An experiment to send a particle back in time could transform our understanding of the subatomic realm, finds Miriam Frankel
WHEN Seth Lloyd first published his ideas about quantum time loops, he hadn’t considered all the consequences. For one thing, he hadn’t anticipated the countless emails he would get from would-be time travellers asking for his help. If he could have his time over again, he jokes, he “probably wouldn’t have done it”.
Sadly, Lloyd, a physicist at the Massachusetts Institute of Technology, won’t be revisiting years gone by. Spoiler alert: no one will go back in time during the course of this article. But particles? That is another matter.
Theoretical routes to the past called time loops have long been hypothesised by physicists. But because they are plagued by impracticalities and paradoxes, they have been dismissed as impossible for just as long. But now Lloyd and other physicists have begun to show that in the quantum realm, these loops to the past are not only possible, but even experimentally feasible. In other words, we will soon effectively try to send a particle back in time.
If that succeeds, it raises the possibility of being able to dispatch, if not people, then at least messages in the form of quantum signals, back in time. More importantly, studying this phenomenon takes us to the heart of how cause and effect really work, what quantum theory means and perhaps even how we can create a successor theory that more fully captures the true nature of reality.
In physics, time loops are more properly known as closed time-like curves (CTCs). They first arose in Albert Einstein’s theory of general relativity, which says space-time can bend. Hypothetically, if you could curve it enough, it would close in on itself, creating a pathway to the past. The only fly in the ointment is that generating such extreme curvature would require something with a heck of a lot of mass rotating very fast. In practice, that probably means a black hole – not exactly something we could create in a lab.
Then again, all of that applies to relatively large time loops, the sort of thing a human could – in principle – jump into. But what about something that instead operates at the tiniest scales? Breakthrough experiments in quantum mechanics, the strange set of laws that governs reality at the subatomic level, have shown we could conceivably make the mathematical equivalent of a time loop in this domain. It would be called a quantum CTC.
For a long time, physicists snorted at the idea of a time loop in the quantum realm, largely because it isn’t compatible with the way time is thought to operate in this framework. Time is believed to work startlingly differently in quantum mechanics compared with general relativity. Indeed, this disjoint is one of the biggest hurdles physicists face in trying to unite relativi
