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.

I’ve chosen a biggie this week folks, Einstein’s General Theory of Relativity. And boy it really is a biggie! It would take volumes and volumes of books to take you through everything, and years and years of research and study for me to write it. So, I’m going to have a go and see if I can put together some kind of short comprehensive guide for you. Ready?

Imagine you’re standing on Earth, feeling gravity pull you toward the ground. For centuries, scientists believed gravity was an invisible force acting across space. Then, in 1915, Albert Einstein popped his two penneth in and proposed something that was truly astonishing. Einstein proposed that gravity isn’t really a force at all, and that instead, massive objects such as planets, stars, and galaxies actually bend the fabric of space and time itself, and what we experience as gravity is actually the result of moving through this curved spacetime.

In a flash, Einstein had overturned centuries of thinking and by doing so he gave humanity a completely new picture of reality.

Before General Relativity, Isaac Newton’s theory of gravity was king, and his equations successfully explained falling apples, planetary orbits, ocean tides, and the motion of the Moon.

However, for everything that Newton’s laws could explain, there were still some puzzles remaining. Newton’s explanation of gravity appeared to act instantly across vast distances, while Einstein’s Special Relativity showed that nothing can travel faster than light. There were also subtle discrepancies in Mercury’s orbit that Newton’s theory couldn’t fully explain, and Einstein believed there had to be a deeper explanation, and there was.

Einstein’s breakthrough began with a simple thought experiment.

Imagine you’re trapped inside a windowless lift (elevator to my American readers).

In one scenario, the lift (elevator) is sitting on Earth, and in another, it’s floating in deep space, but it is accelerating upward. In both cases, you feel pressed against the floor. But, without looking outside, you can’t tell which situation you’re in.

This observation became known as the Equivalence Principle, and that is gravity and acceleration are locally indistinguishable, and for Einstein, this was the clue that unlocked a completely new understanding of gravity.

This is where spacetime comes in, and it is so breathtaking in its simplicity it is mind blowing.

Einstein realised that space and time are not separate things, but instead, they form a single four-dimensional structure which he called spacetime.

Now then, you have to think of spacetime as the stage on which everything in the universe exists and moves. And, to do so, I’m going give you a classic analogy.

Imagine a giant trampoline stretched tight, and as it is stretched tight, it is flat. If you now place a bowling ball in the centre of the trampoline it creates a dip, and if you now roll a tennis ball nearby, its path curves as it rolls.

The tennis ball isn’t being directly pulled by the bowling ball, all it is doing is simply following the shape of the surface, and this is how planets move through spacetime that has been curved by stars and other massive objects.

I know it’s not perfect, but this analogy captures Einstein’s revolutionary insight that it is mass that changes the geometry of the universe.

So, to your first question, I know you have loads. How does gravity really work?

And the answer to this is when most people think about gravity, they imagine an invisible force pulling objects together, however, Einstein saw something completely different.

Here’s another simple analogy (there’s going to be a few of these, so stick with me).

Imagine it’s nighttime and you are driving along a mountain road. Although you keep the steering wheel pretty straight, the road curves left, looking from above, someone might reasonably think that an invisible force has pushed your car sideways when in reality, you’re simply following the shape of the road.

According to General Relativity, planets are doing exactly this, and the Earth isn’t being pulled around the Sun by a mysterious force, it is moving along the curved road of spacetime created by the Sun. The sun is the bowling ball on the trampoline, and the tennis ball is the Earth. Geddit? Of course you do!

Ok, let’s look how that applies to the whole of the cosmos.

At the heart of General Relativity is a set of mathematical relationships called the Einstein Field Equations, and physicists often summarise them this way, and again, it is incredibly brilliant in its simplicity.

Matter and energy tell spacetime how to curve, and curved spacetime tells matter how to move, and these equations connect the contents of the universe to the shape of the universe itself! Cool, huh!

These equations can describe everything from planetary motion to black holes and even the evolution of the cosmos.

And that neatly leads us on to the next question you are bursting to ask.

“If gravity isn’t a force, then why doesn’t the Earth fly away from the Sun?”

And for that answer we have another simple analogy.

Imagine you are throwing a ball. If you throw it gently it lands on the ground nearby to you, however, if you throw it harder, it travels further and if you were to throw it harder and fast enough, the Earth’s surface curves away beneath it as quickly as it falls. The ball keeps falling but never reaches the ground, and that is essentially what an orbit is.

Earth is constantly falling through curved spacetime around the Sun.

One of General Relativity’s strangest predictions is that time does not flow at the same rate everywhere, and near a massive object, time passes more slowly, and the further away, time passes more quickly.

OK, analogy number 3? 4? I’ve lost count already.

Imagine time as a river. Far from massive objects, the river flows quickly, and near a massive object, the current slows. Two people that have been travelling through different parts of the river can reunite and discover that different amounts of time have passed for them.

