trc (07/31/82)
Could some relativity expert explain something that's been puzzling me? If velocity and acceleration are relative (IE no true fixed reference) then how come, on a spinning wheel, an ant (fixed in the wheel's frame) experiences an apparent acceleration, seemingly induced by the spinning of the external universe? This makes it appear that there is some rotational reference frame, at the very least. Perhaps there is some sort of local reference frame, based on causality being limited to the speed of light? IE, the ant on the wheel can't really spin the universe by spinning the wheel. TT TTT TTT TTT TTT TTTTTTTTTTooooooooommmmmmmm CCCCCCrrrrraaaavvveer houti!trc
doug (08/01/82)
In answer to the question posed: Why does an ant feel forces while spinning on a table in the absence of a universe to make reference to? (I think that is a fair restatement of the question). Let me quote from "Space and Time in the Modern Universe" by P.C.W. Davies of King's College, London: "What is the origin of these inertial forces? Newton attributed them to the space in which the acceleration was taking place. If this is correct, then even if all the contents of the universe are removed except for the roundabout, the centrifugal forces would still appear when the roundabout is rotated relative to the surrounding space. The existence of intertial forces could therefore be taken as a refutation of the relationist position and the establishment of the physical reality of space." Of course Newton believed, as did most physicists until the beginning of the century, in the existence of the ether. In the modern view, even though the presence of the ether has been carefully disproven, the fact that space has structure has been experimentally verified. Space is not just an abstract, collection of geometric points into which the universe fits. Space is a physical entity itself, which physical properties. One of these properties is that objects which spin relative to space experience inertial forces. Doug Lerner doug@uwisc
doug (08/01/82)
One more thing. The poser of the question also said that "if velocity and acceleration are really relative..." I think, and someone will correct me if I'm wrong, that while velocity is certainly relative, that acceleration is not. I think acceleration is something that is fixed. As a matter of fact, it is precisely the experiment mentioned that partially verifies that: the spinning ant is accelerating. We can measure that acceleration, even in the absence of other frames of reference, by the fact that the ant experiences a force. Newton's laws are, for this reason, written in terms of accelerations. doug@uwisc