Professor Miles Padgett FRS, is a Royal Society Research Professor and also holds the Kelvin Chair of Natural Philosophy at the University of Glasgow. He leads an optics research team covering a wide spectrum from blue-sky research to applied commercial development, funded by a combination of government, charity and industry.
His research team, covers all things optical from the basic ways in which light behaves as it pushes and twists the world around us, to the application of new optical techniques in imaging and sensing systems. They are currently using the classical and quantum properties of light to explore: the laws of quantum physics in accelerating frames, microscopes that see through noise, shaped light that overcomes diffraction-limited resolution, endoscopes the width of a human hair and new ways of imaging in 3D.
Does God Play Dice ?
Towards the end of the 19th Century many scientists thought that science itself was complete. However, in 1905 Einstein published three areas of work that caused everyone to think again and set a significant fraction of the physics agenda ever since.
Einstein’s work on relativity transformed Newtonian mechanics into a universe where the speed of light was a constant, leading directly to the phenomena of length contraction, mass transformation and time dilation. Most recently his work leading to the detection of Gravitational Waves in which my colleagues here in Glasgow played such a big part. Einstein’s work on explaining the Brownian motion of microscopic particles showed the discrete, atomic, theory of mater. However, beyond these two agenda changing discoveries, it is for his work on the photoelectric effect for which he was awarded the 1921 Nobel Prize for Physics. His understanding of this effect explained how a low intensity of light at a high frequency could release an electron from a metal surface where a higher intensity of light at a lower frequency could not. This suggested that light was quantised and could under the right conditions behave like a stream of particles rather than a commonly understood wave. In this discovery, Einstein drove the birth of a new physics – namely that of quantum mechanics.
Quantum mechanics is one of the most successful theories every proposed, making highly accurate predictions both in fundamental science and in the underpinning of many of today’s technologies and consumer electronics such as the satellite navigation on your phone or in your car. However, although highly accurate as a predictive tool, some aspects of quantum mechanics have opponents. The fact that light has both particle-like and wave-like properties seems odd, but all scientists and philosophisers agree these are simply convenient models that we use to describe the way that light actually behaves. The concerns over quantum mechanics and its implications lie much deeper, and to understand them we must consider a few simple experiments.
One of the simplest experiments to demonstrate the wave-like property of light is Young’s double slits. Two narrow slits, illuminated by a single-wavelength source create many alternate bright and dark fringes on a distant screen. This is an example of interference where light waves emanating from each of the slits combine constructively, crest with crest, to give a bright fringe or destructively, crest with trough, to give a dark fringe. Strangely, if the light intensity is reduced so that the photons leave the source one at a time, the interference pattern persists! But how can one photon apparently interfere with itself? We can only conclude that the photon effectively passes through both slits simultaneously! It is found that any attempt to detect which slit the photon goes through results in a perturbation of the system and the interference pattern is spoiled. This last fact is again strange but totally explainable in terms of classical interactions – it is not the main cause of dispute.
To be clear, Einstein had no issue with the concept of wave-particle duality. The dispute instead arises from a different question. The interference pattern comprises many bright fringes, and after passage through the slits the single photon might be detected in any one of these fringes with a roughly equal probability. But when is the decision as to which specific fringe the photon will be detected in be made? In a pre-determined universe, we could argue that this dilemma had been foreseen and the destiny of every single photon and in which fringe it will be detected is already decided. This outcome and the outcome of all future events are set by the highly complicated, and too complicated to know, initial conditions. In this type of universe, if you do exactly the same experiment twice and you will get exactly the same answer. The other extreme interpretation of the laws of the universe maintains the element of random chance for as long as possible, with the decision only been resolved at the moment of measurement by an observer. Throughout his life Einstein was a strong supporter of the pre-determined variety of quantum mechanics where the outcome of any experiment was precisely determined by the initial variables, even though some of them were fundamentally hidden from the observer. Bohr championed the alternative form of quantum mechanics where random chance meant that, even if perfectly repeated, the same experiment could give different outcomes. In essence Bohr’s view was that the universe was one very large lottery. Einstein’s concerns with the potential role of random chance in the universe were summarised in his quote that “God does not play dice with nature”.
To an experimentalist, the dispute between the interpretations of Einstein and Bohr perhaps seems a trifle academic. Bohr believed that outcome was not decided until measurement by an observer. Einstein believed that outcome was decided at initiation but the key variables were hidden and hidden so well that the laws of physics made them impossible to measure. So even in Einstein’s universe the outcome could not actually be known until measurement by an observer. These two viewpoints are significantly different in philosophy but seem pragmatically indistinguishable! However, in the famous EPR paradox, Einstein, Podolsky and Rosen ́ realized that maybe the two interpretations could be distinguished and in the 1960s John Bell formulated this test into the Bell inequality – the dispute was now open to experimental resolution!
Most experiments investigating the Bell inequality have used special light sources that emit photons in pairs. These photons are emitted in opposite directions and when apart can be subject to simultaneous measurements by two different observers. In the 1980s Alain Aspect (an honorary graduate of Glasgow) completed a number of groundbreaking experiments measuring the polarisation of the photons, showing that their orientation is only decided at the moment of measurement by an observer. Furthermore, the measurement of either photon instantaneously gives knowledge of the state of the other. This is called quantum entanglement and is sometimes referred to as “spooky actions at a distance”. Most scientists agree that Aspect’s original experiments and similar experiments in various laboratories worldwide offer overwhelming support to the interpretation of quantum mechanics by Bohr. For this work Alain, alongside John Clauser, and Anton Zeilinger were award the 2022 Nobel Prize in Physics.
Quantum entanglement is not just a new physics but it changes the understanding of our role in the universe, one in which the future is not yet decided.