Melvyn Bragg and guests discuss the theoretical physicist Dirac (1902-1984), whose achievements far exceed his general fame. To his peers, he was ranked with Einstein and, when he moved to America in his retirement, he was welcomed as if he were Shakespeare. Born in Bristol, he trained as an engineer before developing theories in his twenties that changed the understanding of quantum mechanics, bringing him a Nobel Prize in 1933 which he shared with Erwin Schrödinger. He continued to make deep contributions, bringing abstract maths to physics, beyond predicting anti-particles as he did in his Dirac Equation.With Graham Farmelo Biographer of Dirac and Fellow at Churchill College, CambridgeValerie Gibson Professor of High Energy Physics at the University of Cambridge and Fellow of Trinity CollegeAndDavid Berman Professor of Theoretical Physics at Queen Mary University of LondonProducer: Simon Tillotson

Melvyn Bragg and guests discuss the origins of horses, from their dog sized ancestors to their proliferation in the New World until hunted to extinction, their domestication in Asia and their development since. The genetics of the modern horse are the most studied of any animal, after humans, yet it is still uncertain why they only have one toe on each foot when their wider family had more, or whether speed or stamina has been more important in their evolution. What is clear, though, is that when humans first chose to ride horses, as well as eat them, the future of both species changed immeasurably.With Alan Outram Professor of Archaeological Science at the University of ExeterChristine Janis Honorary Professor in Palaeobiology at the University of Bristol and Professor Emerita in Ecology and Evolutionary Biology at Brown UniversityAnd John Hutchinson Professor in Evolutionary Biomechanics at the Royal Veterinary CollegeProducer: Simon Tillotson

Melvyn Bragg and guests discuss the flow of particles from the outer region of the Sun which we observe in the Northern and Southern Lights, interacting with Earth's magnetosphere, and in comet tails that stream away from the Sun regardless of their own direction. One way of defining the boundary of the solar system is where the pressure from the solar wind is balanced by that from the region between the stars, the interstellar medium. Its existence was suggested from the C19th and Eugene Parker developed the theory of it in the 1950s and it has been examined and tested by a series of probes in C20th up to today, with more planned.With Andrew Coates Professor of Physics and Deputy Director in charge of the Solar System at the Mullard Space Science Laboratory, University College LondonHelen Mason OBE Reader in Solar Physics at the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Fellow at St Edmund's CollegeAndTim Horbury Professor of Physics at Imperial College LondonProducer: Simon Tillotson

Melvyn Bragg and guests discuss what happens when parents from different species have offspring, despite their genetic differences. In some cases, such as the zebra/donkey hybrid in the image above, the offspring are usually infertile but in others the genetic change can lead to new species with evolutionary advantages. Hybrids can occur naturally, yet most arise from human manipulation and Darwin's study of plant and animal domestication informed his ideas on natural selection.With Sandra Knapp Tropical Botanist at the Natural History MuseumNicola Nadeau Lecturer in Evolutionary Biology at the University of SheffieldAndSteve Jones Senior Research Fellow in Genetics at University College LondonProducer: Simon Tillotson

Melvyn Bragg and guests discuss the work and ideas of Dorothy Crowfoot Hodgkin (1910-1994), awarded the Nobel Prize in Chemistry in 1964 for revealing the structures of vitamin B12 and penicillin and who later determined the structure of insulin. She was one of the pioneers of X-ray crystallography and described by a colleague as 'a crystallographers' crystallographer'. She remains the only British woman to have won a Nobel in science, yet rejected the idea that she was a role model for other women, or that her career was held back because she was a woman. She was also the first woman since Florence Nightingale to receive the Order of Merit, and was given the Lenin Peace Prize in recognition of her efforts to bring together scientists from the East and West in pursuit of nuclear disarmament.With Georgina Ferry Science writer and biographer of Dorothy HodgkinJudith Howard Professor of Chemistry at Durham UniversityandPatricia Fara Fellow of Clare College, CambridgeProducer: Simon Tillotson

Melvyn Bragg and guests discuss how scientists sought to understand the properties of gases and the relationship between pressure and volume, and what that search unlocked. Newton theorised that there were static particles in gases that pushed against each other all the harder when volume decreased, hence the increase in pressure. Those who argued that molecules moved, and hit each other, were discredited until James Maxwell and Ludwig Boltzmann used statistics to support this kinetic theory. Ideas about atoms developed in tandem with this, and it came as a surprise to scientists in C20th that the molecules underpinning the theory actually existed and were not simply thought experiments. The image above is of Ludwig Boltzmann from a lithograph by Rudolf Fenzl, 1898With Steven Bramwell Professor of Physics at University College LondonIsobel Falconer Reader in History of Mathematics at the University of St Andrewsand Ted Forgan Emeritus Professor of Physics at the University of BirminghamProducer: Simon Tillotson

Melvyn Bragg and guests discuss theories about the origins of teeth in vertebrates, and what we can learn from sharks in particular and their ancestors. Great white sharks can produce up to 100,000 teeth in their lifetimes. For humans, it is closer to a mere 50 and most of those have to last from childhood. Looking back half a billion years, though, the ancestors of sharks and humans had no teeth in their mouths at all, nor jaws. They were armoured fish, sucking in their food. The theory is that either their tooth-like scales began to appear in mouths as teeth, or some of their taste buds became harder. If we knew more about that, and why sharks can regenerate their teeth, then we might learn how humans could grow new teeth in later lives. With Gareth Fraser Assistant Professor in Biology at the University of FloridaZerina Johanson Merit Researcher in the Department of Earth Sciences at the Natural History Museumand Philip Donoghue Professor of Palaeobiology at the University of BristolProducer: Simon Tillotson

Melvyn Bragg and guests discuss how members of the same species send each other invisible chemical signals to influence the way they behave. Pheromones are used by species across the animal kingdom in a variety of ways, such as laying trails to be followed, to raise the alarm, to scatter from predators, to signal dominance and to enhance attractiveness and, in honey bees, even direct development into queen or worker. The image above is of male and female ladybirds that have clustered together in response to pheromones. With Tristram Wyatt Senior Research Fellow at the Department of Zoology at the University of OxfordJane Hurst William Prescott Professor of Animal Science at the University of Liverpooland Francis Ratnieks Professor of Apiculture and Head of the Laboratory of Apiculture and Social Insects at the University of SussexProducer: Simon Tillotson

Melvyn Bragg and guests discuss the remarkable achievement of Aristotle (384-322BC) in the realm of biological investigation, for which he has been called the originator of the scientific study of life. Known mainly as a philosopher and the tutor for Alexander the Great, who reportedly sent him animal specimens from his conquests, Aristotle examined a wide range of life forms while by the Sea of Marmara and then on the island of Lesbos. Some ideas, such as the the spontaneous generation of flies, did not survive later scrutiny, yet his influence was extraordinary and his work was unequalled until the early modern period.The image above is of the egg and embryo of a dogfish, one of the animals Aristotle described accurately as he recorded their development.WithArmand Leroi Professor of Evolutionary Development Biology at Imperial College LondonMyrto Hatzimichali Lecturer in Classics at the University of CambridgeAndSophia Connell Lecturer in Philosophy at Birkbeck, University of LondonProducer: Simon Tillotson

Explore the life of Emmy Noether, a renowned mathematician of the 20th century. Learn about her groundbreaking contributions to subatomic physics and her connection between conservation laws and symmetry. Discover her impact on the understanding of physics and mathematics, and her resilient legacy in teaching and theoretical physics.