science thought that confuses me greatly

time.

almost every single physics process looks the same backwards in time and forwards in time.

people have learnt to tell which one is which on a large scale, something attributed to entropy (the idea that things always get messier). so when a glass smashes into a million pieces you can tell if a video shows it going forwards in time or backwards.
even though entropy makes it clear in our understanding, there's no good explanation why this is the case.

(for counter example, take a ball, throw it up and catch it, and that video will look identical forwards and backwards.)

and it's not so simple when mathematically looking at processes. eg an electron going forwards in time looks identical to a positron (it's antiparticle) going backwards in time. EXACTLY the same. some dude postulated that there could be just one electron, and it goes forwards and backwards a lot to make it look like there are lots of electrons and positrons!

as for things travelling at the speed of light, they don't even travel through time!

all forces and interactions have no sense of time. as in, stuff can happen forwards or backwards, and you can't tell them apart*. many interactions don't even include time as part of their equations. eg the gravitational attraction between the earth and the moon, or the repulsion of the same end of a magnet.

things that no one knows include:
why time goes forward
whether time is continuous, or made up of very small steps

thinking about these things is painful to my mind.


*there is one fundamental force which isn't perfectly symmetrical and explains why there is more matter than antimatter. iirc it's called CP violation, and this violation means when you add time in there, it might look different forwards and backwards. i remember something here then explaining there being more matter than antimatter but even if i merember that correctly, that's for another day.