by Ronald Green
The desire to know what will be, to rattle the cage of the future, of time itself, is close to being a human obsession. Of all the dimensions, it is time – a perpetually dangling bait – that we cannot do anything about. Stories of time travel are a manifestation of this frustration and the concomitant belief that it is possible to get to the future “if only.” In science fiction, the ‘if only’ is usually materialized through some sort of mechanical device that allows people to make that time journey. Einstein’s special law of relativity adds theoretical hope to that possibility, since it shows that time runs faster or slower depending on one’s location in space. It is the reason physicist Brian Greene states categorically that “time travel is absolutely possible” (1) and that “relativity lays out a blueprint for time-travel to the future.” (2) Unfortunately, it doesn’t. We can’t slow down or speed up the rate of our personal time. Our clocks and watches do not go slower or faster for us, no matter where we are or how fast we travel. Clocks moves faster or slower, not for each of us, but relative to an observer. Putting it more verbosely, a clock moving relative to an observer ticks more slowly than an identical clock at rest relative to that observer.
Any notion of time in the universe needs to be seen in the light of that principle. So if you could ever speed away to the stars at close to the speed of light and come back after five years, although everyone on earth would be fifty years older, you would still have aged only five years. And, yes, it might be nice to note that you had added on only five years while everyone else was fifty years older, but the fact is that you would not have really changed anything in your personal time span; you would still be only five years older than you were when you left. And sadly, despite the fact that everyone you knew on earth would be 50 years older than when you had left 5 years previously, it would not mean that you would live longer than had you stayed on Earth; you would live out your lifespan, with no connection to what had happened on Earth in your absence.
In fact, you wouldn’t even think of it as having “traveled in time” if there weren’t anything to compare it to; if you weren’t later in contact with those who had aged at a different rate. That time is relative is almost a cliché but not, it seems, when spinning the tale of time travel. In a colorful video, particle physicist Brian Cox says that “moving clocks move slower.” (3)They don’t. They do so only in comparison with other clocks moving at different speeds. What Cox’s demonstration shows is that “time is personal,” as he points out, but not that the person who is the subject of Cox’s demonstration “has traveled in time.”
As we have seen, our getting five years older in outer space while everyone else’s age increases by fifty years here on Earth, has to do with our own personal time. It is not time travel. Imagine two friends, Mary and Carol, both 25 years old. Mary was the lucky astronaut who traveled into space for five years at close to the speed of light. When Mary returned to earth she was 30 years old and found Carol to be an old woman of 75. That would be quite a shocking reunion, but somewhat mitigated by Carol thinking she could ask Mary – who, theoretically, had time-traveled – what was going to happen in the future, perhaps to know the winning lottery numbers that were still to come. Mary would look at her in amazement, for lottery numbers that had come up were obviously not in Carol’s future. But in a strange twist, it seemed to Mary that it was Carol, who had traveled in time; after all, Carol had lived through 50 years while Mary had been through a mere 5 years. And they would both be amazed to hear Brian Greene stating that “If you wanted to leapfrog into the future, to see what the world will be like a million years from now, it could theoretically be done.”(4) Mary would, in fact, have no idea what Carol’s future will be, since she had not traveled to the future. Time had not gone more slowly for Mary. When she came back after five years, those five years were in her past, her memory.
The notion that there is such a journey into the future is patently ridiculous. Mary didn’t travel forward in Carol’s time, or indeed in anyone’s time. If in 2020 she traveled close to the speed of light for 5 years, for her it would be 2025, while for Carol it would be the year 2070. So the notion that one could travel into the future and tell those left behind what the future would be, is preposterous. In fact, when Mary got back, Carol would already have passed that year 50 years earlier! Mary didn’t travel forward in her time any differently to what we all do as time passes. Neither of their clocks moved faster or slower.
To further illustrate how nonsensical the time travel notion is, had Mary moved forward in time, we could likewise claim that she subsequently moved back in time when she returned to Earth, so changing the future that she had just been to! In fact, where she had been was in the past for her, not in the future. And it certainly wasn’t Carol’s future. By the same token, we could say (if we wanted to further mangle our minds) that for Carol to then do the same trip would be for her to travel to Mary’s past.
If the mind game (which is what time travel is) just set out was confusing, it shouldn’t be surprising. The fact that it is deemed to be theoretically possible, however, should be. Traveling back in time is generally accepted as impossible, due to certain implications, like the “grandfather paradox.” Imagine going back in time and murdering your grandfather, before you were born. If you’ve killed your grandfather, you’ve prevented your own existence, and if you never existed, how could you have traveled back in time in order to do it? If that means that backward time travel is impossible, why would physicists conceive forward time travel as theoretically possible? The implications would be just as absurd, since the paradox of backward time travel is inherent in Mary’s venture into space: If Mary died after returning to Earth, supposedly “from the future,” she wouldn’t have been able to make the journey, not having been been alive in order to do it, so presenting us with another version of the paradox.
Beneath the concept of time travel is the implication that the future is where we have not yet arrived, This is, in fact, what Einstein’s theory of special relativity suggests: by destroying any notion of universal time and having time relative to location in space, the future is supposedly already out there waiting for us, and which we simply can’t see until we get there. “Are we there yet, are we there yet, are we there yet?” asks Bart over and over in a car ride with his family in the television cartoon series The Simpsons. If the fact that we are moving linearly towards the future implies that we will eventually arrive, it has no connection to time travel as is popularly envisaged. If it is time travel, though, it means that it is – and always has been – in Mary’s future to make the journey: predestined, in other words.
