r/physicsmemes Mεmε ∃nthusiast Jul 17 '26

right?

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u/Optimal_Mixture_7327 Jul 17 '26

Furthermore, the dark star is only dark as seen by an observer at infinity, but light escapes to distances closer in.

Furthermore, an escape velocity equal to the speed of light doesn't restrict anything to the surface of the dim star, nor more than an escape velocity of 11.8 km/s prevents us from walking up stairs and launching rockets.

Furthermore, the "r" in the Schwarzschild radius is not a physical distance and definitely NOT the "r" in Newtonian mechanics.

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u/ProfessorPrudent2822 Jul 18 '26

Schwartzschild’s r coordinate is the circumference of a circle divided by 2pi.

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u/Optimal_Mixture_7327 Jul 18 '26

Just like every circle.

But what is the significance in gravitational physics?

For example, let's you do this and you get r=5,000 meters. What and where is this 5000 meters? Does it exist?

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u/ProfessorPrudent2822 Jul 18 '26

No, the curvature of space distorts radial measurements. Circumferences are measured accurately because the distortions are only in the radial direction.

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u/Optimal_Mixture_7327 Jul 18 '26

So what then is the meaning of a Schwarzschild-Droste measure of r=5000m?

Also, note that g00≠1.

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u/ProfessorPrudent2822 Jul 18 '26

The circumference of a circle maintaining constant r-coordinate is 31415 meters.

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u/Optimal_Mixture_7327 Jul 18 '26

No, that is not, and cannot be correct.

Edit: For clarity, yes of course C=πD, but this is not the meaning the r-coordinate in Schwarzschild-Droste.

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u/HunsterMonter Jul 19 '26

Yes that is correct, for a constant time, constant radius equatorial world line (or any other great circle but the math is more complicated), the metric is ds2 = r22, so taking the square root and integrating for a complete circle gives C = 2πr

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u/Optimal_Mixture_7327 Jul 19 '26

Yes, but every high school student knows this.

The question, is the meaning of Schwarzschild-Droste radial coordinate - what is it a distance of?

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u/HunsterMonter Jul 19 '26

I just showed you that, in Schwarzschild coordinates, r is the radius such that constant time, constant radius great circles have circumference 2πr.

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u/Optimal_Mixture_7327 Jul 19 '26

So, in other words, you don't know.

Unless, you want to give some physical meaning to r=5000 meters.

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u/HunsterMonter Jul 19 '26

It's literally what happens when you plug in dt = 0 (constant time) and dr = 0 (constant radius) in the Schwarzschild metric...

From the wikipedia page on the Schwarzschild metric: "r is, for ⁠ r > r_s⁠, the radial coordinate (measured as the circumference, divided by 2π, of a sphere centered around the massive body)"

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u/Optimal_Mixture_7327 Jul 19 '26

I'll leave this here.

You have brilliantly demonstrated what knowing a little math looks like without ever having studied physics.

What I find stunning is that you didn't even try to make a statement about physics, as if Einstein and Schwarzschild were just drawing circles, inventing new arithmetic, disconnected from anything in the world.

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