donald (08/02/82)
The ant on the rotation wheel problem is a variation on Newton's bucket and brings up the ugly question of Mach's principle. The argument is that if you have a spinning bucket full of water, inertial effects cause the surface of the water to become concave, thus showing that it is the bucket, not the rest of the universe that is "really" moving. If I recall Gardner's "Relativity for the Million" correctly, General Relativity comes to the rescue with the following explanation: the rest of the universe is rotating about the bucket, which generates a gravitational field (indistinguishable from accelaration according to the General theory), thus causing the surface of the water to go concave. Note that this does not violate Special Relativity by requiring the rest of the universe to spin faster than light: the requirement that no signal be propagated faster than C is satisfied. Contrary to Doug Lerner's statements, the inertial effects caused by the spinning bucket (or wheel) are not a property of movement "relative to space time" (whatever that means). Consider this though: what if the bucket were the only object in the universe and it was started spinning (relative to what, you might ask!), would the surface go concave? Don Chan
bobr (08/02/82)
in a universe in which the bucket is the only object....... ...it is absolutely meaningless to say that the bucket rotates. (Correspondingly, it is meaningless to say that there is an absolute space; there is only motion RELATIVE TO SOMETHING.) Christoph Bobrowski utcsrgv!bobr
doug (08/03/82)
re: Don Chan I quoted from P.C.W. Davies who maintains that the inertial effects are due to acceleration relative to space (not space-time).
doug (08/03/82)
Christoph Bobrowski says, "..in a universe in which the bucket is the only object...it is absolutely meaningless to say that the bucket rotates." The whole difference between a mathematician's approach to the problem and a physicist's is that the physicist will say that the above statement is incorrect. The physicist will say it is not meaningless. Space is a physical entity with physical properties and one *can*, in fact, rotate with respect to it!
donald (08/04/82)
Doug's assertions that "space is a physical entity with physical properties and one can, in fact, rotate with respect to it" are (begging Doug's pardon) mistaken. How can you say that when what you are advancing is essentially the ETHER of olden times? What physical properties DOES space have, and how does one determine if one is "moving" with respect to "space"??? For if you could do that, then you've discovered the very "preferred frame of reference" that both Special and General relativity deny! Don Chan ~p
doug (08/05/82)
re: to Don Chan (i) I am just quoting from some physics books, such as that one from Dr. Davies. My impression is that the books are right, i.e. that they represent our current understanding of physics. (ii) That space has physical properties does not, as Mr. Chan says, mean I said there is an ether. Just because space has physical properties does not mean an invisible, all pervasive fluid exists in all space! (iii) You *can* test some of the physical properties of space! One property of space gives us inertial forces, such as in the spinning ant problem. Another property of space is that it contracts and expands depending on our motion relative to it. Another property of space is that it seems to be generally expanding out, and has been, since the "big bang": the stuff of the universe is not being blown out to fill empty, existing space - the space itself is expanding! There are measurements to this effect! There are other properties of space that I am not familiar with, such as it's curvature in the presence of massive objects, etc. But it is *clear* that space itself has physical properties and is a real thing.
Physics:retief (08/05/82)
As far as the question of "is space is physical thing".
Yes, you can ask any particle Physist that. For example, did
you know that vacuum is NOT empty! The most perfect vacuum God could
ever make would have "virtual particles" zipping in and out of existance.
Vacuum can exert a pressure on you! It is known is the Casimir effect
(or something like that.) In a vacuum two parallel metal plates would
attract each other due to the pressure of vacuum (over and above the
gravitational and electrical attractions they have.)
- Dwight -doug (08/06/82)
re: vacuum and virtual particles Not only that - did anyone read the Scientific American article a couple of years back about the "Decay of Vacuums". Seems vacuums can spontaneously decay INTO particles! Not virtual ones either!
laurir@sri-unix (08/19/82)
Hmmm. It seems that the universe really is spinning. In the July 29 issue of Nature, P. Birch makes a strong case, based on the polarization of distant radio sources vs their apparent shape. This was abstracted in the August 7 issue of Science News. The apparent rate of rotation is 10^-13 radians per year.