developmental trajectory upon adoption, children adopted beyond the age of four were locked into a mode of behaviour reminiscent of that of a child on the autistic spectrum. To understand why this comes about, the process of normal brain development needs to be understood. Interestingly, recent research on empathy reveals how activities and interactions in play provide an insight into who children become [311].

The impact of learning, and education, experience and motivation and the associated physical changes in the brain are beyond the scope of this piece. Suffice it to say that we are born into our respective environments with a low level of consciousness, but with a brain that has a massive capability to develop in response to our experiences, interactions and passions. Initial massive synaptogenesis is replaced by widespread pruning, as we shape our own brains. As we develop, we pass the milestones identified by psychologists until we reach adulthood. In doing so, a set of regions closely associated with the very processes that we feel make us who we are exhibits *depressed activity* when an adult is involved in goal-oriented tasks. This 'default network', is believed to support mental activity that is introspective, self-referential, stimulus independent, self-projecting, etc [312]. The architecture of this network both strengthens and becomes more well developed from late childhood into early adulthood, at a time when, unsurprisingly, introspective and stimulus-independent thought is developed most strongly [312].

**15.4. Consciousness and physics**

Understanding consciousness is one of the greatest challenges of our time. In the context of mathematics, having asked what breathes the fire into equations, Stephen Hawking pointed out:

> The usual approach of science of constructing a mathematical model cannot answer the questions of why there should be a universe for the model to describe. Why does the Universe go to all the bother of existing?

Stephen W Hawking [8]

The aforementioned stages in the development of a human brain represent the highest known level of the development of consciousness across the spectrum of animals down to birds and even insects, depending on one's definition of consciousness. It is absurd to say that all of Nature is conscious, even down to the level of an inorganic mineral, but consciousness surely grows out of related natural phenomena (see [253] for a philosophical perspective). If the ordinary inanimate phenomena of nature are analogous to the incoherent oxidation of inorganic matter, consciousness is analogous to the coherent blazing conflagration of a widespread forest fire.

Although the 'perennial appeal of quantum approaches to consciousness' is dismissed in 'Putting Descartes before the horse' [313] and in Tegmark's more comprehensive consideration of decoherence time [314], these authors have noted that there might indeed be a need for new physics. Seth explained, 'the means by which neurochemical brain activity engenders subjective conscious experience... can still seem entirely mysterious, and perhaps requiring scientific revolution rather than evolution'.

The adoption of an interdisciplinary, holistic perspective will be required to comprehend the integrated operation of white and grey matter, as promoted by Haroutunian [264]. In respect of such an approach, the Director of the American National Institute of Health noted:

> ... new advances in computer science, math, and imaging and data visualization are empowering us to study the human brain as an entire organ, and at a level of detail not previously imagined possible in a living person.

> Some have likened this new ability to the difference between listening to the string section (evaluating an isolated part of the brain) versus listening to an entire orchestra (the whole organ). If you listen only to the string or percussion section, you'll gain a pretty good understanding of how that particular group of instruments sounds. However, that experience would not compare to the experience of listening to the whole orchestra and chorus perform Beethoven's Symphony No. 9, the Ode to Joy.

Francis Collins in 'The Symphony Inside Your Brain' [315]

We postulate that, if the orchestra represents the brain and its operations, then the appropriate analogy for consciousness is the music as it is carried through the air.<fnref n="26" />

Our title recasts, in the context of consciousness, Stephen Hawking's reflection on how mathematical physics is able to encapsulate vibrant physical concepts:

<footnote n="26">²⁶ Roland Allen, private communication.</footnote>
Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe?

Stephen W. Hawking [8]

Our response is that just as Nature breathed fire into the equations, it is Nature that breathes the fire of
consciousness into our brains, a response that reflects the scientists' quest, as exemplified in a 2021 Nova
broadcast on entanglement, when Anton Zeilinger, a coauthor on this paper, remarked: 'I am just trying to
understand Nature'.

16. What philosophers should really be thinking about by Roland Allen and Suzy Lidström

Philosophy over the centuries has changed its meaning, and philosophy in the twenty-first century essentially means the struggle for clear thinking. It is obvious that the profound difficulties of human society have been and continue to be consequences of misunderstandings and even delusions, so—as is already widely recognised—philosophers can provide guidance for a better future by pointing out the common errors in practical matters, ranging from grossly harmful cultural and religious practices to the more subtle nuances of ethical behaviour. Here, we would like to extend this enterprise to the highest imaginable levels in trying to interpret what is correct and erroneous in the scientific enterprise and what are the deepest underlying principles of this enterprise.