This isn’t science fiction; this is science fact, as GPS satellites must account for relativistic time effects to provide accurate navigation. To really delve into time, you should read The Order of Time by Carlo Rovelli. This book is a brilliantly understandable read, and you’ll have your head wrapped around time in no time. And it’s got Smurfs in it!

Something else General Relativity told us is that light can bend. Before Einstein, all the science nerds simply assumed that gravity only affected matter. General Relativity predicted that gravity affects light as well.

And to explain it, you guessed it, another analogy.

Imagine you are riding a bicycle along a painted line on the ground, you aren’t intentionally turning. You are simply following the path beneath you. Light behaves similarly. It travels along the straightest possible path through spacetime, and if spacetime itself is curved, the path of light appears bent. Observations of the famous solar eclipse of 1919 confirmed this prediction and also made Einstein world famous.

The bending of light creates one of astronomy’s most powerful tools, and that is gravitational lensing. This is where massive objects can bend light from distant galaxies and act like giant lenses.  If you hold a wine glass in front of a distant light, the image behind it appears stretched, distorted, or sometimes, even duplicated, and galaxies do the same thing to light, and astronomers use these natural lenses to study objects that would otherwise be too distant to see.

General Relativity also predicts objects so dense that spacetime becomes dramatically distorted, and, you’ve guessed it, as these are one of my favourites, it predicted black holes, and with them, their ever brilliantly baffling event horizons, which leads me to my next analogy, the river model of black holes.

Imagine a fish swimming upstream, and way ahead of it is a waterfall. While it is far away from the waterfall the little fishy can easily fight against the current, however, when it gets closer to the waterfall, the water moves faster, until eventually there is a point where the current exceeds the fish’s maximum swimming speed. Beyond that point, escape is impossible, and the little fishy is heading over. A black hole’s event horizon is similar. Beyond it, spacetime is being dragged inward so strongly that not even light can escape. But you know all this, I’ve written about it loads.

General Relativity also predicts that moving massive objects can create disturbances in spacetime itself and these are known as gravitational waves.

Drop a pebble into still water and waves spread outward in all directions. When massive objects such as black holes collide, they create ripples in spacetime that travel across the universe. And those clever scientists detected these waves directly in 2015, thus confirming another of Einstein’s remarkable predictions.

General Relativity has become the foundation of modern cosmology, and it helps explain the Big Bang, the expanding universe, the evolution of galaxies and the large-scale structure of the cosmos itself, and in many ways, modern astronomy is the story of applying Einstein’s theory to increasingly larger scales.

Unfortunately, Einstein’s theory, as brilliant as it is, isn’t perfect. Even though every major experimental test has supported General Relativity for more than a century, physicists know the theory is incomplete as it struggles to explain what happens at the centre of black holes, the earliest moments of the Big Bang, and how gravity works at quantum scales, that pesky teeny tiny stuff again, and finding a theory of quantum gravity remains one of the greatest challenges in science today.

If you remember only one thing about General Relativity, make sure it’s this

For more than 200 years, Isaac Newton’s theory of gravity successfully explained the motions of objects on Earth and the movements of the planets. In Newton’s view, gravity is a force that pulls objects toward one another. The Earth is pulled toward the Sun, the Moon is pulled toward the Earth, and an apple falls because the Earth exerts a gravitational force on it. This picture works remarkably well for most everyday situations and remains extremely useful today.

Einstein’s insight was way more radical.

According to General Relativity, gravity is not fundamentally a force at all. Instead, mass and energy change the geometry of the universe itself by curving spacetime. Planets, stars, galaxies, and even light move through this curved spacetime, following its natural paths. What we perceive as the force of gravity is actually the consequence of moving through a universe whose geometry has been distorted by matter and energy.

A useful way to picture the difference is to imagine a ball rolling across a landscape. Newton would describe the ball’s motion as being caused by forces acting upon it. Einstein would direct your attention to the shape of the landscape itself. If the ground is curved, sloped, or warped, the ball’s path changes naturally as it follows the terrain. In a similar way, planets orbit stars because they are moving through curved spacetime, not because an invisible force is reaching out and pulling them around.

This seemingly simple change in perspective transformed our understanding of the cosmos. From it emerged the prediction of black holes, the discovery of gravitational waves, the realisation that time can flow at different rates in different gravitational environments, and the modern picture of an expanding universe. More than a century after Einstein introduced his theory, it is still one of mankind’s greatest achievements, and it remains humanity’s best description of gravity and one of the most profound (I had to get that word in here somewhere) insights ever achieved in science.

And that my faithful readers, is Einstein’s Theory of General Relativity for you in a nutshell. For anyone who just wanted the 1905 Special Theory, it’s the short, sharp one about speed and simultaneity, no bendy trampolines required. You can pretend the last ten minutes didn’t happen, and, if you’re lucky, and you really want it, Special may get its own piece in a few weeks’ time.