Genady

If this is not about the theory of relativity as in physics, then it should not be in the Physics forum. It should go elsewhere.

Try Aspirin.

No, Einstein didn't say this.

Stop here. They are not.

Two wrongs here: 1. It is not so according to Einstein. 2. It is not what Einstein called, the happiest thought.

Yes, it looks so. This is about what And And what Dawkins wrote in 1996 (I just repeat it here as it has been quoted above):

Yes. These wheels appear as cellular components. But not as components of a multi-cellular organ. I think that both Dawkins and @mistermack have explained it well. (It's fun to imagine an aquatic animal that moves by such a motor.)

However, such a design works in bacteria. Not a human design, with a spinning wheel, powered by and made of the bacterial components. PS. Not for moving blood, of course, in bacteria, but for a different function.

Turned out that Richard Dawkins wrote an article 26 years ago exactly about this question. (Why Don't Any Animals Have Wheels? | Live Science)

Nothing. The discussion between @Lorentz Jr and @Markus Hanke has moved to GR and then to SR some time ago.

Right! But I've expected the microbial answer and that's why have specified animal organs in the OP. It didn't help, though. Yes, this might be a fundamental obstacle. If we designed a pump for moving blood around, aka heart, it would certainly have a few wheels in it. Dawkins: Why Don't Any Animals Have Wheels? | Live Science:

From your link: I wonder, why. The OP question refers to a variety of wheels used in a variety of human designed mechanisms.

That was the subject of the OP question. There are wheels in microbial world, but not in macroscopic world of animal organs. Why? For clarity: What about it? This:

Animal organs. I am not sure there is a problem, yet. OP just states a fact.

Wheels are ubiquitous in human designed mechanisms but absent in animal organs. Why is it so?

This is not SR. This is a wordplay.

Right. The slowdown of aging is not physical as long as they are in transit. Only when they meet again, their ages can be physically compared. The result is the consequence of the entire history between their separation and their meeting. There are two different worldlines with two different lengths. The acceleration is just an indication of the difference. If the histories are different, why to expect the outcome to be the same?

I think it is more than this, it determines who ages more. Let's say A is on Earth and B is in the spaceship. When B reaches the turning point, it doesn't turn, but instead A accelerates to a faster relative speed and catches up with B somewhere behind that point. Then it will be B who has aged more, right?

The difference is that the spaceship accelerates while Earth does not.

Why is this straightforward question a brain teaser or puzzle?

An active ball joint mechanism (ABENICS) enhanced by interactions of spherical gears: https://ieeexplore.ieee.org/ielx7/8860/9556521/9415699/supp1-3070124.mp4?arnumber=9415699

I see that one can find spirals here and there. I don't see that they are favored.

About 15 years ago I watched an MIT seminar on issues in cosmology. A professor leading it has offered his prediction that in 10 years we will know what dark matter is and we will know that dark energy doesn't exist. 15 years later, we don't know much more than we did then, do we?

What is it?

Is it a cosmological comoving frame, the one where CMB is isotropic?