Recent content by AxelBoldt

I just noticed that search engines index these FAQs. If you search on Bing for "timescape cosmology" you'll find a link to the PhysicsForums thread and a misleading snippet quoting the FAQ.

If you switch off your ad blocker, you'll see two Google ads in the middle of the FAQ. Note that the ad blocker (uBlock Origin) does not remove the FAQ, only the Google ads.

Closed threads sometimes contain terrible FAQ sections at the bottom; these are not shown to logged-in users. See for instance here: https://www.physicsforums.com/threads/what-are-the-latest-thoughts-about-the-timescape-cosmology.964801/ The discussion in the thread is reasonable, but the FAQ...

I assume it's possible to convert matter into energy, and vice versa. I assume the same is true for antimatter. I also assume there is no such thing as "anti-energy". Shouldn't it thus be possible to convert matter into antimatter, by first converting the matter into energy, and then the...

I was thinking of a perfectly symmetrical and flat "vane" with pivot exactly in the middle. I think by now we all agree that such a vane won't align itself with the flow, so my initial assumption was wrong. But what will it actually do? Approach a stable equilibrium near the 90 degree angle...

Well, on the NASA page they define AC as the point where the moment doesn't vary with angle; then they claim that the location of AC doesn't vary with angle either, and that the moment is zero at AC for symmetrical airfoils. In any event, something still seems fishy to me. In the symmetrical...

If it is true that the lift is always larger at the leading edge, then the situation with the vane being parallel to the flow (angle of attack=0) is an unstable equilibrium: deviate a bit from it, and the resulting torque will pull you farther away. On the other hand, the situation of the vane...

That's true, in the absence of perfect symmetry, the 90-degree situation is certainly not stable. But suppose the vane is already almost parallel to the flow, let's say angle=15 degrees or so. Then my question would be: is the torque acting to increase or decrease the angle? I guess that...

You're right of course, "vane" is the word, thanks. I guess what I'm asking is: why is the situation with the vane being orthogonal to the flow not a stable equilibrium? If we deviate a bit from it, so that the vane-flow angle is a bit smaller than 90 degrees, then as you say there's a lift...

Let's say we have a symmetrical weather wane that's free to rotate about its vertical axis, and a uniform fluid flows in some horizontal direction d. Then, I assume, the wane will align itself parallel to d, but why? Is there a simple argument or does one have to carry out a numerical simulation...

One way to understand the gravitational field within and without a massive shell is with the Gauss Law. I wrote up an explanation https://www.physicsforums.com/showpost.php?p=2334147&postcount=9".

The fact that the external gravitational field of a sphere is the same as the gravitational field of a point source located at the sphere's center is a consequence of the Gauss Law. That law states that the total flux of any gravitational (or electric) force field through any closed surface is...

Yes, that's the heart of my problem. Assuming totally elastic collisions, the balls will never "stick together", but there could still emerge regions of very high density, where balls are kicking each other like crazy, with the whole region being held together by gravity. And these are the...

Is that true? I thought matter isn't subject to the expansion. If it were, then the whole tethered-galaxy idea would fall flat, wouldn't it? The galaxies would just fly apart as they have always done, connected by an expanding tether.

For a gas and dust cloud to collapse under gravity to form a star and planets, is the concept of friction important at all? In other words: let's consider a large number of billiard balls, no internal degrees of freedom, participating in completely elastic collisions, with random initial...