The Science of Time Travel: Is Time Travel Possible? (#117)

When I was brainstorming topics for this week, I decided I wanted to do something scientific, like another astronomy episode, because we haven’t had one of those for a while. One of our most popular episodes on this podcast is the one Ellie did about glitches in time, so I thought I’d use that as inspiration today and talk about time travel!

I’m going to attempt to cover the science of time travel and whether it’s possible. (I say “attempt” because I am not a physicist and some things go way over my head, so I’m going to try and condense things like the theory of relativity into easy chunks.) I’m also going to look at time travel in books and the media, and when it started being explored.

The science of time travel

Technically, we’re constantly travelling through time at the very impressive rate of one second per second. Whether it feels like it or not, like if you’re sitting watching paint dry or doing something fun that makes an hour feel like 5 minutes, you're pretty much always moving through time at the same speed.

The idea of time travel has captivated people for ages in the form of sci-fi books and films (more on that later). And although many people are fascinated by the idea of changing the past or taking a leap into the future, no one has ever demonstrated the kind of back-and-forth time travel seen in science fiction, or proposed a method of sending a person through significant periods of time that wouldn't destroy them in the process.

As Stephen Hawking pointed out in his book Black Holes and Baby Universes, “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future."

Science does support some amount of time-bending, though, and this is where I try to explain Albert Einstein's theory of special relativity.

Einstein’s theory of special relativity

Einstein’s theory of special relativity was published in 1905, and is still one of the most important papers ever published in the field of physics. In basic terms, the theory proposes that time is an illusion that moves relative to an observer. An observer travelling near the speed of light will experience time, with all its aftereffects (boredom, ageing, etc.) much more slowly than an observer at rest.

Special relativity is an explanation of how speed affects mass, time and space. The theory includes a way for the speed of light to define the relationship between energy and matter — small amounts of mass (m) can be interchangeable with enormous amounts of energy (E), and this is defined by the classic equation E = mc^2.

Special relativity applies to "special" cases — it's mostly used when discussing huge energies, ultra-fast speeds and astronomical distances, all without the complications of gravity. Gravity wasn’t added into the theory until 10 years later, in 1915, when he published a paper on the theory of general relativity, which explored how special relativity is affected once gravity is involved.

In the theory of special relativity, as an object approaches the speed of light, the object's mass becomes infinite and so does the energy required to move it. That means it is impossible for any matter to go faster than light travels.

Before Einstein, astronomers (for the most part) understood the universe in terms of three laws of motion presented by Isaac Newton way back in 1686. These three laws are: 

1. Objects in motion or at rest remain in the same state unless an external force imposes change. This is also known as the concept of inertia.
2. The force acting on an object is equal to the mass of the object multiplied by its acceleration. In other words, you can calculate how much force it takes to move objects with various masses at different speeds.
3. For every action, there is an equal and opposite reaction.

Newton’s laws formed the basis for our understanding of mechanics and gravity, but some things couldn't be explained by Newton's work: for example, light.

Scientists realised that light behaves strangely, and they began to try and shoehorn the odd behaviour of light into Newton's framework for physics. In the 1800s, scientists supposed that light must be transmitted through some medium, which they called the "luminiferous ether." That hypothetical ether had to be rigid enough to transfer light waves like a guitar string vibrates with sound, but also completely undetectable in the movements of planets and stars.

Researchers set about trying to detect that mysterious ether. In 1887, physicist Albert A. Michelson and chemist Edward Morley calculated how Earth's motion through the ether affected how the speed of light is measured, and unexpectedly found that the speed of light is the same no matter what Earth's motion is. 

If the speed of light didn't change despite the Earth's movement through the ether, they concluded, there must be no such thing as ether to begin with: light in space moved through a vacuum. That meant it couldn't be explained by classical mechanics. Physics needed a new paradigm. Enter Einstein.

Einstein was only 16 years old when he started questioning the odd behaviour of light. Up to this point, it was questioned whether physics as a whole would have to change depending on a person's speed, and their vantage point. But instead, Einstein sought a unified theory that would make the rules of physics the same for everyone, everywhere, all the time. This led to his eventual musings on the theory of special relativity.

Einstein theorised through several thought experiments that time is relative; that it moves differently for objects in motion than for objects at rest. Meanwhile, the speed of light, as observed by anyone anywhere in the universe, moving or not moving, is always the same.

