Misconceptions in Relativity

A paper may well refer to previous papers. These are the paper's references and they will be listed at the end of the paper. A paper will quite possibly be *referred to* by papers in the future. These are the paper's citations. Obviously, these will not be listed in the paper itself, but in the grand database of papers, a citation index can be built. In my day, the lanl online database was in its infancy, and citations were looked up in the very large SCI volumes in the library (Scientific Citation Index). A paper's "importance" is often measured by its number of cites. Of course, cites can be for good or bad reasons. A lack of cites is usually a measure of irrelevance or "so wrong, not even worth a reply".If we go back to Lieu and Mittaz you will see on the right hand side of the page "References & Citations", and under this the link to cited by where you will find the four citations. Nearly all of the literature points to CMBR gravitational lensing being just beyond the level of current detetction; hence Lieu and Mittaz being all but ignored. But here is a counterexample: Smith K, Zahn O, Dore O, Detection of Gravitational Lensing in the Cosmic Microwave Background, Phys.Rev.D76:043510,2007

According to General Relativity (and most related gravitational theories) gravitational effects propegate at the speed of light. This is consistent with the limited number of experimental/observational tests carried out.Edited by Admin, : Fix quote.

I have always found this notion really intriguing.If the sun were to (hypothetically) just be "vanished" away we would continue to orbit as normal for 8 minutes or so? Is that correct?How can we visualise this situation in terms of spacetime curvature? Does ST sort of "spring back" into flatness?

Speed of light. It's a natural constant for the speed with which any part of the universe can influence any other part, and that includes gravity.--Percy

I'm fond of pointing out that if you invoke magic to set up your experiment, you cannot expect meaningful results! You cannot simply vanish away the mass of the Sun using any physics of which we are aware.However, answering in the spirit of your question, GR is a *local theory*. Space-time geometry at a point depends on the mass/energy distribution at that point. There is no action at a distance. The space-time curvature 7 light minutes out from the Sun is not a direct result of the Sun's mass, but of the space-time curvature slightly less than 7 light minutes out. Think of the old rubber sheet analogy. So the space-time will relax outwards from the centre, and will take eight minutes to affect the space-time around us.

I think Slevesque knows what a citation is. English is not his first language, and I think he didn't know that "cite" is short for "citation."--Percy

Oh of course. But then nor can we look at cats in boxes without actually looking at them. Sometimes hypothetically invoking magic in "what if" scenarios can be a valid method of seeking understanding from those with superior knowledge. IMHO. So the topology (is that the right use of the term?) of spacetime itself "reconfigures" itself at the speed of light? Is that what we are saying here? Or have I misunderstood?

We use geometry - topology refers to the connectedness of a space, and is a whole new subject within General relativity - but yes, that is the idea. However, it is very difficult to test directly.

How could it be tested in principle? Ignoring practical limitations for one second.Would the observation of a black hole forming be a method of establishing this somehow?If this is not the sort of topic intended just tell me to bugger off and I'll stop pestering you.

It can't Because you can't magic mass/energy into and out of existence. All you can do is shift it around. And I don't need to tell you that there is a limit to how fast you shift mass/energy around. Hence the problem...All you can do is look (indirectly) at gravitational waves. These do definitely appear to propegate at the speed of light. Not really. If the Sun collapsed to a black hole today, what gravitational effect would this have on us?

Oh bugger! I see. Yep.At the risk of totally talking out of my arse....(and when did that ever stop anyone round here?).....What if we wormholed the Sun away somehow? Is that an even in principle notion or is it just utter bollocks? I know almost nothing about gravitational waves. I thought they could be indirectly detected by astronomical observations of certain (highly massive) binary star systems. BUT I must admit I did also think that there were experiments in place to try and measure such things directly. Is this not the case? Well roughly 7-8 mins after this event I am "conjecturing" that the earth would be sucked into this black hole?

No. The gravitational field here would be just the same as it was. The event horizon of that black hole would be (pure guess here) a few kms only. Outside that we'd just keep orbiting like we do now.

This feels like two kids waving their arms enthusiastically in class to get the teachers attention!!!! No way Nosy!!We have gone from a region of massive but finite density to a region of infinite density. We cannot just keep orbitting unaffected. Can we?The 7-8 mins taken to affect us is the event horizon.Cavediver who is right and who is wrong? Necks are on the line here

Yes it is, but the wormhole is still just an extension of space-time, so the Sun will start moving off through the worm-hole at finite speed, so its gravitational effect will simply diminish, not instantaneously vanish - and of course, the space-time curvature caused by the Sun will be dwarfed by that of the worm-hole that carries it away!! Exactly - and by looking at the enegy loss in those systems, we can calculate the speed of the gravitational waves. Not at all - as Nosy explains...

*We* haven't - we are a long way from the Sun. All we care about is how much mass/energy is contained within our orbital radius. We don't give a damn how dense it is, just so long as it is a spherically symmetric distribution. Blow the Sun up, and as long as the entire debris cloud remains within our radius, and remains spherically symmetric, we'll keep orbiting. Likewise, should the Sun collapse. The only difference the collapse will make is that we can know get much closer to the interesting parts of the Solar space-time: r=2M (the event horizon, about 3km for the Sun), and r=0 (the singularity). Normally, we cannot encounter these regions of space-time as they don't exist, as the Sun occupies that space.