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By Root 775 0

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