Russian cosmonaut Sergei Krikalev spent over 803 days on the ISS, orbiting the Earth at 7.66 km/s, and as a result, he arrived on Earth 0.02 seconds into the future. Of course, it may not meet the standards of science fiction movies like Back to the Future where you hit the button on a flux capacitor and land up at your dad’s prom night, but for all intents and purposes, it is time travel.
Before we figure out how Sergei landed into the future, and how you’ve done the same countless times in your life, there’s a more fundamental question that we need to tackle.
What is Time anyway?
The best answer for this possibly comes from Einstein, who after a lot of thought, simply said: “Time is what the clock measures.”
This is the generally accepted definition of time in physics. Now, it may sound like an oversimplification, but it conveys an important truth: Time is a measurable and relative quantity. This is how Sergei leapt into the future, by travelling at high speeds for a long time, time slowed down for him relative to an observer on earth. In other words, he experienced time dilation. You’ve experienced time dilation as well, the last time you took a flight, drove around in your car or even you went for a jog – the only difference is that time dilation in those cases is insignificant.
Time is the fourth dimension woven into the three dimensions of space to form the spacetime continuum which permeates every corner of the universe
This stretchy picture of time may seem counter-intuitive, but that’s only because we humans perceive time as a standard, linear quantity. In reality, time slows down, speeds up, and even bends over itself forming a loop, depending on the location of the observer. While working on the General Theory of Relativity, Einstein coined the term spacetime to depict how time is tied in with the three dimensions of space to form a four-dimensional “fabric” – on this fabric lie all gravitational objects such as planets and stars. Gravity, in fact, bends and distorts this fabric, the stronger the gravity, the bigger the distortion in spacetime.
In his book, Quantum Theory Cannot Hurt You, Marcus Chown states that you can only go back in time so far as when the first time machine was invented, anything before that is unrecorded history. This explains why we aren’t overrun by tourists from the future – doesn’t mean time travel is not possible – just that they can’t reach us in a time before we invent it.
To us, time always flows from past to present and into the future. Scientists call this the arrow of time. Nature makes going back in time difficult in comparison to moving forward, if not impossible. First, you have to travel faster than the speed of light to travel back in time, but doing so is prohibited by the laws of physics. Another reason that past time travel is trickier is because entropy increases with time, i.e., things move towards mode disorderly states. You see a teacup fall and break, but you would never see a shattered teacup join itself together.
How to Time Travel
Let’s say you’re not satisfied with moving just 0.02 seconds into the future like Sergei, and you want to travel 500 or maybe 5000 years into the future. How do you do it?
Recall that time is relative, the faster you travel in space, the slower time ticks for you. So one way into the future is to travel really fast, not rocket ship fast, more like speed of light fast. Sure, we can’t travel faster than light, but there’s nothing stopping us from getting close to its speed. In order to travel 500 years into the future, you would have to travel at 99.99% the speed of light for 7 years—let’s say you do this in a spaceship. Since light travels at 299792458 m/s, your spaceship must travel at 299762478.754 m/s. The fastest rocket ship we’ve built has a rather modest top speed of 16,260 m/s.
Time travel is simple. Step 1: Find a really fast spaceship. Step 2: Try not to die
Here we begin to understand the physical limitations of our current technology when it comes to time travel. A problem of speed is really a problem of energy. Theory of relativity also states that mass increases with velocity, so as our spaceship approaches the speed of light, it gets heavier and heavier, and to push it, you must expend more and more energy, the more energy you need, the more fuel you must carry, which adds even more mass—pretty soon the amounts of energy you need to keep it going reach unimaginable levels. Far beyond anything that we’re capable of generating. Even if we manage to go nuclear, there isn’t enough radioactive fuel in the world. One candidate could be matter-antimatter reactions, in which 100% of the mass is converted into energy, but the technology just isn’t there yet.
As hard as the energy problem may be, it’s not the only problem of time travel. Time travel involves time travellers, and human bodies just aren’t built to handle the kind of acceleration needed to make this hypothetical 7-year trip work. An average person can withstand about 5 G (49 m/s2) before losing consciousness, if we try to go too fast, our bodies will be torn apart by the centrifugal force. At this rate of acceleration, our time travellers would have to spend about 70 days strapped into their seats, being pushed at their physical limits. In order to overcome this, cryonics and suspended animation could one day be used to put travellers in a state of hibernation in hypersleep chambers, but so far it exists only in fiction.
If you think about it, these are just two big problems that are the most apparent, we haven’t even talked about the material science, trip trajectory, on-ship life support, communication systems, and a hundred other things that have to come together to make this work. But speed isn’t the only way…
An artist’s impression of a black hole’s singularity sucking in matter from a nebula, it’s probably not the brightest idea to get too close to one of these
Black holes, Wormholes & Warp drives
So far our search for a time machine has been limited to what we can do with the technology available to us, but does nature have some neat tricks up its sleeve? Yes, it does.
Travelling close to the speed of light is one way of slowing down time, the other way is to be in the presence of a strong gravitational field. How strong? The kind that exists near black holes. The strong gravitational field of a black hole bends the space around it, so instead of a straight path, light has to traverse a longer distance along a curvature to get from one point to another. Obviously, a longer would mean lower speed of light, right? No. This is where is gets bizarre: Time will slow down just enough to make sure the speed of light remains constant. The stronger the gravitational field, the higher the curvature, and the greater the time dilation. In fact, if you get to a region within the black hole called singularity where gravity approaches infinity and spacetime collapses, the lifetime of the entire universe will unfold outside while only a fraction of a second will pass for you. This mind-bending phenomenon of gravity time dilation sounds like a sci-fi trope, but is rigidly enforced everywhere in nature. As seen in the movie Interstellar, it is possible to use this time dilation effect for time travel, for every hour that Cooper spent close to the black hole Gargantua, 7 years passed outside its gravitational field, by this math, you would only need to spend 70 hours around the event horizon to experience a 500-year time dilation.
