A Simplified Introduction to Einstein's Theory of Relativity

Einstein's theory of relativity is as simple as it is groundbreaking, but how exactly does it work? Let's take a very quick look.
Christopher McFadden

Don't be scared, as complex as the Theory of Relativity seems to be; it's surprisingly simple. In this short article, we will attempt to explain what Einstein proposes to give you some insight. So, without further ado, here is our simplified explanation of the Theory of Relativity.

Technically speaking

When we refer to the "Theory of Relativity" what we actually mean is general relativity. Special relativity is a "special case" of general relativity. The combination of these two principles helps explain many subjects varying from the motion of the planets, the effect of gravity on light to the existence of black holes.

Special relativity states that the laws of physics, and thus the universe is the same for all equally "fast" observers. In the vacuum of space, the speed of light is a constant independent of any observer.

But what about acceleration and gravity? Einstein spent a decade musing on this. In 1915, he triumphantly produced his General Theory of Relativity. He determined that massive objects in space will cause warping or distortion of space-time which we all "feel" as gravity.

Thinking outside the box

Einstein, with his unusual way of thinking, assumed that experimental observations were correct. This was the complete opposite of his contemporaries'  thoughts. In the late 19th Century, physicists were all searching for something called the "ether". Ether was believed to be the medium that light traveled through. It had become, in essence, the quest for the holy grail. Einstein realized that his peers' obsession with the task was getting in the way of progress. His solution was to simply remove it from the equation. He assumed the laws of physics would work regardless of how things were moving. A strategy that did not conflict with what experimental and mathematical data has revealed.

In 1905, Albert Einstein developed his Special Theory of Relativity. His groundbreaking work invalidated centuries of accepted scientific thinking, as well as changing how we perceive the world around us.

As its name suggests this theory is only applicable for special cases, i.e. when both objects are moving with constant or uniform speed.

Einstein explained that the relative motion of two objects should be the frame of reference rather than an external, esoteric "etheric" reference system. By way of example, say you were an astronaut in a spaceship, observing another spaceship at a distance. The only thing that matters is how fast you and your observed target are moving with respect to each other. One snag, however, special relativity only applies if you are traveling in a straight line and not accelerating. If acceleration takes place, General Relativity needs to be applied.

The theory is based on two fundamental principles:

Relativity - The laws of physics do not change. Even for objects moving at inertial, constant speed frames of reference.

The speed of light - It is the same for all observers regardless of their relative motion to the source of light.

Einstein's work creates a fundamental link between time and space. We intuitively envisage the universe as three-dimensional (up and down, left and right, forwards and backward) but also with a time component or dimension. The combination of these makes the 4-D environment we experience.

If you were to move fast enough through space, any observations you made about space and time would differ from anyone else moving at a different speed than you. As the difference between speeds increased, so would the observed differences.

It's all relative

Now, imagine you are in a spaceship with a laser in your hand. The laser beam shoots directly up to the ceiling, strikes a mirror and gets reflected back to the floor into a detector. Remember now that the ship is in motion, let's say at around half the speed of light. Relativity states that this move makes no difference to you, you can't "feel" it (just like on Earth as it is spinning on its axis and hurtling through space around the sun).

But here comes the twist:

An external observer, however, would witness something very different. If they could "see" into your ship, they would notice that the laser beam travels "up" at an angle, strikes the mirror and then travels downwards again at another angle to hit the detector. The observer would notice that the light path would be longer and at a more pronounced angle than you would observe in your ship. More importantly, the time taken for the laser to reach the detector would be different. Given that the speed of light is constant, how can you both reach the same conclusion that proves this theory? Clearly, the passage of time must be different for you and the external observer.

What the hell? This phenomenon is known as time dilation. In the above example, time must be "moving" faster for you compared to that of the slower observer. This simple example allows us to visualize Einstein's theory of relativity, whereby space and time are intimately linked.

As you can imagine such an extreme variance in the passage of time would only be appreciably noticed at very great speeds, especially close to the speed of light. Experimentation carried out since Einstein's revelations have validated his theory. Time and space are perceived differently for objects moving near the speed of light.

Mass, energy and the speed of light

Einstein certainly didn't rest on his laurels. Also in 1905, he applied his principles of relativity to produce the famous equation e=mc2. This innocuously simple equation expresses the fundamental relationship between mass (m) and energy (e). Pretty neat.

This little equation found that as we approach the speed of light, c, the objects mass balloons. So you get to travel really fast but your mass increases in relation to your speed. Bummer. At its extreme, if you were traveling at the speed of light both your energy and mass would be infinite. As you already know, the heavier the object, the harder it is; thus more energy needed, to speed it up. So by this token, it's impossible to exceed the speed of light.

Einstein's legacy

Until Einstein, mass and energy were seen as completely separate things. His work proved that the principles of the conservation of mass and energy are part of a bigger, more unified conservation of mass-energy. Matter, therefore, can be turned into energy and vice versa due to the fundamental connection between them. That is, frankly, amazing.


To summarize, firstly, there is no "absolute" frame of reference, hence the use of the term "relativity". Secondly, the speed of light is constant for whoever measures it, whether in motion or not - I know crazy right? Lastly, the speed of light cannot be exceeded, it is the universal "speed limit".

Got it? Great. No? Don't worry if you didn't, it is, by its very nature, counter-intuitive. The greatest discoveries in science are often found in the realms outside of our "common sense".

Via dummies.com

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