In the last post, I introduced the notion of time travel into the past and pointed out that the potential for a temporal paradox would prevent time travel that allowed changes to the past. Is it possible, as some have argued, to affect, influence, or participate in the past without changing the past?
First, there is the Butterfly Effect. You can get the general idea by watching the movie of the same name, but the idea is simple: small events get amplified over time into much larger consequences. The Butterfly Effect is actually a mathematical result of chaos theory, whereby nonlinear relationships cause effects that seem to be entirely unpredictable over a long enough time period. Perhaps one of the most fascinating papers I ever read was this one (“Gargantuan chaotic gravitational three-body systems and their irreversibility to the Planck length”), in which the authors show that even the largest objects in the universe are fundamentally unpredictable over a long enough time period. They analyze initial conditions that are precise down to the Planck length, because that length, according to physicists, is the smallest length that has any possible physical meaning in the universe. So if precision down to the Planck length can’t predict the motions of supermassive black holes, then nothing can.
The Butterfly Effect is a problem for time travel into the past, and here’s why. You certainly can’t go back in time to kill one of your ancestors, as that would prevent your own existence and cause a temporal paradox. But what if instead you did something seemingly inconsequential, but that event caused another event that caused another event… that prevented your own existence. In the first installment of Back to the Future, Marty pushes his future father out of the road so that he didn’t get hit by a car, but of course that caused a chain of events that led to Marty’s future mother falling for him instead of his future father.
But what about something more subtle? Say you want to go back in time just to observe your ancestors – you have no intention of killing or even interacting with them. Say you’re just walking around and watching from a distance. A malarial mosquito starts buzzing around you and you swat it away. If you hadn’t been there, it would have just fed on some nearby rat. But your swatting at it stimulates it; angry and hungry, it flies in the direction of your ancestors and stings one. Maybe that ancestor dies from malaria before he reproduces. Maybe he just gets sick and misses his opportunity to mate with the woman who would have been your ancestor. Or maybe nothing serious happens at all. Maybe he was already in the middle of having sex with her when he gets stung by the mosquito, and the brief moment he spends smacking the mosquito is enough to change which sperm cell ultimately fertilizes her egg. That alone would be enough to change the identity of their descendants, and it would (arguably) prevent YOU from existing.
Physicists and philosophers who argue that time travel into the past is possible usually understand the Butterfly Effect and might retort something like, “Time travel into the past is possible as long as you don’t do anything, even something small that could chaotically amplify, that would change the facts of the present.”
Here’s the problem: THERE IS NO EVENT, NO MATTER HOW SMALL OR SEEMINGLY INSIGNIFICANT, THAT WOULD SATISFY THAT REQUIREMENT.
As I discuss in this paper, information in the universe is embedded in the relationships, correlations, and entanglements among and between all the particles and fields throughout the universe. Whether the amount of information in the universe is constant or changing is a fundamental question. Those who believe in the assumption of U (that quantum wave states always and universally evolve linearly) believe that the information content of the universe is fixed. As you know if you read this blog or my papers (such as this or this), I have shown that U is false and that the information content of the universe is increasing – specifically, collapse of the quantum wave state upon irreversible correlation events is what produces new information. Nevertheless, whether the information content of the universe is constant or changing, we can all agree that at the present moment there is a certain amount of information in the universe (and it is a staggeringly large amount).
In other words, there are a huge number of “facts of the present,” and every one of them correlates to some event in the past. Every single event in the past gets embedded as a fact in the information structure of the universe. (Those who believe in U would say that every event in the past is already embedded in the information structure of the universe, both past and future, in a block universe.)
