There has been a break in the videos and missives of the TikTok time travelers whose apocalyptic warnings and upcoming events crumble as the dates arrive, so this is a great time for a real physicist to weigh in on the science of time travel. An in-depth article in The Conversation by Barak Shoshany, an Assistant Professor of Physics at Ontario’s Brock University, has been making the rounds this week for a good reason – Professor Shoshany declares that backwards time travel is definitely possible and you can step on a butterfly, kill your grandfather, marry your mother, destroy the time machine and do all of those other paradoxical things without harm … but with one important ‘if’.
“The stomp of your foot, on one mouse, could start an earthquake, the effects of which could shake our earth and destinies down through time, to their very foundations.”
-- from “A Sound of Thunder” by Ray Bradbury
As with all time travel discussions, Shoshany starts with Einstein’s theory of relativity and the fact that one can create time travel equations that are compatible with relativity, but going from relativity to reality is the hard part. A physical time machine needs negative energy – which can theoretically be created with quantum mechanics but the limit to minute quantities – and that ‘theoretical’ part – are huge hinderances. However, possible means possible, according to Shoshany.
“I guess you guys aren’t ready for that yet. But your kids are gonna love it.”
-- from “Back to the Future”
In “Back to the Future,” Marty McFly seemingly steps on a butterfly when he plays “Johnny B. Goode” at a dance before Chuck Berry even wrote it, yet nothing cataclysmic seemed to happen in the future as a result. Shoshany points out that a paradox is a purely theoretical concept that points towards an inconsistency in the theory itself – a consistency paradox like the butterfly effect “don’t merely imply time travel is a dangerous endeavor, they imply it simply cannot be possible.” Stephen Hawking supported this idea but could never prove it. Shoshany says one way around this is to declare that you can go back in time but you can’t change it. He seems to think that this is not only boring science fiction but no fun. It’s also not necessary.
“The idea is very simple. When I exit the time machine, I exit into a different timeline. In that timeline, I can do whatever I want, including destroying the time machine, without changing anything in the original timeline I came from. Since I cannot destroy the time machine in the original timeline, which is the one I actually used to travel back in time, there is no paradox.”
The big “If” in Shoshany’s time travel solution is “If history can be split into multiple histories.” A side “if” is “and if our universe allows it.” He believes that “General relativity and quantum mechanics tell us that time travel might be possible, but if it is, then multiple histories must also be possible.”
What do you think of this ‘solution’ to the possibility of time travel? It’s certainly not a proof – Shoshany admits his next step is to find a concrete theory of time travel with multiple histories that is fully compatible with general relativity. While that is still not proof of time travel, it is proof that consistency paradoxes cannot prevent it.
Shoshany didn’t kill the butterfly or his grandfather … just a lot of time travel movies.