Talk:Galilean invariance

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Contents 1 einstein's elevator vs. einstein's cabin 2 examples of inertial frames in the lead paragraph 3 Tidal forces example 4 Torpid matter

[edit] einstein's elevator vs. einstein's cabin

Google searches for einstein's elevator and einstein's cabin give:

1. Results 1 - 100 of about 442 for "einstein's elevator".
2. Results 1 - 3 of 3 for "einstein's cabin".

All three results for einstein's cabin refer to the scientist's own residence, not his thought experiments.

This article says Einstein's elevator is used in Einstein and Infeld (1938), which I have somewhere. The article would benefit from any references in which einstein's cabin is used in a thought experiment.

As there appears to be a low-level edit war over this matter, here are two questions:

1. What did Einstein say?
2. Why can't there be two examples, contrasting the inertial and accelerated scenarios?

The article currently reads: "In special relativity, one considers Einstein's cabins, cabins that fall freely in a gravitational field." Is it really necessary to consider so many cabins? I find one cabin charming, but a whole rain of them frightening. :-)

--Jtir 16:38, 23 September 2006 (UTC)

(transfered from User talk:Mct mht)

thanks for the heads up. both "cabin" and "elevator" can be found in the literature. they convey the same idea. no objections from me if you strongly prefer elevatorvand wanna put it back. Mct mht 19:12, 23 September 2006 (UTC)

I like cabin in this example, because the cabin is in free fall. An elevator has a cable that can be used to accelerate it, so it provides a somewhat different example. Do you have a particular source, such as textbook, that uses cabin. I would like to add some references to this article and a title and author is all I would need. If you like, you can put it here and I will do the edit. --Jtir 19:43, 23 September 2006 (UTC)

[edit] examples of inertial frames in the lead paragraph

The lead paragraph reads very nicely until the last sentence: The fact that the earth on which we stand orbits around the sun at approximately 18 km/s offers a somewhat more dramatic example [of an inertial frame].

The Earth has a gravitational field, is subject to earthquakes, and is orbited by a moon, so it is not an example of an inertial frame.

Examples of nearly inertial frames in a gravitational field are the Vomit Comet and the International Space Station. As the article notes later, microgravity is still present in those frames.

--Jtir 18:58, 23 September 2006 (UTC)

The Earth also rotates. The ship example is triply qualified by assuming the ship moves "... at constant speed, without rocking, on a smooth sea...". Perhaps something similar could be done for the Earth example.--Jtir 17:36, 24 September 2006 (UTC)

I'm not sure how relavent this is, since i'm having trouble understanding Newtonian Relativity, the earth does rotate around the sun, as does the International Space Station around the earth, meaning that they are accelerating, as acceleration is not only speed but the direction of motion as well. So, take that into consideration, whoever understands this better than i.--The Sporadic Update 19:08, 28 September 2006 (UTC)

Indeed. The Earth also follows a (nearly) circular orbit. It has a constant speed of 18 km/s, but is continuously accelerating to follow a curved path; it is an example of a decidedly non-inertial frame. TenOfAllTrades(talk) 19:27, 3 October 2006 (UTC)

I think that what may have been meant originally is that the orbital acceleration is effectively "cancelled" by the gravitational field of the Sun. The same thing may be said about satellites in Earth orbit. The article needs to help sort this out. --Jtir 19:50, 3 October 2006 (UTC)

[edit] Tidal forces example

The example used to show the locality of some frames of reference says:

"In general, the convergence of gravitational fields in the universe dictates the scale at which one might consider such (local) inertial frames. For example, a spaceship falling into a black hole or neutron star would be subjected to tidal forces so strong that it would be crushed. In comparison, however, such forces might only be uncomfortable for the astronauts inside (compressing their joints, making it difficult to extend their limbs in any direction perpendicular to the gravity field of the star). Reducing the scale further, it might have almost no effects at all on a mouse. This illustrates the idea that all freely falling frames are locally inertial (acceleration and gravity-free) if the scale is chosen correctly."

I don't think that this describes tidal forces at all well. Objects in freefall, as described, are stretched: not crushed. I'll give it a few days, and if there's no objection, I'll reword the paragraph quoted above, and reference the spaghettification article. Andy Ross 08:48, 14 June 2007 (UTC)

In a sufficiently large freefalling lab, objects will be stretched in the direction of the centre of attraction, but crushed in directions perpendicular to that. See for instance Taylor and Wheeler's Exploring Black Holes - Introduction to General Relativity, Chapter 2:

"Consider, for example, the plight of an experimental astrophysicist freely falling feet first toward a black hole. As the trip proceeds, various parts of the astrophysicist’s body experience different gravitational accelerations. His feet are accelerated toward the center more than his head, which is farther away from the center. The difference between the two accelerations (the tidal acceleration) pulls his head and feet apart, growing ever more intense as he approaches the center of the black hole. The astrophysicist’s body, which cannot withstand such extreme tidal accelerations, suffers drastic stretching between head and foot as the radial distance drops to zero.

But that is not all. Simultaneous with this head-to-foot stretching, the radial attraction toward the center funnels the astrophysicist’s body into regions of space with ever-decreasing circumferential dimension. Tidal gravitational accelerations compress the astrophysicist on all sides as they stretch him from head to foot. The astrophysicist, as the distance from the center approaches zero, is crushed in width and radically extended in length. Both lethal effects are natural magnifications of the relative motions of test particles released from rest at opposite ends of free-float frames near Earth (Chapter 1, Section 8)."

DVdm 11:00, 14 June 2007 (UTC)

Ah yeah, I read that paragraph completely wrongly. We might as well delete this section from the talk page, as it's resolved. Andy Ross 14:44, 14 June 2007 (UTC)

Adding a link to spaghettification is a good idea — I only learned of the term by reading this discussion. BTW, a talk page is normally archived only when it gets too long. --Jtir 15:13, 14 June 2007 (UTC)

... and no need to remove this section from the talk page. Cheers! DVdm 15:16, 14 June 2007 (UTC)

[edit] Torpid matter

According to Evans and Starrs^*, Galilean invariance will not hold for certain configurations of matter. — DIV (128.250.204.118 03:17, 18 June 2007 (UTC))

^*{“Emergence of a stress transmission length-scale in transient gels”; Journal of Physics: Condensed Matter; Institute of Physics; 18 March 2002; 14 (10): pp. 2507–2529.}