all the relevant forces we could successfully predict the results. But when an atom in radioactive material decays, there is no way to explain why it did so, or to predict when it was going to do so, even if our knowledge of the physical state of the material was perfect.
Uncertainty at the quantum level is not just a result of human ignorance: it’s inherent, as a basic feature of nature. This is one of the few times Einstein got it wrong: God does play dice with the universe!
What does it all mean?
The core idea that Einstein, among many others, struggled with is that there doesn’t seem to be, at the base of the physical world, a definite reality. The nature of reality at the quantum scale seems to be that it is not observer-independent. It’s as if there is no reality until we interact with it. We appear somehow to construct the world in the process of looking at it.
This presents a challenge to my own materialistic outlook. I interpret mind, spirit etc. as a fortuitous outworking of matter. I did always assume, however, that there was some solid, independent, fundamental ‘thing that is there’, no matter how tiny, at the base of matter. It seems we can’t get hold of that reality without changing it or even determining it.
What I do find comforting is that this repeatable, reliable (if baffling) science meshes well with my view of language. Don Cupitt taught me that there can be no God’s-eye-view: our world and reality itself can be apprehended only through humanly-created language. Like a one-language dictionary, this world is outsideless. Every word or idea refers to some other entry within the dictionary.
Our human signs (language in its broadest sense), like the intervention of the scientist, help determine the world we encounter.
The famous physicist Niels Bohr won the Nobel Prize for Physics in 1922 for his contributions which were essential to modern understandings of atomic structure and quantum mechanics. Bohr made no bones about the implications of what he and others were discovering, as these quotes (by, or attributed to, Bohr) reveal: 6.
Everything we call real is made of things that cannot be regarded as real.
We are suspended in language in such a way that we cannot say what is up and what is down. The word ‘reality’ is also a word, a word which we must learn to use correctly.
We must be clear that when it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images and establishing mental connections.
[F]rom our present stand-point, physics is to be regarded not so much as the study of something a priori given, but as the development of methods for ordering and surveying human experience.
And, as a nod to SOFiA’s aim to explore openly in a non-dogmatic fashion:
Every sentence I utter must be understood not as an affirmation but as a question.
And so…
It may be worth concluding with a couple of quotes about Bohr to console those who, like me, are entranced by quantum physics but are mightily taxed by the effort to get one’s head around it.
Many a time, a visiting young physicist (most physicists visiting Copenhagen were young) would deliver a brilliant talk about his recent calculations on some intricate problem of the quantum theory. Everybody in the audience would understand the argument quite clearly, but Bohr wouldn’t. So everybody would start to explain to Bohr the simple point he had missed, and in the resulting turmoil everybody would stop understanding anything. Finally, after a considerable period of time, Bohr would begin to understand, and it would turn out that what he understood about the problem presented by the visitor was quite different from what the visitor meant, and was correct, while the visitor’s interpretation was wrong. 7.
I have been getting sporadic flashes of feeling that I may actually be starting to understand what Bohr was talking about. Sometimes the sensation persists for many minutes. It’s a little like a religious experience and what really worries me is that if I am on the right track, then one of these days, perhaps quite soon, the whole business will suddenly become obvious to me, and from then on I will know that Bohr was right but be unable to explain why to anybody else. 8.
Note
A very readable general introduction to this subject is Phillip Ball’s 2018 book Beyond Weird: Why Everything You Thought You Knew about Quantum Physics Is Different.
References
- See video at https://phys.org/news/2019-11-quantum-physics-reality-doesnt.html
- Cited in David Mermin (1985) ‘Is the moon there when nobody looks? Reality and the quantum theory’ https://physicstoday.scitation.org/doi/pdf/10.1063/1.880968
- See https://www.forbes.com/sites/startswithabang/2019/09/11/the-most-unpopular-interpretation-of-quantum-physics-may-make-all-the-others-irrelevant/
- See Brian Koberlein ‘Real and Unreal’, https://archive.briankoberlein.com/2015/06/04/real-and-unreal/index.html
- A word of caution on this, from Brian Cox (The Quantum Universe: Everything that can happen does happen): “Saying that every atom in the Universe is connected to every other atom might seem like an orifice through which all sorts of holistic drivel can seep.” Indeed, a great deal of new-age drivel has arisen from misunderstanding and misapplying quantum science concepts.
- http://www.greatthoughtstreasury.com/author/niels-bohr-fully-neils-henrik-david-bohr
- George Gamow on Niels Bohr in The Great Physicists from Galileo to Einstein (1961)
- Quoted in Philip Ball (2018) Beyond Weird: Why Everything You Thought You Knew about Quantum Physics Is Different
Disclaimer: views represented in SOFiA articles are entirely the view of the respective authors and in no way represent an official SOFiA position. They are, however, intended to stimulate thought, rather than present a final word on any topic.
Photo from Wikipedia Commons
Figures 1 & 2 are from snappygoat.com
