What does Quantum Mechanics actually say about the nature of reality?

Credit: Alamy.

Graham writes …
 
I mused long and hard about what to write about this month, and then I saw an interesting article in Nature on this, a very intriguing topic. The article, by a senior reporter for Nature Elizabeth Gibney (1), tries to analyse the results of a recent survey of quantum mechanics (QM) practitioners and theorists on whether QM tells us anything about the nature of the subatomic world that it is used to investigate.
 
At a recent event to commemorate the 100th anniversary of the theory, eminent specialists in the field came together to argue about the issues. To gain an insight into how the wider community interprets quantum physics in its centenary year, Nature carried out a survey on the subject. They emailed more than 15,000 researchers whose recent papers involved quantum mechanics, and also invited attendees of the Centenary Meeting, held on the German island of Heligoland, to take the survey. The 1100 or so responses they received showed how widely researchers vary in their understanding of the most fundamental features of quantum theory and experiments.

Credit: CRC Press.

My own association with QM began in 1971 – only 46 years after its inception – as an undergraduate student. The Institution in which I studied mathematical physics had a research focus on QM topics, with the consequence that we undergrads got rather a lot of QM-related teaching in our course. My scientifically immature attitude to QM at the time was that it seemed to be a very successful theory in terms of predicting the outcome of experiments, but intuitively it made no sense. I guess everything else that I had encountered in my studies up to that point stemmed from classical physics, which did make sense of the underlying reality of the classical world – of which people are of course a part. This came as a bit of a shock, and the issue arose about how to cope with QM to achieve a pass in my final exams! A typical pragmatic approach for an undergraduate student …? I decided that I would simply use quantum theory without engaging with what it means — the ‘shut up and calculate’ approach (or more formally, an epistemic approach). As a consequence, I ultimately developed a dislike of QM, tending to believe that there was an underlying reality in the quantum world that the existing theory was not able to reveal. A result of all this was that when I began my in PhD studies in 1972 I had decided that I would not engage with QM – instead I embarked on an enjoyable three years of research on the topic of Einstein’s theory of gravity (his general theory of relativity) which is inherently a classical theory.

Credit: izquotes.com.

It's interesting to note that Einstein had a similar attitude to QM that I had unknowingly adopted as an undergraduate (it is also fair to say that we had very different motivations!). Despite the fact that he was one of the originators of QM, Einstein became troubled by what he perceived as an incomplete picture of reality that QM presented. All of his criticisms of the theory throughout his life stemmed from the notion that he believed that there was an underlying reality that was sciences’ job to uncover.
 
However, despite Einstein’s misgivings, it is undeniable that the mathematics of QM work beautifully, as witnessed by its successful application in the development of many recent technologies, such as nuclear engineering, medical imaging, computer chip manufacture and, indeed, the relatively new science of quantum computing. It has also provided the most accurate predictions of the outcome of experiments of any physical theory (see for example the discussion of the ‘muon g-2 experiment’ in the blog post of March 2022 – to see this, click the date on the archive list on the right hand side of the screen). So, should it just be regarded as an epistemic theory which tells us little about the nature of reality? This is one of the many questions posed to experts in the recent survey.

Schrödinger's wave equation is the pillar of contemporary quantum mechanics.

But before we get into that, let’s take a brief look at how QM theory works. The most common approach is the so-called Copenhagen Interpretation which could be regarded as the standard “textbook” view. This was developed by Niels Bohr and Werner Heisenberg in the 1920s, and is named after the university at which they did their seminal work. Other eminent physicists also played a major role in this endeavour, in particular the German physicist Erwin Schrödinger, who developed his wave equation which is central to QM theory. An object’s behaviour is characterized by its wavefunction, which is a mathematical expression calculated using Schrödinger’s equation. The wavefunction describes a quantum state (the particles’s position or spin, for example) and how it evolves as a cloud of probabilities. As long as it remains unobserved, a particle seems to spread out like a wave, interfering with itself and other particles. According to this interpretation, a quantum particle exists in a fuzzy state of many possibilities until a measurement is made. Only when you look – through an experiment or observation – does the wavefunction ‘collapse’ into a definite outcome. However, the issue of what counts as a ‘measurement’, and why the act of observation should change reality has long been discussed by physicists. In the survey, the Copenhagen Interpretation was the most popular preference, comprising 18% of experts who were confident or fairly confident in making their choice.

