Thursday, June 01, 2017

What should everyone know about physics?

A couple of weeks ago, Sean Carroll made an offhand remark (the best kind) on twitter about what every physics major should know.  That prompted a more thoughtful look at our expectations for physics majors by Chad Orzel, with which I broadly agree, as does ZapperZ, who points out that most physics majors don't actually go on to be physics PhDs (and that's fine, btw.)  

Entertaining and thought-provoking as this was, it seems like it's worth having a discussion among practicing physicist popularizers about what we'd like everyone to know about physics.  (For the pedants in the audience, by "everyone" I'm not including very young children, and remember, this is aspirational.  It's what we'd like people to know, not what we actually expect people to know.)  I'm still thinking about this, but here are some basic ingredients that make the list, including some framing topics about science overall.
  • Science is a reason- and logic-based way to look at the natural world; part of science is figuring out "models" (ways of describing the natural world and how it works) that have explanatory (retrodictive) and predictive power.   
  • Basic science is about figuring out how the world works - figuring out the "rules of the game".  Engineering is about using that knowledge to achieve some practical goal.  The line is very blurry; lots of scientists are motivated by and think about eventual applications.
  • Different branches of science deal with different levels of complexity and have their own vocabularies.  When trying to answer a scientific question, it's important to use the appropriate level of complexity and the right vocabulary.  You wouldn't try to describe how a bicycle works by starting with the molecular composition of the grease on the chain....
  • Physics in particular uses mathematics as its language and a tool.  You can develop good intuition for how physics works, but to make quantitative predictions, you need math.
  • Ultimately, observation and experiment are the arbiters of whether a scientific model/theory is right and wrong, scientifically.  "Right" means "agrees with observation/experiment whenever checked", "wrong" means "predicts results at odds with reality".  Usually this means the model/theory needs an additional correction, or has only a limited range of applicability.  (Our commonplace understanding of how bicycles work doesn't do so well at speeds of thousands of miles an hour.  That doesn't mean we don't understand how bikes work at low speeds; it means that additional effects have to be considered at very high speeds.)
  • There are many branches of physics - it's not all particle physics and astrophysics, despite the impression you might get from TV or movies.
  • Physics explains light in all its forms (the microwaves that heat your food; the radio waves that carry your wifi and cell phone traffic; the light you see with your eye and which carries your internet data over fibers; the x-rays that can go through your skin; and the really high energy gamma rays that do not, in fact, turn you into an enormous green ragemonster).  
  • Physics includes not just looking at the tiniest building blocks of matter, but also understanding what happens when those building blocks come together in very large numbers - it can explain that diamond is hard and transparent, how/why water freezes and boils, and how little pieces of silicon in your computer can be used to switch electric current.  Physics provides a foundation for chemistry and biology, but in those fields often it makes much more sense to use chemistry and biology vocabulary and models.  
  • Quantum mechanics can be unintuitive and weird, but that doesn't mean it's magic, and it doesn't mean that everything we don't understand (e.g., consciousness) is deeply connected to quantum physics.  Quantum is often most important at small scales, like at the level of individual atoms and molecules.  That's one reason it can seem weird - your everyday world is much larger.
  • Relativity can also be unintuitive and weird - that's because it's most important at speeds near the speed of light, and again those are far from your everyday experience.   
  • We actually understand a heck of a lot of physics, and that's directly responsible for our enormous technological progress in the last hundred and fifty years.
  • Physicists enjoy being creative and speculative, but good and honest ones are careful to point out when they're hand waving or being fanciful. 
I'll add more to this list over time, but that's a start.... 


Peter said...

On "whether a scientific model/theory is right and wrong", are we better to say that a "model/theory is more or less accurate"? Furthermore, the accuracy of a theory is determined by the accuracy of its best model, which we usually can only exhibit for the simplest cases. There are other pragmatic issues, particularly tractability: if we can prove that accurate models exist but the algorithms we have to construct usefully accurate models require decades of supercomputer time, we may rarely use the theory (this is more-or-less equivalent to your discussion of levels of complexity, but I somewhat prefer to think in terms of tractability, for which complexity is not the only issue).

I also have worries about the nature of "explanation", particularly the relationship with the also complex "intuition", but I like your list a lot.

Tahir said...

I also think it is important that the taxpayers who fund science understand that research is slow and unpredictable. Everyday citizens should realize that discovery does not work by just deciding what it is you want to discover, and then discovering it. At the same time, it is also critical that these same taxpayers realize that, even though the eventual benefits of basic, curiosity-driven inquiries are not usually immediately foreseeable, the historical evidence for the return-on-investment is undeniable. Simply put, such blue-skies research is critical for maintaining the long-term health, sustainability and competitiveness of the national economy. I think that if the general public understood this better, the federal agencies that award research grants might be a bit more open to supporting high-risk, high-reward directions.

Douglas Natelson said...

Peter, fair points. I would love it if the lay populace understood that it's overly simplistic to call a scientific theory "wrong". By the criteria that often gets used, Newton's laws are wrong, but it's better to say that they have a limited regime of validity, Regarding me bringing up intuition, I was trying to bridge the idea that you can understand pieces of physics on a gut level (e.g., that the path of a ball takes an approximate parabola so that you can catch it, or that a more massive car will have a harder time stopping), but quantitative prediction (really at the heart of the science) requires math.

Tahir, agreed. Science as taught in schools usually leaves out the false starts and blind alleys, for multiple reasons. That can give the misleading impression that we can plan for discovery, and that progress (however defined) is inevitable.

David Brown said...

"By the criteria ... often ... used, Newton's laws are wrong, but it's better to say that they have a limited range of validity." It seems that Milgrom's MOND might have identified a new problem with Newton's laws.
According to Stacy McGaugh: Either:
(1) Most of the Mass in the Universe is Invisible (Dark Matter), or
(2) Dynamical Laws must be Modified (MOND).
The Basic Issue, The MOND Pages, Stacy McGaugh
Consider 5 conjectures:
(1) The empirical successes of MOND indicate that supersymmetry needs to be replaced by MOND-compatible supersymmetry.
(2) Gravitons and gravitinos have D-brane charges that constitute empirical evidence that D-branes and alternate universes influence gravitational accelerations.
(3) Gravitinos are MOND-chameleon particles that have variable effective mass depending upon nearby gravitational acceleration.
(4) For galactic dynamics, most of the mass-energy of dark matter particles has the form of MOND-chameleon particles that have variable effective mass depending upon nearby gravitational acceleration. The empirical successes of MOND can be explained as follows: Replace the -1/2 in the standard form of Einstein’s field by a term which represents an apparent (but not real) failure of general relativity theory. The apparent failure is caused by ignoring the existence of MOND-chameleon particles. In other words, replace the -1/2 by -1/2 + MOND-chameleon-tracking-function — how might this explain MOND? In the range of validity of MOND, assume that MOND-chameleon-tracking-function is roughly a constant = sqrt((60±10)/4) * 10^–5 . Outside the range of validity of MOND, assume that MOND-chameleon-tracking function is roughly = 0 except for an unspecified transition range. An easy scaling argument shows that this amounts to boosting the gravitational redshift in such a way that there appears to be a universal acceleration constant as postulated in MOND.
(5) It is possible to mathematically define a D-brane corresponding to any plausible MOND-chameleon-tracking function.
For more thoughts on the foundations of physics and dark matter, see:
Triton Station: A Blog about the Science and Sociology of Cosmology and Dark Matter, Stacy McGaugh

Anonymous said...

Would LOVE to read more posts like this on your blog!