My physics masters couldn't possibly be grand mages.

"A spectrometer!" Char's eyes lit up. "With this, we can see ghosts?"

"You should first think about where you can get a hyperspectral instrument, and a portable one at that," Tesla said, pouring cold water on Char's enthusiasm.

Char's smile froze on his face.

Tesla was right; they're stuck in the tunnel, surrounded and unable to move. Forget about finding a spectrometer.

"So what do we do now?" Xia asked helplessly.

Tesla didn't answer; he simply sat on the sofa and spoke to the computer on the table.

"A, give me a list of what you need to make a portable, small hyperspectral eyepiece and the technologies involved."

The first practical spectrometer for chemical analysis was developed in 1859.

For much of the 20th century, it was a bulky laboratory instrument, but as time progressed, intelligent algorithms replaced most of the physical components, enabling revolutionary miniaturization.

In the early 21st century, it was already handheld and classified as "general data" in this computer belonging to 2072.

Based on existing data and offline databases, the core components and technologies required to manufacture a portable hyperspectral imaging eyepiece are as follows:

Little A began listing them one by one. Xia leaned over and glanced at the screen, which was full of technical terms and parameters; his eyes quickly became unfocused.

「1：分光模块（核心）。可选方案一：透射式衍射光栅。需要纳米级精度的光栅刻线（每毫米600-1200线）。可选方案二：......2：光电传感器：......3：信号处理与算法：现有条件：本机算力充足，可解决。......」

Tesla carefully examined the information displayed on the computer screen and continued.

We currently have one computer on hand...

"There's one more phone," Xia reminded her, "a Nokia phone model 2071."

Tesla then asked, "Can these electronic devices be used to manufacture them?"

"The 2071 model of the Nomi phone has integrated some spectral detection technology, which can use the camera to achieve non-invasive, home-based rapid testing."

For example, non-invasive blood sugar level detection can be performed by irradiating the skin with near-infrared light, or non-invasive cancer risk screening can be conducted using saliva samples taken over a few minutes.

Disassembling this phone can meet most of your needs.

Tesla looked at Char.

Xia was also a little confused, but then he smiled helplessly, "The technology in electronic devices is becoming more and more advanced, and many of them may never be used in a lifetime."

As he spoke, he took his phone out of the storage compartment and handed it to Tesla.

"What about the key to the alchemy workshop that the Vilvia family gave you before?" Tesla continued to ask.

Char also took the key out of his storage space and handed it to Tesla.

Tesla found his box in the corner and put his computer and phone inside.

"Give me a day, and I can have the sample made by this time tomorrow."

"Will it work?" Xia asked.

Tesla didn't answer. He simply clicked the lock on the box shut, picked it up, and walked to the door. Then he stopped, turned to the side, and slightly raised his chin.

The eyes, deep in their sockets, were as calm as a stagnant pool.

"Who do you think I am?"

Tesla's voice was soft, even softer than usual. Yet every word was clear and distinct.

"My name is Nikola Tesla."

After saying that, he turned around, pushed open the door, and stepped into the sunlight. The door closed behind him with a very soft "click."

The living room fell silent.

Char stood there, opened his mouth, then closed it again. He turned to look at Einstein.

Einstein, holding his coffee cup, gazed in the direction Tesla had disappeared, a slight smile playing on his lips.

"He says one day, and that's it." Einstein took a sip of coffee, head bowed. "You're learning, and so are we."

"I don't know much about machinery, but I do know that the most difficult part is precision manufacturing."

Einstein slowly rose and stood in front of the blackboard, quietly looking at the words written on it.

"Alchemy can achieve an astonishing level of precision. It can overcome the vast majority of obstacles encountered."

Char did not say anything more.

Einstein stared at the word "ghost" on the blackboard and whispered.

"Next, we need to figure out what this is."

Char seemed to remember something, slapped his forehead, and exclaimed, "Oh no! We let him take the computer!"

"No need." Einstein tapped his temple with his finger. "I've already memorized it all."

"Did you notice the way those corpses on the ground died?"

Char recalled for a moment, "Burnt scars?"

"Actually, besides high temperatures causing 'burns,' low temperatures can also cause them," Einstein said.

"Rapid freezing causes the water inside cells to instantly freeze into ice crystals, piercing the cell membrane. This is different from high temperatures directly destroying the structure of proteins, but the end result is the same: tissue necrosis."

As necrotic tissue loses moisture, it becomes shriveled, blackened, and hardened, resembling charcoal. This typical appearance is medically termed a "charred" change.

After a pause, Einstein stroked his chin. "The temperature on this ghost's body should be infinitely close to absolute zero."

He looked at Xia Ya and said, "Go find Tesla later and have him improve those hyperspectral goggles so that they can capture the mid- and far-infrared wavelengths and even longer wavelengths."

Char tilted his head in confusion.

Einstein smiled and added, "If that thing were really close to absolute zero, the wavelength of its radiation would be far beyond what ordinary sensors can capture."

Your phone is probably doomed. Because that's for measuring blood sugar, not for catching ghosts.

Xia was taken aback for a moment, then nodded. "I understand."

Einstein put the pipe back in his mouth and mumbled, "As long as the wavelength matches, it's a live target dressed in ice."

He picked up the chalk and wrote on the blackboard in front of him.

[Approaching absolute zero]

"This is its first characteristic," Einstein whispered, "and it should also be its primary means of attacking people."

He frowned slightly. "Invisible to the naked eye but visible on a special spectrum, able to pass through walls..."

He recalled the final scene in the video he had just watched, where those things kept crashing into the armored vehicle, "but... they had a physical form."

"You said before that it couldn't get through armored vehicles?"

Char nodded. "We don't know why either."

"Composite armor?"

Xia opened his eyes. "How did you know?"

A glint flashed in Einstein's eyes for a moment, and some clues quickly pieced together in his mind.

He took the pipe from his mouth, and a barely perceptible smile curved the corners of his lips.

"I think I know what that is."