by Patricia Kane
At the risk of sounding cliché, walking into Mark Wilde’s office as the 4:00 pm sun filtered through the window was like entering another dimension. I arrived early, having been warned that the room would be difficult to find. As I weaved my way through the Byzantine hallways of the physics building, I kicked myself for never taking the opportunity to explore it before. Scientific papers plastered to bulletin boards lined the walls, and I overheard a young lady explaining, to another student’s bewilderment, the existence of particles even smaller than the sub-atomic ones we were taught about in high school (protons, neutrons, electrons, etc.) It’s not every day you get to overhear someone getting their mind blown.
Nested four floors above the cacophony of campus, Wilde’s office is serene. A large window takes up much of the northern wall, with a white board lining the adjacent one. For much of our conversation, Wilde would have his back to me, trying to illustrate the complexities of time travel.
Yeah. Time travel.
Based on what I gathered from my time spent in Wilde’s office and my cursory knowledge of philosophy, physics seems to be a pretty Kantian discipline. That is to say, it is in the business of determining the rules. Rules by which the entire universe is governed; in essence, a Theory of Everything. Sound familiar?
Wilde: You know, around the early 1900s, that’s when Einstein had these great contributions to science. And one of the things he established is called the general theory of relativity. And this sort of tells you a different way of thinking about gravity. He came up with these equations that tell you how matter and the gravitational field interact. And one thing that was realized is this theory of general relativity conflicts with the theory of quantum mechanics, which is the set of laws which governs the small things. So quantum mechanics governs the behavior of atoms, photons, electrons, whatever. Anything in the universe is subjected to the theory of quantum mechanics. But also, if we want a theory of everything, they have to be subjected to the laws of general relativity as well. But these two theories conflict, and so there’s been some exploration of what a theory of quantum gravity would be.
Within the parameters of Einstein’ theory of general relativity, which describes how matter interacts with gravity and is one of the foundational theorems (read: rules) of modern physics, the possibility of time travel is not excluded. So, scientists began to explore time travel’s potentials. Actually, the first recorded mention of time travel is found in a Hindu text from 700 BCE, but modern physicists have the luxury of exploring the potential of time travel mathematically.
Investigations into the plausibility of time travel have led physicists to identify two paradoxes: the grandfather paradox and the Shakespeare paradox. In the grandfather paradox, the time traveler goes back in time to before his grandfather and grandmother ever met, and kills his grandfather. Therefore, it wouldn’t have been possible for the time traveler to ever have been born in the first place. So how, then, can he travel back in time in the first place? (Why he wants to kill his grandfather is an entirely different question.)
Physicist David Deutsch proposed a model of time travel that he asserted would resolve the grandfather paradox. Essentially, he argues, when you go back in time you are going into other universes, affecting things in those parallel timelines rather than in your original universe. It is referred to as a closed timelike curve.
Wilde and his team wrote two papers that discussed the ramifications of Deutsch’s model. One of the big red flags, to Wilde and his colleagues, raised by Deutsch’s model, is that with it you can violate the uncertainty principle. When measuring particles on a quantum level, you can determine their position or momentum, but you can’t precisely measure both at the same time. By measuring the particle’s momentum, you lose information related to its position, and vice versa.
Wilde: The uncertainty principle, it says something different from what you’d expect based on our everyday experiences. Like, if we make our rulers or measuring devices more and more accurate, we can get our information about whatever to be more and more precise. So, for example, every particle, every atom, electron, has some features about it that you can measure. So one of them would be where is it, and another would be how fast is it going. So, position and momentum. And you’d think that if we built a measuring device to be more and more accurate or precise somehow then we could learn exactly where it is and exactly how it’s going, at the same time. But, this Heisenberg uncertainty principle says well, no, you can’t. If you learn exactly where the particle is located, then you won’t be able to… you’ve sort of lost all knowledge of how fast it’s going.
So maybe you have heard of that?
Me: Well, it makes sense. I was thinking, it’s kind of like observing something changes the measurement?
Wilde: That is an aspect of quantum mechanics. So, if you make an observation you do disturb the thing that you are observing. So that’s related to this Heisenberg uncertainty principle.
And then, conversely, if you try to figure out exactly how fast its going, you’ll sort of lose all information about where it was. So, and then there’s like a tradeoff. It doesn’t have to be extremes. But maybe, learn somewhat precisely where it is and you can learn somewhat precisely how fast it’s going, but there’ll always be some noise in your measurements.
So, what we showed is that with Deutsch’s model, if time travel behaved according to that model, then you could violate the uncertainty principle.
Because quantum mechanics and the uncertainty principle are both widely regarded as valid, it points to the Deutsch model as being incomplete. A second paper done by Wilde and his colleagues strengthened the first result by pointing out that the model also would violate the no-copying, or no-cloning, theorem of quantum mechanics.
Wilde: Related to the uncertainty principle there’s something called the no-copying theorem of quantum mechanics. So, you know, you can copy information as you please.
You can copy what’s called classical information, or ordinary information, as you please. I mean like that’s how we communicate with computers, and it’s nice to have backups. But, in quantum mechanics, you can’t copy generally. So if you have what’s called the quantum information encoded into some particle, then you cannot copy the state of that particle onto another one perfectly while retaining the original copy of the state. So, that’s different from how we copy ordinary, classical information. You can encode classical information onto a particle. Like you can say the atom is in its ground state or its excited state and that can correspond to a bit, you know a zero or one. But then there are these possibilities where it can be somewhat in the ground state, somewhat here and there at the same time, and so those kinds of things you can’t copy generally. So that’s called the no-copying theorem and it’s related to the uncertainty principle. So that’s also a core principle of quantum mechanics, and what we did in the later work was to show, well, if you have this Deutsch wacky time machine then you could violate the no copying theorem as well. And so it suggests that… maybe there’s something wacky about… maybe there’s something wrong with the Deutsch model.
Me: Maybe it’s incomplete or…?
Wilde: Incomplete, yeah.
Modern physicists are grappling to reconcile the theory of general relativity with the laws of quantum mechanics. Physics is a very principled science. Rather than discovering or creating new information, physicists really are seeking to uncover the truths by which the universe operates. They are merely obscured rather than yet-to-exist.
After further chatting about a myriad of other quantum mechanics principles, the current state of academic research, and time travel in pop culture, unfortunately Prof. Wilde had to excuse himself to attend a lecture on black holes. I walked back to my car in silent contemplation. I am not unfamiliar with thinking about the loftier possibilities of the universe, but to discuss time travel as if it were an inevitable possibility is quite humbling. I hadn’t ever really considered the fact, and it is a fact, that all over the world there are men and women tucked away in quiet university offices, chipping away at that which holds us back from accessing time travel and a succinct theory of everything.
Above is an illustrated example of the grandfather paradox. The x-axis represents space, and the y-axis represents time. The pink dot represents the murderous grandchild, and the red dot represents the grandfather. Universe 1 and universe 2 are seen divided by a dashed vertical line. In Deutsch’s model, entering a wormhole to the past would send you across this division and into a separate universe, rather than the past of your original one. Therefore, you could kill your grandfather and continue moving forward through time.
As an aside, one of the movies recommended to me by Prof. Wilde was Predestination starring Ethan Hawke. It is based off of the short story “All You Zombies” by Robert A. Heinlein written in 1958. After our conversation, I went home and watched it. I would highly recommend it to anyone interested in time travel, paradoxes, or Ethan Hawke. I would like to sincerely thank Mark Wilde for taking the time out to speak with me and for the inevitable ensuing existential crisis.
Editorial note – Edited by Dr. Paige Jarreau