The recent 50th anniversary of Doctor Who makes it seem only fitting that we have a chat about time travel.
The Doctor manages it with the help of his TARDIS, some amazing, high budget sound effects and a fancy bow-tie, but can it actually be done?
'Is time travel possible?'
The short answer is yes and no. There are flaws in time travel for creatures as large as we. Were we smaller, smaller than an atom, we might be able to utilise the many tiny wormholes that reside within the nooks and crevices of time in a place called the quantum foam. Just as physical objects aren't as smooth or flat as we perceive them to be, time is also full of these gaps and creases. It's here that the quantum foam exists. Wormholes can connect not only two different points in space, but two different points in time. Trouble is they're only one billion-trillion-trillionths of a centimetre across and extremely short-lived, forming, collapsing and then reforming within the foam, which means they're pretty useless for human transportation.
'Well, what if we somehow made one bigger?'
One of the primary answers to this, apart from not having a wormhole enlarger, is positive feedback. It works in much the same way as a microphone causes feedback: the sound enters the microphone and loops through the system and amplified sound shouts from the speakers. The speaker sound is then picked up by the microphone, creating a positive feedback loop whereby the noise continues to circulate, getting louder and louder each time until a hero moves the microphone or the sound system explodes. The process is similar with the embiggening of a wormhole; instead of sound, a radiation feedback loop is formed which would possibly tear the wormhole apart. So enlarging tiny wormholes is probably, for the time being, a no-go.
'What about some sort of ship or device then, like the TARDIS?'
You'll be pleased to know that we have already built a time travel device of sorts: the Large Hadron Collider in Switzerland. Although you won't be able to buy a ticket to the future, scientists have shown that the speeds at which particles are moved at within the LHC (close to the speed of light) allow for the slowing of time for the particle relevant to the passing of time for the spectator. Pions have a long and luxurious life of approximately 26 nanoseconds – 26 billionths of a second. But within in the dizzying spin of the LHC, the pions get close to the speed of light and time for the pion slows relative to the scientists observing it and we perceive their decay to last 30 times longer.
'So all we need to do is go really fast then?'
Really, really fast. Time dilation as described by Einstein's theory of relativity means that the passing of time between two events isn't constant from one observer to another. Rather, it depends on the relative speeds of the observers' reference frames. That is to say, that time moves at what seems a correct speed for both observers, but they will see their counterpart's passage of time as moving faster or slower, depending on differing effects. A good example of this is the Twin paradox (see below). This thought experiment involves a set of identical twins, one who takes a journey in a space ship travelling near the speed of light. The space-faring twin returns to Earth years later to find that the other twin has aged considerably. Both twins have witnessed the passing of time, but time for the twin in the space ship passed slower in comparison to the twin on Earth due to the speed at which they were travelling. Voila! Welcome to the future.
The problem is going back. Travelling at speeds close to the speed of light, it's possible to travel into the future. But there's no way to reverse that method to take you into the past (if you could go faster than the speed of light you could potentially go back in time, but that would be breaking the universal speed limit and is a story for another time). Wormholes could possibly take you back but we've already discussed them and death by excessive radiation is painful. I'm assuming being crushed inside a wormhole is equally unpleasant.
Additionally, there is the problem of paradox. The most famous one is the Grandfather Paradox where you go back in time and shoot your own grandfather before your father was ever conceived. Therefore, your father would never be born, so neither would you. So who's shooting your grandfather then, hmm? A simpler example is you go back one minute in time and shoot yourself. Same paradox. An inability to travelling backwards in time is good for preventing paradoxes but bad for getting home from the future. For now, potential time travel seems to be a one way ticket. It also means Marty McFly would never have been able to play matchmaker to his own parents, which is perhaps a more sobering revelation than it should be.
This is only scratching the surface and it seems that despite a Time Lord's ability to meander through the wibbly wobbly timey wimey stuff we humans haven't the capacity to do it ourselves, no matter how fancy you look in a fez. But the possibility of some sort of time travel is there, so keep dreaming.