Our first example is the separate concepts of a multiverse and the anthropic principle, which it is now fashionable to either accept or abhor for what are usually the wrong reasons. For example, in what is otherwise still the best broad treatise on cosmology [316], a footnote states

> It is unclear to one of the authors how a concept as lame as the 'anthropic idea' was ever elevated to the status of a principle.

This is a psychological and even emotional rather than scientific statement. In the other direction, the concept of landscape has come to be regarded as a positive feature of, and even justification for, string theory, but other theories may also have landscapes and the landscape may not be necessary to understand why our universe seems favourable for the development of intelligent life [317]. Furthermore, string theory itself has developed largely because its mathematical beauty is appealing to human theorists. (The beauty of a theory is not directly relevant to its physical correctness.) So what are the valid arguments that would lead to accepting the potential reality of a multiverse? The first would be experimental verification. For example, in the Everett interpretation of quantum mechanics—which implies one type of multiverse—demonstration of entanglement in many different contexts and with higher and higher levels of macroscopic physical systems would imply that the standard quantum formulation does apply to all physical systems including human observers. If the multiverse is an unavoidable logical implication of an accepted physical theory then acceptance of that theory inescapably implies an acceptance of the multiverse. In the vast number of papers that have been written on the interpretation of quantum mechanics it is evident that nearly all betray lapses in clear thinking on some level. So, to review, philosophy can help the scientific enterprise, by emphasising what are valid and invalid logical arguments.<fnref n="27" /> It is invalid to rule out a multiverse, or any physical theory because one dislikes it. It is invalid to cherish a physical theory because one finds it intellectually appealing. It is valid to accept a physical theory if it is confirmed by experiment or if it is inevitably implied by accepted theory. There are, of course, many other examples besides the multiverse of how these principles apply and should be routinely employed in science.

In addition to reminding scientists of what is invalid and valid reasoning, philosophers may look for principles that are too deep to be considered in normal scientific thinking and publications. One such principle is this: How do we understand what systems or concepts emerge to be dominant in Nature or human society? For example if there is a multiverse with 10$^N$ potential universes, with N greater than 1000, how do we estimate that such a universe will be stable at all and will be sufficiently stable to harbour intelligent life? If we assume the best version of Nature currently available, namely the path integral description of either quantum field theory or a deeper theory, then a solution is provided by the power of the exponential function in either the Lorentzian formulation, with large actions killed off by rapid oscillations, or the Euclidian formulation, with large actions killed off by exponential decrease. Even an extremely large number of unstable universes lose in probability to those that are stable.

<footnote n="27">²⁷ We consider Richard Feynman and Murray Gell-Mann to be the greatest theoretical physicists of their generation. Feynman is on record as saying with regret that he initially disliked the gauge theory of the standard model because he regarded its asymmetry as ugly [318]. One of us speaking to MGM mentioned that the primal feelings of human consciousness are not fully explained by the current mathematical laws of physics to which Gell-Mann responded 'That's crazy talk' [319].</footnote>
One can imagine extending this principle to weighting the factor involving the action by a similar factor which expresses the probability for the development of intelligent life. In a multiverse scenario this provides a quite respectable foundation for even the anthropic principle. In this context it should be mentioned that Stephen Weinberg predicted the approximate value for the dark energy [93] before it was discovered [76, 320].

This general idea, of the stability of an occurrence dominating the sheer number of occurrences also explains many other observations in Nature and human society. Why is a single kind of molecule, DNA, the basis of all life on Earth, whereas an infinite number of molecules can be formed from the common elements? Because DNA has been proved to be stable on a timescale of billions of years. Why is the Riemann hypothesis still regarded as the greatest problem in mathematics [4], as it was in 1900 [321] and again in 2000 [322]? Because the worldwide community of mathematicians is justifiably in awe of the profound connections between number theory and the deepest other aspects of mathematics.

Our chief points are: (1) As scientists who have observed at close hand and in publications that even the best scientists often display remarkable philosophical naïvete in their reasoning, we believe that some interaction between philosophers who understand science and the scientific community could have a true positive impact. (2) Another potential role for philosophers is in considering the deepest principles behind science—such as those given immediately above—in clarifying these issues, Richard Feynman was profoundly sceptical of philosophers, thinking that philosophers would provide vacuous explanations for scientific facts, possibly thinking of Hagel and his apparent belief that philosophical arguments can limit the number of planets. But 21st century philosophers with greater sophistication and an understanding of the real fundamental principles in science can offer some positive influence in scientific thinking.

