Relativity theory and quantum mechanics are both very counterintuitive – human brains have evolved via experiencing the world as three-dimensional, time flowing, and for physical stuff to behave “normally”.
Consider one of the most intriguing aspects of our reality... “length contraction” as defined in relativity theory – physical objects don't have an absolute length. Surely 1 metre is 1 metre, right? ... the answer is both yes and no.
The faster that a physical body travels through space relative to being at absolute rest in the universe, the slower its own time runs, which is called “time dilation”.
Absolute rest: The cosmic microwave background (CMB; radiation) in the universe; there must be a potentially fastest clock speed (see post by: James Overduin). Not to be confused with... there is no preferred reference frame...
Also follow this link to see another discussion about the so-called “Global Rest Reference Frame” (GRRF) (see 2nd and 3rd posts by: Yurij V. Baryshev)
In contrast to time dilation “length contraction” occurs, whereby both the comoving and non-comoving observer experience each other to really physically exist as being shorter. The amount depends on their relative (i.e., to each other) difference in speed.
The physical length of a comoving observer as experienced by themselves remains unchanged (“proper length”). Likewise, the same is true for a comoving observer’s own time (“proper time”) – a comoving observer always seems normal to themselves.
For example, if you hold a 1 metre ruler it will measure exactly 1 metre from your perspective (standing position), i.e., you are the comoving observer, as you would be to a spaceship if you were onboard; so simply moving with the object.
However, if somebody else travels past you super-fast (substantially close to lightspeed) whilst they are holding a 1 metre ruler, its length would exist to be considerably shorter from your perspective, albeit it would be rather difficult to measure as it raced past.
The strange reality as described above has nothing to do with becoming shorter due to increased air pressure for example, but is in fact entirely due to the weirdness of relativistic effects – this happens in the vacuum of space just as it does inside a planet’s atmosphere.
The word “experience” as used here means to exist as really physically being either longer or shorter according to different observers' relative speeds to each other – there is no such thing as absolute time or length in space... at least from the perspective of how individual observers experience reality.
Optical effects also exist such as "Terrell rotation" as described by Roger Penrose and James Terrell.
The Schrödinger equation which governs how the wave function develops for quantum mechanical systems... is yet another strange phenomenon.