We know from Einstein that my clock and your clock will not run at the same rate. Time (or rate of our clocks) depends on acceleration and/or gravity. If my clock accelerates and/or is in a strong gravitational field then it will run slower than your clock. The stronger the gravity/acceleration the slower the time. So time is relative. Quran 22.47 compares time on Earth with time at Paradise/Hell (1 day vs 1000 years); while Quran 70.4 compares time on Earth with time in wormholes (1 day vs 50,000 years).
When angels travel by a wormhole (miaaraj in Arabic معراج) they experience different time than ours:
٤ تَعْرُجُ الْمَلَائِكَةُ وَالرُّوحُ إِلَيْهِ فِي يَوْمٍ كَانَ مِقْدَارُهُ خَمْسِينَ أَلْفَ سَنَةٍ
Here angels will experience 1 day while humans will measure it as 50,000 years (time vs. time and not time vs. distance). This can happen if angels pass through wormholes. This is a picture of a wormhole.
The Quran says that angels use these wormholes to reach any place in the universe. An observer outside gravitational fields sees those angels passing by him at 99.99999...% of the speed of light (relativistic speeds, very close to the local speed of light), that is at 299792.457999...km/s. But when those angels enter the gravitational field of a wormhole this observer still sees those angels entering the wormhole at 99.99999...% of the speed of light however that would no longer be at 299792.457999...km/s. This is because in the presence of gravity the speed of light becomes relative. If he sees the speed of light in a wormhole at 10 meters/sec, for example, then he would see those angels traveling at only 9.999999...meters/sec, flapping their wings in slow motion, reddish in color and very dim (and in turn they would see him in fast motion, bluish in color and very bright). A smaller time dilation (say 100 or 1000 years vs. one day) can also occur if we stand on Earth and angels stand on a neutron star (or resist their fall into a neutron star; gravitational time dilation). This makes their velocity relative to us to be zero (and not 299792.457999... km/s). But naturally occurring wormholes connect distant black holes; however at a lower time dilation (100 or 1000 years vs. one day) the two entrances of the wormhole cannot connect; hence a wormhole cannot form. So with the time dilation stated in the Quran (50,000 years vs. 1 day) and the fact that angels are moving then those angels have to be crossing a wormhole. The neutron star or any other scenario cannot do. Learn more: Wormholes.
How could an illiterate man who lived 1400 years ago have known time dilation?
The Quran (Koran, the book of Islam) contains scientific knowledge that could not have been known 1400 years ago. It ranges from basic arithmetics to the most advanced topics in astrophysics. You are invited to go through those miracles and judge for yourself.
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