Genady

Thank you. I'll work on being more precise in writing.

You're right, the first one was a bad statement. But I've fixed it just a couple of posts later,

I think it is a convention, because in the last 15 or so years since I started to study this part of physics, I don't remember seeing a different use of these concepts.

You guys just repeat what I say. I know all this, and I was talking about when we FEEL gravity, we FEEL acceleration. When we free fall, we don't feel gravity and we don't feel acceleration. We FEEL zero acceleration then, which is different from non-zero acceleration that we feel when we don't free fall. I guess, you simply misread what I said. For example, you said, and I said earlier, The same. Is my English so bad? Was not it clear that in "we" I mean "human body"?

So, we feel acceleration, and we feel difference in acceleration, including the difference between zero and not-zero acceleration.

We feel the effect of gravity, but not the gravity. This effect is not distinguishable from the effect of upward acceleration without gravity.

Yes, I feel it. It's a sense. And it is not one of the original five.

Suggestion to split the discussion of senses from the original thread and to move it to a biology forum.

Isn't a change from not free-falling to free-falling an acceleration?

There is a little thing in my vestibular system that stops pushing on some receptor and gives me feeling of the fall. If I accelerate upward, it pushes stronger and gives me feeling of accelerating up. Etc.

I do.

If you say so.

And that's why I've mentioned gravity/acceleration. When we free fall we sense acceleration. Of course, technically speaking we are too small to ever sense gravity as it manifests itself in tidal forces. Because you've responded to the mention of echolocation.

Sense - Wikipedia has an in-depth discussion of the topic (senses, not god).

I understand that these are examples of perception rather than sensation.

In Antarctica, yes. In Sahara, no.

I think there are four. Taste and smell are variations of the same sense. OTOH, some animals sense Earth magnetic field. Some animals sense electric field. These are two other senses. One more: sense of gravity/acceleration. Isn't it another sense?

Yes, it is true. I remember some things and I don't remember some details of some other things. As I said, "many years ago"... Yes, I use different expressions in different posts. So?

"Closer to eternity"? You are always infinitely far from eternity, by definition. Regardless of how you measure a "distance to eternity", super computers cannot simulate eternity either.

I don't think we can simulate eternity on computer.

LOL. BB: Boltzmann Brain, Big Bang.

Relativity as formulated in terms of observable measurements demands the theoretical existence of points in space and time. The resolution of the singularity as a point in Einstein's Relativity is a consequence of the intrinsic limitations of observable measurements. Einstein could not make heads or tails of the singularity. This is why: to resolve the singularity as a point in spacetime is objectively incorrect. The singularity is a wave in spacetime with no upwards limit to its frequency You did not answer my question. You did not even relate to it in your reply. Did you understand my comment?

Hmm... Reminded me that the OP, @Commander, hasn't acknowledged my answer above to the last puzzle.

I suspect that they never had or lost pigmentation on the underside because it is not needed. Also possible, that an ancestor of rays was pelagic with a light underside like in manta rays, for example. The stingrays evolved to be bottom dwellers but retained the light underside of the ancestor.

Neither countershading nor "light against surface" explanations seem applicable to bottom dwelling animals, such as stingrays and countless others with similar dark-top-light-bottom coloration.