Buzz Blog

Why Physics Students Need Philosophy Too

Monday, February 13, 2012
Quantum Mechanics is notoriously difficult, and the subject has raised some deep questions about the natural world. Perhaps the best known example is the Schrodinger's cat thought experiment that poses the following question: does quantum mechanics imply that a cat can be both dead and alive at the same time?

Physics education researchers have found that professors often glance over or sidestep fundamental questions like this, and it's hindering students' understanding. Touching on these interpretive questions not only makes students more excited about physics, but it also leads to a better understanding of the underlying physics principles.

Schrodinger's cat is placed in a box with a bottle of poison, and the release of poison depends on the wavefunction of a radioactive isotope. Image Credit: Dhatfield

Charles Bailey and his colleagues at the University of Colorado Boulder compared various teaching styles for a modern physics course. Through their research, the team wanted to analyze student perspectives on the measurement problem in quantum mechanics: what does wavefunction collapse mean?

And the measurement problem is perhaps best illustrated by the unfortunate case of Schrodinger's cat:

Consider one cold, calculating physicist who places a cat, a bottle of poison, a Geiger counter to detect radiation, and a decaying radioactive atom in a box. The single atom has a half-life of 53 minutes, meaning that after 53 minutes there is a 50-50 chance that it will emit radioactive particles upon measurement. Before measurement, however, the system remains in a superposition of the decayed state and the undecayed state.

The traditional quantum mechanics interpretation seems to suggest that the superposition follows a chain. The Geiger counter, poison, and ultimately the cat are all in a superposition of states before someone looks inside the box. Quantum mechanics might imply that the cat is both poisoned (dead) and alive at the same time. Counterintuitive indeed!

Other interpretations suggest that the universe branches in these cases, implying that there is a new universe in which the cat lives and another where the cat dies. Both universes are equally real, and there's no universe where the cat is both alive and dead.

But the researchers found that the point is not teaching the "correct" interpretation. Instead, simply allowing students to explore these types of interpretations improved their understanding of the fundamental physics behind quantum mechanics.

The CU researchers looked at four different classes over time that addressed these issues with varying approaches and dedicated class time. They found that explicitly addressing these interpretive questions led students to better understand what quantum probability really means.

At the beginning and end of each semester, Bailey and his team asked students if they agreed or disagreed with the following statement:

The probabilistic nature of quantum mechanics is mostly due to the limitations of our measurement instruments.

Regardless of one's quantum mechanical interpretations, a wide majority of physics professors agree that this statement is incorrect. Quantum probability is not the same as experimental uncertainty.

Bailey and his team found that exposing students to more philosophical questions caused students to disagree with this statement, meaning that they could discern the aforementioned distinction between classical uncertainty and quantum probability.

In addition to better student achievement, this style of teaching also led to more enthusiastic students: After the class, 98% of students thought quantum mechanics was an interesting subject compared to 70% for students in previous classes.

Posted by Hyperspace

Charles Baily said...

One of the important points of our research (and which I think is missing from this article) is that these kinds of questions are no longer a matter of "philosophy". The underlying question behind "Schrodinger's Cat" is whether a superposition state represents the actual state of individual systems (the cat is both dead and alive - "quantum uncertainty"), or just the statistics behind measurements made on a large ensemble of identically prepared systems (the cat is always either dead or alive; we just can't know which until we open the box - "classical ignorance"). A more realistic example is the double-slit experiment – does each electron pass through either one slit or the other as a localized particle, or does it pass through both slits as a delocalized wave? Experiments from the last few decades have helped to clarify this issue - they tell us that the superposition state is real, and that strictly statistical interpretations of quantum mechanics do not agree with experimental evidence.

We (instructors and students) should care about what the "collapse of the wave function" means because it has everything to do with why we describe quantum systems in terms of superpositions, and what it means to make a quantum measurement. An incorrect interpretation of a measurement on a superposition state is to claim that each individual system could only have been in just one of the many possible states before the measurement, and that the measurement revealed this "unknown reality" to us - in essence, that's saying that probabilities in quantum mechanics are purely statistical in nature, and that superposition states reflect our classical ignorance of "hidden variables" - properties of an individual system that are unknown to us. These kinds of misconceptions are exactly what we meant to address in the development of our new curriculum.

I think a statement like: "Who cares what the collapse of the wave function means? We have made a measurement, so let's move on." is a learned response - instructors often (sometimes inadvertently) promote a "Shut up and calculate!" kind of attitude for a variety of reasons. One is that many instructors feel introductory students don't have the requisite sophistication to appreciate the nature of the "measurement problem" in quantum mechanics, and so they sidestep the questions or act like they're unimportant. We've found that students develop attitudes and opinions about such questions on their own, whether an instructor addresses them or not, but that explicit instruction has a measureable impact. When instructors are not explicit, student beliefs tend to be intuitively classical in nature (in other words, wrong).

Our curriculum was designed to give students the required conceptual tools to articulate and confront their own classical expectations, while also promoting greater understanding of what a quantum state represents, and the difference between classical ignorance and quantum uncertainty. We did this in part by presenting experimental evidence that contradicts their intuitions; we argued for quantum uncertainty rather than just telling them how they're supposed to think. Students appreciate that we care about what they are thinking, the experiemnts were interesting to them and promoted a lot of discussion, and there was an overwhelmingly positive response (98%) from a class of around 100 students. Modern physics courses that fail to address these issues in a meaningful way typically result in many students being less interested in quantum mechanics than they were before they started learning about it.

Long answer short: No, there was absolutely no discernable "pushback" from our students. I think that attitude you're describing can stem from a belief that these really are still "philosophical" questions, and can't be put to experimental test, something our students knew wasn’t true.

Thursday, February 16, 2012 at 4:29 PM

Bogolese said...

I wonder whether or not there was any "push back" by students as these discussions came up. I am inspired to ask based on a conversation I overheard in the Undergradute Physics room at my university yesterday where one student was essentially saying "Who cares what the collapse of the wave function means? We have made a measurement, so let's move on."

I, for one, welcome these discussions and think it behooves us as physicists to always push the envelope of our understanding.

Thank you for this fine article.

Thursday, February 16, 2012 at 9:54 AM

Hyperspace said...