Peter4075

560 reputation

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	location	United Kingdom
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visits	member for	1 year, 11 months
	seen	May 10 at 17:51
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I live in Essex, England.

	560 reputation	bio	website		visits	member for	1 year, 11 months
18 badges		location	United Kingdom		seen	May 10 at 17:51

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Feb 23	asked	Why no basis vector in Newtonian gravitational vector field?
Feb 22	accepted	Trying to understand Laplace's equation
Feb 22	comment	Trying to understand Laplace's equation @mtrencseni - Now I think I get it. I was making assumptions that I shouldn't (I was assuming I could ignore y and z and find the Laplacian of -Gm/x). I popped the real phi into the WolframAlpha calculator took second partial derivatives, added them all up and there was zero. Brilliant. Thank you very much.
Feb 22	comment	Trying to understand Laplace's equation @mtrencseni - thank you, but that doesn't answer my question. How/why does Laplacian equal zero when the radius doesn't equal zero. I'm thinking that if I differentiate phi twice wrt r I should be getting zero. Is that correct? How does that give zero? I'm a physics novice so don't worry about making your answer too simple.
Feb 21	asked	Trying to understand Laplace's equation
Feb 5	awarded	Tumbleweed
Jan 31	awarded	Teacher
Jan 31	answered	Getting started general relativity
Dec 21	comment	Proper distance and embedding diagrams? I actually found a useful reference to this at physics.ucsd.edu/students/courses/winter2011/physics161/…. I managed to input the integral equation into Excel and come up with the same results as the author. I then calculated the difference between coordinate height and proper height of Mount Everest, due to the Earth's gravitational field, is 0.0000062m. You'd hardly notice. Another productive day spent!
Dec 21	accepted	Proper distance and embedding diagrams?
Dec 21	comment	Proper distance and embedding diagrams? Thanks. I quite like that picture of the two concentric circles because I can then easily see how it builds into the kind of upside down witch's hat shape of an embedding diagram. But how does he calculate the circumference of the inner circle to be 0.99999999x2pi miles less than the outer circle? Does it involve some nasty integral of the proper distance equation I gave in my question? My definition of "nasty integral" is pretty all encompassing.
Dec 21	comment	Proper distance and embedding diagrams? thought I should point out that at my level nothing is trivial.
Dec 21	comment	Proper distance and embedding diagrams? Thank you. The context of the question was trying to understand a really simple embedding diagram in the form of a picture of a couple of 1 mile apart concentric circles around the Sun at pitt.edu/~jdnorton/teaching/HPS_0410/chapters/…. He says, "for each mile that we come closer to the sun, the circle does not lose 2π miles in circumference; it loses only (0.99999999)x2π miles". How does he work that out from the equation?
Dec 21	comment	Proper distance and embedding diagrams? Thank you. The context of the question was trying to understand a really simple picture of a couple of concentric circles around the Sun at pitt.edu/~jdnorton/teaching/HPS_0410/chapters/…
Dec 20	asked	Proper distance and embedding diagrams?
Dec 19	comment	Difference between coordinate and proper distance in Schwarzschild geometry @genneth - thanks for that. I'm assuming there's no problem in measuring r with my ruler in the "flat space" circles at the bottom of the diagram?
Dec 19	comment	Difference between coordinate and proper distance in Schwarzschild geometry Well, if r was Euclidean I'd use a ruler.
Dec 19	asked	Difference between coordinate and proper distance in Schwarzschild geometry
Nov 13	comment	Inertial frames of reference @Ben Crowell - sorry, I can't see your last point. If I jump off a cliff, I see the sun accelerating at g. OK. But how does that violate Newton's first law?
Nov 13	comment	Laws of physics and general relativity Luboš Motl - many thanks. You must admit it takes some talent to draw not only the wrong conclusion but the totally opposite wrong conclusion! I can see how the experiments would proceed identically in a freely falling frame, but surely the observer would make very different measurements in frames in different gravitational fields (eg a ball thrown on the Moon will travel further than one thrown with the same force on the Earth). I can see that the laws of physics are the same on the Earth and Moon, but how does GTR allow us to derive those laws from different sets of measurements?