Timeline for Reference frame doubts about isotropy
Current License: CC BY-SA 4.0
18 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Nov 5, 2023 at 15:55 | comment | added | John Rennie | @GiorgiLagidze Yes I agree. | |
| Nov 5, 2023 at 15:27 | comment | added | Giorgi Lagidze | My final assumption is that by universe, we mean everything(space + every "body" included in the space) while by space, Landau means empty space(no "body" included in it). Wouldn't you agree ? | |
| Nov 5, 2023 at 15:24 | comment | added | Giorgi Lagidze | I think what landau means by "space" is ALWAYS empty space then - i.e without its constituent members and for sure, in such empty space, we got inertial frame and isotropy too. Should I follow Landau in that sense that when "space" is mentioned in terms of "homogeneity/isotropy", I should always treat space as empty one ? Problem with this approach is that in Barbara Ryden's book, she says that on small scale, universe is non-isotropic and uses gravity example. (see ibb.co/TrrsBYS). Though, she mentions "universe" and not "space". | |
| Nov 5, 2023 at 15:14 | vote | accept | Giorgi Lagidze | ||
| Nov 5, 2023 at 14:59 | comment | added | John Rennie | @GiorgiLagidze it's the external force that's anisotropic, not the space that the force acts in. | |
| Nov 5, 2023 at 12:53 | comment | added | Giorgi Lagidze | I agree that space is inertial and external force is acting. but space is also non-isotropic and that's where I see the landau's contradiction. I'm not saying that space is non-inertial or anything. contradiction comes about isotropy where landau says that in inertial frame, space is isotropic. | |
| Nov 5, 2023 at 12:45 | comment | added | John Rennie | That's a matter of opinion. Landau's opinion is different from yours. I have to say that I agree with Landau. I would say the space is inertial but there is an external force acting. | |
| Nov 5, 2023 at 12:43 | comment | added | Giorgi Lagidze | When we say that ground frame is inertial, we don't take gravitational field out of that frame, right ? so in that frame, space consists of gravitational field. no ? because if we take gravitational field out of that frame, then it's not "ground" frame anymore :D | |
| Nov 5, 2023 at 12:39 | comment | added | John Rennie | You and Landau mean different things by space and that's what is causing the confusion. You are taking space to mean everything including the gravitational field field and Landau is taking it to be what's left if we remove the gravitational field. That is Landau is treating the gravitational force as an external force like the springs in my example. There is no real difference here - it's a matter of terminology. | |
| Nov 5, 2023 at 12:33 | comment | added | Giorgi Lagidze | Ok, I re-read it now. the problem that I have in my mind is the following. We treat ground frame as inertial, we also know that in that frame, space is NON-isotropic, which clearly contradicts what Landau said about inertial frame being isotropic. What's your thought on this ? | |
| Nov 5, 2023 at 12:30 | history | edited | John Rennie | CC BY-SA 4.0 |
Typo
|
| Nov 5, 2023 at 12:29 | comment | added | John Rennie | @GiorgiLagidze Oops, yes, that's a typo. Thanks :-) | |
| Nov 5, 2023 at 11:43 | comment | added | Giorgi Lagidze | @JohnRennie I think, before I read it in deep thoughts, i found a mistake (not sure)in your answer. you say:"So space itself is still a non-inertial frame but there are external forces acting." - shouldn't you have said that space is inertial ? | |
| Nov 5, 2023 at 11:35 | comment | added | John Rennie | @GiorgiLagidze I have extended my answer to try and address the question in your comment. | |
| Nov 5, 2023 at 11:34 | history | edited | John Rennie | CC BY-SA 4.0 |
Respond to comment
|
| Nov 5, 2023 at 11:18 | comment | added | KarimAED | The space itself can still be considered isotropic. The gravitational force in classical mechanics doesn't arise from an anisotropy of space, but rather from stuff (i.e. earth) within that space. This is very different from the treatment chosen in GR, where space(-time) itself is distorted due to massive objects. | |
| Oct 9, 2023 at 11:34 | comment | added | Giorgi Lagidze | Thanks for the great answer. Though, I want to confirm something and ask it maybe in a clearer way: If in newtonian mechanics, we say ground frame is an inertial frame, then how does the "inertial frames are isotropic" obeyed ? because clearly, $mgy$ is not isotropic. | |
| Oct 9, 2023 at 10:25 | history | answered | John Rennie | CC BY-SA 4.0 |