Welcome to this week’s installment of the From Eternity to Here book club. Chapter Six is entitled “Looping Through Time.” It’s about both the logical paradoxes presented by time travel, and some of the obstacles to actually building a time machine (closed timeline curves) in general relativity.
Excerpt:
Everyone knows what a time machine looks like: something like a steampunk sled with a red velvet chair, flashing lights, and a giant spinning wheel on the back. For those of a younger generation, a souped-up stainless-steel sports car is an acceptable substitute; our British readers might think of a 1950s style London police box. Details of operation vary from model to model, but when one actually travels in time, the machine ostentatiously dematerializes, presumably to be re-formed many millennia in the past or future.
That’s not how it would really work. And not because time travel is impossible and the whole thing is just silly; whether or not time travel is possible is more of an open question than you might suspect. I’ve emphasized that time is kind of like space. It follows that, if you did stumble across a working time machine in the laboratory of some mad inventor, it would simply look like a “space machine”—an ordinary vehicle of some sort, designed to move you from one place to another. If you want to visualize a time machine, think of launching a rocket ship, not disappearing in a puff of smoke.
There might not be too much new to say about this chapter, as part of it appeared as an excerpt in Discover and we’ve already talked about that. But maybe you weren’t reading that post, in which case, it’s new to you!
There were three main goals in this chapter. The first was to explain what time travel would and would not be, in the context of general relativity — in particular, it would be just another form of travel through spacetime, not involving any disappearing and rematerializing at some other point in the past. The second was to go through some of the possible ways to make closed timelike curves (with wormholes or cosmic strings) and see how difficult it really was.
But the third and most interesting goal was to connect time machines to the arrow of time and entropy. At this point in the book we’ve only introduced these concepts somewhat casually — the careful exploration of entropy is in Part Three, which begins next week — so one could argue that a more logical presentation would have delayed this discussion for later. But sometimes there are considerations beyond logic; in particular, once we built up momentum with the entropy discussion, a digression on time travel would have seemed like wandering too far afield. That was my feeling at the time, anyway.
This is a really interesting aspect of time travel, which I think is dramatically under-emphasized in discussions about it: the real reason why traveling backwards in time makes us nervous is that it becomes impossible to define a consistent arrow of time. The arrow is very ingrained in how we think about the world, including the sense that the past is set in stone while we can still make choices that affect the future. In the presence of a time machine part of our personal “future” is already in the “past,” which seems to compromise our free will.
So be it! Our free will was always an approximation, if we are good materialists who believe in the laws of physics. But it’s a highly useful approximation. It’s always worth emphasizing, when you start talking about the paradoxes of time travel: the simplest and most plausible way out is to imagine that the universe doesn’t (and won’t ever) actually have any time machines.
Right here:
http://blogs.discovermagazine.com/cosmicvariance/2010/01/12/from-eternity-to-here-book-club/
I thought that the Gate into Yesterday was an interesting thought experiment. Using a human to illustrate the point of course makes the scenario highly unlikely, especially as you indicated that every atom in his body needs to line up and every speck of dust needs to be the same on either side of the gate.
However, I think that the scenario becomes less far-fetched when you consider individual particles, which travel as waves unless measured. Since we are uncertain about the exact position of a particle at any given moment, we are unable to determine whether it crosses through the gate in the same exact position each time. Also, we are uncertain about the precise time that the particle crosses through the gate. In fact, at some point on the particle’s world line, the probability distribution of its location in spacetime would span both sides of the gate. Does that concept conflict with any known laws of physics or would that be permitted?
Also, closed timelike curves can exist without containing particles, thus also eliminating the issues that you discussed.
Just for fun, could we have an example that is even more odd than your scenario? Could the stranger that you describe, whose life story you described as “a one-day loop repeated ad infinitum,” actually walk through the gate facing forwards on one day, then walk through the gate facing backwards the next day (in his world line), then restart the loop by walking through forwards again such that he lines up exactly with his past self? In this way, we create a two-day loop, occurring within a single day, perhaps playing out in two parallel universes. I understand only a little about how “spin” works in quantum physics, but isn’t this the concept of how some particles must rotate twice to return to their original position?
Regardless, if closed timelike curves were to exist, we can be certain that the conditions would not permit the evolution of conscious beings, so paradoxes in terms of free will not occur.
Uh, Sean, couldn’t you have come up with a more original title and cover art for the book -http://www.facebook.com/group.php?gid=76903743708&_fb_noscript=1 ? That one is from March 2009…
Corey– I don’t think that switching from particles to waves makes any difference. Waves have definite values at every event, just like particles; you shouldn’t think of waves as “particles whose positions we don’t precisely know.” But your complicated example is perfectly reasonable (at least within the confines of this thought experiment).
Stevie– Unfortunately not; by the time we knew about Viola’s book, the die had been cast, as it were.
Sean,
I’m disappointed, you haven’t answered my question: Do you think we can change the future by your actions, or do you think we just have the impression we can change it? Saying you believe in an “effective” kind of free will from our inability to predict the future seems to mean to me you do not believe you can change the future (as in: it’s entirely determined), which I would argue means simply you don’t believe in free will. Best,
B.
PS: A comment subscription feature for the posts here would be handy. I tend to forget about comments I’ve made if I don’t get replies by email.