Does the plectrum you play with make a difference in your guitar sound? Is it just the thickness of the pick or does materials also make a difference if at all?
What would be the science behind this?
Oh god yes it makes a difference. And thickness, shape, and material all make a difference. You can easily do tons of home experiments on this. Go buy a selection of picks at your local music store and then find some coins. Coins have been used as picks by some famous guitarists and have a very distinct sound.
The science (a little bit at least): A good first approximation of the sound that a guitar string makes is the shape that is made by the string when it is moved from its rest position right before it is released. With any given pick, plucking closer to the bridge makes a different kind of triangular shape from picking close to the 12th fret, and the difference in tone there is obvious.
Now think about the shape of the point of the triangle where the pick is (the triangle is formed by the bridge, nut, and pick, with the string making up two sides and the rest position of the string making the third). If the pick is more rounded and/or more flexible, that point of the triangle will be more rounded. If the pick is hard and/or more pointed, that point of the triangle will have a sharper pointed shape.
When the string is released, the sharper shapes are released also and form tiny little waves that run up and down the string. The sharper the point, the more and tinier waves are formed. They don't last as long as the overall wave motion of the string, but they are picked up by the pickup and/or transferred to the body by the bridge and are clearly audible. More tiny waves are also created by plucking closer to the bridge as opposed to closer to the 12th fret. The tiny ripples give a brighter sound, and a lack of tiny ripples gives a darker, more rounded sound.
This is also tied into the difference in sound between bridge and neck pickups in terms of how big the little ripples are in relation to the larger overall string motion.