Kamui is a Sound Technique - PROOF using the laws of physics!!!
Let me say that this is a scientific explanation that may not be comprehended by 10year-olds and by people with half(or less) a brain. But if you're not one of those - keep reading.
Also this is a simplified explanation so it is easier to understand
First off lets start by me explaining that every single atom in the universe is 99.99999999999999999999999999999% empty space.
So then why are objects solid, why can we touch them?
Well actually you can't touch anything. Since actually touching something would mean that your atoms touch the atoms of what you're touching, and your and the objects atoms would merge due to the forces of atomic attraction.
This is what happens with liquids and gasses - they merge and become a new liquid or gas (or when a solid is dissolved in a liquid). Simply because when two atoms touch the electrons of one element start orbiting around the first element so both elements are connected by their electrons.
If you want to really touch something you'd have to become a liquid or gas, the object to become liquid or gas also and the two of you to merge together (or to be dissolved in acid..yuk..).
Then why you can touch objects and they feel solid?
The reason is that in the Universe two objects vibrating on the same friquency can't occupy the same physical space.
So when you get your atoms super close to the atoms of another object this law kicks into place meaning that both your atoms and the atoms of that object, can't both be occupying the same space. So the only solution is for the atoms of one of the object to move. And the atoms of the object that has the leas amount of energy just give in and move.
The thing you perceive as touch is simply you feeling the force with which the object is pushing back(Newton's law that every force has an opposite bur equal force(meaning when you punch a wall the wall punches your hand back with the exact same force you punched it first))
But what does all of this have to do with anything? - you might ask
Bare with me.
From this law comes something else: two objects can occupy the same space if they resonate/vibrate on a different frequency.
Meaning the objects are in different dimensions.
This is why objects that are in other dimensions(vibrating on a different frequencies) do not exist in out dimension.
Because for an object to exist in our dimension it has to vibrate on a specific frequency. And when it's not on that exact frequency it simply does not exist in out dimension (but still is in the same place in the Universe) - it exists in another dimension.
This is why there is the theory of the infinite parallel universes(actually the reason for that theory is another principle in the quantum physics relating to the spin of particles, but what I mean is that this shows that it is possible for infinite Universes to exist in the same place at the same time) - they all exist in the exact same place, but vibrate on different frequencies, so in every dimension there is only one Universe that exists.
This also means that if you knew that the frequency of some other dimension is and if you can make a certain object vibrate on that exact frequency - then you'll make it stop exist in out dimension and make it exist in another dimension(this is how Walter does it in the tv series - Fringe).
This is how Obito goes intangible and travels between dimensions - he changes the frequency on which his atoms vibrate.
Back to my point:
Sound is simply a wave moving trough matter(and yes air is matter - it is made out of different gasses (in outer space where there is no space but is rather vacuum - there is no sound)). And waves make objects vibrate(vibrations are simply waves passing trough an object) so basically sound can make objects vibrate and vice versa - vibrating objects make sounds (all you girls reading this turn on your vibrators and you know you can hear them "buzz-ing" )
So the right sound on the right frequency can make an object vibrate on the frequency of another dimension
Meaning Obito's using sound(waves moving through matter) to change the frequency of vibration of his own particles and the particles of other things, and make them stop existing in out dimension, but makes them exist in another dimension. When he wants to come back - he simply has to make his particles vibrate on the frequency of our dimension.. and voila!
You're welcome NB. I hope you now understand how Kamui works and that it actually possible(but how to do it is far beyond our grasp).
Btw using physics I can write a thread explaining why it is possible to use chakra(energy) to create matter - meaning using chakra to create fire/water/earth/ every single jutsu is scientifically possible!
If you wish I write such a post please post it in the comments, saying that you want that explanation.