Special relativity

Physics is the same for every perspective and the speed of light is constant

What is it?

This clip from Channel 4’s wonderful comedy Father Ted actually explains the basics of the Special Theory of Relativity (aka Special Relativity) quite nicely:

Special Relativity is all about frames of reference. In the clip, Father Ted explains to Dougal that the toy cows in his hands are ‘small, but the real ones out there are far away’.

Special Relativity takes things one step further to include motion. It states that everything is moving relative to everything else.

Let’s explain this with the help of the Red and Blue LEGO figures and the video (at the top of this post) . 

Blue is on the ground as Red flies by. Let’s say Red flies past at 30 mph. Blue would say Red is travelling at 30mph. Blue is right.

But from Red’s frame of reference (or perspective), Blue is travelling at 30 mph. 

Under Special Relativity, Red is also right. Both frames of reference are correct. It could be that Red is moving at 30 mph or it could be that Blue (and the Earth and everything on it) is moving in the opposite direction at 30 mph.

But this rule only applies under certain special circumstances (hence, the name special relativity). This special circumstance is that you must be travelling at the same speed and in a straight line. Namely, you have to travel at a constant velocity. (Velocity describes both the speed and direction of travel.)

If you change your direction or speed, then special relativity no longer applies. But general relativity does (more on that in the next post).

Now, imagine your car is moving at close to the speed of light and turns on its headlights. What does that mean?

Would the light leaving the headlamps now be moving at a speed greater than the speed of light to the external observer?


Remember, everything is moving relative to one another. The Earth is moving, our galaxy is moving and the light is moving relative to your motion and that of the car.

The Special Theory of Relativity is based on two principles:

  1. Relativity: the laws of physics don’t change, even for moving objects
  2. Speed of light: the speed of light is the same for every observer, regardless of how they move relative to the source of light.

So what?

Special Relativity has many consequences for our entire understanding of the Universe. These are:

  1. Energy and mass are interchangeable: The world’s most famous equation E=mc² results.
  2. Time dilation: Time moves relative to the observer. Time moves more slowly when you are moving, compared to when you are standing still
  3. Length contraction: The length of moving object measures shorter than its measured length when it is at rest.

These are three pretty huge concepts to get your head around. (So, we’ll deal with those in some upcoming posts.)

What else?

The Special Theory of Relativity was developed by Albert Einstein in 1905. Before Einstein, we understood the Universe in terms of Newton’s laws of motion.

Which is fine. Special Relativity is only really needed when you’re dealing with extremely small objects or objects moving at extremely fast speeds. Otherwise, you can use Newton’s laws of motion.

Newton’s laws of motion describe the motion of everyday objects, travelling at slow speeds (compared to the speed of light). But they deal with absolute time and motion. In other words, they assume two observers moving in different directions would see events unfold in the same way.

Cracks had started to appear in Newton’s three laws of motion before Einstein’s Special Theory of Relativity.

In 1865, James Clark Maxwell published a set of equations to explain electromagnetism. However, these equations implied that the two observers moving in different directions would NOT see events unfold in the same way. This contradicted Newton’s established laws of motion.

A new mathematical concept was later devised to preserve Maxwell’s equations when moving from one frame to another. This is known as the Lorentz transformation.

The Lorentz transformation implies that time and length can change, depending on your frame of reference. 

Einstein wondered if the Lorentz transformation was just a maths trick, or if there was something more significant about it.

Using the Lorentz transformation to all of time and space, Einstein realised time and space are not absolute and change their properties, depending on your frame of reference. So, Special Relativity was born.

However, Einstein realised Special Relativity doesn’t fit with Newton’s description of gravity. So, the General Theory of Relativity was later developed by Einstein.

But scientists are still running into problems as Special Relativity (which describes large objects) is completely incompatible with Quantum Mechanics (which describes small objects).

A recent article from The Guardian describes the impact of this problem:

“Relativity gives nonsensical answers when you try to scale it down to quantum size, eventually descending to infinite values in its description of gravity. Likewise, quantum mechanics runs into serious trouble when you blow it up to cosmic dimensions. Quantum fields carry a certain amount of energy, even in seemingly empty space, and the amount of energy gets bigger as the fields get bigger. According to Einstein, energy and mass are equivalent (that’s the message of E=mc2), so piling up energy is exactly like piling up mass. Go big enough, and the amount of energy in the quantum fields becomes so great that it creates a black hole that causes the universe to fold in on itself. Oops.”

Want more?

The implications of Special Relativity are fascinating and complicated. This video explains why you can’t turn on your headlights at light speed and how Special Relativity could affect our very existence:

Relativity for the Questioning Mind

A series of problems, guiding you through Relativity.

What’s So Special About Special Relativity?

Outlining the impact of Special Relativity.

The Reality Frame by Brian Clegg

A tour-de-force of Special Relativity and much more!

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