Hello everybody, apologies for the three-day wait for this one, I’ve been away with the family over the weekend, during which I have managed to properly injure my back. Despite the pain though, it’s time for another promised article, the one on Special Relativity, and for this one I’m going to start with a classic.

Imagine you’re sitting on a train moving smoothly through the countryside, and while you’re sitting you happen to pull out a tennis ball, you conveniently have with you, and toss it to a friend sitting opposite you. Everything feels normal, the ball travels over the table exactly as expected, and your friend catches it. Now imagine I am standing beside the tracks watching as you make the same throw as the train passes by me. To you, the ball moved at about 20 mph, however, to me, observing from outside, the ball moved at the train’s speed plus the speed of the throw.

For centuries, this simple idea seemed obvious, and it was the foundation of Isaac Newton’s view of the universe. Then, in 1905, a 26-year-old patent clerk named Albert Einstein published a paper that changed everything. In that paper, he came to a startling conclusion. Space and time are not fixed but actually bend to accommodate one unbreakable rule. And that rule is:

The speed of light is always the same.

You all know this, I’ve written about it plenty of times in my articles now, but at the time, this was revolutionary.

299,792,458 metres per second, which is around about 186,000 miles per second, fast enough to travel around the earth about 7 times in one second, and fast enough to reach the moon in about 1.3 seconds. What makes light particularly special though, is every observer measures it moving at exactly that speed, regardless of their own motion.

I know this sounds impossible but just suppose you are standing still and you are measuring a beam of light from a torch (flashlight to my American readers) and I come racing towards it at 99% of light speed. Common sense tells us that I should measure the light moving only a little faster than me, but I don’t. And neither do you. What we both actually measure is exactly the same speed. Nature is somehow protecting this value, but it’s doing it at a price. Reality itself must change.

The first casualty of this is time. Now then, Newton believed time flowed identically everywhere, however, Einstein discovered that time behaves more like an elastic material. That is, the faster you move, the slower your clock runs compared with someone standing still, and physicists call this time dilation.

Imagine two identical clocks. One of them is safe and sound on Earth, hanging on your kitchen wall while the other is flying through space at enormous speed. If you were to retrieve the travelling clock after its journey it would show that less time has elapsed than the one on your kitchen wall. It has literally experienced less time, not because it has malfunctioned, and not because of some kind of optical illusion. It is showing less time because time itself has passed more slowly for it.

There are tiny particles produced high in the Earth’s atmosphere called muons which without relativity should decay long before reaching the ground, but lots of them survive. Why do they survive? Because they travel near light speed, and their internal clocks run slower. Exactly as Einstein predicted.

On to the next question. If time can stretch, what about space? Well, it turns out space makes sacrifices as well, as an object moving rapidly relative to you becomes shorter in the direction of travel, and physicists call this Length Contraction.

Imagine a spacecraft that is 100 metres long while resting in its space port. After undocking and zooming off to Alpha Centauri, as it approaches light speed, a stationary observer sees it become shorter and shorter, while the crew onboard experience nothing unusual, as from their perspective, it is the rest of the universe that has changed. And this is one of relativity’s strangest lessons. There is no single universal viewpoint as different observers can measure different lengths and times and still all be correct.

Moving on, most people believe they know what “at the same time” means. Einstein showed that they don’t.

Imagine a train, again. There’s a massive thunderstorm raging overhead and bolts of lightning strike both ends simultaneously. To someone standing on the station platform, they would see both flashes arrive at the same moment, so to them the lightning strikes happen simultaneously. However, to someone sitting on the moving train, because it is moving towards one flash and away from the other, the flashes reach them at different times, and to them the strikes were not simultaneous.

So, which person is correct in what they witness? The answer is both of them, because simultaneity itself depends on motion, and this idea was a shock for physicists as it meant that even the ordering of events can depend on your frame of reference.

Ok, let’s move on again. Before Einstein, space was space, time was time, and small furry creatures from Alpha Centauri were small furry creatures from Alpha Centauri.

Sorry, got a bit keyboard happy there, forget the small furry creatures from Alpha Centauri

Back to space and time, before Einstein, these were always perceived as separate things, but after Einstein, physicists realised they are really aspects of a single entity. That’s right, you’ve guessed it. Spacetime

Every event in the universe has three spatial coordinates and one time coordinate, and together they form a four-dimensional reality called spacetime. You are not merely travelling through space, what you are doing is continuously moving through spacetime. Right now. At this very moment.

This brings us back to the equation that everyone knows, especially if you are a regular reader of my stuff, and are aware of the trouble I had when trying to produce a quick mid-week article on it recently.

E = mc²

Perhaps the most famous equation in history. And to translate it into plain English, without all the complicated maths this time, it basically says that mass and energy are different forms of the same thing. Matter can become energy, and energy can become matter, and the conversion factor is the speed of light squared.

Since light speed is enormous, even tiny amounts of matter contain staggering amounts of energy, and as you regulars know, this principle powers the Sun, nuclear reactors, nuclear weapons, along with many other processes throughout the universe.

While we are on the subject of the speed of light, let’s take a look at it.

Science fiction often treats the speed of light as something that can eventually be exceeded, Special Relativity, however, says otherwise. As an object accelerates its energy increases, the energy required for further acceleration rises dramatically, and near light speed the required energy approaches infinity. Because infinite energy is impossible, a material object of mass can never quite reach the speed of light. Photons can travel at light speed because they have no rest mass (remember my equations the other week?). Everything else though must remain below that cosmic speed limit.

Perhaps the most famous story in relativity is the Twin Paradox. Ready for another analogy? Good, here we go.

Imagine identical twins. One of them stays on Earth, while the other is on our imagined spaceship travelling to Alpha Centauri at near light speed before eventually returning home. When they meet again, the Earth-bound twin is older, while the space-faring twin is younger.

This sounds impossible as shouldn’t each twin see the other moving? The key to this is that the traveller accelerates, turns around and changes reference frames and the Earth twin does not. When all the mathematics is done, the traveller genuinely ages less. Which is why Matthew McConaughey has hardly aged at all when he returns from his interstellar travels and visits his aged daughter. And what is really mind boggling about this is that if fast enough journeys were possible, your future descendants could be older than you after your return!

Moving along once more, many people think relativity is only useful to astronomers, whereas in reality you and I are using it all the time as we drive around to unknown destinations, or have a nosey at our phone to see where the kids are. You’ve guessed it, I’m talking about GPS navigation

GPS satellites carry incredibly precise clocks and those clocks do not tick at exactly the same rate as clocks on Earth so engineers must correct for relativistic effects or navigation errors would rapidly build up. Every time your phone finds a location, a small victory for Einstein is taking place!

So, what did Einstein really teach us?

More than a century later, Special Relativity remains one of the most thoroughly tested ideas in science. Experiments involving high-speed particles, atomic clocks and modern technology continue to confirm its predictions. But at its deepest, the lesson isn’t a mathematical one, it’s a philosophical one.

For thousands of years humans assumed time was universal, space was fixed, and simultaneous events were absolute. Einstein showed us that none of these assumptions are true and the universe is stranger, more elegant and far more astonishing than common sense ever imagined.

Special Relativity’s big reveal is that space and time aren’t the fixed stage we assume they are as they are flexible, they stretch, they bend around each other, and they do it all in service of one stubborn rule. And that rule is the speed of light never changes, no matter who’s watching.