This thought eventually changed the world forever.

The boy's name was Albert Einstein.

So what really was that thought?

And how could one thought change the world so thoroughly?

Well in this video we're discussing Einstein's theory of relativity and more importantly how he got there.

We cannot start talking about Einstein without discussing classical physicist Sir Isaac Newton.

In 1687, Newton published his book Mathematical Principles of Natural Philosophy.

In the book, he described history, laws of motion, and the law of gravity.

His book is considered one of the most important writings in the history of mankind.

After watching an apple fall, he asked himself a question.

If the apple falls, does the moon also fall?

These questions led him to discover the law of gravity.

According to his law of gravitation, every object in the universe enacts its own force of attraction on another object.

And this force is the reason why we are stocked on earth, why the moon orbits the earth, and indeed why the universe exists.

But that didn't answer all of Newton's questions, like why do objects attract each other, what is the source of gravity, and why does Mercury's orbit jiggle?

The rest remained a mystery until the era of Albert Einstein.

In 1915, Einstein published his theory of general relativity, or in other words, his In this theory, Einstein explained gravity and its source.

He answered all the questions that Newton couldn't, like Mercury's precession.

Let's take a step back to 1905 to truly grasp where these different pieces of the puzzle come to life.

This year, Einstein published his theory of special relativity.

It deals with the speed of light and the motion of objects.

It was fine with non-accelerating objects, but the theory did not apply when gravity was present, or if the object was accelerating.

He couldn't quite figure the rest out until one fine day in 1907.

He was observing a window washer on a ladder and had an epiphany.

He thought about what he would be experiencing while falling.

He imagined himself there and realized that while falling, the ground would not be pushing him so he would be in a free fall.

That put some of the pieces together, but still he wasn't fully finished.

He began to think about falling again.

This time he imagined himself in a room with no windows.

On the surface of the earth, you would weigh whatever your weight is now.

But imagine if the room was in a spacecraft, moving in an upward direction with the same 9.8 meters per second squared as here on the ground.

In that moment, if you were to weigh yourself, you would weigh the same as you do on earth.

Einstein realized that the observer would not be able to tell if he's on a spaceship or on the surface of the earth.

That is because he would be moving at the exact same rate of acceleration.

There would be no way to tell the difference.

This phenomenon is called the equivalence principle, which states that an object that is accelerating free of any gravitational pull is essentially no different than the same object that is stationary but affected by gravity.

In other words, something moving in space with no gravity has the same mass as something on earth that is not moving.

In his special theory of relativity, he could only deal with non-accelerating objects and the speed of light.

Let's pause here for a second.

This means that at that point, he knew how to unify his theory of special relativity with gravity.

Luckily, he didn't stop there.

He did another thought experiment.

This time he imagined what would happen if he pointed a laser beam from one side of the room towards the other.

If the room were in a spaceship that was moving upwards with the same acceleration rate as earth, the height of the beam would slightly be lower on the other side of the room.

As the floor moved up, it curved the light and caused it to bend down.

But this principle only applied in space.

When he thought about the same experiment on earth, the light would appear to be straight.

Of course, these two scenarios only happened in his brain.

Let's just take a brief moment to appreciate how brilliant he is to be able to conduct that experiment, all of that, in his brain.

Back to the topic.

Einstein wasn't convinced though, as this violated the principle of equivalence.

The acceleration of the room on the spaceship was the same as under the influence of gravity.

The height should be the same in both cases.

He realized that the only possibility could be that the light beam must be bending under the influence of gravity.

But how could this be?

What does this bending of light mean?

Well, thanks to his incredible imagination and deep thinking, he came up with an answer.

He considered that maybe the light was following the shortest and straightest path that it could, but that in space, there was no visual, objective straight path and maybe there was an inherent curvature.

This idea revolutionized the entire course of scientific history.

That space somehow gets curved in the presence of gravity and it creates an illusion that the light beam is being curved.

He hypothesized that whenever matter or energy present in space, it creates a curvature.

There's one really popular video on YouTube.

Imagine a trampoline with two types of masses, a heavy chunk of metal and a piece of marble.

When the metal is placed on the trampoline, it bends it.

Then the piece of marble is rolled on the surface.

It orbits the heavy chunk of metal.

In this scenario, we can see that no force is acting on the piece of marble, but it is still revolving around the heavy metal.

Einstein went even further and said that the same thing happens when the earth is orbiting the sun.

The sun acts like a chunk of metal and creates a curvature in space.

The same can be applied to all objects in the universe.

So according to Einstein, matter and energy warp space-time, making nearby objects fall towards it.

Another remarkable part of his theory is that it doesn't just include the three dimensions of space, but also time.

This is why we use the term space-time.

But how does time come into play?

Well, it comes from his previous theory, the theory of special relativity.

The speed of light is the same for the observers in any frame of reference.

Either the object is moving or in a state of rest.

Again, imagine a light beam traveling from point A to point B.

If we place a massive object like our sun between these two points, the sun will curve the path and the distance from point A to point B will increase.

So it will take more time for light to move from A to B than in the absence of the sun.

But that concept violates the theory of special relativity, which says that the speed of light is independent of the frame of reference.

This means that regardless of whether the light beam is traveling freely or under the gravitational field, it must take the same time for it to travel from A to B.

So for special relativity to be valid, time must slow down.

In other words, time moves slowly when you are near a massive object like in space, and moves faster when you are away from it.

For example, time for us on Earth is slower than the astronauts on the ISS, which has been directly confirmed through many experiments.

This phenomenon is called gravitational time dilation.

Back a century ago, no one was taking Einstein's ideas seriously.

But just four years after the publication of his general theory of relativity, astronomer Sir Arthur Eddington performed an experiment to test it out.

If the general relativity was correct, then the sun would warp the space around it, causing the light beams to bend and the position of the stars behind the sun to change.

Eddington followed this principle, and he found that Einstein was right.

That phenomenon is now called the gravitational lensing.

This was the first proof of Einstein's theory, and it took the world by storm.

Today we have found lots of evidence that proves his theory.

The predictions made by his theory a hundred years ago are now being confirmed today.

They have created a new window to look into the universe.

Thank you so much for watching this video.

I'm Harry and I'm so glad you made it to the end.

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I'll see you again next week for more curiosity videos.

Bye bye.