# 17. How can scientists address misinformation? Science, survival and the urgent pursuit of truth by Steven Goldfarb

We live in challenging times. At the time of writing, the world is attempting to navigate its way through rapid climate change [323], a global pandemic [324] and economic collapse [325], all in the backdrop of increasing socioeconomic inequality [326] and depleting resources [327]. It is at times like this that we turn to scientists for solutions, and to our world leaders to provide resources and guide the implementation of these solutions.

Indeed, international teams of scientists have heeded the call, coordinating their efforts to find solutions that are both effective and safe, then communicating them to the world leaders. Researchers in climate science, epidemiology and economics have all spoken up, noting the importance and urgency of the problems at hand, and offering paths forward. Those of us in fundamental research support these efforts, through public talks and editorials, and sometimes by advocating to politicians and other key stakeholders.

Despite these efforts, the scientific advice is not always heard. Worse, even when it is heard, it is often ignored. Why? Although most of the world's leaders have realised the urgency of the situation and have used their skills at communication and consensus-building to motivate their citizens to work together for the common good. Others clearly have not.

By not taking action or by taking inappropriate action, these leaders are endangering their own constituents, future generations, and quite possibly an entire species. As scientists, we have a moral obligation to expose these misdeeds and to inform the population of the actions that need to be taken. Unfortunately, this is not easily done.

Toward the end of the last century, scientists working at CERN, the international particle physics laboratory in Geneva, Switzerland, developed a communication application designed to facilitate the sharing of scientific documentation around the globe [328]. This tool, the World-Wide Web, has more than served that purpose, allowing instant sharing of information not only between science institutions and laboratories, but between individuals everywhere. As the reader is well aware, a wealth of information, knowledge and wisdom is now available, quickly and affordably, to nearly everyone on this planet.

Ironically, it is this very tool that is at the heart of the problem. Belligerent and/or ignorant parties are able to use social media platforms on the web to disseminate false or misleading information rapidly, while posing as reliable sources. Those with a natural penchant for communication, and minimal training, are able to exploit naïve, ill-informed or simply trusting audiences on these platforms, whether to push a political or financial agenda or simply to wreak chaos for the fun of it.

Anyone with an understanding of science or history knows that, in most cases, facts and evidence eventually do come to light. However, a lot of damage can occur in the meantime. Thus, many scientists are taking proactive approaches to address the issue. Some have honed their communication skills, interacting through the traditional media, while others (often younger researchers) develop social media strategies to effectively disseminate scientific advancements and knowledge.
Surprisingly, as a particle physicist, I find myself dedicating nearly as much time to current political discourse as I do to current research. There is a lot at stake and we, as scientists—people who have dedicated our lives to the pursuit of truth—cannot afford to ignore it. At the heart of the issue is the human ability to discern truth from fiction. Those with well-amplified, far-reaching communication platforms have the ability to disguise lies as truth and vice-verse, confusing audiences and undermining public trust in science. This is a hard battle to fight.

In one of my more recent presentations [329], I spent a significant time describing the complex and rigid process researchers follow to go from basic idea to publications. My hope is to instil an appreciation for science that can transcend the disinformation that bombards us every day, by explaining the effort required to attain truth, and thus the value of science to humanity.

Such efforts can have an important effect in the short term, but in many cases there are simply too few science communicators or resources to battle with professional liars. It is much easier to spout untruths at random, than to do research and present the results to the public in a convincing manner. This problem is compounded when the sources of the misleading information are in positions of power or are members of the professional media.

Fortunately, our nature provides a path for a long-term, sustainable solution. Human beings have a natural affinity for science and discovery, especially at a young age. Our DNA provides us with the means to address certain basic needs: finding food, building shelter, making babies, and seeking a better understanding of our universe. It is the last capability that allows us to develop and improve the tools needed by future generations to survive.

This instinct motivates us to create art, music and literature, and pursue science. It is driven by our inherent curiosity, but goes deeper than that, in that it compels us to share our findings with our family, friends and fellow inhabitants. That is, we are all scientists from the day we are born. As discussed in an earlier contribution to this paper, as soon as our eyes open, we look around, take in our environment and try to make sense of it.

Our current environment, however, does not always provide equally fertile ground to develop this ability. What varies from person to person is our understanding of the existing knowledge base, the proposed models to describe it and make predictions, and the methodologies employed to build these models from the data. The knowledge and skill sets we attain depend on individual capability, experience and access to quality education. Thus, there is an important socio-economic aspect we cannot ignore.

