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.