The pioneers of QM: Niels Bohr, Erwin Schrödinger and Werner Heisenberg. Credit: Public Domain.

Another approach is the Many-Worlds Interpretation, introduced by American physicist Hugh Everett III in 1957, and this got rid of wavefunction collapse issue altogether. Instead, every time a quantum choice is made, the universe splits. In one world, the particle is here and in another it is there. Both outcomes are real, but we only experience one branch of the ever-multiplying multiverse. I don’t know what you might think of this, but I have always considered it to be totally crazy – but nevertheless it was favoured by 8% (confident or fairly confident) of the survey respondents.
 
To give an impression of the diversity of opinions about QM theory among the practitioners and theorists, 9 interpretation options were offered in the survey (I have only discussed three of them for the sake of brevity), and in some instances equal numbers of respondents took diametrically opposing views, showing how widely researchers vary in their understanding of the most fundamental features of quantum mechanics. Interestingly 10% of respondents agreed with me and opted for the epistemic (information-based) approach.
 
I think if you asked many of the physicists attending the Centenary Meeting if QM was wrong, most of them would say something like ‘it’s incomplete’. From what we have said, I think this is reasonable as there is certainly something of value in the theory. However, some scientists are rather more outspoken. In this latter group I would include Roger Penrose, an eminent theoretical physicist and professor Emeritus at Oxford University, and Lee Smolin, an American physicist with associations with Yale and Pennsylvania State Universities and co-founder of the innovative Perimeter Institute of Theoretical Physics at Waterloo, Canada. In their writings (2), (3) (4), they have both been unequivocal in their opinions that the current theory is simply wrong. But then, at the end of the day, why is this important?

Einstein's theory of gravity is very different from Newton's vision. In Einstein's theory mass and energy tell space and time how to curve and the curvature of space and time tell mass and energy how to move. Credit: Veritasnewspaper.org.

The key to answering this question is the fact that there are currently two main pillars of modern physics – quantum mechanics (the theory of the very small) and Einstein’s theory of gravity (general relativity – the theory of the very large). Both of these theories were launched during an amazingly productive decade of the twentieth century from 1915 to 1925. Both theories have stood the test of time remarkably well. However, all attempts to unify them into a ‘theory of everything’ – a theory of quantum gravity have so far failed. So, when we look at problems where the domains of the two theories overlap – such as at the initial instant of the Big Bang, or at the centre of a black hole where gravity and quantum effects are both very relevant – we do not have a theory to describe what is happening. And this is not just a recent problem. The physics community have been struggling with this for a century – and efforts continue. But what if Penrose and Smolin (and others I’ve not mentioned) are right in their belief that QM is wrong. Then our efforts at unification are doomed.
 
So at the end of the day we have a quantum theory that doesn’t say very much that the experts can agree upon about the underlying reality of the world of molecules, atoms and elementary particles. And that the current version of QM may be an inappropriate starting point for the process of unification.
 
Recently I heard, or read, a quote from someone – I can’t remember who – ‘Maybe we should give up on the process of trying to quantise gravity, and try gravitising quantum mechanics instead’. I’m actually not sure what they meant by ‘gravitising’, but I understand and appreciate the sentiment.
 
Graham Swinerd
 
Southampton, UK
August 2025
 
(1) Elizabeth Gibney, Nature, Vol. 643, pp. 1175-1179, 31 July 2025.
(2)* Roger Penrose, Fashion, Faith and Fantasy in the new physics of the Universe, Princeton University Press, 2016.
(3) Lee Smolin, The Trouble with Physics, Penguin Books, 2006.
(4) Lee Smolin, Einstein’s Unfinished Revolution: the search for what lies beyond the quantum, Penguin Books, 2019.
 
* Warning: The publisher’s blurb about this book suggests that the content is suitable for the layperson. It is not.