Harry T. Jones explores the establishment of time travel in fiction and fact.
“People assume that time is a strict progression of cause to effect, but actually, from a non-linear, non-subjective viewpoint, it’s more like a big ball of wibbly wobbly, timey wimey stuff.” – The Doctor
These words were famously uttered by David Tennant’s Tenth Doctor, but they do little to clarify the physics of space-time and how we may use such knowledge to travel through time. While open to interpretation, my partial understanding of the phrase is that from a perspective outside our own experience of space-time, different points in time may be viewed simultaneously rather than one after another, or sequentially in the order in which they occur (as we experience them). So, could we use the underlying principle of such a perspective, that the past and future tangibly exist in a form we just can’t access yet, to travel into another time?
Although the concept of time travel has been implied in works dating as far back as the 9th century BCE (namely in the Hindu Mahabharata), it was only popularised in 1895 with H.G. Wells’ novel The Time Machine. This story not only exemplified the use of a machine to travel through time, but also demonstrated the ability to move forward and backward in time with intent. Time machines have since been ingrained in pop culture, including icons such as the TARDIS (Doctor Who) and the DeLorean (Back to the Future). Such machines are fictional, but some theoretical physicists postulate that specific circumstances could enable time travel.
Albert Einstein was amongst the first to scientifically discuss how time could be experienced differently. His 1905 theory of special relativity proposed that measurements of space and time vary, depending on the relative speed at which the measurer is travelling. The theory posits that as a traveller’s speed approaches the speed of light (299,792 km/s), time for the traveller starts to slow down, meaning that time appears to move more quickly for everything else. This idea is termed ‘velocity time dilation’. Assuming we could reach anywhere near such great speed, this could be one way of travelling into the future more quickly than our current rate of one second per second.
German mathematician Hermann Minkowski later used Einstein’s theory to link the three dimensions of space to time, a fourth dimension, in the single four-dimensional concept of space-time. This model of the Universe is the one we use today, and it led into the second part of Einstein’s relativity theory in 1915: general relativity. This theory views gravity as a warping of space-time created by massive objects, such as planets. The greater an object’s mass, the stronger the gravity. The stronger the gravity, the more slowly time moves (compared to further away from the gravity source). This effect is termed ‘gravitational time dilation’. It explains why travelling past objects with strong gravitational fields causes a relative slowing in time for the traveller, as seen in the film Interstellar, where one hour on a planet near a black hole equalled seven Earth years.
Einstein theorised another temporal disruption in the form of wormholes: folds in space-time that produce a tunnel-like shortcut between two different points in space-time. Unlike black holes, wormholes have not been observed in space hence remain theoretical to this date. Even if it were possible to reach/create a wormhole for time travel, there remains the problem of creating a machine to survive the journey through one, as well as the uncertainty of when and where it may lead.
While theoretically possible through a traversable wormhole, travelling into the past is less probable than travelling to the future. Moving faster than light would produce the effect of moving backwards in time, but it is physically impossible for an object to exceed the speed of light (the universal speed limit) as that would require infinitely high energy. Hypothetical particles called tachyons are believed to always travel faster than light, so perhaps they could be harnessed to send signals, if not people, back in time? Such particles warrant further investigation, as does a long-sought unified theory that couples general relativity with quantum mechanics – a link that may reveal new insights into space-time.
The logistical impacts of backward time travel remain contentious. Most famously, the ‘grandfather paradox’ theorises someone travelling back in time then killing their young grandfather, averting the traveller’s mother/father’s birth, thus preventing the traveller from ever existing. If the traveller never existed, how could they have killed their grandfather? Stephen Hawking cited this paradox as evidence of backward time travel being impossible, in a rule termed the ‘chronology protection conjecture’. Other theories suggest the Universe may somehow compensate for the grandfather’s death, ensuring the time traveller’s existence, or that the death creates a new timeline that branches off to create a parallel Universe, as suggested in Avengers: Endgame.
In conclusion, time travel is likely to remain confined to works in theoretical physics and science-fiction for the foreseeable future, but is a subject that will undoubtedly be further explored. Time is an intriguing concept, yet its ‘wibbly wobbly’ nature (through space-time disturbances/dilations) may have applications that help mankind travel throughout the stars.
From Issue 22: the Dark Side of Science