The Instrument Knows Nothing

The Senior Common Room had been unusually quiet for several minutes, a circumstance that Mr Blottisham regarded with suspicion.

He had been reading the latest series on measurement and had arrived at the conclusion that something had gone seriously wrong with scientific education.

“I must say,” he announced, putting down his copy of the journal, “that I find the whole business rather excessive. Six articles to explain that when you measure something, you get a measurement.”

Professor Quillibrace looked up from his tea.

“An admirably economical summary, Mr Blottisham. Unfortunately, it omits almost everything the articles were about.”

“Precisely my point. The author has managed to turn a perfectly straightforward activity into an elaborate philosophical obstacle course. We measure something, the instrument gives us a reading, and we know the result. What more is there to say?”

Miss Stray, who had been listening, put down her cup.

“Perhaps we should begin with the instrument. Does it know what it has measured?”

“Of course not,” said Blottisham. “It's an instrument.”

“Then how does it produce a reading?”

“It registers the result.”

“And what makes the result a reading?”

Blottisham paused.

“The result is the reading.”

Professor Quillibrace stirred his tea.

“We appear to have reached the heart of the difficulty rather sooner than expected.”

“I don't see any difficulty.”

“That, Mr Blottisham, is not the same thing.”

Miss Stray smiled.

“Consider a thermometer. The liquid rises to a particular position. That is a physical event. We can observe the position, but to call it twenty degrees we must relate it to the markings on the scale.”

“Obviously.”

“And the scale must be calibrated, and the instrument must be functioning correctly. So the position of the liquid and the temperature reading are related, but they aren't quite the same thing.”

“It's a thermometer, Miss Stray. What else would it be doing?”

“Being a thermometer, I should imagine,” said Quillibrace.

Blottisham looked dissatisfied.

“I still don't see why we need to distinguish the measurement from the reading.”

“Because one occurs physically and the other involves interpretation.”

“Interpretation!” Blottisham exclaimed. “Now we're interpreting thermometers? Whatever happened to science?”

“Science,” said Quillibrace, “appears to have survived the experience.”

Miss Stray continued.

“The distinction becomes more interesting when we consider a Geiger counter. A particle interacts with the detector, producing an electrical pulse. The instrument registers the event, perhaps with a click. The pulse can be stored. But the physical event, the detector's response and the record of that response are not identical.”

“It's a click,” said Blottisham. “You hear it. You count it. You know how much radiation there is.”

“Do you?” asked Quillibrace.

“Certainly.”

“Does the click tell you which particle produced it?”

“Not necessarily.”

“Does it tell you the energy of the particle?”

“Not necessarily.”

“Does it tell you whether the instrument is working properly?”

Blottisham frowned.

“Not necessarily.”

“Then perhaps the click is not quite the complete scientific account you initially supposed.”

Miss Stray leaned forward.

“The record can preserve information about an event without anyone interpreting it. But information and meaning aren't the same thing.”

Blottisham looked from one to the other.

“I should have known. We've now reached the stage where information doesn't mean anything.”

“An interesting possibility,” said Quillibrace. “Though not quite the one under consideration.”

He put down his cup.

“A signal can carry information because its physical properties distinguish possible states or outcomes. That can be described without assuming that anyone understands the signal. Meaning requires something further: interpretation within an organised system of distinctions and relationships.”

“Then who decides what it means?” asked Blottisham.

“Not necessarily a single person,” said Miss Stray. “The interpretation depends on the instrument, the procedures, the concepts being used, and the scientific context in which the result is understood.”

“So meaning is whatever scientists agree it means?”

“No,” said Quillibrace. “That would be an unfortunate conclusion. The material world places constraints on what can reasonably be interpreted. Agreement cannot make a detector register an event that did not occur.”

“Then the world decides?”

“The world does not submit its readings to the journal for approval,” said Quillibrace. “But neither does it write the conclusions for us.”

Miss Stray smiled.

“Perhaps the important point is that the record doesn't interpret itself. The physical event constrains the record, and the record constrains the interpretation. Meaning emerges through the relationship between them.”

Blottisham leaned back.

“I see. So the instrument is innocent, the record is ignorant, and the scientist is responsible.”

“An unexpectedly charitable assessment of scientific practice,” said Quillibrace.

“I don't intend it charitably. I intend it as an indictment. The instrument should tell us what happened!”

“Perhaps you would like to interview it.”

“I should certainly like to know why it refuses to answer a straightforward question.”

“Most instruments,” said Quillibrace, “are commendably reticent.”

Miss Stray turned a page in the journal.

“There is another issue. The series argues that a reading doesn't automatically become scientific evidence. Its significance depends on how it relates to other readings, experimental procedures and theoretical explanations.”

“Surely a measurement is evidence!”

“It can be,” said Quillibrace. “But consider a single reading from an instrument that has not been calibrated. What does it establish?”

“That the instrument has produced a result.”

“Indeed. And what does that result establish about the world?”

Blottisham hesitated.

“Possibly nothing.”

“An excellent beginning.”

Blottisham was beginning to look offended.

“I must say, Professor, that this approach threatens to undermine the entire scientific enterprise. If every measurement has to be questioned, nothing will ever be established.”

“On the contrary. Questioning measurements is one of the ways science establishes things.”

“Then why don't scientists simply trust their instruments?”

“Because they are scientists, Mr Blottisham.”

Miss Stray laughed.

“But I think the quantum measurement problem is where the distinction becomes particularly important. A physical interaction produces an outcome, and the apparatus registers it. Yet the relationship between the quantum description of a system and the definite outcome obtained through measurement remains a difficult problem in physics.”

Blottisham looked relieved.

“Ah! At last, something straightforward. Surely that is just a question of someone looking at the particle.”

Quillibrace closed his eyes briefly.

“No.”

“But someone has to observe it.”

“An instrument can register an outcome without a human being looking at it.”

“Then the instrument is the observer!”

“Only if you are determined to make the word mean something different from what you previously thought it meant.”

Blottisham considered this.

“I don't see why physicists make such a fuss. If a detector registers something, then something has happened. If nobody reads the detector, that is their problem.”

“Quite,” said Quillibrace. “And the measurement problem is not solved by your decision to stop reading about it.”

Miss Stray looked thoughtful.

“Perhaps the series is asking us to distinguish two questions. What happens physically when a measurement takes place? And how does the outcome become meaningful within scientific knowledge?”

“Two questions?” Blottisham exclaimed. “There is only one question. What did we measure?”

“Which,” said Quillibrace, “is sometimes a surprisingly difficult question to answer.”

There was a brief silence.

Blottisham picked up the journal again.

“I shall write to the editor. I shall recommend that the next instrument be equipped with a small explanatory notice. Something along the lines of: ‘This apparatus has measured the following, and it means the following, for the following reasons.’”

“An admirable proposal,” said Quillibrace. “Perhaps the instrument could also provide the peer review.”

“And why not?”

“Because it would first have to understand the paper.”

Blottisham looked triumphant.

“Exactly! At last we agree. The instrument knows nothing.”

Miss Stray smiled.

“Which is rather the point, isn't it?”

Quillibrace returned to his tea.

“Indeed. And it seems that, for once, Mr Blottisham has arrived at the correct conclusion by an entirely incorrect route.”