In his new book Time Travel: A History, James Gleick presents a valuable literary history of the idea of time travel while also highlighting the various paradoxes associated with the topic. Time travel too often can be considered an unserious aspect of physics and the subject only of speculative fiction. By writing this book, Gleick indicates that the paradoxes inherent in the concept should be taken seriously because solving those problems could lead to a more consistent scaffold of understanding for the role of time in theoretical physics. Gleick’s book creates an important history for the concept of time travel and makes the paradoxes clear. The author seems to have written the book in part to bring attention to the topic of time travel in the hopes that other thinkers will take the subject seriously and look for solutions to the paradoxes.
Could an observer be preserved in a current psychological state, and sent into a physical reality of the past or future, and do so without causing any logical paradoxes or violating any laws of physics?
To begin, H.G. Wells still does not get enough credit for his genius. The man single-handedly invented the discipline of World History, pioneered the “invading aliens” genre, and can be fairly credited with introducing the concept of scientific time travel literature. Gleick indicates that the widespread use of trains made humans realize that their relationship to distance differed depending on speed—it was only a matter of thinking about time before someone realized that our relationship to time also differed depending on speed. Gleick writes that Wells did not bother himself much with the physics as “He was just trying to gin up a plausible-sounding plot device for a piece of fantastic storytelling” (p. 4). Yet it is possible to see how the creativity of both Wells and Einstein branched off from the same concepts.
A scientific concept of time travel originated with Wells, but philosophical and poetic musings about time and its effects preceded the great man. Gleick showcases an impressive collection of quotes about time from Tennyson, Poe, and Laplace. The second chapter then highlights “time travel” as a pop-culture phenomenon explored by Mr. Peabody, Mark Twain, and Woody Allen. The point of this discussion appears to be to point out that The Time Machine by H.G. Wells turned time travel into a mechanistic possibility when he moved beyond a concept from his earliest work titled The Sleeper Awakes that featured a man simply sleeping for a long time in a comfortable chair. “Machines improved upon magic armchairs” writes Gleick and “By the last years of the nineteenth century, novel technology was impressing itself upon the culture” (p. 31).
The most interesting section of the book comes when Gleick tries to frame the idea of time and the future itself in the context of the Age of Exploration:
No one bothered with the future in 1516. It was indistinguishable from the present. However, sailors were discovering remote places and strange peoples, so remote places served well for speculating authors spinning fantasies… William Shakespeare, whose imagination seemed limitless, who traveled freely to magical isles and enchanted forests, did not—could not—imagine different times. The past and present are all the same to Shakespeare: mechanical clocks strike the hour in Caesar’s Rome, and Cleopatra plays billiards (p. 35).
The idea of the future as a thing to be strived for should be seen as intertwined with the concept of discovery and the evolution of the scientific method. After crediting Isaac Asimov with developing the idea of “futurism” as a concept denoting the imagining of a speculative time (as if the future was analogous to an island that one sailed to), Gleick then heaps some more importance on a well-known historical cause, that of the printing press. “It began in earnest with the Gutenberg printing press, saving our cultural memory in something visible, tangible, and shareable. It reached critical velocity with the Industrial Revolution and the rise of the machine-looms and mills and furnaces, coal and iron and steam—creating, along with so much else, a sudden nostalgia for the apparently vanishing agrarian way of life” (p. 41).
Was that it? Or was it that humans, in those early years of science and steam, simply did not know just exactly where the boundaries of scientific achievement could be drawn? Just a few decades before Wells, Mary Shelley wrote of using science to raise the dead. Perhaps Frankenstein’s monster and Wells’s Time Machine both stem from an initial period of wonder and naïvete about what might be achieved using science and technology.
The various paradoxes associated with time travel make appearances throughout the narrative, but as in all of his books Gleick has demonstrated a gift for understanding the boundaries of his arguments. He merely presents the paradoxes as philosophical artifacts, but this is where ordinary science can be used to solve the paradoxes.
Can you go back in time and kill your grandfather, therefore eradicating your own existence? This is the grandfather paradox, and it can be expanded into any “change the past” plot of a science fiction story. Gleick references the philosopher Larry Dwyer, who sees a similar problem with all time travel scenarios:
They all make the same error, according to Dwyer. They imagine that a time traveler could change the past. That cannot happen. Dwyer can live with other difficulties created by time travel: backward causation (effects preceding their causes) and entity multiplication (time travelers and time machines crossing paths with their doubles.) But not this. “Whatever else time travel may entail,” he says, “it does not involve changing the past” (p. 229).