Underlying it all is the covert presumption that there is a future real or correct time that can be used as a marker to show that time moves faster or slower. But there is no universal time; time depends upon where one is. Time is always in relation to someone else’s time, so Mary’s time is always relative to Carol’s time. If, when Mary was traveling away from the Earth, the Earth was destroyed, she could not be considered as traveling in time, since there would be no way of comparing. In any case, there could never be a way of comparing two times; she would simply be 5 years older. There would, in other words, be no observer to make the comparison.
And that brings us to the matter of the observer, There has to be one, since the clocks move faster or slower relative to an observer. But – and this is a big but – it would not be possible to communicate with someone in another “time zone,” because the length of time it took for the communication to travel between two time zones would equal the gap in time between them. There could never be an observer “in real time,” because (1) there is no one real time, and (2) there is no location that an observer could be in order to see both locations simultaneously (in our case, Carol and Mary), since those times could not be simultaneous.
It’s time, then, to put to rest myths regarding time travel. As much as we would like it to happen, it will not be as physicist Paul Davies (who has written extensively about time travel) says, namely that perhaps in 100 years it will be normal to travel in time (5). Thus, when he asks “Might time travel one day become commonplace too?” the answer should be a resounding “no.”
Like Santa Claus, time travel to the future is an idea, pleasant in its promises. But unlike Santa Claus, which we eventually grow out of, the romantic possibilities inherent in traveling to the future, from The Time Machine of H. G. Wells to TV’s Doctor Who, have never been put out to pasture. The question is not whether it will ever be technically possible, but whether the notion “travel to the future” itself makes sense, even in terms of science fiction.
The problem with time travel is, as pointed out, inherent within a fundamental misstating of how time works according to Einstein’s general law of relativity. It is where a populist and romantic view of time travel prevails over the more pedantic actual physics.
It has been stated as fact that time goes faster at a higher elevation than at a lower elevation on Earth. Physicist Carlo Rovelli tells us in his books and talks for general audiences that “it is a fact that a watch positioned higher will go faster than a watch lower down.” This is measurable, he says, and goes on to say that it can be measured by a super-sensitive atomic clock, even when the difference in height is only 30 centimeters. This fact, he says, has nothing to do with Einstein’s theories. (6) But it is not fact, and it has everything to do with Einstein’s theories.
While time does go faster on Earth at a higher elevation, relative to an observer, it does so due to gravitational time dilation, a consequence of Einstein’s theory of general relativity, which posits that the gravity of a massive body – such as the Earth – warps the space-time around it, causing the flow of time to speed up or slow down depending on its distance from the mass.
For Jack at the top of a mountain, 5 minutes will be 5 minutes on his watch, as it would be for Bill in a valley. Jack’s clock will have moved faster relative to Bill’s, and this will be seen by comparing atomic clocks brought together from each, albeit too small to be ordinarily noticeable. For differences in time to be noticeable, the process could be sped up by Jack or Bill flying into space at close to the speed of light – and we are back to our “time travel” saga with all its attendant fallacies.
It all comes down to the fact that that no two events happen at the same time, and clocks tick at different rates according to where they are in relation to an observer. We don’t, then, need to get into the realm of time travel or the notion that higher clocks tick faster than lower ones to understand the principle that all clocks run at different rates, depending on where the observer is located. In other words, clocks are accurate only to themselves! When clocks are coordinated to give an accurate time – as they are for GPS – the clocks on a number of satellites in orbit above Earth are averaged to an incredible accuracy in order to provide location information anywhere on or near the Earth’s surface at a point on the Earth, i.e. the observer.
We can think of the observer as an independent arbitrator. And the fact that each observer is an independent arbitrator provides another perspective of what is meant by “time is relative”: it is different for each observer, who is necessarily in a different location from every other observer.
It is clear why people do not time travel when they speed away at close to the speed of light. And it is clear why people who are at a higher location do not grow older faster than those living lower down. The simple point is that their personal clocks run at the same rate for each of them; they do not speed up or slow down.
A clock, then, is never wrong. It is wrong only in the sense that it (always) shows a different time than on another clock. This should be clear from Einstein’s special theory of relativity, in which there is no universal time, and hence that time is uniquely personal; the relativity of time, being observer-bound, does per se not encompass notions of right or wrong time.
The uniquely personal implications for time travel are part and parcel of the theories of relativity. Even if there is as yet no consensus as to what time is and/or how it can be defined, or even if time does not necessarily exist within physical equations, surely the very notion of time travel can be brought up only by discarding the relativity of time. Failing that, it seems that we will continue to frustratingly dream on about traveling to the future and thereby extending our lives.
Ronald Green is a former lecturer in linguistics and philosophy at Tel Aviv and Oxford, the author of Time To Tell: a look at how we tick (iff Books 2018) and Nothing Matters: a book about nothing (iff Books 2011), and 13 ESL books used worldwide. He has lectured and given workshops in Europe, North and South America and the Middle East on linguistics, philosophy and the use of the Internet in education. His articles on philosophy have appeared in a number of journals, while his short stories have been published in several literary journals. He is active in taking philosophy down from “the ivory tower,” showing its connection to science, and explaining it in terms that are popularly understood.
(2) “The Time We Thought We knew.” The New York Times, Jan, 1, 2004.
(5) How to Build a Time Machine. Penguin. 2003.