What does this all mean? Imagine a speck of dust a million years ago that might have flown in direction A or direction B depending, for example, on some quantum event. There is necessarily some physical experiment that can be done today whose outcome would depend on whether that speck of dust went in direction A or B, whether or not anyone could figure out which experiment to perform. In other words, that seemingly insignificant event from a million years ago is currently embedded in the information structure of the universe, which means that there is some feature of the universe today that depends on that event. That feature may (or may not) be small, depending on the extent to which it was chaotically amplified, and it may not even be possible to know what kind of experiment would differentiate that feature. But the point is that the happening of that event a million years ago MUST be physically embedded in the universe today. (If it wasn’t, then it didn’t happen – which is the point I’ve been trying to make over and over regarding measurement and physical irreversibility.)
So here’s the problem. If it was possible for you to travel back in time, even if you tried your best not to change the past, your mere physical presence would be a fact that gets embedded in the future information structure of the universe. The universe, after all, is constantly measuring you and everything about you. Trillions of air molecules are bouncing off your body every second. Infrared radiation (heat) is constantly being emitted and absorbed by your skin. Even your own minuscule gravitational field causes changes around you that constantly provide evidence of your presence.
Even if all you wanted to do was watch your ancestors in the past, and you were somehow able to send back just your eyeballs (or, even better, a tiny camera), your eyes work by focusing and absorbing photons on your retina, and the FACTS about whether or not particular photons in the past got absorbed get permanently embedded in the universe. At a bare minimum, in order to see something, you have to change the trajectory and/or presence of photons. These would constitute events, the occurrence of which would forever affect the universe.
Therefore, there is no way to send anything physical to the past, no matter how small or seemingly insignificant – and thus there is no way to directly observe the past – without affecting the present. If, for example, it was possible to travel to the past in some kind of super-insulated vehicle that managed to affect nothing in the past, except that it allowed a few photons to be absorbed by your retinas so that you could see something, the facts about the absorption of those few photons would be embedded in the universe in a way that would reveal measurably distinct outcomes at any time after, including the present.
So here’s what I’ve pointed out so far:
· You can’t travel back in time to kill an ancestor because this would cause a temporal paradox.
· You can’t travel back in time to do much at all because, thanks to the Butterfly Effect, almost anything you do will get chaotically amplified over time in a way that somehow prevents your existence or your ability or willingness to travel back in time.
· Nevertheless, if you did travel back in time (or sent something back in time), even if you did nothing, your very presence in the past would get embedded as permanent facts of the universe. The way the universe is right now (or 10 minutes ago or 10 years ago) depends on whether you travel back in time.
The last point is key.
Imagine that there is some conscious experience you had 10 minutes ago (at time T) that depended, at least to some small extent, on whether you time traveled to the past. Assume for simplicity that you observed something as blue – i.e., there is a fact, embedded in the universe, about your conscious experience of seeing blue at time T. However, if you travel into the past, your mere existence gets embedded in the universe in such a way that, at time T, you instead experience seeing red. So what did you actually experience?
Some may reply that this is just a case of memory erasure or modification, but in these papers (here and here) I argue that retroactive modification of conscious states is impossible. I’m not saying someone can’t misremember something. I’m saying that conscious states are history dependent, that the facts of past conscious states get embedded in future states much as past physical events get embedded in future states of the universe. If there is indeed a fact about what I am consciously experiencing now, then it evidences facts about previous conscious states. It is therefore impossible for me to experience my present conscious state (which, indeed, I am experiencing) if my past conscious states are not facts and can instead be retroactively changed. And because time travel into the past implies the ability to retroactively change my own past conscious states, then it would imply that there is no fact about my current conscious state (and also no fact about the current physical state of the universe). This is false.
Therefore, time travel into the past in any form, even just to observe it, is impossible.
One nagging objection to the above analysis is this: it all hinges on one’s ability to choose whether to travel into the past. If there’s no free will, then it is either true or false that I’ve already traveled into the past. But if I have, then that fact is already embedded in the present universe and there is nothing I can do to prevent myself from traveling into the past. Of course, I believe that we have free will, but it remains one of the problems I am currently working on.
I’ve said enough for this post. What other objections or thoughts do you have?
 But then where would the image information be sent? On that note, if you were able to send back just your eyeballs to the past without the rest of your body and brain to consciously experience it, then what’s the point?