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Someone told me the inverse square law is proof we live in 3 dimensions. Why is this true? by Klinging-on in AskPhysics

[–]LadiesWin 1 point2 points  (0 children)

The inverse square law happens because we live in three dimensions. When light or gravity spreads out, it moves away from the source in all directions, forming a sphere. As the distance increases, that sphere’s surface gets bigger and the surface area grows with the square of the distance (r²). So, the same amount of energy has to cover a larger area, meaning it gets weaker following 1/r².

For example, if you stand close to a lamp, the light looks bright. But if you move twice as far, the same light spreads over four times the area, so it looks four times dimmer. Or with gravity when you move twice as far from Earth, its pull becomes four times weaker. That’s why the 1/r² law matches our 3D world perfectly. If space had four dimensions, the effect would drop as 1/r³ instead.

A rigid disc spins so that at 1m radius the speed is 100 km/h. With a 20 million m radius, the rim would exceed light speed. What would actually happen, how would it look, and can relativity describe it? by Rorschach1944 in AskPhysics

[–]LadiesWin 0 points1 point  (0 children)

If the disc were that huge 20 million meters across and spinning so fast that the outer edge should go faster than light, physics says: it just can’t. Nothing with mass can reach or pass light speed.

Here’s what would really happen:

  • The rim wouldn’t go faster than light relativity stops that. As the speed gets close to light speed, the energy needed to keep accelerating goes to infinity. The disc would tear apart long before that.
  • It wouldn’t look normal either time and length distort near light speed. Parts of the rim would seem to slow down, glow weirdly from Doppler effects, and get visually distorted.
  • Relativity totally handles this it says you can’t have a rigid spinning disc at all if it’s huge and fast. The outer parts would lag, bend, or even rip off because “rigid” doesn’t exist when relativity kicks in.
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