Prologue: For everyone to easily understand

Ⅰ: Beginning
1.
Black Hole Journey

II: Einstein's special theory of relativity
2.
Running Light
3.
Redefining time and space
4.
New common sense

III: Einstein's general theory of relativity
5.
Newton's irrationality
6.
Redefining Gravity

Ⅳ: The meaning of relativity
7.
black hole
8.
Expanding Universe

Epilogue: The Trace You Left in the Universe

Acknowledgements

General relativity is proven after 104 years with successful black hole observation!

On April 10, 2019, the word 'black hole' appeared on the real-time search list from the morning and did not disappear until late at night.
It was because of the news that night that the world's first image of a black hole would be revealed.

At 10 p.m., evidence and images of the black hole were finally revealed.
A photo of the black hole's strong gravity distorting the light from surrounding celestial bodies and wrapping it in red made the front page of a portal site.
The black hole photographed is located at the center of the giant galaxy M87, 55 million light-years away from Earth, and is said to weigh 6.5 billion times the mass of the sun.
We can finally see the true form of a black hole, something we have only heard about and imagined in the past.

104 years ago, when black holes were not only impossible to observe but even their existence was not certain, Einstein published his general theory of relativity, which states that spacetime is warped by gravity.
And after 104 years, his theory was proven to be completely correct.

Einstein was right!
The theory of relativity is easy and fun to learn without complicated formulas and equations!

If you are a math dropout (a person who has given up on math) or science dropout (a person who has given up on science), and the words “theory of relativity” scare you and make you want to avoid the book, I hope you don’t feel that way.
"What is the Theory of Relativity?" is different from other scientific theory books that claim to be "easy" to read and are filled with formulas and equations.
This book is written really easily and has no formulas or equations (though they appear occasionally for readers who haven't given up on prime numbers, but you can still understand the content without them). If the content seems even a little difficult, it kindly includes illustrations to hold on to our souls trying to escape into space like a black hole pulling light.

Even the incomprehensible concept of general relativity, which states that 'time and space are one and space-time is warped by gravity', and the special theory of relativity, which contains the world's most famous formula E=MC², can be perfectly understood by reading 'What is the Theory of Relativity?'
In particular, the special theory of relativity
1.
The laws of nature are the same for everyone.
2.
The speed of light is the same for everyone.
If you just accept these two principles, it's very easy to understand.
This book discusses black holes, which are receiving renewed attention due to successful observations, and raises the question of what it would be like if we traveled to a black hole.
Most people believe that if the Sun suddenly becomes a black hole, it will instantly suck in the Earth and other planets in our solar system and destroy them.
But astrophysicist Geoffrey Bennett, author of "What is Relativity?", says, "Black holes don't suck in."
Even if the sun becomes a black hole, the Earth will still orbit around that black hole.
If even light cannot escape a black hole due to its strong gravity, why is Earth safe?

If you read "What is the Theory of Relativity" you will understand why.
Aren't you curious?