As young children, we are fascinated by the beautiful blue sky. We share that fascination with those around us, who confirm that they also see a blue sky, and teach us the name of the colour. Before long, we wonder why the sky is blue. Then, if we are fortunate, after years of formal education, we might learn the formalisations needed to understand the transmission and scattering of sunlight [330], optics, electromagnetism and waves.

Unfortunately, somewhere along the way, between kindergarten and elementary electromagnetism courses, many lose the thread connecting the initial thrill of discovery to the formal education required to develop a deep understanding of its meaning. Great teachers recognise this and do what they can to bring that thrill back to the classroom. Some are able to relate a lesson to their own research, or to current science headlines. But this is not always an easy task, and often the latest headlines involve seemingly complex topics unfamiliar to the teacher or the students. Furthermore, many teachers do not have access to that information.

This is where informal science education can make an impact. Much modern research is anchored in basic concepts. An appreciation of this enables those who are active in public engagement to convey the fundamental aspects of recent advancements in language that is accessible to the general public: Dark Matter and conservation of momentum, the Higgs boson and a cocktail party, gravitational waves and billiard balls on a sheet, viral infection and dominoes. By working together with formal educators, these scientists can bring the excitement of current research to the classroom and use these concepts to catalyse the learning process.

Although the current reach of informal science programs is still rather limited, they are growing in size, scope and worldwide reach. The International Particle Physics Outreach Group (IPPOG) [331], for example, runs the International Particle Physics Masterclass and Global Cosmics programs reaching tens of thousands of students in 60 countries around the world. These programs partner active researchers with secondary school teachers to give their students the possibility to learn what it is like to be a scientist today.

The students become actively involved in the research, analysing actual data from current particle physics or astrophysics experiments. This has the effect of re-igniting that flame of curiosity from childhood or, in some cases, fanning existing flames sufficiently to spark interest for future studies. Most importantly, students learn the methodology employed by scientists to explore data and to address the complex problems they are trying to solve. That is, they re-learn the scientific process and the value of evidence-based decision making.

This is no small step. Students exposed to these opportunities develop an appreciation for science and the scientific process. Through improved understanding of the thought processes they, as individual citizens, are better prepared to sift through the mountains of lies they are presented each day, to find the facts they need to fuel their decisions. As they mature, they will be better able to choose appropriate sources of information, and
will demand the replacement of deceptive leaders by people keen to govern based on evidence and valid argumentation.

It would be an exaggeration to think that such efforts will have dramatic effects in the short-term. Trolls are certainly here to stay. There will always be people who feel sufficiently disenfranchised to want to break existing power structures through lies and deception. Only significant global economic and political change can address the underlying issues there. However, the effect of their weapons can be greatly reduced in the long-run through education and improving the fundamental understanding of science by future generations. And this battle is being fought today.

## 18. Can we find violations of Bell locality in macroscopic systems? by Bryan Dalton

To Einstein [51], the Copenhagen quantum interpretation of what happens when we first measure an observable $\Omega_A$ in one sub-system A with outcome $\alpha$, and then immediately measure an observable $\Omega_B$ in a second well-separated sub-system B with outcome $\beta$ seemed counter-intuitive, implying 'instantaneous action at a distance' during the two-step measurement process. This has been known since the 1930s as the EPR paradox. According to the Copenhagen interpretation, after the first measurement, the quantum state is changed, conditioned on the outcome of the first measurement. As a result, the reduced density operator describing the original state for sub-system B would have changed instantaneously to a different state, despite no time having passed in which a signal could have travelled between the two well-separated sub-systems. This effect is referred to as steering [332]. Of course if $\Omega_A$ was immediately measured a second time, it is easy to show that the outcome $\alpha$ would occur with probability 1. For the Copenhagenist, this raises no issues, since the quantum state is not regarded as a real object, but only a means of determining the probabilities of the outcomes of measuring observables (the outcomes being the real objects which are created by the measurement process on the prepared quantum state). That the quantum state changes as a result of the measurement of $\Omega_A$ with outcome $\alpha$, merely signifies the probability changing from its previous value for the original preparation process, to now being unity for a new preparation process in which the second part involves measuring $\Omega_A$ with outcome $\alpha$. If we now measure the second sub-system observable $\Omega_B$ the conditional probability for outcome $\beta$, given that measurement of $\Omega_A$ in the first sub-system A resulted in outcome $\alpha$, will now be determined from the new conditioned quantum state. In general this will be different from the probability of outcome $\beta$ resulting from measurement of observable $\Omega_B$ for the original quantum state. However, using Bayes' theorem the joint probability for outcomes $\alpha$ for $\Omega_A$ and $\beta$ for $\Omega_B$ can be determined to be the standard Copenhagen expression for the joint measurement probability for the measurement of the two observables in the separated sub-systems if the measurements had been made on the original quantum state totally independently of each other and in no particular order. As far as we know, the predictions based on the Copenhagen version of quantum theory are always in accord with experiment. But to Einstein and others, the Copenhagen theoretical picture was philosophically unsatisfactory. The question arose: is it really necessary to invoke the Copenhagen picture involving the instantaneous change to the quantum state as a result of the first measurement (the 'collapse of the wave function') to describe what happens, or is there a simpler picture based on classical probability theory—and involving what we now refer to as hidden variables—that could also account for all the probability predictions of quantum theory?

