Time Travelers Never Miss a Party…

After a bit of an interlude, let’s get back to where I left off from Space and Time Travel Post 5. What is unique about time travel is that unlike other flights of fantasy, how we speculate on time today does not just speculate on the future, it also speculates on the past. Or as Prof. Stephen Hawking once asked in A Brief History of Time, “where are the tourists from the future?”.

On June 28th, 2009, Stephen Hawking threw a Party that, he believes, disproved time travel. The party was exclusively for time travelers and the invitations were sent out after the party was over. Now, there are other explanations. Perhaps, in the future, time travel is possible but highly restricted. Perhaps Hawking’s invitations are absent from any future historical record or as you would say “lost in the mail”. Perhaps in the future, no one likes him. But, as he probably intended, his experiment does illustrate one of the scientific issues with time travel. If it is ever possible, we could see the evidence now.

Let’s say I built a spaceship in my garage capable of faster-than-light travel. When I drunkenly brag about it at your dinner party, you could ask me, “Why have we not seen faster-than-light travel before?”. I would burp and smugly answer that mine is the first. Unfortunately, he reads this blog and then asks, “but isn’t faster-than-light also time travel? Then when spaceships are built in the future, we should see them today.” At this point, my liquid courage wanes, and I whisper, “Not too loud. The time assassins will show up and kill us.”

Then again, maybe there is evidence of time travel. The internet is now full of websites claiming to have pictures of time travelers. Old pictures of people looking like they are holding cell phones, Starbuck coffee cups, etc. Yeah, I’m skeptical too. Time traveler photos are the new bigfoot. But heh, that’s why this is the “Creative Guide to…” series. However, that is a topic better suited to the “Creative Guide to Conspiracy Theories” (and you bet I want to write a few of those).

But back to actual science or what passes for it when talking about time travel. One issue with Hawking’s time traveler party experiment is that he is assuming self-consistent time travel (you can’t kill your grandfather). In paradoxical time travel (you can kill your grandfather), no one can show up until we hear Stephen brag about it, build and a time machine, and then go back in time to wipe the smug smile off his face. At which point history is rewritten, and the news is about how Hawking is no fun at parties.

So now here’s the question. After changing history do they remember Hawking bragging that no one showed up or him admitting time travel existed? Do the party goer’s future “past” selves still need to go the party? And if they don’t, does the timeline return to Hawking being the first nerd to brag about NOT having anyone at his party? And then, do we go back to a party full of time travelers, and so on.

Forgive me. That was more confusing than a sequel to the Inception movie. That’s the kind of craziness you get with paradoxical time travel. So, let me try to break it down. Assuming you can change the past:

1. Once the past is changed, do actions still need to be taken to keep it changed?
2. Who does or does or does not remember that history was changed?
3. What is the relationship between the past and the future? In other words, if you change the past the same way, do you get the same future?

The accurate answer is that no one knows, so you are able to come up with any answers you want. However, be warned, because how you deal with these questions can make the difference between you winning critical acclaim or descending into comic ridiculousness. So, here is my take.

The first question of changes persisting depends on how the time traveler gets to the past. In the post about teleportation, I pointed out that a way to avoid violating conservation laws is to say the time traveler is “reconstructed” in the past as opposed to actually traveling there. In this case, changes to the past never affect the time traveler (because he was already reconstructed). The new timeline continues whether or not the time traveler repeats his actions. However, if the traveler is not reconstructed, he is not independent of changes to the timeline, and so changes do not persist on their own.

The second question of what is remembered is even trickier. A reconstructed time traveler would remember the original timeline. But, if not reconstructed, from a perspective of pure physics, memories are only physical processes and would change as everything else in the timeline. To suggest, otherwise, is to say that there is a special relationship between the dimension of time and consciousness beyond our current understanding. There are some interpretations of quantum mechanics that align with such a position, but we are really talking about philosophy at this point. We are describing how Schrodinger’s cat sees himself inside the box.

For the third question of the relationship between the past and future, that is entirely a discussion on how you see free will and individual actions affecting history. A purely deterministic view would say that if the initial conditions are the same, the timeline will be the same. The view that randomness and free will guide events would say each timeline evolves differently.

So, in our example, if time travelers don’t go to the party, in the deterministic view Hawking’s behaves exactly as before. The time travelers would then be motivated to go to the party. If after hearing how boring the party was, the younger versions of the time travelers decide to skip it, we return to what happened in the first timeline. And so we get an oscillation between the two timelines.

However, if the universe is more random, Hawking behaves differently in each timeline, and so changes the behavior of the time travelers. Here events could settle into a single final timeline or spread into an infinite number of timelines. (If this is sounding like multiverse time traveling, it could be, but that is a topic for another post.)

The third question also dwells into the role of individuals in history. By definition, paradoxical time travel allows us to affect the past of individuals, including ourselves. But what about larger changes to history? That depends on how much you believe individuals can affect history, to begin with. There are two competing views of history — that history changes based on “trends and forces” or the Great Man theory. With trends and forces, killing Hitler does not prevent World War II because economics, sociology, and culture remain the same. In the great man view, World War II requires Hitler. Of course, the answer is probably somewhere in between, but how you approach that question can define your creative use of paradoxical time travel.

The time traveler could find he can only change history slightly where trends and forces pull history back to what is familiar. Then again, he could be “great man” of the Great Man theory that steps on a butterfly and changes all of history as in Ray Bradbury’s “A Sound of Thunder”.

One answer to the “tourists from the future” question is that a time machine is needed to arrive in the past. Kip Thorne and colleagues have researched time travel (sorry I meant closed time-like curves) using wormholes. The wormhole could have one end move through time slower than the other. (Simple to do for us who know about special relativity and time dilation). One end of the wormhole would be in the past, and so could be used to travel back and forth into the past. The catch (among many others) is that you cannot travel into the past before the wormhole was created. This would mean you cannot observe any effects of time travel until there is a time machine. Scientists love this idea because as we learned in the last post if you can’t observe it, it does not violate science.

And then, of course, there is the possibility that when we go into the past and change it, we create another universe. That though, is what we previously described as multiverse time travel, which has its own problems. However, that’s something to discuss in another post.