Physicists Have Built A Time Machine Simulator
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, but at an earlier moment in time. 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.
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.