"I can prove that a true time machine is impossible. It will never exist," the young professor said, looking out at his students.
It was his very first lecture. He was nervous, and to steady the slight tremor in his hands, he adjusted the heavy horn-rimmed glasses on the bridge of his nose—an old gift from his father. They carried the faint, lingering scent of leather and home, a familiar smell that helped him stand a little taller.
"The theory of relativity allows us to venture into the future," he continued, his voice dropping into a near-confidential tone. "But I am prepared to wager that returning to the past is impossible. The way back is closed to us forever."
The students listened, breathless. The professor leaned against the lectern, shifting his weight forward.
"Let's experiment. You can record me on your cameras. Right now, I invite any of you, or your distant descendants, to enter this hall. Appear this very instant and prove me wrong. Three. Two. One."
Silence fell over the auditorium. In the corner, the old wall clock ticked with a methodical, cozy rhythm.
"As you can see, nothing happened," the professor said, offering a good-natured shrug. "Well, that's all for today. Good evening, friends. I'll see you tomorrow."
The students began to filter out, the room filling with the clatter of chairs and the hum of cheerful voices. As the professor gathered his notes from the desk, he suddenly flinched. In the very last row, shrouded in shadow, sat an old man. His face looked hauntingly familiar to the professor, like a forgotten dream from early childhood.
"Excuse me, sir… have we met before?"
The old man didn't answer. He stood up abruptly and nearly ran from the hall. The professor hurried after him, hoping to help or perhaps just finish the thought, but the hallway was empty. It was drowning in the long shadows of the evening.
"Did you see an elderly man come through here?" he asked a pair of students who were scribbling notes as they walked.
"What man, Professor? There's been no one here but us."
"But I just…" He cut himself off and gave a sheepish smile. "I must be tired. Never mind me."
He returned to the empty classroom. The evening sun gilded the dust motes dancing in the air, and the hall fell perfectly still.
"I remembered myself differently," a soft, cracking voice whispered from behind him.