What’s the catch? Well, you have to make sure you hover around the event horizon and no closer—if you happen to fall into the black hole’s gravity, your body would be torn down into a stream of subatomic particles, like being passed through a microscopic shredding machine, this effect is known as spaghettification. Moreover, the closest black holes we know of are thousands of light years away. It would be easier to do a 7-year time hopping trip than using a black hole. If only there was a shortcut we could take…
If black holes bore you, there are also these funnel-shaped portals called wormholes, though we haven’t spotted one in nature yet, these will most likely kill you too
Wormholes are hypothetical shortcuts in spacetime that can connect extremely long distances such as billions of light years, different points in time, and even different universes! They’re hypothetical, and we have never found one. Scientists like Kip Thorne, who have worked extensively on the science of wormholes, say that one thing we need to ensure is the presence of net negative energy within the wormhole in order to keep its walls from collapsing, killing our time traveller in the process. This means that first we have to extract all energy and create pure vacuum, and then suck some more energy out of the portal. It’s like creating an energy debt, but there are limitations on how much energy we can extract from a region that already has none, at least according to classical physics.
In his paper on Space and Time Warps, Stephen Hawking explains how it may be possible to build a wormhole, “Energy is rather like money. If you have a positive bank balance, you can distribute it in various ways. But according to the classical laws that were believed until quite recently, you weren’t allowed to have an energy overdraft. So these classical laws would have ruled out us being able to warp the universe, in the way required to allow time travel. However, the classical laws were overthrown by Quantum Theory, which is the other great revolution in our picture of the universe, apart from General Relativity. Quantum Theory is more relaxed, and allows you to have an overdraft on one or two accounts. If only the banks were as accommodating. In other words, Quantum Theory allows the energy density to be negative in some places, provided it is positive in others.”
Let’s go back to the spaceship, and see if we can get it to travel faster than light. In 1994, Miguel Alcubierre devised a model called Alcubierre drive which involves warping the space in and around the ship to arrive at destinations faster than light without depending on your own kinetic energy, a bit like surfing a wave. Alas, this is the most speculative of concepts that we’ve discussed so far. NASA was working on it at some point but have since stopped. A statement released by them says, “Science fiction writers have given us many images of interstellar travel, but travelling at the speed of light is simply imaginary at present. In the meantime, science moves forward. And while NASA is not pursuing interstellar flight, scientists here continue to advance ion propulsion for missions to deep space and beyond using solar electric power. This form of propulsion is the fastest and most efficient to date. There are many “absurd” theories that have become reality over the years of scientific research. But for the near future, warp drive remains a dream.”
Alright, so time travel isn’t exactly easy, but you already knew that. We’ve discussed the limitations of science, but time travel also comes with Catch-22 brand of logical problems.
Let’s say you manage to get your hands on a sweet time machine, and also, you hate your grandfather for reasons best known to you. You get in the machine, dial back a few decades, and go and kill your grandfather before he fathered your father. Now wait a second, if your grandfather died before your father was conceived, how did you exist to be able to come back and kill your own grandpa? This conundrum is aptly called the Grandfather Paradox, and it’s just one of the many problems we encounter when thinking about time travel.
According to the many-worlds interpretation, our universe may be just one of the countless other universes and alternate realities that exist, in case you were feeling important
Opinion is divided on the matter, some physicists say that an yet undiscovered physical law will stop us from changing the past. Another, more interesting explanation says that an alternate reality will branch off when you kill your grandfather, you don’t exist in that reality but that doesn’t stop you from existing in the one you came from. This many-worlds interpretation suggests the existence of a multiverse where anything that can happen, does happen.
Sometimes, objects and information can get stuck in time. This is known as the Bootstrap Paradox. Suppose a time traveller comes back in time and explains relativity to Einstein, who then builds a theory around the idea—his theory gains popularity and soon becomes one of the most discussed scientific concepts in the history of mankind. Then, in the future, the time traveller who came back, learns about it in school. Now, who really created the theory? Clearly not Einstein, because the time traveller informed him about it, but also not the time traveller, who didn’t know about it until he learned it from a textbook. In the movie Somewhere in Time, an old woman gives Christopher Reeves a pocket watch in 1972, later Reeves travels back to the year 1912 and gives the watch back to the woman who is now a young girl. Where did the watch really originate? In both these cases, the information and object are trapped within an infinite loop of cause and effect, with no clear origin outside of that loop.
Time travel is one of those big unsolved scientific questions that forces us to think deeply, and then discover and invent things to turn our vision into a reality. Obviously, barring the exception of stumbling upon a functional time machine, it’s unlikely that we’re going to crack the code anytime in the near future, but some day in the distant future, taking a trip in time may be as easy as calling a cab to go downtown. Till then, you can always just catch another flight to arrive a few nanoseconds into the future.
This article was first published in the July 2017 issue of Digit magazine. To read Digit’s articles first, subscribe here or download the Digit app for Android and iOS. You could also buy Digit’s previous issues here.