Have you wondered if time travel is possible? The pinnacle of most Science Fiction movies is centered around the main character entering a Time Machine and finding themselves in an unknown era, either awed by the futuristic flying cars or terrified by the dinosaurs that once ruled Earth.
In truth, as far as we know one could never go back to an era where dinosaurs lived. However, traveling to the far future is not completely out of the question.

To understand how one may time travel, first we have to understand some of the fundamental properties of our universe. Light demonstrates one of these fundamental properties. The speed of light is the fastest speed for any object. More accurately, all objects move at the speed of light, unless they have mass, which makes it harder for them to accelerate to the speed of light.

Suppose you are on a subway platform and you watch a train pass to your left. From your perspective, the train has passed at some speed towards the left. However, a passenger inside the train does not see the same thing as you. Instead, they would see you moving to the right of them. From their perspective, they are just standing while the entire world moves past them. Of course, we know that the train is moving due to common sense and understanding of the world. Yet, without this understanding, it is no longer obvious who should be considered moving. An astronaut looking at Earth would say that it’s spinning, but you would not say you are currently spinning, nor would you see the world around you spin.

Though it may be an uncomfortable idea, the universe is relative and perspective matters.

So why is this important? The universe’s relativity changes not only the directionality but also other critical elements such as time and space. Yes, that’s right! Time is also relative! Objects that move faster and faster experience time slower. So to be a time traveler, all you have to do is get up and walk.

Time Dilation

Compared to a motionless object, you would experience a phenomenon known as time dilation. Time Dilation is the process in which moving objects experience time slower. However, the effects at walking speed would be near negligible, you would barely experience a shift in time. However, in situations with further extremes – such as objects traveling close to the speed of light – the effects of time dilation become more apparent.

Suppose you are moving at ⅘ times the speed of light, a decade that passes for you would equate to sixteen years for an unmoving observer. You would have aged ten years, while a motionless observer would have experienced sixteen years in the same duration of time. At even greater extremes, such as the event horizon of a black hole (the outermost edge of a black hole) that sees extreme gravitational forces, in the few seconds that pass while you are in the event horizon, perhaps millions of years will have occurred on Earth. After just a few moments at the event horizon, you would have lived past innumerable events and perhaps even the end of humanity.

Such is the nature of time dilation. Objects that move faster experience time slower. As you approach the speed of light, the effects of time dilation become more exaggerated, until you reach the speed of light, where time and space become irrelevant and the entire universe collapses into a singular point. This is why objects with a mass can never accelerate to the speed of light – they can only approach it. Only massless particles, such as gluons, can move at the speed of light. Unfortunately if moving extremely fast is the only way to travel to ‘far away’ galaxies, then I’m afraid we are out of luck as those galaxies will take thousands of years to travel to, even at light speed.

But why does time dilation occur in the first place? The truth is that they simply do. There is no concrete explanation; it is just the nature of our universe. In the same way that everything with mass attracts each other, aka gravity, objects that move at faster speeds experience time faster. A while ago, a twin went up into space and after he returned, he was younger due to time dilation.

Loss of Simultaneity

This phenomenon depicts a very important idea in special relativity: the loss of simultaneity. Objects lose their simultaneous nature in a moving frame of reference. For example, if you have a moving train, then the front and back ends of the train are no longer moving at the same time. If you were to have two clocks at the ends of a moving train, special relativity shows that the clock at the rear end would be ahead in time compared to its front-end counterpart.

Let’s say there were 2 individuals, Alice and Bob. Alice is on a platform watching a train go by, while Bob is on the train. Suppose that while Bob was on the train, he turned on a laser that shot two beams in opposite directions, hitting the two clocks on the opposite ends of the train. When this laser hits the clocks, the clocks stop ticking and record the time. In Bob’s frame of reference, the train is still and the rest of the world is moving, so the laser simply goes to the left and the right, hitting the clocks at the same time.

On the contrary, Alice would say that one of the clocks was hit before the other. Since the light was shot while the train was in motion, the rear end of the train was moving toward the beam of light while the front end was moving away from it. By this logic, because the train is moving, the light should hit the back of the train first since it is moving toward the laser. But wouldn’t this create a discrepancy? If the clock on the rear end of the train is hit with the light first, then it would stop sooner and the clocks would not read the same time.

This is where the loss of simultaneity factors in. The rear clock stops sooner, but it is also ahead of time. In the time it takes for the laser to reach the front clock, the clocks read the same amount of time. The rear clock was ahead of time and also stopped before the front clock, giving the latter sufficient time to reach the same time.

Although Alice and Bob see fundamentally different things – Alice says one clock stops first and Bob says both clocks stop at the same time – the result is the same. Both clocks read the same time, so there was no point where either of them was wrong. They experienced the world in different ways, but the ultimate conclusion was the same.

Time is a strange phenomenon that we still don’t fully understand. It seems wondrous and mind-boggling. One truth is that when things move very fast they experience time slower. New technology is not necessary to build a Time Machine, just improvements on current technology to allow one to travel close to the speed of light. When objects move, their rear ends are ahead of time as opposed to the front ends. We don’t notice the concept in our day-to-day lives, as the effects are so small that they are negligible. Though, the beauty of physics is searching these unknowns and making extraordinary discoveries while questioning the mundane. When you start to make the conditions more extreme, physics allows even time travel to happen!

When you start to make the conditions more extreme, physics allows even time travel to happen!