"...The above are my views and suggestions."

"Sender, Li Feiyu."

After finishing this letter.

Xu Yun let out a gentle sigh and placed the pen aside.

Although it was still unclear why and through what channel this letter reached him.

But based on the situation previously mentioned by Little Niu in the letter.

Perhaps...

This is some sort of activation method for outside assistance?

That is, when Little Niu encounters a problem he finds difficult to solve, there is a certain possibility that he can contact me, engaging in cross-dungeon correspondence.

It's just unclear whether this method has a fixed cycle, more crucially, it's uncertain...

Whether there is any reward?

After all, according to Halo's previous performance.

Although it can be somewhat stingy, it has always adhered to the basic principle of "no pain, no gain."

Xu Yun then turned his attention back to the letter, starting the final proofread.

The purpose for Little Niu writing this letter was actually very clear:

The core issue was that for some unknown reasons, he had developed some doubts about the particle theory of light.

Maybe it was some phenomenon suggested by an outsider.

Maybe it was some clues that Little Niu discovered himself.

In any case, the impact on Little Niu was considerable. 𝒻𝑟ℯℯ𝑤𝑒𝑏𝑛𝘰𝓋𝑒𝓁.𝒸𝑜𝘮

Therefore, in this reply.

After much deliberation, Xu Yun finally chose to mention the wave-particle duality of light.

In the previous life, all friends who read textbooks would know.

The so-called wave-particle duality of light is actually very simple in textbook terms:

Light is both a wave and a particle.

In future generations, as long as you have received compulsory education, you would have come across this term.

But historically.

The exploration of the nature of light was actually a very tortuous affair.

Before Little Niu.

The wave theory of light actually had a lot of currency.

This theory was put forward by Huygens, and its number one fan was Hook, who was previously mentioned and greatly mistreated.

Hook supported the wave theory of light because he discovered the phenomenon of light interference, which is the colorful soap bubbles we usually see.

These beautiful bubbles are actually thin-film interference of light, and interference is undoubtedly a characteristic of waves.

So naturally, Old Hu believed that light is a wave.

Later, Little Niu ventured into new territory, proposing the particle theory.

And with the growing prestige of the Duke, scientists formed the view of "two whatevers":

Whatever the Duke supported was right, whatever the Duke opposed was wrong.

Thus, the particle theory thrived while the wave theory declined.

Then came a person named Thomas Young who conducted an initial interference experiment.

This experiment has now entered middle school textbooks; its ingenious design is a classic in physics:

Place a piece of paper with a pinhole in front of a lit candle, obtaining a point light source.

Place another piece of paper with two narrow slits in front of the point light source.

Then on the screen behind this paper, you would see alternating light and dark fringes.

By adjusting the distances between the paper and the screen, and between the double slits, you could change the spacing of the interference fringes.

The importance of this experiment ranks among the top in the history of science, so much so that when confirming electron wave properties later, scientists directly adopted the same experimental principle.

Thus, the double-slit interference experiment holds two spots among the ten classic physics experiments.

By 1808.

Marius discovered the phenomenon of light polarization.

Later, with the discovery of Poisson's spot, the wave theory of light saw a resurgence, achieving complete victory.

Then came Faraday and Maxwell. In classical physics—that is, the world visible to our naked eyes—the definition of light basically stopped here.

After another 80 years, Hertz discovered electromagnetic waves.

The photoelectric effect.

This key dividing line between the macro and micro world in human history was discovered for the first time.

During research on photoelectric effect, Thomson discovered the electron through the study of cathode rays.

At that time, Thomson exclaimed as if he saw something extremely surprising, that there existed such small things in the world!

Thus advancing human research into another micro world.

The subsequent developments became quite straightforward.

In 1900.

Planck proposed the quantum hypothesis.

In 1902.

Lenard summarized the photoelectric effect.

In 1905.

Lao Ai, Einstein entered the stage, summarizing the wave-particle duality of light.

After Lao Ai came Lai Debro, who proposed the hypothesis of matter waves.

He pointed out that not only photons possess wave-particle duality, but all microscopic particles do.

In 1927.

American physicist Davidson, a year after de Broglie's paper was published, completed the electron diffraction experiment.

Proving that electrons have wave properties, wave-particle duality as a basic characteristic of matter was confirmed, de Broglie's idea was vindicated.

Since then up to now, front-end physics has been primarily researching the truth behind the phenomena observed by these sages.

Meanwhile.

The entire debate over the nature of light lasted for hundreds of years, requiring a large accumulation of accomplishments.

Therefore, Xu Yun had no worry that his hints to Little Niu would steal Einstein's thunder, because in physics, proposing a concept doesn't equate to the truth.

Just like Little Niu's accomplishments in the field of universal gravitation.

Before Little Niu, many had already discovered 'gravity' or 'gravitational' phenomena, such as the famous official Ximen Qing.

But discovering a phenomenon means nothing, further argumentation and calculations are the tough parts.