= It is OK to treat things as black boxes
Nature <science is the reverse engineering of nature>[is a black box, right]?
You don't need to understand the <from first principles> derivation of every single phenomena.
And most important of all: you should not start learning phenomena by reading the from first principles derivation.
Instead, you should see what happens in experiments, and how matches some known formula (which hopefully has been derived from first principles).
Only open the boxes (understand from first principles derivation) if the need is felt!
E.g.:
* you don't need to understand everything about why <SQUID devices> have their specific <I-V curve> curve. You have to first of all learn what the I-V curve would be in an experiment!
* you don't need to understand the fine details of how <cavity magnetrons> work. What you need to understand first is what kind of <microwave> you get from what kind of input (<DC current>), and how that compares to other sources of <microwaves>
* <lasers>: same
Physics is <doing physics means calculating a number>[all about predicting the future]. If you can predict the future with an end result, that's already predicting the future, and valid.