This assumption is always based on the rather squishy premise that time travel would mess up a logical sequence of causations. The solution to the Grandfather Paradox, or “entity multiplication,” has more to do with the travel than with the time. For all of time travel’s many paradoxes, the most basic problem with the concept has been missed. A person traveling back in time would be adding matter to the universe in a way that the Law of Conservation forbids. The matter that makes up a person existed in a different form in the past, so traveling backward in a closed system would be physically impossible. To actually travel backward to an exact past, one would have to unravel one’s own body to do so. A time machine would actually travel back into a past where its component parts existed before being assembled. This would add matter to that universe without actually creating any new matter. To actually travel to the past would be to return to our component parts, eventually star dust.
In other words, you did exist when your grandfather did, just not in a physical shape with a consciousness. To actually travel back to the time of your grandfather, you would need to revert to that state, and would not be capable of altering anything.
Calendar Problem: Future Time Travel
Science fiction hacks have been repeating the old saw that “one can travel into the future just by standing around” for years without actually contemplating what that means. No, you cannot travel into the future by just standing around because the concept of “you” is always in flux. You are moving into the future along with everyone and everything else. Consciousness may be a slippery concept, but it may be thought of as existing in Julian Barbour’s “nows,” which consist of three-second chunks of awareness.
The question is not whether it is possible to travel into the future, but whether or not it is possible to send your current consciousness into a radically different future environment? Is it possible for a nine-year-old boy to travel 30 years into the future as a nine-year-old boy rather than arrive there as a thirtynine- year-old man? That is, could a nineyear- old be sent 30 years into the future to meet his 39-year old self? No, as this would, again, violate the Law of Conservation. Even if the nine-year-old dies somewhere on that continuum, the “stuff” that the nine-year-old is made of will exist in some form in the future, and the molecules that the person is made of cannot exist side-by-side in different formations as this would actually add matter to a closed system.
Multiverses (if this is a paradox)
What if the universe is not a closed system, but an open one that interacts with other universes in a multiverse? This question must be framed by the other time-travel paradoxes. Gleick notes the work of John Hospers here:
Time travel a la Wells is not just impossible, it is logically impossible. It is a contradiction in terms. In an argument that runs for four dense pages, Hospers proves this by power of reason. “How can we be in the 20th century A.D. and the 30th century B.C. at the same time? Here already is one contradiction….it is not logically possible to be in one century of time and in another century of time at the same time.” (p. 222).
Hospers is correct. Even though he does not invoke the Law of Conservation, he comes to the correct conclusion through simple logical analysis. However, since he continues to think of human beings as singular he makes this mistake that Gleick simply records:
Time is simple for Hospers. If you imagine that one day you are in the twentieth century and the next day your time machine carries you back to ancient Egypt, he retorts, “Isn’t there a contradiction here again? For the next day after January 1, 1969, is January 2, 1969. The day after Tuesday is Wednesday (this is analytic—‘Wednesday’ is defined as the day that follows Tuesday)” and so on. And he has one final argument, the last nail in time travel’s logical coffin. The pyramids were built before you were born. You didn’t help. You didn’t even watch. “This is an unchangeable fact,” says Hospers and adds, “You can’t change the past. That is the crucial point: the past happened, and you can’t make what happened not have happened.” (p.223).
We must think of the past as a compilation of particles and waves. You may not have participated in building the pyramids, but the stuff you are made of did exist at the time that the blocks got stacked, and so the physical substance that makes you up did participate, in a small way, in the making of a particular past. Again, you were always here; you just weren’t you.
One proposed workaround here is the multiverse theory. Gleick highlights the work of Hugh Everett III, who developed an adolescent interest in science fiction and studied physics under John Wheeler at Princeton. Gleick sums up Wheeler’s theory:
So what if, he asks—encouraged by Wheeler, who is open as always to the weird and paradoxical—what if every measurement is actually a branching? If a quantum state can be either A or B, then neither possibility is privileged: now there are two copies of the universe, each with its own observers. The world really is a garden of forking paths. Rather than one universe, we have an ensemble of many universes. The cat is definitely alive in one universe. In another the cat is dead. “From the viewpoint of history,” he writes, “all elements of a superposition (all ‘branches’) are ‘actual,’ none any more ‘real’ than the rest.” Protective quotation marks run rampant. For Everett, the word real is thin ice atop a dark pond:
When one is using a theory, one naturally pretends that the constructs of the theory are “real” or “exist.” If the theory is highly successful (i.e., correctly predicts the sense perceptions of the user of the theory), then the confidence in the theory is built up and its constructs tend to be identified with “elements of the real physical world.” This is however a purely psychological matter.
Nonetheless, Everett had a theory, and the theory made a claim: everything that can happen does happen, in one universe or another. (pp. 142–1143).
Everett’s multiverse theory simply reifies the concept of probability. A particle can be described as it exists in one position, but since it could be in an infinite number of other positions, the current position must be compared with all other probabilistic states. This does not mean those states actually exist at any given time. Here’s the confusing part, though, and the part that unlocks a concept of time travel that obeys what we know about physics. Probabilities for the past and the future can exist in any number of nearly infinite states. As Gleick had stated earlier in the book, “Physics is made of mathematics and words, always words and mathematics. Whether the words represent ‘real’ entities is not always a productive question” (p. 112).