A first question is whether the results for any quantum states describing two sub-systems can also be described by hidden variable theory. One whole class of quantum states that can be so-described are the separable states [333]. Here the initial process involves preparing each separate sub-system in a range of sub-system quantum states, each choice being specified according to its probability. However, the results for the joint measurement probability outcomes for $\Omega_A$, $\Omega_B$ are of the same form as in local hidden variable theory. So as the separable states can all be given a local hidden variable theory interpretation, it follows that any state that cannot be so interpreted must be a non-separable or entangled state. However, Werner [333] showed that there were some entangled states that could be interpreted in terms of local hidden variable theory. Particular examples were the so-called Werner states [333], which are mixed states specified by a single parameter, involving two sub-systems with equal dimensionality. This means that the division of quantum states into separable or entangled ones does not coincide with their division into Bell local and Bell non-local. The separable states are examples of quantum states that can be also described by local hidden variable theory, and are characterised by both sub-systems being associated with a so-called local hidden quantum state [334] which is specified by the hidden variables $\lambda$. Clearly within local hidden variable theory we could also have the situation where only one of the two sub-systems, B say, is associated with a local hidden quantum state from which the measurement probability for the outcome for $\Omega_B$ is determined; for the other sub-system, A, the corresponding probability for outcome $\alpha$ for $\Omega_A$ is not determined from a local hidden state. Another situation is where neither sub-system is associated with a local hidden quantum state. Both of the latter situations involve entangled quantum states, whilst still being described by local hidden variable theory. States where there are no local hidden states are referred to as
<figure><img src="image_18.png" /><figcaption>**Figure 12.** Based on a local hidden variable theory interpretation of quantum theory, John Bell (pictured here at CERN in 1982) derived a famous inequality involving correlation functions for the measurement outcomes of spin component measurements on two spin 1/2 sub-subsystems, that could be tested against the predictions of quantum theory in the case of an entangled singlet state of the combined system. This inequality was shown theoretically to be violated for certain spin component choices, and this result was later confirmed in experiments. Credit: CERN PhotoLab.</figcaption></figure>

EPR steerable states [334]. They allow for the possibility of choosing the measurement for observable $\Omega_A$ to steer sub-system B such that the outcome for measuring $\Omega_B$ can be chosen in advance. The EPR steerable states are all entangled, and include those that are Bell non-local as well as some that are Bell local and entangled (named after John Bell, shown in figure 12), and are said to exhibit EPR correlations. Bell non-locality is the most general form of hidden variable theory for describing the two sub-systems. Here there are no separate hidden variable dependent probabilities for sub-system observable measurements. To determine whether a state is Bell non-local it must be shown that a Bell inequality—derived from the basic hidden variable expression for the joint probability—is violated.

As pointed out recently [335], there are a multitude of Bell inequalities that can be derived even for bipartite systems, depending on the number of observables considered in each of the two sub-systems and on the number of different outcomes for each observable. One of the earliest of these was the famous CHSH Bell inequality [336]. Here there were two different observables $\Omega_{A1}$, $\Omega_{A2}$ and $\Omega_{B1}$, $\Omega_{B2}$ for each sub-system, and measurement of any observable was restricted to two outcomes—which we choose to be +1/2 and -1/2. The CHSH inequality is $|S| \le 1/2$, where $S = \langle\Omega_{A1} \otimes \Omega_{B1}\rangle + \langle\Omega_{A1} \otimes \Omega_{B2}\rangle + \langle\Omega_{A2} \otimes \Omega_{B1}\rangle - \langle\Omega_{A2} \otimes \Omega_{B2}\rangle$. Suitable physical systems for which this inequality can be tested include two spin 1/2 sub-systems, with components of the spins along various directions being the observables since the measured outcome is either +1/2 or -1/2. Another suitable physical system is two modes of the EM field as the two sub-systems are each occupied by one photon, with the mode polarisation being the observable and the outcome being +1/2 or -1/2 depending on whether the outcome is right or left circular polarisation, or up or across for linear polarisation. These examples are both microscopic systems. Experiments testing the CHSH inequality have been carried out since the 1970s (see [335] for a recent review), and a violation of the inequality has now been convincingly demonstrated following numerous improvements to remove possible loopholes by means of which the inequality might not really be violated.