As a quick aside: What does E = mc^2 mean?

One of the most famous and well-known equations in all of human history, E = mc^2, translates to "energy is equal to mass times the speed of light squared." In other words, energy (E) and mass (m) are interchangeable. They are, in fact, just different forms of the same thing. 

But they're not easily exchanged. Because the speed of light is already an enormous number, and the equation demands that it be multiplied by itself (or squared) to become even larger, a small amount of mass contains a huge amount of energy. For example, according to the show NOVA on PBS, “If you could turn every one of the atoms in a paper clip into pure energy — leaving no mass whatsoever — the paper clip would yield [the equivalent energy of] 18 kilotons of TNT. That's roughly the size of the bomb that destroyed Hiroshima in 1945.”

Time dilation

As part of his theory of relativity, Einstein developed the idea of time dilation by re-envisioned space itself. He coined the phrase “spacetime,” fusing the three dimensions of space and one dimension of time into a single term. Instead of treating space as a flat and rigid place that holds all the objects in the universe, Einstein thought of it as curved and malleable, able to form gravitational dips around masses that pull other objects in, just as a bowling ball placed in the centre of a trampoline would cause any smaller object placed on the trampoline to slide towards the centre.

The closer an object gets to the centre of the dip, the faster it accelerates. The centre of the Earth’s gravitational dip is located at the Earth’s core, where gravitational acceleration is strongest. According to Einstein’s theory, because time moves more slowly as you move faster through space, the closer an object is to the centre of the Earth, the slower time moves for that object.

This basically means that time moves relative to the observer — an object in motion experiences time dilation, meaning that when an object is moving very fast it experiences time more slowly than when it is at rest.

An example of this is when astronaut Scott Kelly spent nearly a year aboard the International Space Station starting in 2015, he was moving much faster than his twin brother, Mark Kelly, who spent the year on Earth’s surface. Due to time dilation, Mark Kelly aged a little bit faster than Scott. Since Scott wasn't moving near lightspeed, the actual difference in ageing due to time dilation was negligible (around 5 milliseconds). But at speeds approaching the speed of light, the effects of time dilation could be much more apparent.

A more dramatic example of time dilation can be seen in the film Interstellar, when Matthew McConaughey and his crew land on a planet with an extreme gravitational field caused by a nearby black hole. Because of the black hole’s intense gravitational influence, time slows dramatically for the crew on the planet, making one hour on the surface equal to seven years on Earth. This is why, when the crew returns to Earth, Matthew McConaughey’s daughter is an old woman while he appears to be the same age as when he left.

So why hasn’t humanity succeeded in making drastic leaps forward in time? The answer comes down to velocity. In order for humanity to send a person years into the future, we would either have to take advantage of the intense gravitational acceleration caused by black holes, or send the traveller rocketing into space at close to the speed of light (about 1 billion km/h). With our current technology, jumping a few microseconds into the future is all humans can manage.

But if technology one day allows us to send a human into the future by travelling close to the speed of light, would there be any way for the traveller to use time dilation to return to the past and report her findings? Unfortunately, Dr. Jaymie Matthews, professor of astrophysics at the University of British Columbia, said, “Interstellar travel reaching close to the speed of light might be possible, [but] this voyage is one way into the future, not back to the past.”

Relating this back to time travel

However, right now, we obviously don't have the technology to travel anywhere near the speed of light. But with the precision of modern technology, time dilation does actually affect human engineering. One example of this is GPS (which, if you didn’t know, stands for Global Positioning System).

We use GPS satellites to help us figure out how to get to new places. NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations?

GPS satellites orbit around Earth at about 8,700 miles (14,000 km) per hour. This slows down GPS satellite clocks by a small fraction of a second. However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Einstein's theory of general relativity says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground. This results in the clocks on GPS satellites experiencing time at a rate slightly faster than 1 second per second.

Luckily, scientists can use maths to correct these differences in time. If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day.

Other time travel theories

Some researchers have proposed other solutions that could allow jumps back and forth in time. These theories share one major flaw: as far as scientists can tell, there's no way a person could survive the kind of gravitational pulling and pushing that each solution requires, so even if we could send a person through one of these methods, it might get quite messy.

Wormholes

Wormholes, also known as Einstein-Rosen bridges or white holes (as opposed to black holes), are theoretical "tunnels" through the fabric of space-time that could connect different moments or locations in reality to others.

