As humans, we have always challenged the boundaries of nature and the universe around us.
Compared to our fellow living creatures – which are still held under the ropes of nature for their survival – our species had a different approach. We shaped the world around us in order to fit our needs. The formation of cities, harnessing energy from fossil fuel and renewable resources, the invention of modern medicine, the manufacturing of cars and planes, the creation of artificial intelligence: all these are but a small sample of what humans have been able to do since the inception of our species approximately 200,000 years ago.
Our innate knack for discovering and making new things is perhaps best represented through pop culture. The genre of science fiction in particular has been a mainstay in bookstores as well as in theatres. Some are captivated by the action sequences from Blade Runner or The Matrix, while others prefer the horrors provided by the dinosaurs in the Jurassic Park series. Flocks of moviegoers are enthralled by the Star Wars series, or by movies such as Star Trek, Back to the Future, and Interstellar. In the case of the latter three movies, they are bound together by a single premise: the possibility of time travel.
Who would not want it, anyway? Imagine having the ability to see how your future self will turn out, which will likely relieve you from all the pressures caused by your present-day anxieties and responsibilities. For some people, time travel may even serve as a means for them to correct their wrongs in the past: mending broken relationships, passing college entrance exams, or even just cutting out the day you were humiliated in front of your crush way back in high school. Indeed, the idea of time travel presents us with seemingly limitless possibilities.
However, one question still remains: Is time travel even possible in the first place?
To answer this question, we need to take a look at the brainchild of arguably the most popular scientist in history, Albert Einstein: the ideas concerning the special and general theories of relativity. If you recall, a Filipina high school student, Hillary Diane Andales, recently made an explainer video on this for an international competition. Watch the video to jog your memory a bit:
Time to ponder
For starters, the special theory of relativity shows that the laws of physics are the same for non-accelerating objects. This holds true for as long as they have the same inertial frames — places wherein Newton’s laws of motion work. In space, however, there exists an infinite number of reference frames. Thus, according to Einstein, the speed of an object can be recorded differently based on the frame of reference used.
For example, if your car is moving at a constant speed of 50 km/hr, you will not sense any movement as you sit inside. It would seem like you are not moving at all. An observer on the road, however, would see that you and your car are actually moving at the aforementioned speed.
This connection between space and time prompted Einstein to coin the term space-time, which operates on the notion that the universe has three space dimensions (up/down, left/right, and forward/backward) and one time dimension.
This allowed Einstein to determine that the laws of physics apply to all non-moving and moving observers at constant speed. He also showed that the speed of light within a vacuum (or space completely devoid of matter) is the same for all observers, regardless of their speed relative to the light source, and that the speed of light itself (299,792 kilometers per second) sets the limit for the speed of any object. The theory is named “special” because it can only be observed when objects are moving at a constant speed and direction in space.
Einstein’s theory of general relativity, on the other hand, aimed to address this gap by adding acceleration, which denotes a change in movement speed, to the mix. He then found out that massive objects in space (such as planets) can stretch space-time. Picture a dimple reminiscent of a heavy ball in a trampoline:
The dimple would then cause lighter objects to spiral towards it, which is essentially how gravity works. Gravity, of course, is the most popular example of acceleration – one g is equal to 9.8 m/s2.
The question is: How can these theories of Einstein explain or allow time travel?
Before continuing, it is important to understand that as humans, we tend to perceive the movement of time in two directions: forward or backwards.
First, let’s focus on traveling forward in time. With Einstein’s theory of special relativity stating that time and space are connected, it also allows for the fact that time is relative to the observer. This phenomenon, called time dilation, means that an object in motion experiences time more slowly in comparison to an object that isn’t moving.
According to NASA, a hypothetical example would be if a 15-year-old boy were to fly into space at 99.5 percent of the speed of light for five years (from the perspective of the astronaut). When he returns to Earth, he would only feel like he is 20 years old. However, his classmates would already be 65 years old, due to the fact that they were moving more slowly than the astronaut. In other words, time moved faster for them. In essence, you can say that the boy astronaut in this example has indeed already traveled forward in time. After all, he returned to a place that, instead of being five years older as he would perceive, has actually aged fifty years. Based on this explanation, it is theoretically possible to travel forward in time.
What about traveling to the past, though?
A “hole” new method of time travel
The concept of wormholes came from the theory of general relativity presented by Einstein and physicist Nathan Rosen. In an article for Forbes, astrophysicist Ethan Siegel suggested that an immensely strong, dense, positive energy fluctuation would create curved space in one fashion, while an immensely strong, dense, negative energy fluctuation would create a curve in an opposite manner. The connection of the two curves would then create a wormhole, which may have the ability to transfer one particle from a certain point in space-time to another. The wormhole would essentially have two “mouths” connected by a single “throat.”
However, for this to work in a scale that can transport humans from one point in space-time to another, it would require two things that have not yet been discovered: (1) an actual wormhole — with black holes as its “mouths” — and (2) a huge amount of negative matter or energy.
Okay, so this traversable wormhole should theoretically have the ability to transport a human from one point to another in space. How about the time aspect, though?
In the same article, Siegel indicated that traveling back in time would necessitate that one end of the wormhole be suspended in space, close to motionless, relative to another object. The other end, however, would then need to be pushed off via a journey that’s close to the speed of light relative to the other end. Given the fact that the other end would be moving continuously for, say, a year, once a person enters that moving end, that end would have aged 40 years, while the motionless end would only have aged a year.
Siegel explores this possibility, but also provides a logical reason why it wouldn’t work:
“If, 40 years ago, someone had created such a pair of entangled wormholes and sent them off on this journey, it would be possible to step into one of them today, in 2017, and wind up back in time at the mouth of the other one… back in 1978. The only issue is that you yourself couldn’t also have been at that location back in 1978; you needed to be with the other end of the wormhole, or traveling through space to try and catch up with it.”
All in all, despite taking all challenges and limitations into consideration, traveling back in time using this method would still not be possible. First, no evidence of the kind of negative matter or energy this requires exists, and the technology for this setup probably wouldn’t even be within reach for the foreseeable future. To add, such a wormhole would not be stable, and may collapse very quickly. This would certainly not be enough to allow transportation from one time frame to another.
Plus, to make time travel possible, one would have to move faster than the speed of light in a vacuum. However, Einstein’s equations demonstrate that to move at the speed of light, an object would need to have both infinite mass and zero length. Based on our present-day understanding of Einstein’s theories and the technology on hand, this method of time travel is physically impossible.
With the way humans have progressed, it isn’t so surprising that some who aim to transform science fiction into reality succeed at doing so. For example, who could have anticipated the transition from the old Nokia 3310 to the bezel-less and considerably more advanced iPhone X? Indeed, such progress was made within the realm of physics, through applied knowledge in electronics and engineering.
Time travel, however, is a different story. For now, we can only imagine ourselves walking on an unlit street, struggling to know which way we are actually heading. –MF
Author: Rafael Ambag
A science kid at heart, Paeng aims to spark the interest of the common man in science through science journalism and organizing science camps for elementary children. He is an incoming freshman under UP Diliman’s BS Molecular Biology and Biotechnology program.