Monday, March 11, 2013
ALIENS!
This articles came out this morning from MIT, detailing the work of a team of scientists in the UK on the remains of a meteorite, which landed in Sri Lanka this December. I don't want to give away any punch lines or anything (I'm going to), and you should read it yourself, but they're claiming to have found non-terrestrial microbial fossils on the meteorite!
Thursday, March 7, 2013
Mercury's Perihelion Advance
I seem to read a lot about (or, at least, I used to, when I had time) Mercury's anomalous orbit. It feels like every physics book I picked up (not counting text books) that talked about astronomy, cosmology, or gravity, mentioned the anomalous orbit of Mercury. I never actually looked up what that meant until just now (none of said books explained what "perihelion advance" meant. Probably should have looked it up five years ago). Anyhow, here's about Mercury's orbit, kinda neat stuff!
First off, what is anomalous about Mercury's orbit? It looks something like this, over time (obvious exaggerated but you get the point):
So what? Don't all the planets do that? Yeah, but Mercury's advance is 43.1" per 100 years greater than the calculated value. So, in other words, Mercury's perihelion is 43.1 arcseconds away from its expected position every 100 years. This anomaly was first measured in 1859 by Urbain Le Verrier. Initially there were numerous possible solutions presented (including the theory that there was another, smaller, planet inside Mercury's orbit). At one point or another, it was even claimed that such a planet had been observed (they named it Vulcan). It is now clear that there are no other planets inside Mercury's orbit (I think we would have found them, by now); so what's causing the shift?
First, a little more about the orbit: we can measure the precession of Mercury to be 574.10±0.65 arcseconds/century. So what contributes to that? 531.63 ±0.69" can be attributed to the gravitational pulls from the other planets. 0.0254" are due to the not-quite-spherical shape of the sun (it's measurably oblate). This leaves 42.98 ±0.04" unaccounted for.
The solution was finally reached by Einstein's theory of General Relativity. It turns out that the equations behind Newton's gravitational theory (which predict no precession for the orbit in the absence of other celestial bodies) are slightly off from those designed in Einstein's gravitational theory. The differences are more noticeable in Mercury's orbit because of how close Mercury is to the sun. The other planets also have measurable perihelion advances, but they're substantially smaller than Mercury's.
When you run through the equations for Newtonian gravitation, you get the orbit of an ellipse. If, on the other hand, you work out the orbit using General Relativity you get something really ugly that I don't want to type up. If you're interested in the math you can find it all here, which, incidentally, is a good read on the subject, even if you're not interested in the math. So there you have it! Mercury's orbit is anomalous because Newtonian Gravitation is not quite correct.
References:
Perihelion Advance image by - European Space Observatory
Tests of General Relativity - Wikipedia
First off, what is anomalous about Mercury's orbit? It looks something like this, over time (obvious exaggerated but you get the point):
So what? Don't all the planets do that? Yeah, but Mercury's advance is 43.1" per 100 years greater than the calculated value. So, in other words, Mercury's perihelion is 43.1 arcseconds away from its expected position every 100 years. This anomaly was first measured in 1859 by Urbain Le Verrier. Initially there were numerous possible solutions presented (including the theory that there was another, smaller, planet inside Mercury's orbit). At one point or another, it was even claimed that such a planet had been observed (they named it Vulcan). It is now clear that there are no other planets inside Mercury's orbit (I think we would have found them, by now); so what's causing the shift?
First, a little more about the orbit: we can measure the precession of Mercury to be 574.10±0.65 arcseconds/century. So what contributes to that? 531.63 ±0.69" can be attributed to the gravitational pulls from the other planets. 0.0254" are due to the not-quite-spherical shape of the sun (it's measurably oblate). This leaves 42.98 ±0.04" unaccounted for.
The solution was finally reached by Einstein's theory of General Relativity. It turns out that the equations behind Newton's gravitational theory (which predict no precession for the orbit in the absence of other celestial bodies) are slightly off from those designed in Einstein's gravitational theory. The differences are more noticeable in Mercury's orbit because of how close Mercury is to the sun. The other planets also have measurable perihelion advances, but they're substantially smaller than Mercury's.
When you run through the equations for Newtonian gravitation, you get the orbit of an ellipse. If, on the other hand, you work out the orbit using General Relativity you get something really ugly that I don't want to type up. If you're interested in the math you can find it all here, which, incidentally, is a good read on the subject, even if you're not interested in the math. So there you have it! Mercury's orbit is anomalous because Newtonian Gravitation is not quite correct.
Newton. Because kitties named after physicists.
References:
Perihelion Advance image by - European Space Observatory
Tests of General Relativity - Wikipedia
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