They were first theorised in 1916, and then the theory was elaborated upon in 1935 by Einstein and physicist Nathan Rosen. They used the theory of general relativity to elaborate on the idea of white holes, proposing the existence of "bridges" through space-time. These bridges connect two different points in space-time, theoretically creating a shortcut that could reduce travel time and distance.

But despite taking up a lot of space in science fiction, no wormholes of any kind have been identified in real life. Einstein's theory of general relativity mathematically predicts the existence of wormholes, but none have been discovered to date.

Infinite cylinder theory

Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder) where you could take matter that is 10 times the sun's mass, then roll it into a very long, but very dense cylinder. The Anderson Institute, a time travel research organisation, described the cylinder as "a black hole that has passed through a spaghetti factory."

After spinning this black hole spaghetti a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around the cylinder — could travel backwards in time on a “closed, time-like curve”.

The major problem is that in order for the Tipler Cylinder to become reality, the cylinder would need to be infinitely long or be made of some unknown kind of matter. At least for the foreseeable future, this kind of endless interstellar spaghetti is beyond our reach.

Time donuts

Theoretical physicist Amos Ori proposed a model for a time machine made out of curved space-time — a donut-shaped vacuum surrounded by a sphere of matter.

“The machine is space-time itself,” Ori explained. “If we were to create an area with a warp like this in space that would enable time lines to close on themselves, it might enable future generations to return to visit our time.”

There are a few caveats to Ori's time machine. First, visitors to the past wouldn't be able to travel to times earlier than the invention and construction of the time donut. Second, and more importantly, the invention and construction of this machine would depend on our ability to manipulate gravitational fields at will — a feat that may be theoretically possible, but is certainly beyond our immediate reach.

Suspended animation

One that definitely comes from the realms of science fiction is the idea of time travel via suspended animation. One way to time travel to the future may be to slow your perception of time by slowing down, or stopping, your bodily processes and then restarting them later.

Bacterial spores can live for millions of years in a state of suspended animation, until the right conditions of temperature, moisture, and food kick start their metabolisms again. Some mammals, such as bears and squirrels, can slow down their metabolism during hibernation, dramatically reducing their cells’ requirement for food and oxygen. But could humans ever do the same?

Though completely stopping your metabolism is probably far beyond our current technology, some scientists are working towards being able to induce a short-term hibernation state lasting at least a few hours. This might be just enough time to get a person through a medical emergency, such as a cardiac arrest, before they can reach the hospital.

In 2005, American scientists demonstrated a way to slow the metabolism of mice (which do not hibernate) by exposing them to minute doses of hydrogen sulphide, which binds to the same cell receptors as oxygen. The core body temperature of the mice dropped to 13°C and metabolism decreased 10-fold. After six hours the mice could be reanimated without ill effects. Unfortunately, similar experiments on sheep and pigs were not successful, suggesting the method might not work for larger animals.

Another method, which induces hypothermic hibernation by replacing the blood with a cold saline solution, has worked on pigs and is currently undergoing human clinical trials in Pittsburgh. So maybe some of the things we see in sci-fi aren’t actually too far away.

To summarise…

Time travel is indeed a real thing, but it's not quite what you've seen in sci-fi stories. Under certain conditions, it is possible to experience time passing at a different rate than the 1 second per second that we know. But big time jumps are definitely out of our reach — for now, at least.

My own personal summary is that it’s all very confusing and I’m still not sure I fully understand it, hence why I am not a quantum physicist. 

For the most part, time travel remains the domain of an ever-growing array of science fiction books and media.

Science Fiction

Time travel has captivated countless science fiction writers and spurred a genre so extensive that Wikipedia lists over 400 titles in the “Movies about Time Travel” category. In franchises like Doctor Who, Star Trek, and Back to the Future, characters climb into some wild vehicle to blast into the past or spin into the future. Once the characters have travelled through time, they grapple with what happens if you change the past or present based on information from the future (which is where time travel stories intersect with the idea of parallel universes or alternate timelines).

I was going to cover this in this episode and give you a whole timeline of time travel within the sci-fi genre, but I realised when I started researching it that it can be its own entire episode in itself (unless this one turns into a 2-hour epic).

So that’s what I’m going to do, only I’m going to expand on it and do a whole timeline of the sci-fi genre as a whole, diving more in-depth into when specific tropes like aliens, time travel, space, etc were introduced. So look out for that coming at some point soon!