Albert Einstein said, "It takes three dimensions to describe a point!" He was missing the
'point' of cosmological and quantum "entanglements" (unaware of the meaning, and possibly even the existence, of "superposition" and "entanglement" at the time):
Tesseract:
en.wikipedia.org
Particularly this
'point' of entanglement (if you know for certain the velocity you cannot know for certain position! If you know for certain position you cannot know for certain velocity):
en.wikipedia.org
A few more for good measure:
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Einstein's observer is at one of these outside points, the observable imaged traveler the center point (1/2), and the invisible un-observable traveler traveling light's coordinate point SPACETIME's past-future histories' invisible un-observable universe, the other two points of a 4-point triangulation of points:
en.wikipedia.org
Just in case you still don't see the 'point' of entanglement and triangulation of points:
en.wikipedia.org
In recession away, one color the visible observable traveler, the other color the invisible un-observable traveler . . . both travelers the same traveler (in oncoming, reverse the direction of arrows, to the vertex). The visible observable traveler in the Einsteinian observer's observable universe, in receding away, will observably slowly, or swiftly, fall behind in space and time of the invisible un-observable traveler in light's coordinate point past-future histories SPACETIME! Only to reverse the process in oncoming). The warning label and message at the bottom of most modern automobile rearview mirrors (object reality is always asymptotically ahead of, out front of, in the future of, what you observe per the speed of light . . . whether in going away or in oncoming) will tell you the same thing I just told you above.