This was a lot of fun BUT, I really wish that the Es could also have been entered as [MC]s. That is, I'd prefer a little Schrödinger and/or Heisenberg thrown in with my Einstein. I mean, if E really does equal MC squared, then let it. (Using the NYTX iPad app, I left rebuses in the grid when I checked my work, and did not get the "Well Done!" until they were all Es.) I get it, though - All the Es, and the only Es in the grid, mind you, are contained within the thematic material, and in order to read the thematic material correctly, they must be taken as Es. It's really a thing of beauty, this puzzle, but I still think Einstein himself would have been ok with the possibility of MCs being accepted.
OK, with that slight complaint out of the way, I can get on with the praise. Well, wait a minute, let's just mention ILIAC (40A: Near the hip) and NARCO (37A: Stash seeker) too. There. Done. I'm fine with them, really. The greatness of the rest overshadows these minor inconveniences.
So, we've established that the theme is excellent, but that's not all, the cluing and fill are also largely top-notch. TOOL (20D: Drive (along)), ([MC])JOB (59D: Unfulfilling work), and COPS (53D: Admits (to)) feel nicely modern and/or hip (at least to this geezer), and NASA (55D: Org. with lofty goals?), SWI([MC])AP (1D: Top in the pool?), and PIP (34D: One of hearts?) are given slightly tricky, fun clues. The fill also includes such non-standard, interesting words as SUNDRY (49D: Miscellaneous), THORAX (48D: Chest), and the lovely PLUTARCH (44A: "Parallel Lives" writer).
In short, it's just the kind of puzzle we enjoy, and an excellent start to "the turn." Here's hoping tomorrow and Saturday deliver as well!