Correcting the Paradoxes and Coming to a Time Travel Potentiality
To clear up the paradoxes that Gleick highlights we can begin by looking at a clock and looking at a ruler. A clock is just a ruler bent into a circle; rulers measure distance but what do clocks measure? The best answer is that clocks measure movement. Einstein’s big insight was to see that an observer’s experience of time, like his experience of distance, differs depending upon how fast the observer is moving. Now, if time is the measurement of movement, this would indicate that the absence of movement equals the absence of time.
In quantum calculations, the singularity “before” the Big Bang is expressed as T=0. Pick this equation up and look at it closely and you’ll see that what it really says is that T=Movement, and that zero movement equals zero time. Having said this, we must define what is meant by “time travel.” By time travel, we likely mean that a human observer could be sent, preserved in a current psychological state, into a physical reality of the past or future and do so without causing any logical paradoxes or violating any laws of physics. Is this possible? Yes, it’s possible, but not in a way that validates most science fiction scenarios.
First, we must understand that from our perspective where a particle or wave was three seconds ago and where it will be three seconds from now contain the same probabilities. We are never sure of the past of anything, but if a salt shaker is sitting on the table the odds that it was on the table three seconds ago and the odds that it will be on the table three seconds from now are the same. Those probabilities can change with the arrival of evidence. If I see a salt trail from the counter to the table, that might indicate that the salt shaker was on the counter recently and if someone picks the salt shaker up, that shakes the probabilities, but a docile salt shaker will likely stay in place.
Okay, from this we must understand that the universe does not have a single history, but a series of probable histories that are discerned from using current evidence. As Paul Davies wrote in a 2007 article in New Scientist on “The Flexi-Laws of Physics”:
As Hawking has emphasized, it is a mistake to think there is a single, well-defined cosmic history connecting the big bang to the present state of the universe…. Rather, there will be a multiplicity of possible histories, and which histories are included in the amalgam will depend on what we choose to measure today. “The histories of the universe depend on the precise question asked,” Hawking said in a paper with Thomas Hertog…In other words the existence of life and observers today has an effect on the past. “It leads to a profoundly different view of cosmology, and the relation between cause and effect,” claims Hawking.
Think of right now as a box that connects to other boxes of past probabilities. There are infinite boxes, but only the boxes that include a history that leads to the development of life on the planet, and you as an observer, are lit up. There are several different probabilities that could have led to the universe as we observe it, but we accept only the particular pasts that led to our existence because all others would be illogical. We can discard any past paths that did not lead to the extinction of dinosaurs, because dinosaurs are not here. There are still plenty of paths that could have led to life on the planet, and in the absence of specific evidence, some are equally probable.
Continue thinking about those boxes and this idea can be connected to Relativity Theory. Picture a line with an observer at rest represented at the bottom. His frames of particles and waves (the foundation of all the stuff around him) are divided into relatively small sections. For the purpose of simplicity, let’s imagine a two-year timeline divided into 730 boxes. Now, imagine two larger boxes on top of the timeline that take up the entirety of the timeline (two big boxes on top and 730 small boxes on bottom). The two big boxes represent an observer moving at nearly light speed. The two observers move through their respective boxes and then, meet at the end of the timeline.
Our faster observer only moved through two boxes of particles and waves, while our slower one moved through 730 boxes. They experienced different rates of particle and wave movements, so the observer who moved at nearly light speed only went through two boxes of movement while the slower observer experienced 730 boxes of movement. Once they both revert to the same speed however, the faster observer would be in the future in a more or less preserved state.
What we are talking about here, then, is preserving an observer in a current psychological state while finding a way to move all of the particles and waves around him or her either into a future or past state. This means that time travel is possible to an extent but within certain limits. First of all, to repeat, the Law of Conservation forbids a traveler from moving back to an exact past in a closed system. However, since the laws of physics are reversible, with enough energy it would be possible to reverse particles and waves to a previous state that is probably like the state they were in previously, with the exception that the matter that makes up the traveler would be absent from the past.
This would not create any paradoxes, because this really just puts particles into a probable past state (with the exception of the matter making up the traveler) but in future motion. This is logically sound but physically unlikely (to say the least). Stephen Hawking has always stated that thinking about time travel might be useful even if time travel proves to be ultimately impossible, which, for all practical reasons, it is. However, straightening out these logical paradoxes might lead to a greater understanding of time and space, and for that, those of us with an interest in philosophy should be grateful to James Gleick for giving the topic a serious history.
About the Author
Dr. Chris Edwards is a frequent contributor to Skeptic and the author of several books.