However, apart from situations involving two super-conducting qubits, the CHSH inequality only establishes Bell non-locality in microscopic systems. As quantum theory was originally formulated to treat microscopic systems, merely showing that the Copenhagen interpretation was needed for microscopic systems leaves open the possibility that hidden variable theory could still be used to explain experimental effects in macroscopic systems. The latter, after all, normally lie in the domain of classical physics where quantum theory is not usually required. Hence there is an interest in finding quantum systems on a macroscopic scale for which Bell inequalities can be derived, and for which violations might be both predicted and found experimentally. There are examples from the 1980s of Bell inequalities applied to macroscopic systems, though no experimental tests have yet been carried out. In [337] a system consisting of two large spin s sub-systems was considered allowing for measurements of any spin component to have outcomes from -s to +s in integer steps. For an overall singlet pure state in which measurement of a spin component in one sub-system leads to the opposite outcome when the same spin component was measured in the other, a Bell inequality involving spin components along three unit vectors a,b,c of the form $s|\langle S_{Aa} \rangle - \langle S_{Bb} \rangle| \ge \langle S_{Aa} \otimes S_{Bc} \rangle + \langle S_{Ab} \otimes S_{Bc} \rangle$ was found. This was shown theoretically to be violated for coplanar unit vectors, where a,b make an angle $\pi - 2\theta$
with each other and the same angle $\pi/2 + \theta$ with $c$, provided $0 < \sin \theta < 1/2s$. This is a very small range of violating angles if $s$ is large enough for the system to be considered macroscopic, and the required singlet state would be difficult to create. In [338] two sub-systems each containing two bosonic modes were considered. An overall entangled state with a large number $N$ of bosons was studied, and a Bell inequality found involving sub-system boson number-like observables for each sub-system. These were given by linear combinations (specified by a parameter $\theta$) of its pair of mode creation operators and raised to power $J$, times a similar expression involving the annihilation operators. For $J = N \rightarrow \infty$ the inequality is violated for finite $\theta$ if $3g(\theta) - g(3\theta) - 2 > 0$, where $g(\theta) = \exp(-J\theta^2/2)$. Although suitable $\theta$ can be found, the measurement of the observables for large $J = N$ would be difficult. Subsequently, Leggett and Garg [339] developed a test for macroscopic quantum coherence based on the mean value of products of pairs of observables for the two sub-systems, but now taken at three different times.

More recently, the interest in finding Bell non-locality in macroscopic systems has revived [340, 341]. This is in part due to experimental progress in the study of ultracold atomic gases, which are macroscopic systems for which a quantum description is required. These include ultracold bosonic gases, where large numbers of bosonic atoms may occupy each mode, creating Bose-Einstein condensates. For studying bipartite Bell non-locality, two mode systems such as those for bosons trapped in a double potential well, or for bosons in a single well but with two different spin states are available. A four mode bipartite system involving two modes associated with different internal states in each well can also be prepared [342] using atom-chip techniques. Multipartite systems in which each two-state atom is located at a different site on an optical lattice have also been created [343]. For ultracold fermionic gases the situation is not so clear, for although many fermion systems would be macroscopic, each mode could only be occupied by fermions with differing spins and hence many modes would be involved making it difficult to devise bipartite systems. Recent discussions of Bell non-locality in many-body systems are presented in [335, 344–346] and [347]. These contain examples of multipartite Bell inequalities, with applications to systems such as $N$ two state atoms located at different sites in an optical lattice. Here each identical atom $i$ is treated as a distinguishable two-mode pseudo-spin sub-system. Measurements on one of two chosen spin components $M_{i0}$ or $M_{i1}$ for the $i$th atom sub-system are considered, the two possible outcomes being designated as $\alpha_i = \pm 1$. Defining $S_0, S_{00}, S_{11}$ and $S_{01}$ involving the mean values of single measurements on individual spins or joint measurements on different spins, a Bell inequality $2S_0 + S_{01} + 2N + (S_{00} + S_{11})/2 \ge 0$ has been derived [344]. Bell correlations based on this inequality have been found [347] in systems involving 5 x $10^5$ bosonic atoms. In these systems the indistinguishability of the identical atoms and the effect of super-selection rules that rule out sub-system states with coherences between different boson numbers can be ignored, as there is just one atom in each separated spatial mode on each different lattice site. However, the symmetrisation principle and the super-selection rules are important in regard to tests for quantum entanglement and EPR steering [348, 349] in situations where the sub-systems must be defined via distinguishable modes rather than non-distinguishable atoms, and where there is multiple mode occupancy. The derivation of testable Bell inequalities for this common situation is an ongoing issue.

