If time is one of the four dimensions of the Universe, why can we only travel forward in time?
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It’s true that we live in four dimensions, with three dimensions of space and one of time.
However, the time dimension is different to the three space dimensions because of the way we choose to define it and the way the Universe is constructed.
Nevertheless, one of the remarkable features of physics is that travel into the past does appear to be possible.
A wormhole could make backwards time travel possible. Credit: Mark Garlick / Science Photo Library / Getty Images
According to Einstein, time flows more slowly in stronger gravity.
Imagine two people, one on Earth and the other near a black hole, where time flows more slowly because of the stronger gravity.
We view them on Monday, but by the time the person on Earth reaches Friday, the person near the black hole has only reached Wednesday.
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If there was a bridge between the two – and Einstein’s theory permits one known as a wormhole – it would be possible for the person on Earth to travel back from Friday to Wednesday.
Marcus Chown is an award-winning writer and broadcaster and a former radio astronomer at the California Institute of Technology in Pasadena. He is the author of Breakthrough: Spectacular stories of scientific discovery from the Higgs particle to black holes (Faber & Faber, 2021).
