#3 "Open the Yellow Notebook and Set Off on a Journey to the Universe"

"Say When!" is a phrase used when pouring a drink, meaning "tell me when to stop" or "let me know when it's just right." I will be writing essays that you can enjoy slowly, just like savoring your favorite drink.

There is a yellow spiral notebook on the living room bookshelf.
Brown stains are scattered here and there, and the once-vibrant color has faded. The ink of the title, "Science of the Earth and Universe," written in handwriting that hadn't quite matured into an adult's, is also blurring and fading away. It has been over 30 years since I used that notebook.

I hated physics until high school. So, when I found out in college that I had to take at least one physics lecture as a mandatory general education requirement, I was stunned. Among the few options available, the one I reluctantly chose was a course called "Science of the Earth and Universe." Looking back now, the professor must have known from the start that the majority of the students hated physics.

"At this moment, we do not know whether this universe is an expanding universe or a contracting universe."
Standing at the podium in the tiered lecture hall, the professor began to speak quietly in a clear, resonant voice.

If it is an expanding universe, it will continue to expand forever; if it is a contracting universe, it will shrink once it passes a critical point, eventually becoming smaller than a millimeter, smaller than a micron, and finally turning into a point. My brain was so breathless with excitement that my imagination couldn't keep up with a spectacle more magnificent than any movie. Since it was a class from 30 years ago, the cutting-edge debates of that time have now been settled, and the expanding universe has been declared the winner.

What captured my heart was not which theory was correct.

I had assumed that the universe was a space that extended infinitely and was the fundamental framework at the very edge of this world. I had never even questioned the idea that an absolute space called the "universe" existed first, with stars and the Earth inside it, and that the universe was a boundless space.
Just as I never wondered why rain falls from the sky or why the moon orbits the Earth, it was an unshakable, self-evident premise for me.

The very concept that the universe could expand or contract was a surprise, but to expand means there is space outside the universe into which it can expand, and if it turns to contraction, a vast space will remain afterward. Either way, it means there is an "outside of the universe."
Just as there is a sky far above our heads, and beyond that sky—the layer called the atmosphere—there is the universe. Outside the universe, there is a space that completely envelops it. If so, there might be another space that wraps around that space, and another one after that... It means there is a possibility of dreaming of the next layer, and the one after that, like the layers of an onion.

It was such a shock, as if I had acquired an "eye" to see outside the universe.
Until then, like a frog in a well, I only knew the sky directly above the well, but I felt I had found a perspective to jump out of the universe and look down from far away.

It was the moment I realized that physics, which had been nothing but a series of incomprehensible formulas without a shred of interest, was a discipline filled with immense romance. If I had been able to take a class like this in high school, perhaps I wouldn't have loathed physics so much. With that slight bitterness, I began to look forward to the next class.
Every week, I opened the lecture hall door as if I were opening a door to the universe.

Even after that, the professor continued to explain cutting-edge astrophysics in a way we could understand.

It was also in this lecture that I learned about neutrinos.
The fact that there exists a ghost-like substance that passes through any matter and cannot be caught. That it is constantly raining down from the universe, and that hundreds of trillions of them are passing through our bodies every second. I felt dizzy at a truth that far exceeded my imagination. Later, when Dr. Koshiba won the Nobel Prize in Physics for his research on neutrinos, I felt a little proud.
The Doppler effect, the tunnel effect, the alpha-beta-gamma theory, the Stefan-Boltzmann law... While excited by the wonders of the universe that various theories revealed, I kept taking notes with all my might. I wrote desperately, trying not to miss a single word the professor said.

When I flip through that yellow spiral notebook, the excitement and heat of those days still come through. Like Doraemon's "Anywhere Door," whenever I open that yellow notebook, I can set off on a journey to the universe from that large lecture hall at any time.

I was very surprised to hear the news of Dr. Masatoshi Koshiba's passing. I pray for the repose of his soul, as another great giant of knowledge has departed on his journey.