Recent content by dchartier

Hello, I'm self-studying Ta-Pei Cheng's Relativity, Gravitation and Cosmology. Problem 2.8 is the following: Two spaceships traveling in opposite directions pass one another at a relative speed of ##1.25 \times 10^8## m/s. The clock on one spaceship records a time duration of ##9.1 \times...

Thanks, it's good to know I'm not missing something obvious here. I shouldn't have used the approximation symbol, I should've used the equals sign. I was just converting from Cartesian to polar. Again, thanks for checking my calculation.

On page 252 of Kleppner's Introduction to Mechanics (2d. Ed.) in Example 7.5, Kleppner analyzes the forces and torques on a uniform rod that have been bent into a quarter-circle of radius R and length PI*R/2. His diagram is attached. Kleppner writes, "[t]he center of mass is halfway along the...

I understand, thanks!

This may be a very stupid question, but what's the difference between ##R(t)^3## and ##R^3(t)##?

Whoops, sorry, I miswrote -- I meant an open (negative curvature) universe, not a closed universe. Is an open universe necessarily infinite?

Perhaps I misunderstand, but I thought (based on the FAQ, https://www.physicsforums.com/showthread.php?t=506986) that flat and closed geometries implied that the universe was infinite. Is that not the case?

Thanks, that's very helpful. When speaking of flatness or curvature, I presume you mean flatness/curvature of space-time, not just space? Also, what sort of physical effects would you expect to see from an open or closed universe due to its curvature? For example, if you created a huge...

Meaning of a "Flat," "Open," and "Closed" Universe Hello all, I'm reading up on cosmology and the potential shapes of the universe based on its density in relation to the critical density. When one says that a universe is "flat," what precisely does this mean? Does this mean that the...

Many thanks, gentlemen! I'm still working on solidifying these higher dimensional and non-Newtonian concepts in my mind, but I'm slowly getting there. Your explanations are much appreciated.

Thanks, so let me see if I understand this properly in terms of GR. If I were to imagine this in space-time, in the absence of any other forces acting on an object, that object will move inertially on an geodesic path through space-time toward the source of the gravity. This is the "natural"...

Thanks, this is very helpful. Another perhaps silly question (and A.T. may have already answered this as simply being a postulate of the theory), but under GR what causes the free fall to begin with? Again, thanks guys!

Hello, I've been doing some relativity self-study and have what may be a silly question but one that has me scratching my head. I understand conceptually that under GR, gravity is not a force but rather the effect of objects following the straightest possible path (a geodesic) over curved...