"Professor Thomson, I'm here to save your Plum Pudding theory."

J. J. Thomson looked up from his desk. The stranger wore gentleman's clothing, but they were dirty and disheveled. His deep-set gray eyes sparkled with intelligence.

Thomson grunted and dropped his pen into its well. "Who the devil are you? And how did you get into my office at this hour?"

"I'm a friend of Herbert Wells."

"What's he teach? Physics? Chemistry?"

"He's a writer. Perhaps you read his chronicle of my exploits a few years back?"

Thomson looked over his glasses at the untidy man. "Can't say I've had the opportunity."

The stranger shrugged. "Pity. In the future, your Plum Pudding theory--"

"Stop calling it that. The term is a gross oversimplification of my model."

"Oh dear. Do I have my history wrong? Aren't you the physicist who said that the atom is like a plum pudding?"

Thomson drew back in indignation. "I never uttered such rubbish. My model proposes a diffuse positively charged cloud through which negative corpuscles revolve."

"The point is," the stranger said, his gaunt face hardening with resolve, "next year Lord Rutherford will design an experiment that shows your model to be wrong."

"Ernest Rutherford? My old student? Brilliant man, but no Lord."

"Not yet. He won't get the title until a few years after he proposes his nuclear model of the atom."

Thomson leveled a sharp gaze at the stranger. "And how would you have knowledge of the future?"

"I've been there. That's what I've been trying to tell you. Rutherford's work will lead to a new theory called quantum mechanics. It's nearly an inverse of your model, a central positive nucleus surrounded by a negatively charged cloud."

Thomson raised his eyebrows. "And Ernest does all this?"

"No, but he gets it started by disproving your model. And you have to stop him."

"Have I, indeed? Young man, even if I believed you, why on Earth would I want to impede scientific progress?"

"You don't know the terrible things I've seen." The stranger's face reflected the pain of his memories. He faltered, staring straight ahead as though seeing the horrors of the future once more.

Thomson looked into the unfortunate man's haunted eyes and his heart softened. "Stiff upper lip, old boy. Tell me what you found."

"It's Rutherford's nucleus! Once you have the nucleus, you can split the nucleus, and then... you've no idea the horrors mankind unleashes--will unleash--with that theory."

"But my good man," Thomson said, "even if I were to dissuade Ernest from his experiment, someone else will find this nucleus."

"Ah, but you're wrong. The new theory that arises in the future, quantum mechanics, says that reality exists only as a set of probabilities, none of which are truly real until observed. So don't you see? The nucleus didn't exist until Rutherford searched for it. Upon his measurement, nature rolled the dice and they came up nucleus. In essence, he created the nucleus by observing it."

Thomson struggled with the odd notion. "So you're saying that if Ernest doesn't do his experiment--"

"Then nature doesn't have to decide on the location of the atom's positive charge, and it can remain diffuse. The Plum Pudding atom becomes reality."

"So all I have to do is write a letter dissuading Ernest and my model of the atom becomes true." The ghost of a smile played across Thomson's lips.

The stranger's eyes lit up. "Yes! Will you do it?"

"But it's all nonsense." Thomson threw up his arms and laughed. "Of course I shan't write to Ernest with such rubbish."

The stranger grabbed Thomson's arm in a grip like iron. "But you must! Consider this," the man said, and his face became cunning, "your letter cannot do any harm. If I'm wrong, someone else will discover the nucleus. But if I'm right, you'll have saved the future. You must send that letter!"

"Oh, very well," Thomson conceded. "If it means that much, then I shall send it. He'll likely ignore it anyway."

The man grabbed Thomson's hand and pumped it vigorously. "Thank you, Professor. You won't regret it." With a start, he withdrew his hand. "I must be off. I can use my machine to find out...."

Thomson never heard the end of the sentence, because the man was already out the door and trotting down the hallway. With a wry smile, Thomson watched him retreat from the Cavendish Laboratory. After a long moment, he returned to his desk and pulled a fresh piece of paper from a drawer. He lifted his pen from its well and wrote, "Dear Ernest," at the top of the page. He paused, allowing the pen to hover over the page. With a sigh, he reminded himself that he had given his word.

His head snapped up when the door to his office flew open and the stranger burst through. In the few moments he had been gone, his hair had thinned and his eyes had acquired the first hint of crow's feet. "Put down that pen!" he shouted.

"Good sir, did you not moments ago convince me to write this letter?"

In a voice just short of hysterical, the stranger said, "You've no idea the damage mankind will do with your Plum Pudding model."

Jay Werkheiser

The Plum Pudding Paradox