# Why Time Travel Is So Hard?

The notion of time travel is rooted in the early 20th century physics of Albert Einstein.  In the 1880s, two American scientists, Albert Michelson and Edward Morley set out to measure how the speed of light was affected by Earth’s motion through space. They discovered, to their amazement, that the speed of Earth made no difference to the passage of light through space (which they called the ether). No matter how fast you travel, the speed of light remained the same.

How could this be? Surely if you were travelling at half the speed of light and the beam from a torch passed you, the speed of the light from the torch would be seen as travelling slower than if you were stationary. The answer is definitely no! The speed of light is ALWAYS 300 million metres per second.

Einstein gave careful consideration to the observation. If light is always measured to be the same velocity, no matter how fast you are travelling, then something else must be changing to accommodate the speed of light. In 1905, Einstein formulated a theory he called ‘Special Relativity’ and described that ‘something else’ as ‘spacetime’. In order for light to remain constant, space and time (hence ‘spacetime’) have to vary and so they must be inextricably linked. It’s an extraordinary conclusion – but one that has passed every conceivable test for almost 100 years since it was proposed. Quite simply, as you approach light speed, time runs slower and space shortens.

The result is often referred to as the ‘twin paradox’. Imagine two twins on Earth. One takes a trip into space on a rocket that propels him very close to the speed of light. The other remains on Earth. For our space-travelling twin, time starts to slow as he reaches a speed very close to that of light. However, for the twin on Earth, time passes normally. Finally, when our space-travelling twin returns home years later, a strange thing has happened, his Earth-bound twin has gone grey. He has aged normally since his time was not slowed. Welcome to the realm of time travel.

There is a set of mathematical formulae known as the Lorenz transformation equations that can calculate the so-called ‘time dilation’ as one approaches the speed of light. All you simply do is plug in your speed and how long you travel for and bingo! You can calculate how much more time will pass on Earth while you are off gallivanting around the universe.

‘Ah! Fantastic,’ I hear you cry, ‘time travel is easy!’ All we need to do to travel into the future is leap on a spacecraft that travels at the speed of light, so that time onboard stops compared to time on Earth. And if we want to travel into the past, all we need do is put our foot down and accelerate beyond the speed of light, that way our clock would actually start running backwards.

Well, unfortunately it isn’t quite that easy. Einstein predicted with Special Relativity that nothing could travel faster than the speed of light. Why? It’s to do with mass. As you approach the speed of light, your mass increases. This is because space is shortening – it’s squeezing up, so to speak, and so any matter is also squeezed. Quite simply, the faster you go the more squeezing takes place and the heavier you become. And the heavier you become the more force you need to move; or to put it another way, the harder you have to push to go faster. At the speed of light your mass becomes infinite, and so does the energy required to push you. Sadly then, you’ll never travel at or faster than the speed of light.

While travelling at a velocity close to the speed of light (say 95% as in our example above) will allow you to travel into the future, travelling into the past seems impossible. But is it?

Special Relativity is so called because it deals with the special circumstance of what happens to mass, space and time when travelling in a straight line at a constant speed – it doesn’t take into consideration gravity and acceleration. In 1915, Einstein published his thesis on ‘General Relativity’. General Relatively does take into consideration gravity and acceleration.

Though Sir Isaac Newton discovered gravity, he couldn’t explain it. Einstein does explain it, within the formulae of General Relativity. This too has important consequences for time travel. Einstein proposed that mass distorts space, or more correctly the fourth dimension of spacetime. It’s simple to visualize if you think of space as a flat rubber sheet, like a trampoline. Placing a bowling ball on the trampoline will cause it to sag and become curved. The bowling ball distorts our two-dimensional rubber sheet. This is exactly what mass does to space.

The Sun is massive and so it curves space. Anything that passes close to the Sun will be affected by the curve, just as a marble rolled along our trampoline will be affected by the depression made by the mass of the bowling ball. The essence of General Relativity is that mass tells space how to bend and space tells matter how to move. The mass of the Sun, a planet or indeed any mass in the universe can bend the fabric of space. Furthermore, the greater the mass, the greater the distortion of space. This is important for time travel – because space and time are related, if you change one, you change the other. So if you bend space then you will bend time!

It’s apparent then that to bend time you must bend space – and to bend space you need a mass. Black holes are the biggest objects in the universe. They form when massive stars collapse at the end of their lives. They are thought to be ubiquitous within our galaxy. Black holes are so massive that they distort space to such a degree not even light can escape. Perhaps then, black holes can distort spacetime in such a way that we could move through them and end up in another place at another time.

Unfortunately not. As astronomer and leading black hole expert Kip Thorne pointed out, though travelling into a black hole does slow time, any traveller would be destroyed by any of a number of processes. In particular, by little bits of space matter that the black hole swallows and accelerates to the speed of light, increasing their mass incredibly, and then rains down on any spaceship destructively.

But Kip has had another idea pertaining to time travel. In the 1980s, astronomer Carl Sagan phoned Kip Thorne for help. Sagan was writing his novel, Contact, and needed a scientifically viable way for his heroine to travel to the star Vega. Vega is some 26 light-years from Earth, so even travelling at the speed of light (which we know is impossible) it would take 26 years to reach the star. Sagan wanted his heroine to get there in a matter of hours. Thorne confirmed Sagan’s suspicions that a black hole wouldn’t work. But he came up with another suggestion – the wormhole.

A wormhole is a theoretical shortcut for travel between two points in the universe. It works like this. Imagine the universe to be two-dimensional (like the rubber sheet below). The quickest way to get from Earth to Vega (both of which lie on the rubber sheet) is to travel in a straight line between the two. Right? Wrong! If our rubber sheet happened to be folded over so that Vega and Earth are opposite each other, Earth lying above Vega, then the quickest way would be to bridge the gap between them by burrowing a hole (the wormhole) from Earth down to Vega. This wormhole may only be a few kilometres in length, yet it would cut out the 26 light-year distance along the surface of our rubber sheet.

The implications of wormholes for time travel are astounding. If you can travel from Earth to Vega in just minutes you are effectively travelling faster than the speed of light. Light takes 26 years going the slow way through spacetime, while you just nip through the hyperspace between in minutes. Anyone who travels faster than light has the ability to travel back in time.

While wormholes may exist in the sub-atomic (quantum) realm, getting one to form, grow large enough, and remain open long enough so we may use it is a formidable task and one for a very distant civilisation.

The truth about time travel is that while, rather excitingly, current physics does allow time travel, albeit with a number of caveats, the physical practicality seems ages away. But once mastered, perhaps this famous limerick, will become reality. Only time will tell.