Time travel is a staple of science fiction, but what does physics have to say about the dangers of altering the past? In shows like Doctor Who, characters often face catastrophic consequences when they interfere with history. However, physicists argue that such paradoxes are not actually possible.
According to theoretical physicist Barak Shoshany, a paradox indicates a flaw in our theory rather than a real event. 'A paradox isn't really something you can create,' he says. Instead, if a theory predicts a paradox, that theory must be incorrect. There are multiple ways to resolve time travel paradoxes, ranging from parallel universes to quantum phenomena, but the physics of time ensures we cannot create contradictory situations.
The simplest solution is that time travel to the past is impossible. Our experience of time is one-directional, moving from Monday to Tuesday without ever going back. While space allows movement in any direction, time seems to flow only forward. This 'arrow of time' may be a fundamental property of the Universe, but quantum mechanics challenges this view.
Quantum physicist Vlatko Vedral notes that on quantum scales, processes can be reversed. 'If you can excite an atom from one state to another, then quantum mechanics says you should be able to, with 100% efficiency, go backwards.' This suggests that time's arrow may not be fundamental but emerges from other phenomena. Cosmologist Katie Mack adds that space and time might be emergent properties, possibly even illusory, with a more fundamental mathematical framework underlying the Universe.
Ludwig Boltzmann's explanation for irreversibility in large-scale processes, such as a coffee cup shattering, points to the statistical likelihood of disorder increasing. Astrophysicist Emma Osborne illustrates this with headphones: 'It takes more energy to untangle your headphones, whereas there's infinite ways they can be tangled.' This concept helps explain why time seems to move forward, even if time travel remains a theoretical puzzle.