This section has been adapted from the more technical and more comprehensive treatment of this topic given in [350] (CC BY 4.0).

## 19. What is the source of quantum nonlocality? by Ana Maria Cetto

‘That one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one another, is to me so great an absurdity that, I believe, no man who has in philosophic matters a competent faculty of thinking could ever fall into it.’ wrote Isaac Newton, in a letter to Richard Bentley in 1692.

We live in a world full of ‘signs’ reaching us from the distance: The glitter of stars, the sound and strike of thunder and lightning, the pull of the Earth under our feet… Ancient deities, once endowed with supernatural powers to unleash such actions at a distance, have been left idle by the principle of locality, stating that objects are directly influenced only by their immediate surroundings. We have filled the void with a variety of fields surrounding the objects and mediating between them, to account for such actions no matter how distant the source-provided the speed of the message is not larger than the speed of light. Locality has been established as a basic tenet of physics

Or has it?

Quantum mechanics is conventionally said to have posed a challenge to locality. Bohm’s rendering of the Schrödinger formalism is overtly nonlocal. Bell’s theorem is widely interpreted as quantum mechanics outlawing local realism. (For other, not Bell-related inequalities that are violated by quantum mechanics, see [351].) Experiments designed to test Bell-type inequalities with a pair of entangled particles (or photons)
produce results consistent with the quantum predictions, suggesting the ruling out of local hidden-variable theories and consequently, of local realism altogether.

However, as is argued by a significantly increasing number of authors, this quite dramatic conclusion is based on a certain class of hidden variables pertaining only to the particle pair involved in the experiment. This is an unwarranted restriction, since, more generally, the variables could in addition describe a background field or medium that interacts with particles and detectors and intervenes in the measurement process. And, as demonstrated in [352], such ‘background-based’ theories can in principle reproduce the quantum correlations of Bell-type experiments.

An instance of a background-based theory is stochastic electrodynamics, the theory developed on the basis of the interaction of particles with the vacuum radiation field. The quantum features, as described by the Heisenberg and Schrödinger formalisms, emerge as a consequence of this permanent interaction [353] (for a comprehensive account of the first three decades of Stochastic Electrodynamics, see [354]). In a bipartite system, the particles become entangled by resonating to common field modes; the invisible, intangible vacuum acts as a mediator, and in turn becomes influenced by the particles [355, 356]. De Broglie’s wave has an electromagnetic character: It is the modulated wave made up of the background waves at the Compton frequency in the particle’s rest frame, with which the moving particle interacts resonantly (see [353] Ch. 9).

Is quantum mechanics the only instance in which the dynamics of particles is influenced by the surrounding medium, producing such ‘nonclassical’ behavior? Recent experiments with droplets bouncing on the surface of a vibrating liquid (see, e.g. [357, 358]) demonstrate that a background field can lead to a surprisingly wide range of quantum-like effects in the macroscopic, hydrodynamic realm, too. With each new bounce, the droplet contributes to form the pilot wave that moves along with it on the surface of the vibrating liquid.

In stochastic electrodynamics, as in the fluid mechanical quantum analogue, by hiding the underlying field element the description of the particle’s behaviour becomes nonlocal. Of course, stochastic electrodynamics, although intriguing, has not been shown to replace either the qualitative or extremely precise quantitative predictions of quantum electrodynamics, but it reproduces them while providing a physically sound picture for the quantum formalism. And it explains the origin of the apparent nonlocalities.

