Suppose you traveled back in time and stopped your grandparents from ever meeting.
This would create a paradox since you never would have been born if your grandparents never met. You've prevented your later birth, so you shouldn't exist anymore.
This is called the "Grandfather Paradox," and it's an infamous one among physicists.
Even though Einstein's famous theory of general relativity actually allows for time travel, the Grandfather Paradox gets in the way. According to Einstein, a gravitational field with enough force (like the one generated by a black hole) could bend space-time enough to fold it back on itself.
This bending could create a path through space-time that returns to its original starting position. It's essentially a time-travel loop. Physicists call it a closed time-like curve, or CTC.
CTCs frustrate physicists because they come with all kinds of paradoxes, like the Grandfather Paradox. If you entered a CTC, traveled back in time and stopped your grandparents from meeting, you would never come back out of that CTC. The principles of cause and effect collapse.
Einstein's predicted CTCs are part of our conventional understanding of physics, but could never allow for time travel without a paradox.
Enter quantum mechanics.
While Einstein's general relativity describes the macro world, like planets and galaxies, quantum mechanics describes the micro world of things like atoms and particles. The two sets of laws do not get along well, and physicists are still working on reconciling them.
The math behind quantum mechanics suggests that time travel through a CTC is not only possible, but could be done without creating any paradoxes. So while a person (a macro object) can't time travel without creating a paradox, something much smaller, like a single particle (a micro object), could.
Back in the 1990s, theoretical physicist David Deutsch was the first person to realize this, and he figured out a way to get around the paradox.
In the world of quantum mechanics, the rules are a lot more fuzzy than conventional physics. If a quantum particle, like a photon or an electron, entered one of these time travel loops, it would have to emerge on the other side as that same identical particle. But when a quantum particle enters a CTC, there's no set outcome, only a spread of probabilities that the particle will emerge or not. So a particle that enters a CTC with a 50% chance of coming back out will only fail to make it back out of the CTC half the time. It's a crazy solution, but that 50/50 chance is good enough to solve the paradox according to the laws of quantum mechanics.
No one has discovered a CTC or successfully built one, so time travel is still not possible. But physicists at the University of Queensland in Australia have built a system that can mimic how a quantum particle would behave if it passed through a CTC and interacted with a younger version of its self. They've effectively built a time machine simulator.
The team of physicists simulated a particle traveling through a CTC by firing pairs of entangled light particles through a circuit. Entangled particles are created from the same parent particle, so they are identical to each other and any force that acts on one immediately affects the other. The entangled particles passed through a circuit and hit a polarized beam splitter that broke them apart so they could interact with each other. Think of it has you meeting the younger version of yourself right at the entrance to a time travel loop.
The physicists encoded the polarization of each particle pair they tested before sending it through the time machine simulator, so the polarization of any particles that emerged could be measured and compared to the original to make sure it was in fact the same particle.
So what happened when the simulated past and present versions of the particle met each other? The interaction was paradox-free, and the quantum particles came out of the mock time machine in exactly the same way they entered it.
Time travel isn't possible yet, but this simulation means it could be. The experiment also fit both the laws of general relativity and quantum mechanics, demonstrating that the two bodies of law could actually be compatible.
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