I promised Physics and Whiskey that I would look into time travel and then try to provide some information about what I would accept as convincing evidence for time travel. The first step in this process is to have a look at what, exactly, time travel means. And that means having a look at the time dimension, if indeed such a thing is real. From that we can deduce if time travel is possible, and from there have a ponder about what might constitute convincing evidence. We need to figure out whether it is conceptually possible first because of that old mantra: extraordinary claims require extraordinary evidence; if time travel really is tantamount to a suspension of the laws of physics the evidence needs to be vast and concrete, else any other explanation of the evidence will do.
(Also, 5 points to anyone that gets the reference in the title. Points not redeemable.)
Is Time Travel Possible (within the laws of physics)?
It might be wise to first consider what a dimension is. My father tends to liken dimensions to degrees of freedom; we have three dimensions of freedom: up and down, forward and back, left and right (or X, Y and Z). Therefore we are three dimensional. Images on your computer screen can only go up and down or left and right (X and Y) therefore they are two dimensional. We are not free to move back and forward through time (yet) and so we are not four dimensional.
The question for time travel, then, looks pretty easy: Can we create a way to be free along the time dimension? Unfortunately, it is not that simple. The first question is whether time is actually a dimension at all. Many people (myself included) thought that Einstein had answer that question by demonstrating that time could be manipulated with high energy (very high speeds or very high density matter like black holes). This is called Einstein’s Theory of Special Relativity, and is the most robustly tested theory in physics. To not use Special Relativity, and to use the physics that dominated before it (Newtonian physics) would mean that a GPS system—including the one you use in your car—would lose 10 feet of accuracy every day. Relativity is true, and time can be manipulated.
But I, and the scientific community, may have jumped the gun in assuming that time is a dimension. There is an argument that relativistic phenomena are better described in a three dimensional matrix, with time being a tool to measure rate of change (here). Time is nothing more than a ruler. You may be familiar with experiments where one clock is left not moving (stationary) and another clock is put in a high-speed jet. The result is that the clock in the high-speed jet runs slower. The four dimensional model (where time is a dimension) explains this by saying that the high-speed jet warps time slightly, so the clock doesn’t run slowly; time itself runs slowly. In their paper, Sorli and Fiscaletti argue that it makes more sense to imagine the clocks are measuring their relative position in three dimensional space, only. But this may be cutting too deeply with Occam’s Razor.
Alternatively, there is also a theory put forward by Stojkovic et al in 2010 that time, as a dimension, precipitated out of a spatial dimension. And that it is most definitely a dimension. Debates, discussing these options and many more still rage on and there is still no leading answer (so far that I can tell… but I am willing to be corrected). (The debate rages on at CosmoQuest.org.)
So, is time travel prohibited by the laws of physics? Well, no. If the latter option I offered is true then it should be possible to dissolve the time dimension back into a spatial dimension and travel through it. If time is a dimension, there should be a way to describe it as a degree of freedom; so we should be able to travel through it. We just don’t know how to do it (yet). If time is not a dimension then we should not be able to treat it like a degree of freedom*. There is the philosophical question of time paradoxes: if a person goes back in time they would change the course of their history and thus not time travel. If that can’t be resolved then any evidence is likely to be reasoned away: the more extraordinary a claim the more extraordinary the evidence has to be.
*there is an important note here about probability. I have committed myself to neither “time travel is possible” nor “time travel is not possible”; I do not know whether it is possible. What I have said is that given certain models, so far as I can tell, it would be possible; given other models it would not be possible. This is an important comment about probability that I may come back to in a later post (or just share the Atheist Experience episode on the issue).
What evidence might convince me that time travel had happened?
One would assume that if time travel were to happen in the future then we would see evidence of it now. After all, we’re talking about people going backwards in time, so it is fair to assume that is when the evidence would be. It is possible that the account of the unsinkable ship, Titan, is evidence of time travel having happened, but it is so far outside our normal observation and expectation that we deny the evidence. For a little reading on Titan, the novella from 14 years before the Titanic sank about how a ship called Titan that hit an iceberg and sank, read the Wikipedia article on it. There are no Biblical prophecies evidenced and fulfilled quite this well. Some of the numbers are similar but off, and the nature of the sinking is quite different: Titanic went hull-first; Titan capsized. But, would a perfect prophecy really convince me? I suspect not. Would a dozen prophecies convince me? … No.
This is where it gets confusing, because I don’t know why I wouldn’t accept this as good evidence. Perhaps the authors’ response to being told “a ship will sink and thousands will die”—to write a novella and not attempt to save those lives—doesn’t make sense to me. However, perhaps the author didn’t believe the time traveller was a time traveller… so the book makes perfect sense; the author thought they were being told a delightful story, not a foretelling of the future. And this prophecy isn’t written in code, either. But it’s not presented as a prophecy. But if the author didn’t believe the time traveller was a time traveller, why would it be presented as a prophecy? It was just a story a man heard.
I would dismiss Victorian-aged pictures of people with iPhones as PhotoShopped. (Occam’s razor demands it! Plus, why would they be trying to use their iPhone without a Wi-Fi connection or a satellite?) Herein begins to lie our problem: Occam’s Razor.
Occam’s Razor is a great tool. It approximates to don’t assume more than you have to, to account for the evidence. The result of that is the evidence you do have points you only as far as you need to go. If a wise man apportions his beliefs to the evidence, Occam’s Razor is a way of figuring out exactly what that evidence says. But Occam’s Razor cuts deep around issues like God and time travel. The more outlandish a claim seems (i.e. the further it sways from the normal paradigm we accept) the more we wield Occam’s Razor. And modern technology has meant that faking evidence always seems more likely than the outlandish thing. Extraordinary evidence and all that.
To be convinced of time travel I think I would have to see a person do it. I would have to be allowed to set certain challenges (“Come back with yourself at two different stages visibly later in your life. Oh, marvellous. Now, come back halfway through this sent—. Excellent.”) I am not quick to believe and I think I would be fast to cut out most other evidence with Occam’s Razor. But I cannot justify why I find the claim so outlandish as to do that.
Share this post, in the hope that one day it becomes famous and someone can come back in time and stop me from writing it. Oh, time travel; you so paradox-ridden.