"Feynman's Six Physics Stories" is a compilation of six easy-to-understand lectures from the Feynman Lectures on Physics, lectures that Feynman gave to undergraduates at the California Institute of Technology.
Since then, people who were fascinated by Feynman's physics lectures have requested to hear the next lecture, and six additional chapters have been extracted and published as Six Not So Easy Pieces: Another Story of Feynman's Physics.
This book presents 'Feynman-style lectures' on 'relativity, symmetry, and spacetime', but in fact, as the title suggests, it is 'not so easy'.
But if you're not confident that you understand the theory of relativity, whether you're a specialist or not, this book will provide you with one last chance to conquer it.

The theory of relativity opened a new chapter in physics by introducing symmetries that were previously unimaginable, but recently, it has been discovered that existing symmetries that were 'taken for granted' do not actually exist.
This fact, discovered by Lee, Yang, and Wu in 1957, sent shockwaves through the physics world - the main idea was that the laws of physics changed when the left and right sides of some basic natural phenomena were swapped.
However, Feynman created a new theory that included all of these asymmetrical elements, thereby raising the level of understanding of nature to a new level.

For physics to advance, mathematics must advance as well.
Because new physical theories require new mathematical tools to explain them.
Once the mathematics is organized, physics becomes much simpler.
Vector calculus is a representative example.
Originally, three-dimensional vectors were developed to mathematically describe the properties of space, which allowed physicists to perfectly describe Newton's laws of motion in a space without any direction.
In other words, the space we live in has rotational symmetry.
And all of this unfolds before our eyes as if it were an everyday event through Feynman's eloquence.


In the theory of relativity, even time is affected by symmetry transformations, so a calculation method for four-dimensional vectors is required.
This book unravels Feynman's complex concept of four-dimensional vectors in detail, providing a vivid explanation of how time, space, energy, and momentum are intertwined in a four-dimensional relative space.

--- In the introduction

So, what is the philosophical significance of the theory of relativity? Considering only the ideas and inferences physicists have proposed through the theory of relativity, it can be summarized as follows: First, even laws that have reigned as truth for a long time and yielded accurate results can easily be proven wrong in the face of new experimental results.
While it was certainly a shocking event that Newton's laws were wrong, that doesn't mean that physicists of Newton's time were careless.
In the past, the speed of objects that were physical objects was so slow compared to the speed of light that relativistic effects were barely noticeable.
But this one incident forced scientists to become incredibly humble.
Because I learned the painful lesson that "anything can turn out to be wrong at any time!"


Second, no matter how unfamiliar ideas like time passing slowly in a moving system may be to us, we cannot deny them simply because we don't like them.
It is not personal preference that determines whether a theory is accepted or not, but only experimental results.
The reason we're going on and on about this absurd idea is because it matches up well with experimental results.

Finally, as physicists studied the theory of relativity, they realized how useful the concept of 'symmetry in the laws of physics' was.
To put it a little differently, we can deepen our understanding of nature by finding transformations that do not change the laws of physics even when certain transformations are applied.
As we learned before, the fundamental laws of motion do not change their form when the coordinates are rotated.
And as explained in the previous chapter, the laws of physics are invariant under a special transformation of spacetime called the Lorentz transformation.
In general, transformations that do not change the form of the fundamental law provide us with useful information.
So, what is the philosophical significance of the theory of relativity? Considering only the ideas and inferences physicists have proposed through the theory of relativity, it can be summarized as follows: First, even laws that have reigned as truth for a long time and yielded accurate results can easily be proven wrong in the face of new experimental results.
While it was certainly a shocking event that Newton's laws were wrong, that doesn't mean that physicists of Newton's time were careless.
In the past, the speed of objects that were physical objects was so slow compared to the speed of light that relativistic effects were barely noticeable.
But this one incident forced scientists to become incredibly humble.
Because I learned the painful lesson that "anything can turn out to be wrong at any time!"


Second, no matter how unfamiliar ideas like time passing slowly in a moving system may be to us, we cannot deny them simply because we don't like them.
It is not personal preference that determines whether a theory is accepted or not, but only experimental results.
The reason we're going on and on about this absurd idea is because it matches up well with experimental results.

Finally, as physicists studied the theory of relativity, they realized how useful the concept of 'symmetry in the laws of physics' was.
To put it a little differently, we can deepen our understanding of nature by finding transformations that do not change the laws of physics even when certain transformations are applied.
As we learned before, the fundamental laws of motion do not change their form when the coordinates are rotated.
And as explained in the previous chapter, the laws of physics are invariant under a special transformation of spacetime called the Lorentz transformation.
In general, transformations that do not change the form of the fundamental law provide us with useful information.

--- p.140~141