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General Science / Re: Would you tell the world if you knew an asteroid was coming to Earth?
« on: 15/05/2021 21:47:41 »I'm intrigued by the idea that, if I told people about it, they would listen to me.
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I'm intrigued by the idea that, if I told people about it, they would listen to me.
Below is a popsci article that explains what happens and below that I’ve compared it with a similar effect due to a moving charge.
https://medium.com/starts-with-a-bang/what-is-the-speed-of-gravity-8ada2eb08430
You might want to follow the link in this postThanks for the suggestion, I will check it out.
My colleagues do it every day, converting the mass of an electron-positron pair into the energy of two photons.I was thinking someone would say something like that. Energy and mass are both used in the stress-energy tensor for GR. It's fine to convert one to the other. The problem is only when you create one from nothing (or remove one and leave nothing). It might seem that I have been quite unkind to you, Alancalverd, but that was not my intention.
those two events are outside each other's causal light conesYes, I agree and I've always liked the explanation using light cones and causal effects. You've presented the explanation very well.
I have concluded that the concept is about existence and well being of future conscious entities.OK. Actually, it does sound reasonable.
So if we create a mass ex nihilo, its gravitational field will propagate isotropically at c,and also,
The speed of physical changes in a gravitational or electromagnetic field
so discussing Newtonian rigid motion is off topic, no?I'm only guessing what was being considered as a rigid body and by whom.
My example was simply the proper length of a trainBut someone mentioned rigid objects, not sure who and it was days ago.
I think I need a citation somewhere that this is a valid form of rigid motionSome citations were given in an earlier post. It's not that it is "valid" in the sense of being the best definition, it isn't. It's just that this idea of rigidity can and has been considered by Physicists and still seems to be a view of "rigidity" that ordinary people may want to have ---> quickly leading to problems, including transmission of force at speeds ~ infinity etc. Nothing that would be a problem in Newtonian mechanics - but in SR nothing vibrates, moves or transmits that fast.
You use the word ‘sacrifice’Can't keep every characteristic that a rigid body may have in Newtonian mechanics when you make the transition to SR. Not all of them are important. Someone mentioned Violation of the Equiv. principle but that isn't a thing to worry about in SR, that's a GR thing.
Not necessarily... (is a reference frame involved)My original reply was intended to show that a suitable frame to measure that length (in the ordinary way with a ruler) can be constructed. However, this assumes Talanum would want to define and measure length that way - see later.
The unique proper length is a frame independent quantity and thus serves as the length property of the object. This is the length that Talanum probably meansThis is one of the more important items. Is Talanum willing to consider "proper length" as a property of the object? I'm only guessing but most people just want to consider length as something they can measure with a ruler. I guess we won't know unless Talanum replies.
By classic, you mean non-relativistic?Yes. I'm old, so "classical mechanics" is just Newtonian mechanics. New definitions sometimes include SR as classical.
If you you don't think that is accurate then you don't think Special Relativity is accurate which, considering all of the rigorous testing SR has undergone, seems like you are on the wrong side of science.
Again my point is not about idealized Born rigid objects, but rather about ordinary real objects where the two endpoints are effectively stationary relative to each other. Such objects have a proper length, which is a property of the object, not a frame dependent relation.I'm not following this, which is probably my fault. "The two endpoints are stationary relative to each other" - so there is a frame of reference involved, we can pick the front end to be the origin of the frame and the back end has 0 velocity in that frame. So it is the frame where the object is stationary (+ or - some translation). Yes, then there is a unique length you will observe for the object, although it seems that you have told me (up to translation) which reference frame we were using and I also know about the motion of the object in that frame (it is stationary).
A rigid object has a proper length,
When a train is moving with respect to a stationary observer, he will measure an object on the train as shorter than an observer on the train would measure it. However if he later climb onto the train and measure the object again he will find it longer than he measured it previously: a contradiction for a global observer (Gods eye view). One can't say the object was really shorter.
I can see that my approach so far has been wrong. Instead of my making statements, perhaps it would be better, if I quote from a source. Here is the definition of the equivalence principle taken from the Wikipedia article on equivalence: