October 21, 2004

Weighty issue

Well, further evidence, should you need it, that the General Theory of Relativity actually works. New satellite tracking data has just confirmed frame dragging as a real effect. But the coolest thing is that they used a highly accurate gravity map of Earth to compensate for minute local effects. It turns out that Chileans, Icelanders, Indonesians and Turks have something in common:


Note, this is not the same as weighing more. This gravity map does not compensate for the effect of centrifugal force felt at the equator. Charles and I have sparred before on whether people at the pole weigh more or less, but not as effectively as this lot, who manage to quote some actual measurements. The verdict: You weigh 0.5% more at the poles than on the equator.

Posted by Stefan at 12:32 PM GMT

I thought it was the "Theory of General Relativity", not the "General Theory of Relativity".

Posted by: Sterling on October 21, 2004 12:50 PM

That settles it then, I am never going to the Poles. In fact, I'm going to head to the equator so I no longer have to lie about my weight!!!

Posted by: michelle on October 21, 2004 01:17 PM

So, wait, because they're on the Equator, and because gravity is "heavier" there, Indonesians and Peruvians think they're fatter than they really are? No more of those "diet" pisco sours.

Posted by: mike on October 21, 2004 02:12 PM

Sterling, you can call it what you like, but General Theory of Relativity is the most used.

Posted by: Stefan Geens on October 21, 2004 04:41 PM

They're different things. General Relativity is a specific thing, as is special relativity. The general theory is a combination of both of these, I guess? But the warping of spacetime - which this new satellite was launched to test - was initially described in Einstein's Theory of Special Relativity.

My recollection is that Einstein needed to describe Special Relativity to disprove the Newtonian model of the universe, because the latter could not be reconciled with General Relativity. Specifically, if nothing can move faster than light, how is it that gravitation can have instantaneous effect? Einstein then described curved space in his Theory of Special Relativity, and it was proved empirically - I recall - before 1930 by measure the distortion of light around the sun during a solar eclipse.

Posted by: Sterling on October 21, 2004 07:54 PM

Yup, correct on every point there, except for the "deflection of light" thing which is bogus. Trust me.

Posted by: Simon on October 21, 2004 10:02 PM

Simon - I looked this up and there's all sorts of information about "gravitational lensing", like this. Are you saying that gravitational lensing is bogus, or that the specific experiment/eclipse I mentioned was mistaken?

Posted by: Sterling on October 22, 2004 12:23 AM

Sterling, I thought you were trolling - the part about deflection of light was really the only thing that was correct. My apologies.

In short: Special relativity is just a special case of general relativity. Warping of spacetime is described by the latter - the former deals with "flat" spacetime. The Newtonian model of the universe didn't fit together with Maxwell's theory of electromagnetism (which predicts the finite speed of light), and the special theory of relativity was needed to fix this. Gravitational effects propagate with the speed of light.

The general theory has as its base the equivalence of gravitation and acceleration, and all it does is, more or less, to describe how spacetime is "curved". It was vindicated both by the observation during the eclipse, as well as by making better predictions regarding the orbit of Mercury. (And in modern times by lots and lots of other measurements.)

Posted by: Simon on October 23, 2004 12:17 PM