In the 19th century, James Maxwell discovered that all electromagnetic radiation, including visible light, traveled at a certain fixed speed. This speed is the ultimate speed limit of the universe. As far as we know, there is absolutely no way to exceed this speed. [1] One interesting property of light is that it will appear to travel at the same speed regardless of relative velocities between two different observers. This is a bit counter-intuitive. If you are traveling at 40 miles an hour, and a car in the other lane is traveling in the opposite direction at 40 miles an hour it would seem there is approximately an 80 mile per hour difference between your two velocities. Similarly, if you are traveling at 40 miles per hour and the car next to you going in the same direction is also traveling at 40 miles per hour, it would seem like the car adjacent to yours is stationary. However, regardless of how fast you may go, light will always appear to go at the same speed. This concept is the basis of special relativity.

Special relativity claims that observers in relative motion will encounter the phenomena of time dilation and space contraction. In other words, if I am moving with a certain velocity relative to you, the clock on my arm will tick slower than the clock on your arm. Relative motion also has an effect on perceived simultaneity. Also, the length I measure of a vessel in motion from inside will be different than the length a stationary observer calculates.

But what exactly does stationary mean? Is there some uniform medium that permeates the entire universe which we can measure our speed against? At one point, physicists believed in such a medium, termed aether. Today, the definition of stationary has changed a bit. Anyone who remains at a constant velocity can make the claim of being a stationary observer. Velocity is meaningless unless a relative reference point is also given.

Next, lets define what exactly speed is. Speed is the measure of some distance covered in a certain duration. Termed as such, this is an insipid definition. Keep in mind, however, that distance is intrinsically linked with space, and duration is intrinsically linked with time. Revamping our understanding of speed, as the constancy of the speed of electromagnetic radiation forced us to do, consequently required us to revamp our understanding of both space and time.

Time dilation and space contraction occur even when you are simply driving your car. The reason that they are not noticeable is because they occur on an extremely small scale. The closer one gets to the speed of light, the more noticeable the effects of time dilation and space contraction become.

In order to show why time dilates lets devise a special kind of clock. This clock consists of two parallel mirrors facing each other. In the middle of these two mirrors we will put a photon, the smallest quanta of light. This photon will bounce off of the two mirrors indefinitely. We will define the time that the photon takes to traverse the vertical distance between the two mirrors as a billionth of a second. Once we start the clock up, the photon keeps bouncing back and forth. It is a simple contraption, yet a very accurate clock. Now, what happens if instead of watching the photon bounce between the two mirrors while the clock is stationary compared to us, we set it in motion? Once the clock is set in motion, we will notice that the photons take longer to traverse the distance in between the two mirrors. The reason for this is that the photon must not only traverse the vertical distance between the two mirrors, it must now also traverse the horizontal distance that the mirrors have traveled in the time it takes the photon to traverse the vertical distance between the two mirrors. A stationary observer will notice the photon traveling at an angle. Keep in mind that the photon is traveling at the same speed whether it is stationary compared to an observer or whether it is moving compared to an observer. Since the photon has to travel further when in motion relative to us at its original speed, it will take longer for it to cycle between the mirrors. Note that the faster the clock is moving compared to an observer the further the photon will have to travel, therefore the more time will dilate. Also, note that since the clock has a constant speed it has the right to make the claim that it is stationary. Since the photon is moving with the same speed as the mirrors, from the clock’s perspective the photon will bounce back and forth between the mirrors in the same time that it did when it was stationary compared to us. Essentially this shows that an observer moving with the same speed as the clock will notice a different cyclical rate than an observer moving at some other speed.

As I mentioned before, relative motion also affects perceived simultaneity. To demonstrate this lets set up a different scenario. Let’s have two people sitting at opposite ends of a train facing each other equidistant from a central light source. Once the light reaches each person, he must immediately stand up and begin doing jumping jacks. To an observer on the train, it will look like each individual begins doing jumping jacks simultaneously as expected; however to an observer outside the train it would seem that the person at the back of the train began doing jumping jacks before the person near the front of the train. Both observers on the train and observers outside the train would be correct in their assertion. The reason for this is because to an outside observer the individual at the back of the train is moving towards the light wave while the individual at the front of the train is receding away from the light wave; therefore the light wave has to travel a shorter distance to the individual at the back of the train. Since light moves at the same speed in all directions, it will arrive at the back of the train first since it has to travel a shorter distance to get their, and therefore the individual at the back of the train will seem to have started his jumping jacks before the individual at the front of the train. This example shows how relative motion affects perceived simultaneity of events.


Based on our understanding of relative motion’s effect on time, we can easily see the effects it has on space as well. For instance, let’s say there are two people, Sov, and KA, Sov in a car and KA outside. When the car’s front bumper passes a certain marker both people start a stopwatch, and when the cars rear bumper passes the same marker both people stop their stopwatch. Both people calculate the length of the car by multiplying the speed of the car by the time elapsed in between the front and rear bumpers passing the marker. Since from Sov’s perspective he is stationary, he knows that KA’s watch is running slower than his own, therefore he knows that KA will have calculated the car’s length as shorter than what Sov would have calculated it to be. Essentially, relative motion causes space to contract. As a car moves faster and faster, the distance between its front bumper and rear bumper lessens.


It is an important fact about our universe that space and time are intertwined. It is not correct to think of them as two separate ideas, but rather as one single idea – spacetime. Time is simply the fourth dimension. In fact, it may help to describe the effect of motion on spacetime.


Again, we’ll use a car analogy. If a car is traveling supposed to travel 100 miles west, and it’s traveling at 100 miles per hour towards the west, it is safe to assume that the car will reach its destination in one hour. However, if some of that speed were deflected to a southward direction so the car was moving southwest, it would take the car longer to travel 100 miles west. And if it were moving completely in a southern direction, it would never complete its journey towards the west. Similarly, all matter is constantly traveling through spacetime. In fact, all matter travels through spacetime at the speed of light. Our motion is simply far more directed in the time dimension than in the three spatial directions most of the time. Note, the faster we move in the three spatial directions the slower we move in the time dimension. Also, note that since photons move at light-speed through the three spatial dimensions, they do not move through the time dimension at all. That means a photon that has been around since the big bang has not aged at all.