The Physics and Metaphysics of Time
Why so many paradoxes regarding time?
It’s a curious thing. Every time travel sci-fi story I can think of almost always ends up with some sort of paradox or plot hole.
Consider the popular Back to the Future series. Marty McFly travels back in time with Doc Brown and accidentally prevents the situation that caused his parents to meet. This creates a paradox in which the most probable timeline is one in which he is never born. But if he were never born, there would’ve been no Marty McFly to go into the past to prevent his parents from meeting. Which would mean that his parents actually would meet and go on to have Marty. So which is it, does Marty prevent his parents from meeting or not? This is called the Grandfather paradox, stipulated of the age old question “What if I went back in time and killed my grandparents?” Or more humorously, in Futurama, “What if I was my own grandfather?”
Same [Credit]
Another common time-travel paradox is called the causal loop. Think of the Terminator series. Humanity develops Skynet, an artificial intelligence that plummets the world into nuclear holocaust. John Connor rallies the survivors to lead a war against the machines. In retaliation, Skynet sends back a terminator to kill Sarah Connor (and then in the sequel both Sarah and a young John Connor). But in doing so, the remnants of the Terminator inspire human scientists to begin developing Skynet. The actions of the future directly cause the actions of the past which cause the actions of the future and so on. And it’s kind of a relevant question because the causal loop paradox kind of questions the multiverse theory. Is there a universe in our set of multiverses that just circles back on itself or needs to circle back on itself in order to even exist?
Even in the physics world, the multiverse theory is kind of one version of the parallel timelines idea that pervades pop culture. Basically, this view of time says that all possibilities for events exist in separate universes. Like in the movie the Butterfly Effect or Back to the Future 2. My favorite example, hands down, is the episode Remedial Chaos Theory in TV show Community. In this episode, the seven members of a study group roll a die to see who goes and gets pizza. The roll of the die creates six separate timelines in which a different person leaves for the pizza. All sorts of hi-jinks ensue, it’s a great episode, you should go watch it right now.
There really isn’t an explicit paradox in this, depending on how it’s done. If it is like in Community or in the Butterfly Effect (surprisingly), where each timeline is its own world than there isn’t an obvious paradox, at least to me. If a parallel timeline is done like in Back to the Future 2 where a future self and a past self can occupy the same time, for example future Biff talking to younger Biff, then there is a problem. If the universe is a closed system, we must conserve mass, meaning the total mass of the universe can’t increase or decrease. If a future version of you comes to visit you, than the total mass of the universe has increased. This isn’t so much a paradox as it is a violation of the laws of physics, but I think we can throw it in the same pile.
Why is it that even theoretically it is so hard to find a self-consistent, logical time-travel story that doesn’t induce a paradox? For example, a common criticism of the Harry Potter series is why didn’t Dumbledore use the time-turner to go back in time and prevent the rise of Voldemort in Harry Potter and the Prisoner of Azkaban? Some franchises try to get around some of these paradoxes. Some sci-fi stories try to address the causal loop problem by using something like astral projections or visions of the future, something can’t influence a direct action on the past. But these franchises don’t recognize that even a vision from the future can incite that future as self-fulfilling prophecies.
I think I have an answer. It’s kind of convoluted, but I think it gets at why so many of these paradoxes occur.
Alright, so in order to try to understand why these paradoxes occur I made my own. I posted this guy on facebook, so let’s see how you fare with this:
Imagine a time machine. The rules of this time machine are that you can only time travel while the time machine is active and existing. So if the time machine was made in 2018 you could not go to 2016.
Now, the physicist who crafted this time machine is smart, but vicious. He realizes that if the universe is eternal, and human nature is what it is, that many people will try to travel back to the first moment the time machine is active to claim that they were the first to appear out of the time machine. Over an infinite trajectory, this could mean billions of people pour out of the time machine as soon as it is activated. To stop this, he places an automated turret, a gun, pointed at the time machine as negative incentive for future generations.
Now the physicist doesn’t want people to not use the time machine. After all he made it to be used. He simply wants it to be manageable, so at some time he must remove or disable the gun turret.
When does he disable the turret?
Now I want to clear up a few things before we move forward, because when I originally posted this many (most) seemed to miss the point. I don’t care what the time machine actually is — it can be a Delorean, it can be a train, it could be a giant Hot Pocket. I don’t care, as long as you cannot travel to before the time travel device was created. And this is because I was trying to close off one paradox called the Time tourism paradox. Basically the time tourism paradox asks if time travel is possible, where are all the time travelers? My solution is that the mechanism that allows time travel will only allow time travel while that mechanism exists. We don’t see time travelers because we haven’t invented time travel yet!
The second landmine I want to diffuse is that the turret doesn’t actually matter. I got a lot of responses about how future people will have body armor or how you can’t defend the space with just a turret. It doesn’t have to be a turret. He could hard wire it to explosive or poison gas or a nuclear bomb. The important bit is that it is a negative incentive. There just has to be some disincentive to try to prevent future travelers from overwhelming the machine on startup.
So, with all that out of the way, how does this explain why there are so many time paradoxes?
The answer is that time is information.
First, keep this definition in your pocket: information is the physical orientation of matter.
Alright, imagine a universe with only two point masses, point A and point B, with point B revolving around point A. You have a view port into this universe but every time you open this view port the two point masses do not seem to have moved. Could you say that time had passed in this universe? No, you could not. The information that you have observed, the two points positions, was the same. The physical orientation of these points was the same. Because the information was the same each observation you cannot claim time has passed. Maybe this is because what we call “time” is actually just a given physical orientation of matter. In our two-point universe, time seems to repeat. But in our large, complex and chaotic universe time never repeats because there are infinitely many positions any given particle could assume.
This is
it, your whole universe [Credit]
You might be asking “Ok, but what does that have to do with the turret paradox?” The important bit is to recognize that the negative incentive I bolded is really only information. The turret is never actually fired. The time machine is never actually turned on. The only thing that happens is that the scientist physically orients the world prior to turning on the time machine. This propagates information to the future. The turret doesn’t even have to be loaded! The only thing that needs to happen to prevent overwhelming the time machine with the time traveling equivalent of “First!” commenters on Youtube is that future peoples need to believe that there is a credible threat. Even if it doesn’t exist. The issue is that as soon as this turret is revealed to be empty or revealed to not be a credible threat, that information travels into the future too. Which means that as soon as this physical orientation that prevented this information overload from occurring is undone, that overload is basically guaranteed to happen.
This is why I stressed on the incentives. Incentives only work via information. I have to know that I stand to gain something or lose something in order to make a decision. The information comes first. The mere knowledge of some value is inherently useful by itself. The knowledge of the existence of the time machine is enough to cause people to try to storm one moment in time. The knowledge of a disincentive might be able to prevent some people from storming the time machine. This whole paradox actually speaks to relevant issues today, like how to manage this massive influx of data and immigration policies (although we deal with immigration across space and not time).
I have no idea how to address this time travelers paradox, I just thought it was a good way to introduce a thought I rarely see discussed directly in time-travel sci-fi. What is the role of information across time?
Maybe you can figure it out.