The Senior Common Room at St Anselm's had acquired that peculiar afternoon stillness in which tea cooled unnoticed and metaphysical certainties approached extinction.
Professor Quillibrace sat beside the fire.
Miss Elowen Stray was reading with an expression suggesting increasing concern for the fate of reality.
Mr Blottisham entered carrying an apple.
He looked triumphant.
Quillibrace immediately narrowed his eyes.
"You've brought fruit."
"Yes."
"You appear confident."
"Very."
Quillibrace sighed.
"Oh dear."
Blottisham sat down and held up the apple.
"I've solved the matter."
"The matter of reality?"
"Gravity."
Silence.
Elowen slowly looked up.
"Gravity."
"Yes."
Blottisham waved the apple cheerfully.
"At last we reach firm ground."
Quillibrace stared at him.
"My dear fellow, nothing in recent history suggests this outcome."
Blottisham continued undeterred.
"Gravity is obvious."
He released the apple.
It fell neatly into his lap.
"There."
He spread his hands.
"Things fall because something pulls them downward."
Quillibrace looked at him sadly.
"Ah."
Elowen closed her notebook with measured caution.
Quillibrace folded his hands.
"For centuries, Mr Blottisham, everyone would have agreed with you."
Blottisham looked pleased.
"I knew it."
"Do not celebrate prematurely."
Quillibrace stood.
"The classical picture is wonderfully straightforward."
He began pacing slowly.
"There are objects."
"Yes."
"There are forces."
"Yes."
"There are trajectories."
"Yes."
"And there is a geometrical arena within which these things interact."
Blottisham nodded happily.
"Beautiful."
Quillibrace turned.
"And general relativity quietly murders the central character."
Silence.
Blottisham frowned.
"The central character?"
"Force."
The apple nearly slipped from Blottisham's hand.
"Force?"
"Gravity ceases to be one."
Long silence.
Blottisham blinked.
Then blinked again.
"No."
Quillibrace sat down.
"I'm afraid so."
"No."
"Yes."
"No."
"Mr Blottisham, repeating the word does not reconstruct Newtonian mechanics."
Elowen looked thoughtful.
"Because freely falling objects are no longer being acted upon."
Quillibrace nodded.
"They are simply following geodesics."
Blottisham stared.
"Following what?"
"The locally coherent trajectories permitted by the relational organisation of spacetime."
Silence.
"I preferred force."
Quillibrace smiled faintly.
"Naturally."
He leaned forward.
"The extraordinary move Einstein makes is this: under gravity, bodies are not being compelled."
Blottisham looked deeply suspicious.
"They aren't?"
"No."
"They're simply moving in the locally coherent way available to them."
Blottisham stared.
"So planets aren't being pulled around stars?"
"No."
"The apple isn't being dragged downward?"
"No."
Silence.
Blottisham looked slowly at the apple.
Then back at Quillibrace.
"So gravity..."
"...yes?"
"...has been pretending to be a force."
Quillibrace considered this.
Then nodded.
"That is unexpectedly acceptable."
Elowen laughed softly.
She leaned forward.
"So what looked like causal interaction becomes structural coherence."
"Precisely."
"And motion becomes actualisation within relational constraints rather than response to external cause."
Quillibrace pointed approvingly.
"Very good."
Blottisham looked uneasy.
"I don't like where this is heading."
"Really?"
"You're removing causes now."
Quillibrace frowned.
"No."
"I'm not?"
"No."
"We are reorganising them."
Blottisham narrowed his eyes.
"I suspect this may be one of those distinctions professors make before stealing something."
Quillibrace ignored him.
"The problem lies with a particular picture of causation."
He stood and wandered toward the windows.
"Mechanistic thinking imagines independent objects connected by transmitted influence."
"Pushes and pulls."
"Yes."
"But general relativity dissolves the separation between object and environment."
He turned.
"So motion no longer requires externally imposed cause."
Silence.
"It emerges through relational coherence."
Elowen's eyes widened.
"So gravity disappears locally."
Quillibrace nodded.
"The equivalence principle."
"Because a freely falling observer experiences weightlessness."
"Exactly."
Blottisham frowned.
"One moment."
He sat upright.
"If gravity vanishes depending on one's frame..."
"Yes?"
"...that seems extremely suspicious."
"Why?"
"A proper force ought to remain where one left it."
Quillibrace stared at him.
Then very slowly smiled.
"My dear Mr Blottisham..."
"Yes?"
"...that is actually rather good."
Blottisham looked delighted.
A silence settled over the room.
Rain brushed softly against the windows.
Finally Blottisham looked down at the apple in his hand.
"So the apple does not fall because something reaches out and drags it downward."
"No."
"It falls because falling is simply the locally coherent trajectory available within the relational organisation of the world."
Silence.
Quillibrace looked at him.
Elowen looked at him.
Blottisham looked at the apple.
"My God."
After several moments he frowned.
"Though one issue remains."
Quillibrace looked tired already.
"Naturally."
"If gravity isn't a force..."
He glanced toward the dining hall door.
"...what exactly explains the force with which Cook insists on serving cabbage every Thursday?"
Quillibrace removed his spectacles.
"My dear fellow," he said quietly, "there are trajectories from which escape appears locally impossible."
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