Common sense tells us that influencing the past is impossible -- what's done is done, right? Even if it were possible, think of the mind-bending paradoxes it would create. While tinkering with the past, you might change the circumstances by which your parents met, derailing the key event that led to your birth. Such are the perils of retrocausality, the idea that the present can affect the past, and the future can affect the present. Strange as it sounds, retrocausality ... has been debated for decades, mostly in the realm of philosophy and quantum physics. Trouble is, nobody has done the experiment to show it happens in the real world, so the door remains wide open for a demonstration. It might even happen soon. Researchers are on the verge of experiments that will finally hold retrocausality's feet to the fire by attempting to send a signal to the past. It should all be doable with the help of a state-of-the-art optics workbench and the bizarre yet familiar tricks of quantum particles. If retrocausality is confirmed -- and that is a huge if -- it would overturn our most cherished notions about the nature of cause and effect and how the universe works.