Wonders of the Universe. Andrew Cohen

and up/down, as he added a fourth direction – past/future. Hence spacetime is referred to as four-dimensional, with time being the fourth dimension.

      A full explanation of this is beyond the scope of this book, suffice to say that Einstein was forced into this bold move primarily because Maxwell’s equations for electricity and magnetism were incompatible with Newton’s 200-year-old laws of motion. Einstein abandoned the Newtonian ideas of space and time as separate entities and merged them. In Einstein’s theory there is a special speed built into the structure of spacetime itself that everyone must agree on, irrespective of how they are moving relative to each other. This special speed is a universal constant of nature that will always be measured as precisely 299,792,458 metres (983,571,503 feet) per second, at all times and all places in the Universe, no matter what they are doing. This is critical in Einstein’s theory because it stops us doing something strange in spacetime; if past/future is simply another direction like north/south, why can’t we wander backwards and forwards in it? Why can we only travel into the future, not the past?

      In Einstein’s theory of relativity it is the existence of this unanimously agreed special speed that makes time direction different to that of space and prevents time travel. In this sense, the special speed is built into the fabric of space and time itself and plays a deep role in the structure of our universe. What does it have to do with the speed of light? Nothing much! There is a reason why light goes at this speed, and it seems to be a complete coincidence. In Einstein’s theory, anything that has no mass is compelled to travel at the special speed through space. Conversely, anything that has mass is compelled to travel slower than this speed. Particles of light, photons, have no mass, so they travel at the speed of light. There is no deep reason we know of why photons have to be massless particles, so no deep reason why light travels at the speed of light! We only call the special speed ‘light speed’ because it was discovered by measuring the speed of light.

      The key point is that the speed of light is a fundamental property of the Universe because it is built into the fabric of space and time itself. Travelling faster than this speed is impossible, and even travelling at it is impossible if you have mass. It is this property of the Universe that protects the past from the future and prevents time travel into the past

TIME TRAVEL

      Without realising it, we are all travelling back in time by the most miniscule amount. The consequence of light travelling fast, but not infinitely fast, is that you see everything as it was in the past. In everyday life the consequences of this strange fact are intriguing but irrelevant. It may be strictly true that you are seeing your reflection in the mirror in the past, but since it takes light only one thousand millionths of a second to travel thirty centimetres (twelve inches), the delay is all but invisible. However, the further away we get from an object, the greater the delay becomes. Although over tiny distances the effect is always utterly negligible, it should be obvious that once we lift our eyes upwards to the skies and become astronomers, profound consequences await us.

      A rare sight; in this picture Earth’s crescent moon is visible above Venus (bottom) and Jupiter (right) in the night sky. As light takes longer to reach Earth from other planets and moons, depending on how far away they are, we see further into their respective pasts.

       © JASON REED/Reuters/CORBIS

      Look up at the Moon and you are looking at our closest neighbour a second in the past, because it is on average around 380,000 kilometres (236,120 miles) away; perceptible certainly, but not important. However, take a look at the Sun and you really are beginning to bathe in the past.

      The Sun is 150 million kilometres away (93 million miles) – this is very close by cosmic standards, but at these distances the speed of light starts to feel rather pedestrian. We are seeing the Sun as it was eight minutes in the past. This has the strange consequence that if we were to magically remove the Sun, we would still feel its heat on our faces and still see its image shining brightly in the sky for eight minutes. And because the speed of light is actually the maximum speed at which any influence in the Universe can travel, this delay applies to gravity as well. So if the Sun magically disappeared, we would not only continue to see it for eight minutes, we would continue to orbit around it too. We are genuinely looking back in time every time we look at the Sun.

      However, this is just the beginning of our time travelling. As we look up at the planets and moons in our solar system, we move further and further into the past. The light from Mars takes between four and twenty minutes to reach Earth, depending on the relative positions of Earth and Mars in their orbits around the Sun. This has a significant impact on the way we design and operate vehicles intended for driving on the surface of Mars. When Mars is at its furthest point from Earth it would take at least forty minutes to be told that a Mars Rover was driving over a cliff and then be able to tell it to stop, so Mars Rovers need to be able to make up their own minds in such situations or must do things very slowly. Jupiter, at its closest point to Earth, is around thirty-two minutes away, and by the time we journey to the outer reaches of our solar system, the light from the most distant planet, Neptune, takes around four hours to make the journey. At the very edge of the Solar System, the round-trip travel time for radio signals sent and received by Voyager 1 on its journey into interstellar space is currently thirty-one hours, fifty-two minutes and twenty-two seconds, as of September 2010.

      But look beyond our solar system and the time it takes for light to travel from our nearest neighbouring stars is no longer measured in hours or days, but years. We see Alpha Centauri, the nearest star visible with the naked eye, as it was four years in the past, and as the cosmic distances mount, so the journey into the past becomes ever deeper

TO THE DAWN OF TIME

      When filming a series like Wonders of the Universe, the locations are chosen to be visually spectacular, but they must also have a narrative that enhances the explanation of the scientific ideas we want to convey. Occasionally, the locations deliver more. There is a resonance, a symbiosis between science and place that serves to amplify the facts and generates something deeper and more profound on screen. For me, the Great Rift Valley was such a place.

      We arrived in Tanzania on 10 May 2010 for the first day of filming. After a brief overnight stay close to the airport at Kilimanjaro, we were driven out into the Serengeti in vintage dark green Toyota Land Cruisers, complete with exaggerated front cattle bars and shovels tied to the rear doors. The landscape is unmistakably African; the warm, damp light still wet from the rains illumines plains seemingly too vast to fit on our planet. The horizon, darkened by scattered thunderclouds stark against the early summer skies, is simply more distant than it should be. The rains have brought with them journeys, and as you drive you experience first-hand the thousand-mile migration of the Serengeti wildebeest. The relentless advance of these herds creates ruts in the drying savannah along the precise and ancient roads that always seem to run at right angles to your direction of travel, shaking the Land Cruisers to the edge of their design tolerance. Zebras, giraffe and Grant’s gazelles graze, unconcerned, as our intrepid film crew rattles by.

      The Great Rift Valley is not just an extraordinary geological feature…there is more to this place because the echoes of the history of humanity ring louder across these plains than anywhere else on the planet.

      The Great Rift Valley, Tanzania, is one of the most spectacular geological locations on Earth. The summer skies were darkened by rainclouds, but these soon departed to reveal dusty, unmistakably African landscapes and breathtaking vistas.

      The Great Rift Valley, Tanzania, is one of the most spectacular geological locations on Earth. The