Could we really travel through time? The short answer is yes! That’s right. We all have the ability to leap forward through time, even if we don’t realize it. If you wanted to jump to the future, you can simply go to sleep and wake up a few hours later right? Of course that’s cheating the question. We want to be able to go back and redo those silly mistakes we made many years ago. who doesn’t wish they could go back and talk to their 15-year-old self? Tell them not to make the mistakes you did (or they will). Or perhaps you’d rather travel 500 years into the future and see those flying cars we were promised by the year 2000. Fortunately, time travel is theoretically possible.
In fact there is no law in physics that prevents time travel. That’s right, according to all the laws of physics we know, it’s perfectly plausible to travel through time at will. But as the saying goes, with great power comes great responsibility. Time travel is an extremely dangerous endeavor with devastating consequences, and is also riddled with paradoxes.
For example, what if I traveled back in time and prevented World War II? Sounds like a brilliant idea right? Countless lives would be saved, I’d be hailed as a hero! Not necessarily. Although I’d be saving lives, I’d also be destroying others. What about all the technology that we rely on today that was developed during the war, such as jet engines and nuclear power. In fact the world map could be completely different. We could actually be worse off than simply leaving history as it was.
Another famous time travel paradox is the grandfather paradox, which basically states that if we were to say for instance, I travel back to a time before my parents were born and prevent my grandfather from meeting my grandmother. My parents couldn’t have met and thus, I would never have been born. So how could I have gone back in time to prevent my grandparents from meeting in the first place.
So we can see that time travel could be a bad idea, but let’s say we really want to go back in time, how would we do it? Well first we need to understand how time works.
Time is something we are all very familiar with, we all know what it is, but yet we can’t see it, touch it, we can’t seem to interact with it in any way, we can only observe it. Isaac Newton thought that time was constant and never deviated, which would of course make time travel impossible. Even Einstein believed it was impossible, yet it is his equations that make it possible. Einstein theorized that space and time are inexplicably linked in what he refers to as “space-time”. So in theory, if I was to warp space with something extremely powerful like a black hole, I would also be warping time. While this appears to be true and scientists continue to explore its possibilities, the real possibility for time travel, seems to be in his other theory; Relativity. In fact time travel using relativity isn’t just a theory, it’s actually been done, several times! Now you probably think I’m crazy but the secret seems to be in going extremely fast.
According to Einstein’s theory of general relativity, no object with mass may travel faster than the speed of light, which is an astonishing 299,792,458 metres per second in a vacuum (that’s no air). That’s an incredible 1,080 million kilometres an hour! So according to General Relativity, we can only travel at 99.99% the speed of light. But lets say for instance that I’m sitting at the back of a plane travelling at the speed of light and I walk to the front of the plane from the back at a rate of 10 kilometres an hour. I’ll leave the equations out of this, but that would mean that my speed plus the speed of the plane would mean that I’m travelling at 1,080 million and ten kilometres an hour, which is 10km/h faster than light, right? Wrong. According the theory of relativity, time would actually slow down for me to prevent me from travelling faster than light. Sounds weird doesn’t it.
Of course this scenario would never happen in real life, as I have already said we can’t travel at or faster than the speed of light. In fact you would need more than an infinite supply of fuel to travel faster than light, which is obviously impossible.
So then what is relativity? Einsteins theory of relativity basically states that if I’m driving down the road at 50km/h and drive past someone standing on the side of the road, to them I would appear to be travelling at 50km/h, I’m sure you already know this. However to me in the car, they too would travel past me at 50km/h. But then if someone where to drive beside me in another car travelling at 50km/h, to them I would appear to be completely stationary. Hard to believe? It’s the same thing that makes it look the car driving beside you on the highway sometimes seems to be floating just outside your window. I’ll leave the complicated equations out of this for now, but you can use simple addition and subtraction to confirm this theory. If you’re travelling in the same direction as the object you’re observing, in this case the other car, you subtract your speed from theirs. So 50km/h minus 50km/h is equal to 0km/h. so they would appear to be stationary. If we’re traveling in the opposite direction we add our speed to theirs. So 50km/h plus another 50km/h is equal to 100km/h. So if we drove past the other car at the same speed in the opposite direction, they would appear to be travelling at 100km/h.
So how does all this work, and more importantly, how did they actually manage to travel through time? Well as it turns out, the faster we travel, the slower time will pass. This was proven when scientists placed an atomic clock, which is an extremely accurate clock down to one billionth of a second, on the space shuttle and observed it its behavior. Before launch the clock was perfectly synced with another atomic clock here on earth, and once the shuttle had returned to earth they put the two clocks together. They then discovered the clock from the shuttle was slightly behind the clock that remained on earth. meaning time had past slower for the clock on the shuttle than the clock that remained on Earth.
So basically astronauts are in fact also time travellers. Sergei Krikalev, the current record holder for the longest time spent in space (about 804 days or 2.2 years) is actually half a second behind the rest of the world. furthermore, scientists have actually calculated that if we could orbit the Earth at 99.99% the speed of light for 7 whole years, we would actually land back on earth 500 years in the future. This effect is known as Time Dilation. What a spectacular realisation! Time travel may actually be within our reach after all. The only problem is inventing a machine or vehicle capable of travelling that fast. Unfortunately it looks like that might be just out of our reach for now. We would actually need the power of a whole star to get to that speed.
But what about travelling back in time? So far neither general or special relativity allow for travelling backwards in time and it appears as though no other laws of physics will allow travel to the past. Such travel to the past, as stated above would lead to paradoxes that could result in the collapse of an entire universe, or a completely new one to be formed. This is part of the Many World Interpretation of Quantum Theory. There are other theories to travel both forward and backwards in time such as black holes and wormholes, but I’ll discuss those in the future.
So for now it looks like we could theoretically travel to the future, but unfortunately we can’t go back and tell our younger self not to buy that car from that dodgy looking salesman, or to buy stocks in Apple and Microsoft when they first start out. So we’ll just have to settle for travelling forwards in time at the same rate we always have, and just to wait to see what the future holds.
Daral is an Australian based physicist specialising in Astrophysics and Quantum Theory. He has a keen interest in the theoretical side of these topics and writes frequently to share his knowledge with those who are not so mathematically minded.