The reason the world we live in is not like that but like this is because the constants of physics have exactly these values.
If it weren't for that, neither I nor you would be able to exist in this world.
At the root of all our existence are the natural laws of physics and universal constants.
We can understand the universe because physics is the same everywhere in the universe.
The incomprehensibility of this world is because physics is the same everywhere in the universe.
--- p.10, from “Introductory Remarks”

I promised to meet my friend, who always keeps his promises, at 8 in the morning. When I arrived at the meeting place and looked at the clock, it was exactly 8 o'clock, but my friend was still nowhere to be seen.
But my friend showed me his watch, which showed 8 o'clock, even though he arrived at 9 o'clock on my watch.
So, in a world where the speed of light is slow, if you want to make a promise to meet somewhere and when, you also have to agree on how fast you need to go to that place.
A world where the speed of light is fast, or rather, a world where we move much slower than the speed of light, is a very convenient world for us.
I like the world better now, where it's easy to set appointment times and where time flows the same for all of us, no matter where we move every day.
--- p.28-29, from “Chapter 1 If the speed of light is the same as the speed at which I am moving”

Gravity only pulls, never pushes.
The electromagnetic force is stronger than gravity, but for large objects with a lot of charge, the repulsive and attractive forces add and subtract, so the total magnitude of the electromagnetic force may not be that large.
However, gravity can only increase as more matter gathers and gathers to form a large object, which can grow to an enormous size.
Electromagnetic forces have virtually no effect on the moon's orbit around the Earth.
We can accurately explain the motion of the moon using gravity alone.
When it comes to the force acting between large objects that make up the universe, we only need to think about gravity.
Just as dust gathers to form a mountain, gravity can grow enormously.
Gravity is a weak but powerful force.
--- p.45, from “Chapter 2: In a World Where Gravity Is 100 Times Greater”

Sungnyung is best enjoyed warm, and sikhye is best enjoyed cold.
If you put your mouth on it, you can vividly feel the heat or cold.
Even with such a clear difference, it is not easy to express it in the form of a quantitative number called 'temperature' (Professor Jang Ha-seok, a philosopher of science, describes in detail the struggles of numerous scientists to standardize temperature in his book 'The Philosophy of Thermometers').
Just as there needs to be a standard convention, such as '1 meter is this long', to allow us to compare different lengths, temperature also needs some convention.
Looking back, there were many interesting suggestions.
Suggestions include the melting point of butter, the temperature on the hottest day of summer, or the temperature in the basement of the Paris Observatory in France.
There was even a crazy suggestion to use the temperature of the hottest water you could stand to put your hand in as a reference.
Even Newton, who was revered by all physicists, proposed a reference point called the temperature of human blood, which now appears to be unreliable as it changes constantly.
Everyone's body temperature changes slightly throughout the day, and in the case of women, their body temperature changes regularly according to their menstrual cycle.
--- p.78, from “Chapter 4: When does water boil and when does blood become hot”

Boltzmann's entropy is what solved this paradox.
What it means to say that entropy increases now seems self-evident.
According to S=kBlogW, in the macroscopic world, an increase in S means that events with a greater probability of occurrence (i.e., events with a greater W) are more likely to occur.
This means that in the macroscopic world, only the change in the direction in which the cup breaks is observed, because the W corresponding to the state in which the glass pieces are scattered on the floor is much larger than the W corresponding to the state in which the exact same glass pieces are neatly gathered together to form a cup.
The second law of thermodynamics, also known as the law of increasing entropy, can be rephrased as "whatever is likely to happen, will happen."
This is precisely what Boltzmann made us realize.
--- p.118, from “Chapter 6: Why Things Happen”

There is one more experiment I remember from my college days about the charge of electrons.
This is an experiment conducted by American physicist Robert Millikan using oil drops.
The charge of an oil droplet is measured by using the gravitational force acting on the charged oil droplet, the electric force due to the applied electric field, and the air resistance acting on the moving oil droplet.
By measuring the charge of the charged oil droplets, we can see that it is an integer multiple of a certain value, and using this, we can calculate the elementary charge e of an electron.
My conclusion from this experiment was that Millikan really had a good eye.
Because it was really difficult to track and measure the speed of one of the many oil droplets visible to the eye.
Of course, you can get good results if you are not a person who is not good at experiments like me.
--- p.150-151, from “Chapter 8: It was the electrons that separated us”

Pi is a number obtained by measuring the circumference C of a circle and dividing it by the diameter D (𝝅=C/D).
It is very important that the pi p is given by the ratio of two lengths C and D.
For this very reason, whether the circumference and diameter are measured in the current international standard unit of length, the 'meter', or the American 'inch', which still does not follow the international standard, or the 'ja' of the Joseon Dynasty, or the 'cubit', the unit of length in the Christian Old Testament, the pi always yields the same value.
(The unit of length is the same in both the numerator and denominator of 𝝅 = C/D, so it is reduced, and therefore 𝝅 is a unitless number, that is, a number without a ‘dimension.’) Since pi 𝝅 is a dimensionless number, any civilization with sufficient mathematical knowledge, whether it is an ancient civilization on Earth or an extraterrestrial civilization living somewhere in the universe, must know what 𝝅 is.
If you tell them that the 2 o'clock direction is an angle of 𝝅/3, even aliens will understand.
--- p.165, from “Chapter 9: What Existed Before the Universe?”

The Bohr radius plays an important role in determining the size of an atom, and the Bohr radius is proportional to the reciprocal of the electron mass.
That is, if the mass of an electron increases by a factor of about 1030, the Bohr radius decreases by a factor of 10-30.
As the size of all atoms decreases in this way, the size of all objects decreases rapidly, just as the charge of electrons decreases.
A world where the size of the Earth is smaller than the size of a proton.
--- p.183, from “Chapter 10: It’s Fortunately the Earth is Bigger than an Atom”

What if superconductivity could be realized at the temperatures and pressures we experience in our everyday lives? Superconductivity is already widely used around us.
Magnetic resonance imaging (MRI), used in hospitals, places the body within a very strong magnetic field and creates images by measuring the magnetic moments of water molecules within the body. Creating a very strong magnetic field is crucial for MRI equipment to function.
For this purpose, MRI uses the principle of electromagnetism.
It is to create a magnetic field by flowing a large current.
However, there are limits to creating powerful electromagnets from ordinary materials.
Because of the electrical resistance of the material, when a large current is passed through it, not only does a huge amount of energy dissipate as heat, but a massive cooling device is also required to resolve this.
This is why many MRI devices today use superconducting magnets to create strong magnetic fields.
Superconductors have the advantage of having exactly 0 electrical resistance, so there is no energy loss due to heat. However, there is the problem that very low temperatures must be maintained to maintain the superconductivity phenomenon.
Modern MRI machines are kept at low temperatures by using expensive liquid helium.

--- p.197-198, from “Chapter 11: Physics of Levitating Through Walls”