Severian

There are of course differences between direct and indirect measurements. Often in physics one is forced to assume particular things and then say, 'if this assumption is true, our measurement shows...'. I think this is quite reasonable, as long as one remembers that assumptions have been made. This is true with gravity as well. In addition to the direct observations of the 'speed of gravity', one can show that gravity must travel at (or very very near to) the speed of light just by thinking what would happen if it didn't. As I pointed out earlier in this post, gravity must be mediated by a particle, which I call the 'graviton'. This graviton does not have to be fundamanetal - it could be a composition of other particles (or strings!) if you like - the important point is that this must exist in a low energy (ie. < Planck mass) effective theory. Now, lets further as what would happen if the graviton did not travel at c? To do this, the graviton would have to have non-zero mass. How non-zero depends on how close to c it travels. Now, it is relatively easy to show in QFT that any force mediated by a massive particle will give a (classical) force-distance dependance of: [math]F \propto \frac{1}{r^2} e^{-mr}[/math] (where I am setting c=1, r is the distance from the source and m is the mass of the mediator). This was first shown by Yukawa. You can see that any mass causes an exponential fall-off of the force. In fact, this is what happens to the weak nuclear force. It is 'weak' because the particles mediating it are massive, so we end up with an exponential decline of the force with distance. Of course, one could imagine that m is so small that one can expand [math]e^{-mr} \approx 1-mr[/math] giving the Newtonian result plus a small correction, but one must remember that gravity has been tested on atronomical scales. The values of r at which gravity has been tested are huge, so m would need to be very small indeed to make m*r a small number. So, it is reasonable to say that m must be very small and that gravity propagates at (very near to) the speed of light.

Have a look at this: http://map.gsfc.nasa.gov/m_uni/uni_101bb2.html

I agree with pretty much everything that Martin (and JaKiri) said. I only have one clarifying comment: The twin paradox is normally formulated assuming Bob is at rest in a frame with no gravity - so he is feeling no force, while Bill feels the acceleration 'force' when he turns round. If Bob were on Earth the whole time and Bill maintained an acceleration/decelaration of 1g the entire time of his travel, their clocks at the end would show the same time - they would have aged the same amount because one could not tell their frames apart from the forces they feel.

Give me any definition of 'forever' and I can give you a value of the frictional force which is small enough to allow the thing to spin 'forever'.

University of Glasgow: William Thomson, Lord Kelvin, returned at the age of 22 to the University where he had studied and took up the chair of Natural Philosophy (Physics), a post he held for 53 years. Arguably the pre-eminent scientist of the nineteenth century, he enjoyed an international reputation for theoretical and practical research across virtually the entire range of the physical sciences. Adam Smith, economist, philosopher and author of The Wealth of Nations, was only 14 when he started as a student at Glasgow. In 1751 he returned as Professor of Logic, transferring to the Chair of Moral Philosophy shortly afterwards. Joseph Black taught both chemistry and medicine in the eighteenth century and introduced a modern understanding of gases. John Logie Baird, one of television's pioneers, was attending the University when the First World War intervened. James Watt conducted some of his early experiments with steam power while working at the University. William Macquorn Rankine, pioneer of modern thermodynamics, wrote the first authoritative textbooks on engineering. Nobel laureates: alumnus Sir William Ramsay received the Nobel Prize in Chemistry in 1904 for his discovery of inert gases which established a new group in the periodic table. Frederick Soddy lectured at the University in the early 1920s. He was awarded the Nobel Prize in Chemistry in 1921 for his work on the origin and nature of isotopes. graduate John Boyd Orr campaigned for an adequate diet for the people, starting during the First World War; his food plan produced a better nourished population than ever before. He was awarded the Nobel Peace Prize in 1949 for his work with the United Nations. graduate Sir Alexander Robertus Todd received the Nobel Prize in Chemistry in 1957. His research led directly to the understanding of nucleic acids. Sir Derek Barton, Regius Professor of Chemistry in the mid-1950s, received the Nobel Prize in Chemistry in 1969 for his work on conformational analysis. Sir James Black, who worked at the University's Veterinary School during the 1950s, was awarded the Nobel Prize in Chemistry in 1988 for discoveries of important principles for drug treatments.

It does. Gravity goes like [math]\sim \frac{1}{r^2}[/math] and the surface of a sphere goes like [math]\sim r^2[/math]. I don't see your problem.....?

That isn't very true. go look at the mathematics boards here and you will see what I mean. They use a lot of words...

"Collapsing the wavefunction" perhaps?

Well, we did it - we bought an ipod mini (in silver). Thanks for all your help guys.

Sorry I have not gotten back to this. Incredibly busy at the moment.... Will go read the paper sometime soon...

Why is that a fault? You don't have to use it.

OK, I am now swithering between the Apple iPod Mini 4 GB MP3 Player and the Creative Labs ZEN Micro 5 GB MP3 Player...

Thanks for the info guys. I think I should do a little more research online but now I know what to look for....

My wife asked for an iPod for Christmas. I have heard good things and bad things about iPods. Does anyone here have any first hand experience? What are the pros and cons of ipods compared to different similar MP3 (or whatever) players? Advice please...

But YT and I are now free to maim and slaughter at will... Edit for the authorities: That was a joke....

Ahhh.... all becomes clear...

Time to close this spamfest I think....

WMAP has given us the best estimates for the age of the universe so far. Take a look here: http://map.gsfc.nasa.gov/m_uni/uni_101age.html

I don't mean to steal your thunder, but how about a more interesting paper on gravity posted today: http://www.arxiv.org/abs/hep-ph/0412109 "Lagrangian formalism of gravity in the Randall-Sundrum model"

That is actually a rather negative statement. One is usually discouraged from staying on at the same place, so the best people move on...

From its acceleration due to gravity...

NASA don't use GR for that - Newtonian gravity is good enough on a planetary scale.

Yes, quarks and leptons are point particles as far as all experimental evidence so far shows, in the sense that their size is less than about [math]\hbar c /100GeV \approx 0.2 fm[/math]. Of course, if string theory is true, then they are not point particles but are composed of strings, but there is no evidence for that yet.

Well, a burgler and a mass murderer are two different things....