Grandfather Paradox: Explained in Simple Words

Imagine that you build a time machine, allowing you to travel into the past. Once there, what would you do? Would you quietly slip notes to eminent scientists and scholars, helping them to fill the gaps in their scientific research? Would you try to prevent major historical events, such as world wars or tragic terrorist attacks? Or would you choose to meet and interact with the younger version of your parents, or perhaps even your grandparents? If a time machine were to actually exist, then you could, in theory, interact with your grandfather, talk to him, give him life advice, or even lift his vintage car's keys to take it for a spin. However, if you suddenly had the horrible idea of killing him, you wouldn't be able to do it. Why is that? Because of the grandfather paradox. Our classical interpretation of time conceptualizes it as a straight line. The present is a point somewhere in the middle, while the past stretches in one direction, and the future extends the opposite way. As time passes, the individual in question progresses towards the future. Pretty straightforward, right? Well, actually, not so much. The grandfather paradox is a logical problem arguing that if time is truly a straight line, then from a logical standpoint, traveling to a pastime would be impossible. Let's imagine that an exceptional scientist—let's call her Clara—invents a time machine. Using that machine, she travels to the past and meets her grandfather, in a year before the birth of Clara's father. Theoretically, if Clara chose to, she should be able to kill her grandfather. Obviously, this would be a terrible thing to do, but there would be no physical, logical, or time-travel-related impediments that could prevent her from doing so. Even so, she cannot kill her grandfather. Why is that? Quite simply, if Clara kills her grandfather before the birth of her own father, then her father would never be born. This would, in turn, prevent the event of Clara's own birth, so she would cease to exist. This, however, would also mean that the time machine she used was never invented, since she invented the machine. This lack of a machine—or Clara—would mean that Clara's grandfather was never killed. But since Clara does exist in the present, with her time machine, she can travel to the past to kill her grandfather. However, if she does that, she would cease to exist and her time machine would never be invented. This is what scientists call the grandfather paradox. It characterizes a hypothetical situation in which a time traveler travels into their past and ends up causing an event, such as the killing of their grandfather, which makes their act of traveling to the past and altering it, therefore, impossible. What this means is that if you go back in time and kill your grandfather, you don't exist, which means that your grandfather cannot be killed. As you can see, there are two realities occurring in parallel here. Although all this talk about time travel and the grandfather paradox is purely hypothetical, there is real evidence of two realities occurring in parallel at the same time. This is a phenomenon in the quantum world that scientists refer to as superposition. Superposition is a fundamental principle of quantum mechanics in which a system can be in multiple states at the same time. Just as a spinning coin is both heads and tails simultaneously, a quantum system can be in multiple states at the same time, meaning that it can have more than one reality occurring in parallel. You may want to check out our video on quantum entanglement, whose link is present in the top right corner of the screen or in the description of this video. If the principle of multiple realities is applied to the grandfather paradox, what you end up with is something called a closed time loop. In this system, your grandfather is both dead and alive at the same time, and by extension, so are you. As you can imagine, this is deeply confounding because it doesn't make sense according to our classical one-dimensional interpretation of time, which dictates that time is a straight line progressing in only one direction, and it cannot have multiple dimensions. However, this is exactly why science fiction literature and media love the concept of time travel and the multidimensionality of time. A sci-fi Hollywood movie released in 2020 called Tenet ingeniously explores the grandfather paradox. One of the main characters of the movie says the line, what's happened, happened, multiple times to suggest that altering the events of the past is impossible since they have already occurred. The grandfather paradox is considered the primary and most popular objection to the logical possibility of backwards time travel. Our current understanding of time and the laws of physics dictate that if someone travels to the past, they would somehow have to be able to work their way around the grandfather paradox. Now, what if time is not one-dimensional? What if time, just like space, has more than one dimension to it? If that's the case, then not only could the grandfather paradox be avoided, but it also means that there may be another copy of our world running in parallel to ours.