## 20. How much of physics have we found so far? by Anton Zeilinger

The 20th century saw the discovery of two new big fields, the relativity theories and quantum mechanics. Could it be that similar, even larger discoveries are waiting around the corner? My, certainly not logically convincing, argument is the following. First we have to consider science in the modern way. In my eyes, it starts with the invention, if it is possible to say that, of the Renaissance point of view of the role of humankind in the Universe. During the Renaissance, humans started to dare to ask Nature questions. As I see it, a significant input was the rising self-esteem of humans as you see them in the gigantic change of portraits painted before and after the beginning of the Renaissance. Another interesting discovery was the discovery of Laws of Nature. Prior to the Renaissance, laws were God-given, and humans were not supposed to meddle in His work. Finally, we need to recall the great discovery that mathematics is the language of Nature. All these concepts led over the last few hundred years to immense discoveries, many new fields of science. We all are familiar with the development, step by step, of physics. But given that the development in science is of such a young nature compared to the history of humanity, I find it rather unlikely that we have discovered all the physics there is. I find it even unlikely that much of what we do know now will stand in the distant future. And, concluding, I hope that I am still alive when some young chaps discover the next great field.

## 21. Coda by Suzy Lidström

We conclude with the enduring voice of Stephen Hawking (see figure 13) as it was broadcast into space in a final message from the Cebreros antenna in Spain towards IA 0620-00, the closest black hole to Earth. Hawking’s daughter, Lucy, described her father’s message as being one ‘of peace and hope, about unity and the need for us to live together in harmony on this planet’<fnref n="28" />.

Hawking’s message was directed at the young people whose task it will be to advance scientific frontiers and resolve the major challenges facing the world:

‘I am very aware of the preciousness of time. Seize the moment. Act now. I have spent my life travelling across the Universe inside my mind. Through theoretical physics I have sought to answer some of the great questions but there are other challenges, other big questions which must be answered, and these will also need a new generation who are interested, engaged and with an understanding of science.

<footnote n="28">²⁸ http://esa.int/About_Us/Art_Culture_in_Space/ESA_honoured_to_take_part_in_Hawking_tribute.</footnote>
<figure><img src="image_19.png" /><figcaption>**Figure 13.** Stephen Hawking in zero gravity. Credit: NASA. Hawking's final words were broadcast into space set to music by the Greek composer Vangelis after his remains were laid to rest between those of Sir Isaac Newton and Sir Charles Darwin: 'We are all time travellers journeying together into the future. But let us work together to make that future a place we want to visit. Be brave, be determined, overcome the odds.'</figcaption></figure>

How will we feed an ever-growing population, provide clean water, generate renewable energy, prevent and cure disease and slow down global climate change? I hope that science and technology will provide the answers to these questions, but it will take people, human beings with knowledge and understanding to implement the solution...

When we see the Earth from space we see ourselves as a whole; we see the unity and not the divisions. It is such a simple image, with a compelling message: one planet, one human race...

We must become global citizens...

It can be done. It can be done.'

After millennia of struggles in hundreds of cultures around the world to understand the Universe and our place in it, we are extremely fortunate to be living in a time when clarity is beginning to emerge. Our worldview is vastly grander than the narrow human-centred fantasies of past centuries. This article is meant to provide a microcosm of the best ideas that are surging through our current intellectual environment at the highest level. And as Hawking implies, with unparallelled eloquence, a central message is that equally grand challenges await even the youngest scientists who are just beginning to confront these issues today.

## Acknowledgments

The work of I K S, S F, O S, and A F was supported as part of the Quantum Materials for Energy Efficient Neuromorphic Computing (Q-MEEN-C) Energy Frontier Research Center (EFRC), funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0019273.

C F M, L A M C and P F thank Professor Lou Massa for his critical reading of their contribution to the manuscript and are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), Mount Saint Vincent University, Université Laval, and Saint Mary's University for financial and material support. A A C would also like to acknowledge the financial support provided by NSERC of Canada

I K S, S F, O S and A F acknowledge the support received as part of the Quantum Materials for Energy Efficient Neuromorphic Computing (Q-MEEN-C) Energy Frontier Research Center (EFRC), funded by the U S Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0019273.

## Data availability statement

This is a perspective article that does not present novel data. The data that support the findings of this study are available upon reasonable request from the authors.
ORCID iDs

Roland Allen https://orcid.org/0000-0001-5951-4717

Ana María Cetto https://orcid.org/0000-0001-6006-1102

Alan A Coley https://orcid.org/0000-0001-5482-6703

Bryan J Dalton https://orcid.org/0000-0002-1176-7528

Peyman Fahimi https://orcid.org/0000-0002-3824-5017

Chérif F Matta https://orcid.org/0000-0001-8397-5353

Sam Patrick https://orcid.org/0000-0001-9239-6310

Suzy Lidström https://orcid.org/0000-0003-0050-413X

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