October 25, 2021

I, Science

The science magazine of Imperial College

Universe-ending paradoxes aside, Andrew McMahon discusses what physics has to say about time machines

Darren Tunnicliff Time 1024w 4469318003_4ff51615fa_bIn H.G. Wells’s 1895 classic, The Time Machine, a Victorian inventor successfully creates a machine that allows him to navigate the ‘fourth dimension’, almost as easily as we do normal 3D space. The protagonist then travels to the year 802,701 AD, confronts a dystopian future where humanity consists of two warring factions and then returns to his own time to share the tale. Wells’s novel has inspired countless works of science fiction, from Doctor Who to episodes of Star Trek. But was there any scientific substance to what he was positing? Could human beings one day travel through time?

What is fascinating is that the first leg of the time traveller’s journey is completely consistent with our understanding of modern physics. Einstein’s theory of special relativity, published in 1905, tells us that if an object is moving close to the speed of light (at relativistic speeds), some interesting effects start to take hold. One of these effects is time dilation – the faster an object moves, the slower time passes for it relative to other objects. This is not an illusion, but a fundamental law of nature that is experimentally observed in particle accelerators everyday. To give an example, say I build a train that goes on a track around the Earth, and that this train can move at relativistic speeds. If I get on the train and it zips off at 90% of the speed of light for an hour, as measured by a clock on my wrist, then according to special relativity I may have only felt an hour go by but everyone else on Earth will have aged by approximately 2½ hours. If I travel at 99% of the speed of light for an hour, then everyone else will have experienced approximately seven hours relative to me. What time dilation means then is that if you can travel at higher and higher relativistic speeds you can effectively travel farther and farther into the future. So technically, if the Victorian inventor built something like my relativistic train and if he goes fast enough for long enough, almost no time at all may pass for him and yet the rest of the Earth may age by around 800,000 years. Time travel into the future – easy!

The return leg however, is where we hit a snag or two….

streetwrk Relativity 6948479263_399db1a5fa_oYou travelled 800,000 years into the future, did some sightseeing and now you want to go back and tell all your friends about it. This is where our Victorian inventor would encounter a slight problem. Physics as we currently understand it does not allow for travelling back in time except for in a few very speculative cases. One such option is using a wormhole, a hypothetical tunnel connecting two different points in time and space. Although these objects are considered to be consistent with general relativity, which is Einstein’s extension of special relativity taking gravity into account, the scientific consensus is that they would not occur naturally but would have to be engineered using some pretty hard-to-come-by technology. For example, one theory of how a traversable wormhole might be created says that you would need a Jupiter’s worth of ‘exotic matter’, matter with strange new properties such as negative mass, in order to open one up and keep it open. This kind of manipulation of matter may be possible in the far future, but not now. In terms of backward time travel being a realistic prospect in the near future, therefore, physics seems to tell us that it is not possible.

Other even more speculative theories exist concerning how to travel back in time. The most speculative, and most likely to be impossible is faster than light travel. It can be shown using the equations of relativity that if an object existed which always moved faster than light, such as the hypothetical ‘tachyon’ particle, it would indeed move backwards through time. The reality however seems to be that there is no physical justification for such an object existing and it would be very difficult or impossible to observe experimentally. Even if it did exist, you would not be able to use it to travel through time yourself, since you have mass and relativity forbids you from travelling as fast as light, never mind faster than it! The best you could hope for is to learn how to manipulate these tachyonic particles and send information back through time using them, but even this is doubtful according to current theories. Even if you could send such messages, this is not the kind of time travel that Doctor Who and Star Trek fans will get excited about.

So, what does physics say about travelling around at will in the fourth dimension? Well, it is time travel Jim, just not as we know it.

This is online-only content relating to the Autumn 2014 iScience magazine #29 themed on ‘Time’.  Read the magazine; other online-only content.  Images from Flickr under Creative Commons license: (Top:  …Time… by Darren Tunnicliff; Bottom: Relativity by streetwrk.com)