There is no "lower level" at which to explain it: Things that are good at making copies of themselves will tend to be more prolific. Restating Darwin in terms of thermodynamics takes a simple idea and complicates it without adding explanatory power, which makes it a poor theory.
Here is how you "say 'Life' in physics": Life is a pattern which obeys four rules:
- Reproduction: The pattern makes copies of itself. - Inheritance: The copies share traits of the original. - Variation: The copies can be different from the original. - Selection: The success of the pattern making more copies depends on the traits which it inherits.
That's it. A tree is alive. A person is alive. A stone is not alive. A virus is alive.
And an idea is alive. It is a pattern in matter which replicates (I can tell you an idea; now there's one in my head AND your head), it inherits (your idea is similar to mine), it varies (you may reinterpret the idea, or combine it with your own ideas), and it selects (you will spread ideas you think are good, and discard those you think are bad). From this arises technology, or religion. We may, with a twist of the mind, think of either as an organism in its own right living in the habitat of human consciousness. (Perhaps all of our species' success can be attributed to the fact that our brains support a symbiotic form of life which can adapt far, far faster than DNA can).
And so by this definition even a solar panel is alive (and the "problem" of physically differentiating the two evaporates); it is just a stranger form of life that spends part of its existence as the plans for its production, coded in the configuration of matter in its blueprints, computer files, and human brains; and whose life cycle includes a stage where it is physically manifests and makes it self useful to its human hosts by providing them electricity. Its design is copied and mutated; some better at providing power for less effort, and thus more likely to propagate and eventually out-compete weaker designs.