I present to you
Inter-dimensional travel: the novel

I believe the theory that there is in fact an infinite amount of dimensions, but I do not think that any of them has existed long enough to invent inter-dimensional travel or rather they have not mastered it yet to the point they can enter any dimension of their choosing; thus explaining why a spacey-looking man hasn't revealed himself and said "I'm from an alternate reality!"

I believe its a problem of time; (for reference I'm calling our inter-dimensional travel inventor the Good Professor)

First off the Good Professor would have to locate a gate or medium in order to travel, then he needs to be able to control it. I believe it would take a "Godly" amount of power to link two separate spaces of respective dimensions. So, he maybe require some form of synthesized element that generates enough power; I imagine it would be a big hassle, especially if the Good Professor's dimension had occurrences of Einstein, his Mass–energy equivalence formula, and Nuclear weaponry. Finally the prof. would need a Guinea Pig crazy enough to walk through the inter-dimensional gate, as a non-perfect dimensional gate would infinitely rip you apart among the infinite possible dimensions and choices humans make

Which came first, the chicken or the egg?

You don't need a gate for that, it happens naturally to all of us. Just as there is a certain uncertanty(i love saying that) in an elementary particle's position in our observable 3D world, the particle is not always well localized in higher dimensions either, hence the wave properties.

Long story short, your elementary particles keeep seeping into paralel universes for super short amounts of time whenever they display wave-like properties which is more or less all the time.

You are already a smear on a higher dimension rather than a point.

@Chestnut_Rice egg mutated